GENERATING STATION POWER REQUIREMENTS BASIC INFORMATION

Power requirements for unit auxiliaries at startup (or shutdown) are dependent on the size and type of unit and the degree of reliability. The design should include considerations for continuous load, short time and transient overload, and a margin for future growth.

Continuous Capability

The continuous capability should supply maximum power requirements of all electrical auxiliaries in any combination in which they may be used during plant operation from the station service system.

Where more than one unit is supplied from a common station service system, consideration should be given to the possibility that a unit in normal operation may trip from full load and transfer its shutdown auxiliaries to the common station service system during the startup of another unit's auxiliaries.

Instead of designing a system to supply auxiliaries requirements for two units, it may be acceptable to impose engineering or administrative load limitations. The system should also be able to react in the event that station load is suddenly lost.

Short-Time Overload Capability

Certain auxiliaries served by station service systems may be required, for limited periods, to carry loads greater than those existing at plant full load.

Examples are circulating water pumps filling the system before full syphon conditions are established, induced-draft fans handling cold air, motor-driven startup boiler feed pumps that will be replaced by turbine-driven pumps at a more advanced stage of startup, and electric boilers not used during full-load operation.

Affected equipment or components of the service system should be capable of carrying these overloads for appropriate periods without producing voltage profiles or temperature rises outside prudent limits.

While reviewing equipment load requirements that are greater during starting than at full-load operating conditions, equipment whose load requirements are lower during starting should also be noted.

A typical example would be the forced-draft fans of a unit, which operate substantially below nameplate rating during startup. Incorporating both types of load in a common portion of the system may reduce the overall supply requirements.

Transient Overload Capability

Station service systems should have the capability of supplying electrical auxiliaries during the starting or restarting of the largest load. When the largest load is a motor driving high-inertia loads, such as an induced-draft fan, the starting transient duration may be up to 60 s.

Voltage profiles during such transients are not required to satisfy criteria established for continuous operation or short-time overload, but they must be adequate to assure that no running motor will stall and no motor-starter contactor will drop out. Acceptable equipment operating voltage ranges should be confirmed with the manufacturer.

If load tap-changing (LTC) transformers are used in the power system, it should be assumed that these tap changers do not operate fast enough to apply significant correction during such transients. The system should be capable of supplying unit auxiliaries required during an automatic transfer of the buses from the normal to the startup/shutdown source.

Voltage profiles immediately following such transfers should meet the same criteria as those established for the motor-starting transient. Within approximately 60 s, the profiles should meet criteria established for continuous operation. If transformers with LTC are used, it should be assumed that significant correction is not applied until after the 60 s period.

Margin for Load Growth

Station service system design should allow for auxiliaries that may be added after the plant is placed in service. An acceptable margin for future loads should be included in transformer and feeder cable capacity and space should be provided for switchgear and motor control center additions.

Short-circuit withstand capability of switch-gear buses should be sufficient to avoid transformer voltage regulation problems and allow for future growth. This last margin generally will be created by increasing transformer impedance at the expense of voltage regulation. For that reason, it is often impractical to make large allowances for future load growth.