abb gcb - disjuntor de gerador

8/18/2019 ABB GCB - Disjuntor de Gerador

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Product Brochure

Generator Circuit-Breakers Application Guide


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Edited by

ABB Switzerland LtdHigh Voltage ProductsDepartment: High Current SystemsBrown Boveri Strasse 5CH-8050 Zurich / Switzerland

Text: Dieter Braun, Giosafat Caval iere,

Kurt Dahinden, Mirko Palazzo


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Table of contents

1 Introduction 6

3 Design of generator circuit-breakers 93.1 Interrupting chamber 9

3.2 Hydraulic spring operating mechanism 11

3.3 SF 6-gas density monitoring system 11

3.4 Disconnector 12

3.5 Earthing switch 12

3.6 Starting switch (for gas turbine power plants) 12

3.7 Short-circuiting connection 13

3.8 Current transformer 13

3.9 Voltage transformer 13

3.10 Ferroresonance damping device 13

3.11 Surge capacitor 14

3.12 Surge arrester 143.13 Connecting zone 15

3.14 Phase enclosure 16

3.15 Control and supervision 16

2 History of the development of generator circuit-breakers 7

5 Selection of generator circuit-breakers 185.1 Duties of generator circuit-breakers 18

5.2 Requirements for generator circuit-breakers 18

5.3 Selection of generator circuit-breakers 18

5.3.1 Rated maximum voltage 19

5.3.2 Power frequency 195.3.3 Rated continuous current 19

5.3.4 Rated dielectric strength 20

5.3.5 Rated short-circuit duty cycle 20

5.3.6 Rated interrupting time 20

5.3.7 Rated closing time 20

5.3.8 Short-circuit current rating 20

5.3.8.1 System-source short-circuit current 20

5.3.8.2 Generator-source short-circuit current 23

5.3.8.3 Required closing, latching, and carrying capabilities 27

5.3.8.4 Required short-time current-carrying capability 27

5.3.9 Transient recovery voltage rating 27

5.3.9.1 First-pole-to-clear factor 285.3.9.2 Amplitude factor 28

5.3.9.3 Power frequency recovery voltage 28

5.3.9.4 Rated inherent transient recovery voltage 29

5.3.9.5 System-source faults 30

5.3.9.6 Generator-source faults 30

5.3.9.7 Calculation of TRV in case of terminal faults 30

5.3.10 Rated load current switching capability 32

5.3.11 Capacitance current switching capability 32

5.3.12 Out-of-phase current switching capability 32

5.3.13 Excitation current switching capability 34

5.3.14 Rated control voltage 34

5.3.15 Rated mechanism fluid operating pressure 34

4 Standard for generator circuit-breakers 17


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6 Application of generator circuit-breakers 356.1 Power plant layouts 35

6.1.1 Thermal power plants 35

6.1.2 Gas turbine power plants 35

6.1.3 Hydro power plants 356.1.4 Pumped storage power plants 36

6.2 Advantages of generator circuit-breakers 38

6.2.1 Simplified operational procedures 38

6.2.2 Improved protection of the generator and the main and unit transformers 38

6.2.3 Increased security and higher power plant availability 38

6.2.3.1 Transformer failures 39

6.2.3.2 Short-time unbalanced load condition 41

6.2.3.3 Generator motoring 42

6.2.3.4 Synchronizing under out-of-phase conditions 42

6.2.4 Economic benefit 43

Table of contents

8 Case study 1: Impact of the method of connecting a generator to the high-voltage grid on the availability of a power plant 458.1 Power plant layout 45

8.1.1 Layout of extra high-voltage substation 47

8.1.2 Layout of high-voltage substation 48

8.1.3 Generator circuit-breaker 48

8.1.4 Station transformer 48

8.2 Data for availability calculations 48

8.3 Simulations 48

8.4 Simulation results 49

8.5 Economic evaluation 50

7 Maintenance of generator circuit-breakers 44

References 57

9 Case study 2: Interrupting capability of generator circuit-breakers in case of delayed current zeros 529.1 Generator circuit-breaker model adopted for the simulations 52

9.2 Generator terminal faults 53

9.3 Out-of-phase synchronising 55

9.4 Conclusions 56


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AUX G

MT

EHV HV

UT ST GCB

AUX

G

MT

EHV HV

UT ST

1 Introduction

A major objective of all power plant operating companies isthe achievement of the highest possible plant availability atthe lowest possible cost. Obviously, how a generator is con-nected to the high-voltage grid and how the power supply to

the unit auxiliaries is secured has a decisive influence on theavailability of a power plant. Two basically different ways of connecting a generator to thehigh-voltage transmission network are in use today, namelythe connection without a circuit-breaker between the genera-tor and the low-voltage terminals of the main transformer (i.e.the "unit connection") and the connection with a generatorcircuit-breaker (Figure 1). The layout with a generator circuit-

breaker has several advantages over the unit connection, e.g.:

– simplified operational procedures

– improved protection of the generator and the main and unittransformers

– increased security and higher power plant availability

– economic benefit

ABB generator ci rcuit -breakers are suitable for application inall kinds of new power plants such as fossil-fired, nuclear, gasturbine, combined cycle, hydro and pumped storage powerplants as well as for replacement or retrofit in existing powerstations when they are modernized and/or extended.

Figure 1: Layout of a thermal power plant without generator circuit-breaker a) and with generator circuit-breaker b)

a) b)

LegendMT Main transformer

UT Unit transformer

ST Station transformer

GCB Generator circuit-breaker

EHV Transmission system

HV Sub-transmission system

AUX Unit auxil iaries


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2 History of the development of generator circuit-breakers

During the sixties, when there was a trend towards higher unitratings and, consequently, increased use of phase-segregated generator busducts, ABB developed a circuit-breaker which could meet these new requirements. This wasthe first circuit-breaker designed to be installed in the run ofgenerator busducts (Figure 2).Since the delivery of the first specific purpose generatorcircuit-breaker in 1970, there has been a continuous develop-ment of this piece of power plant equipment. At the beginningthe circuit-breakers consisted of three metal-enclosed, phase

segregated units using compressed air as operating and arc-extinguishing medium.In the 1980’s SF 6 generator circuit-breakers were successfullyintroduced into the market. The design of these circuit-break-ers was a three-phase system in single-phase enclosures,supplied fully assembled on a common frame with operatingmechanism and control equipment. Mainly the economicalaspect and reasons of reliability and maintainability convincedcustomers of this modern arc-extinguishing medium.

Figure 2: Air blast generator circuit-breaker type DR mounted in the run of an isolated phase bus

Originally conventional distributioncircuit-breakers were used to connectthe generator to the step-up transformer.With the increasing output of the genera-tors, the required ratings exceeded theload currents and short-circuit levels ofthe switchgear available. Therefore theunit connection became the acceptedstandard power plant layout.


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Figure 4: Generator circuit-breaker type HEC 7 based on SF 6 technology and self-blast principle

Figure 3: SF 6 generator circuit-breaker type HECS-130R for openinstallation

Current transformer

Voltage transformer

Interrupting chamber

Disconnector

Surge arrester

Figure 5: View into one pole of a generator circuit-breaker system

In the 1990’s SF 6 generator circuit-breakers were specificallydeveloped for open installation, i.e. without enclosure. Thissolution was introduced to allow quick and easy installationeven for projects with very small space requirements (Figure 3).

Today SF 6 generator circuit-breakers with rated currents upto 24’000 A with natural cooling and up to 57’000 A withforced air cooling, respectively, and with short-circuit breaking

Another development has been the integration of all the

associated items of switchgear into the generator circuit-breaker housing. Series disconnectors, earthing switches,short-circuiting connection, current transformers, single-bushing voltage transformers, protective capacitors and surgearresters can be mounted in the enclosure of the generatorcircuit-breaker (Figure 5). Depending on the type of powerplant additional items like starting switches (for gas turbine

currents up to 210 kA are available. This breaking capacitycorresponds to the highest short-circuit breaking current everachieved with a single SF 6 interrupting unit. The developmentwas made possible by using the most advanced SF 6 self-blast principle. With this achievement modern SF 6 generatorcircuit-breakers can now be delivered for generating units withratings up to 2’000 MVA (Figure 4).

and hydro power plants) can also be fitted in the generator

circuit-breaker housing. This greatly improved functionalityallows simpler and more economic power plant layouts.Beside a substantial reduction of the first costs this newsolution - being fully factory assembled and tested - alsomakes possible considerable savings in time and expendituresfor erection and commissioning.


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3 Design of generator circuit-breakers

ABB generator ci rcuit -breaker systems are three-phase sys-tems with a SF 6 circuit-breaker and a disconnector in single-phase enclosures, supplied fully assembled on a commonframe, with operating mechanisms and control equipment.

In addition to the circuit-breaker and disconnector, thegenerator circuit-breaker systems are available with earthingswitch, starting switch, short-circuiting connection, currentand voltage transformers, surge capacitor and surge arrester.

The single l ine diagram of a generator circuit-breaker systemis depicted in Figure 6.

All the components are integrated and mounted in the phaseenclosures (Figure 5). The generator circuit-breaker system isdesigned for welded connections to the isolated phase busenclosures. Each enclosure is made of aluminium and capable

of carrying the induced return current. The phase distance can be selected to suit the busbarspacing in the power plant.

Figure 6: Typical single line diagram of a generator circuit-breaker system

1 Generator circuit-breaker 2 Line disconnect switch 3 Earthing switch 4 Starting switch for SFC connection 5 Manual short-circuiting connection

(only with generator side earthing switch)(by removal of cover)

G

7

8

6

3

4

1 2 7

5

3

6 9

8

10

6 Surge capacitor 7 Current transformers 8 Voltage transformers 9 Surge arrester10 Motor-operated short circuiting link

3.1 Interrupting chamber Within the interrupting chamber SF 6 gas is used for both arcextinguishing and internal insulation. The external insulationis air. For current interruption the self-blast principle is usedwhich represents an optimised design to achieve a signifi-cant reduction in operating energy. The main advantages ofemploying SF 6 gas as interrupting medium with self-blastprinciple can be summarised as follows:

– the arc-voltage of the circuit-breaker is high enough toensure current zeros in case of fault currents with delayed

current zeros without the need of delaying the tripping

– the pressure of SF 6 which is needed for interruption de-pends on the magnitude of the current

– an efficient operation can be achieved with a smaller oper-ating mechanism due to lower energy consumption duringcontact movement

– a gentle interruption of small inductive currents can beobtained thus reducing the risk of chopping the arc andgenerating subsequent overvoltages

– SF6 gas can be monitored


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The interrupting chamber of a generator circuit -breaker is depicted inFigure 7. On the left side the terminal is visible. The contacts are oper-ated by a shaft passing through the vertical support insulator.

The design of SF 6 generator circuit-breakers consists of two separatecontact systems, one for current carrying and one for arc interruption(Figure 8).

During the interruption process the current has to commutate from the

nominal contact system to the arcing contact system. This avoids wearand erosion of the current carrying contacts and ensures trouble-freecurrent carrying even after a large number of operations.

Figure 7: Interrupting chamber of agenerator circuit-breaker

a Circuit-breaker “CLOSED”

b Initiation of opening movement (transfer of current from the main contactsto the arcing contacts)

c Separation of arcing contacts with interruption of small currents supportedby puffer action

Separation of arcing contacts with interruption of large currents supportedby the thermal effect of the current arc itself to build up the pressure in the

heating volume

d Circuit-breaker “OPEN”

Arc Extinguishing Technology:Mode of operation of the interrupting chamber of the type HECS circuit-

breaker systems

a

b

c

d

7 6 8 3 1

5 4 2

Figure 8: Contact systems of anSF 6 generator circuit-breaker anddescription of a current interruption

procedure

1 Terminals 2 Cylindrical coil 3 Fixed arcing contact 4 Moving arcing contact

5 Fixed main contacts 6 Moving main contact 7 Puffer 8 Heating volume


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3.2 Hydraulic spring operating mechanism The hydraulic spring operating mechanism combines theadvantages of a hydraulic operating mechanism with those ofa spring energy storage system (Figure 9).Energy storage is accomplished with the aid of a disk springassembly, with the advantages of high long-term stability, reli-ability and non-influence of temperature changes.

Tripping of the operat ing mechanism and energy output arebased on proven design elements of the hydraulic operating

technique, such as control valves and hydraulic cylinders. The operating mechanism is based on the so-called differen-tial piston principle.For the closing operation the piston head side is isolated fromthe low pressure and simultaneously connected to the highpressure oil volume.

As long as the pressure is maintained, the piston remains inthe “closed” position. A pressure controlled mechanical

interlock prevents movement of the piston to the “open” posi-tion in case of a pressure drop.For the opening operation, the piston head side is isolatedfrom the high pressure and simultaneously connected to thelow pressure oil volume.

The charging state of the spring disk assembly is controlledby switching elements, actuating the pump motor to immedi-ately maintain the oil pressure.

A non-return valve between pump and high-pressure oilvolume prevents pressure loss in the event of a pump outage. The hydraulic system is hermetically sealed against atmo-sphere. The mechanically operated position indicator providesreliable indication of the circuit-breaker position.

The drive operates all three circuit-breaker poles simultane-ously by mechanical linkages, thus keeping the switching timedifference between the poles to a minimum.

High pressure 1 Breaker operating rodLow pressure 2 Energy storage device

Figure 9: Hydro-mechanical spring operating mechanism a) and its schematic diagram b)

3.3 SF 6-gas density monitoring system The breaking capacity of an SF 6 circuit-breaker and thedielectric withstand level across its open contacts is depen-dent upon the density of the SF 6-gas. Under the condition ofconstant volume the gas density is independent of the gastemperature, while the pressure varies with the temperature.It is therefore more practical to measure and use the gasdensity rather than the pressure for circuit-breaker supervisionpurposes.

The density moni tor operates according to the reference-

volume-density principle. The density of the gas in the circuit-breaker chamber is compared with the density of the gas ina sealed reference gas volume. When the gas density dropsbelow the specified value, the density monitor signals the lossof SF6-gas in several steps.Since the gas volumes of the three circuit-breaker poles areconnected via the refilling pipe only one SF 6-gas densitymonitor per circuit-breaker is required to supervise the gasdensity.

a) b)


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3.4 Disconnector The switchgear concept provides a disconnector fitt ed inseries with the circuit-breaker. It is placed on the transformer-side of the circuit-breaker and within the same enclosure. Thedisconnector is a tubular telescopic unit and it is equippedwith a drive which operates through a mechanical linkage allthree poles. This layout provides easy access and simplifiesmaintenance. In the open position of the disconnector the iso-

lating air distance can be clearly seen through an inspectionwindow. The moving contact is motor driven. A lockingfeature prevents motor operation while the disconnector isbeing manually operated. A mechanically driven positionindicator is provided in a visible position and a crank handleis provided for manual operation. The view of a disconnectorbeing in the open position is depicted in Figure 10.

Figure 10: View of a disconnector in the open position

3.5 Earthing switch The earthing switch can be provided on either one or bothsides of the system. The switch and its connections aredesigned for protective earthing purposes, i.e. it is rated forthe full fault current but has no current making or continuouscarrying capacity.

The design of blade type (for generator circuit-breaker

systems type HECS and HEC 7/8 up to 160 kA) or of tubulartelescopic type (for generator circuit-breaker systems typeHEC 7/8 up to 210 kA) is depicted in Figure 11.

The earthing connection is made via the system enclosure. The moving contact is motor driven.

Figure 11: Blade type a) and tubular telescopic type b) earthing switches

a) b)

3.6 Starting switch (for gas turbine power plants) A start ing switch can be provided on the generator-side ofthe system. It is designed for being employed for the start-up

of the machine from a static frequency converter (SFC). Themoving contact is motor driven.


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3.7 Short-circuiting connection The short-circuiting connection helps to expedite the test-ing and adjustment of the power plant protection system. Itcan be provided manually mounted for the use between thecircuit-breaker and the disconnector of the system or motoroperated. In the former case the cover of each phase

enclosure has to be removed to allow the fitting of the short-circuiting bar. In the latter case the short-circuiting link is usedin conjunction with the earthing switch installed on thegenerator-side of the circuit-breaker (for generator circuit-breaker systems type HECS).

Figure 12: Ring core current transformer

3.9 Voltage transformer Single-phase voltage transformers can be provided on eitherone or both sides of the circuit-breaker system. Up to threevoltage transformers can be fitted at each side and each volt-age transformer can be supplied with one or two secondarywindings, depending on the class and output power required(Figure 13). The secondary windings are permanently wiredback to terminal blocks in the control cubicle.

3.10 Ferroresonance damping deviceIn order to prevent the occurrence of ferroresonance ABBgenerator circuit-breaker systems are equipped with a

damping device installed in the open delta formed by thetertiary windings of the three voltage transformers on thetransformer-side of the generator circuit-breaker (see Figure14 and Figure 15).Ferroresonance is characterised by a periodic displacementof the potential of the system neutral in a three-phase systemwith an isolated neutral. These so-called relaxation oscillationsare caused by discharging and recharging the capacitancesto ground via magnetising inductances of e.g. single-poleinsulated voltage transformers and the periodic repetition ofthis process. The magnetic core is temporarily subjected tosaturation during these phenomena. As a consequence of

saturation high currents are flowing through the primarywindings of the voltage transformers that heat up thesewindings and often lead to the destruction of the voltagetransformer. In practice the ferroresonant oscillations may be

initiated by momentary saturation the core of the inductiveelement resulting from e.g. switching operations or other type

of events leading to an unbalance in the system. The insertion of a ferroresonance damping device in the opendelta of the residual voltage windings (tertiary windings) of aset of voltage transformers is a very efficient solution for thedamping of second subharmonic relaxation oscillations. Thisdevice basically consists of a saturable coil (damping coil)paralleled by a group of resistors with a relatively high resis-tance. For power frequency voltages, i.e. in case of persistentsingle-phase-to-ground faults, the saturable inductance worksin the linear range of the magnetising characteristic and car-ries only a small current thus avoiding any thermal overloadingof the voltage transformer as well as of the inductance itself.

