Power system station services

Power System Station Services The other major system of a nuclear plant is the electric power system. The normal electric power system comprises a main power output transformer, unit and service transformers, and a switchyard. This system steps up (increases) the generator output voltage to match the electric utility s grid requirements for... 

The station services power supplies are classified according to their required levels of reliability. The reliability requirement of these power supplies is divided into four classes that range from uninterruptible power to power that can be interrupted with limited and acceptable consequences. The electric power system station services comprise the supply systems described below. 

Generic Safety Issue (GSI) 051 in NUREG-0933 (Reference 1), identifies the susceptibility of the Station Service Water System (SSWS) to fouling which leads to plant shutdowns and reduced power operation for repairs. 

Automatic transfer system In order to ensure continuity of supply in the event of a failure of either the unit or system power, an automatic transfer system is incorporated on the station service buses. Transfer of load from one service transformer to the other is accomplished by... 

NFPA 850, Par 7.8.6 recommends that oil-filled main, station service, and startup transformers at power generation plants not meeting the separation or fire barrier recommendations should be protected with automatic water spray or foam-water spray systems. Additionally, it recommends that substations and switchyards located at the generating facility and utilizing combustible oil-filled equipment should be protected by fire hydrants where practical and consideration should be given to water spray protection of transformers critical to the transmission of the generated power. 

The frequency target for the loss of DHR function is less than about 10 per year. Since the degree of redundancy of the DHR equipment in the water/steam system is determined only by operational and plant availability, and since the water/steam plant is not available after loss of station service power, the DHR reliability is essentially that of the DRC systems. Because of the high reliability target an appropriate degree of redundancy and diversity has been introduced into the DRC systems to exclude common mode failures from design basis considerations. 

The normal power supply system (level A) supplies all those components performing non-safety classified functions which are not needed after a loss of station service power (LOSSP). 

The Eukushima accident has reinforced the awareness, already well appreciated by designers, of the importance of DHR systems and of the necessity that they continue to operate, even following loss of station service power. 

Apart from the generic reactor coolant pump seal question, station blackout frequencies at PWRs are determined by the plant-specific electric power system design and the design of other support systems. Battery depletion times for the three PWRs were projected to be shorter than for the two BWRs. A particular characteristic of the Surry plant is a gravity-fed service water system with a canal that may drain during station blackout, thus failing containment heat removal. When power is restored, the canal must be refilled before containment heat removal can be restored. 

These applications have considerably more stringent performance requirements than any other application. Circulating water pumps, boiler feed pumps, forced-draught (FD) and induced-draught (ID) fans, pulverizers (ball mills) and condensate pumps are components in a thermal power station that may require extra safety in a standard motor to make it able to fulfil these requirements and withstand abnormal service conditions and system disturbances. Abnormal operating conditions may be one or more of the following ... 

Figure 13.21 A typical powerhouse generation and transmission system, also illustrating power distribution to unit and station auxiliary services...

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