8-2 C70 Capacitor Bank Protection and Control System GE Multilin8.1 OVERVIEW 8 THEORY OF OPERATION8Figure 8–1: BANK OVERVOLTAGE PROTECTION FOR GROUNDED AND UNGROUNDED BANKSFor 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. For instance elements shown in the ungrounded banks in the above figurewill be overstressed when the quantities below exceed nine times the element safe operating voltage limit.(EQ 8.1)These quantities are what the bank phase overvoltage protection measures when the bus source is set for wye VTs, theauxiliary channel of the neutral source is set for a neutral voltage, and the bank overvoltage protection ground setpoint isset to ungrounded.When the neutral voltage of an ungrounded wye capacitor bank is not measured or for some reason it is desired not to usethe 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 VX is:(EQ 8.2)As such, the A-string voltage is:(EQ 8.3)Similarly, the B-string and C-string voltages are:(EQ 8.4)These quantities are what the bank phase overvoltage protection measures when the bus source is set for wye VTs, andthe 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 zerosequence voltage are unknown. However, as shown immediately above, string voltages can be determined for anungrounded balanced bank using only phase-to-phase bus voltages. In the case of a grounded bank, we can calculate thatthe A-string voltage is:834744A1.CDRGrounded bankV AABCV B V CVXV AABCV B V CUngrounded bankV A V X– RMS , V B V X– RMS , or V C V X– RMSV X13--- V A V B V C+ +( )=V A V X– RMS V A13--- V A V B V C+ +( )– RMS=13--- 2V A V B V C–– RMS=13--- V AB V CA– RMS=13--- V BC V AB– RMS and 13--- V CA V BC– RMS
