standards / manuals / guidelines for small hydro development - AHEC

4.1 Type and RatingThe current transformer should be of outdoor type, single phase, oil immersed, selfcooled and suitable for operation in 3 phase solidly grounded system (11 kV CTs will beas per Para 1.6 above).Each current transformers should have the following (table 2.4.1) particulars under thesite conditions for the system under design (typical values upto 72.5 kV system aregiven).Table 2.4.1i) Nominal system voltage 66 kV 33 kV 11 kVii) Highest system voltage 72.5 kV 36 kV 12 kViii) Frequency 50 Hz 50 Hz 50 Hziv)Insulation level (kV Peak)(based on system insulation coordination)Impulse withstand test voltage with 1.2/50micro-second, + ve and – ve wave to earthand between polesv) One minute power frequency (wet) withstandvoltage against ground and between poles.vi)vii)viii)Short time current rating(based on system studies)Rated dynamic current peak(based on system studies)Total minimum creepage of CTs bushings(based on environment)4.2 Details of Current Transformer325 kV 170 kV 75 kV140 kV(rms)70 kV (rms) 28 kV (rms)31.5 kA 31.5 kA 12.5 kA78.75 kA 2.5 time of short time currentrating viAs per Para 2.5 of section 1Details of current transformer i.e. current, number, ratio, no. of cores andprotection/metering class based on metering and relaying scheme be specified.4.2.1 Temperature Risea) 36 kV and aboveThe maximum temperature attained by any part of the equipment in service at site undercontinuous overload capacity conditions and exposed continuously to the direct rays ofsun should not exceed the permissible limit fixed by the applicable standard, whencorrected for the difference between the ambient temperature at site and the ambienttemperature specified by the standard.2-7