The application of large shunt
capacitor banks with switched parallel banks in high-voltage
transmission systems involves a number of considerations, one of
which is grounding. It is generally recommended that the neutral of
capacitor banks be grounded only to systems that are effectively
grounded.
In the event of a phase-to-ground
fault, a grounded capacitor bank neutral in an otherwise ungrounded
system may lead to high transient overvoltages in the system and
capacitor bank as a result of restriking of the arcing fault to
ground.
One of the main advantages associated
with neutral grounding concerns the severity of the recovery voltage
across the first pole of the switch to clear, interrupting the
charging current of the capacitor bank.
The recovery voltage across the first
pole to open consists of trapped charges on the capacitors and the
variation in the 60 Hz voltage of the system. Due to system
parameters and capacitor bank size, the recovery voltage can be
approximately two times normal peak voltage when the bank is
grounded.
On an ungrounded bank, the magnitude of
the first peak of the recovery voltage can be as high as three times
the peak system line-to-ground voltage when the bank is switched.
Because recovery voltage is a critical factor in determining the
capability of a switching device to switch capacitive reactive power,
it may be desirable (in terms of switch performance) to ground the
neutral of shunt capacitor banks.
IEEE Std C37.04-1979 and ANSI
C37.06-1997 recommend that both the shunt capacitor bank and the
system be grounded at voltage levels of 121 kV and above. Many
capacitor banks of higher voltage are installed ungrounded, but the
circuit breaker manufacturer should be consulted for the application
of a breaker if these conditions are not met.
While many shunt capacitor banks are
directly connected to a high-voltage substation bus, switched
capacitor banks may be applied to tertiaries of power transformers
that are connected to the line or possibly to the bus. Grounding the
neutral of the wye-connected capacitor bank should be done only on an
effectively grounded system.
For instance, the delta tertiary of the
auto transformer represents an isolated source; grounding the
capacitor bank neutral makes this side of the transformer
capacitively grounded. Overvoltages may be experienced during
line-to-ground faults for certain ratios of X0/X1, depending on
system, transformer, and capacitor bank parameters.
If the neutral is to be grounded on a
system that is not effectively grounded, the application should be
thoroughly analyzed for proper application of surge arresters, bank
configuration, bank switching devices, etc.
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