standards / manuals / guidelines for small hydro development - AHEC

OFAFOil Immersed with forced oil circulation Forced Air CooledTransformers when located in powerhouse should be sited so that unrestricted ambient aircirculation is allowed. The maximum transformer rating should match maximumgenerator rating with forced cooling in dual rating transformers, which are commonlyemployed. The rating of these dual rated transformers is usually as follows:ONAN/OFAFONAN - 60%OFAF - 100%The rating under ONAF condition although not guaranteed should be about 80%.Standby cooling capacity should be provided for different type of forced cooling asfollows as per Central Board of Irrigation and Power Manual on Transformer.ONAN/OFAF-2 – 50% group2 – 100% pump of which one will be standby,2 – Standby fans one in each 50% groupOr3 – 50% group with independent pump andfans out of which one group to act as standby.6.0 Temperature Rise, Overload Capacity And Continuous RatingConservative value of temperature rise, overload capacity and continuous rating oftransformer located in the switchyard should be specified. For the purpose ofstandardization of maximum temperature rise of oil and windings, the following ambienttemperatures are recommended by CBI & P.Cooling mediumMaximum ambient temperatureMaximum daily average ambient temperatureMaximum yearly weighted average temperature: Air: 50 0 C: 40 0 C: 32 0 CWith the above ambient temperature condition the temperature rises for powertransformers as per CBI & P are as given below:Oil 0 CWinding 0 C50 55However more conservative temperature rise are specified for generator, transformers.Reference ambient temperatures and climatic conditions and temperature rise specifiedfor generators transformer at Mukerian stage II in Punjab given in Annexure 3.1.3-4