Voltage Differential (Ansi 87V) - GE C70 Instruction Manual

CHAPTER 9: THEORY OF OPERATION
Similarly, the B-string and C-string voltages are:
These 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 "Ungrd w/o Vn."
In applications where the bus VTs are delta-connected, the bus phase-to-ground voltages and therefore the bus zero
sequence voltage are unknown. However, as shown in the preceding equations, string voltages can be determined for an
ungrounded balanced bank using only phase-to-phase bus voltages. In the case of a grounded bank, we can calculate
that the A-string voltage is:
If we make the assumption that the zero-sequence voltage is negligible, which is reasonable when the system is normal
(non-faulted), then the string voltages for grounded banks are the same as for the ungrounded balanced banks:
These quantities are what the bank phase overvoltage protection measures whenever the bus source is set for delta VTs.

9.1.3 Voltage differential (ANSI 87V)

9.1.3.1 Operating principle
The voltage differential function is based on a voltage divider principle. A healthy capacitor string has a constant and
known match factor between its full tap (typically the bus voltage) and an auxiliary tap used by the protection. Any single
element failure results in a difference between the measured factor and its value when the bank is healthy. The protection
can be used on both grounded and ungrounded banks. For ungrounded banks, the neutral point voltage (V
measured by the relay and used to derive the voltage across the string.
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
OVERVIEW
Eq. 9-4
Eq. 9-5
Eq. 9-6
) must be
X
9
9-3