Page "Emergency power system" Paragraph 14

In recent years, large units of a utility power station are usually designed on a unit system basis in which the required devices, including the boiler, the turbine generator unit, and its power ( step up ) and unit ( auxiliary ) transformer are solidly connected as one unit.

A less common set-up consists of two units grouped together with one common station auxiliary.

As each turbine generator unit has its own attached unit auxiliary transformer, it is connected to the circuit automatically.

For starting the unit, the auxiliaries are supplied with power by another unit ( auxiliary ) transformer or station auxiliary transformer.

The period of switching from the first unit transformer to the next unit is designed for automatic, instantaneous operation in times when the emergency power system needs to kick in.

It is imperative that the power to unit auxiliaries not fail during a station shutdown ( an occurrence known as black-out when all regular units temporarily fail ).

Instead, during shutdowns the grid is expected to remain operational.

When problems occur, it is usually due to reverse power relays and frequency-operated relays on grid lines due to severe grid disturbances.

Under these circumstances, the emergency station supply must kick in to avoid damage to any equipment and to prevent hazardous situations such as the release of hydrogen gas from generators to the local environment.