18-industrial and commercial

8/14/2019 18-Industrial and Commercial

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Introduction 18.1

Busbar arrangement 18.2

Discrimination 18.3

HRC fuses 18.4

Industrial circuit breakers 18.5

Protection relays 18.6

Co-ordination problems 18.7

Fault current contribution

from induction motors 18.8

Automatic changeover systems 18.9

Voltage and phase reversal protection 18.10

Power factor correctionand protection of capacitors 18.11

Examples 18.12

References 18.13

1 8 I n d u s t r i a l a n d C o m m e r c i a l P o w e r S y s t e m P r o t e c t i o n

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18.1 INTRODUCTION

As industrial and commercial operations processes andplants have become more complex and extensive (Figure18.1), the requirement for improved reliability of

electrical power supplies has also increased. Thepotential costs of outage time following a failure of thepower supply to a plant have risen dramatically as well.The introduction of automation techniques into industryand commerce has naturally led to a demand for thedeployment of more power system automation, toimprove reliability and efficiency.

The protection and control of industrial power supply

systems must be given careful attention. Many of thetechniques that have been evolved for EHV powersystems may be applied to lower voltage systems also,but typically on a reduced scale. However, industrialsystems have many special problems that havewarranted individual attention and the development ofspecific solutions.

Many industrial plants have their own generationinstalled. Sometimes it is for emergency use only,feeding a limited number of busbars and with limitedcapacity. This arrangement is often adopted to ensuresafe shutdown of process plant and personnel safety. In

18 Industrial and Commercial

Power Sys tem P rotection

N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e 3 1 7

Figure 18.1: Large modern industrial plant

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other plants, the nature of the process allows productionof a substantial quantity of electricity, perhaps allowingexport of any surplus to the public supply system ateither at sub-transmission or distribution voltage levels.Plants that run generation in parallel with the publicsupply distribution network are often referred to as co-generation or embedded generation. Special protection

arrangements may be demanded for the point ofconnection between the private and public Utility plant(see Chapter 17 for further details).

Industrial systems typically comprise numerous cablefeeders and transformers. Chapter 16 covers theprotection of transformers and Chapters 9/10 theprotection of feeders.

18.2 BUSBAR ARRANGEMENT

The arrangement of the busbar system is obviously very

important, and it can be quite complex for some verylarge industrial systems. However, in most systems asingle busbar divided into sections by a bus-sectioncircuit breaker is common, as illustrated in Figure 18.2.Main and standby drives for a particular item of processequipment will be fed from different sections of theswitchboard, or sometimes from different switchboards.

The main power system design criterion is that singleoutages on the electrical network within the plant willnot cause loss of both the main and standby drivessimultaneously. Considering a medium sized industrialsupply system, illustrated in Figure 18.3, in more detail,it will be seen that not only are duplicate supplies andtransformers used, but also certain important loads aresegregated and fed from Essential Services Board(s)(also known as Emergency boards), distributedthroughout the plant. This enables maximum utilisation

of the standby generator facility. A standby generator isusually of the turbo-charged diesel-driven type. Ondetection of loss of incoming supply at any switchboardwith an emergency section, the generator isautomatically started. The appropriate circuit breakerswill close once the generating set is up to speed andrated voltage to restore supply to the Essential Servicessections of the switchboards affected, provided that thenormal incoming supply is absent - for a typical dieselgenerator set, the emergency supply would be availablewithin 10-20 seconds from the start sequence commandbeing issued.

The Essential Services Boards are used to feed equipmentthat is essential for the safe shut down, limited operationor preservation of the plant and for the safety ofpersonnel.

This will cover process drives essential for safe shutdown,venting systems, UPS loads feeding emergency lighting,process control computers, etc. The emergencygenerator may range in size from a single unit rated 20-30kW in a small plant up to several units of 2-10MWrating in a large oil refinery or similar plant. Large

18

IndustrialandCommercialPowerSystem

Protection


NC

NO

Bus section C - Essential suppliesEDG - Emergency generator

A

A

NO B

NC0.4kV

C

NC

NOB C

NO

NO

NO

0.4kV

6kV

0.4kV

33kV

NO

NO

EDG

6kV

110kV

NO

A B

A B

C

*

*

*

*

*

*

* - Two out of three interlock

BA

Figure 18.3: Typical industrial power system

Transformer21

Transformer

interlockmechanical or electrical

HV supply HV supply1 2

2 out of 3

Figure 18.2: Typical switchboard

configuration for an industrial plant

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financial trading institutions may also have standbypower requirements of several MW to maintaincomputer services.

18.3 DISCRIMINATION

Protection equipment works in conjunction withswitchgear. For a typical industrial system, feeders andplant will be protected mainly by circuit breakers ofvarious types and by fused contactors. Circuit breakerswill have their associated overcurrent and earth faultrelays. A contactor may also be equipped with aprotection device (e.g. motor protection), but associatedfuses are provided to break fault currents in excess of thecontactor interrupting capability. The rating of fuses andselection of relay settings is carried out to ensure thatdiscrimination is achieved i.e. the ability to select andisolate only the faulty part of the system.

18.4 HRC FUSES

The protection device nearest to the actual point ofpower utilisation is most likely to be a fuse or a systemof fuses and it is important that consideration is given tothe correct application of this important device.

The HRC fuse is a key fault clearance device forprotection in industrial and commercial installations,whether mounted in a distribution fuseboard or as partof a contactor or fuse-switch. The latter is regarded as avital part of LV circuit protection, combining safe circuit

making and breaking with an isolating capabilityachieved in conjunction with the reliable short-circuitprotection of the HRC fuse. Fuses combine thecharacteristics of economy and reliability; factors thatare most important in industrial applications.

