UPTEC E 17 013

Examensarbete 30 hpOktober 2017

Grounding of distrubution grids

High impedancegrounding compared to solid

grounding with Fault Current Limiter

Maria Kättström

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

High impedancegrounding compared to solidgrounding with Fault Current Limiter

Maria Kättström

Today cables replace overhead lines in distribution systems and cause higher system capacitance and higher capacative fault currents. This fault current, in the fault location, is limited by a Petersén coil and a resistance in parallel when a fault occurs in so called “high impedance grounded systems” which are commonly used in Europe. The high impedance however has the disadvantage that it needs to be adjusted to the capacitance in the system in order to optimize the limitation of the fault current.

Another option is to use a solidly grounded system with a so called “Fault Current Limiter” (FCL) instead, on the outgoing terminals of the transformer feeding the system. The FCL interrupts the outgoing current on the load side of the transformer that feeds the system, in case of a fault, by forcing the current to a zero with a counter voltage. The FCL in this master thesis triggers on 1.5 times higher current than nominal current. After fault clearing the FCL is reclosed. The FCL has the advantage that it does not have to be adapted to the capacitance in the system. It should be noted that the current in all of the phases (even the healthy phases) are interrupted downstream the FCL. It also has the ability to interrupt the single line-to-ground fault before it develops into a three phase fault.

This thesis presents a comparison between the high impedance grounding and the FCL. The characteristics of the two grounding principles are investigated in a distribution system from Vattenfall. The assessment is done via simulations in the program PSCAD with three types of grounding of the transformer; high impedance grounding, solid grounding (without FCL) and solid grounding with FCL. The system is simulated with the faults “single line-to-ground” and “three phase short circuit” respectively, even if a line-to-line can occur. The results from the simulations show that the FCL gives a short fault duration time and a possibility to limit both single line-to-ground faults and three phase faults. The high impedance grounding on the other hand is able to limit single line-to-ground faults whereas it is generally known that not able to limit three phase short circuits.

ISSN: 1654-7616, UPTEC E 17 013Examinator: Mikael BergkvistÄmnesgranskare: Juan de SantiagoHandledare: Elisabeth Lindell

Sammanfattning

Kablar ersätter i dagsläget luftledningar i distributionssystem och orsakar en högre kapacitans ochen högre kapacitiv felström. Den felström som uppstår när ett fel har inträffat begränsas av enPetersénspole parallellt med en resistans. Detta kallas "hög impedansjordning" och används van-ligen ute i Europa. Dock har den höga impedansjordningen nackdelen att den behöver stämmasav till kapacitansen i systemet för optimerad felströmsbegränsning.

Ett annat alternativ är istället att använda ett direktjordat system, med en så kallad "Felströms-Begränsare" (engelska "Fault Current Limiter" FCL) på de utgående terminalerna på systemetsmatande transformator. FCL:en begränsar, i händelse av ett fel, den utgående strömmen på trans-formatorns lastsida, genom att forcera ner strömmen till noll med hjälp av en motspänning. FCL:ensom användes i examensarbetet utlöses vid 1,5 gånger högre ström än den nominella strömmen.Efter felavhjälpning återställs FCL:en till ursprungsläget. FCL:en har den fördelen att den intebehöver anpassas till kapacitansen i systemet men strömmen i alla faser (även de friska faserna)begränsas nedströms om FCL:en. Den har också förmågan att förhindra en övergång från enfas-till-jord-fel till ett trefasfel.

I examensarbetet presenteras en jämförelse mellan höga impedansjordningen och FCL:en. De tvåbegränsningsprincipernas egenskaper undersöks i ett distributionssystem från Vattenfall. Studienär gjord via simuleringar i programmet PSCADmed tre typer av jordning på transformatorn; Peter-sénspolejordning, direktjordning (utan FCL) och direktjordning med FCL. Systemet är simuleratmed felen "enfas-till-jord-fel" respektive "trefas kortslutning", även om "tvåfas-fel" kan uppstå.Resultaten från simuleringarna visar att FCL:en ger en kort varaktighet av felet och har möjlighetatt begränsa både enfas-till-jord-fel och trefasfel. Petersénspolen däremot kan begränsa enfas-till-jord-fel till jord medan det är allmänt känt att den inte kan begränsa trefaskortslutningar.

3

Acknowledgements

Jag vill börja med ett stort tack till min handledare Elisabeth på ABB. Du är duktig, smart, nog-grann och har humor, tack för ditt stöd och handledning. Jag kommer sakna de tidiga morgnarnamed benhård träning tillsammans med dig och Jonas på Friskis och Svettis med en frukost somavslutning efter passen. Jag kommer framför allt sakna ditt skratt när jag köpte vitkål till ex-trapris.

Jag vill också tacka Lars Jonsson, som hjälpte till med sin kunskap om felströmsbegränsare ochditt tålmodiga sätt bolla idéer om simuleringar tillsammans med Elisabeth. Jag vill även tackaLars Liljestrand som i början av examensarbete hjälpte mig förstå hur PSCAD fungerar och hurfelhantering av ett system fungerar.

Tack Daniel och Fredrik som tog hand om mig på Vattenfall och gav alla mina frågor om distribu-tionsnät ett svar. Och framför allt gav mig inputs av vad ett energibolag vill få ut av detta. Ochtack till Juan, du förtjänar också ett tack som var min ämnesgranskare under min tid på ABB.Sist men inte minst Magnus, du är så hjälpsam och duktig. Tack allihopa!!!

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Contents

1 Introduction 71.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.2 Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.3 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 Theory 82.1 Earthing standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2 Earthing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1 High impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.2 Solidly grounded system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3 Fault Current Limiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.4 Faults in the system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.4.1 Three phase short circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.4.2 Single line-to-ground fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4.3 Back-fed earth fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3 Simulations 133.1 PSCAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.2 The "Ideal system" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.3 The "Real system" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.4 The "Radial system" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.5 "Ideal back-fed earth fault" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4 Results 194.1 The "Ideal system" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.1.1 Single line-to-ground fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.1.2 Three phase short circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.1.3 Comments on the FCL in the "Ideal system" . . . . . . . . . . . . . . . . . 22

4.2 The "Real system" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.2.1 Single line-to-ground fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.2.1.1 Secondary substation "A" . . . . . . . . . . . . . . . . . . . . . . . 234.2.1.2 Secondary substation "AI1" . . . . . . . . . . . . . . . . . . . . . 254.2.1.3 Secondary substation "O1" . . . . . . . . . . . . . . . . . . . . . . 26

4.2.2 Three phase short circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2.2.1 Secondary substation "A" . . . . . . . . . . . . . . . . . . . . . . . 274.2.2.2 Secondary substation "AI1" . . . . . . . . . . . . . . . . . . . . . 284.2.2.3 Secondary substation "O1" . . . . . . . . . . . . . . . . . . . . . . 30

4.2.3 Comments on the FCL in the "Real system" . . . . . . . . . . . . . . . . . 304.3 The "Radial system" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.3.1 Single line-to-ground fault at "Radial system 1" . . . . . . . . . . . . . . . 314.3.1.1 Secondary substation "A" in the "Radial system 1" . . . . . . . . 314.3.1.2 Secondary substation "AI1" in the "Radial system 1" . . . . . . . 34

4.3.2 Three phase short circuit at "Radial system 1" . . . . . . . . . . . . . . . . 364.3.2.1 Secondary substation "A"in the "Radial system 1" . . . . . . . . . 374.3.2.2 Secondary substation "AI1" in "Radial system 1" . . . . . . . . . 39

4.3.3 Single line-to-ground fault which develops into a three phase short circuit in"Radial system 1" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.3.3.1 Fault in secondary substation "A" . . . . . . . . . . . . . . . . . . 424.3.3.2 Fault in secondary substation "AI1" . . . . . . . . . . . . . . . . . 48

4.4 Back-fed earth fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514.4.1 Simulated with an FCL on transformer T1 . . . . . . . . . . . . . . . . . . 514.4.2 Simulated with high impedance grounded transformer T1 . . . . . . . . . . 52

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5 Summary of the simulation 535.1 Summary of the "Ideal system" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535.2 Summary of the "Real system" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535.3 Summary of the "Radial system" . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6 Conclusions 56

7 Further work 57

8 Appendix 1: The "Real system" with faults in secondary substation "O1" 60

6

1 Introduction

This master thesis is performed in cooperation with Vattenfall and investigates the possibilities ofa solidly grounded distribution system with a FCL. Currently, distribution system transformers aregrounded using Petersén coils parallel with resistances, which are tuned to the grid, and resistancein parallel. A well tuned Petersén coil parallel with resistance is able to limit the fault currentfor a single line-to-ground fault and is adjusted to the capacitance in the system, to compensatefor the capacitive earth fault current. Since cables today are replacing overhead lines, the systemcapacitance increases and thus the size of the Petersén coil also increases. But by using a solidlygrounded system with Fault Current Limiter (FCL) instead of a Petersén coil, the potential tolimit the fault current independent of the type of faults increases.

The most common type of fault in distribution systems, which is single line-to-ground faults foroverhead line systems, can develops to three phase faults in cables system. How large the limitingeffect of the FCL is has been studied by simulations in PSCAD together with grid data of Vattenfall.

1.1 BackgroundTraditionally the impedance grounding has been used in overhead lines system to limit singleline-to-ground faults. It dominates the market in Scandinavia and Europe since around eightyyears. The impedance grounding using Petersén coils in parallel with resistance in the transformerneutral will minimize fault currents at the fault location due to single line-to-ground faults, whichis required due to safety regulations. The fault is cleared by an upstream circuit breaker. Two orthree phase faults and short circuits are causing high currents which are also interrupted by theupstream circuit breaker, but are not limited by the Petersen coil. [1] [2][12] [13]

Cables are replacing overhead lines for distribution systems. The cables will improve the availabilityof the distribution system since the number of faults will be reduced. One drawback of cable systemsis that the few faults that occur could cause large capacative earth current and longer reparationtimes than faults on over head lines. Another drawback of cable systems is the large capacitancerequiring a high power of the neutral reactor.[3] [14]

An improved fault handling in cable systems could be achieved by replacing the high impedancegrounding by solid grounding in combination with fault current limiters for fast short circuit currentinterruption. A fast fault clearing will have the benefit of minimizing the damages at the location ofa fault and all parts the fault current flow through. The FCL also enables the circuit breakers usedon outgoing feeders to be replaced by load break switches together with smaller surge protectorwhich will reduce the cost further. [15]

1.2 AimThe aim of this study is to analyse what benefits and conditions there are of using a fault currentlimiter and solidly grounded systems instead of impedance grounded systems. The benefits andpossibilities with solidly grounded systems with fault current limiter shall be examined. This studyalso shows the fault handling in a distribution grid from Vattenfall in simulations in PSCAD, withand without FCL. Single line-to-ground fault and three phase short circuit in a distribution systemare studied to get a more trustworthy result for the grounding systems. Last but not least thestandards and safety regulations available for fault handling are also going to be studied.

1.3 LimitationsIn this master thesis the limitions are:

• The faults which are taken into account in this study are: single line-to-ground faults andthree phase faults

• The grid in these simulations is a typical Swedish MV rural network

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2 Theory

Grounding of a transformer in a distribution system can be done in a number of ways. In thisinvestigation the solid grounding with and without FCL and the high impedance grounding arestudied. The main purpose of the grounding is safety and protection for plants, humans andanimals. The grounding concepts need to follow different standard independently of the groundingof the transformer. [9] [16] [17]

2.1 Earthing standardsGrounding of a plant, or in this case grounding of a transformer, has the meaning to protectthe system, plant, transformer and components. A well working grounding lets the earth currentand leakage currents flow to earth without risk for damages [16]. For a high voltage plant withvoltage over 1000 V AC or 1500 V DC, it applies to non-solid grounded systems that one ortwo phase ground faults disconnect automatically. But plants without overheads lines and witha nominal voltage below 25 kV, has exceptions and only need to signal the single line-to-groundfault automatically. But for a solid ground system the regulation tells that the system needs todisconnect the fault within 0.5 s. [17]

2.2 EarthingThe purpose of a grounded system is including increased safety and a common reference pointfor the voltage level at each side of the transformers in the system [4]. The idea is also that thegrounding system will not be used except in faults state and if the load in the three phases isnot balanced. When the faults happen, the grounding systems gives higher personal safety, andprotection for equipment. This also applies at lightning strikes. So earthing the system will reducethe magnitudes of over voltage transients and make it easier to locate the fault. But the damagein the system depends on how long the fault time is and the size of the fault current [5].

