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  1. Electrical Systems

  1. Main Generators and Appurtenances

The main generators convert mechanical energy of turbines to electrical energy. Although this basic concept is common in both fossil fuel plants and nuclear plants, speeds of nuclear plant turbines are half those of fossil fuel plants, 1,500 rpm (for 50 Hz area) or 1,800 ipm (for 60 Hz area), since volumetric flow rates of steam to turbines of nuclear plants are much larger than those of fossil fuel plants having similar outputs, due to lower pressure and temperature of steam fed to nuclear plant turbines. Subsequently, quadrupole generators are employed instead of bipole generators used in fossil fuel plants. The diameter and weight of a quadrupole generator rotor are approximately 1.5 times and 2 times of those of a bipole generator of the same output, respectively.

Hie main generators of large nuclear plants are directly cooled by fluids flowing in generator coils (internal cooling method). Cooling medium or media are determined by selecting from two options, i.e. water or hydrogen gas, based on the comparison of the cooling efficiencies, maintainability and economy of the systems using these media. The result will be affected by the generator size.

A cross-sectional view of a typical generator for a 3-loop plant is shown in Figure 3.10.1, and the specifications of typical 3- and 4-loop plant main generators are given in Table 3.10.1.

  1. Appurtenances of the main generators

The frame structure of the main generators serves as a pressure vessel for cooling hydrogen gas. The structure has shaft seal mechanisms around the rotor shaft at the locations where the shaft penetrates the frame. A hydrogen gas controller is used for initially charging hydrogen into the generator frame and for the hydrogen gas pressure control during the normal generator operation. A sealing oil controller is used for continuously supplying sealing oil to the shaft seal mechanisms. In the case where the generator stator is cooled by water, a stator cooling water controller is used to monitor and control the temperature, pressure and quality of the cooling water.

  1. Generator excitation system

A brushless excitation system is applied to PWR plant generators selected from various generator excitation systems. The brushless excitation system, as its concept shows in Figure 3.10.2, consists of an alternating current exciter (AC

Figure 3.10.1 Main generator cross sectional view (example)

NSRA, Japan

3-114

Chapter 3 Systems of PWR Nuclear Power Plants

Table 3.10.1 Main Generator Specification (example)

Plant Output(MWe)

l,180(4-Loop)

890(3-Loop)

Generator Type

horizontal, cylindrical revolving field, totally-enclosed self ventilated, three-phase synchronous

Rated Outpnt(kVA)

1,310,000

990,000

Rated Voltage (V)

24,000

23,000

Frequency (Hz)

60

60

Rotation Speed (rpm)

1,800

1,800

Power Factor

0.9 (lagging)

0. 9(Iagging)

Cooling Method

Stator

Water cooled

Hydrogen internally cooled

Rotor

Hydrogen internally cooled

Excitation

Brushless excitor

Brushless excitor