HRC fuses remain consistent and stable in their breakingcharacteristics in service without calibration andmaintenance. This is one of the most significant factorsfor maintaining fault clearance discrimination. Lack ofdiscrimination through incorrect fuse grading will resultin unnecessary disconnection of supplies, but if both themajor and minor fuses are HRC devices of proper design

and manufacture, this need not endanger personnel orcables associated with the plant.

18.4.1 Fuse Characteristics

The time required for melting the fusible elementdepends on the magnitude of current. This time is knownas the pre-arcing time of the fuse. Vaporisation of theelement occurs on melting and there is fusion betweenthe vapour and the filling powder leading to rapid arcextinction.

Fuses have a valuable characteristic known as cut-off,

illustrated in Figure 18.4. When an unprotected circuitis subjected to a short circuit fault, the r.m.s. currentrises towards a prospective (or maximum) value. Thefuse usually interrupts the short circuit current before itcan reach the prospective value, in the first quarter tohalf cycle of the short circuit. The rising current isinterrupted by the melting of the fusible element,

subsequently dying away dying away to zero during thearcing period.

Since the electromagnetic forces on busbars andconnections carrying short circuit current are related to thesquare of the current, it will be appreciated that cut-offsignificantly reduces the mechanical forces produced bythe fault current and which may distort the busbars and

connections if not correctly rated. A typical example ofcut-off current characteristics is illustrated in Figure 18.5.

18


Protection


1250A710A500A400A

200A125A80A

50A35A25A16A

6A

2A

800A630A

315A

0.1 1.0 10 100 500

0.1

1.0

10

100

1000

Prospective current (kA r.m.s. symmetrical)

Cutoffcurrent(peakkA)

Figure 18.5: Typical fuse cut-off current

characteristics

Ip

1 Cycle

Arcingtime

Pre-arcing time

Total clearance time

Current trace

Curve of asymmetrical

prospective short-circuitcurrent

TimeStart of

hort-circuit

Figure 18.4: HRC fuse cut-off feature

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It is possible to use this characteristic during the design

stage of a project to utilise equipment with a lower faultwithstand rating downstream of the fuse, than would be

the case ifcut-off was ignored. This may save on costs,

but appropriate documentation and maintenancecontrols are required to ensure that only replacement

fuses of very similar characteristics are used throughout

the lifetime of the plant concerned otherwise a safety

hazard may arise.

18.4.2 Discrimination Between Fuses

Fuses are often connected in series electrically and it is

essential that they should be able to discriminate with

each other at all current levels. Discrimination is

obtained when the larger (major) fuse remainsunaffected by fault currents that are cleared by the

smaller (minor) fuse.

The fuse operating time can be considered in two parts:

i. the time taken for fault current to melt theelement, known as the pre-arcing time

ii. the time taken by the arc produced inside the fuse

to extinguish and isolate the circuit, known as the

arcing time

The total energy dissipated in a fuse during its operationconsists of pre-arcing energy and arc energy. The

values are usually expressed in terms ofI2t, where Iis the

current passing through the fuse and t is the time in

seconds. Expressing the quantities in this manner

provides an assessment of the heating effect that thefuse imposes on associated equipment during its

operation under fault conditions.

To obtain positive discrimination between fuses, the totalI2tvalue of the minor fuse must not exceed the pre-

arcing I2tvalue of the major fuse. In practice, this meansthat the major fuse will have to have a rating

significantly higher than that of the minor fuse, and this

may give rise to problems of discrimination. Typically,the major fuse must have a rating of at least 160% of

the minor fuse for discrimination to be obtained.

18.4.3 Protection of Cables by Fuses

PVC cable is allowed to be loaded to its full nominal

rating only if it has close excess current protection. Thisdegree of protection can be given by means of a fuse link

having a fusing factor not exceeding 1.5, where:

Fusing factor =

Cables constructed using other insulating materials (e.g.

paper, XLPE) have no special requirements in this respect.

Minimum Fusing Current

Current Rating

18.4.4 Effect of Ambient Temperature

High ambient temperatures can influence the capabilityof HRC fuses. Most fuses are suitable for use in ambienttemperatures up to 35C, but for some fuse ratings,derating may be necessary at higher ambienttemperatures. Manufacturers' published literature

should be consulted for the de-rating factor to beapplied.

18.4.5 Protection of Motors

The manufacturers' literature should also be consultedwhen fuses are to be applied to motor circuits. In thisapplication, the fuse provides short circuit protection butmust be selected to withstand the starting current(possibly up to 8 times full load current), and also carrythe normal full load current continuously withoutdeterioration. Tables of recommended fuse sizes for both

direct on line and assisted start motor applications areusually given. Examples of protection using fuses aregiven in Section 18.12.1.

18.5 INDUSTRIAL CIRCUIT BREAKERS

Some parts of an industrial power system are mosteffectively protected by HRC fuses, but the replacementof blown fuse links can be particularly inconvenient inothers. In these locations, circuit breakers are usedinstead, the requirement being for the breaker tointerrupt the maximum possible fault current

successfully without damage to itself. In addition tofault current interruption, the breaker must quicklydisperse the resulting ionised gas away from the breakercontacts, to prevent re-striking of the arc, and away fromother live parts of equipment to prevent breakdown. Thebreaker, its cable or busbar connections, and the breakerhousing, must all be constructed to withstand themechanical forces resulting from the magnetic fields andinternal arc gas pressure produced by the highest levelsof fault current to be encountered.

The types of circuit breaker most frequently encounteredin industrial system are described in the following

sections.

18.5.1 Miniature Circuit Breakers (MCBs)

MCBs are small circuit breakers, both in physical size butmore importantly, in ratings. The basic single pole unit isa small, manually closed, electrically or manually openedswitch housed in a moulded plastic casing. They aresuitable for use on 230V a.c. single-phase/400V a.c.three-phase systems and for d.c. auxiliary supplysystems, with current ratings of up to 125A. Containedwithin each unit is a thermal element, in which a bimetal

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18.5.3 Air Circuit Breakers (ACBs)

Air circuit breakers are frequently encountered onindustrial systems rated at 3.3kV and below. Modern LVACBs are available in current ratings of up to 6.3kA withmaximum breaking capacities in the range of 85kA-120kA r.m.s., depending on system voltage.