2.2.1 High impedance

The impedance grounding of a transformer can be done by a Petersén coil. The Petersén coil iseffective in case of single line-to-ground faults, often encountered i overhead line systems, where itlimits the fault current so no arc can be shaped. This applies to short-term disconnection of theresistance that is parallel to the coil. The mechanism behind the ability to limit the fault currentcomes from the capability of the Petersén coil to compensate the capacitive fault current in thesystem. This makes the faults manageable until the fault is cleared. The active fault current canbe limited by disconnect the resistance in parallel with the Petersén coil 1 s after the fault occurred,which is within the standard about disconnection. To maintain the ability to reduce the capacitivefault current, the Petersén coil must be adjusted to the grid, i.e. it must be adjusted to the gridcapacitance to ground. This can be determined by the number of overhead lines and cables in thegrid and the length of overhead lines and cables.

The Petersén coil will neutralize the capacitive earth fault current IC by generating the samemagnitude of the inductive current IL but 180 degrees out of phase. From a single line-to-groundfault as shown in Figure 2.2.1a, the size of the coil can be derived from IC where IC is the resultof the phase currents ICR and ICY for the phases R and Y in Figure 2.2.1a, phase voltage, Uphase,between one phase and ground and the system voltage, Usyst (Usyst =

√3Uphase), between the

phases. [8] [6]

IC = ICY + ICR (2.2.1)

ICY =Usyst

1ωC

=

√3Uphase

1ωC

(2.2.2)

ICR =Usyst

1ωC

=

√3Uphase

1ωC

(2.2.3)

ICY = ICR (2.2.4)

8

The current Ic can by the phase diagram also be written :

IC =√

3ICR =√

3ICY (2.2.5)

IC =

√3 ·√

3Uphase1

ωC

= 3ωCUphase (2.2.6)

Since the current into the coil has the same amplitude as the capacitive earth current the balancebetween IC and IL can be written as:

IL = IC (2.2.7)

The size of the coil, L, can be derived by the current trough the Petersén coil

IL =Uphase

ωL(2.2.8)

Uphase

ωL= 3ωCUphase (2.2.9)

L =1

3ω2C(2.2.10)

(a) Single Line-to-Ground fault on phase B [6] (b) Phase diagram [7]

Figure 2.2.1: Petersén coil, fault handling and phase diagram

The resistance which can be found in parallel with the coil in Figure 2.2.2, has the task to handlethe active current to the Petersén coil and together with the Petersén coil, they are called highimpedance grounding. The size of the resistance can be calculated by using the formula:

R =Uphase

IR(2.2.11)

Were IR is the current through the resistance and is typically 5, 10 or 15 A. [20]

Figure 2.2.2: The high impedance grounding at the secondary side of the transformer, withthe resistance R in parallel with the coil L. This reasoning applies to systems consisting mainlyof overhead lines and malfunctioning for systems consisting mainly of cables. When the largecapacitive earth current becomes too high for this reasoning.

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2.2.2 Solidly grounded system

A solidly grounded transformer means that the transformer has the neutral point directly connectedto earth, see Figure 2.2.3. This can decrease the fault voltage transient (compered to a ungroundedsystem), if the transformer reactance is small enough, [5]. But the disadvantage of a solidlygrounded system is that the fault current becomes higher for the solidly grounded system than forthe non-solidly ground system, e.g. a high impedance grounded system. Therefore it is importantthat the breakers in a solidly grounded system switch fast enough to reduce the damage in thesystem, for personal and protection for equipment. For the single line-to-ground fault, the voltageof the faulty phase will go down to zero and the source will feed this type of fault. The two healthyphases will be unaffected. [5]

Figure 2.2.3: The transformer secondary side is solidly grounded.

2.3 Fault Current LimiterThe fault current becomes limited before it rises to the peak value with a Fault Current Limiter(FCL), which can limit several types of faults e.g. single line-to-ground fault, three phase faultand more. In this thesis, the type of FCL which is used is the FCL which forces the current to azero crossing with counter voltage and creates a fast current interruption. The FCL can performmultiple operations and has the ability to perform an open-close operation in a few ms. The tripvalue for limiting can be chosen as e.g. 1.5 times nominal current. It is possible to install theFCL at an incoming feeder of a substation. The FCL also has the advantage to work as a mediumvoltage DC circuit breaker, since the FCL will not wait for the natural zero crossing to break thecurrent as a traditional AC breaker would do. So the FCL will also fulfill the requirements for amedium voltage DC circuit breaker [15]. But independent of the fault all of the phase currents andphase voltages are limited to zero unlike in the other grounding system, where the healthy phasesstill work.

One example of fault handling with an FCL is shown in Figure 2.3.1, where the incoming side ofthe feeder has a protection from an FCL, which limits the fault current before it has risen to themaximum peak value. This also gives the option to exchange the circuit breakers to load breakerswitches at the outgoings feeders in Figure 2.3.1.

The sequence shown in Figure 2.3.1 can be described as follows:

a) A fault occurs and the FCL at the incoming feeder and the load break switch on the faultyoutgoing feeder are tripped

b) The FCL has opened and interrupted the short circuit current

c) The outgoing feeder load break switch is opened and the faulty feeder is disconnected

d) The FCL is reclosed and the bus is re-energized

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Figure 2.3.1: Fault handling using an FCL on the incoming feeder in combination with load breakswitches on the outgoing feeders [15]

2.4 Faults in the systemThe most common faults are single line-to-ground fault and single line-to-ground that turns intoshort circuit faults. One example of a fault, is the lightning strike in overhead lines. This faultstarts as a one phase fault and ends as a short circuit commonly. A similar type of fault couldhappen for cables, if an excavator hits the isolation layer and damages it and one of phases. Thiswill commonly end as three phase fault [21]. So two types of faults that will be discussed in thisthesis are hence single line-to-ground faults and three-phase short circuits and they are presentedbelow.

2.4.1 Three phase short circuit

An ideal three phase fault is shown in Figure 2.4.1 with phase voltage, Uphase, system voltage Usyst

(with a magnitude of√

3 · Uphase), short circuit current, ISC , short circuit power SSC and shortcircuit impedance, ZSC . For the three-phase short circuit the following formulas can be used forcalculations of a balanced system without FCL. With an FCL the fault current is limited.

SSC =√

3 · ISC · Usyst =U2syst

ZSC(2.4.1)

ISC =U2syst√

3 · Usyst · ZSC

(2.4.2)

ISC =Usyst√3 · ZSC

=Uphase

ZSC(2.4.3)

ISCa + ISCb + ISCc = 0 (2.4.4)

Where the short circuit current, ISC , is determined from the phase voltage and the short circuitimpedance upstream from the fault [12].

Figure 2.4.1: An ideal three phase short circuit in a solidly grounded system.

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2.4.2 Single line-to-ground fault

An ideal single line-to-ground fault is illustrated in Figure 2.4.2. The capacitive earth fault currentin a medium voltage high impedance grounded system (10-20 kV) caused by the single line-to-ground fault is calculated from equation 2.2.6. This formula is used instead of the formula derivedfrom the zero-, positive- and negative sequence and calculates the capcative earth current in thesystem [13]. This facilitates comparison later, when the capacitive earth current is measured.

Figure 2.4.2: An ideal single line-to-ground fault in an ungrounded system

2.4.3 Back-fed earth fault

One unusual fault, which can occur in overhead line systems is the back-fed ground fault illustratedin Figure 2.4.3. There one of the three phases in the overhead lines between the two transformersbreaks into two parts. The part nearest transformer T2, falls to the ground. Transformer T2feeds the line that has fallen to ground through the ground connection. Since the transformerT2 is delta-Y connected the fault will be fed by the transformer. The two healthy phases causeunbalance in transformer T2. The fault current goes through the ground and feeds the fault phaseat T2. [11]

The fault current IF can be calculated from the zero-, positive- and negative-sequence which isderived by Charles Legeyt Fortescue [13] according to the formula:

IF =UF

9ZL + 6Rj + 2Z0(2.4.5)

where Z0 is the zero-sequence impedance, UF is the voltage over Rj which corresponds to the phasevoltage and Rj is the fault resistance. The equivalent impedance of the Y-connected transformerT1 is described by ZL (in equation 2.4.5) determined from the equation below: [18]

ZL =U2R

Sactual(2.4.6)

where UR is the rated voltage at transformer T1 and Sactual is the power consumption fromtransformer T1.

Figure 2.4.3: Schematic drawing of back-fed ground fault where transformer T2 back-feeds thefault.

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3 Simulations

The simulations performed in this thesis are based on a system from Vattenfall. Initially, simula-tions will be performed in an "Ideal system" which is a stripped version of the distribution systemfrom Vattenfall with one generator, transformer and three capacitors to ground (one per phase).The next step is a "Real system", which is built as the system in Figure 3.0.1 from Vattenfall withone generator, transformer, cables, overhead lines and loads. Finally the "Radial system" is builtwith a bus with generator and transformer at the incoming feeder and five outgoing feeders witha "Real system" at respective outgoing feeder. Two of the three systems in this investigation, the"Ideal system" and the "Real system", are simulated with high impedance , solid grounding withand without FCL, while the third system, "Radial system", is simulated with Petersén coil andFCL. They are all simulated and built in the simulation and calculation program PSCAD.

The real distribution system from Vattenfall in Figure 3.0.1, can be described as a main branchwith several branches. The main branch consists of secondary substations named by just one letteror one letter followed by the number 1 e.g. "A" or "B1". This is marked with orange colour inFigure 3.0.1. The branches consist of secondary substations named by letters with a higher numberthan 1 e.g. "B2" and "B3".

In the secondary substations "A", "O1" and "AI1" marked in Figure 3.0.1, faults will be simulatedindependently of each other. These three places are of interest since the fault current is high insubstation “A” and lower in substation O1 and low in substation AI1. The faults in the secondarysubstation are simulated one at a time e.g. a single line-to-ground fault in secondary substationO1.

The size of the transformer is the sum of all loads in the systems, see Figure 3.3.3, multipliedwith a factor of two for margin against overload. The transformer is Y-Y connected since it is thegeneral connection in a general system in Vattenfall [20]. The loads consist of households and the"Real system" and the "Radial system" are simulated with maximum load, Pmax and Qmax. Thetype of cables and overhead lines in the system is taken form the reality. The voltage levels at thetransformer, 75/11 kV, also come from an arbitrary transformer 75/10.45 and have been roundedup to an even number to make it easier to calculate the size of the transformer.

One ideal system with back-fed earth fault is also simulated in order to see if it is easier to detectthe fault with solidly grounded Y-Y connected transformer and if the FCL will detect the fault.This system is a stripped version of the system from Vattenfall as the "Ideal system" and theresults shows a fault which is difficult to detect so the ideal3 system was never extended to a fullscale system for this type of fault. All simulations are done with a time step of 25 µs.

Figure 3.0.1: An overview of the "Real system", simulated in this master thesis, with markedmain branch and secondary substations "A", "O1" and "AI1".

13

3.1 PSCADPSCAD has been used for the simulations of the "Ideal system", the "Real system" and the "Radialsystem". PSCAD is an EMTDC based calculation and simulation program, where PSCAD standsfor Power System Computer Aided Design. EMTDC stands for "Electro Magnetic Transient &Direct Current". This program is a graphic simulation program and makes it possible to simulateschematic circuits instead of entering data through text list. It is also used for calculation oftransients, faults and transmission lines etc. The user can alter parameters during the simulationsand it is also compatible to simulate earth faults, [10].

