Controls for Operating Steam Turbine Generators

O. N. B R Y A N T C . G . S T E R R E T T M E M B E R Λ Ι Ε Γ :

D. M. S A U T E R A S S O C I A T E M E M B E R A I E E

T ^ H E I N D U S T R Y IS at present engaged in designing -I- and building turbine generators in ever-increasing

sizes. The operation of turbine-generator units necessitates the use of a multitude of controls.

Basically, the controls of steam turbines can be divided into those for the control of the flow of steam, and those for the protection of the turbine in case of failure of the main steam controls, or of the main oil pump. The block dia­gram. Fig. 1, shows the relation between the component parts of the control system of the turbine.

The main steam controls include the servomotors for moving the governing valves which determine the steam flow to the turbine, the speed-responsive governor, and the speed-changer which adjusts the speed range in which the governor will control the servomotors. A load limit device also is provided to limit the maximum opening of the governing valves to any chosen amount.

The protective controls include the throttle valve, the overspeed governor and its connecting devices for tripping the throttle valve closed whenever the turbine reaches a predetermined overspeed, and the auxiliary oil pump with its pressure responsive controls which cause the pump to start in case of incipient failure of the supply of bearing lubricating oil, or of the supply of high-pressure oil which actuates the valve servomotors.

These comprise the basic controls that are applied to all straight condensing turbines whether tandem-compound or cross-compound. Since cross-compound turbines are not located on the same shaft, an auxiliary governor, an overspeed governor, and a bearing oil trip must be sup­plied for the lower pressure turbine. A reheat turbine will need the following auxiliary controls: interceptor valves, auxiliary governor, reheater stop valves, and inter­locking relays.

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Fig, 1. A block d iagram of the control sys tem for a typical turb ine burgh, Pa.

7 9 8 Byranty Sterrett, Sauter—Steam Turbine Generators

The following accessory controls also are available for operation of any turbine: low-bearing oil-pressure trip, low-vacuum trip, throttle pressure regulator, low-frequency release for load limit, speed compensator, load responsive governing.

The main devices that are available for controlling the generator include stator, gas, and rotor temperature re­corders, electric metering instruments, and capability curves. The temperature readings sometimes are used as the sole guides for operating the generator. However, the manufacturers always have recommended that hydrogen-cooled i^enerators be loaded according to capability curves.

It is generally recognized that the standard stator temperature detectors indicate unusually low temperature rises at higher hydrogen pressures. Thus, the emphasis on loading according to capability curves is based on the fundamental limitation of the stator temperature de­tectors. These curves take into consideration the aging eflfects (jf total temperature as well as temperature rise which causes differential expansion of the component parts. Temperature readings, therefore, should be taken and recorded, but their primary purpose should be to indicate incorrect operation or changes in the generator.

A hydrogen-cooled turbine generator is provided with a gas and oil system. The primary functions of the hydrogen control system are to provide for scavenging and filling the generator housing; to maintain the gas within the gener­ator housing within predetermined purity, pressure, and temperature limits and free of moisture; and to give warning of improper operation of the generator or failure of the hydrogen control system. These functions must be performed for running and standstill conditions.

The most common types of protective devices used on generators are differential relays, loss-of-field relays, and Held ground detectors. Over and above these three devices there are relays which provide protection for excessive negative sequence currents, out-of-step operation, anti-motoring, overvoltage, overspeed, single phase, vibration, malfunctioning of cooling system, bearing temperature, and fire.

Turbine-generator controls can be provided for remote indication and these indicators also can be provided with limit switches to operate warning devices. Supervisory instruments can be provided for the turbine generator to detect shaft vibration, thrust bearing wear, and differential expansion between the turbine casing and the turbine rotor. This information can be recorded continuously and any marked deviation can be noted and investigated. Digest of paper 54-154, "Controls for Operat ion of Steam Turbine-Generator Uni ts ," recommended by the A I E E Committee on Power Generation and approved by the A I E E Committee on Technical Operations for presentation at the A I E E Winter General Meeting, New York, N . Y., January 18-22, 1954. Published in A I E E Power Apparatus and Systems, February 1954, pp . 79-88, O. N . Bryant is with Westinghouse Electric Corporation, Philadelphia, Pa., and C. C. Sterrett and D. M. Sauter arc with Westinghouse Electric Corporation, East Pitts-

ELECTRIGAL ENGINEERING