GB2176248A - Turbine control - Google Patents

Abstract

A turbine set 10 has in series an HP turbine 12, a re-heater 14, an IP turbine 16, an LP turbine 18, and a condenser 20. An HP bypass 22 connects the outlet of a boiler 24 to the inlet of the reheater 14 and an LP bypass 26 connects the outlet of the reheater 14 to the condenser 20. A parameter, e.g. the HP turbine exhaust pressure, of the set 10 is monitored to enable a ventilating flow for the set 10 to be determined. At least one steam admission valve is controlled in response to the determined ventilating flow to maintain a flow of working fluid through the HP turbine 12 substantially equal to the determined ventilating flow. The ventilating flow is that flow of working fluid which is required to maintain the temperature of the working fluid of the HP turbine exhaust equal to or below a maximum acceptable temperature, whenever operation of the bypass system would otherwise cause that temperature to be greater than the maximum acceptable temperature. <IMAGE>

Description

SPECIFICATION Turbine control The invention relates to turbine control.

The use of turbine sets for driving electricity generators, compressors, ships' drives etc. is well known. The working fluid for the turbines is usually steam but it can be any other suitable thermal fluid depending upon the source of heat. In the description below, it is to be understood that where the context permits, the use of the word "steam" is intended to include "or other suitable thermal fluid".

One form of turbine set, in which the working fluid is steam or other suitable thermal fluid, comprises in series a high pressure (HP) turbine, a working fluid reheater, at least one intermediate and/or low pressure (IP/LP) turbine and a condenser for the working fluid; a working fluid bypass system having a high pressure (HP) bypass connected between the source of hot working fluid and the inlet to the reheater and a low pressure (LP) bypass connected between the outlet from the reheater and an inlet to the condenser; which form of turbine set is hereinafter referred to as a "turbine set as hereinbefore defined".

A turbine set as hereinbefore defined has valves as follows: for the HP turbine, steam admission valves consisting of at least one emergency stop valve and at least one governor valve to control the flow of steam thereto; for the IP/LP turbine, steam admission valves consisting of at least one reheat emergency stop valve and at least one intercept valve to control the flow of steam thereto; and for the HP and LP bypasses, respective control valve and desuperheater means whereby, during use of the bypasses, pressures in the steam boiler and in the reheater are controlled as desired.

In a turbine set as hereinbefore defined, the bypass system allows steam to flow, via the reheater, from the boiler to the condenser.

The boiler can thus generate steam under pressure largely independently of the operation of the turbines. For example, prior to starting the turbines, the firing of the boiler can be brought up to a level at which it is stable before the turbines are- brought up to speed.

Alternatively, the bypass system can respond to sudden changes in load demand, e.g. a generator load rejection in an electrical power generation application, to maintain the boiler firing rate and steam flows at suitable levels until the load demand is restored and the turbines are restarted.

If the turbine set as hereinbefore defined is started without using the bypass system, steam is introduced into the turbines until, at a flow of typically 3%5% of the continuous maximum rating (CMR) flow, the power generated by the turbines is sufficient to drive the turbines at their rated speed, e.g. 3000 RPM, without generating useful power. Thereafter, increases in the flow of steam through the turbines result in the generation of useful power.

During such a start the reheater pressure and hence the HP turbine exhaust pressure varies in direct proportion to the steam flow through the turbine, maintaining a constant pressure ratio over the HP turbine and achieving satisfactory blading efficiences and exhaust steam temperatures.

If the turbo-generator set as hereinbefore defined is started with the bypass system in service, the reheater is pressurised by the steam flow through the bypass system. Consequently, the HP exhaust pressure is abnormally high during the turbine start, the pressure ratio across the HP turbine is reduced from design and the blading and exhaust steam temperatures can increase above acceptable limits.

A similar problem can occur during load shedding if the conditions are such that the bypass system comes into operation.

One typical approach to solve this problem has been to provide an auxiliary bypass from the HP exhaust to the condenser, using a nonreturn valve between the HP turbine exhaust and the reheater to prevent steam flow backwards from the reheater. This arrangement reduces the HP exhaust pressure and hence the steam temperatures to acceptable levels.

Alternative approaches have also employed connections from the HP turbine to the condenser. These are used to evacuate the HP turbine or to create a small reverse cooling flow of steam, and the turbine set is run up to speed and loaded entirely by steam flow through the IP/LP turbines.

In another approach described in UK Patent No. 918779, it has been proposed to provide a ventilating flow of steam through all of the turbine sections using special starting valves.

The starting valves open and close in response to the load demand upon the turbine set. Although such a system may give a gross solution to the problem, it does not accommodate fluctuations in the HP turbine exhaust pressure, for example, caused by changes in the load, steam temperature and pressure etc.

which in turn affect the required ventilating flow.

Disadvantages of such systems is that they can require the provision of additional pipework, valves and desuperheaters and can necessitate a relatively complex control arrangement to transfer to or from normal governor valve control.

It is an object of the present invention to provide a method of controlling a turbine set (as hereinbefore defined) which avoids the aforementioned disadvantages.

According to the present invention, a method of controlling a turbine set (as herein before defined) comprises monitoring a parameter of the turbine set from which the ventilating flow (as hereinbefore defined) is determinable, determining the ventilating flow using said monitored parameter and controlling in response to said determined ventilating flow at least one steam admission valve to maintain a flow of working fluid through the high pressure turbine substantially equal to said determined ventilating flow.

In this specification the term "ventilating flow" is herein defined as that flow of working fluid which is required to maintain the temperature of the steam of the HP turbine exhaust equal to or below a maximum acceptable temperature, whenever operation of the bypass system would otherwise cause that temperature to be greater than said maximum acceptable temperature. The ventiling flow is regulated by monitoring a parameter of the turbine set and calculating the required ventilating flow from the monitored parameter.

There are a number of alternative parameters which may be used to calculate the required ventilating flow, including working fluid pressure and temperature, turbine metal temperature and generated power. It is usually calculated from measurement of the HP turbine exhaust pressure, taking into account the desired turbine speed and power.

All turbines must be controlled such that the speed and overall power generated are equal to the desired values. To achieve this, there is some flexibility in deciding what proportion of the total power each high, intermediate or low pressure turbine contributes. This flexibility is incorporated in the control of the HP turbine and the calculation of the required ventilating flow.

Ventilating flow is only required when the turbine set as hereinbefore defined operates at low loads and the bypass system causes the HP turbine exhaust pressure to be higher than normal. At high level loads the pressure drops over the HP turbine blading created by the normal steam flow enables sufficient energy to be extracted from the steam for the HP turbine exhaust steam temperature to stay below the maximum acceptable temperature. There is a condition, therefore, at which the normal steam flow just exceeds the required ventilating flow. When the turbine is operating at higher loads than this condition, the HP turbine flow need not be specially controlled to ventilate.Since the bypass system can cause the steam pressure at the HP turbine exhaust to vary over a large range, independent of the turbine, both the required ventilating flow and the point at which the normal steam flow just exceeds the ventilating flow can vary greatly.

The principles of HP turbine behaviour which enable the invention to work are that the work done per unit mass of steam decreases and the calculated ventilating flow increases as the HP turbine exhaust pressure increases.

These two effects can be balanced against each other, such that for different HP turbine exhaust pressures, the flow through the HP turbine is sufficient to satisfy the ventilating flow requirements, while the power generated by the HP turbine remains reasonably constant.

If the steam flow is controlled to achieve substantially constant HP turbine exhaust temperature, the power generated by the HP turbine will vary slightly with varying exhaust pressure. It is often more appropriate to calculate the ventilating flow so that the HP turbine produces the same power with various exhaust pressures, in which case the temperature of the steam at the HP turbine exhaust varies within a limited range, but is always below the maximum acceptable temperature. It is clear that a number of variations on this same theme are possible.

If the ventilating flow is calculated to produce constant HP turbine power, there is some flexibility when choosing the value of power produced. The difference between the total power desired and the power produced by the ventilating flow can be accommodated by increasing or decreasing the power generated by the IP/LP turbines.

In the preferred embodiment the ventilating flow is calculated such that the HP turbine produces the same amount of power as it would if the bypass system was not in use and the turbine was operating at fuli-speed with no generated useful power. This criterion causes the value of calculated ventilating flow to vary typically between 5% and 25% of CMR HP turbine flow as the HP turbine exhaust pressure varies between 10% and 100% of its CMR value, respectively. With the ventilating flow calculated on this basis, it is clear that the power produced by each individual turbine is satisfactory, when operating at the full speed, no useful power generated load condition with the bypass system in use. As more power is desired, the steam flow through the IP/LP turbines is increased while the HP turbine flow remains equal to the appropriate ventilating flow. When the normal steam flow corresponding to the desired load begins to exceed the ventilating flow actually passing through the HP turbine, the HP turbine flow begins to increase and the HP turbine begins to contribute more power to the total power generated.

When starting the turbine, the ventilating flow produces more power than is required for controlling acceleration of the turbine up to full speed. The flow through the HP turbine is therefore increased progressively until the speed and power of the turbine is such that the full ventilating flow can be accommodated.