3.2 The "Ideal system"The ”Ideal system" in Figure 3.2.1 consists of one generator feeding a transformer. The transformersecondary side is grounded via a high impedance or solidly grounded. The solidly grounded systemis investigated with and without an FCL installed. The three capacitances to ground on the loadside of the transformer are representing cables and overhead lines in the "Real system". The sizeof the transformer is determined from the total sum of the loads in the system multiplied with afactor of two and rounded to 7 MVA. The multiplication with a factor of two is due to the marginfor overload, in Vattenfall’s systems. The voltage rating of the transformer is chosen according tothe arbitrary transformer namely 75/11 kV. To keep it simpler in this "Ideal system", the loadsalong the system and the secondary substations are removed. This simple system makes it simplerto investigate the grounding systems on a theoretical level and also gives a picture of how thesystems are able to manage a fault.

With the same size of capacitors between each phases and ground, independently of the groundingsystem, the nominal load current becomes same for the three systems, around 8 A, which makesit possible to compare the three grounding systems each other shown in Figure 3.2.1. The voltageand current on the load side of the transformer are measured during the simulation, for singleline-to-ground fault and three phase short circuit.

The capacitive earth current (Icap) and the incoming current through the high impedance ground-ing (Iind) are simulated for the single line-to-ground fault and the system with Petersén coilgrounding. These measurements are shown in Figure 3.2.2. The value of the Petersén coil, inFigure 3.2.2, is adjusted to the capacitance in the system, done by a single line-to-ground fault.The single line-to-ground fault shown in Figure 3.2.2 is simulated with a breaker between one ofthe phases on the load side of the transformer and ground, which closes at a given time. The sizeof the coil in Figure 3.2.2 is adjusted to 0.6 H parallel with a resistance and the resistance to 1Ω, in series with the coil, which represents the resistance in the Petersén coil. The value of theresistance is set to any number. The "Ideal system" is simulated when a single line-to-ground faultoccurs and when a three phase short circuit occurs. The last alternative is shown in Figure 3.2.1with high impedance grounding and solid grounding with and without FCL. The fault is simulatedwith one breaker between each phases and one breaker between the phase and ground in the singleline-to-ground fault

The solidly grounded transformer shown in Figure 3.2.1b, has the neutral point directly connectedto ground. The FCL is combined with the solidly grounded transformer and this case is shownin Figure 3.2.1c, with the breakers SWA/SWB/SWC and surge arresters for each phase modellingthe FCL. The FCL triggers on a 1.5 times higher current than nominal out of the transformer.When it detects a fault, the breaker for each phase opens and the current limits through the surgesarresters. In this manner, the current will flow from the transformer through the surge arrestersand become limited to zero when the circuit breakers opens. All of the simulations for the idealsystem are presented in section 4.

14

(a) The "Ideal system" with Petersén coilgrounding (b) The "Ideal system" with solidly grounding

(c) The "Ideal system" with a FCL

Figure 3.2.1: "Ideal system" with three different groundings and with three phase short circuit

Figure 3.2.2: The measurements for the "Ideal system" when a single line-to-ground fault occurs,with measured current and voltage on the load side of transformer together with measured earthcurrent and incoming coil current.

3.3 The "Real system"The "Real system" is built in a single line view with resistances, inductors, capacitors to ground andcapacitors between phases and loads that create the realistic network according to the data fromVattenfall, representing cables, overhead lines and loads. Cables and overhead lines are simulatedby a resistance in series with an inductance and capacitances between phases and to ground. Thesize of the components are chosen to represent a cable or an overhead line depending on the type ofoverhead line or cable and the length of it, i.e. the length between two secondary substations. Thisis illustrated in Figure 3.3.1 and Figure 3.3.2 shows what one of the branches looks like in moredetail. The households, or loads, have been built-up by a resistance in parallel with an inductance.Sub-branches with loads are represented by white boxes along the main branch in Figure 3.3.1. Theloads which are connected directly to the main branch are not packaged into any white boxes, butplaced visibly along the main branch. All grounding of capacitances and the transformer ground inthe system are connected to one common point. The distribution of loads is shown in Figure 3.3.3,where the total load for each branch and the main branch are presented. Voltage and current onthe load side of the transformer is measured similar as for the "Ideal system" together with theincoming current for the Petersén coil and earth current.

This system is also simulated with high impedance grounding, solidly grounding with and withoutan FCL. The faults in the system is simulated as; single line-to-ground fault and three phase short

15

circuit and the location of the fault is in the secondary substations "A","O1" and "AI1". Thesesecondary substations are marked with red in Figure 3.3.1. When, where and which types of faultthat is simulated depends on the sequence control, which selects type, timing and location of thefault. This sequence control is represented by the grey boxes in Figure 3.3.1.

The Petersén coil is adjusted to a single line-to-ground fault in secondary substation "A" and hassubsequently been simulated for both the three phase short circuit and the single-line-to groundfault for each of the three investigated fault locations. A solid grounded system both with andwithout with FCL is also simulated with the same type of faults and in the same secondarysubstations as for the previous types of groundings.

Figure 3.3.1: The main branch of the real system with sub-branches in the white boxes

Figure 3.3.2: Details of the sub-branch "AC" represented by a white box in Figure 3.3.1 in theend of the main branch, with overhead lines between the substations "AC1" to "AC2", "AC2" to"AC3" and "AC3" to "AC4" .

16

Figure 3.3.3: The loads in the system divided in groups of main branch and sub-branches.

3.4 The "Radial system"Finally a radial structure has been implemented through parallel coupling of several similar systems;"Radial system 1", ... ,"Radial system 5", all of which have been linked to a common bus that isfed by a transformer of 11 kV. The size of the transformer has been increased to 11.7 MVA and thefeeding generator is set to the same voltage level as the “Real system” at 75 kV, on the incomingside of the bus. So the "Radial system" is built with a generator, a transformer, a bus and fivesubsystems, see Figure 3.4.1. In this "Radial system" the four larger systems; 1, 2, 3, and 5, aresimilar to the system in the "Real system". The fifth system is smaller compared to the otherradial systems in terms of load and complexity, in order to see what happens if the "Radial system"has one smaller system, "Radial system 4". The results for the healthy systems are similar to eachother, independently of the size of the systems, so one of the four healthy systems are representedin the Results.

In conjunction with the structure of the radial system, each system is illustrated as a white boxwith a breaker between the box and the bus. The voltage, current and power is measured at theincoming side of each box. The power consumption for each system is presented in Figure 3.4.1,together with an overview of the radial system and the total power consumption is 5.7 MW resp.1.2 MVAR.

The "Radial system" is only simulated with high impedance grounding and solid grounding incombination with an FCL, since these two groundings are of interest because the solidly groundedsystem without FCL is not able to limit the faults in previous simulation. The healthy systemshave also been reconnected 30 s after the fault according to Vattenfall’s fault handling when theFCL is reclosed [21]. It should be noted that with an FCL it is expected re-connection could beperformed substantially faster though.

The faults in the simulations are single line-to-ground fault, three phase fault and a combinationof the two faults, which starts as a single line-to-ground fault and thereafter develops into a threephase fault. The faults are simulated in the secondary substations "A" and "AI1" in the topsystem. The simulation in "A" is performed to show the fault handling in a radial system and thesimulation in "AI1" is performed to investigate if the fault would be found and limited when itoccurs further out in the system.

The faults "single line-to-ground fault", "three phase short circuit" and "single line-to-ground faultwhich develops into a three phase short circuit" are simulated as faults in the "Radial system 1".After the disconnection caused by the fault, four healthy systems are connected again. While the"Radial system 1", with fault, will stay disconnected for the rest of the time in the simulation.

17

Figure 3.4.1: The "Radial system" with an FCL and with one "Radial system" in each box."Radial system 1", "Radial system 2", "Radial system 3" and "Radial system 5" are the system inFigure 3.3.1. The measured voltages and currents on the incoming side of each system are marked

3.5 "Ideal back-fed earth fault"The "Ideal back-fed earth fault" is simulated with FCL and High impedance grounding on thefed Y-Y connected transformer together with ideal overhead lines, see Figure 3.5.1 and Figure3.5.2. The Delta-Y connected transformer is solidly grounded which is a common grounding fora transformer connected between system and load. The load on the secondary side of the Delta-Y connected transformer is simulated with the capacitance based on the system from Vattenfall.With a solidly grounded Y-Y connected transformer is the expectations to detect this type of faulteasier than with Petersén coil.

Figure 3.5.1: The "Ideal back-fed earth fault" with high impedance grounded transformer

Figure 3.5.2: The "Ideal back-fed earth fault" simulated with solid grounding with FCL

18

4 Results

For all simulated cases voltage and current on the secondary side of the transformers are measured.The results are presented below and also as number in paragraph Summary. Some of the figuresare located in Appendix 1 instead to give a clearer view of the result. (The affected figures are theplots over secondary substation "O1" in the "Real system".)

The time at which the fault occurs is by default 0.1225 seconds but for some of the figures thesingle line-to-ground fault develops into three phase short circuit. These faults which not developsinto larger fault are plotted between 0.1 seconds and 0.3 seconds if nothing else is specified. For thesingle line-to-ground fault which develops into a three phase fault the simulation time is longer tokeep the 30 seconds wait time before re-connection, which is the recommendation from Vattenfall,in order to limit the damages on rotating machines at re-connection. For the y-axis most of thefigures have the span of ± 20 kV and ± 20 kA to facilitate comparison. When peak values ofvoltage or current are higher, larger ranges are used though.

4.1 The "Ideal system"In the "Ideal system", simulations of a single line-to-ground fault and a three phase short circuithave been made both for the high impedance grounded and the solidly grounded case, with andwithout FCL. All voltages and currents are measured at the transformer terminals on the load sideof the transformer and the fault occurs at 0.1225 seconds.

4.1.1 Single line-to-ground fault

Figure 4.1.1 presents voltages and currents from the simulations of a single line-to-ground faultin a high impedance ground system (Figure 4.1.1a and 4.1.1b), a solidly grounded system (Figure4.1.1c and 4.1.1d) and a solidly grounded system with FCL (Figure 4.1.1e and 4.1.1f). The currentsin Figure 4.1.1 are magnified in Figure 4.1.2. The nominal current before the fault occurs is 14Arms, 20 Apeak and the nominal voltage is 6.35 kVrms and 9 kVpeak. Looking at the top figures,Figure 4.1.1a and 4.1.1b, the characteristic limitation of a fault with a high impedance groundedtransformer can be seen. The voltage in the two healthy phases gets a higher amplitude, 15.5kVpeak instead of 9 k Vpeak, which equals a factor of

√3 while the voltage in phase c is zero due to

the fault. The fault current in phase c becomes limited by the coil from the peak current on 120A to ± 15 A at 0.38 s. Where as in the two healthy phases the current in the two healthy phasesincreased from nominal 20 Apeak to 30 Apeak as can be seen in the enlarged in Figure 4.1.2a.

Below the high impedance grounded the solidly grounded system is presented in figures Figure4.1.1c and Figure 4.1.1d with the voltage and current out of the transformer. Also here the voltagein phase c, becomes zero due to the fault as with the high impedance grounding. But the faultcurrent in phase c (shown in Figure 4.1.2b) is not limited. This causes a higher fault current. Forthe healthy phases the amplitude of the current stays at the nominal values as shown in Figure4.1.2b.

The last grounding system is presented in Figure 4.1.1e and Figure 4.1.1f. There the influence ofFCL is shown, which limits the transformer current on the load side to zero for all three phases, seeFigure 4.1.2c. Unlike the other two grounding systems the transformer current has one transientat each phase, which is short. The duration of the fault current is approximately 1 ms andmuch shorter than for the high impedance grounding. The voltage on the secondary side of thetransformer stays on the nominal value, with only a small transient at the instant when the faultoccurs.

19

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VTa

VTb

VTc

(a) Voltage on the load side of the transformerwith a high impedance grounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[kA

]

-20

-15

-10

-5

0

5

10

15

20

T1a

T1b

T1c

(b) Current on the load side of the transformerwith a high impedance grounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VTa

VTb

VTc

(c) Voltage on the load side of the transformerwith solidly grounding

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[kA

]

-20

-15

-10

-5

0

5

10

15

20

T1a

T1b

T1c

(d) Current on the load side of the transformerwith solidly grounding

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VTa

VTb

VTc

(e) Voltage on the load side of the transformerwith an FCL

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[kA

]

-20

-15

-10

-5

0

5

10

15

20

T1a

T1b

T1c

(f) Current on the load side of the transformerwith an FCL

Figure 4.1.1: The transformer current and voltage, simulated in the “Ideal system" for threedifferent groundings when a single line-to-ground fault occurs

20

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-0.15

-0.1

-0.05

0

0.05

0.1

T1a

T1b

T1c

(a) Expansion of the current on the load side ofthe transformer with high impedance grounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-5

-4

-3

-2

-1

0

1

2

3

4

5

T1a

T1b

T1c

(b) Expansion of the current on the load side ofthe transformer with solid grounding

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

T1a

T1b

T1c

(c) Expansion of the current on the load sideof the transformer with an FCL in combinationwith solid grounding

Figure 4.1.2: Zoom of transformer current, simulated in the “Ideal system" for three differentgroundings and single line-to-ground fault.

4.1.2 Three phase short circuit

For a three phase short circuit, the measured currents and voltages can be seen in Figure 4.1.3.Figure 4.1.3b shows that the high impedance is not capable of limiting the three phase short circuit.The fault current becomes 20 Apeak for all phases. The same thing also happens for the solidlygrounded transformer. With the FCL the fault current becomes limited to zero due to the FCL,see Figure 4.1.3f and some current transients are detectable for the FCL in the same way as forthe single line-to-ground fault. The voltage on the load side of the transformer is unchanged forthe FCL compared to high impedance grounding and solid grounding system where it goes downto zero due to the fault.

21

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VTa

VTb

VTc

(a) Voltage on the load side of the transformerwith a high impedance grounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[kA

]

-20

-15

-10

-5

0

5

10

15

20

T1a

T1b

T1c

(b) Current on the load side of the transformerwith a high impedance grounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VTa

VTb

VTc

(c) Voltage on the load side of the transformerwith solid grounding

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[kA

]

-20

-15

-10

-5

0

5

10

15

20

T1a

T1b

T1c

(d) Current on the load side of the transformerwith solid grounding

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VTa

VTb

VTc

(e) Voltage on the load side of the transformerwith an FCL

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

T1a

T1b

T1c

(f) Current on the load side of the transformerwith an FCL

Figure 4.1.3: The transformer current and voltage, simulated in the "Ideal system" for threedifferent groundings and three phase short circuit fault.

4.1.3 Comments on the FCL in the "Ideal system"

In order to understand how the FCL works in the system, both voltages across (Ealim, Eblim, Eclim)and currents through (ISWA, ISWB , ISWC) the breakers (SWA, SWB and SWC) are illustratedtogether with the transformer voltages (VTa, VTb, VTc) and currents (T1a, T1b, T1c) on the loadside, see Figure 4.1.4. The voltage across the FCL when a fault has occurred is smaller than thephase voltage on the load side of the transformer initially, but it becomes equal with each other.However, the voltage downstream the FCL is zero. This is shown in Figure 4.1.4a e.g. where theEAlim is the voltage across the FCL in phase a and VTa is the phase voltage on the load side of

22

the transformer for same phase. That occurs when the FCL limits the fault current.

The transformer current and breaker current in Figure 4.1.4b and Figure 4.1.4d are equal andoverlap. But when the FCL opens after 0.7 ms which approximately is 0.1235 s, in Figure 4.1.4d,after the fault had occurred at 0.1225 s, the transformer current for phase c, Ic, will discharge tozero instead for the breaker current ISWC which forces down to zero.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VTa

VTb

VTc

EAlim

EBlim

ECLim

(a) The voltage across the FCL and on the loadand the load side of the transformer

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

T1a

T1b

T1c

ISWA

ISWB

ISWC

(b) The current in the FCL and on the load sideof the transformer

Time [s]

0.12 0.121 0.122 0.123 0.124 0.125 0.126 0.127 0.128 0.129 0.13

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VTa

VTb

VTc

EAlim

EBlim

ECLim

(c) Enlargement of FCL and transformer volt-ages which become limited after 0.7 ms after thethree phase short circuit has occurred

Time [s]

0.12 0.121 0.122 0.123 0.124 0.125 0.126 0.127 0.128 0.129 0.13

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

T1a

T1b

T1c

ISWA

ISWB

ISWC

(d) Enlargement of FCL and transformer cur-rent which become limited after 0.7 ms after thethree phase short circuit has occurred

Figure 4.1.4: The FCL current and voltage, simulated in the "Ideal system".

4.2 The "Real system"In the "Real system", the three grounding systems are simulated to see how they react in caseof a fault (single line-to-ground fault and three phase short circuit) in a real system with loads.With this larger system the high impedance was increased to 0.9 H (instead of 0.6 H as for the"Ideal system"). The trip value for the FCL also has increased with the bigger system current.The nominal current is 78 Arms, corresponding to 111 Apeak and the FCL trips on 1.5 times thisvalue, i.e. 166.5 A. Faults are simulated in substations "A", "AI1" and "O1" (which is found inAppendix 1).

4.2.1 Single line-to-ground fault

4.2.1.1 Secondary substation "A"

Results from simulation of a single line-to-ground fault in secondary substation "A" are shown inFigure 4.2.1. The nominal current is 111 Apeak and the nominal voltage is 9 kVpeak, but whenthe fault occurs the high impedance will limit the current in the faulted phase. The current in the

23

two healthy phases are marginal increased over the nominal value and the voltage in the healthyphases will increase. The fault current in the solid grounded system is not limited and increasesto 12 kA and thereafter decreases. Both the voltage and the current of the two healthy phasesremain of the nominal value when the fault occurs. So with a solidly grounded transformer thetwo healthy phases are unaffected when the fault occurs which means the amplitude of the healthyphases are the same as before the fault, see Figure 4.2.1c and Figure 4.2.1d. In Figure 4.2.1f thethree phase currents are limited on the load side of the transformer when the fault occurs. Thislimitation 0.7 ms is much faster than with high impedance grounded or solid grounded, but this ishard to compare FCL with high impedance grounded or solid grounded, since they are dependingon relays and breakers.

The voltage in the fault phases will drop to zero when the fault is limited by the high impedance.But the two healthy phases are increased from 9 kV with a factor of

√3 to 15.5 kV. Similar

phenomenon occurs with the solid grounded system, with the fault phase decreased to zero andthe two healthy phases at the nominal value instead for the higher amplitude at 15.5 kV. The voltagewith the solid grounded. transformer combined with an FCL is still on the nominal value whenthe fault occurs, see Figure 4.2.1e. This shows that the voltage across the FCL and downstreamthe FCL becomes zero, see paragraph 4.2.3. This will cause that the consumers downstream theFCL are not fed by the two healthy phases, which can be able with the high impedance and thesolid grounded system without an FCL.

24

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(a) Voltage on the load side of the transformerwith high impedance.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

T1a

T1b

T1c

(b) Current on the load side of the transformerwith high impedance.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(c) Voltage on the load of the transformer withsolidly grounding without FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[kA

]

-10

-5

0

5

10

T1a

T1b

T1c

(d) Current on the load side of the transformerwith solidly grounding without FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(e) Voltage on the load side of the transformerwith solidly grounding combined with an FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

T1a

T1b

T1c

(f) Current on the load side of the transformerwith solidly grounding combined with an FCL.

Figure 4.2.1: Voltage and current on the load side of the transformer in the "Real system" withthree grounding systems and a single line-to-ground fault in secondary substation "A". The mea-surement points are marked in Figure 3.2.2

4.2.1.2 Secondary substation "AI1"

All of the three grounding systems react similar for a single line-to-ground fault in substation "AI1"as a single line-to-ground fault in substation "A", but the fault current is smaller and thereforefault current transients are smaller for each grounding. This is most clearly visible for the solidgrounded transformer, see Figure 4.2.2. The smaller fault current is visible by comparing the solidgrounding system for the two substations, Figure 4.2.1d with Figure 4.2.2d. This shows how a fault

25

near the end of the main branch in secondary substation "AI1" is smaller than 0.3 kA and thereforemore difficult to detect, compared to the fault in substation "A" with higher fault current, 12 A.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(a) Voltage on the load side of the transformerwith high impedance.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

en

t [k

A]

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

T1a

T1b

T1c

(b) Current on the load side of the transformerwith high impedance.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(c) Voltage on the load of the transformer withsolidly grounding without FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-0.3

-0.2

-0.1

0

0.1

0.2

T1a

T1b

T1c

(d) Current on the load side of the transformerwith solidly grounding without FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(e) Voltage on the load side of the transformerwith solidly grounding combined with an FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

en

t [k

A]

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

T1a

T1b

T1c

(f) Current on the load side of the transformerwith solidly grounding combined with an FCL.

Figure 4.2.2: Voltage and current on the load side of the transformer in the "Real system" withthree grounding systems and a single line-to-ground fault in secondary substation "AI1". Themeasurement points are marked in Figure 3.2.2

4.2.1.3 Secondary substation "O1"

The simulated fault current transient when the fault occurs in secondary substation "O1" is betweenthe results for "A" and "AI1", 1.5 kApeak in the solidly grounding case, see Appendix 1, where theresults of the simulations in secondary substation "O1" are presented.

26

4.2.2 Three phase short circuit

4.2.2.1 Secondary substation "A"

Neither the high impedance grounding nor the solid grounding is capable of limiting the faultcurrent in a three phase short circuit, see Figure 4.2.3b and Figure 4.2.3d. The phase currents arebalanced, which means that the sum of the three phases will be zero (Ia + Ib + Ic = 0). Sincethe sum is zero, the capacitive earth current is also zero which means that no current goes upthrough the neutral point of the transformer and the current upstream the fault location becomesnot limited. This result in a higher fault current, 12 kApeak, for both of the grounding systems.In neither of the grounding systems the capacative fault current decreases to zero, as comparedto case of the single line-to-ground fault, where the fault current decreases to zero. The FCL iscapable of limiting this type of fault, which is shown in Figure 4.2.3f. The voltage on the loadside of the transformer nearly becomes zero for the high impedance grounding and solid groundingwhen the fault occurs near the transformer, see Figure 4.2.3a and Figure 4.2.3c. The amplitudeof the voltage will increase when the fault is further out in the system, see next case with threephase short circuit in substation "AI1".

27

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(a) Voltage on the load side of the transformerwith high impedance grounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[kA

]

-20

-15

-10

-5

0

5

10

15

20

T1a

T1b

T1c

(b) Current on the load side of the transformerwith high impedance grounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(c) Voltage on the load side of the transformerwith solid grounding

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[kA

]

-20

-15

-10

-5

0

5

10

15

20

T1a

T1b

T1c

(d) Current on the load side of the transformerwith solid grounding

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(e) Voltage on the load side of the transformerwith an FCL

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

T1a

T1b

T1c

(f) Current on the load side of the transformerwith an FCL

Figure 4.2.3: Voltage and current on the load side of the transformer, simulated in the "Realsystem" with high impedance grounding, solid grounding, FCL respectively with a three phaseshort circuit fault in secondary substation "A".

4.2.2.2 Secondary substation "AI1"

With the three phase short circuit further out in the "Real system", see Figure 4.2.4, the voltage outof the transformer is not as significantly affected as for the three phase short circuit in secondarysubstation "A". The fault current is not as much affected as for the three phase short circuitin substation "A". The peak value of the current is 0.28 kApeak instead of 12 kApeak, for thehigh impedance grounding and the solid grounding system in Figure 4.2.4a and Figure 4.2.4c

28

respectively. The transformer voltage on the outgoing side of the transformer is not zero, whichhappened for the same fault in substation "A" in Figure 4.2.3 where it drops with more than 90%. The FCL shown in Figure 4.2.4e and Figure 4.2.4f limits the fault current to zero on the otherhand.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(a) The voltage on the secondary side of thetransformer in the "Real system" with highimpedance grounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

T1a

T1b

T1c

(b) The current on the secondary side of thetransformer in the "Real system" with highimpedance grounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(c) The voltage on the secondary side of thetransformer in the "Real system" with solidlygrounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

T1a

T1b

T1c

(d) The current on the secondary side of thetransformer in the "Real system" with solidlygrounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(e) The voltage on the secondary side of thetransformer in the "Real system" with an FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

T1a

T1b

T1c

(f) The current on the secondary side of thetransformer in the "Real system" with an FCL.