This type of breaker operates on the principle that the arcproduced when the main contacts open is controlled bydirecting it into an arc chute. Here, the arc resistance isincreased and hence the current reduced to the pointwhere the circuit voltage cannot maintain the arc andthe current reduces to zero. To assist in the quenchingof low current arcs, an air cylinder may be fitted to eachpole to direct a blast of air across the contact faces asthe breaker opens, so reducing contact erosion.

Air circuit breakers for industrial use are usuallywithdrawable and are constructed with a flush frontplate making them ideal for inclusion together with fuse

switches and MCBs/MCCBs in modular multi-tierdistribution switchboards, so maximising the number ofcircuits within a given floor area.

Older types using a manual or dependent manual closingmechanism are regarded as being a safety hazard. Thisarises under conditions of closing the CB when a faultexists on the circuit being controlled. During the close-trip operation, there is a danger of egress of the arc fromthe casing of the CB, with a consequent risk of injury tothe operator. Such types may be required to be replacedwith modern equivalents.

ACBs are normally fitted with integral overcurrentprotection, thus avoiding the need for separateprotection devices. However, the operating timecharacteristics of the integral protection are oftendesigned to make discrimination withMCBs/MCCBs/fuses easier and so they may not be inaccordance with the standard dependent time

characteristics given in IEC 60255-3. Therefore,problems in co-ordination with discrete protection relaysmay still arise, but modern numerical relays have moreflexible characteristics to alleviate such difficulties.ACBs will also have facilities for accepting an externaltrip signal, and this can be used in conjunction with anexternal relay if desired. Figure 18.6 illustrates thetypical tripping characteristics available.

18.5.4 Oil Circuit Breakers (OCBs)

Oil circuit breakers have been very popular for many

years for industrial supply systems at voltages of 3.3kVand above. They are found in both bulk oil andminimum oil types, the only significant difference beingthe volume of oil in the tank.

In this type of breaker, the main contacts are housed inan oil-filled tank, with the oil acting as the both theinsulation and the arc-quenching medium. The arcproduced during contact separation under faultconditions causes dissociation of the hydrocarboninsulating oil into hydrogen and carbon. The hydrogenextinguishes the arc. The carbon produced mixes withthe oil. As the carbon is conductive, the oil must bechanged after a prescribed number of fault clearances,when the degree of contamination reaches anunacceptable level.

Because of the fire risk involved with oil, precautionssuch as the construction of fire/blast walls may have tobe taken when OCBs are installed.

18.5.5 Vacuum Circuit Breakers (VCBs)

In recent years, this type of circuit breaker, along withCBs using SF6, has replaced OCBs for new installations

in industrial/commercial systems at voltages of 3.3kVand above.

Compared with oil circuit breakers, vacuum breakershave no fire risk and they have high reliability with longmaintenance free periods. A variation is the vacuumcontactor with HRC fuses, used in HV motor starterapplications.

18.5.6 SF6 Circuit Breakers

In some countries, circuit breakers using SF6 gas as thearc-quenching medium are preferred to VCBs as the

18


Protection


0.011 10

Current (multiple of setting)

Time(s)

Inverse

Very Inverse

Ultra Inverse

Short Circuit

.1

1

10

100

1000

20

Figure 18.6: Typical tripping characteristics

of an ACB

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18


Protection


C

A B C N

B CA

A B C

CBA

A B C

(h)

(g)

(f)

(e)

(d)

(c)

(b)

(a)

CTconnections

Resi ual-urrent

lements

SystemPhaselements

Type offault

Notes

Ph. - E

3Ph. 4w

3P . 4w

3Ph. 4w

Ph. 3w

Ph. 3w

3Ph. 3w

Ph. 3w Ph. - Ph. Petersen coil andunearthed systems.

Ph. 3wr

Ph. 4w

Earth-fault settingsmay be less than full

load

Earth-fault settingsmay be less than ful

loa


load

(i) Ph. - Ph.(ii) Ph. - E(iii) Ph. - N

(i) Ph. - Ph.(ii) Ph. - E(iii) Ph. - N

(i) Ph. - Ph.(ii) Ph. - E*(iii) Ph. - N

(i) Ph. - Ph.(ii) Ph. - E*

(i) Ph. - Ph.(ii) Ph. - E

(i) Ph. - Ph.(ii) Ph. - E


load, but must begreater than largest

Ph. - N loa

Phase elements mustbe in same phases at

ll stations.Earth-fault settingsmay be less than full

load

* Earth-fault protectiononly if earth-faultcurrent is not lessthan twice primary

operatingcurrent

Ph. = phase ; w = wire ; E = earth ; N = neutra

Figure 18.7: Overcurrent and earth fault

relay connections

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replacement for air- and oil-insulated CBs. Somemodern types of switchgear cubicles enable the use ofeither VCBs or SF6-insulated CBs according to customerrequirements. Ratings of up to 31.5kA r.m.s. fault breakat 36kV and 40kA at 24kV are typical. SF6-insulated CBsalso have advantages of reliability and maintenanceintervals compared to air- or oil-insulated CBs and are

of similar size to VCBs for the same rating.

18.6 PROTECTION RELAYS

When the circuit breaker itself does not have integralprotection, then a suitable external relay will have to beprovided. For an industrial system, the most commonprotection relays are time-delayed overcurrent and earthfault relays. Chapter 9 provides details of the applicationof overcurrent relays.

Traditionally, for three wire systems, overcurrent relays

have often been applied to two phases only for relayelement economy. Even with modern multi-elementrelay designs, economy is still a consideration in terms ofthe number of analogue current inputs that have to beprovided. Two overcurrent elements will detect anyinterphase fault, so it is conventional to apply twoelements on the same phases at all relay locations. Thephase CT residual current connections for an earth faultrelay element are unaffected by this convention. Figure18.7 illustrates the possible relay connections andlimitations on settings.

