The essential electrical system is the power source for the mechanical post-trip heat removal plant. Normally power is supplied by the national grid system, but should this fail, the diesel generators are switched in by the post-trip sequencing equipment. The system is energised at all times, since in addition to its role in post-trip heat removal, it is also a secure source of supply to many systems which are normally opera­tional, and thus it is an integral part of the overall station electrical system.

The overall arrangement of the system is shown in Figs 2.116 and 2.117. In a similar manner to the

Fig. 2.116 Main and essential electrical systems

other components of the post-trip heat removal systems, the essential electrical system comprises two diverse X and Y systems, with each system consisting of four independent trains of equipment (A to D), one for each reactor quadrant. The X and Y systems are designed to be electrically independent.

There are eight diesel generators on the station generating at 3.3 kV, each generator supplies one train on each reactor, i. e., the AX generator provides power to both the AX train on reactor 7 and the AX train on reactor 8. The X diesels are rated at -■2 MW and the Y diesels at 6.7 MW.

The 11 kV boards associated with the A and В quadrants are supplied from the 132 kV grid system through a three-winding station transformer. Those associated with the C and D quadrants are supplied through individual unit transformers and the gen­
erator transformer from the 400 kV grid system; the main generator is connected via a 23.5 kV gen­erator circuit-breaker, which allows the generator and unit transformers to remain in service with the unit shutdown.

The 11 kV switchboards provide a grid supply to the 3.3 kV essential auxiliaries boards. A single 11/

3.3 кV auxiliary transformer is connected to the 3.3 kV X board of each train, and an interconnector from this board is arranged to supply the 3.3 kV Y board of the same train. If however the 11 kV supply should be lost whilst the PTSE is active, this interconnector is opened and the X and Y boards are then each supplied by their respective diesel generators.

Each 3.3 kV essential auxiliaries board supplies an associated 415 V essential services board. Motor drives at the 3.3 kV level are indicated on Fig 2.117,

all other post-trip motor drives are at the 415 V level.

The 415 V essential services boards also provide power to the battery-backed no-break systems. The X train no-break systems comprise a 220 V DC sys­tem for switchgear closing, а ПО V DC system for switchgear control and a 415 V З-phase AC system for essential motor drives, the main guard lines and instrument supplies.

The Y train no-break systems consist of ПО V DC for switchgear control and ПО V AC single-phase for
instrumentation. Spring-closing is employed for the Y train switchgear.

After every reactor trip, all diesel generators are started and X to X or Y to Y interconnectors are opened. However, whilst the grid supply is available it remains the preferred source of power to the es­sential electrical system; if this should be lost to any

3.3 kV essential auxiliaries board, then the normal supply route to that board is disconnected, all major loads are cleared at 3.3 kV and 415 V level and the diesel generator connected before the loads are again

reconnected. All actions are carried out automatically by the PTSE.