For second subharmonic voltages however the inductancesaturates and absorbs active power sufficient to damp out therelaxation oscillations. This power is dissipated in the resis-tance associated with the damping coil.

3.8 Current transformer A ring core current transformer can be provided on eitherone or both sides of the circuit-breaker system (Figure 12).Depending on the class up to three cores per current trans-former can be accommodated. The secondary windings arepermanently wired back to terminal blocks in the controlcubicle.

Figure 13: Single-phase voltage transformers


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3.11 Surge capacitor Surge capacitors are fitted on both sides of the generatorcircuit-breaker system to provide additional protection againstovervoltages and to support arc extinction in the circuit-breaker by transient recovery voltage limitation (Figure 16).

The surge capacitors are used to reduce the rate-of-rise ofthe transient recovery voltage from the very high prospec-tive values (and at the same time to increase the time delayfrom the very low prospective values) to values the generatorcircuit-breaker can cope with. The capacitors are thereforeto be considered as an integral part of the generator circuit-breaker.

Figure 14: Insertion of a ferroresonance damping device (DE6) in theopen delta of the residual voltage windings (tertiary windings) of aset of voltage transformers (voltage transformer with one secondary

winding)

3.12 Surge arrester Surge arresters can be fitted on the transformer-side of thegenerator circuit-breaker system, to provide protection forthe equipment connected to the generator busbar againstovervoltages. Metal-oxide surge arresters with silicon housing

are installed in ABB generator circuit-breaker systems (Figure17). Metal-oxide surge arresters have a highly non-linearresistance characteristic. At service voltage a predominantlycapacitive low current flows. Any voltage increase leads to arapid increase of the current, thereby limiting any further risein the voltage. When the voltage decreases, the conditionreverts to its essential non-conducting state.

Figure 15: Insertion of a ferroresonance damping device (DE6) in theopen delta of the residual voltage windings (tertiary windings) of aset of voltage transformers (voltage transformer with two secondary

windings)

Y-connecti on of the primary windings

open delta connection of the tertiarywindings

R S T

DE 6

Earth FaultProtection Relay

Y-connecti on of the primary windings

open delta connection of the tertiarywindings

R S T

DE 6

Earth FaultProtection Relay

Y-connecti on of the secondarywindings

Figure 16: Surge capacitor Figure 17: Metal-oxide surge arrester


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a)

b)

c)

d)

3.13 Connecting zone The connecting zone is designed to provide a detachable(bolted) connection between the generator circuit-breakerlife parts and the conductors of the adjacent isolated phasebus (IPB) or busduct. The main components of the connect-ing zone are depicted in Figure 18 and Figure 19. The flexibleconnections shall be designed for:

1) carrying the rated continuous current and the rated short-time withstand current without exceeding the maximumpermissible t emperatures

2) ensuring that the dielectric strength requirements are met

3) compensating expansion and contraction of the conductordue to temperature changes

4) compensating vibrations and withstanding the stresscaused during switching operations

5) withstanding the mechanical stress resulting from electro-dynamic forces in case of short-circuit currents

6) providing a low resistance, safe and stable electricalconnection

ABB recommended type and arrangement of flexible copperstraps responds to these requirements as follows:

1), 2) & 5) Fully type tested together with the generator circuit-breaker to prove that the stringent requirements imposed bythe relevant IEC and IEEE standards with regard to dielectricstrength, hottest spot temperature and mechanical stress are

fully met. The special shape easily adapts to different distances between terminals ensuring that dielectric strength require-ments are always met.

3), 4) & 5) Flexible type employing laminates with pressure-welded contact ends designed and tested for high mechanicalstress.

6) Silver plated contact ends with high requirements oncontact surface evenness and material properties.

Figure 19: Connection between one pole of a generator circuit-breaker and the associated phase bus

Figure 18: Main components of the connecting zone

a) Flexible copper straps b) Fastening and securing bolts & nuts c) Terminal with silvered contact surfaces

for welded connection to the conductorof the IPB or busduct

d) Support ring for withstanding themechanical stress and to reduce thecontraction of the connectors resultingfrom electro-dynamic forces in case ofshort-circuit currents


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3.14 Phase enclosure The magnetic field in the neighborhood of the connection be-tween generator and transformer may have adverse effects onequipment and building steel if the current exceeds a certainvalue. The values of magnetic fields outside of the generatorcircuit-breaker housing could induce voltages and currentswhich in turn might produce undesired heating effects.For this reason, and to avoid electromagnetic forces between

the current-carrying busbars the generator circuit-breakersystem is designed for welded connections to the isolatedphase bus enclosures.Each single phase enclosure is made of aluminium and

capable of carrying the induced return current thus minimisingthe impact of the magnetic field. In order to avoid pollutiondue to ingress of dust and moisture, the generator switchgearenclosure is designed to allow air tightness and to withstand aslight internal overpressure.Inspection windows are provided in the phase enclosures nearto the disconnector, earthing switch and starting switch, to

allow visually checking of the position of each of them.Occasionally, the busbars in power plants are not enclosedand in general, effects of magnetic fields for small generatorcontinuous current is usually of no concern.

3.15 Control and supervision All cont rol and supervisory apparatuses are mounted in thecontrol cubicle. An active mimic diagram is provided with

position indications and the integrated local control of thecircuit-breaker and all other switching apparatuses. In thecontrol cubicle there is also installed equipment for local/ remote changeover facilities and counters for CO operationsof the circuit-breaker and pump starts of the circuit-breakerdrive.


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4 Standard for generator circuit-breakers

IEEE Std C37.013-1997 (R2008) “IEEE Standard for AC High-VoltageGenerator Circuit Breakers Rated on a Symmetrical Current Basis” coversthe requirements applicable for generator circuit-breakers [1]. It is theonly standard worldwide specifically relating to generator circuit-breakers.

Therefore generator ci rcuit -breakers have to be designed and tested inaccordance with [1] and its amendment IEEE Std C37.013a-2007 “IEEEStandard for AC High Voltage Generator Circuit Breakers Rated on aSymmetrical Current Basis - Amendment 1: Supplement for Use withGenerators Rated 10–100 MVA” [2]. Since no other national or inter-national standard on generator circuit-breakers exists, this standard isused worldwide. Specifically, IEC publication 62271-100 “High-voltageswitchgear and controlgear – Part 100: Alternating-current circuit-break-ers” does not apply to generator circuit-breakers as it explicitly excludesgenerator circuit-breakers from its scope [3]. Circuit-breakers that havebeen designed and tested in accordance with IEC 62271-100 do notmeet the stringent requirements imposed on generator circuit-breakers

and therefore are not suitable for the use as generator circuit-breakers.Contrary to general purpose circuit-breakers covered by IEC 62271-100generator circuit-breakers have two fault ratings, i.e. the system-sourceshort-circuit current interrupting capability (in case of a fault between thecircuit-breaker and the generator) and the generator-source short-circuitcurrent interrupting capability (in case of a fault between the circuit-breaker and the transformer).

The stresses imposed on generator ci rcuit -breakers differ from the stresses imposed on gen-eral purpose circuit-breakers mainly in the followingrespects:

1. The degree of asymmetry of the system-sourceshort-circuit current is in the order of 60% to 80%.

2. The degree of asymmetry of the generator-source short-circuit current is in the order of 90%to 150%, i.e. the generator-source short-circuitcurrent may exhibit delayed current zeros (degree

of asymmetry > 100%).

3. The rate-of-rise of the transient recovery voltageafter the interruption of a system-source short-circuit current may be as high as 6.0 kV/µs.

4. The rate-of-rise of the transient recovery voltageafter the interruption of a generator-source short-circuit current may be as high as 2.2 kV/µs and thecorresponding time delay may be extremely short(< 0.5 µs).

The test quanti ties given in IEC 62271-100 for theshort-circuit tests do not adequately cover theabove requirements. The only standard which cov-ers the requirements for generator circuit-breakersis IEEE Std C37.013-1997 (R2008). This standardin particular covers the requirements imposedon generator circuit-breakers regarding the d.c.component and the degree of asymmetry of thefault currents (including the case of fault currentswith delayed current zeros) and the characteristicsof the transient recovery voltages (rate-of-rise, timedelay and peak value).