18.7 CO-ORDINATION PROBLEMS

There are a number of problems that commonly occur inindustrial and commercial networks that are covered inthe following sections.

18.7.1.1 Earth Fault protection

with residually-connected CTs

For four-wire systems, the residual connection of threephase CTs to an earth fault relay element will offer earthfault protection, but the earth fault relay element mustbe set above the highest single-phase load current toavoid nuisance tripping. Harmonic currents (which may

sum in the neutral conductor) may also result in spurioustripping. The earth fault relay element will also respondto a phase-neutral fault for the phase that is not coveredby an overcurrent element where only two overcurrentelements are applied. Where it is required that the earthfault protection should respond only to earth faultcurrent, the protection element must be residuallyconnected to three phase CTs and to a neutral CT or toa core-balance CT. In this case, overcurrent protectionmust be applied to all three phases to ensure that allphase-neutral faults will be detected by overcurrentprotection. Placing a CT in the neutral earthing

connection to drive an earth fault relay provides earthfault protection at the source of supply for a 4-wiresystem. If the neutral CT is omitted, neutral current isseen by the relay as earth fault current and the relaysetting would have to be increased to prevent trippingunder normal load conditions.

When an earth fault relay is driven from residuallyconnected CTs, the relay current and time settings mustbe such that that the protection will be stable during thepassage of transient CT spill current through the relay.Such spill current can flow in the event of transient,asymmetric CT saturation during the passage of offsetfault current, inrush current or motor starting current.The risk of such nuisance tripping is greater with thedeployment of low impedance electronic relays ratherthan electromechanical earth fault relays whichpresented significant relay circuit impedance. Energisinga relay from a core balance type CT generally enablesmore sensitive settings to be obtained without the risk ofnuisance tripping with residually connected phase CTs.When this method is applied to a four-wire system, it isessential that both the phase and neutral conductors arepassed through the core balance CT aperture. For a 3-wire system, care must be taken with the arrangement ofthe cable sheath, otherwise cable faults involving thesheath may not result in relay operation (Figure 18.8).

18


Protection


Cable gland /sheathground connection

Cable gland

a le ox

Relay does not operate

(a) Incorrect

Relay operates

(b) Correct

I >

I >

I >

Figure 18.8: CBCT connection for

four-wire system

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18.7.2 Four-Wire Dual-Fed Substations

The co-ordination of earth fault relays protecting four-

wire systems requires special consideration in the case oflow voltage, dual-fed installations. Horcher [18.1] hassuggested various methods of achieving optimum co-ordination. Problems in achieving optimum protection

for common configurations are described below.18.7.2.1 Use of 3-pole CBs

When both neutrals are earthed at the transformers andall circuit breakers are of the 3-pole type, the neutralbusbar in the switchgear creates a double neutral to

earth connection, as shown in Figure 18.9. In the eventof an uncleared feeder earth fault or busbar earth fault,with both the incoming supply breakers closed and thebus section breaker open, the earth fault current willdivide between the two earth connections. Earth fault

relay RE2 may operate, tripping the supply to the healthy

section of the switchboard as well as relay RE1 trippingthe supply to the faulted section.

If only one incoming supply breaker is closed, the earthfault relay on the energised side will see only aproportion of the fault current flowing in the neutralbusbar. This not only significantly increases the relay

operating time but also reduces its sensitivity to low-level earth faults.

The solution to this problem is to utilise 4-pole CBs thatswitch the neutral as well as the three phases. Thenthere is only a single earth fault path and relay operation

is not compromised.

18.7.2.2 Use of single earth electrode

A configuration sometimes adopted with four-wire dual-fed substations where only a 3-pole bus section CB isused is to use a single earth electrode connected to themid-point of the neutral busbar in the switchgear, asshown in Figure 18.10. When operating with both

incoming main circuit breakers and the bus sectionbreaker closed, the bus section breaker must be openedfirst should an earth fault occur, in order to achievediscrimination. The co-ordination time between theearth fault relays RF and RE should be established atfault level F2 for a substation with both incoming supplybreakers and bus section breaker closed.

When the substation is operated with the bus sectionswitch closed and either one or both of the incomingsupply breakers closed, it is possible for unbalancedneutral busbar load current caused by single phaseloading to operate relay RS1 and/or RS2 and inadvertently

trip the incoming breaker. Interlocking the trip circuit ofeach RS relay with normally closed auxiliary contacts onthe bus section breaker can prevent this.

However, should an earth fault occur on one side of the

busbar when relays RS are already operated, it is possiblefor a contact race to occur. When the bus section

breaker opens, its break contact may close before the RSrelay trip contact on the healthy side can open (reset).Raising the pick-up level of relays RS1 and RS2 above themaximum unbalanced neutral current may prevent thetripping of both supply breakers in this case. However,the best solution is to use 4-pole circuit breakers, and

18


Protection


Supply 1

RS1

F1

F

RS2

E

RF

Supply 2

I >

I >

I >I >

N

Figure 18.10: Dual fed four-wire systems:

use of single point neutral earthing

RE1

IF

IF

IF2 IF2

IF2

IF2

RE2

Neutral busbarSupply 1 Supply 2

Bus section CB

Figure 18.9: Dual fed four-wire systems:

use of 3-pole CBs

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independently earth both sides of the busbar.

If, during a busbar earth fault or uncleared feeder earthfault, the bus section breaker fails to open whenrequired, the interlocking break auxiliary contact willalso be inoperative. This will prevent relays RS1 and RS2from operating and providing back-up protection, with

the result that the fault must be cleared eventually byslower phase overcurrent relays. An alternative methodof obtaining back-up protection could be to connect asecond relay RE, in series with relay RE, having anoperation time set longer than that of relays RS1 andRS2. But since the additional relay must be arranged totrip both of the incoming supply breakers, back-upprotection would be obtained but busbar selectivitywould be lost.

