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.
The capacitor units in fuseless capacitor banks are similar to those used for externally fused banks. In the capacitor bank, individual capacitor units are connected in series with each other from the phase terminal to the neutral terminal.
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).
In-ternally fused capacitor unit consists of a large number of individual capacitor elements that are discon-nected when an element breakdown occurs. Internally-fused capacitor units are subject to overvoltage across elements within the unit as internal fuses blow and remove elements from a parallel group.
Most capacitor fuses have a maximum power frequency fault current that they can interrupt. These currents may be different for inductive and capacitively limited faults. For ungrounded or multi-series group banks, the faults are capacitive limited.