standards / manuals / guidelines for small hydro development - AHEC

7.0 Transformer StudiesIt is recommended that following studies be carried out for transformers 72.5 kV andabove from economic considerations.a) Type of coolingb) Insulation levelc) Departure from normal design impedance8.0 Electrical Characteristics8.1 Type of Transformer and Operating ConditionsAll transformers, should be oil immersed and may be either core or shell type and shouldbe suitable for outdoor installation. Normally oil immersed transformer should beprovided with conservator vessels. Where sealed transformers are specified, there will beno conservator but adequate space will be provided for expansion of oil withoutdeveloping undue pressure.Transformers designed for mixed cooling should be capable of operating under thenatural cooled condition upto the specified load. The forced cooling equipment shouldlcome into operation by pre-set contacts in WTI and the transformer will operate as forcedcooled unit.Transformer should be capable of remaining in operation at full load for 10 minutes afterfailure of blowers without the calculated winding hot-spot temperature exceeding 150 0 C.Transformer fitted with two coolers each capable of dissipating 50% of the losses atcontinuous maximum rating (CMR) should be capable of remaining in operation for 20minutes in the event of failure of the blower associated with one cooler without thecalculated winding hot-spot temperature exceeding 150 0 C.8.2 Continuous Maximum Rating and OverloadsTransformers provided with mixed cooling should comply, as regards its ratingtemperature rise and overloads, with the appropriate requirements of IS: 2026 whenoperating with natural cooling and with mixed cooling.All transformers, except where stated should be capable of operation continuously, inaccordance with IS: loading guide at their CMR and at any ratio. In case bi-directionalflow of power is required, that should be specifically stated by the purchaser.Temperature rise test should be performed at the tapping as desired by the purchaser. Ifnothing has been stated by the purchaser, the test should be carried out at the tapping withthe highest load losses.3-5