Type: Critical Essays
Sample donated: Sam Francis
Last updated: September 20, 2019
Generator circuit experience conditions that are muchmore severe and demanding than normal distribution or transmission circuits.The requirements of generator circuit breaker – installed between generator andstep-up transformer differs from that of distribution or transmission circuit breakers.
Electric utility identifies the need for generator circuit breakers (GCB) toprotect generating stations which drives the development of first industrystandard. In 1993, Switchgear Committee of the Institute of Electrical andElectronics Engineers (IEEE) developed and issued a special industry standard (IEEEC37.013) to address these special requirements of generator circuit breaker.After de-regulation of the utility industry and emergence of small packagedpower plants, the demand for small generator circuit breaker is also increasingday by day.For generator application circuits, special considerationis given generally to following parameters: • Ratedmaximum voltage• RatedDielectric strength• Continuouscurrent rating• Short-circuitcurrent rating• Out-of-phasecurrent switching• Transientrecovery voltage requirement Above mentioned demanding requirements and requiredcapabilities of generator circuit breaker are discussed in following paragraphsin this paper.
II. International standards (IEEE C37.013 &IEC/IEEE 62271-37-013)The IEEE Std. C37.013 wasintroduced in 1993 and is the first only applicable standard for generatorcircuit breakers in installations with a rated power over 100 MVA and up tomore than 1000 MVA. This standard applies to all ac high-voltage generatorcircuit breakers rated on a symmetrical current basis that are installedbetween the generator and the transformer terminals.
In this standard themandatory type testing as well as constructional and operational requirementsare defined. In 2007,a supplement(IEEE C37.013a) was published focusing theneeds of smaller generator circuits ranging from 10 MVA to 100 MVA. A newworking group WG52 was established in 2009, after a decision from IEC and IEEEboards, in order to create a joint standard under IEC 62271-37-013. In 2015International Standard IEC/IEEE 62271-37-013 has been released which isprepared by a joint working group comprised of members both from IEC and IEEE technicalcommittee.
Dual Logo is assigned to this standard. The major benefit of thisstandard is that the manufacturer can built one product to cover both IEC andIEEE markets.III. configuration of generator circuits The simple single linediagram of a generator circuit breaker connected between a generator and step-uptransformer along with auxiliary transformer & auxiliary motor is shown infigure 1.Generator circuit breaker is located close to generator andtransformer to minimize the power loss of system and connected with largecross-section Conductor to reduce the impedance.
Figure 1.Typical generator circuit in power plant IV. Ratings and special requirements A. Ratedmaximum voltage The rated voltage of agenerator circuit-breaker is the highest r.m.s.
voltage for which the generatorcircuit-breaker is designed and is the upper limit for operation. Generally, rated maximumvoltage is equal to 1.05 times of the generator’s maximum operating voltage.The nominal voltage classes of medium-voltage metal enclosed switchgear forgenerator application is mentioned in section 5.
4.1 of IEEE C37.013a andsection 4.1 of IEC/IEEE 62271-37-013 are7.2 kV, 12 kV, 17.5 kV, 24 kV, 36 kV & 38kV. B.
Rateddielectric strength When the generator connectedto generator circuit breaker is OFF and circuit breaker is open then voltageacross open contacts of circuit breaker is equal to system voltage. As generatorstarts and slowly picks up speed, the generator frequency and output voltage increasesslowly causing voltage across open contacts of circuit breaker to vary. Whenthe system and generator voltage are out-of-phase then the high voltage whichcan reach up to 2.5 times of rated line-to-ground voltage of system appearsacross open contacts of generator circuit breaker. So, dielectric design ofcircuit breaker must withstand this overvoltage.
The rated dielectricwithstand of a generator circuit breaker is its voltage withstand capabilitywith specified magnitudes and standard wave shapes .The dielectric strengthrequirement for different rated voltage values of generator circuit more than100 MVA is given(Table I) in section 5.4.1 of IEEE C7.013. TABLE ISchedule of dielectric strength for ac generatorcircuit breakers and external insulation C.
Ratedcontinuous current Generatorcircuit breakers generally carry high continuous current for extended period oftime. For large generators continuous current ranges from 6.3kA up to 20kA.Thishigh continuous current causes excessive heating.
