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Handbook of Turbomachinery Second Edition Revised - Ventech!

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auxiliary governor to momentarily assume control <strong>of</strong> the turbine by<br />

decreasing the control oil pressure, causing the governing and interceptor<br />

valves to close rapidly.<br />

With the governing and interceptor valves closed, the speed <strong>of</strong> the unit<br />

will decrease at a rate depending on the residual load left on the generators.<br />

As the unit reaches a speed, which will be determined by the setting <strong>of</strong> the<br />

valve speed changer, the interceptor valves will regulate the flow <strong>of</strong> steam,<br />

holding the speed <strong>of</strong> the unit constant.<br />

As the pressure in the reheater circuit decreases, the interceptor valves<br />

will continue to open. The speed will then tend to drop and the governing<br />

valves open to control at the no-load speed corresponding to the setting <strong>of</strong><br />

the speed changer.<br />

Electro-Hydraulic Control Systems<br />

In the early 1960s, the large turbine generator manufacturers recognized the<br />

need for a faster and more accurate steam turbine control system. Solidstate<br />

electronic techniques had successfully proven their reliability in other<br />

fields such as aerospace and computers. This technology and the use <strong>of</strong> an<br />

even-higher-pressure hydraulic system were combined in the development <strong>of</strong><br />

the Electro Hydraulic Control System [126–128].<br />

EH Control Systems provide the following main advantages:<br />

Improved reliability<br />

Improved load and frequency control<br />

Simplified interfacing with external systems<br />

More compact valve actuators<br />

Today, complete remote-control operation, supplemented by diagnostic<br />

monitoring, automatic startup, and interlocking functions, greatly overshadow<br />

the basic control system. Since these more modern functions require<br />

a high degree <strong>of</strong> flexibility, steam turbine engineers recognized the need for a<br />

digital computer to be used as the heart <strong>of</strong> a new control system. Large<br />

steam turbine manufacturers now provide fully digital Electro Hydraulic<br />

Control Systems.<br />

The spring-loaded steam control and throttle valves are positioned by<br />

hydraulic actuators, which receive their motive fluid from a fire-retardant<br />

pressurized fluid supply system. In the event <strong>of</strong> a trip contingency, a highpressure<br />

emergency trip system is interfaced to the lube oil systems by means<br />

<strong>of</strong> adiaphragm or mechanical trip valve (Fig. 64).<br />

The digital controller positions the throttle and governing valves by<br />

means <strong>of</strong> electro-hydraulic servo loops. In the event <strong>of</strong> a partial load drop,<br />

energizing a solenoid valve on the appropriate valve actuators closes the<br />

Copyright © 2003 Marcel Dekker, Inc.

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