Theory Of Operation; Overview; General Operation; Bank Phase Overvoltage (Ansi 59B) - GE C70 Instruction Manual

C70 Capacitor Bank Protection and Control System
Chapter 9: Theory of operation

Theory of operation

This chapter outlines advanced information on operation.

9.1 Overview

9.1.1 General operation

Static capacitor banks are constructed with a large number of individual capacitor elements connected in a series/parallel
arrangement designed to distribute the system voltage equally among the elements. Equal distribution of voltage is
important as the rate of internal insulation degradation is a highly non-linear function of element voltage, with
degradation accelerating very rapidly with overvoltage greater than 10%. To obtain balanced voltages in spite of
manufacturing tolerances in the capacitance of the elements, the elements are positioned in the bank according to their
actual capacitance such that parallel groups have a uniform total capacitance, and a good voltage balance is obtained.
However, once in service, individual elements can fail randomly upsetting the balance, increasing the stress on other
elements, and causing their early failure. Early failure of an element due to unbalanced voltage predictably further
unbalances the bank, leading to cascading failure.
The purpose of capacitor unbalance protection is to detect unbalance caused by element failures as soon as practical, and
to either alarm the condition so that repair can be scheduled for a convenient time, or to automatically remove the bank
from service to prevent deterioration in the healthy elements pending repair. A brut force method is to directly measure the
voltage across each element, operating on detecting an abnormally high voltage on any element. However, this approach
is seldom economic due to the large number of voltage transducers required. Thus, the C70 supports various other
methods, the suitability of each dependant on the capacitor bank configuration, and on the VTs and CTs available.
The challenge faced by all capacitor unbalance protections not using the brut force method is the increased sensitivity
required due to the less than ideal number of monitoring points, and compensation for the increased effects of inherent
unbalance due to manufacturing tolerance and due to system voltage unbalance.

9.1.2 Bank phase overvoltage (ANSI 59B)

The capacitor unbalance protection functions discussed in the following sections are intended to detect failures in the
capacitor bank itself that can result in a few capacitor elements being overstressed. However, these unbalance protection
functions do not detect overstressed elements in a healthy bank that can result from external problems. Protection against
overstressing elements from system overvoltages is instead provided by the bank phase overvoltage protection.
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 9-1