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control system monitors and controls temperature, pressure, flow and conductivity of pure water for cooling the stator coils.

  1. Generator excitation system

While there are various types of excitation systems, recent nuclear power plants mainly employ thyristor excitation systems and brushless excitation systems. Figure 2.10.2 shows typical diagrams of such excitation systems.

The thyristor excitation system provides excitation power, which is taken from the terminal of the main generator through the excitation transformer and converted to direct current by thyristor rectifiers. The good response of this system contributes to power system stability enhancement

The brushless excitation system provides the main generator field with power which is output from a rotating armature type AC exciter and converted to direct current by revolving rectifiers. This system does not need any sliding contacts such as brushes.

  1. Thyristor excitation system

(b) Brushless excitation system

Figure 2.10.2 Excitation system

  1. Major Transformers and Switchyard System

  1. Major transformers

A three-phase, two-winding transformer is installed as a main transformer. The main transformer steps up the voltage of electrical power generated by the main generator to a transmission line voltage for power transmission.

Unit auxiliary transformers and startup transformers are also installed. The unit auxiliary transformers distribute part of the generated power to the plant auxiliary equipment during normal operation, stepping down the generator voltage to that of the unit high-voltage bus (6.9 kV). During plant startup and shutdown, the startup transformers are used to supply power to the plant auxiliary equipment, stepping down the external power transmission line voltage to match the unit high-voltage bus.

  1. Switchyard system

A switchyard, which is located generally on the site of a nuclear power plant, serves as a relay station of electrical power between the nuclear power plant and an external power system.

Recent large capacity nuclear power plants typically have a transmission line voltage of 500 kV.

The external power system is connected to the plant electrical power supply system via two or more transmission lines so that external power may be surely available for the plant auxiliary equipment during plant startup and shutdown.

Use of gas insulated switchgear has prevailed in recent power plants for better performance such as higher tolerance against salt damage.

  1. Plant Auxiliary Power System

  1. Plant auxiliary power supply system

The plant auxiliary power supply system is configured to secure, at any time, the electrical power supply required by the plant auxiliary equipment to operate, i.e., in plant normal operation, during startup and shutdown, and in accident conditions such as a LOCA. This supply system also allows electrical power generated by the main generator to be transmitted to the external power grid.

Figure 2.10.3 shows a single line diagram

NSRA, Japan

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