Generator rating • Short-circuit current rating • Out-of-phase

Generator circuit experience conditions that are much
more severe and demanding than normal distribution or transmission circuits.
The requirements of generator circuit breaker – installed between generator and
step-up transformer differs from that of distribution or transmission circuit breakers.
Electric utility identifies the need for generator circuit breakers (GCB) to
protect generating stations which drives the development of first industry
standard. In 1993, Switchgear Committee of the Institute of Electrical and
Electronics Engineers (IEEE) developed and issued a special industry standard (IEEE
C37.013) to address these special requirements of generator circuit breaker.
After de-regulation of the utility industry and emergence of small packaged
power plants, the demand for small generator circuit breaker is also increasing
day by day.

For generator application circuits, special consideration
is given generally to following parameters:

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•              Rated
maximum voltage

•              Rated
Dielectric strength

•              Continuous
current rating

•              Short-circuit
current rating

•              Out-of-phase
current switching

•              Transient
recovery voltage requirement


Above mentioned demanding requirements and required
capabilities of generator circuit breaker are discussed in following paragraphs
in this paper.



II.  International standards (IEEE C37.013 &
IEC/IEEE 62271-37-013)

The IEEE Std. C37.013 was
introduced in 1993 and is the first only applicable standard for generator
circuit breakers in installations with a rated power over 100 MVA and up to
more than 1000 MVA. This standard applies to all ac high-voltage generator
circuit breakers rated on a symmetrical current basis that are installed
between the generator and the transformer terminals. In this standard the
mandatory type testing as well as constructional and operational requirements
are defined. In 2007,a supplement(IEEE C37.013a) was published focusing the
needs of smaller generator circuits ranging from 10 MVA to 100 MVA. A new
working group WG52 was established in 2009, after a decision from IEC and IEEE
boards, in order to create a joint standard under IEC 62271-37-013. In 2015
International Standard IEC/IEEE 62271-37-013 has been released which is
prepared by a joint working group comprised of members both from IEC and IEEE technical
committee. Dual Logo is assigned to this standard. The major benefit of this
standard is that the manufacturer can built one product to cover both IEC and
IEEE markets.

III.       configuration of generator circuits

        The simple single line
diagram of a generator circuit breaker connected between a generator and step-up
transformer along with auxiliary transformer & auxiliary motor is shown in
figure 1.Generator circuit breaker is located close to generator and
transformer to minimize the power loss of system and connected with large
cross-section Conductor to reduce the impedance.


Figure 1.
Typical generator circuit in power plant


IV.       Ratings and special requirements


A.      Rated
maximum voltage


The rated voltage of a
generator circuit-breaker is the highest r.m.s. voltage for which the generator
circuit-breaker is designed and is the upper limit for operation.


Generally, rated maximum
voltage is equal to 1.05 times of the generator’s maximum operating voltage.
The nominal voltage classes of medium-voltage metal enclosed switchgear for
generator application is mentioned in section 5.4.1 of IEEE C37.013a and
section  4.1 of IEC/IEEE 62271-37-013 are
7.2 kV, 12 kV, 17.5 kV, 24 kV, 36 kV & 38kV.


B.      Rated
dielectric strength


When the generator connected
to generator circuit breaker is OFF and circuit breaker is open then voltage
across open contacts of circuit breaker is equal to system voltage. As generator
starts and slowly picks up speed, the generator frequency and output voltage increases
slowly causing voltage across open contacts of circuit breaker to vary. When
the system and generator voltage are out-of-phase then the high voltage which
can reach up to 2.5 times of rated line-to-ground voltage of system appears
across open contacts of generator circuit breaker. So, dielectric design of
circuit breaker must withstand this overvoltage.


The rated dielectric
withstand of a generator circuit breaker is its voltage withstand capability
with specified magnitudes and standard wave shapes .The dielectric strength
requirement for different rated voltage values of generator circuit more than
100 MVA is given(Table I) in section 5.4.1 of IEEE C7.013.