The maximum temperature limitationsare mentioned in International standard IEEE C37.013. So, these circuitbreakers require effective cooling system. Traditionally these circuit breakerswere cooled by natural convection of the ambient air, fan cooling is also apopular option where circuit breakers carry more than normal current for shortperiod of time during high power demand.
Nowadays, Generator circuit breaker isusually integrated in phase isolated bus duct and separate cooling systems (e.g.Water cooling) are used to cool GCB. In case of failure of cooling system thecontinuous current need to be decreased, figure 2 illustrates the procedure insuch cases. Figure 2—Effect of various cooling failures and subsequent load reductions on generatorcircuit breaker temperature. D. Short-circuit current ratingsand interrupting capabilities Generator circuit breaker experience severe condition ofshort-circuit currents containing high values of not only symmetrical but alsoasymmetrical currents.
In a generator circuit, there are two short-circuitsituations, both having different short circuit current values. These fault currentconditions occur by faults at locations “a” & “b” are shown in figure 3 andare known as: a) System-sourceFaults or Transformer-fed Faults (location a) b) Generator-sourceFaults or Generator-fed Faults (location b) Figure 3.Single line diagram of generator circuit Generator source short-circuitfault has no direct relation with system-source short-circuit fault.Practically, the system-source short-circuit current is higher than that of thegenerator source short-circuit current because the transformer and the systemcombined short-circuit reactance is lower than the subtransient and transientreactances of the generator.
So, System-source short-circuit current is higherand is specified as the rated short-circuit current of a generator circuitbreaker. a) System-sourceFaults These type of faults resultsnot only in high symmetrical fault current but also asymmetrical currents havingsevere DC components. The low impedance value of transformer and short busesrunning from transformer to generator through circuit breaker contribute littleto limit the fault current and results in high value of fault current.
Thedecay of DC component depends on X/R ratio (reactance to resistance ratio) of system,and generator circuits have higher X/R ratio of around 50 resulting in DC decaytime constant of 133ms.The values of the dc component in percent of the peakvalue of the symmetrical short-circuit current are given in Figure 3 forprimary arcing contact parting times in milliseconds. Figure 4—Asymmetrical interrupting capability: DC component in percentage of the peakvalue of the symmetrical three-phase system-source short-circuit currentb) Generator-sourceFaults Generator-source faults alsoinclude symmetrical and asymmetrical fault currents.
These faults are lower in magnitude thanSystem-source faults but with much higher degree of asymmetry, the character ofthe fault current is determined by the type of the generator. The symmetricalshort-circuit current value is significantly lower than the system-sourceshort-circuit current. The decay of AC component of this short-circuit currentdepends on subtransient and transient time constant of generator. The asymmetricalshort-circuit current consist of both AC and DC components .The AC componentdecays normally depending on transient and subtransient time constant ofgenerator.
The DC component decaysdepends on the short-circuit time constant “Ta” (Ta= Xd?/?Ra, where Xd? is thedirect axis subtransient reactance and Ra represents the armature resistance).The severe condition of fault current comes from the very high X/R (reactanceto resistance) ratio of the circuit and the operating conditions of thegenerator, both combine to produce a DC component of the fault current reachingas high as 100% which causes asymmetrical fault current peak to shoot high. Ifprior to fault, the generator is operating in under-excited condition withleading power factor causing fast decay of AC component and slow decay of DCcomponent results in another demanding condition of “Delayed current zeros” having the first current zero delayed forseveral cycles as shown in figure 5, connecting additional resistance in serieswith armature resistance forces DC component to decay faster and prevent delayedcurrent zeroes. The arcresistance of the fault and the circuit breaker arc resistance after contact separationalso helps in reducing DC time constant as shown in figure 5, where DCcomponent decays much faster after contact separation because of arc resistancebetween contacts of circuit breaker. As generator circuitbreakers rely on current zero crossing to interrupt the fault.so, it shouldwithstand longer arcing times and greater thermal, electrical and mechanicalstresses during fault clearing. The vacuum interrupters are well suited to thisrequirement because they retain the ability to interrupt even after the contactmotion has ceased and have the capability to withstand very long arcing timesduring the delayed current zero condition.
