Stress specific to the protection of capacitor banks by fuses, which is addressed in IEC 60549, can be divided into two types: Stress during bank energization (the inrush current, which is very high, can cause the fuses to age or blow) and Stress during operation (the presence of harmonics may lead to excessive temperature rises).
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.
The fuse for an individual unit in a capacitor bank must withstand the energy contributed to the failed unit by other capacitors in the same phase group. Short circuit (interrupting) – Must be greater than the short-circuit current that will ow when the capacitor unit is shorted.
This fuse is used for capacitor banks with a large number of parallel capacitors. It can be used on applications with essentially infinite parallel stored energy, as long as sufficient back voltage can be developed to force the current to extinguish.
Eaton’s Cooper PowerE series bus-mounted expulsion-type capacitor fuse provides highly reliable, economical protection for capacitor banks where medium-energy-interrupting ability is required. See Table 1 for electrical ratings. The fuse tube is constructed of bone-grade fibre overwrapped with epoxy-bonded filament-wound fiberglass.
Consequently, short circuit protection for fuseless capacitor banks is the same as for fused capacitor banks and is generally provided in the form of phase and ground time-overcurrent relaying. Where available, the relaying is generally connected to current transformers located at the capacitor bank breaker.