An example of protection of a typical dual-fedswitchboard is given in Section 18.12.3.

18.8 FAULT CURRENT CONTRIBUTIONFROM INDUCTION MOTORS

When an industrial system contains motor loads, themotors will contribute fault current for a short time.They contribute to the total fault current via thefollowing mechanism.

When an induction motor is running, a flux, generated bythe stator winding, rotates at synchronous speed andinteracts with the rotor. If a large reduction in the statorvoltage occurs for any reason, the flux in the motorcannot change instantaneously and the mechanical

inertia of the machine will tend to inhibit speedreduction over the first few cycles of fault duration. Thetrapped flux in the rotor generates a stator voltage equalinitially to the back e.m.f. induced in the stator beforethe fault and decaying according to the X/R ratio of theassociated flux and current paths. The induction motortherefore acts as a generator resulting in a contributionof current having both a.c. and d.c. components decayingexponentially. Typical 50Hz motor a.c. time constants liein the range 10ms-60ms for LV motors and 60-200ms forHV motors. This motor contribution has often beenneglected in the calculation of fault levels.

Industrial systems usually contain a large component ofmotor load, so this approach is incorrect. Thecontribution from motors to the total fault current maywell be a significant fraction of the total in systemshaving a large component of motor load. Standardsrelating to fault level calculations, such as IEC 60909,require the effect of motor contribution to be includedwhere appropriate. They detail the conditions underwhich this should be done, and the calculation methodto be used. Guidance is provided on typical motor faultcurrent contribution for both HV and LV motors if therequired data is not known. Therefore, it is now

relatively easy, using appropriate calculation software, todetermine the magnitude and duration of the motorcontribution, so enabling a more accurate assessment ofthe fault level for:

a. discrimination in relay co-ordination

b. determination of the required switchgear/busbar

fault rating

For protection calculations, motor fault levelcontribution is not an issue that is generally is important.In industrial networks, fault clearance time is oftenassumed to occur at 5 cycles after fault occurrence, andat this time, the motor fault level contribution is muchless than just after fault occurrence. In rare cases, it mayhave to be taken into consideration for correct timegrading for through-fault protection considerations, andin the calculation of peak voltage for high-impedancedifferential protection schemes.

It is more important to take motor contribution intoaccount when considering the fault rating of equipment(busbars, cables, switchgear, etc.). In general, the initiala.c. component of current from a motor at the instant offault is of similar magnitude to the direct-on-linestarting current of the motor. For LV motors, 5xFLCisoften assumed as the typical fault current contribution(after taking into account the effect of motor cableimpedance), with 5.5xFLC for HV motors, unless it isknown that low starting current HV motors are used. Itis also accepted that similar motors connected to abusbar can be lumped together as one equivalent motor.

In doing so, motor rated speed may need to be taken intoconsideration, as 2 or 4 pole motors have a longer faultcurrent decay than motors with a greater number ofpoles. The kVA rating of the single equivalent motor istaken as the sum of the kVA ratings of the individualmotors considered. It is still possible for motorcontribution to be neglected in cases where the motorload on a busbar is small in comparison to the total load(again IEC 60909 provides guidance in this respect).However, large LV motor loads and all HV motors shouldbe considered when calculating fault levels.

18.9 AUTOMATIC CHANGEOVER SYSTEMS

Induction motors are often used to drive critical loads. Insome industrial applications, such as those involving thepumping of fluids and gases, this has led to the need fora power supply control scheme in which motor and otherloads are transferred automatically on loss of the normalsupply to an alternative supply. A quick changeover,enabling the motor load to be re-accelerated, reducesthe possibility of a process trip occurring. Such schemesare commonly applied for large generating units totransfer unit loads from the unit transformer to thestation supply/start-up transformer.

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When the normal supply fails, induction motors thatremain connected to the busbar slow down and thetrapped rotor flux generates a residual voltage thatdecays exponentially. All motors connected to a busbarwill tend to decelerate at the same rate when the supplyis lost if they remain connected to the busbar. This isbecause the motors will exchange energy between

themselves, so that they tend to stay synchronised toeach other. As a result, the residual voltages of all themotors decay at nearly the same rate. The magnitude ofthis voltage and its phase displacement with respect tothe healthy alternative supply voltage is a function oftime and the speed of the motors. The angulardisplacement between the residual motor voltage andthe incoming voltage will be 180 at some instant. If thehealthy alternative supply is switched on to motorswhich are running down under these conditions, veryhigh inrush currents may result, producing stresseswhich could be of sufficient magnitude to cause

mechanical damage, as well as a severe dip in thealternative supply voltage.

Two methods of automatic transfer are used:

a. in-phase transfer system

b. residual voltage system

The in-phase transfer method is illustrated in Figure18.11(a). Normal and standby feeders from the samepower source are used.

Phase angle measurement is used to sense the relativephase angle between the standby feeder voltage and themotor busbar voltage. When the voltages are

approximately in phase, or just prior to this conditionthrough prediction, a high-speed circuit breaker is usedto complete the transfer. This method is restricted tolarge high inertia drives where the gradual run downcharacteristic upon loss of normal feeder supply can bepredicted accurately.

Figure 18.11(b) illustrates the residual voltage method,which is more common, especially in the petrochemicalindustry.

Two feeders are used, supplying two busbar sectionsconnected by a normally open bus section breaker. Eachfeeder is capable of carrying the total busbar load. Each

bus section voltage is monitored and loss of supply oneither section causes the relevant incomer CB to open.Provided there are no protection operations to indicatethe presence of a busbar fault, the bus section breaker isclosed automatically to restore the supply to theunpowered section of busbar after the residual voltagegenerated by the motors running down on that sectionhas fallen to a an acceptable level. This is between 25%and 40%, of nominal voltage, dependent on thecharacteristics of the power system. The choice ofresidual voltage setting will influence the re-acceleration current after the bus section breaker closes.