There are no difficulties with excessive steam temperatures during this operation because the run-to-speed is usually of short duration and is a period during which the HP turbine is brought up to its normal working temperature.

The present invention also includes a turbine set (as hereinbefore defined) which is operable by the method according to the invention.

An electrical power generation unit incorporating a turbine set as hereinbefore defined will now be described to illustrate the invention by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a schematic arrangement of the unit; Figures 2 and 3 are graphs showing typical behaviour of the governor and intercept valves during a hot start of the turbine set both without and with the bypass system, respectively.

As shown in Figure 1, the electrical power generation unit 10 has a turbine set 11 which has in series an HP turbine 12; a reheater 14; an IP turbine 16; an LP turbine 18; and a condenser 20. A steam bypass system has an HP bypass 22 connected between the outlet of a main boiler 24 of the unit 10 and the inlet to the reheater 14 and an LP bypass 26 connected between the outlet from the reheater 14 and the condenser 20. An electrical power generator 28 is connected to the common shaft 30 of the turbines 12, 16, 18 to be driven by them.

Flow of steam from the boiler 24 to the HP turbine 12 is controlled by at least one emergency stop valve 32 and at least one governor valve 34. A non-return valve 36 is located in the exhaust of the HP turbine 12 to prevent steam flowing backwards from the reheater 14 to the turbine 12.

Flow of steam from the reheater 14 to the IP and LP turbines 16, 18 is controlled by at least one reheat emergency stop valve 38 and at least one intercept valve 40.

The HP and LP bypasses 22, 26 have respective control valves 42, 44 and desuperheater units 46, 48. Alternatively, the desuperheaters can be integral with the control valves.

A control system (not shown) controls the operation of the turbine set 11, the control system being any suitable computer or like system.

Operation of the set 11 will now be described.

The aspects of the control system to be described below are restricted to those aspects associated with the main functions of the governor and intercept valves 34, 40 to illusrrate the sequencing of those valves and a means of controlling the ventilating flow.

The signal which opens the turbine governor and intercept valves 34, 40 is usually derived from measurement of the turbine speed, and comparison with an adjustable target value.

The difference, or error, is then processed to provide signals to position the valves 34, 40.

By suitable adjustment of the target value, the desired turbine speed or load can be obtained.

The error signal has been referred to here as the load demand signal (LDS) and the processed signals for the valve positions as the governor valve position signal (GVPS) and intercept valve position signal (IVPS), respectively.

Control of the governor and intercept valves 34, 40 is automatically selected in one of two alternative modes, depending on whether the bypass system 22, 26 is in use or not.

When the bypass system is out of commission, the GVPS is made the same as the LDS i.e. GVPS = LDS. The intercept valve signal IVPS is offset and typically given a gain of 4:1 relative to the demand, i.e. IVPS = (4 X LDS + offset). With this sequence (see Figure 2), the intercept valve 40 always opens in advance of the governor valve 34, and so, plays no part in controlling the steam flow through the turbines 12, 16, 18, except during major transients such as load rejections. The governor valve 34 controls the steam flow through the turbines 12, 16, 18, with flow and hence load, approximately proportional to the load demand signal (LDS).

When the bypass system 22, 26 is operating, however, the reheater 14 is pressurised and the intercept valve 40 must control the flow through the IP and LP turbines 16, 18.

Furthermore, because of the increased backpressure on the HP turbine 12, the HP turbine flow must be increased at low loads to the ventilating flow.

To achieve the required control, the intercept valve 40 is arranged to operate over the first part of its travel with a gain ratio of 1:1 and with no offset relative to the demand signal, i.e. IVPS = LDS. This gives fine control over the flow through the IP and LP turbines 16, 18 during starting and low load operation.

At a fixed value of IVPS, the intercept gain is changed to 4:1. This ensures that the intercept valve 40 becomes fully open as soon as possible, to minimise losses in thermal efficiency. The fixed value is chosen such that the generator 28 is always synchronised to the grid at this time and the reheater pressure is only at a moderate level, i.e. load control is no longer particularly sensitive to intercept valve position. (See Figure 3).

As mentioned above, the governor valve 34 has to be controlled to provide a ventilating flow to the HP turbine 12. The required ventilating flow increases with rising HP turbine exhaust pressure, but the poweroutput per unit mass of flow decreases with rising HP turbine exhaust pressure. It is therefore possible to provide adequate ventilating flow at different exhaust pressures, without changing the power generated in the HP turbine 12. For example, at full speed no load, the nominal turbine flow when operating without bypass is approximately 3%5% CMR flow. If the bypass system 22, 26 raises the reheater to full rated pressure, the ventilating flow to the HP turbine 12 increases to 25% CMR flow to provide the required ventilation, but there is no change in the net power output. The flow through the HP bypass 22 is correspondingly reduced.The governor valve 34 is therefore controlled as follows.

The turbine governor unit of the control system calculates the ventilating flow requirement for full speed at the prevailing reheater pressure and the corresponding governor valve position. The valve 34 is then opened linearly to this position during the run to speed, i.e. proportional to LDS. When full speed is attained, the valve lift is restricted to this opening even though the LDS may continue to increase and is only adjusted if there is a change in reheater pressure, i.e. a different ventilating flow requirement or change in supply pressure, i.e.

a different flow per unit of valve lift. On increasing LDS, therefore, there is no further increase in HP turbine power output and the increase in total power is obtained only by an increase in flow through the IP and LP turbines 16, 18. During this period, fluctuations in the reheater pressure caused by the changing requirements of the iP and LP turbines 16, 18 and of the LP bypass 26 cause corresponding fluctuations in the ventilating flow.

When the LDS exceeds the HP turbine ventilating flow requirement, the governor valve 34 will start to open according to its normal gain ratio i.e. GVPS = LDS. The HP turbine 12 will now begin to generate additional power to suit the demand.

Point A in Figure 3 represents the point at which the HP turbine just reaches full speed.

The line A-B in Figure 3 represents the period during which the governor valve 34 is controlled to provide ventilating flow, the overall power output of the set being decreased by opening the intercept valve 40 further. The line A-B is usually not horizontal because the HP turbine exhaust pressure changes as the LDS changes and consequently the required ventilating flow changes. At point B, the LDS just equals the calculated GVPS for the ventilating flow requirement.

When the load demanded from the turbine set 11 changes rapidly, the bypass system 22, 26 may come into operation. If the turbine set 11 subsequently operates at a sufficiently low load with the bypass system in use, there will be a need for a ventilating flow of steam. The ventilating flow corresponding to the prevailing HP turbine exhaust pressure is always calculated and the opening of the governor valve 34 is automatically increased to pass this flow whenever the calculated ventilating flow is greater than the flow actually passing through the HP turbine 12, provided that the desired turbine speed and overall load would not be exceeded. Therefore, ventilation is provided whenever necessary, even after a rapid load change. The control of the governor and intercept valves 34, 40 subsequently behave in the manner shown in Figure 3.

It will be appreciated that, in the above description, the relationship governing the opening of the governor valve 34 and the intercept valve 40 is an example only and other relationships are possible. Furthermore, at low loads, the HP turbine 12 may be controlled using the emergency stop valve 32 instead of or as well as the governor valve 34.

As discussed previously, the ventilating flow can be used to maintain the HP turbine exhaust temperature substantially constant. In that instance, fluctuations in the power generated by the HP turbine 12 will be accommodated by changes in the power generated by the IP and LP turbines 16, 18. Other methods of calculating a suitable ventilating flow are possible. The ventilating flow need not be arranged such that the power produced by the HP turbine is constant or that the HP turbine exhaust steam temperature is constant.

Rather, it is only important that the HP turbine exhaust steam temperature does not exceed a maximum acceptable temperature. It is also possible to use a different parameter to calculate the approximate ventilating flow than HP turbine exhaust steam pressure.

Although the turbine set 11 has been described in combination with a generator 28, it will be appreciated that the invention applies equally to turbine sets used in other applications, e.g. to drive a compressor.

Claims (7)

1. A method of controlling a turbine set (as hereinbefore defined) comprising monitoring a parameter of the turbine set from which the ventilating flow (as hereinbefore defined) is determinable, determining the ventilating flow using said monitored parameter and controlling in response to said determined ventilating flow at least one steam admission valve to maintain a flow of working fluid through the high pressure turbine substantially equal to said determined ventilating flow.

2. A method according to claim 1, in which the ventilating flow is selected to maintain the power generated by the high pressure turbine substantially constant, the temperature of the working fluid at the high pressure turbine exhaust being allowed to vary.

3. A method according to claim 1, in which the ventilating flow is selected to maintain the exhaust temperature of the high pressure turbine substantially constant, the power generated by the high pressure turbine being allowed to vary.

4. A method according to any one of the preceding claims, in which the working fluid is steam.

5. A method according to claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

6. A turbine set (as hereinbefore defined) which is operable by the method as claimed in any one of the preceding clajms.

7. A turbine set according to claim 6 substantially as hereinbefore described with reference to the accompanying drawings.