GB2453849A

GB2453849A - Steam power plant with additional bypass pipe used to control power output

Info

Publication number: GB2453849A
Application number: GB0818829A
Authority: GB
Inventors: Andreas Rodewald
Original assignee: EOn Kraftwerke GmbH
Current assignee: EOn Kraftwerke GmbH
Priority date: 2007-10-16
Filing date: 2008-10-14
Publication date: 2009-04-22
Anticipated expiration: 2028-10-14
Also published as: GB0818829D0; GB2453849B; GB2453849A8

Abstract

A steam power plant comprises a steam generator 1, a steam turbine and a condenser 8, wherein at least one bypass pipe 9,11 and/or a turbine start-up pipe 27 are provided for bypassing at least one pressure stage of the steam turbine. At least one further bypass pipe 10,12 is provided for the output control of the steam power plant, and which is connected in parallel to at least one pressure stage 4,5,6 of the steam turbine, said further bypass pipe 10,12 being designed to limit the mass flow of steam supplied to said at least one pressure stage 4,5,6 of the steam turbine connected in parallel. The bypass pipe 10,12 may discharge into a condenser, and may comprise a quick-action shut-off valve 16,22, a pressure reducing device 18,24, or a cooling device 20,26 to inject water into the steam. A method for controlling the output of a steam power plant is also described.

Description

Steam Power Plant and Method for Controlling the Output of a Steam Power Plant The invention relates to a steam power plant comprising a steam generator, a steam turbine and a condenser, wherein at least one bypass pipe and/or a turbine start-up pipe for bypassing at least one pressure stage of the steam turbine is provided. Furthermore, the invention relates to a method for controlling the output of such a steam turbine.

In general, modern thermal power plants having hypercritical steam generators are able to perform only relatively moderate load ramps solely by means of measures such as throttling the turbine valves or condensate stoppage or preheater switch-off, respectively. However, the grid connection conditions of the Transmission Code presently effective in Germany requiring 2% increase of output within 30 sec cannot be satisfied by these means. In other electric grids, depending on the size of the grid and the structure of the generating units, even from modern hypercritical condensation power * plants high load gradients may be required for frequency support, for example in the British Grid Code with a **** requirement of 10% increase of output within 10 sec.

It is the object of the present invention to provide a steam power plant which is able to fulfill even the extreme grid connection conditions mentioned above. In particular a **** corresponding method for controlling the output of a steam *..: power plant is to be provided which allows fulfilling the grid connection conditions mentioned above.

This object is achieved by means of a steam power plant having the features of claim 1 and a method having the features ofclaim 13.

The subclaiins relate to preferred and advantageous embodiments of the invention.

The steam power plant according to the invention is essentially characterized in that it comprises at least one partial flow bypass pipe serving for the output control of the steam power plant, and which is connected in parallel to at least one pressure stage of the steam turbine, said partial flow bypass pipe being designed for a partial flow of a maximum allowable steam mass flow to be supplied to said a least one pressure stage of the steam turbine connected in parallel.

The method according to the invention is correspondingly essentially characterized in that at. least one bypass pipe, a turbine start-up pipe, and/or at least one partial flow bypass pipe are being used for output control of the steam power plant, wherein said at least one partial flow bypass pipe is connected in parallel to a high pressure section of the steam turbine or a medium pressure section and/or a low pressure section of the steam turbine and is designed only for a partial flow of the maximum allowable steam mass flow to be supplied to the respective section/sections of the *:*: turbine.

By partially opening at least a conventional bypass pipe and/or by opening the at least one partial flow bypass pipe and/or by opening the turbine start-up pipe, according to the invention the net output of a steam power plant can thus be lowered by an amount which is to be provided for frequency ::::; support, if necessary.

In the case of a demanded increase of output the method according to the invention provides for partial or complete closing of the at least one conventional bypass pipe and/or the at least one partial flow bypass pipe and/or the turbine start-up pipe.

The solution according to the invention is supplementary to other measures of primary control which should be implemented fully at first with respect to maximum efficiency and low CO2 emissions.

Basically the invention consists of using a bypass system (which is referred to as a control bypass system in the following) for controlling the output of a steam turbine plant. The control bypass system may be the same system as the conventional bypass system or the turbine start-up pipe, respectively, which have to be provided anyway for start- up and for protection of the plant, or may be a bypass system of its own which is designed only for a part of the entire maximum bypass mass flow and which is installed in parallel to the conventional bypass system.

In modern hypercritical steam power stations, based on today's knowledge and experience, an increase in output of about 5-6% by means of condensate stoppage and throttling the turbine control valves can be envisaged. If a certain electric grid requires an output increase of 10% for frequency support, the bypass system would have to be able to * take up and reduce about 5% of the nominal mass flow at the ***.

respective steam turbine inlet. The absorption capacity and thus the structural size of a partial flow bypass provided for this purpose is thus only about 5% of the normal bypass.

For the period of standby operation the efficiency is thus decreased from about 46% to 43.44%. The specific fuel consumption during standby operation with maximum primary control band and thus the variable operation costs rise in this case by 5.88%. However, the costs of power generation have a substantially lower increase since the fuel costs only account for a certain part of the overall costs of power generation. This is due to the comparatively high investment costs of a coal power plant and other fixed costs. If necessary, costs for wear of the control bypass (partial flow bypass) have to be added.

According to a preferred embodiment of the invention the partial flow bypass pipe is connected in parallel to a high pressure section of the steam turbine and is designed only for a partial flow of the maximum allowable steam mass flow which can be supplied to the high pressure section.

As an alternative or supplementary hereto, another preferred embodiment of the invention provides for the partial flow bypass pipe to be connected in parallel to a medium pressure section and/or a low pressure section of the steam turbine and to be designed only for a partial flow of the maximum allowable steam mass flow to be supplied to the medium pressure section or the low pressure section.

Regarding control engineering it is preferable that according to another preferred embodiment of the invention the at least one partial flow bypass pipe is provided with its own shut-of f valve. If necessary, the partial flow bypass pipe can * .2.5 thus be completely separated from the remaining water-steam circuit. Moreover, it is advantageous for the reliability of the output cçntrol according to the invention if according to a preferred embodiment of the invention the shut-off valve in the at least one partial flow bypass pipe is a quick-action **,...

* 3� shut-of f valve. In the present context, a quick-action shut-off valve is a shut-off valve which can be shut off substantially quicker than the valves commonly used in * bypasses which mainly are quick opening. For example, as a shut-off valve in a partial flow bypass pipe a quick-action shut-off valve with a switching time of less than 5 sec, preferably less than 2 sec is used.

Furthermore it is useful if according to another preferred embodiment of the invention the at least one partial flow bypass pipe is provided with at least one device for reducing steam pressure (throttle) . The device for reducing steam pressure (throttle) here preferably has an adjusting device by means of which the mass flow of steam may be adjusted.

The device for reducing steam pressure may also be called a control valve or throttle.

According to a further embodiment of the invention the at least one partial flow bypass pipe has at least one cooling device for cooling steam. Cooling is preferably effected by injecting of water into the steam.

In the following the invention will be explained in detail with reference to the drawing which illustrates an exemplary embodiment. The single figure of the drawing shows a simplified circuit diagram of a steam power plant arranged according to the invention. Several components which may be present in such processes conventionally or optionally, which, however, are not necessary for explaining the present invention, for example a regenerative feed water heater, are not shown.

As is known per se the steam power plant comprises a steam generator 1 with a superheater 2, a steam turbine, a condenser 8, a feed water tank (de-aerator) 30, and a feed water pump 31. By supplying heat energy generated by burning * 3@ fossil fuels, e.g. coal, water is evaporated in the steam generator 1. Upon expansion in the steam turbine the steam provides the energy to drive a generator 7. The steam * exiting the steam turbine is condensed in the condenser 8 at a pressure below atmospheric pressure. The condensate is collected in the feed water tank 30 and fed back to the steam generator 1 by the feed water pump 31, thus providing a closed water-steam circulation.

The steam turbine is divided into several pressure stages.

It comprises a high pressure section 4, a medium pressure section 5, and a low pressure section 6 which drive the generator 7 via a common shaft.