In order to cover the stringent requirements whichare imposed on generator circuit-breakers the typetests listed in Table I have to be performed ongenerator circuit-breakers in accordance with IEEEStd C37.013-1997 (R2008).

Description of Test Standard Clause

Rated continuous current carrying tests IEEE Std C37.013 Cl. 6.2.1

Rated dielectric strength IEEE Std C37.013 Cl. 6.2.2

Short-time current-carrying capability IEEE Std C37.013 Cl. 6.2.3

Short-circuit current rating IEEE Std C37.013 Cl. 6.2.3

Rated transient recovery voltage IEEE Std C37.013 Cl. 6.2.4

Rated standard operating duty IEEE Std C37.013 Cl. 6.2.5

Rated interrupting time IEEE Std C37.013 Cl. 6.2.6

Shor t- circui t cur ren t w ith de layed cur rent zeros IEEE Std C37.013 Cl. 6.2.7

Load current switching tests IEEE Std C37.013 Cl. 6.2.8Out-of-phase current switching tests IEEE Std C37.013 Cl. 6.2.9

Mechanical endurance life IEEE Std C37.013 Cl. 6.2.10

Excitation current switching tests IEEE Std C37.013 Cl. 6.2.11

Sound level tests IEEE Std C37.013 Cl. 6.2.12

EMC tests IEEE Std C37.013 Cl. 6.2.13

TABLE I: LIST OF TYPE TESTS FOR GENERATOR CIRCUIT-BREAKERS ACCORDIN G TOIEEE Std C37.013-1997 (R2008)


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5 Selection of generator circuit-breakers

5.1 Duties of generator circuit-breakers The main duties of a generator circuit-breaker are as follows:

– synchronise the generator with the main system

– separate the generator from the main system (switching off

the unloaded/lightly loaded generator) – carry and interrupt load currents (up to the full load currentof the generator)

– interrupt system-source short-circuit currents

– interrupt generator-source short-circuit currents

– interrupt fault currents due to out-of-phase conditions up toout-of-phase angles of 180°

5.2 Requirements for generator circuit-breakers The requirements imposed on generator circuit-breakers

greatly differentiate from the requirements imposed on generalpurpose transmission and distribution circuit-breakers.Due to the location of installation of a generator circuit-breaker high technical requirements are imposed on thecircuit-breaker with respect to:

– rated current

– short-circuit currents (system-source and generator-source)

– fault currents due to out-of-phase conditions

– degree of asymmetry of fault currents, fault currents withdelayed current zeros

– rate-of-rise of the recovery voltages

Circuit-breakers are only capable of providing satisfactoryservice when they are capable of fully meeting these require-ments.Specifications must therefore fully reflect the technical andreliability requirements and equipment, confirming to suchspecifications, must be designed and tested in full accor-dance with recognized and relevant standards.

5.3 Selection of generator circuit-breakers According to IEEE Std C37.013-1997 (R2008) the ratings andrequired capabilities of a generator circuit-breaker are the fol-

lowing ones: – rated maximum voltage

– power frequency – rated continuous current – rated dielectric strength – rated short-circuit duty cycle – rated interrupting time – rated closing time – short-circuit current rating – transient recovery voltage rating – rated load current switching capability

– capacitance current switching capability – out-of-phase current switching capability – excitation current switching capability – rated control voltage – rated mechanism fluid operating pressure


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5.3.1 Rated maximum voltage The rated maximum voltage is the generator ci rcuit -breaker’supper limit for operation and it is selected so that it is higherthan or equal to the maximum operating voltage of thegenerator.

5.3.3 Rated continuous current The rated continuous current of a generator circuit-breaker

is the designated upper limit of current in r.m.s. amperes atpower frequency, which it shall be required to carry continu-ously without exceeding any of the limitations designatedin IEEE Std C37.013-1997 (R2008). Due to their installationlocation generator circuit-breakers have to be able to carrycontinuously load currents of very high magnitude. Thesecurrents place a severe stress on the conductors, connec-tions and contacts. In order to guarantee that the switcheshave a high degree of reliability and a long service life, theymust be so designed that limits for temperature increase arenot exceeded. In order to make optimal use of the conductormaterial employed in the circuit-breaker, the power losses

have to be minimized and the transfer of heat from theconductor path to the environment must be intensified.

The maximum value of load current which the circui t-breakershall be able to carry continuously can be calculated by usingthe following formula:

The current carrying capability of a generator circuit-breaker

depends on the operating condition at the specific location.When assessing the current carrying capability of a generatorcircuit-breaker special attention shall be paid to the followingitems:

– power frequency

– design temperature of the isolated phase bus to which theterminals of the generator circuit-breaker are connected(normally these temperatures are 105 °C (or 90 °C) for mainconductor and 80 °C (or 70 °C) for enclosure)

– ambient temperature

– installation location (indoor or outdoor)

– colour of the enclosure of the generator circuit-breaker

In special cases the isolated phase bus is equipped with itsown forced cooling system. In such a case also the technicalparameters of this cooling system shall be taken into accountin the assessment of the current carrying capability of thegenerator circuit-breaker.In Figure 20 the current carrying capability of the generatorcircuit-breaker type HECS-100L is displayed.

Imax is the maximum r.m.s. value of the current which thegenerator circuit-breaker shall be able to carrycontinuously

Figure 20: Current carrying capability curves for generator circuit-breaker type HECS-100L operating at a power frequency of 50 Hz andisolated phase bus temperatures of 90 °C / 70 °C (conductor / enclosure respectively)

-25.0 -20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 Ambient Temperature (°C)

11000 12000 13000 14000 15000 16000 17000 18000 19000 20000

C u r r e n

t ( A r m s )

indoor outdoor (RAL 9010)

min

max3 V

SI

n=

5.3.2 Power frequency The rated frequency for generator ci rcuit -breakers is 50 Hz or60 Hz, depending on the system power frequency in whichthe generator circuit-breaker is installed.

S n is the rated power of the generator V min is the minimum operating voltage of the generator


20/6020 | ABB

5.3.4 Rated dielectric strength The rated dielectric strength of a generator circuit-breakeris selected in accordance with the Table II depending on itsrated maximum voltage.

Rated maximumvoltage [kV rms ]

Power frequencywithstand voltage

[kV rms ]

Lightning impulsewithstand voltage

[kV peak ]

5 20 608.25 28 75

8.25 / 15 38 95

15.5 50 110

27 60 125

38 80 150

TABLE II : RATED DIELECTRI C STRENGTH OF GENERATOR CIRCUIT-BREA KERS IN ACCORDANCE WITH IEEE S td C37 .013a-2007

5.3.5 Rated short-circuit duty cycle The rated short-circuit duty cycle of a generator circuit-breaker is two unit operations with a 30 min interval betweenoperations (CO–30 min–CO) [1].

5.3.6 Rated interrupting time

According to IEEE Std C37.013-1997 (R2008) the rated inter-rupting time of the generator circuit-breaker is the maximumpermissible interval between the energizing of the t rip circuitat rated control voltage and rated fluid pressure of the operat-ing mechanism and the interruption of the main circuit in allpoles on an opening operation.

A typical interrupting time for ABB generator c ircuit-breakersis about 3 cycles.

The interrupting t ime is the sum of the opening t ime (i.e. thetime interval between the energizing of the opening circuit andthe mechanical separation of the arcing contacts) and the arcing time (i.e. the time interval between the contact separation in

the first pole and the final arc extinction in all poles).

5.3.7 Rated closing time

According to IEEE Std C37.013-1997 (R2008) the ratedclosing time of the generator circuit-breaker is the intervalbetween energizing of the close circuit at rated control voltageand rated fluid pressure of the operating mechanism and theclosing of the main circuit.

5.3.8 Short-circuit current rating

5.3.8.1 System-source short-circuit current According to IEEE Std C37.013-1997 (R2008) the system-source short-circuit current of a generator circuit-breakeris the highest r.m.s. value of the symmetrical component ofthe three-phase short-circuit current. It is measured from the

envelope of the current wave at the instant of primaryarcing contact separation and is the current that the generator

circuit-breaker is required to interrupt at the rated maximumvoltage and rated duty cycle when the source of the short-circuit current is from the power system through at least onetransformation.

The a.c. component of the system-source short-circuit currentcan be calculated by using the following formula:

eqac

X

V I =

3max

Iac is the a.c. component of the fault current Xeq is the equivalent reactance of the circuit referred to

the LV-side of the step-up transformer

V max is the maximum r.m.s. value of the applied voltageprior to fault (it can generally be considered equal to

the maximum service voltage of the HV-system re-ferred to the LV-side of the step-up transformer)


21/60 ABB | 21

The required asymmetrical system-source interrupting capa-bility of a generator circuit-breaker is composed of the r.m.s.symmetrical current and the percentage d.c. component. Thevalues of the d.c. component are expressed in percent of thepeak value of the symmetrical short-circuit current and aremeasured at the primary arcing contact parting time. The pri-mary arcing contact parting time can be generally considered

equal to the sum of 1/2 cycle (protection system trippingdelay) plus the minimum opening time of the particulargenerator circuit-breaker.

