[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Since C S in the case of tertiary-winding-connected shunt reactor is relatively smaller than C S for a
shunt reactor directly connected to the high-voltage bus, tertiary-connected reactors normally experience
overvoltages of lower magnitude.
Vacuum circuit breakers have a constant I ch , which is weakly affected by the capacitance seen across
its terminal. To obtain current chopping characteristics of a vacuum breaker, the breaker manufacturer
should be consulted.
IEEE C37.015-1993 provides helpful information for evaluating overvoltages caused by circuit breaker
current chopping. [7]
Current chopping can be mitigated by employing an opening resistor with the circuit breaker. The
shunt reactor can also be protected against current-chopping overvoltages by a surge arrester installed
across the reactor terminals.
FIGURE 2.9.34 Single-phase equivalent circuit for a star-connected and solidly grounded shunt-reactor bank.
C b = equivalent capacitance across the breaker terminals, F
C S = system equivalent capacitance, F
u L–G = system line-to-ground voltage, kV
The parameter k a in Equation 2.9.37 provides a relative indication of the magnitude of the overvoltage
for this type of reactor connection. Circuit damping effects are neglected.
(2.9.37)
where
k a = per-unit parameter that indicates the relative magnitude of the overvoltage due to current
chopping
V reactor_chopping = maximum peak of the overvoltage across the reactor terminals after current chopping, kV
V L–G = maximum peak line-to-ground system voltage, kV
I ch = magnitude of current chopped, A
C 1 = total capacitance on the reactor side, µF
= angular power frequency, rad/s
Q = shunt-reactor-bank three-phase MVAR
For SF 6 , bulk-oil, and air-blast circuit breakers, I ch is a function of the high-frequency stray and grading
capacitances in parallel with the circuit-breaker terminals [12]. Equation 2.9.38 shows this relationship.
where
k
a
2
3.
I
V
ch
1 ka
2. . CQ .
1
I
ch
.
C
reactor _ chopping
VL
G
(2.9.38)
= chopping number (AF –0.5 ), which depends on the circuit breaker construction and arcextinguishing
media
C CB = equivalent capacitance across the circuit breaker terminals for circuit represented in
Figure 2.9.34, F
C
CB
CB
CS.
CL
Cb
C C
S
L
(2.9.39)
2.9.4.2 Restrike
When interrupting small inductive current just before the natural current zero, in a circuit with a critical
combination of source-side and load-side capacitances and inductances, the voltage across the circuit
breaker terminals may exceed its transient-recovery-voltage (TRV) capability and lead to circuit breaker
restrikes. This process can repeat several times until the gap between the circuit breaker contacts becomes
sufficiently large so that its dielectric withstand exceeds the voltage across the circuit breaker terminals.
Each time the circuit breaker restrikes, a transient overvoltage is imposed on the reactor. This type of
overvoltage has a very fast rate of rise of voltage that can distribute nonlinearly across the reactor winding
turns. Current chopping can also increase the magnitude of transient overvoltages produced by restrike.
Multiple restrikes can excite a resonant oscillation in the reactor winding. This can lead to highfrequency
overvoltages between some coil winding turns. Equation 2.9.40 approximates the relative
magnitude of the first restrike overvoltage for the reactor shown in Figure 2.9.34.
k1
k 1
a
CS
C
L
CS
C
L
V
k
V
reactor _ restrike
LG
(2.9.40)
When reactors are not solidly grounded, higher restrike voltages can occur. IEEE C37.015-1993 [7]
provides detailed information for evaluating this type of overvoltage for reactors with different types of
grounding.
Restrike can be mitigated by the use of a surge arrester across the circuit-breaker terminals or by the
use of a synchronous opening device with the circuit breaker. Overvoltages caused by restrike can be
limited by adding a surge arrester across the reactor terminals. If multiple restrikes excite the natural
frequency of the reactor winding, employment of an RC circuit can change the resonant frequency of
the circuit and avert high-frequency overvoltages between coil winding turns.
2.9.5 Current-Limiting Reactors and Switching Transients
2.9.5.1 Definitions
Transient-recovery voltage (TRV) is the voltage that appears across the contacts of a circuit-breaker pole
upon interruption of a fault current. The first time that the short-circuit current passes through zero
after the circuit breaker contacts part, the arc extinguishes and the voltage across the circuit breaker
contacts rapidly increases. If the dielectric strength between the circuit breaker contacts does not recover
as fast as the recovery voltage across the contacts, the circuit breaker will restrike and continue to conduct.
There are various sources of the high rate of rise of transient-recovery voltage, which can potentially
cause the circuit breaker to restrike, e.g., transformer impedance, short-line fault, or distant-fault reflected
waves. In cases where the fault current is limited by the inductance of a current-limiting reactor, a high
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC