1 Governing systems
The main functions of the governing system of a large turbine-generator unit used for electricity production via an extensive power network are:
- To contain the speed rise within acceptable limits should the unit become disconnected from the load.
- To control the steam valve positions (and hence the load generated) in response to signals from the operator, or from a separate station automatic control system.
- To control the initial run-up and synchronisation of the machine.
- To assist in matching the power generated to that demanded by responding to network frequency changes.
The first of these functions is a vital one for the safety and availability of the plant. Consider a unit at full-load supplying a power network through its generator con nections. If these connections are opened, either by a power system fault or by the unit's own protection system, the steam flow at the instant of disconnection exceeds the steam flow necessary for steady state opera tion at 50 Hz (3000 r/min) by the amount necessary to generate full-load. The excess energy input must be reduced rapidly if an unacceptable overspeed is to be avoided. The governor performs this function by re sponding to the initial change in speed to close the steam valves.
Separate overspeed trips (described in Section 3.5 of this chapter) are provided to guard against possible governor failure and ensure that the safety of the plant and personnel is always maintained. The governor supplements this safety function and, since it maintains the unit at the running speed, ensures the continuity of unit supplies from its own generator as well as the ability to reconnect the generator to the power system. The initial transient speed rise following such a load rejection, contained by the governor, is pri marily due to two factors:
- The stored energy of steam within the turbine and its associated pipework.
- The time taken by the turbine valves to close in response to the sensed overspeed.
It is always well inside the overspeed trip setting and a full analysis is given in Heilbronn .
An electrical governing system for a typical turbine-generator with multiple steam admission paths comprises many elements, as depicted in Fig 2.1. Since it includes at least one closed-loop control function, the machine and network characteristics form an integral part of the system. The primary feedback is of turbine shaft speed which is usually measured by a toothed-wheel and probes at the HP end of the machine. This signal is processed by a modular electronic system, often mounted in a cubicle quite remote from the turbine, to form output signals which are directed back to each steam valve on the turbine. The processing is complex and is subject to detailed variations for each application; it generally includes the following:
- The speed/load characteristics of the machine when synchronised.
- A predetermined relationship between the high pressure (HP) and interceptor valve position.
- Facilities for operator control.
- Features to limit the maximum speed of the machine.
- Features to limit the output in the event of abnormal operating conditions.
- Features to permit routine proving and testing of the system.
The above functions are described in detail in Sections 1.2 and 1.3 of this chapter.
Fitted to each steam valve is a relay, whose function is to convert the low power electrical signal formed by the processing equipment into the movement of the steam valve. Since the mechanical forces involved are substantial (150 kN) and the time for full stroke may be less than 200 ms in the case of a load rejec tion, several stages of hydraulic amplification may be necessary. Conversely, in order to obtain fine control over load (or speed when unsynchronised), the gov erning system needs to be very sensitive and capable of moving the valves to within about 0.2% of the required position. The needs of high resolution and the ability to amplify small electrical signals, necessitates the use of precision hydraulic components with fine clearances. Although earlier mechanical/hydraulic governing systems shared the lubricating oil supply, adequate reliability of the precision systems is only assured by the use of a separate high quality fluid supply unit. Various configurations of valve relay and typical fluid supply unit characteristics are described later in this section. One of the features of an electrical governing system is that since the conversion to mechanical movement is made at the steam valve relay, all other interfaces are electrical. This facilitates connections to station automatic control systems, alarm systems, data process ing systems, switches and indicators both on the operator's desk and at the governor cubicle. All these other systems are closely associated with either the opera tion or maintenance of governing systems.