The standard value for the t ime constant of the decay of thed.c. component is 133 ms. For time constants different than133 ms, the following formula can be used:

Idc is the d.c. component of the fault currentIac is the a.c. component of the fault current

τ

cpt

acdc eII−

= 2

eq

eq

R

X =

ω τ

Xeq is the equivalent reactance of the circuit referred tothe LV-side of the step-up transformer

ω is equal to 2 π f with f being the power frequency

When the fault current is asymmetrical it is characterized by adegree of asymmetry which is defined as follows:

a is the degree of asymmetry.

ac

dcI

Ia

2=

The typical course of the system-source short-circuit currentand of its degree of asymmetry are shown in Figure 21 andFigure 22, respectively. It is understood that the degree ofasymmetry of the system-source short-circuit current is gen-

erally monotonically decreasing with time as the a.c. compo-

nent of the fault current is usually constant. Its value dependson the opening time of the circuit-breaker and on the relaytime of the protection system and it assumes a typical valueof 75% at the primary arcing contact parting time.

Time (ms)

C u r r e n

t ( p u

)

Iac

Idc

t cp

2

-1

-0.5

0

0.5

1

1.5

2

0 10 20 30 40 50 60 70 80 90 1000.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

Time (m s)

D e g r e e o

f a s y m m e t r y

( % )

0 10 20 30 40 50 60 70 80 90 100

Figure 21: Prospective system-source short-circuit current Figure 22: Degree of asymmetry of the system-source short-circuitcurrent

t cp is the primary arcing contact parting timeτ is the time constant of the decay of the d.c.

component and it can be calculated by using the following formula:

R eq is the is the equivalent resistance of the circuitreferred to the LV-side of the step-up transformer


22/6022 | ABB

The system-source short-circuit current is generally fed by theHV-system and by the motors connected to the LV-side of theunit auxiliary transformer (see Figure 23). The a.c. componentof the system-source short-circuit current for the power plantlayout depicted in Figure 23 can be calculated by using thefollowing formula:

G

Step-Up Transformer

GeneratorCircuit-Breaker

M

HV-System

Unit Auxiliary

Transformer

Motors GeneratorFigure 23: (right side) Typical power plant layout with a step-uptransformer and a unit auxiliary transformer

( )+

++

=

AUXT LV AUXT

HV AUXT

rM

rM

LR

rMGSUT sys

ac

X V

V

S

V

I

I X X

V I

2

_

_2

max 11

3

V max is the maximum r.m.s. value of the applied voltageprior to fault (it can be considered equal to themaximum service voltage of the HV-system referredto the LV-side of the step-up transformer)

Xsys is the equivalent reactance of the HV-system referredto the LV-side of the step-up transformer

XGSUT is the short-circuit reactance of the step-uptransformer referred to the LV-side of the step-uptransformer

V rM is the rated voltage of the motors connected to the

LV-side of the unit auxiliary transformer

S rM is the rated apparent power of the motors connectedto the LV-side of the unit auxiliary transformer

ILR / I rM is the ratio of the locked-rotor current to the ratedcurrent of the motor

X AUXT is the short-circuit reactance of the unit auxiliarytransformer referred to the HV-side of the unitauxiliary transformer

is the transformation ratio of the unit auxiliarytransformer

The d.c. component of the system-source short-circuit current for the power plant layout depicted in Figure 23 can becalculated by using the following formula:

( )( )

+

+

+= ++

−−

AUXT M

cp

GSUT sys

cp t

AUXT LV AUXT

HV AUXT

rM

rM

LR

rM

t

GSUT sys

cpdc e

X V

V

S V

II

e X X

V tI τ

τ

2

_

_2

max 11

32

R sys the equivalent resistance of the HV-system referredto the LV-side of the step-up transformer

R GSUT the resistive component of the short-circuitimpedance of the step-up transformer referred to theLV-side of the step-up transformer

XM / R M the X/R ratio of the motors connected to the LV-sideof the unit auxiliary transformer

( )( )GSUT sys

GSUT syssys + GSUT

RR

X X

+

+=

ω

τ

+

+

=+

AUXT LV AUXT

HV AUXT

rM

rM

LR

rM

M

M

AUXT LV AUXT

HV AUXT

rM

rM

LR

rM

AUXT M

R V

V

S

V

I

I

X

R

X V

V

S

V

I

I

2

_

_2

2

_

_2

ω

τ

R AUXT the resistive component of the short-circuitimpedance of the unit auxiliary transformer referred

to the HV-side of the unit auxiliary transformerω is equal to 2 π f with f being the power frequency

V AUXT_HV V AUXT_LV


23/60 ABB | 23

The degree of asymmetry of the fault current measured at thecontact parting time is:

In some cases a three-winding transformer is used to connect

two generators to the HV-system (see Figure 24). In this casethe system-source short-circuit current has three contribu-tions, i.e. it is fed by the HV-system, by the motors connectedto the LV-side of the unit auxiliary transformer and by theother generator through the step-up transformer. Specialattention shall be paid to this scheme because the degree ofasymmetry of the system-source short-circuit current can bevery high depending on the reactances and time constants ofthe generator. In some cases the current wave-shape mightshow delayed current zeros (i.e. degree of asymmetry higherthan 100%).

( ) ( )ac

cpdccp

I

tIta =

2

G

Step-Up Transformer

GeneratorCircuit-Breakers

M

HV-System

Generator

G

Generator Motors

Unit Auxiliary Transformer

Figure 24: Power plant layout with a three-winding step-uptransformer and a two-winding unit auxiliary transformer

5.3.8.2 Generator-source short-circuit current According to IEEE Std C37.013-1997 (R2008) the generator-source short-circuit current of a generator circuit-breakeris the highest r.m.s. value of the symmetrical component ofthe three-phase short-circuit current. It is measured fromthe envelope of the current wave at the instant of primaryarcing contact separation that the generator circuit-breakershall be required to interrupt, at rated maximum voltage andrated duty cycle when the source of the short-circuit current

is entirely from a generator through no transformations. Thegenerator-source symmetrical short-circuit current is usuallylower than the system-source symmetrical short-circuitcurrent.

The generator-source symmetrical short-circuit current can becalculated using the following simplified formula for no-loadconditions:

+−+−= −ddd

t

ddrG

rGmGsymgen xxx

exx V

S V I d

11'1

'1

''1

3''

2τ −te d'τ

Igen sym is the a.c. component of the generator-source short- circuit current

V mG is the maximum generator line-to-line voltageS rG is the rated power of the generator

V rG is the rated voltage of the generatorxd is the pu value of the direct-axis synchronous

reactance

x' d is the pu value of the direct-axis transient reactancex" d is the pu value of the direct-axis subtransient

reactanceτ ' d is the direct-axis transient short-circuit time constantτ "d is the direct-axis subtransient short-circuit time

constant

If the fault initiation takes place when the voltage in one phasepasses through zero the resulting fault current in that phaseexhibits the maximum degree of asymmetry. The a.c. compo-nent decays with the subtransient and transient time

constants of the generator; the d.c. component decays withthe armature time constant τ a. The armature time constantcan be calculated with the following formula:

aa

R

X

π τ

22=

f

τ a is the armature time constant X2 is the negative-sequence reactance of the generator

f is the power frequencyR a is the d.c. armature resistance


24/6024 | ABB

The value of X2 can be approximated by:

The generator-source asymmetrical shor t-circuit current for the phase with the highest asymmetry, the generator being in theno-load mode, can be calculated by the following simplified formula:

If the a.c. component of the fault current decays faster thanthe d.c. component, it can happen that for a certain period oftime following the initiation of the fault the magnitude of thed.c. component of the fault current is bigger than the peakvalue of its a.c. component. In such a case the degree of

asymmetry of the fault current is higher than 100% thus lead-ing to delayed current zeros. The typical course of the degreeof asymmetry of the generator-source short-circuit current isshown in Figure 25.

