Tender Document Volume - 2 - BPDB

5. TRANSFORMERS
5.1 GENERAL
The transformers shall be designed and tested in accordance with IEC 76
(1) Generator Step-up Transformer
Single phase, oil immersed, self cooled / forced air cooled (ONAN / ONAF),
outdoor use type for stepping up the voltage from 11 KV to 230 KV with on load
tap changer having uniform insulation.
The maximum continuous rating of the transformer shall meet at any taps a total
output of the generator, as defined in Vol. 2 of 3 clause 4.1.2 Generator Rating,
which is connected with the transformer in series as shown in the attached single
line diagram. The maximum continuous rating of each transformer (one threephase
bank of three single-phase Transformers out of four single-phase
Transformers and Vector Group = Ynd 11 ) shall be 120% (one hundred and twenty
per cent) of the corresponding MVA (pf=0.8) of the Guaranteed Net Generator
output at Site condition of 35 0 C, 1.013 bar and 98% RH.
The self cooled capacity shall not be less than 75 % of forced air cooled capacity.
(2) Station Transformers
Three phase, oil immersed type, self air cooled (ONAN) for stepping down the
voltage from 6.6 kV to 415 V with off load tap changer. The capacity of each
auxiliary transformer shall enable to supply 120% of the required power for the
unit 415 V system.
(3) Unit Auxiliary Transformer
Three phase, oil immersed type, self air-cooled (ONAN) for stepping up the
voltage from 15 kV to 6.6 KV with off circuit tap changer. The capacity of station
transformer shall enable to supply start-up power, medium voltage loads and
common station load.
5.1.1 REQUIREMENT FOR CHARACTERISTICS
(1) Efficiency
The transformers shall be of highest efficiency that the Contractor can attain.
(2) Temperature Rise
The temperature rise of the windings shall not exceed 55 0 C when measured by he
resistance method, after circulating the rated current at rated frequency in the
windings under test.
The temperature rise of top insulation oil shall not exceed 55 0 C when measured
by a thermometer in an oil filled thermometer pocket on the cover or in the outlet
pipe to the cooler, and the method of the test of temperature rise will be decided
in accordance with IEC 76-2 .
(3) Dielectric Test Voltage
The transformers shall withstand the following test voltages in accordance with
IEC 76-3.
Ghorasal 200 MW to 300 MW Gas Turbine Peaking Power Plant Project (Vol-2 of 3)
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