When voltage differential is used for a fuseless capacitor bank, the bottom can in each phase is a single element protection module (PM). The voltage differential relay (87V) is connected to look at the difference between the bus voltage and the protection module voltage (see Figure 4).
The protection of shunt capacitor bank includes: a) protection against internal bank faults and faults that occur inside the capacitor unit; and, b) protection of the bank against system disturbances. Section 2 of the paper describes the capacitor unit and how they are connected for different bank configurations.
The function of fuses for protection of the shunt capacitor elements and their location (inside the capacitor unit on each element or outside the unit) is a significant topic in the design of shunt capacitor banks. They also impact the failure modality of the capacitor element and impact the setting of the capacitor bank protection.
An individual fuse, externally mounted between the capacitor unit and the capacitor bank fuse bus, typically protects each capacitor unit. The capacitor unit can be designed for a relatively high voltage because the external fuse is capable of interrupting a high-voltage fault.
For all types of capacitor banks, protection against overvoltages that are caused by excessively high system voltage is generally provided by a high speed overvoltage relay connected to the substation bus voltage transformers. This relay trips the capacitor bank breaker or vacuum interrupter before capacitor damage can occur.
The objective of the capacitor bank protection is to alarm on the failure of some minimum number of elements or units and trip on some higher number of failures. It is, of course, desirable to detect any element failure. II. ELEMENT AND UNIT FAILURES EXAMINED