For improving the efficiency of the cyclic steam power process an intermediate superheating of the partially expanded steam is provided. Accordingly the high pressure section 4 of the steam turbine is connected to the medium pressure section 5 through a steam pipe comprising an intermediate superheater 3. At least one control valve 13 is arranged upstream of the high pressure section 4. Moreover, upstream of the high pressure section 4 or the control valve 13, respectively, a quick-action shut-off valve (shut-off valve) may be arranged which is not shown for simplifying the circuit diagram.

As a standard feature of such steam power plants a bypass pipe 9 is connected in parallel to the high pressure section 4 of the steam turbine. A shut-off valve 15, a pressure reducing device (control valve) 17, and a cooling device 19 are arranged in the bypass pipe 9. a..

Furthermore, at least one control valve 14 may be arranged upstream of the medium pressure section 5 of the steam turbine, upstream of which another shut-of f valve (quick- 3� action shut-of f valve) is arranged which is not shown in the drawing. a. *

Between the intermediate superheater 3 and the control valve 14 the steam pipe is provided with a branching to which a bypass pipe 11 is connected bypassing the medium pressure section 5 as well as the low pressure section 6. Also in this bypass pipe 11 which opens into the condenser 8, a shut-off valve 21, a pressure reducing device (control valve) 23, and a cooling device 25 are provided.

Furthermore, the steam power plant schematically shown has a turbine start-up pipe 27. The turbine start-up pipe 27 is branched of f the steam pipe between the high pressure section 4 and the intermediate superheater 3 and discharges into the condenser 8. Thus the turbine start-up pipe 27 presents a bypass bypassing the intermediate superheater 3 as well as the medium pressure section 5 and the low pressure section 6.

Again, a pressure reducing device (control valve) 28 and a cooling device (water injection) 29 are arranged in the turbine start-up pipe 27.

To increase the primary controllability the steam power plant according to the invention is provided with an additional turbine bypass system which may also be called a control bypass system. The figure illustrates the principle of the output control according to the invention by means of a possible arrangement of the control bypass system in the water-steam circuit.

In the example shown, the control bypass system is divided into a HP control bypass (A) and one or multiple (depending * on the number of intermediate superheaters) MP/L1P bypasses. ****

However, also a system having only one control bypass for the high pressure section 4 or only for the low pressure/medium pressure sections 5/6 of the steam turbine is conceivable, the latter variation providing the simplest solution in terms of mechanical complexity. ****

*:*. The control elements of the control bypass system, particularly in the case of a partial flow control bypass, may be especially adapted for output control, i.e. comprise quick-action valves 16, 22 and actuators as well as valves 18, 24 suited for Continuous operation in an (partly) open state. The main advantage here is the low absorption capacity required for the partial flow bypass which allows for the fittings 16, 18, 20; 22, 24, 26 to be considerably smaller with greater control ranges, thus achieving an improved controllability. Otherwise, the configuration of the bypass control systems is analogous to conventional bypass control systems and consists of one or more pressure reducing devices 18 or 24, respectively, and one or more cooling devices 20 or 26, respectively, e.g. water injectors.

The throttled and cooled steam is either re-fed to the turbine in a low pressure stage or directly condensed in an own condenser, i.e. a separate condenser, or the main condenser 8, which in this case is to be designed for continuous operation with this additional feed flow.

By means of shut-off valves 16 or 22, respectively, the control bypass system as a partial flow bypass can be completely separated from the remaining steam circuit such that supply or efficiency influences on the overall process by possible wash-out or other defects in the control bypass can be minimized.

The single figure of the drawing shows that the bypass pipe connected in parallel to the high pressure section 4 of the steam turbine as well as the bypass pipe 9 is configured as a partial flow bypass. Accordingly, the bypass pipe 10 is designed only for a partial flow of the maximum allowable steam mass flow to be supplied to the high pressure section 30* 4.

Furthermore, the figure shows that also the bypass pipe 12 * ** connected in parallel to the medium pressure section 5 and the low pressure section 6 of the steam turbine as well as the bypass pipe 11 and discharging into the condenser 8, is configured as a partial flow bypass. This bypass pipe 12 is

B

thus only designed for a partial flow of the maximum allowable steam mass flow to be supplied to the medium pressure section 5.