2

''''

2qd X X X

+=

X" d is the direct-axis subtransient reactance of thegenerator

X”q is the quadrature-axis subtransient reactance of thegenerator

( ) ( )−++−+−+−= −−−− texx

exx

tx

exx

exx V

S V I aadd t

qd

t

qdd

t

dd

t

ddrG

rGGmasymgen ω ω

τ τ τ τ 2cos''1

''1

21

''1

''1

21

cos11

'1

'1

''1

3

2 '''2

ω is equal to 2 π f with f being the power frequency x” q is the quadrature-axis subtransient reactance in pu

Since x "d is approximately equal to x "q for turbo generators, the equation can be written as follows:

( )−+−+−= −−− ex

tx

exx

exx V

S V I add t

dd

t

dd

t

ddrG

rGGmasymgen ω

τ τ τ

''1

cos11

'1

'1

''1

3

2 '''2

00.02 0.04 0.06 0.08 0.10 0.12 0.140

time [s]

100

50 D e g r e e o

f A s y m m e t r i e

[ % ]

150

Figure 25: Typical course of the degree of asymmetry of thegenerator-source short-circuit current


25/60 ABB | 25

In addition the a.c. component of the generator-source short-circuit current and its degree of asymmetry can vary if thegenerator is unloaded or delivering power with lagging powerfactor (i.e. working in the over-excited mode) or leading powerfactor (i.e. working in the under-excited mode) prior to fault.

Typical fault current wave-shapes are depicted inFigures 26, 27 and 28.

The magnitude of the a.c. component of the fault currentwhich is fed by the generator is typically about 80% of the

magnitude of the a.c. component of the system-source short-

circuit current and typically shows a degree of asymmetrymeasured at the primary arcing contact parting times are inthe order of 130% (see Figure 25). Special attention shouldbe paid if the generator is loaded with leading power factorbefore fault initiation. In such a case the degree of asymmetryof the fault current can reach very high values and exceed130%.In order to accurately simulate the behaviour of the genera-tor in case it is loaded prior to fault computer simulations are

necessary.

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

Figure 26: Prospective generator-source short-circuit current(generator unloaded prior to fault initiation) - fault initiation at U A = 0

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

Figure 27: Prospective generator-source short-circuit current(generator delivering power with lagging power factor prior to faultinitiation) - fault initiation at U A = 0

Figure 28: Prospective generator-source short-circuit current(generator delivering power with leading power factor prior to faultinitiation) - fault initiation at U A = 0

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

Additional resistance in series with the armature resis tanceforces the d.c. component of the short-circuit current todecay faster. Such additional resistance may be the connec-tion from the generator to the fault location, but especiallythe circuit-breaker arc resistance after contact separation. Ifthere is an arc at the fault location, this arc resistance furtherreduces the time constant of the d.c. component from the

initiation of the fault. The values of these additional seriesresistances are normally high enough to force a fast decay ofthe d.c. component of the short-circuit current so that currentzeros are produced.

In case of a short-circuit current with delayed current zerosthe capability of a circuit-breaker to interrupt a given short-circuit current can be considered as being demonstrated ifthe generator circuit-breaker is capable of forcing the currentto zero within the time interval in which it is able to interrupt acurrent (i.e. within the maximum permissible arcing time).

According to IEEE Std C37.013-1997 (R2008) demonstrating

the capability of a generator circuit-breaker to interrupt short-circuit currents with delayed current zeros may be difficult andlimited in high power testing stations. Since various designs


26/6026 | ABB

of generators behave differently, it may not be possible tosimulate the required current shape in the testing station.

Therefore the capability of a circuit-breaker to interrupt ashort-circuit current with delayed current zeros can be ascer-tained by calculations that take into account the effect of thearc-voltage of the circuit-breaker on the prospective short-circuit current. The arc-voltage model used for this purpose hasto be derived from tests (IEEE Std C37.013-1997 (R2008),Clause 6.2.7). The technical data of the actual generator shall

be used for these computations.

According to IEEE Std C37.013-1997 (R2008), Clause7.3.5.3.5.3 the following two cases shall be investigated:

1) Generator at no-load with the generator circuit-breakerclosing into a three-phase fault. In the computation the arc-voltage of the generator circuit-breaker starting at contactseparation shall be taken into account.

2) Generator in service with leading power factor. An arcingfault is assumed in at least two phases. In the computation

the arc-voltage at the fault location starting at the initiation ofthe fault and the arc-voltage of the generator circuit-breakerstarting at contact separation shall be taken into account.

Further the following two situations shall be considered for aparticular generator-source short-circuit current in case of athree-phase fault (IEEE Std C37.013-1997 (R2008), Clause6.2.7.2):

1) Fault initiation at voltage zero in one phase which impliesthat the current in the corresponding phase exhibits themaximum degree of asymmetry.

2) Fault initiation at voltage maximum in one phase whichimplies that the current in the corresponding phase issymmetrical.

The arc-voltage of a circuit -breaker depends on the instanta-neous value of the current and on the type of the extinguish-ing medium, its pressure, the intensity of its flow and thelength of the arc. The u arc -i characteristic of one break of thecircuit-breaker has to be derived from short-circuit currentinterrupting tests. To be able to investigate the behaviourof the circuit-breaker during the interruption of short-circuitcurrents with delayed current zeros the arc-voltage versuscurrent characteristic has to be transferred into a mathematical

model. From the arc-voltage u arc (i,t) and the current i(t) thearc resistance R arc (i,t) can be obtained. In order to model thebehaviour of the SF 6 circuit-breaker a non-linear time-varyingresistance of the value R arc (i,t) has to be inserted into thesimulation at the time of the separation of the contacts of thecircuit-breaker.

Figures 29 to 32 show examples of the corresponding cal-culation results. Figures 29 and 30 represent the case of thegenerator being under no-load condition with the generatorcircuit-breaker closing into a three-phase fault. In the compu-tation the arc-voltage of the generator circuit-breaker starting

at contact separation is taken into account. Figure 29 repre-sents the case with fault initiation at voltage zero and Figure30 represents the case with fault initiation at voltage maxi-mum in one phase. Figures 31 and 32 represent the case ofthe generator being in service with a leading power factor. Inthe computation the arc-voltage at the fault location startingat the initiation of the fault and the arc-voltage of the genera-tor circuit-breaker starting at contact separation is taken intoaccount. Figure 31 represents the case with fault initiationat voltage zero and Figure 32 represents the case with faultinitiation at voltage maximum in one phase.

As the maximum calculated arcing time (i.e. 20.9 ms, see

Figure 29) is shorter than the maximum arcing time of thegenerator circuit-breaker of concern it can be concluded thatthe circuit-breaker is capable of interrupting these faultcurrents showing delayed current zeros.

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

t cp

Figure 29: Interruption of generator-source short-circuit current with aSF

6 generator circuit-breaker

– generator unloaded prior to fault initiation – fault initiation at U A = 0 – contact parting time t cp = 39 ms – arcing time = 20.9 ms

Figure 30: Interruption of generator-source short-circuit current with aSF

6 generator circuit-breaker

– generator unloaded prior to fault initiation – fault initiation at U A = max – contact parting time t cp = 39 ms – arcing time = 20.7 ms

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

t cp


27/60 ABB | 27

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

t cp

Figure 31: Interruption of generator-source short-circuit current with aSF 6 generator circuit-breaker

– generator delivering power with leading power factor prior to faultinitiation

– fault initiation at U A = 0 – contact parting time t cp = 39 ms – arcing time = 18.2 ms

Figure 32: Interruption of generator-source short-circuit current with aSF 6 generator circuit-breaker

– generator delivering power with leading power factor prior to faultinitiation

– fault initiation at U A = max – contact parting time t cp = 39 ms – arcing time = 18.6 ms

]

-250.0

-187.5

-125.0

-62.5

0.0

62.5

125.0

187.5

250.0

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30[s]

t cp

In some cases the arc-voltage of the generator circuit-breakeris not high enough to force current zeros within the maximum permissible arcing time of the circuit-breaker (this can hap-pen for example if a vacuum interrupter is employed as agenerator circuit-breaker). In such a case a solution whichis sometimes adopted is to delay the tripping signal to thegenerator circuit-breaker. It has to be noted that this solution

is not recommendable because the longer fault arcing timemight lead to severe damages to power plant equipment withconsequent long downtime for repair. A better approach con-sists in choosing a generator circuit-breaker which is provento be able to interrupt the fault current without the aid of anyintentional tripping delay.

5.3.8.3 Required closing, latching, and carryingcapabilities

The short-circuit current into which the generator circuit -breaker must close is determined by the higher value of eitherthe system-source short-circuit current or the generator-source short-circuit current. In the majority of applications thesystem-source short-circuit current is higher than thegenerator-source short-circuit current.

According to IEEE Std C37.013-1997 (R2008) the generatorcircuit-breaker shall be capable of the following:

a) Closing and latching any power frequency-making current(50 Hz or 60 Hz) whose maximum crest (peak making current)does not exceed 2.74 times the rated symmetrical short-circuit current or the maximum crest (peak making current) ofthe generator-source short-circuit current, whichever is higher.

b) Carrying the short-circuit current for a time of 0.25 s.

5.3.8.4 Required short-time current-carrying capability According to IEEE Std C37.013-1997 (R2008) the generatorcircuit-breaker shall be capable of carrying for a time equal to1 s, any short-circuit current, whose peak value does notexceed 2.74 times the rated short-circuit current, as deter-

mined from the envelope of the current wave, at the time ofthe maximum peak, and whose r.m.s. value determined overthe complete 1s period, does not exceed the rated short-circuit current considered above.

