standards / manuals / guidelines for small hydro development - AHEC

Fig. 3.6 Reclosing – Close Open Reclose TimeThe rated interrupting time of a circuit breaker is the maximum permissible intervalbetween the energization of the trip circuit at rated control voltage and rated mechanismpressure and the interruption of the current in the main circuit in all poles. It is used toclassify breakers of different speed.For line-to-ground faults, the interrupting time is estimated to exceed the ratedinterrupting time by 0.1 cycle. For asymmetrical faults, it is estimated that theinterrupting time may exceed rated time by an additional 0.2 cycle. Hence, for groundedasymmetrical faults, the last phase to clear is estimated to be 0.3 cycle slower than therated interrupting time. Additionally, rated interrupting time may be exceeded duringextreme cold weather or when the breaker has been closed for an extended period of time.Also, the breaker may be slower at the lower limits of control voltage and/or mechanismstored energy. These interrupting times are in the range of several milliseconds havesystem stability implications.The rated interrupting time may be exceeded for close-open operations. The increase ininterrupting time on close-open operation may be important from the standpoint ofpossible system instability. For low values of current, these considerations are lessimportant.6.0 Co-ordination of rated valuesCo-ordination of rated voltages, short circuit breaking current and rated normal currentfor guidance as per IS 13118 for rated voltage 33 kV and above as commonly used are asfollows. (Table 3.4).Table 3.4Ratedvoltage(kV)3.6 101625407.2 812.516256012 812.516Rated shortcircuitbreakingcurrent (kA)400400400 630630630400400 630630630 125012501250Rated normal current (A)12501250125012501600160016001600160012501250 160025002500 400025002500 40004-15