The partial flow bypass pipes 10, 12 are each provided with a shut-of f valve 16 or 22, respectively. The shut-off valve 16, 22 is configured as a quick-action shut-off valve. Downstream the shut-off valve 16, 22 a device 18, 24 for reducing the pressure of the steam is arranged. The pressure reducing device 18, 24 may be configured as a control valve.

Preferably the pressure reducing device 18, 24 is provided with an adjusting means for adjusting the mass flow of steam.

Moreover, the partial flow bypass pipes 10, 12 are provided with a cooling device 20, 26 for cooling the steam, preferably for injecting water into the steam.

Since the bypass pipes 10, 12 are designed only for a partial flow of the steam mass flow flowing through the high pressure section 4 and the medium pressure section 5, respectively, of the steam turbine, these pipes 10, 12 and their integrated valves 16, 22 and devices 18, 20, 24, 26 can be designed with relatively small dimensions as compared to the normal bypass systems (9, 15, 17, 19; 11, 21, 23, 25) * The operation of the output control according to the **** *.... invention will be described in detail in the following. ****

When the power plant is to be operated in the power grid for primary control with a maximum primary control band, the net output of the power plant is reduced by opening the control bypass byan amount which should be available for frequency * *. support, if necessary (standby operation) . If the grid requires an increased output later, this is achieved by closing the control bypass (possibly partly only). The advantages of this output control are the very short response -10 -time and the minor influencing of thermal parameters in the range of the steam generator and the turbine so that no major wear of material or control problems are to be expected here.

Furthermore, operation of the control bypass system requires no recovery time after application as does condensate stoppage, for example.

For power plants which are used for primary control only occasionally, this is a supplementary measure for fulfilling the grid requirements for frequency support without major expenditure in terms of devices and control systems.

The functional principle is based on dissipation of the power to be provided for primary control during standby operation.

This means that during standby operation the electrical nominal output of the power plant is not reached and the efficiency of the power plant is reduced. The amount of the bypass mass flow thus is to be reduced to the necessary amount, which will be even smaller, the larger the portion of the requirad output increase is which can be reached by other measures, such as condensate stoppage or switching of f of preheater and throttling the turbine.

Carrying out the invention is not limited to the embodiment illustrated in the drawing. Rather a number of variations can be envisaged which make use of the invention as defined in the patent claims even when configured differently. Thus a steam power plant designed according to the invention may have only one or several MP/LP control bypasses (B) or only * 30 the HP control bypass (A) **.s * S S... S. *

S S S S **

-11 -List of Reference Numerals 1 steam generator 2 superheater 3 intermediate superheater 4 high pressure section (HP section) medium pressure section (MP section) 6 low pressure section (LP section) 7 generator 8 condenser 9 HP bypass pipe (= high pressure bypass pipe) HP control bypass pipe 11 MP/LP bypass (= medium pressure/low pressure bypass pipe) 12 MP/LP control bypass pipe 13 HP control valve 14 MP/LP control valve HP bypass shut-off valve 16 HP control bypass shut-off valve 17 HP bypass pressure reducing device 18 HP control bypass pressure reducing device 19 HP bypass cooling device HP control bypass cooling device 21 MP/LP bypass shut-off valve *2 22 MP/LP control bypass shut-off valve * 23 MP/LP bypass pressure reducing device *.

24 MP/LP control bypass pressure reducing device MP/LP bypass cooling device 26 MP/LP control bypass cooling device 27 turbine start-up pipe 28 control valve 29 cooling device : 30 feed water tank (de-aerator) 31 feed water pump