5.3.9 Transient recovery voltage rating The transient recovery voltage is the voltage appearing across

the open contacts of the generator circuit-breakerimmediately after current interruption. The characteristics

of the generator and of the associated step-up transformer

dictate the wave-shape of the inherent TRV for various duties.


28/6028 | ABB

A three-phase fault is the most severe case and gives themaximum short-circuit current and the maximum TRV rate.

The neutral of the generator is not solidly grounded, thus thephase-to-ground fault current is not significant.

The typical power plant layout is shown in Figure 23, wherethe generator and the step-up transformer have essentiallythe same rating. For TRV calculations the contribution ofauxiliary transformer to the fault current can be neglected as itis a minor source of short-circuit current.

The TRV shall be calculated after the interruption of a sym-metrical current as any asymmetry in the current wave-shapewould lead to a less severe TRV. At the interruption of theshort-circuit current with maximum asymmetry, the transientoscillation of the recovery voltage will be very small or evennon-existent since at the moment of short-circuit currentinterruption, the normal frequency voltage value may be verysmall or zero.

5.3.9.1 First-pole-to-clear factor When interrupting any symmetrical three-phase current thefirst-pole-to-clear factor k pp is the ratio of the power frequencyvoltage across the first interrupting pole before current inter-ruption in the other poles, to the power frequency voltage

occurring across the pole or the poles after interruption in allthree poles [3]. Standard value for generator circuit-breakersis 1.5. The first-pole-to-clear factor can be calculated byusing the following formula:

10

0

2

3

ZZ

Zk pp +

=

k pp is the first-pole-to-clear factorZ0 is the equivalent zero-sequence impedance of the three-phase circuit

Z1 is the equivalent positive-sequence impedance of thethree-phase circuit

In practical applications the step-up transformer is Ynd

connected and the star point of the stator winding of thegenerator is usually grounded via a high resistance.

The zero-sequence impedance of such a system is much

higher than Z1 thus leading to

5.12

3

2

3

0

0

10

0 =≈+

=

Z

Z

ZZ

Zk pp

5.3.9.2 Amplitude factor The amplitude factor k af is ratio between the maximumexcursion of the transient recovery voltage to the peak valueof the power frequency recovery voltage [3].

Standard value for generator circuit-breakers is 1.5 withoutconsidering any capacitance connected at the terminals of thegenerator circuit-breaker.

5.3.9.3 Power frequency recovery voltage The power frequency recovery voltage is the recovery voltageafter the transient voltage phenomena have subsided [3].

The magnitude of the power frequency recovery voltage whichis imposed on the first pole which clears the current is 1.5

higher then the power frequency voltage. The second andthird poles clear the current at the same time and the power

frequency recovery voltage which is imposed on each of themis √3/2 times the power frequency voltage.


29/60 ABB | 29

5.3.9.4 Rated inherent transient recovery voltage The rated inherent transient recovery volt age is the reference voltage thatconstitutes the limit of the inherent transient recovery voltage of circuitsthat the generator circuit-breaker shall be capable of withstanding underfault conditions and shall be defined by an oscillatory wave-shape havinga TRV rate-of-rise, time delay (t d ) and peak voltage (E 2 ) [1].

The waveform of transient recovery voltages approximates to a dampedoscillation.

The TRV curve is bounded by three lines:

a) one line starts at the origin of time axis and istangent to the TRV curve with a slope equal to the

TRV rate-of-rise

b) one line is horizontal and is tangent to the TRVcurve at the time of TRV peak T 2.

c) one line starts on the time axis at the rated time

delay (td ) and runs paral lel to the firs t reference l ine

An example of a transient recovery voltage wave-shape is depicted inFigure 33

T2 is the time to reach the peak voltage E 2E2 is the peak value of the TRV t 3 is the intersection point of the tangent to

the transient recovery voltage which startsat the origin of the time axis and to thehorizontal tangent to the TRV curve at thetime of TRV peak T 2

E 2

t d t 3 T 2

TRV TRV rate-of-rise

Figure 33: Inherent TRV curve for first-pole-to-clear for required symmetricalinterrupting capability for three-phase faults

The standard value of E 2 can be calculated with followingformula:

V k k V E ppaf == 84.13

22

V is the rated maximum voltage of the generatorcircuit-breaker.

The rated TRV is the inherent value assuming an ideal generator circuit-breaker. These values may be modified by the generator circuit-breakercharacteristics or by the asymmetry of the current.

A system with a TRV that exceeds the rated values of the generatorcircuit-breaker must be modified in such a way as to lower the TRV. This

is generally achieved by connecting capacitors phase-to-ground on bothsides of the generator circuit-breaker.

The additional capacitance has three effects :

– it decreases the oscillation frequency and theRRRV of the TRV

– it increases the time delay of the TRV

– it increases the peak value of the TRV

If the circuit-breaker requires that the inherent TRVbe modified by the addition of capacitors, then theamount of equivalent capacitance required has tobe given in the test report and on the nameplate [1].


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5.3.9.5 System-source faultsFor system-source faults the maximum value of short-circuit current isobtained for a given transformer when X sys is minimum or assumed tobe zero. It is assumed that the contribution of the auxiliary system tothe fault current is negligible. The natural frequency of the transformer ismuch higher than the natural frequency of the HV-system. The TRV firstoscillates at the prospective value of 1.5 √2 X GSUT Iac , where I ac is the

r.m.s. value of the symmetrical short-circuit currentuntil the second and third poles open. The voltagedrop in the transformer is equal to the total powerfrequency recovery voltage for

X sys = 0. Therefore, the TRV rate is maximum whenthe short-circuit current is maximum [1].

5.3.9.6 Generator-source faultsFor generator-source faults the short-circuit current is generally lowerthan for system-source faults because of the higher reactance of thegenerator windings. Although the short-circuit current is lower forgenerator-source faults than for system-source faults, generator-sourcefaults cannot be ignored because of the short time delay specified inIEEE Std C37.013-1997 (R2008).For a generator-source fed fault occurring at the HV-side of the step-uptransformer the short-circuit current is lower when compared to a fault

at the LV-side of the step-up transformer. The TRVresults from transformer and generator voltageoscillations. The magnitude of each oscillation isapproximately proportional to the transformer andgenerator reactances, respectively. This fault loca-tion can usually be ignored because the resultingstresses on the generator circuit-breaker are muchlower than for faults occurring at the LV-side of the

step-up transformer [1].

5.3.9.7 Calculation of TRV in case of terminal faults TRV calculat ions need to be performed with computer simulations whichallow to model power plant equipment with distributed parameters.

Anyway a simplified single-phase circuit for calculat ing the TRV in caseof interruption of terminal fault currents is depicted in Figure 34 where

V eq is the r.m.s. value of the voltage source and R eq , Leq and C eq are

respectively the values of equivalent resistance, in-ductance and capacitance to ground of the circuitfor assessing the TRV across the first pole to clear(lumped parameters).

GCB

Ceq

L eqR eq

V eq

Figure 34: Single-phase circuit for TRV calculation in case of terminalfaults

V eq can be calculated by using the following expression:

max10

0max 5.12

3 V

ZZZ

V V eq ≈+=

V max is the maximum r.m.s. value of the appliedvoltage prior to fault (it can generally be considered

equal to the maximum phase-to-ground servicevoltage of the HV-system referred to the LV-sideof the step-up transformer and to the maximumphase-to-ground operating voltage of the generator

in case of system-source short-circuit currents andgenerator-source short-circuit currents, respectively)

Z0 is the equivalent zero-sequence impedance of the three-phase circuitZ1 is the equivalent positive-sequence impedance of the

three-phase circuit


31/60 ABB | 31

Leq can be considered equal to 1.5 L 1 where L 1 is the equiva-lent positive-sequence inductance of the three-phase circuit.Following the same procedure R eq is equal to 1.5 R 1 where R 1

is the positive-sequence resistance of the three-phase circuit.