Figure 4.2.4: The "Real systems" simulated with the three grounding systems and three phaseshort circuit in secondary substation "AI1".

29

4.2.2.3 Secondary substation "O1"

As for the single line-to-ground fault the amplitude of the fault current in substation "O1" withthree phase short circuit is lower than in substation "A" but higher than in substation "AI1", 1.65kApeak for solid grounding and high impedance grounding. The voltage drop when the fault occursis also lower when the fault occurs in "O1" then it occurs in "A" but higher then when it occursin "AI1", from 9 kV to 7.56 kV. The results and simulation are presented in Appendix 1.

4.2.3 Comments on the FCL in the "Real system"

The current and the voltage downstream the FCL are limited to zero, which is shown in Figure4.2.5. It shows how the "Real system" current and voltage are forced to zero, when the FCL tripson the fault current. The FCL is modelled as breakers with surge arrestors in parallel. This meansthat the fault current flows through the surge arrestor and becomes limited. Limitation with FCLimplies that all of the three phases are limited instead of the limited fault phase in the system withhigh impedance grounding. Since the two healthy phases are also limited, can they not feed thesystem as the system with high impedance grounding.

,30Figure 4.2.6 classes the transfer quality and the measure for the owner, [19]. The voltage acrossthe FCL is near the 8.9 kV after the fault has occurred, meaning that the voltage downstreamthe FCL is close to zero. And the power quality is in the area for 5>u in Figure 4.2.6 fromEnergimarknadsinspektionen. However, a possible re-connection is faster than the 10 ms which isthe limit for the table in Figure 4.2.6.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[kA

]

-10

-8

-6

-4

-2

0

2

4

6

8

10

Vaftera

Vafterb

Vafterc

(a) The voltage downstream the FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

Iaftera

Iafterb

Iafterc

(b) The current downstream the FCL

Figure 4.2.5: The current through the FCL and the voltage phase to ground downstream the FCLmeasured before, simultaneously as the faults happens and after the FCL has trapped. The faultoccurs at 0.1225 seconds and is a single line-to-ground fault in secondary substation "AI1"

Figure 4.2.6: There shall be no short-term voltage drop with such residual voltage and durationas shown in area C. Grid owners are required to fix short voltage reductions within area B in thetable above to extent the measures are reasonable in relation to the inconvenience for the usersassociated with the short voltage reductions. [19]

30

4.3 The "Radial system"In the "Radial system", the measured incoming voltages and currents at "Radial system 1" and"Radial system 5" are presented together with the outgoing voltages and currents from the feedingtransformer. The time axis is broken into two parts, to give a better view of the fault handling withdisconnection and re-connection. The interruption time starts at 0.7 ms for the FCL (see paragraph4.1.3) and 1 s for high impedance grounding, according to Vattenfall’s standard respectively, afterthe fault occurred. The breakers re-close at 30 s after the disconnection.

4.3.1 Single line-to-ground fault at "Radial system 1"

The "Radial system" is simulated with FCL and high impedance grounding respectively with asingle line-to-ground fault in the "Radial system 1" at secondary substation "A" and respectively"AI1". The simulation time is 0.3 s and the re-connection of the healthy systems takes place 100ms after that fault has occurred.

The simulation is done with breaker on the "Radial system 1", which is set to break at any currentto disconnect the fault. This does not happen in reality, it instead breaks on the zero crossing. They-axes are not limited to ± 20 kV or ± 20 kA. They are instead chosen to give a good overview ofthe measured data.

4.3.1.1 Secondary substation "A" in the "Radial system 1"

The single line-to-ground fault in the secondary substation "A" in the "Radial system 1" is limitedby both the FCL and the high impedance grounding.

In Figure 4.3.1 the transformer is grounded through a high impedance. The simulated voltage onthe load side of the transformer, incoming voltage at "Radial system 1" and "Radial system 5"show same behaviour as for the "Real system", with zero voltage on the fault phase and with highvoltage amplitude on the healthy phases. When the "Radial system 1" is disconnected around 0.22s, the voltage on the load side of the transformer and the incoming voltage at "Radial system 5"will go back to the normal state and the voltage in the "Radial system 1" go down to zero. Thesimulated transformer current in Figure 4.3.1b, on the load side, show a maximum current around700 A and a different behaviour compared to the "Real system". For the "Radial system 1" thefault phase (0.5 A) is not limited before the breaker opens around 0.22 s and the fault current isinterrupted. But the current at the incoming side of the "Radial system 5" is somewhat below thenominal value. All of these figures show a longer fault time and higher amplitude compared to theFCL. This can cause more damage occurred by the fault current.

In Figure 4.3.2 the transformer in the "Radial system" is solidly grounded in combination with anFCL. As is seen in the figure, the FCL limits the current after the fault has occurred. None ofthe outgoing feeders are fed with any current or voltage, before the re-connection of the healthysystems is done. One important part compared to the high impedance grounding, is the short faulttime which is much shorter than high impedance grounding and the voltage on the load side of thetransformer only has a transient compared to the increased amplitude on the healthy phases for thehigh impedance grounding. After the FCL all phases are limited which limits the damage causedby the fault current. Under this time when the FCL works (≈ 0.12s to ≈ 0.22s) the voltage andcurrent are nearly zero and after the "Radial system 1" is disconnected, the transformer currentand "Radial system"s voltage and current are back to nominal values after the system componentsare charged.

31

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(a) The voltage on the load side of the trans-former which is grounded through a highimpedance.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

T1a

T1b

T1c

(b) The current on the load side of the trans-former which is grounded through a highimpedance.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

Va56

Vb56

Vc56

(c) The incoming voltage at "Radial system 5"(healthy subsystem).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

Ia56

Ib56

Ic56

(d) The incoming current at "Radial system 5"(healthy subsystem).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VBRKa

VBRKb

VBRKc

(e) The voltage at "Radial system 1" which hasa single line-to-ground fault in secondary sub-station "A".

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

IaBRK

IbBRK

IcBRK

(f) The current at "Radial system 1" which hasa single line-to-ground fault in secondary sub-station "A".

Figure 4.3.1: The "Radial system" grounded through a high impedance when a single line-to-ground fault occurs in the "Radial system 1" at secondary substation "A"

32

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-15

-10

-5

0

5

10

15

VT1a

VT1b

VT1c

(a) The voltage on the load side of the solidlygrounded transformer with an FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-3

-2

-1

0

1

2

3

T1a

T1b

T1c

(b) The current on the load side of the solidlygrounded transformer with an FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-15

-10

-5

0

5

10

15

Va56

Vb56

Vc56

(c) The incoming voltage at "Radial system 5"(healthy system).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Ia56

Ib56

Ic56

(d) The incoming current at "Radial system 5"(healthy system).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-15

-10

-5

0

5

10

15

VBRKa

VBRKb

VBRKc

(e) The incoming voltage at "Radial system 1"which has a single line-to-ground fault in sec-ondary substation "A".

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-3

-2

-1

0

1

2

3IaBRK

IbBRK

IcBRK

(f) The incoming current at "Radial system 1"which has a single line-to-ground fault in sec-ondary substation "A".

Figure 4.3.2: Measured voltages and currents in the "Radial system" with a solidly groundedtransformer in combination with FCL and a single line-to-ground fault in the "Radial system 1"at substation "A"

33

4.3.1.2 Secondary substation "AI1" in the "Radial system 1"

The single line-to-ground fault in secondary substation "AI1" is simulated with both FCL and highimpedance grounding, see Figure 4.3.3 and Figure 4.3.4. Neither of the two grounding systemslimit the fault and "Radial system 1" is never disconnected because of the low fault current, butin the reality the fault would be disconnected.

In the simulations below in Figure 4.3.3, the "Radial system" has the transformer grounded througha high impedance. The high impedance (Figure 4.3.3) does not limit this week fault clearly visiblein secondary substation “AI1”. The current at the radial system where the fault occurs is visiblein Figure 4.3.3f and the voltage visible in figure 4.3.3e, shows an increase in the fault phase and adecrease in the other phases.

In Figure 4.3.4 the transformer in the "Radial system" is solidly grounded with an FCL. The faultis visible in Figure 4.3.4f, with a higher amplitude on the current in the faulty phase, 270 A. Thissymptom with much higher amplitude of the faulty phase is not clearly visible for the transformerand "Radial system 5", which only show a small alteration. So the amplitude of the phases for theload side of the transformer and "Radial system 5" are not noticeably changed. The FCL is notable to detect the fault and therefore not able to disconnect the fault.

34

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(a) The voltage on the load side of the trans-former, grounded with high impedance.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

T1a

T1b

T1c

(b) The current on the load side of the trans-former, grounded with high impedance.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

Va56

Vb56

Vc56

(c) The incoming voltage at the "Radial system5" (healthy subsystem).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

Ia56

Ib56

Ic56

(d) The incoming current at the "Radial system5" (healthy subsystem).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VBRKa

VBRKb

VBRKc

(e) The incoming voltage at the "Radial system1" which has a single line-to-ground fault in sec-ondary substation "AI1".

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

IaBRK

IbBRK

IcBRK

(f) The incoming current at the "Radial system1" which has a single line-to-ground fault in sec-ondary substation "AI1".

Figure 4.3.3: The "Radial system" simulated with a high impedance grounded transformer and asingle line-to-ground fault of substation "AI1".

35

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(a) The voltage on the load side of the solidlygrounded transformer with a FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

T1a

T1b

T1c

(b) The current on load side of the solidlygrounded transformer with a FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

Va56

Vb56

Vc56

(c) The incoming voltage at the "Radial system5" (healthy subsystem).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

Ia56

Ib56

Ic56

(d) The incoming current at the "Radial system5" (healthy subsystem).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VBRKa

VBRKb

VBRKc

(e) The incoming voltage at the "Radial system1" which has a single line-to-ground fault in sec-ondary substation "AI1".

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1.5

-1

-0.5

0

0.5

1

1.5

IaBRK

IbBRK

IcBRK

(f) The incoming current at the "Radial system1" which has a single line-to-ground fault in sec-ondary substation "AI1".

Figure 4.3.4: The "Radial system" with solid ground system combination with an FCL when asingle line-to-ground fault occurs in the "Radial system 1" at substation "AI1"

4.3.2 Three phase short circuit at "Radial system 1"

The "Radial system" is simulated with high impedance grounding and solid grounding with anFCL when a three phase short circuit occurs in the "Radial system 1" in substation "A" and "AI1"respectively. The fault occurs at 0.1225 s and the bus becomes disconnected with the main circuitbreaker downstream the transformer at 1.12 s with the high impedance grounding and respectivelyaround 0.13 s with the FCL combined with solid grounding. But the time for disconnection inthe reality for high impedance grounded transformer with short circuit is 100 ms after the fault

36

occurred and 100 ms more for a low fault current. The re-connection of the healthy systems andthe bus become at 31.12 seconds with high impedance grounding and respectively 30.13 s with theFCL. But in reality the sick system in the high impedance grounded system become disconnectedand the healthy systems never become disconnected. This is due to a misunderstanding of faulthandling in distribution systems, visible in the Figures. With the FCL the re-connection can bedone faster but it is simulated with the 30 s to prevent damage to rotating machines, as the re-commendation from Vattenfall.

In the simulation the beakers between the "Radial system 1" and the bus are set to break at anycurrent. This does not happen in the reality, where it breaks at the zero crossing. The y-axis inthe plots are adjusted to get a good overview of the measured data.

4.3.2.1 Secondary substation "A"in the "Radial system 1"

The results of the simulations of three phase faults are present in this paragraph. Only the FCLcan limit the three phase short circuit because the high impedance grounding is only able to limitthe single line-to-ground fault with an earth current.

The "Radial system" with a high impedance grounded transformer is presented in Figure 4.3.5.The voltage and current on the load side of the transformer and the incoming voltage and currentat "Radial system 1" and "Radial system 5" are plotted. When the three phase short circuit occursthe high impedance grounding is not able to limit the short circuit current. This is most clearlyvisible in Figure 4.3.5f where the current amplitude increases under the fault (around 1.12 s) to 18kA transients and declining to 12 kA and become zero after the "Radial system 1" is disconnected.

