standards / manuals / guidelines for small hydro development - AHEC

Fig. 3.5 Short-line fault circuit5.13 Rated Supply Circuit characteristicsVoltage equal to the phase-to-earth voltage V/3 corresponding to the rated voltage V ofthe circuit breaker.Short circuit current, in case of terminal fault, equal to the rated short circuit breakingcurrent of the circuit breaker.Prospective transient recovery voltage, in case of terminal fault, given by the standardvalues in IS: 13118.5.14 Rated line CharacteristicsStandard values of rated surge impedance rated peak factor and time should be takenfrom IS: 13118.5.15 Rated out –of-phase breaking currentRated out –of-phase breaking current is required to be specified for generator breaker andas per IS 13118. This provision will provide with following.i) The power frequency recovery voltage should be 2.0/3 times the rated voltagefor earthed neutral systems and 2.5/3 times the rated voltage for other systems.ii) The rated out-of-phase breaking current should be 25% of the rated short-circuitbreaking current.iii) Transient recovery voltage as pre IS: 13118.5.16 Rated Line Charging Breaking CurrentRated Line Charging Breaking Current is required to be specified for feeder circuitbreakers. Standard value of line charging capacity of circuit breakers for thecommonly used voltages are as follows:Table 3.3Rated voltage (kV) Rated line-charging breaking current (A)12 2.536 4072.5 40145 50245 125This implies for overhead line length equal to 1.2 times the rated voltage of the circuitbreaker in kilovolts. In case line length is longer than it may be necessary to specify ahigher value of line charging capacity.4-13