Claims

CLAIMS: 1. A steam power plant, comprising a steam generator (1), a steam turbine and a condenser (8), wherein at least one bypass pipe (9, 11) and/or a turbine start-up pipe (27) are provided for bypassing at least one pressure stage of the steam turbine, characterized in that at least one further bypass pipe (10, 12) is provided serving for the output control of the steam power plant, and which is connected in parallel to at least one pressure stage (4, 5, 6) of the steam turbine, said further bypass pipe (10, 12) being designed only for a partial flow of a maximum allowable steam mass flow to be supplied to said at least one pressure stage (4, 5, 6) of the steam turbine connected in parallel.
2. The steam power plant according to claim 1, characterized in that said at least one further bypass pipe (10) is connected in parallel to a high pressure section (4) of the steam turbine and is designed only for a partial flow of a maximum allowable steam mass flow to be supplied to the high pressure section (4) * **
3. The steam power plant according to claim 1 or 2, **** 25' characterized in that said at least one further bypass pipe (12) **.* is connected in parallel to a medium pressure section (5) and/or **** a low pressure section (6) of the steam turbine and is designed * only for a partial flow of a maximwn allowable steam mass flow to **** ** be supplied to the medium pressure section (5) or the low *3Q pressure section (6)
4. The steam power plant according to claim 3, characterized in that said at least one further bypass pipe (12) discharges into the condenser (8)
5. The steam power plant according to claim 3, characterized in that said at least one further bypass pipe (12) discharges into a separate condenser.
6. The steam power plant according to one of claims 1 to 5, characterized in that said at least one further bypass pipe (10, 12) is provided with its own shut-off valve (16, 22)
7. The steam power plant according to claim 6, characterized in that the shut-off valve (16, 22) is a quick-action shut-off valve.
8. The steam power plant according to claim 6 or 7, characterized in that in addition to the shut-off valve (16, 22) a quick-action control valve is provided.
9. The steam power plant according to one of claims 1 to 8, characterized in that said at least one further bypass pipe (10, 12) has at least one pressure reducing device (18, 24) for reducing the pressure of the steam.
10. The steam power plant according to claim 9, characterized in that said at least one pressure reducing device (18, 24) has an * ** ** adjusting means by means of which the mass flow of steam is ****

S adjustable. **** * S

*
11. The steam power plant according to one of claims 1 to 10, ** SS *S * characterized in that said at least one further bypass pipe ** (10,12) has at least one cooling device (20, 26) for cooling the steam.
12. The steam power plant according to claim 11, characterized in that said cooling device (20, 26) is designed for injecting water into the steam.
13. A method for controlling the output of a steam power plant comprising a steam generator, a steam turbine, and a condenser, wherein at least one bypass pipe (9, 11) and/or a turbine start-up pipe (27) are provided for bypassing at least one pressure stage of the steam turbine, characterized in that said at least one bypass pipe (9, 11), said turbine start up pipe (27), and/or at least one partial flow bypass pipe (10, 12) are being used for output control of the steam power plant, wherein said at least one partial flow bypass pipe (10, 12) is connected in parallel to a high pressure section (4) of the steam turbine or a medium pressure section (5) and/or a low pressure section (6) of the steam turbine and is designed only for a partial flow of the maximum allowable steam mass flow to be supplied to the respective section/sections (4, 5, 6) of the turbine.
14. The method according to claim 13, characterized in that net output of the steam power plant is lowered by an amount which is additionally available for frequency support, if necessary, by partially opening said at least one bypass pipe (9, 11) and/or by opening said at least one partial flow bypass pipe (10, 12) and/or by opening the turbine start-up pipe (27).
15. The method according to claim 13 or 14, characterized in *

</p>..CLME: <p> that, in the case of a demanded increase of output of the steam power plant, said at least one bypass pipe (9, 11) and/or said at *... least one partial flow bypass pipe (10, 12) and/or said turbine S...

start-up pipe (27) are closed partially or completely. S...

****
16. The method according to claim 15, characterized in that the *:Q partial or complete closing is effected by means of quick-action valves (15, 16, 21, 22).
17. The method according to one of claims 13 to 16, characterized in that steam passed through said at least one -. 15- partial flow bypass pipe (10, 12) is re-fed into the turbine in a low pressure stage or condensed in a separate condenser.
18. A steam power plant substantially as hereinbefore described with reference to and as shown in Figure 1.
19. A method for controlling the output of a steam power plant substantially as hereinbefore described with reference to and as shown in Figure 1. * ** * * * * ** *... * * S.,. **.. S...

I

I..... * . *15S * I *. I. * * SS S *I