Cpacitor

Shunt Capacitor Banks are used to an increasing extent at all voltage levels to avoid transfer of reactive power, better use of existing power system, improving voltage stability, and to compensate reactive loads. In case of Breaker switched Capacitor Banks, the probability of high transients associated with capacitor switching increases, as many banks are switched on a daily basis.ZnO Arresters at Capacitor Banks are necessaryTo prevent capacitor failures at a breaker restrikes.To limit the risk from repeated breaker restrikes.To prolong the service life of the capacitors by limiting high overvoltages.To serve as an insurance" against unforeseen resonance conditions which otherwise would lead to capacitor failures.For overall limitation of transients related to capacitor bank switching which can be transferred further in the system and cause disturbances in sensitive equipment.To serve as protection against lightning for capacitor banks connected to lines.The energy the arresters have to withstand in capacitor switching circuits is a function of the bank size, the arrester rating, the performance of the operating switch, and the details of the surrounding network.In case of earth fault, Arresters connected between phase and ground will be charged to max. system voltage and are subjected to TOV conditions. The Arrester TOV is selected based on the TOV to which the capacitors are charged. The energy to be absorbed by the arrester will be equal to ? CV2 where V is the peak capacitor voltage after a restrike which is very large.In India, the system regulations are poor and we should take a 10% value. Most of the Shunt Capacitor banks have floating neutrals-which will raise the accidental voltage on the other two lines to full line voltage and the MCOV will be line voltage x 1.10. Again during night time, the line voltage will rise on account of capacitive load and this rise also will have to be added to MCOV value. This rise is calculable from the S.C.level and the capacitor bank rating. If considerations of capacitor banks lead to a next higher rating for arresters than otherwise, then arresters installed elsewhere in the vicinity must also be upgraded to this voltage rating.For a 33KV Shunt Capacitor bank, the MCOV will be 33 x 1.10 or 36.3 & a 36KV rating for an arrester will be just enough assuming that the capacitor banks are automatically switched off by the over-voltage relay setting at 110%. For a 11KV Capacitor Bank, a 12KV arrester rating is ideal. A 9KV rating is too risky-particularly for areas with wider line regulations & subject to intense thunder storms.The survey results on arrester failures have brought out-that over loading due to under rating of arrester duty are the most prominent cause of failures. On a restrike of a shunt capacitor bank, the voltages can develop to the tune of 2.5 x rated capacitor voltages.This will be discharged down to the residual voltage of the arrester & the arrester will ground a substantial amount of energy since the value of 'C' is very high compared to the value of 'C' on transmission lines. Thus the arresters used for protecting capacitor banks will have to have a large KJ/KV rating & must be of the station