BANGLADESH POWER DEVELOPMENT BOARD TENDER ... - BPDB

5.3 UNIT AUXILIARY TRANSFORMER5.3.1 TYPE AND RATIOThe transformers shall be of three (3) phase, oil immersed, self air cooled (ONAN)type. Nominal no load ratio of delta star connection shall be 11 KV to 6.6 KV.The connection shall be arrangement in vector symbol Ynd 11 according to IEC 76-4 and neutral of star connected low tension winding shall be earthed solidly.The off load tap changer shall be provided on the high tension winding, and theirratio shall be as follows:11 kV ± 2.50 % x5/ 6.6 kV Output5.3.2 OUTPUTUnit auxiliary transformer shall enable to supply 120% of required power on unit6.6 KV for the gas turbine generating unit and required common power on stationservice.5.3.3 IMPEDENCE VOLTAGEThe impedance voltage shall not less than 5 %, but not more than 7.5 % on therated tapping (11 KV/6.6KV) and shall be guaranteed by the Contractor.5.3.4 WINDING AND INSULATIONThe requirements shall be in accordance with section 5.2.4 for LV side.5.3.5 OFF LOAD TAP CHANGERThe off load tap changer shall be provided on 11 kV winding and shall bedesigned to meet the requirements of IEC 76. The tap changer shall be of threephase resistor equipment with rotary diverter switch, and shall be designed toprovide 10 tapping steps, i.e. 11 positions as follows.Central tap: 11 kVStep voltage : 2.50%Upper side : 5 taps (+2.5%)Lower side : 5 taps (-2.5%)All the mechanical operating parts of the gear shall be self lubricated withtransformer oil, no special lubrication being necessary. The tap changercompartment oil shall be isolated from main transformer tank oil, and thecompartment shall be provided with proper protection facilities and accessories.5.3.6 CABLE BOXThe cable boxes shall be provided on both high tension and low tension terminalsto terminate high and low voltage power cables. Proper cable supports and cablecleats shall also be provided.Ghorasal 200 MW to 300 MW Gas Turbine Peaking Power Plant Project (Vol-2 of 3)52