of a 1,100 MWe nuclear power plant. During normal operation, electrical power is generally supplied to the plant auxiliary equipment from the main generator through the two unit auxiliary transformers. During startup and shutdown, it is supplied from the external power system through the two startup transformers. The high-voltage bus system consists of seven unit buses and four common buses. The seven unit buses further consist of four normal buses, two emergency buses and one HPCS bus; and the common buses are shared with the adjacent unit. Items connected to the normal buses are mainly pieces of the plant auxiliary equipment that operate in normal operation. Items connected to the emergency buses and the HPCS bus are those necessary to safely shut down the reactor in case of a LOCA or an external power failure.

Generally common buses are installed to supply power to the auxiliary equipment that is in common with the adjacent unit or to the station ancillary devices that are operated regardless of unit operating conditions.

2. Emergency diesel generator system

The emergency diesel generator system is an essential system to supply electrical power, in case of an external power failure, to the plant auxiliary equipment needed for safe reactor shutdown or to the ESFs.

Redundancy and independency are applied in the design of this system so that a single failure within the system will not lead to a complete power failure of the plant even though external power may not be available.

Additionally this system is designed to have sufficient capacity, even when a single failure has made one diesel generator inoperable, to supply power to the plant auxiliary equipment which needs to be operated for safe reactor shutdown or in case of a LOCA

3. Direct current (dc) power supply system

A DC power supply system is provided to supply power to DC control circuits for plant operation under all operating conditions including emergency plant conditions, and to the emergency auxiliary equipment upon a complete alternating current (AC)

power failure.

The DC power supply system consists of batteries, chargers, DC control centers and DC distribution panels. This system is composed of independent divisions, each of which is electrically and physically separated from the others, to supply power independently to the corresponding DC loads of the reactor safety protection system and the ESF system in compliance with the same divisional separation.

4. Instrumentation and control power supply system

The instrumentation and control power supply system supplies AC power to the instrumentation and control system. This system is generally divided into two categories of power supply and each subsystem is assigned to a proper power supply in accordance with its required power supply specifications and its safety and operational importance.

1. Uninterruptible (vital) AC power supply

This power supply consists of static uninterruptible power supplies which can keep supplying power by switching to a DC power source if input AC power is lost.

It supplies power to operationally important instrumentation and control subsystems such as the turbine control system and the feedwater control system.

2. AC instrument power supply

This system supplies power, from the AC instrument buses connected through transformers to the emergency low-voltage buses and to the instrumentation and control subsystems other than those assigned to the uninterruptible AC power supply.

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NSRA, Japan

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