For example, a setting of 25% may be expected to resultin an inrush current of around 125% of the startingcurrent at full voltage. Alternatively, a time delay couldbe used as a substitute for residual voltagemeasurement, which would be set with knowledge of theplant to ensure that the residual voltage would havedecayed sufficiently before transfer is initiated.

The protection relay settings for the switchboard musttake account of the total load current and the voltagedip during the re-acceleration period in order to avoidspurious tripping during this time. This time can beseveral seconds where large inertia HV drives are

involved.

18.10 VOLTAGE AND PHASE REVERSAL PROTECTION

Voltage relays have been widely used in industrial powersupply systems. The principle purposes are to detectundervoltage and/or overvoltage conditions atswitchboards to disconnect supplies before damage canbe caused from these conditions or to provideinterlocking checks. Prolonged overvoltage may causedamage to voltage-sensitive equipment (e.g. electronics),while undervoltage may cause excessive current to be

18


Protection


(a) In phase transfer method

High speedCB

Phaseanglerelay

Preferrefee er

Standbyfee er

(b) Residual voltage method

M

Fee er No.1 Feeder No.2

M M M

I

>I

I >>>I >

I >

Lockout

Id>

U > U I

P1

I>> I>> I>>

Figure 18.13: Protection of capacitor banks

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economical method. Some industrial consumers applycapacitors to selected motor substations rather thanapplying all of the correction at the main incomingsubstation busbars. Sometimes, power factor correctionmay even be applied to individual motors, resulting inoptimum power factor being obtained under allconditions of aggregate motor load. In some instances,

better motor starting may also result, from theimprovement in the voltage regulation due to thecapacitor. Motor capacitors are often six-terminal units,and a capacitor may be conveniently connected directlyacross each motor phase winding.

Capacitor sizing is important, such that a leading powerfactor does not occur under any load condition. If excesscapacitance is applied to a motor, it may be possible forself-excitation to occur when the motor is switched off orsuffers a supply failure. This can result in the productionof a high voltage or in mechanical damage if there is asudden restoration of supply. Since most star/delta orauto-transformer starters other than the Korndorffertypes involve a transitional break in supply, it is generallyrecommended that the capacitor rating should notexceed 85% of the motor magnetising reactive power.

18.11.3 Capacitor Protection

When considering protection for capacitors, allowanceshould be made for the transient inrush currentoccurring on switch-on, since this can reach peak valuesof around 20 times normal current. Switchgear for usewith capacitors is usually de-rated considerably to allowfor this. Inrush currents may be limited by a resistor inseries with each capacitor or bank of capacitors.

Protection equipment is required to prevent rupture ofthe capacitor due to an internal fault and also to protectthe cables and associated equipment from damage incase of a capacitor failure. If fuse protection iscontemplated for a three-phase capacitor, HRC fusesshould be employed with a current rating of not less than1.5 times the rated capacitor current.

Medium voltage capacitor banks can be protected by thescheme shown in Figure 18.13. Since harmonics increase

capacitor current, the relay will respond more correctly ifit does not have in-built tuning for harmonic rejection.

Double star capacitor banks are employed at mediumvoltage. As shown in Figure 18.14, a current transformerin the inter star-point connection can be used to drive aprotection relay to detect the out-of-balance currentsthat will flow when capacitor elements become short-circuited or open-circuited. The relay will haveadjustable current settings, and it might contain a biascircuit, fed from an external voltage transformer, thatcan be adjusted to compensate for steady-state spillcurrent in the inter star-point connection.

Some industrial loads such as arc furnaces involve largeinductive components and correction is often appliedusing very large high voltage capacitors in variousconfigurations.

Another high voltage capacitor configuration is the split

phase arrangement where the elements making up eachphase of the capacitor are split into two parallel paths.Figure 18.15 shows two possible connection methods forthe relay. A differential relay can be applied with acurrent transformer for each parallel branch. The relaycompares the current in the split phases, using sensitivecurrent settings but also adjustable compensation forthe unbalance currents arising from initial capacitormismatch.

18.12 EXAMPLES

In this section, examples of the topics dealt with in theChapter are considered.

18.12.1 Fuse Co-ordination

An example of the application of fuses is based on thearrangement in Figure 18.16(a). This shows anunsatisfactory scheme with commonly encounteredshortcomings. It can be seen that fuses B, Cand D willdiscriminate with fuseA, but the 400A sub-circuit fuseEmay not discriminate, with the 500A sub-circuit fuseD at higher levels of fault current.

18


Protection


Capacitorbank

Alarm Trip

>IU

Figure 18.14: Protection of double starcapacitor banks

Chap18-316-335 17/06/02 11:22 Page 330


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The solution, illustrated in Figure 18.16(b), is to feed the400A circuit Edirect from the busbars. The sub-circuitfuse D may now have its rating reduced from 500A to avalue, of say 100A, appropriate to the remaining sub-circuit. This arrangement now provides a discriminatingfuse distribution scheme satisfactory for an industrialsystem.

However, there are industrial applications wherediscrimination is a secondary factor. In the applicationshown in Figure 18.17, a contactor having a fault ratingof 20kA controls the load in one sub-circuit. A fuserating of 630A is selected for the minor fuse in thecontactor circuit to give protection within the through-

fault capacity of the contactor.The major fuse of 800A is chosen, as the minimum ratingthat is greater than the total load current on theswitchboard. Discrimination between the two fuses isnot obtained, as the pre-arcing I2tof the 800A fuse isless than the total I2tof the 630A fuse. Therefore, themajor fuse will blow as well as the minor one, for mostfaults so that all other loads fed from the switchboardwill be lost. This may be acceptable in some cases. Inmost cases, however, loss of the complete switchboardfor a fault on a single outgoing circuit will not beacceptable, and the design will have to be revised.