Ceq can be calculated by using the following expression:

thus leading to an underdamped wave-shape of the TRV. The TRV will appear as the superposition of sinusoidal curvesoscillating at different frequencies, i.e. one oscillating at power

C 0 is the zero-sequence capacitance of the three-phasecircuit

C 1 is the positive-sequence capacitance of the three- phase circuit

In all practical applications the following expression is valid:

2

2

4

1

eq

eq

eqeq L

R

CL>

frequency and one oscillating at the frequency imposed by thecircuit parameters:

( ) ( )−=−

)coscos2 2 tet V t TRV u

tL

R

eqeq

eq

ν ω )

where ν can be calculated with following formula:

eqeqeq

eq

eqeq CLL

R

CL≈−=

1

4

12

2

ν 2

2

4

1

eq

eq

eqeq L

R

CL>>assuming

Assuming that cos( ω t) ~ 1 at the time of TRV peak

(being ν >> ω ) the transient recovery voltage can be f inally

−−

)cos12 2 te V

tL

R

eqeq

eq

ν )( ) =t TRV u The TRV peak value occurs at time eqeq CL T = π 2

The corresponding peak value of the TRV is

+=−=−− eq

eqeq

eq

eq

L

CR

eq T L

R

eq 2 e V T e V Eπ

ν 22

2 12)cos122

)

32 10 CCCeq

+= If C0 = C 1 then C eq = C 1 = C 0

represented by the following expression:


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5.3.10 Rated load current switching capabilityDuring normal service of the generator, the load current isreduced to zero before an opening operation of the genera-tor circuit-breaker is initiated. However, the interruption offull load current may be required occasionally for emergencycircumstances or when the synchronous machine is working

in the motor mode in pumped storage power plants. The gen-erator circuit-breaker shall be capable of interrupting thosecurrents and withstanding the TRV appearing across the opencontacts immediately after the interruption of the current.

5.3.11 Capacitance current switching capabilityIEEE Std C37.013-1997 (R2008) considers this as a specialcase where the line or bus capacitance is separated from thegenerator circuit-breaker through transformation.

The generator ci rcuit -breaker normally is not called on toswitch purely capacitive currents.

5.3.12 Out-of-phase current switching capability This capability applies to a generator ci rcuit -breaker used forswitching the connection between two parts of a three-phasesystem during out-of-phase conditions. The assigned out-of-phase current switching rating is the maximum out-of-phase

current that the generator circuit-breaker shall be capable ofswitching at an out-of-phase recovery voltage.Out-of-phase synchronising occasionally occurs in powerplants [4]. The main reasons for out-of-phase synchronis-ing are wiring errors made during commissioning or duringmaintenance when connecting voltage transformers andsynchronising equipment. These wiring errors lead to particu-lar out-of-phase angles, i.e. multiples of 60°el.. E.g. polarityerrors at a voltage transformer cause synchronising at 180°el.out-of-phase angle; phase connection errors lead to 60°el.and 120°el. out-of-phase angles. Besides these particularout-of phase angles any value may be caused by inadequate

settings of the synchronising equipment, e.g. due to an incor-rect value of the closing time of the circuit-breaker.

The TRV appearing immediate ly after the interruption of faultcurrents resulting from out-of-phase synchronising is verysevere with respect to both peak value and rate-of-rise andtime delay. Even though it is recognized that synchronisingwith out-of-phase angle up to 180° might occur, IEEE StdC37.013-1997 (R2008) covers only requirements for amaximum out-of-phase angle of 90°.

The current resulting from out-of-phase synchronizing mightshow delayed current zeros whose causes are totally differentcompared to generator terminal faults. The rapid movement of

the rotor from initial out-of-phase angle δ 0 to δ = 0 results ina very small a.c. component of the fault current and a domi-nant d.c. component when the condition of δ = 0 is reached.

The current resulting from out-of-phase synchronizing has tobe assessed by the aid of computer simulations which allowto model with high level of accuracy power plants equipmentand especially the synchronous machine. As the instant whenthe δ = 0 condition is reached is determined by the movementof the rotor, the inertia constants of turbine, rotor and excita-tion equipment of the generator are of special importance. Asthe fault current to be interrupted by the generator circuit-breaker is characterized by delayed current zeros it extremely

important to prove that the circuit-breaker by means of itsarc-voltage is capable of forcing current to zero within itsmaximum arcing time.

The most important parameters which influence the wave-shape of the fault current resulting from out-of-phasesynchronizing and the occurrence of delayed current zeros arepower plant equipment parameters, out-of-phase angle δ 0,

power frequency of the system and instant when the synchro-nization is initiated.

The wave-shape of the out-of-phase current is depicted inFigures 35 to 40 for different values of δ 0. It is evident that atthe time when δ = 0 the fault current is dominated by a d.c.component. In modern power systems the protection sys-tems sends the tripping signal to the generator circuit-breakerbefore the δ = 0 condition is reached, thus leading to a lesssevere tripping operation. If the tripping is delayed this mightlead to extremely severe interrupting conditions and evenunsuccessful interruption. It is shown in published literaturethat circuit-breakers installed at the HV-side of the step-up

transformer may not be suitable for interrupting fault currentsresulting from out-of-phase synchronizing [5]. Although thearc-voltage of the HV circuit-breaker is of the same order ofmagnitude of the arc-voltage of the generator circuit-breaker,its value referred to the LV-side of the step-up transformer isreduced by the transformation ratio and has practically noeffect on the time constant of the decay of the d.c.component of the fault current.


33/60 ABB | 33

]

-300

-200

-100

0

100

200

300

400[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400

]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400

Figure 36: Prospective out-of-phase fault current – out-of-phase

angle δ 0 = 60°


angle δ 0 = 90°

]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400

]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400

Figure 38: Prospective out-of-phase fault current – out-of-phase angleδ 0 = 120°


]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400


]

-300

-200

-100

0

100

200

300

400

[kA

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40[s]-400


angle δ 0 = 30°


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5.3.13 Excitation current switching capabilityIEEE Std C37.013-1997 (R2008) defines the excitation cur-rent switching capability as the highest magnetizing currentthat a generator circuit-breaker shall be required to switch atany voltage up to rated maximum voltage at power frequencywithout causing an overvoltage exceeding the levels agreedupon between the user and the manufacturer.During normal operation, a generator step-up transformer israrely switched in an unloaded condition. Anyway, consider-

ation should be given to switching of t ransformer excitationcurrent. Excitation current switching is not so much a matterof the generator circuit-breaker capability, but a question ofwhether overvoltages are produced due to current chopping.Due to instabilities of the arc between the circuit-breakercontacts premature current zeros at high frequencies occurfrequently when switching small inductive currents, leading tocurrent chopping. The chopped current flowing in the no-load inductance charges the capacitances of the t ransformerwindings and the capacitances of the connection between thestep-up transformer and the generator circuit-breaker (e.g.

buses or cables). This might results in voltage oscillations ofhigh amplitudes. Modern transformers have a low no-loadcurrent value compared to older designs, and their magneticcharacteristics are such that a relatively low amount of energyis released when current chopping occurs during switching,leading to moderate chopping overvoltages [1]. Choppingovervoltages are produced only on the transformer side of thegenerator circuit-breaker. No overvoltages occur on the

generator side because the inductance of the generator ismuch lower than the magnetizing impedance of the trans-former, and the energy content is low and not of sufficientmagnitude to produce overvoltages [1].

The overvoltage generated by current chopping can be esti-mated with the following formula where it has been assumedthat the energy stored in the magnetizing inductance of thestep-up transformer is transferred to the equivalent capaci-tance without losses. In addition the magnetizing character-istics and the hysteresis loop of the step-up transformer havebeen neglected.

= 2221

21

vCiL eqmageq

mag

C

Liv =⇒

v is the voltage generated by current choppingi is the chopped currentLmag is the magnetizing inductance of the step-up

transformer

C eq is the equivalent capacitance to ground of the step- up transformer windings and the connection of the

step-up transformer to the generator circuit-breakerterminals

The value of chopped current, and consequently the overvol-tages produced, are mainly dependent on the type of gen-erator circuit-breaker. Experience indicates that the currentchopping level of SF 6 self-blast generator circuit-breakers islow and no overvoltages of concern are expected. Further-more, the transformer LV-side is usually protected by surgearresters which reduce these overvoltages. The energy tobe absorbed by the arresters is usually extremely small. In

addition the generator circuit-breaker systems are generallyequipped with capacitors which help to mitigate the transientrecovery voltage appearing after current interruptions. Thosecapacitors are also very effective in reducing the overvoltagesproduced by current chopping. It has to be mentioned thatthe capacitors installed at the generator circuit-breaker termi-nals increase the chopping current level but on the other handthey help reducing the generated overvoltage.

5.3.14 Rated control voltage

According to IEEE Std C37.013-1997 (R2008) the rated con-trol voltage of a generator circuit-breaker is the designatedvoltage that is to be applied to the closing or tripping devicesto close or open the generator circuit-breaker. Rated voltagesand their permissible ranges for the control power supply of

generator circuit-breakers are shown in Table 10 of IEEE StdC37.013-1997 (R2008). Other control voltages may be speci-fied according to other national or international standardsdepending on the point of original installation.

5.3.15 Rated mechanism fluid operating pressure According to IEEE Std C37.013-1997 (R2008) the ratedmechanism fluid operating pressure of a generator circuit-breaker is the pressure at which a gas- or liquid-operated

mechanism is designed to operate. The pressure is allowed tovary above and below its rated value within a specified range.


35/60 ABB | 35

The major demands on the e lectrical layout of power plantscan be summarise

abb gcb - disjuntor de gerador

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