Even here the healthy systems becomes disconnected for the high impedance grounded transformerin contrast to Vattenfall. When the main circuit breaker open, the transformer will not have anyload. The current on the load side of the transformer will decrease to zero and the voltage on theload side of the transformer returns to the nominal value. After the re-connection, the transformercurrent of the load side will take a value at 340 Apeak and feed the radial systems 2, 3, 4 and5. When the fault occurs with high impedance grounding, the transformer fed the fault and the"Radial system 5" decrease to 0 A and after the re-connection the current increases back to thenominal value at 111 Apeak.

For the simulation with same type of fault in the "Radial system 1" with a solidly groundedtransformer in combination with an FCL, in Figure 4.3.6. The voltage and current downstream theFCL are limited to zero at the outgoing feeders before the re-connection as in previous simulations.At the re-connection, the healthy system, resume the nominal value of the voltage and a lower valueof the current.

37

VT1a

VT1b

VT1c

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.2 0.4 0.6 0.8 1

VT1a

VT1b

VT1c

30.1 30.2

(a) The voltage on the load side of the trans-former grounded with a high impedance.

T1a

T1b

T1c

Time [s]

Curr

ent [k

A]

-20

-15

-10

-5

0

5

10

15

20

0.2 0.4 0.6 0.8 1

T1a

T1b

T1c

30.1 30.2

(b) The current on the load side of the trans-former grounded with a high impedance.

Va56

Vb56

Vc56

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.2 0.4 0.6 0.8 1

Va56

Vb56

Vc56

30.1 30.2

(c) The incoming voltage at "Radial system 5"(healthy subsystem).

Ia56

Ib56

Ic56

Time [s]

Curr

ent [k

A]

-0.3

-0.2

-0.1

0

0.1

0.2

0.2 0.4 0.6 0.8 1

Ia56

Ib56

Ic56

30.1 30.2

(d) The incoming current at "Radial system 5"(healthy subsystem).

VBRKa

VBRKb

VBRKc

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.2 0.4 0.6 0.8 1

VBRKa

VBRKb

VBRKc

30.1 30.2

(e) The incoming voltage at "Radial system 1"where a three phase short circuit occurs in sec-ondary substation "A".

IaBRK

IbBRK

IcBRK

Time [s]

Curr

ent [k

A]

-20

-15

-10

-5

0

5

10

15

20

0.2 0.4 0.6 0.8 1

IaBRK

IbBRK

IcBRK

30.1 30.2

(f) The incoming current at "Radial system 1"where a three phase short circuit occurs in sec-ondary substation "A".

Figure 4.3.5: The "Radial system" with the transformer grounded through a high impedance andsimulated with a three phase fault in the "Radial system 1" at substation "A"

38

VT1a

VT1b

VT1c

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

VT1a

VT1b

VT1c

(a) The voltage on the load side of the trans-former with solid grounding system with FCL

T1a

T1b

T1c

Time [s]

Curr

ent [k

A]

-6

-4

-2

0

2

4

6

0.1 0.15 30.05 30.1 30.15 30.2

T1a

T1b

T1c

(b) The current on the load side of the trans-former with solid grounding system with FCL.

Va56

Vb56

Vc56

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

Va56

Vb56

Vc56

(c) The incoming voltage at the "Radial system5" (healthy subsystem).

Ia56

Ib56

Ic56

Time [s]

Curr

ent [k

A]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0.1 0.15 30.05 30.1 30.15 30.2

Ia56

Ib56

Ic56

(d) The incoming current at the "Radial system5" (healthy subsystem).

VBRKa

VBRKb

VBRKc

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

VBRKa

VBRKb

VBRKc

(e) The incoming voltage at the "Radial system1" where a three phase short circuit occurs atsubstation "A".

IaBRK

IbBRK

IcBRK

Time [s]

Curr

ent [k

A]

-6

-4

-2

0

2

4

6

0.1 0.15 30.05 30.1 30.15 30.2

IaBRK

IbBRK

IcBRK

(f) The incoming current at the "Radial system1" where a three phase short circuit occurs atsubstation "A".

Figure 4.3.6: The "Radial system" with a solidly grounded transformer in combination with anFCL when a three phase fault occurs in the "Radial system 1" at substation "A"

4.3.2.2 Secondary substation "AI1" in "Radial system 1"

A three phase short circuit in substation "AI1" in the end of the "Radial system 1" is hard todetect because of the low fault current and limit for the FCL. The high impedance grounding isnot able to limit this type of fault. Since it is only able to limit the earth current occurring for asingle line-to-ground fault.

A three phase fault in secondary substation "AI1" at the "Radial system 1" simulated with atransformer grounded through a high impedance is shown in Figure 4.3.7. Since the high impedancegrounding does not limit the three phase short circuit, only the initially 0.3 s of the simulation

39

is shown. This type of fault becomes disconnected in the reality. Since the fault occurs furtherout in the "Radial system 1" the peak value of the fault current is significantly lower than whenthe fault occurred in substation "A1". On the load side of the transformer the peak value of thecurrent becomes 500 A and in the "Radial system 1" the peak value of the current becomes 300 Aand declines to 200 A. For the healthy system, the current stays close to the nominal value.

The same type of fault is now simulated with solid grounding in combination with FCL. Becausethe FCL does not detect the fault, the voltage and current are not limited, see Figure 4.3.8. Thisdepends on that the current out of the transformer does not rise to the trip value at 1.5 times thenominal current, 450 Apeak. The characteristic of voltages and the currents are the same as forthe high impedance grounding.

40

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(a) The voltage on the load side of the trans-former with high impedance grounding.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

T1a

T1b

T1c

(b) The current on the load side of the trans-former with high impedance grounding

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

Va56

Vb56

Vc56

(c) The incoming voltage at the "Radial system5" (healthy subsystem).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Ia56

Ib56

Ic56

(d) The incoming current at the "Radial system5" (healthy subsystem).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VBRKa

VBRKb

VBRKc

(e) The incoming voltage at the "Radial system1" where a three phase short circuit occurs insecondary substation "AI1".

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

IaBRK

IbBRK

IcBRK

(f) The incoming current at the "Radial system1" where a three phase short circuit occurs insecondary substation "AI1".

Figure 4.3.7: The "Radial system" grounded through a high impedance and a three phase shortcircuit fault in the "Radial system 1" in substation "AI1"

41

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(a) The voltage on the load side of the trans-former with solid grounding with FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

T1a

T1b

T1c

(b) The current on the load side of the trans-former with a solid grounding with FCL

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

Va56

Vb56

Vc56

(c) The incoming voltage at the "Radial system5" (healthy subsystem).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Ia56

Ib56

Ic56

(d) The incoming current at the "Radial system5" (healthy subsystem).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-20

-15

-10

-5

0

5

10

15

20

VBRKa

VBRKb

VBRKc

(e) The incoming voltage at "Radial system 1"where a three phase short circuit occurs in sec-ondary substation "AI1".

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

IaBRK

IbBRK

IcBRK

(f) The incoming current at "Radial system 1"where three phase short circuit occurs in sec-ondary substation "AI1".

Figure 4.3.8: The "Radial system" with solid grounding where a in combination with an FCL whena three phase fault occurs in the "Radial system 1" at substation "AI1".

4.3.3 Single line-to-ground fault which develops into a three phase short circuit in"Radial system 1"

4.3.3.1 Fault in secondary substation "A"

The nominal current and voltage on the load side of the transformer in the "Radial system" is260 Arms and 6.36 kVrms. In this simulation a the single line-to-ground fault occurs at 0.1225sand a three phase short circuit occurs at 0.1425 s. The disconnection occurs 1 s after the singleline-to-ground fault with high impedance grounding. In the reality the faulty part in the "Radial

42

system 1" would be disconnected 100 ms after the fault develops into a short circuit. Re-connectionof the healthy systems occurs at 30.1425 s. For the solid grounding with FCL the disconnectionhappens 0.7 ms after the single line-to-ground fault occurred. The re-connection of the healthysystems happens at 30.1225 s with FCL which depends on the standard from Vattenfall, whenVattenfall try to re-connect the faulty part to see if the fault is cleared.

The high impedance grounding in the "Radial system" limits only the single line-to-ground faultand not the fault current when it changes in to a three phase short circuit at 0.1425. This is shownin Figure 4.3.9 and in the enlargement in 4.3.10. When the system is re-connected the voltageand current of the "Radial system 5" and the transformer current and voltage are asymmetric.The asymmetry slowly becomes symmetric again, as the components such as capacitances andinductances are recharged. The values of the voltages and currents are the same as shown previouslyfor the single line-to-ground fault and the three phase short circuit respectively. In Figure 4.3.11with solidly grounding combined with a FCL, the fault current is already limited to zero before thefault changes to a three phase short circuit which is visible at 0.1425 s. This is also shown moreclearly in the enlargement in Figure 4.3.12. The time when the fault develops into short circuit isan arbitrary time and does not necessarily occurs in reality. So with the FCL the fault will nothave time to become a three phase short circuit, which might happen for the "Radial system"grounded with a high impedance.

As in previous simulations with an FCL, both voltages and currents are nearly zero downstreamthe FCL, see Figure 4.3.11c.

43

VT1a

VT1b

VT1c

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.2 0.4 0.6 0.8 1

VT1a

VT1b

VT1c

30.1 30.2

(a) Voltage on the secondary side of the highimpedance grounded transformer.

T1a

T1b

T1c

Time [s]

Curr

ent [k

A]

-20

-15

-10

-5

0

5

10

15

20

0.2 0.4 0.6 0.8 1

T1a

T1b

T1c

30.1 30.2

(b) Current on the secondary side of thePteresén coil grounded transformer.

Va56

Vb56

Vc56

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.2 0.4 0.6 0.8 1

Va56

Vb56

Vc56

30.1 30.2

(c) Incoming voltage at "Radial system 5"(healthy system).

Ia56

Ib56

Ic56

Time [s]

Curr

ent [k

A]

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.2 0.4 0.6 0.8 1

Ia56

Ib56

Ic56

30.1 30.2

(d) Incoming current at "Radial system 5"(healthy system).

VBRKa

VBRKb

VBRKc

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.2 0.4 0.6 0.8 1

VBRKa

VBRKb

VBRKc

30.1 30.2

(e) Incoming voltage at "Radial system 1" whichhas a fault in substation "A".

IaBRK

IbBRK

IcBRK

Time [s]

Curr

ent [k

A]

-20

-15

-10

-5

0

5

10

15

20

0.2 0.4 0.6 0.8 1

IaBRK

IbBRK

IcBRK

30.1 30.2

(f) Incoming current at "Radial system 1" whichhas a fault in substation "A".

Figure 4.3.9: Voltages and currents simulated in the "Radial system" with a high impedancegrounded transformer and a single line-to-ground fault which develops in to a three phase fault, insecondary substation "A" in "Radial system 1 ".

44

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(a) Voltage on the secondary side of the highimpedance grounded transformer.

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Curr

ent [k

A]

-20

-15

-10

-5

0

5

10

15

20

T1a

T1b

T1c

(b) Current on the secondary side of thePteresén coil grounded transformer.

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

Va56

Vb56

Vc56

(c) Incoming voltage at "Radial system 5"(healthy system).

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Curr

ent [k

A]

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

Ia56

Ib56

Ic56

(d) Incoming current at "Radial system 5"(healthy system).

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VBRKa

VBRKb

VBRKc

(e) Incoming voltage at "Radial system 1" whichhas a fault in substation "A".

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Curr

ent [k

A]

-20

-15

-10

-5

0

5

10

15

20

IaBRK

IbBRK

IcBRK

(f) Incoming current at "Radial system 1" whichhas a fault in substation "A".

Figure 4.3.10: Voltages and currents simulated in the "Radial system" with a high impedancegrounded transformer and a single line-to-ground fault which develops in to a three phase fault, insecondary substation "A" in "Radial system 1 ".

45

VT1a

VT1b

VT1c

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

VT1a

VT1b

VT1c

(a) The voltage on the load side of the solidlygrounded transformer with an FCL.