18


Protection


500A 500A 500A

400A30Aeach

A

B C D

E

F

1000ARating

Rating Rating Rating

RatingRating

(a) Incorrect layout giving rise to problems in discrimination

(b) Correct layout and discrimination

100A 500A400A

30A

A

B C DE

F

500A

1000ARating

Rating Rating Rating Rating

Rating

Figure 18.16: Fuse protection: effectof layout on discrimination

Alarm Trip

A

C

>I

>I

>I

>I

>I

>I

(a)

Alarm Trip

B

(b)

Figure 18.15: Differential protection of splitphase capacitor banks

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18.12.2 Grading of Fuses/MCCBs/ Overcurrent Relays

An example of an application involving a moulded casecircuit breaker, fuse and a protection relay is shown inFigure 18.18. A 1MVA 3.3kV/400V transformer feeds theLV board via a circuit breaker, which is equipped with aMiCOM P141 numerical relay having a setting range of8-400% of rated current and fed from 2000/1A CT s.

Discrimination is required between the relay and both thefuse and MCCB up to the 40kA fault rating of the board.To begin with, the time/current characteristics of both the400A fuse and the MCCB are plotted in Figure 18.19.

18.12.2.1 Determination of relay current setting

The relay current setting chosen must not be less than thefull load current level and must have enough margin toallow the relay to reset with full load current flowing. Thelatter may be determined from the transformer rating:

FLCkVA

kV x 3=

=

=1000

0 4 31443

.A

With the CT ratio of 2000/1A and a relay reset ratio of 95%of the nominal current setting, a current setting of at least80% would be satisfactory, to avoid tripping and/or failureto reset with the transformer carrying full load current.However, choice of a value at the lower end of this currentsetting range would move the relay characteristic towardsthat of the MCCB and discrimination may be lost at low

fault currents. It is therefore prudent to select initially arelay current setting of 100%.

18.12.2.2 Relay characteristic andtime multiplier selection

An EI characteristic is selected for the relay to ensurediscrimination with the fuse (see Chapter 9 for details).From Figure 18.19, it may be seen that at the fault levelof 40kA the fuse will operate in less than 0.01s and theMCCB operates in approximately 0.014s. Using a fixedgrading margin of 0.4s, the required relay operating timebecomes 0.4 + 0.014 = 0.414s. With a CT ratio of

2000/1A, a relay current setting of 100%, and a relayTMS setting of 1.0, the extremely inverse curve gives arelay operating time of 0.2s at a fault current of 40kA.This is too fast to give adequate discrimination andindicates that the EI curve is too severe for thisapplication. Turning to the VI relay characteristic, therelay operation time is found to be 0.71s at a TMS of 1.0.To obtain the required relay operating time of 0.414s:

TMS setting =

=

0 414

0 71

0 583

.

.

.

18


Protection


800A

Auxiliary circuits

Fused contactor

630A

400V

Figure 18.17: Example of back-up protection

10.0

1.0

0.1

.011000 10,000 100,000

Operatingtime(s)

Characteristic for relay

MCCB

Fuse

Operating current (A) to 415V baseOriginal relay characteristic Revised relay characteristic

Figure 18.19: Grading curves forFuse/MCCB/relay grading example

1

3300/415VMCCB400A

LV oar faultlevel = 30k

2000/1A

Fuse

I>>>I

Figure 18.18: Network diagram for protectionco-ordination example fuse/MCCB/relay

Chap18-316-335 17/06/02 11:22 Page 332


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Use a TMS of 0.6, nearest available setting.The use of a different form of inverse time characteristicmakes it advisable to check discrimination at the lower

current levels also at this stage. At a fault current of4kA, the relay will operate in 8.1s, which does not givediscrimination with the MCCB. A relay operation time of8.3s is required. To overcome this, the relaycharacteristic needs to be moved away from the MCCBcharacteristic, a change that may be achieved by using a

TMS of 0.625. The revised relay characteristic is alsoshown in Figure 18.19.

18.12.3 Protection of a Dual-Fed Substation

As an example of how numerical protection relays can beused in an industrial system, consider the typical largeindustrial substation of Figure 18.20. Two 1.6MVA,

11/0.4kV transformers feeding a busbar whose bus-

section CB is normally open. The LV system is solidlyearthed. The largest outgoing feeder is to a motor rated160kW, 193kVA, and a starting current of 7 x FLC.

The transformer impedance is to IEC standards. The LVswitchgear and bus bars are fault rated at 50kA rms. To

simplify the analysis, only the phase-fault LV protectionis considered.

18.12.3.1 General considerations

Analysis of many substations configured as in Figure18.20 shows that the maximum fault level and feederload current is obtained with the bus-section circuit

breaker closed and one of the infeeding CB s open. Thisapplies so long as the switchboard has a significant

amount of motor load. The contribution of motor load tothe fault level at the switchboard is usually larger thanthat from a single infeeding transformer, as thetransformer restricts the amount of fault current infeedfrom the primary side. The three-phase break fault levelat the switchboard under these conditions is assumed tobe 40kA rms.

Relays C are not required to have directionalcharacteristics (see Section 9.14.3) as all three circuitbreakers are only closed momentarily during transferfrom a single infeeding transformer to two infeedingtransformers configuration. This transfer is normally anautomated sequence, and the chance of a fault occurringduring the short period (of the order of 1s) when all threeCBs are closed is taken to be negligibly small. Similarly,although this configuration gives the largest fault levelat the switchboard, it is not considered from either aswitchboard fault rating or protection viewpoint.

It is assumed that modern numerical relays are used. Forsimplicity, a fixed grading margin of 0.3s is used.

18.12.3.2 Motor protection relay settings

From the motor characteristics given, the overcurrentrelay settings (Relay A) can be found using the guidelinesset out in Chapter 19 as:

Thermal element:

current setting: 300A

time constant: 20 mins

Instantaneous element:

current setting: 2.32kA

These are the only settings relevant to the upstreamrelays.

