standards / manuals / guidelines for small hydro development - AHEC

The rated voltage is expressed in kV (rms) and refer to phase to phase voltage.Nominal system voltagekV rmsRated voltage of circuit breakerkV rms11 1222 2433 36132 145220 2455.2 Rated Insulation Level: - Insulation level of power circuit breakers should beselected from standard insulation level listed in IS: 13118. Refer table 2.1 (section-1) forvoltage upto 36 kV and table 2.2 (section-1) for voltages upto 245 kV. For insulation coordinationrefer (Para 2.3.4. of section1).The surge protection of the system should be coordinated with the impulse strength of thebreaker, both across the open contacts and to ground. Attention should also be given toincrease in surge voltage because of reflections which occur at breakers when theircontacts are open, especially where cables are involved.Surge arrestors are generally installed on the bus or on transformers and not on eachcircuit breaker, the surge voltage at the breaker can exceed that at the arrestors. Theamount of the excess depends upon the steepness of the wave front and the distance fromthe circuit breaker to the surge arrestors. When the circuit breaker is in the open position,either intentionally left open or during operation, an incoming surge voltage may bedoubled by reflection at the open contacts. Selection of too low an insulation level forcircuit breakers, if not individually protected by arrestors, may result in line-to-ground, oropen gap insulation failure of the circuit breaker. Use of individual line entrance surgearrestors may be required if the lightning trip-out rate of the line exceeds 1 per year (referIEEE std. 37.010-1999). Refer section 1 for more details.5.3 Rated Frequency: standard power circuit breakers are rated at 50 cycles. Serviceat other frequencies requires special consideration.5.4 Rated Normal Current: rated normal current of a circuit breaker is the rms valueof the current which the circuit breaker shall be able to carry continuously at ratedfrequency. With the rise in temperature of its different parts not exceeding specifiedvalues. Values of rated normal currents should be selected from standard value of normalcurrents as per IS: 13118 which are 400A; 630A; 800A; 1250A; 1600A; 2000A; 2500A;3500A; 4000A; 5000A; 6300A (if required higher values can be selected). These ratingsare based on operation of the circuit breaker or switchgear assembly where the ambienttemperature (measured outside the enclosure) does not exceed 40 0 C and the altitude doesnot exceed 1000 m. Standard equipment may be operated at higher altitude by reducingthe continuous current rating in accordance with the following tables (based on AmericanPractice).4-8