T1a

T1b

T1c

Time [s]

Curr

ent [k

A]

-6

-4

-2

0

2

4

6

0.1 0.15 30.05 30.1 30.15 30.2

T1a

T1b

T1c

(b) The current on the load side of the solidlygrounded transformer with an FCL.

Va56

Vb56

Vc56

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

Va56

Vb56

Vc56

(c) The incoming voltage to the "Radial system5" (healthy system).

Ia56

Ib56

Ic56

Time [s]

Curr

ent [k

A]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0.1 0.15 30.05 30.1 30.15 30.2

Ia56

Ib56

Ic56

(d) The incoming current to the "Radial system5" (healthy system).

VBRKa

VBRKb

VBRKc

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

VBRKa

VBRKb

VBRKc

(e) The incoming voltage to the "Radial system1".

IaBRK

IbBRK

IcBRK

Time [s]

Curr

ent [k

A]

-6

-4

-2

0

2

4

6

0.1 0.15 30.05 30.1 30.15 30.2

IaBRK

IbBRK

IcBRK

(f) The incoming current to the "Radial system1".

Figure 4.3.11: Voltages and currents in the "Radial system" with an FCL and a fault which startsas a single line-to-ground fault and develops into a three phase fault in substation "A" in the"Radial system 1".

46

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(a) The voltage on the load side of the solidlygrounded transformer with an FCL.

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Curr

ent [k

A]

-1

0

1

2

3

4

5

6

T1a

T1b

T1c

(b) The current on the load side of the solidlygrounded transformer with an FCL.

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

Va56

Vb56

Vc56

(c) The incoming voltage to the "Radial system5" (healthy system).

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Curr

ent [k

A]

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

Ia56

Ib56

Ic56

(d) The incoming current to the "Radial system5" (healthy system).

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VBRKa

VBRKb

VBRKc

(e) The incoming voltage to the "Radial system1".

Time [s]

0.11 0.12 0.13 0.14 0.15 0.16 0.17

Curr

ent [k

A]

-1

0

1

2

3

4

5

6

IaBRK

IbBRK

IcBRK

(f) The incoming current to the "Radial system1".

Figure 4.3.12: Voltages and currents in the "Radial system" with an FCL and a fault which startsas a single line-to-ground fault and become limited before the fault develops into a three phasefault in substation "A" in the "Radial system 1".

47

4.3.3.2 Fault in secondary substation "AI1"

For the high impedance grounding the fault current for the single line-to-ground fault is low andthe effect of the high impedance is therefore, see Figure 4.3.13, not clearly visible. Although thehigh impedance would be capable of limiting the fault even further into the system, it can onlylimit a single line-to-ground fault and not the three phase short circuit that the fault develops into.

With a low fault current the FCL can not detect the fault. The transformer current on thesecondary side, just increases from 450 A to 500 A which is not detected by the FCL. This isshown in figures below at 0.1225 s. Since the FCL never detects the fault, the fault will remain anddevelops into a three phase short circuit. For the healthy "Radial system 5", the current remainsto the nominal value. For the "Radial system 1" the fault current is slightly above the nominalvalue. The values of voltages and currents are the same as for the single line-to-ground fault andthree phase short circuit "AI1", when simulated separately in the previous paragraphs.

48

VT1a

VT1b

VT1c

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

VT1a

VT1b

VT1c

(a) The voltage on the secondary side of thetransformer in the "Radial system" with a highimpedance grounded transformer.

T1a

T1b

T1c

Time [s]

Current [k

A]

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.1 0.15 30.05 30.1 30.15 30.2

T1a

T1b

T1c

(b) The current on the secondary side of thetransformer in the "Radial system" with a highimpedance grounded transformer.

Va56

Vb56

Vc56

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

Va56

Vb56

Vc56

(c) Incoming voltage at the "Radial system 5"(healthy system).

Ia56

Ib56

Ic56

Time [s]

Current [k

A]

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.1 0.15 30.05 30.1 30.15 30.2

Ia56

Ib56

Ic56

(d) The incoming current at the "Radial system5" (healthy system).

VBRKa

VBRKb

VBRKc

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

VBRKa

VBRKb

VBRKc

(e) The incoming voltage at "Radial system1" when a fault occurs in secondary substation"AI1".

IaBRK

IbBRK

IcBRK

Time [s]

Current [k

A]

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.1 0.15 30.05 30.1 30.15 30.2

IaBRK

IbBRK

IcBRK

(f) The incoming current at "Radial system 1"when a fault occurs in secondary substation"AI1".

Figure 4.3.13: Voltages and currents in the "Radial system" with high impedance grounded trans-former when a single line-to-ground fault that turns into a three phase short circuit occurs. Thefault is simulated in secondary substation "AI1" in "Radial system 1".

49

VT1a

VT1b

VT1c

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

VT1a

VT1b

VT1c

(a) The voltage on the secondary side of thesolidly grounded transformer in combinationwith an FCL.

T1a

T1b

T1c

Time [s]

Current [k

A]

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.1 0.15 30.05 30.1 30.15 30.2

T1a

T1b

T1c

(b) The current on the secondary side of thesolidly grounded transformer in combinationwith an FCL.

Va56

Vb56

Vc56

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

Va56

Vb56

Vc56

(c) The measured voltage into the "Radial sys-tem 5" (healthy system).

Ia56

Ib56

Ic56

Time [s]

Current [k

A]

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.1 0.15 30.05 30.1 30.15 30.2

Ia56

Ib56

Ic56

(d) The measured current into the "Radial sys-tem 5" (healthy system).

VBRKa

VBRKb

VBRKc

Time [s]

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

0.1 0.15 30.05 30.1 30.15 30.2

VBRKa

VBRKb

VBRKc

(e) The measured voltage into "Radial system1" when a fault occurs in substation "AI1".

IaBRK

IbBRK

IcBRK

Time [s]

Current [k

A]

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.1 0.15 30.05 30.1 30.15 30.2

IaBRK

IbBRK

IcBRK

(f) The measured current into "Radial system1" when a fault occurs in substation "AI1".

Figure 4.3.14: Voltages and currents in the “Radial system” with an FCL and a single line-to-ground fault which develops into a three phase short circuit. The fault is simulated in secondarysubstation "AI1" "Radial system 1".

50

4.4 Back-fed earth fault

4.4.1 Simulated with an FCL on transformer T1

For the back-fed earth fault, see Figure 2.4.3, the faultly phase current at the incoming side of thesecond transformer is presented in Figure 4.4.1. In the figure, the faultly phase changes from thenominal current to a fault current lower than the nominal current, see Figure 4.4.1. In same figure,the instant when the line breaks into two parts corresponds to that the current is zero. When thepart of the line nearest transformer T2 (Delta-Y connected) falls to the ground and becomes fedby the neutral current from transformer T1. The fault current rises from 0 A to 2.47 Arms with anoffset at 0.75 A. Transformer T1 (Y-Y connected) is only able to feed transformer T2 by the twohealthy phases when the line for phase c goes into two parts. This causes a fault current down toground and up in phase c in transformer T2. The voltages and currents on the secondary side oftransformer T1 are shown in shown in Figure 4.4.2. The current in the phase c (with the fault) iszero and the amplitude of the two healthy phases drops. Since the two healthy phases do not riseto a value over 1.5 times the nominal current, the FCL can not find this type of fault.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[A]

-6

-4

-2

0

2

4

6

Iafter

Figure 4.4.1: The fault current measured before the fault, as the fault happens at 0.12s and afterthe back-fed earth fault, with an FCL on the outgoing side of transformer T1.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-10

-8

-6

-4

-2

0

2

4

6

8

10

VTa

VTb

VTc

(a) Outgoing transformer voltage from trans-former T1 (Y-Y connected), which feeds trans-former T2 (Delta-Y connected].

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [A

]

-6

-4

-2

0

2

4

6

T1a

T1b

T1c

(b) Outgoing transformer current from trans-former T1 (Y-Y connected), which feeds trans-former T2 (Delta-Y connected) when a back-fedearth fault occurs in phase C.

Figure 4.4.2: The simulated voltage and current on the load side of transformer T1 with an FCL.

51

4.4.2 Simulated with high impedance grounded transformer T1

With a high impedance grounding of the transformer T1, the fault current in Figure 4.4.3 is similarto the system with FCL in Figure 4.4.1. As in the FCL system, transformer T1 feeds transformerT2 through the two healthy phases a and b, see Figure 4.4.4. Since the current on the load sideof the transformer T1 drops for the healthy phases and the voltage is barely noticeably affected.The fault current into phase c in transformer T2 comes from the ground fed by transformer T2.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Cu

rre

nt

[A]

-6

-4

-2

0

2

4

6

Iafter

Figure 4.4.3: The fault current measured before the fault, as the faults happens and after theback-fed earth fault with a high impedance grounded transformer T1.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Vo

lta

ge

[kV

]

-10

-8

-6

-4

-2

0

2

4

6

8

10

VTa

VTb

VTc

(a) Outgoing transformer voltage from highimpedance grounded transformer T1 (Y-Y con-nected), which fed transformer T2 (Delta-Y con-nected).

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [A

]

-6

-4

-2

0

2

4

6

T1a

T1b

T1c

(b) Out going transformer voltage from highimpedance grounded transformer T1 (Y-Y con-nected), which fed transformer T1 (Delta-Y con-nected) when the fault occurs in phase c.

Figure 4.4.4: The simulated voltages and currents on the load side of transformer T1 grounded viaa high impedance.

52

5 Summary of the simulation

5.1 Summary of the "Ideal system"Current when the fault had occurred with three types of grounding:

Table 1: nominal current: 14 Arms ⇐⇒ 20 Apeak

high impedance Solid ground Solid ground com-bined with FCL

Single line-to-groundfault

• 120 Apeak,decrease to 0 Aon fault phase

• 30 Apeak

healthy phases

• 4 kApeak

on fault phase

• 20 Apeak

healthy phases

0 A all phases

Three phaseshort circuit

4kApeak all phases(the fault current ismuch higher for the

three phase short circuit)

4 kApeak all phases(Unlike just

one phase at asingle line-to-ground fault )

0 A all phases

5.2 Summary of the "Real system"Current when the fault had occurred with three types of grounding:

Table 2: nominal current: 78.5 Arms ⇐⇒ 111 Apeak

high impedance Solid ground Solid ground com-bined with FCL

Single line-to-groundfault "A"

The amplitude of allphases are marginallyincreased

• 12 kApeak

on fault phase

• 0.1 Apeak

healthy phases

0 A all phases

Single line-to-groundfault "AI1"

The amplitude of allphases are marginallyincreased

• 280 Apeak

fault phase

• nominal currenton thehealthy phases

0 A all phases

Single line-to-groundfault "O1"

The amplitude of allphases are marginallyincreased

• 1.5 kApeak

fault phase

• nominal currenton thehealthy phases

0 A all phases

Three phaseshort circuit"A"

12 kA all phases 12 kA all phases 0 A all phases

Three phaseshort circuit"AI1"

280 A all phases 280 A all phases 0 A all phases

Three phaseshort circuit"O1"

1.7 kA all phases 1.7 kA all phases 0 A all phases

53

5.3 Summary of the "Radial system"Current when the fault had occurred with two types of grounding:

Table 3: nominal current: 260 Arms ⇐⇒ 450 Apeak on the load side of the transformer78.5 Arms ⇐⇒ 111 Apeak on the incoming side of the "Radial system 5"78.5 Arms ⇐⇒ 111 Apeak on the incoming side of the "Radial system 1"

high impedance Solid ground com-bined with FCL

Single line-to-ground fault "A"

• 600 Apeak

transformer

• 90 Apeak

at the faultphase andnominal currentat thehealthy phases(healthy system)

• 320 Apeak atthe fault phaseat the"Radial system 1"

0 A all phases andsystems

Single line-to-ground fault "AI1"

• 500 Apeak

transformer

• nominal currentat thehealthy system

• 270 Apeak

at thefault phasenominal currentat thehealthy phases"Radial system 1"

• 500 Apeak

transformer

• nominal currentat thehealthy system

• 270 Apeak atthe fault phaseand nominalcurrent on thehealthy phases

54

Petersén coil Solid ground com-bined with FCL

Three phase short circuit "A"

• 18 kApeak

transformerdeclines to12 kApeak

• Decreases to0 Ahealthy system

• 18 kApeak

"Radial system 1"declines to18 Apeak

0 A all phases andsystem

Three phase short circuit "AI1"

• 550 Apeak forthe transformer

• nominal valueat 111 A peak

• 300 Apeak

"Radial system 1"

• 550 Apeak forthe transformer

• nominal valueat 111 A peak

• 300 Apeak

"Radial system 1"

Single line-to-ground fault whichdevelops into three phase shortcircuit in "A"

• 18 kApeak

transformer

• Decreases to0 Ahealthy system

• 18 kApeak

"Radial system 1"

0 A for all phasesand systems

Single line-to-ground fault whichdevelops into three phase shortcircuit in "AI1"

• 550 Apeak forthe transformer

• nominal valueat 111 A peak

• 300 Apeak

"Radial system 1"

• 550 Apeak forthe transformer

• nominal valueat 111 A peak

• 300 Apeak

"Radial system 1"

55

6 Conclusions

The FCL has the advantage that it can limit faults, whether it is a single line-to-ground fault or athree phase fault. It shows a peak at the current that is bigger than the fault current for the highimpedance grounding, but this depends on the design of the FCL. The FCL has a big advantageto confine the fault which starts as a single line-to-ground fault and could develops into a threephase fault in the "Radial system". Since the fault current is already limited the fault can notdevelop into a three phase fault. While the high impedance in the same situation, with a singleline-to-ground fault which becomes a three phase fault, limits only the fault current in the singleline-to-ground fault and lacks the capability to limit the symmetric three phase fault.

