Medium and high voltage capacitor banks require protection against faults in the capacitor units and in the associated equipment such as feeder cables, series reactors, overhead structures and conductors connecting the various parts of the capacitor bank.
Faults inside capacitor units are generally cleared by internal fuses. The operation of these fuses must be monitored to allow for appropriate alarm and trip instructions to be issued based on voltage stress levels in the capacitor units. The same applies for unfused units with unbalance current providing an indication of the number of shorted elements. Such protection depends on very sensitive detection of relatively small currents in the star points of capacitor banks. External faults are either phase to ground or phase to phase and require more conventional over current and earth fault detection.
Both of these types of protection require current transformers to provide the necessary detection of current in the primary circuit. Appropriate selection of protection current transformers is therefore essential for safe operation of capacitor banks. This selection must be done to assure correct operation of protection while minimising cost and avoiding any false operation of protection, i.e. ensuring continuity of operation wherever possible.
ONE has prepared a brief application note describing this selection process to assist our clients in this process. Please contact us if more information is required.