18.12.3.3 Relay B settings

Relay B settings are derived from consideration of theloading and fault levels with the bus-section breakerbetween busbars A1 and A2 closed. No information isgiven about the load split between the two busbars, butit can be assumed in the absence of definitiveinformation that each busbar is capable of supplying the

total load of 1.6MVA. With fixed tap transformers, thebus voltage may fall to 95% of nominal under theseconditions, leading to a load current of 2430A. The IDMTcurrent setting must be greater than this, to avoid relayoperation on normal load currents and (ideally) withaggregate starting/re-acceleration currents. If the entireload on the busbar was motor load, an aggregate startingcurrent in excess of 13kA would occur, but a currentsetting of this order would be excessively high and leadto grading problems further upstream. It is unlikely thatthe entire load is motor load (though this does occur,especially where a supply voltage of 690V is chosen formotors an increasingly common practice) or that all

18


Protection


>I>Relay 1

>Relay 2

I >

1.6 MVA11 0.4kVZ=6.25%

>II>>

e ay

>I>Relay

Motor cable

160kW

NO

50kA rms

A2 0.4kV

Trip

2500/1 2500 1

2500/1

300 1

Figure 18.20: Relay grading examplefor dual-fed switchboard

Chap18-316-335 17/06/02 11:22 Page 333


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motors are started simultaneously (but simultaneous re-acceleration may well occur). What is essential is thatrelay B does not issue a trip command under thesecircumstances i.e. the relay current/time characteristicis in excess of the current/time characteristic of theworst-case starting/re-acceleration condition. It istherefore assumed that 50% of the total bus load ismotor load, with an average starting current of 600% offull load current (= 6930A), and that re-accelerationtakes 3s. A current setting of 3000A is therefore initiallyused. The SI characteristic is used for grading the relay,as co-ordination with fuses is not required. The TMS isrequired to be set to grade with the thermal protectionof relay A under cold conditions, as this gives thelongest operation time of Relay A, and the re-acceleration conditions. A TMS value of 0.41 is found toprovide satisfactory grading, being dictated by the motorstarting/re-acceleration transient. Adjustment of bothcurrent and TMS settings may be required depending on

the exact re-acceleration conditions. Note that lowercurrent and TMS settings could be used if motorstarting/re-acceleration did not need to be considered.

The high-set setting needs to be above the full loadcurrent and motor starting/re-acceleration transientcurrent, but less than the fault current by a suitablemargin. A setting of 12.5kA is initially selected. A timedelay of 0.3s has to used to ensure grading with relay Aat high fault current levels; both relays A and B may seea current in excess of 25kA for faults on the cable side ofthe CB feeding the 160kW motor. The relay curves areillustrated in Figure 18.21.

18.12.3.4 Relays C settings

The setting of the IDMT element of relays C1 and C2 hasto be suitable for protecting the busbar while gradingwith relay B. The limiting condition is grading with relayB, as this gives the longest operation time for relays C.

The current setting has to be above that for relay B toachieve full co-ordination, and a value of 3250A is

suitable. The TMS setting using the SI characteristic ischosen to grade with that of relay B at a current of

12.5kA (relay B instantaneous setting), and is found to be0.45. The high-set element must grade with that of relayB, so a time delay of 0.62sec is required. The current

setting must be higher than that of relay B, so use avalue of 15kA. The final relay grading curves and

settings are illustrated in Figure 18.22.

18.12.3.5 Comments on grading

While the above grading may appear satisfactory, theprotection on the primary side of the transformer has notbeen considered. IDMT protection at this point will have

to grade with relays C and with the through-fault short-time withstand curves of the transformer and cabling.

This may result in excessively long operation times. Evenif the operation time at the 11kV level is satisfactory,there is probably a Utility infeed to consider, which will

involve a further set of relays and another stage of timegrading, and the fault clearance time at the utility infeedwill almost certainly be excessive. One solution is to

accept a total loss of supply to the 0.4kV bus underconditions of a single infeed and bus section CB closed.

This is achieved by setting relays C such that gradingwith relay B does not occur at all current levels, oromitting relay B from the protection scheme. The

argument for this is that network operation policy is toensure loss of supply to both sections of the switchboard

does not occur for single contingencies. As single infeedoperation is not normal, a contingency (whether fault ormaintenance) has already occurred, so that a further

fault causing total loss of supply to the switchboard

18


Protection


0.01100 1000 10000 100000

Current (A) referred to 0.4kV

Time(s)

.1

1

10

100

1000

Relay

Relay

Re-accelerationRelay A setting

e ay sett ng

Figure 18.21: Grading of relays A and B

0.01100 1000 10000 100000

Current (A) referred to 0.4kV

Time(s)

Relay

RelayRelay B

.1

1

10

100

1000

Value Value ValueParameter

Relay ARelay BRelay C

(a) Relay settings

0.250.1751200s

I>I>

2750A

00A

TMSTMS

Time const

I>> 15000

inst

inst

tinst 0.62s0.32s

Re-accelerationRelay A settingRelay B settingRelay C setting

(b) Grading curves

I> I>>

I> I>>

Figure 18.22: Final relay grading curves

Chap18-316-335 17/06/02 11:22 Page 334


20/20

through tripping of one of relays B is a secondcontingency. Total loss of supply is therefore acceptable.The alternative is to accept a lack of discrimination atsome point on the system, as already noted in Chapter 9.Another solution is to employ partial differentialprotection to remove the need for Relay A, but this isseldom used. The strategy adopted will depend on the

individual circumstances.

18.13 REFERENCES

18.1 Overcurrent Relay Co-ordination for DoubleEnded Substations. George R Horcher. IEEE. Vol.1A-14 No. 6 1978.

18


Protection


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18-industrial and commercial

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