1. The design of electrical distribution systems varies between commercial nuclear power plants, but they all contain the following:
a. High voltage, offsite distribution
b. high voltage and medium voltage, onsite powerc. Low voltage, control and instrumentation distribution.
2. The offsite power system includes two or more power sources capable of operating independently of the onsite power sources and encompasses the grid, transmission lines, transmission line control system, switchyard battery system, etc.
3. The onsite electric power system consists of the main generator, the unit station transformers, the common station service transformer, the diesel generators and the electric distribution system.
4. Class 1E is the safety classification of the electric equipment and systems that are essential to emergency reactor shutdown, containment isolation, reactor core cooling and containment and reactor heat removal etc.
5. The offsite (preferred) power system is, as the name implies, the preferred source of power for class 1E loads.
6. The onsite electric power system is divided into vital and non-vital distribution.
7. Vital distribution system includes shutdown boards, the diesel generators and interconnecting bus work and the loads supplies by this portion are safety-related and are required to place the plant in a safe shutdown condition.
8. The main generator converts the mechanical energy of the turbine into electric energy.
9. Under normal operating conditions, the main generators supply electrical power through distribution buses to the main step-up transformers and the unit station service transformer; during startup and shutdown, power is supplied from the offsite power through the common station service transformer.
10. The standby onsite power is supplied by diesel generators. Each of which is sized to provide all the electric power required to operate the engineered safety features equipment assigned to the corresponding electrical power train.
11. Diesel generators start automatically on receipt of an engineered safety features actuation signal, loss of voltage to the shutdown board or emergency start initiated by the operator.
12. The vital DC control power system is a class 1E system whose safety function is to provide control power for engineered safety equipment, emergency lighting, vital inventors and other safety related DC power equipment for the entire plant.