The generator circuitbreaker should be capable of interrupting asymmetrical faults containing DCcomponent of 110% of the peak value of the symmetrical generator-sourceshort-circuit current for all generator circuit breaker at primary arcingcontact parting. The maximum degree of asymmetry observed in some generators is130% of actual short-circuit current with symmetrical component of only 74% of generator-sourcesymmetrical interrupting current. Otherrequirements like closing, latching, and carrying capabilities, short-timecurrent-carrying capability and Interrupting performance are mentioned insection 5.8.2,5.
8.3 and 220.127.116.11.6 of IEEE C37.013.
Figure 5.Short-Circuit Current with Delayed Current Zeroes. E. Out-of-phase current switchingcapability In generator circuit breakers out-of-phase conditiongenerally occurs when synchronization of the generator with the system isperformed by the generator circuit breaker with an incorrect tripping signal bythe synchronizing device. Generator circuit breaker need not to interrupt underfull phase opposition of 180? and assigned out-of-phase current rating based onan out-of-phase angle of 90? at rated maximum voltage. Generally, maximumcurrent that would have to be switched for an out-of-phase condition is equalto 50% of the short-circuit current rating of the generator circuit breaker.
Out-of-phase recovery voltages, Interrupting time and Inherent TRV parametersare given in section 5.12 and 7.3.6 of IEEE standard C37.013.
F. TRV Requirements As shown in figure 4 the generator and step-up transformerare connected to generator circuit breaker through a short conductor withminimal resistance .so, resistance and stray capacitance of generator circuitis much lower than the normal distribution circuits. These parameters combineto produce very high natural frequency of circuit and results in extremetransit recovery voltage (TRV) with very high rate of rise of recovery voltage(RRRV).
During fault clearing an arcforms around contacts of vacuum interrupter which creates a plasma arc ofaround 50,000?C. After current zero this arc extinguishes and vacuuminterrupter must re-establish dielectric strength across the open gap in orderto withstand this fast-rising TRV. In first phase to clear the fault, the peakvalue of TRV is nearly double the line-to-line voltage of the system, and thecircuit produces that peak voltage within microseconds after the current zero.If the interrupter is able to withstand this fast-rising voltage, then theinterruption is successful. If not, the gap will break down again, and thefault current will flow continuously until the next current zero, when therewill be another opportunity to interrupt. The most critical parameter here ishow fast the TRV is rising across the recovering gap after current zero, tomeasure this a parameter known as Rate of rise of recovery voltage (RRRV) isdefined which is equal to peak value of the transient voltage in kV, divided bythe time it takes to reach that peak value in microseconds, so that the RRRV ismeasured in units of kV/microsecond.
The TRV rate depends on the MVA rating ofthe generator and /or unit transformer, the lower the MVA rating, the lower isthe TRV rate. For typical 15 kV generator circuit breaker the correspondingvalue of RRRV ranges from 3.2 to 4.5 kV/?s. Vacuum interrupters have thecapability to clear high fault currents against these incredibly fast RRRVvalues, without adding capacitors to reduce the rate-of-rise. Above mentioned TRVconditions are extremely severe that even the world’s best high powerlaboratories cannot construct direct test circuits to prove generator circuitbreaker capability.
The only way to prove this capability by high power testingis with the synthetic test method, where two separate sources are used, one toprovide the required short-circuit current and the other to produce therequired transient recovery voltage. The synthetic test is very complicated, asit needs to control the precise operation of two very large power sources andthe test object, as well as one, or sometimes two, auxiliary circuit breakers,to achieve the necessary worst case switching conditions. Synthetic test methodis not only costly but there are also good chances of invalid tests for everyvalid test. If one or more component of the test circuit does not operate atthe right moment in time, the result is an invalid test. Section 5.9 of IEEEStandard C37.013 defines TRV parameters for different MVA Ratings of generatorsand transformers. V.
CONCLUSION Generator circuit breaker requirements are different fromgeneral purpose circuit breakers (e.g. Distribution circuit breakers).Bothsmall and large generator circuits are subjected to unique phenomenon describedin this paper.
Generator and transformers are expensive components and it willbe time consuming to replace if damaged by any fault. So, Generator circuitbreaker should be properly sized to protect the generator as well astransformer. The generator circuit breakers must be designed and tested inaccordance with standards IEEE C37.013 or IEC/IEEE 62271-37-013 as these arethe only International standards used worldwide for generator circuitapplications.