For the single line-to-ground faults with high impedance grounding, the two healthy phases of thevoltage are affected with a higher amplitude of up to

√3 and the faulty phase for high impedance.

But for the solidly grounded system, the healthy phases are unaffected with the disadvantage ofa high fault current. Independently of if it is a single line-to-ground fault or a three phase faultthe current and voltage are zero downstream the FCL when it has triggered. The FCL has theadvantage that shortly after the fault has occurred the fault can be disconnected. With a fastdisconnection a fast re-connection can be done. The re-connection can be done when the fault iscleared and the operation time of the circuit breaker. This means that the voltage reduction willbe of short duration for the customers downstream the FCL. This is something a system with highimpedance grounding is not able to do. This longer fault time for high impedance grounding andsolid grounding, compared to the FCL system, entails a higher risk of damages. Apart from thefault time, high impedance is a good grounding system to handle a single line-to-ground faults andfed the costumers at the phases and healthy systems.

Since the current and the voltage after the FCL, is conveniently low, a load breaker switch canreplace the breaker after the FCL. In addition, the FCL has the ability to limit the fault current ina system regardless of the capacitance in the system. This means that the FCL confines the faultcurrent independent of whether the capacitance is high or low in a distribution system or increasewith the replacement of overhead lines with cables. The only thing to do is change the thresholdvalue for the current on the FCL to fit the size of the distribution system.

The high impedance grounding system is not able to limit the three phase faults, only the singleline-to-ground faults. The same type of fault with low fault current, is not detected by the FCL.Therefore, the results with an FCL become as for a solidly grounded system. When a fault occursfurther out, "AI1" in the "Radial system", the fault current is low. Thus, the effect of the highimpedance grounding is not clearly visible. The voltage on the load side of the transformer becomeless affected with a fault further out in the system.

In the “Radial system”, the fault in substation “AI1” is difficult to find with the low fault current.However, by placing an FCL on the respective outgoing feeder on the bus, a fault further out inthe "Radial system 1" can be found. This has the advantage that the fault current individuallylimitation will occur on each system instead of as in the “Radial system” where all systems wereaffected by one FCL.

Unfortunately, the FCL and the high impedance grounding are not able to limit the back-fed earthfault and limit the current, which can cause problem in form of damage on person, property andanimal.

With the characteristics of short duration time of the fault current and the lack of need for adjustingto capacitance, the FCL is recommend in distribution systems which have high capacitance or faultsthat are or develop into a three phase faults.

56

7 Further work

The possibilities to continue these investigations can be described in form of:

• Simulations with rotating machines or rotating generators in the distribution system

– All loads in the systems are fixed and can therefore handle fast open and close operationwith FCL. It is therefore of interest to see how fast the FCL can be without causingdamage on the rotating machines and rotating generators. If the circuit breakers closetoo fast after they had opened, machines and generators loos the synchronisation

• The rise time of the transients caused by the FCL

– When the circuit breakers open and close, transients occur, which require more detailedmodelling. With a smaller circuit, simulations can be made and a more reliable dataabout rise time and amplitude is obtained

57

References

Internet

[1] Swedish Neutral AB. Swedish Neutral Seminarie 2013, dag 1 http://www.swedishneutral.se/download/Flyer%20SN%20Seminar%202013%20Neutral%20Treatment%20&%20Earth%20Fault%20Protection%20-%20Svenska.pdf, Accessed March 2017

[2] Swedish Neutral AB. Nollpunktsreaktor http://www.swedishneutral.se/download/Swedish%20Neutral%20ASC%20Technical%20Specification%20Svenska.pdf, Accessed March 2017

[3] ABB SafeGrid. Ett säkrare och pålitligare distributionsnät, http://www02.abb.com/global/seitp/seitp161.nsf/0/3929955564747806c125739f004c03b6/\protect\T1\textdollarfile/ABBSafeGrid.pdf, Accsessed May 2017

[4] John C. Pfeiffer. Principles of Electrical Grounding, http://www.pfeiffereng.com/Principals%20of%20Electrical%20Grounding.pdf, 2001, Accessed March 2017

[5] Jignesh. Parmar. Types of Neutral Earthing in Power Distribution, Institution of Engineers(MIE), India,https://electricalnotes.wordpress.com/2012/01/21/types-of-neutral-earthing-in-power-distribution/,2012, Accessed March 2017

[6] Circuit Globe. Peterson coil Grounding,http://circuitglobe.com/peterson-coil-grounding.html, Accessed April 2017

[7] ELECTROTECHNIK. Petersen coil - Princip and Application,http://www.electrotechnik.net/2009/02/petersen-coils-principle-and.htm, AccessedApril 2017

[8] HV Power Measurements & Protection Ltd. Petersen Coils Basic Principle and Applica-tion, 2012, http://www.hvpower.co.nz/TechnicalLibrary/RE+DS/Petersen%20Coils%20%20Basic%20Principle%20and%20Application.pdf, Accessed April 2017

[9] Sverker Johansson, Johan Pålsson. Personfara genom elektrisk ström, Tillämpad fysik och elek-tronik, Umeå Universitet, Umeå. 1999 http://www8.tfe.umu.se/courses/elektro/analog1/distans/litteratur/Komp3-personfara.pdf, Acessed May 2017

[10] Maintoba Hydro international Ltd, PSCAD. https://hvdc.ca/get-to-know-us, AccessedMarch 2017

[11] ABB Oy. Distribution Automation Handbook, Section 8.6 MV Feeder Earth-fault Protection.2010 https://library.e.abb.com/public/948f3fb78a335cb6c125795f0042ef8b/DAHandbook_Section_08p06_Feeder_EF_Protection_757287_ENa.pdf, Acessed May 2017

Books

[12] Gunnar Elfving. ABB Handbok Elkraft, Tredje upplagan. ABB.1993, Asea Brown Boveri(ABB),

[13] J.Ducan Glover, Mulukutla S.Sarma, Thomas J.Overbye. Power System Analysis & Design.fifth edition. 2012, Cengage LearningTM, ISBN-13: 978-1-111-42579-1

[14] S. Satyanarayana, S. Sivanagaraju.Electric power Transmission and Distribution. 2008, Pear-son Education India, ISBN: 8131753174, 9788131753170 , Pearson Education India

58

Publications

[15] Lars Liljestrand, Lars Jonsson, Magnus Backman &Marco Riva. A new hybrid medium voltagebreaker for DC interruption or AC fault current limitation, ABB Corporate Research & ABBs.p.a – Italy, 2016 18th European Conference on Power Electronics and Applications (EPE’16ECCE Europe), 2016

[16] Elsäkerhetsverket, Starkströmsföreskrifterna.1999, ELSÄK-FS 1999:5

[17] Kim Reenaas. Elsäkerhetsverkets föreskrifter och allmänna råd om hur elektriska starkström-sanläggningar ska vara utförda, ELSÄK-FS 2008:1. Elsäkerthetsverket. 2016

[18] Johan Peterson. Jordfelsproblematik i icke direktjordade system. Master thesis. Dept. ofIndustrial Electrical Engineering and Automation Lund University. 2005

[19] Göran Morén. Energimarknadesinspektionens föreskrifter och allmänna råd om krav som skavara uppfyllda för att överföring av el ska vara av god kvalitet, EIFS 2013:1. Energimarknadsin-spektionen. 2013

Interview

[20] Daniel Wall, recurring contact at Vattenfall. 20 February-22 June 2017

[21] Stefan Larsson, contact at Vattenfall. 18 April 2017

59

Appendix 1: The "Real system" with faults in secondarysubstation "O1"

The fault in secondary substation "O1" is simulated with three types of grounding, where two ofthe grounding systems are similar to each other; solidly grounding with and without FCL and thethird system with a Petersén coil between the transformers neutral and ground.

1 Single line-to-ground fault in secondary substation "O1"Two of the three grounding systems in Figure 1, Petersén coil and FCL, limit the fault on theircharacteristic way and the solidly grounded transformer is not cable to limit the fault. The Peterséncoil in Figure 1b limits the fault current and the amplitude of the voltage in the two healthy phasesis increased. For the solidly grounded transformer the healthy phases are unaffected in Figure 1d,but the fault current is 1.5 kA high. This is because the solidly grounded transformer in Figure1d lets the earth current flow into the natural point on the transformer instead of limiting thecurrent as the FCL or the Petersén coil. For the last grounding system with FCL the fault currentis shown in Figure 1f and some peaks which are lower than for the solidly grounded system occurand thereafter the current is zero.

60

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(a) Voltage on the load side of the transformerwith Petersén coil.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

T1a

T1b

T1c

(b) Current on the load side of the transformerwith Petersén coil.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(c) Voltage on the load of the transformer withsolidly grounding without FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

T1a

T1b

T1c

(d) Current on the load side of the transformerwith solidly grounding without FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VT1a

VT1b

VT1c

(e) Voltage on the load side of the transformerwith solidly grounding combined with an FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

T1a

T1b

T1c

(f) Current on the load side of the transformerwith solidly grounding combined with an FCL.

Figure 1: Simulations of the "Real system" with a single line-to-ground fault in secondary substa-tion "O1" simulated with three types of grounding system: Petersén coil 1b, solidly grounded 1dand FCL 1f.

2 Three phase short circuit in secondary substation "O1"Even here the grounding systems will show different results when the fault occurs. The FCL inFigure 2f is the only system that limits this type of fault. The other grounding systems can notlimit the symmetric three phase short circuit. The figures for the solidly grounded and Peterséncoil also show how the transformer reacts when the fault is in the middle of the "Real system".The voltage on the load side of the transformer is not affected as much as when a three phase

61

short circuit occurs near the outgoing terminals of the transformer. In that case a three phasefault causes the voltage to drop almost to zero. Here it only drops from 9 kVpeak to 7.5 kVpeak

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(a) Voltage on the load side of the transformerwith Petersén coil.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

T1a

T1b

T1c

(b) Current on the load side of the transformerwith Petersén coil.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(c) Voltage on the load of the transformer withsolidly grounding without FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

T1a

T1b

T1c

(d) Current on the load side of the transformerwith solidly grounding without FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Voltage [kV

]

-20

-15

-10

-5

0

5

10

15

20

VATa

VATb

VATc

(e) Voltage on the load side of the transformerwith solidly grounding combined with an FCL.

Time [s]

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Curr

ent [k

A]

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

T1a

T1b

T1c

(f) Current on the load side of the transformerwith solidly grounding combined with an FCL.

Figure 2: The "Radial system" transformer current and voltage for the three types of groundingtypes with three phase short circuit in secondary substation "O1"

62