11.2.1 Development of Speed Control Systems
For typical turbine generators up to approximately 50 to 60 MW capacity, adequate speed control is obtained by exercising control over the admission of the high pressure steam to the turbine from the boiler. Supplementary control is provided by conventional flap or piston type non-return valves in the bled steam lines to prevent a back feed of bled steam into the turbine from the heaters after the HP inlet steam is shut off.
The main speed control system (excluding emergency tripping functions) operates as a proportional controller and is sensitive to turbine speed only.
Such a system is capable of handling all normal load variations imposed on the unit including severe transient conditions such as full load rejection without an excessive rise in Speed.
With larger capacity units coupled with advanced steam conditions, however, and particularly when a reheat turbine cycle is employed, a more sophisticated control system with supplementary control functions is required to control the speed adequately under transient loading conditions.
This situation is brought about by the increased quantity of steam contained at any instant in the turbine, reheater and connecting pipework, which is beyond the immediate control of the HP inlet steam valves. As a consequence sufficient energy is available as the trapped steam continues to expand through the turbine after the HP inlet steam has been shut off to cause an excessive speed rise of the unit.
This potential overspeed may be counteracted by incorporating into the following into the system:
A fast response governor system, which may include an acceleration sensitive control function (i.e. a derivative control action), to increase the rate of closure of the HP steam valves.
Interceptor valves to control the admission of steam to the turbine IP cylinder.
An anticipatory control action in the form of a Secondary Governor, which is sensitive to loss of load and is able to initiate action before an actual speed rise occurs.
Forced Closed Non Return Valves in the bled steam supply to the feedwater heaters. Forced closure ensures the valves can be closed more rapidly than if they relied on the reversal of steam flow for their operation (as with conventional non-return valves).
11.2.2 Summary of Speed Control Systems
For convenience the speed control systems installed on turbine generators may be grouped according to unit capacity and
An overspeed or emergency governor Emergency (or runaway) stop valves
Non-return valves in the bled steam lines.
11.2.3 Speed Governor
The speed governor is sensitive to turbine speed only and is provided for synchronising duties to handle the normal load variations imposed on the unit and to limit the speed rise to below 10% above normal in the event of the most severe load rejection.
11.2.4 Governor Control Valves
These valves are under the control of the speed governor and exercise control over all HP steam admitted to the turbine.
11.2.5 Emergency Governor
The Emergency Governor is sensitive to turbine overspeed and acts independently of the speed governor. At 10% overspeed it will trip the emergency stop valves and in most cases cause the immediate closure of the governor control valves.
Modern units normally have facilities for routine on-load testing of the Emergency Governor. This permits the tripping action (but not the adjustment) of the mechanism to be tested without actually tripping the unit. The method usually involves by-passing the trip valve and injecting high pressure oil into the tripping mechanism so that it operates at normal synchronous speed.
11.2.6 Emergency Stop Valves
In addition to being tripped by the emergency governor the emergency stop valves may also be tripped manually or automatically in an emergency.
11.2.7 Bled Steam Non-Return Valves
When the emergency stop valves trip the pressure within the turbine immediately begins to decay toward that of the condenser. The non-return valves therefore prevent steam from entering the turbine as a result of a backflow from within the bled steam line or as a result of drainate flashing to steam as a result of the pressure drop in the feedwater heaters.
For all reheat units which normally exceed 100 MW capacity and for many large non-reheat units a typical speed control system will incorporate the following additional features:
A secondary governor
IP Interceptor valves and IP emergency stop valves Forced closure of bled steam valves.
11.2.8 The Secondary Governor
The secondary governor is a fast acting governor sensitive to heavy load rejection, its purpose being to hold the speed rise down below the setting of the emergency governor. It acts independently of speed and exercises overriding control from the speed governor. Under normal operation the speed governor would take approximately 0.5 seconds to close the
governor valves whereas the secondary governor takes approximately 0.2 seconds.
On non-reheat units without interceptor valves a form of secondary governor (or overspeed limiting device) may be arranged to initiate a momentary closure of the emergency stop valves when a large electrical load loss is detected. After remaining closed for several seconds the emergency valves reopen and speed control reverts to the speed governor.
Large non-reheat units around 100 MW and all reheat units normally have a secondary governor, which acts on the governor, control valves and the IP interceptor valves. The governor, which is initiated electrically, comprises an electrical circuit which is triggered by a "loss of electrical load" signal.
This in turn operates on the governor system to effect rapid closure of the governor and interceptor valves.
In due course when the steam pressure in the turbine falls the secondary governor action ceases, the governor and interceptor valves reopen and control reverts to the speed governor. The unit is then running close to synchronous speed and is ready again to accept load.
11.2.9 The IP Interceptor Valves
These valves, which are installed at the IP cylinder inlet control the steam received direct from the HP cylinder on a non-reheat unit or from the reheater on a reheat unit. Both of the interceptor valves are controlled by the speed governor and both will close instantly on operation of the emergency governor. When under the control of the speed governor the system is arranged so that the closure of the governor valve leads by a small margin the closure of the interceptor valves and conversely the opening of the interceptor valves precedes the opening of the governor valves. This phasing ensures that no steam will pass into the reheater after the interceptor valves have closed and will also allow any steam trapped in the reheater to escape gradually before the governor valves commence to open.
11.2.10 The IP Emergency Stop Valves
The closure of these valves is initiated under the same conditions as for the HP emergency stop valves.
11.2.11 Bled Steam Valves
The forced closure of the bled steam valves is initiated by operation of either the secondary governor or the emergency governor. One form of these valves is held open by compressed air against the force of a spring and is tripped by operation of a trip valve, which releases the air pressure. Usually due to the large water storage at saturation temperature the deaerator bled steam valve only is affected.
11.2.12 Governor Control Valves
The governor control valves may be arranged to regulate the admission of steam to the turbine by either throttle control or nozzle control.
11.2.13 Throttle Control
With throttle control the steam is admitted around the full periphery of the steam inlet belt of the HP cylinder. Usually two or four throttle control valves are employed which operate in parallel.
11.2.14 Nozzle Control
Nozzle control employs a number of nozzle control valves each of which controls the admission of steam to separate groups of nozzles which are located in segments around the HP steam inlet belt. The nozzle control valves are opened and closed in sequence by a series of cams and levers. The camshaft is rotated by a servo-motor under the influence of the speed governor.
Practically all modern turbines of large capacity employ throttle control. The throttle control valves and the emergency stop valve are located in a steam chest interconnected by a short pipe to the turbine inlet belt. Usually two steam chests are installed, one on either side of the turbine.
On some turbines a by-pass system is used whereby one throttle valve takes the turbine up to an economic load (say 80% MCR) whilst the second valve opens to pass steam into a later stage in the HP cylinder and take the turbine up to full load. It is more usual now to make full load the economic load and to dispense with any by-pass system.
11.2.15 HP Emergency Stop Valves
The emergency stop valves are designed primarily to be either fully open or shut. They are held open by oil pressure against the force of a strong spring. In an emergency the oil pressure can be released and the valve closes instantly thus shutting off all HP steam to the turbine.
Emergency stop valves are opened manually and may be closed manually at any desired rate provided the governor oil pressure is established. Controlling the steam flow to the turbine during running up may be performed by slowly opening the emergency stop valve or an integral or separate by-pass valve, which is sometimes provided.
On large units it is usual to provide an automatic recovery system which is arranged to automatically reset the emergency stop valves following an overspeed trip provided no fault condition exists within the unit. By this means the unit is prepared to accept further load more rapidly than is possible when the emergency stop valves have to be reset manually.
11.2.16 Load Pay Off or Unloading Gear
The unloading gear is provided to reduce the load progressively under conditions of high condenser back pressure or low steam pressure. Devices sensitive to these conditions act automatically on the speed governor in a similar manner to the speeder gear.
11.2.17 Summary of Functions Performed by a Speed Control System The speed control system has the following functions to perform:
To hold the unit at the desired speed prior to the generator being synchronised to the high voltage distribution system.
To provide a means whereby the speed of the unit can be varied to permit the generator to be synchronised to the distribution system.
To synchronise the generator the speed of the unit must be adjusted until the frequency of the generator voltage is equal (or very nearly equal) to the frequency of the system, this being one of the conditions which must be satisfied before the generator circuit breaker can be closed safely
To enable the generator load to be varied in the desired manner from zero to maximum load after the unit is synchronised.
When synchronised the speed of the unit is proportional to system frequency, which normally remains practically constant. Hence the control system must be capable of varying the load without a significant corresponding change in speed.
To assist in maintaining automatically a practically constant system frequency when variations occur in the electrical load-imposed on the distribution system frequency to normal.
To limit the speed rise of the unit to an acceptable value if the generator should suddenly lose its electrical load.
To shut off immediately the energy input to the turbine if, for any reason, the speed should rise to 10% above normal synchronous speed.
To reduce the unit load progressively and automatically to alleviate the effects of certain abnormal operating conditions.
Such conditions include the condenser back pressure rising above and the steam pressure falling below pre-determined values.
To shut off immediately the energy input to the turbine at any time should an emergency arise.
This action may be initiated manually by operating an emergency trip switch, or it may be initiated automatically under the following conditions:
High condenser back pressure Low bearing oil pressure
Low bearing oil tank level
Wear or failure of turbine thrust bearing
Electrical fault in generator, generator transformer, or elsewhere requiring the immediate shut down of the unit
High boiler water level.