Chapter 5: Protection of Capacitor Bank and Power Transformer

5.1. Introduction

The function of protection system is to cause the immediate elimination from service of any component of a power system when it suffers from faults; short circuit or when it starts to operate in any abnormal conditions that might cause damage or otherwise disturb the operation of the rest of the system. The relaying equipment is supported in this task by circuit breakers that have ability of isolating the faulty element when they are called upon to do so by the relaying equipment [79]. This chapter presents all types of protection devices on capacitor bank to protect it from high inrush current and transient overvoltage during switching.
For this type of fault, fast protection is provided by the unbalance protection. However, depending on the applied scheme and the fault type, the unbalance current may be out of the normal reliable operating range of the unbalance protection. For example, if a flashover occurs across the entire limb, the current in the neutral connection can be very high. Also for certain capacitor bank configurations, some faults within the bank will not cause an unbalance signal and will remain undetected, for example phase-to-phase Therefore, if the unbalance protection fails to operate, the initial fault may spread until it becomes severe enough to operate the short circuit protection resulting to considerable damage. For these reasons, a backup protection for the unbalance protection is recommended. The negative-sequence over current protection can be used for this purpose to complement the scheme securing the tripping in the above cases.
If the actual value of the variable is higher than the set one, the unit sends the impulse to the time unit.

2) Time unit is discrimination if the change of the measured value is really enough to take an action, such as switching the capacitor and determine number of required steps that should be switched. In other words, the time unit helps to avoid dispensable switching. Over and above, it allows to discharge the capacitor before it`s switched one more time. The delay can last from few seconds up to the few minutes according the setting values.

3) Final control element sends the impulse which will cause the contactor of particular capacitor to switch on, according to the programmed algorithm.

Figure 5.2 shows interface of power factor regulator and figure 5.3 illustrates its schematic diagram. But fig. 5-10 illustrates the full diagram of capacitor bank with power factor regulator and completely protection system. Figure 5.4 show complete protection system devices on capacitor bank and digital