GE C70 Instruction Manual page 522

OVERVIEW
Ideally, a bank overvoltage protection measures the voltage across each capacitor string, allowing an accurate inference
of the voltage across each element in the string, assuming that the string voltage is equally divided across the elements.
This is readily done in grounded wye capacitor banks by simply measuring the three system phase-to-ground voltages. For
instance, assuming that the bank is healthy, the elements shown in the grounded banks of the following figure are each
stressed at one-ninth of system phase-to-ground voltage, allowing the inference that the elements are overstressed when
the system phase-to-ground voltage exceeds nine times the element safe operating voltage limit. The true RMS value of
these phase-to-ground quantities are what the bank phase overvoltage protection measures when the bus source is set
for wye VTs and the bank overvoltage protection ground setpoint is set to grounded.
For a capacitor bank that is ungrounded, the voltage across the string is equal to the difference between the system
phase-to-ground voltage and the bank neutral voltage. Elements shown in the ungrounded banks in the preceding figure
are overstressed when the following quantities exceed nine times the element safe operating voltage limit.
These quantities are what the bank phase overvoltage protection measures when the bus source is set for wye VTs, the
auxiliary channel of the neutral source is set for a neutral voltage, and the bank overvoltage protection ground setpoint is
set to ungrounded.
When the neutral voltage of an ungrounded wye capacitor bank is not measured or you do not want to use the neutral
voltage measurement, the string voltage can still be determined using the assumption that the bank is balanced. For an
balanced ungrounded bank the neutral voltage V
As such, the A-string voltage is:
9
9-2
Figure 9-1: Bank overvoltage protection for grounded and ungrounded banks
X
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
is:
CHAPTER 9: THEORY OF OPERATION
Eq. 9-1
Eq. 9-2
Eq. 9-3