Schedule of dielectric strength for ac generator
circuit breakers and external insulation



C.      Rated
continuous current


circuit breakers generally carry high continuous current for extended period of
time. For large generators continuous current ranges from 6.3kA up to 20kA.This
high continuous current causes excessive heating. The maximum temperature limitations
are mentioned in International standard IEEE C37.013. So, these circuit
breakers require effective cooling system. Traditionally these circuit breakers
were cooled by natural convection of the ambient air, fan cooling is also a
popular option where circuit breakers carry more than normal current for short
period of time during high power demand. Nowadays, Generator circuit breaker is
usually 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 the
continuous current need to be decreased, figure 2 illustrates the procedure in
such cases.


Figure 2—
Effect of various cooling failures and subsequent load reductions on generator
circuit breaker temperature.


                Short-circuit current ratings
and interrupting capabilities


        Generator circuit breaker experience severe condition of
short-circuit currents containing high values of not only symmetrical but also
asymmetrical currents. In a generator circuit, there are two short-circuit
situations, both having different short circuit current values. These fault current
conditions occur by faults at locations “a” & “b” are shown in figure 3 and
are known as:


a)       System-source
Faults or Transformer-fed Faults (location a)


b)       Generator-source
Faults or Generator-fed Faults (location b)


Figure 3.
Single line diagram of generator circuit


Generator source short-circuit
fault has no direct relation with system-source short-circuit fault.
Practically, the system-source short-circuit current is higher than that of the
generator source short-circuit current because the transformer and the system
combined short-circuit reactance is lower than the subtransient and transient
reactances of the generator. So, System-source short-circuit current is higher
and is specified as the rated short-circuit current of a generator circuit


a)       System-source


These type of faults results
not only in high symmetrical fault current but also asymmetrical currents having
severe DC components. The low impedance value of transformer and short buses
running from transformer to generator through circuit breaker contribute little
to limit the fault current and results in high value of fault current. The
decay 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 decay
time constant of 133ms.The values of the dc component in percent of the peak
value of the symmetrical short-circuit current are given in Figure 3 for
primary arcing contact parting times in milliseconds.


Figure 4—
Asymmetrical interrupting capability: DC component in percentage of the peak
value of the symmetrical three-phase system-source short-circuit current

b)       Generator-source


Generator-source faults also
include symmetrical and asymmetrical fault currents.  These faults are lower in magnitude than
System-source faults but with much higher degree of asymmetry, the character of
the fault current is determined by the type of the generator. The symmetrical
short-circuit current value is significantly lower than the system-source
short-circuit current. The decay of AC component of this short-circuit current
depends on subtransient and transient time constant of generator.


The asymmetrical
short-circuit current consist of both AC and DC components .The AC component
decays normally depending on transient and subtransient time constant of
generator. The DC component decays
depends on the short-circuit time constant “Ta” (Ta= Xd?/?Ra, where Xd? is the
direct axis subtransient reactance and Ra represents the armature resistance).
The severe condition of fault current comes from the very high X/R (reactance
to resistance) ratio of the circuit and the operating conditions of the
generator, both combine to produce a DC component of the fault current reaching
as high as 100% which causes asymmetrical fault current peak to shoot high. If
prior to fault, the generator is operating in under-excited condition with
leading power factor causing fast decay of AC component and slow decay of DC
component results in another demanding condition of “Delayed current zeros” having the first current zero delayed for
several cycles as shown in figure 5, connecting additional resistance in series
with armature resistance forces DC component to decay faster and prevent delayed
current zeroes. The arc
resistance of the fault and the circuit breaker arc resistance after contact separation
also helps in reducing DC time constant as shown in figure 5, where DC
component decays much faster after contact separation because of arc resistance
between contacts of circuit breaker.


As generator circuit
breakers rely on current zero crossing to interrupt the, it should
withstand longer arcing times and greater thermal, electrical and mechanical
stresses during fault clearing. The vacuum interrupters are well suited to this
requirement because they retain the ability to interrupt even after the contact
motion has ceased and have the capability to withstand very long arcing times
during the delayed current zero condition.


The generator circuit
breaker should be capable of interrupting asymmetrical faults containing DC
component of 110% of the peak value of the symmetrical generator-source
short-circuit current for all generator circuit breaker at primary arcing
contact parting. The maximum degree of asymmetry observed in some generators is
130% of actual short-circuit current with symmetrical component of only 74% of generator-source
symmetrical interrupting current.  Other
requirements like closing, latching, and carrying capabilities, short-time
current-carrying capability and Interrupting performance are mentioned in
section 5.8.2,5.8.3 and of IEEE C37.013.


Figure 5.
Short-Circuit Current with Delayed Current Zeroes.



E.                      Out-of-phase current switching


        In generator circuit breakers out-of-phase condition
generally occurs when synchronization of the generator with the system is
performed by the generator circuit breaker with an incorrect tripping signal by
the synchronizing device. Generator circuit breaker need not to interrupt under
full phase opposition of 180? and assigned out-of-phase current rating based on
an out-of-phase angle of 90? at rated maximum voltage. Generally, maximum
current that would have to be switched for an out-of-phase condition is equal
to 50% of the short-circuit current rating of the generator circuit breaker.
Out-of-phase recovery voltages, Interrupting time and Inherent TRV parameters
are 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 transformer
are connected to generator circuit breaker through a short conductor with
minimal resistance .so, resistance and stray capacitance of generator circuit
is much lower than the normal distribution circuits. These parameters combine
to produce very high natural frequency of circuit and results in extreme
transit recovery voltage (TRV) with very high rate of rise of recovery voltage


During fault clearing an arc
forms around contacts of vacuum interrupter which creates a plasma arc of
around 50,000?C. After current zero this arc extinguishes and vacuum
interrupter must re-establish dielectric strength across the open gap in order
to withstand this fast-rising TRV. In first phase to clear the fault, the peak
value of TRV is nearly double the line-to-line voltage of the system, and the
circuit produces that peak voltage within microseconds after the current zero.
If the interrupter is able to withstand this fast-rising voltage, then the
interruption is successful. If not, the gap will break down again, and the
fault current will flow continuously until the next current zero, when there
will be another opportunity to interrupt. The most critical parameter here is
how fast the TRV is rising across the recovering gap after current zero, to
measure this a parameter known as Rate of rise of recovery voltage (RRRV) is
defined which is equal to peak value of the transient voltage in kV, divided by
the time it takes to reach that peak value in microseconds, so that the RRRV is
measured in units of kV/microsecond. The TRV rate depends on the MVA rating of
the generator and /or unit transformer, the lower the MVA rating, the lower is
the TRV rate. For typical 15 kV generator circuit breaker the corresponding
value of RRRV ranges from 3.2 to 4.5 kV/?s. Vacuum interrupters have the
capability to clear high fault currents against these incredibly fast RRRV
values, without adding capacitors to reduce the rate-of-rise.


Above mentioned TRV
conditions are extremely severe that even the world’s best high power
laboratories cannot construct direct test circuits to prove generator circuit
breaker capability. The only way to prove this capability by high power testing
is with the synthetic test method, where two separate sources are used, one to
provide the required short-circuit current and the other to produce the
required transient recovery voltage. The synthetic test is very complicated, as
it needs to control the precise operation of two very large power sources and
the test object, as well as one, or sometimes two, auxiliary circuit breakers,
to achieve the necessary worst case switching conditions. Synthetic test method
is not only costly but there are also good chances of invalid tests for every
valid test. If one or more component of the test circuit does not operate at
the right moment in time, the result is an invalid test. Section 5.9 of IEEE
Standard C37.013 defines TRV parameters for different MVA Ratings of generators
and transformers.


        Generator circuit breaker requirements are different from
general purpose circuit breakers (e.g. Distribution circuit breakers).Both
small and large generator circuits are subjected to unique phenomenon described
in this paper. Generator and transformers are expensive components and it will
be time consuming to replace if damaged by any fault. So, Generator circuit
breaker should be properly sized to protect the generator as well as
transformer. The generator circuit breakers must be designed and tested in
accordance with standards IEEE C37.013 or IEC/IEEE 62271-37-013 as these are
the only International standards used worldwide for generator circuit