Explanation: Generator transformers are employed in generating stations to connect the power station to the transmission system. Generator transformers step up the generator output at low voltage to the voltage at which the transmission system operates.

Explanation: In the case of combined cycle gas turbine (CCGT) plants this can mean having four step-up transformers, three associated with the gas turbines (each rated about 150 MW) and one with the steam turbine (rated about 250 MW) on a single unit.

Explanation: An on-load tapchanger is required to allow for variation of the HV system volts and generator power factor. LV volts will generally remain within š5% under the control of the generator automatic voltage regulator (AVR).

Explanation: Generator transformers may be subjected to sudden load rejection due to operation of the electrical protection on the generator. This can lead to the application of a sudden overvoltage to the terminals connected to the generator.

Explanation: The station transformer generally supplies the power station auxiliary system for starting up the boiler/turbine generator unit or gas turbine/generator and for supplying those loads which are not specifically associated with the generating unit, for example lighting supplies, cranes, workshops and other services.

Explanation: According to the UK standards, Impedance must be such that it can be paralleled with the unit transformer at 11 kV to allow changeover from station to unit supplies and vice versa without loss of continuity and without exceeding the permissible fault level for the unit and station switchgear this usually means that it is about 15%.

Explanation: Operating load factor is low, i.e. for much of its life the station transformer will run at half-load or less. Load losses can therefore be relatively high, but fixed losses should be as low as possible.

Explanation: The HV voltage is relatively low, being equal to the generator output voltage, i.e. usually between 11 and 23.5 kV. The LV voltage is usually 11 kV nominal, although on some combined cycle gas turbine stations 6.6 kV is used to supply the unit auxiliaries.

Explanation: Paralleling of unit and station transformers during changeover of station and unit supplies can result in a large circulating current between station and unit switchboard. This generally adds to the unit transformer load current, and subtracts from that of the station transformer.

Explanation: in the case of the generator transformer, operating load factor is high, so that load losses and no-load losses should both be as low as is economically practicable. (Except in some nuclear stations, where two fully rated unit transformers are provided per unit for system security purposes.)