CN212716793U

CN212716793U - Steam turbine

Info

Publication number: CN212716793U
Application number: CN202021453928.2U
Authority: CN
Inventors: 刘玉海; 张高智; 雷富刚; 梁双荣; 康立强; 李超; 裴英凯; 王洪波; 孙锡岩; 孟祥浩; 王印; 周新飞; 郭亚兰
Original assignee: State Power Xingyang Coal Electricity Integration Co ltd; Guodian Longyuan Energy Saving Technology Co Ltd; Guodian Science and Technology Research Institute Co Ltd Taiyuan Branch
Current assignee: State Power Xingyang Coal Electricity Integration Co ltd; Guodian Longyuan Energy Saving Technology Co Ltd; Guodian Science and Technology Research Institute Co Ltd Taiyuan Branch; Guodian Xingyang Coal and Electricity Integration Co Ltd
Priority date: 2020-07-21
Filing date: 2020-07-21
Publication date: 2021-03-16
Anticipated expiration: 2030-07-21

Abstract

The utility model provides a steam turbine, its characterized in that, this steam turbine includes: the steam communication pipeline is communicated with the intermediate pressure cylinder and the low pressure cylinder, and a communication pipe valve is arranged on the steam communication pipeline; the steam bypass pipeline is communicated with the intermediate pressure cylinder and the low pressure cylinder, a bypass valve is arranged on the steam bypass pipeline, and the communication pipe valve is connected with the bypass valve in parallel; a steam bypass line between the bypass valve and the intermediate pressure cylinder branches off to a steam line to the heat supply network heater.

Description

Steam turbine

Technical Field

The utility model relates to a steam turbine power generation field, concretely relates to steam turbine.

Background

The steam turbine in the prior art comprises a medium pressure cylinder, a heating network heater and a low pressure cylinder, wherein an output pipeline of the medium pressure cylinder is divided into two paths, one path is a heating pipeline and is communicated with the heating network heater. The other path is a steam communication pipeline which is communicated with the low-pressure cylinder, and a communication pipe valve is arranged on the steam communication pipeline. A steam bypass is arranged between the intermediate pressure cylinder and the low pressure cylinder, a bypass valve is arranged on the steam bypass, and the bypass valve is connected with the communicating pipe valve in parallel.

In the prior art, the opening degree of the communication pipe valve is controlled by the middle discharge pressure, the opening degree of the bypass valve is controlled by the flow control of the steam bypass, and when the middle discharge pressure is reduced to the pressure corresponding to the minimum allowable opening degree of the communication pipe valve, the steam flowing from the intermediate pressure cylinder to the low pressure cylinder completely passes through the steam bypass. When the exhaust pressure is low, the specific volume of the steam is large, which causes large volume flow in the steam bypass, exceeding the design value of the steam bypass and causing pipeline vibration. The control of the bypass valve and the communicating pipe valve is based on different parameters and has complex control logic, and frequent valve action is easily caused because of no compensation between the bypass valve and the communicating pipe valve. The bypass valve is always in an open state, and when the steam turbine breaks down, the steam in the pipeline flows backwards through the steam bypass and enters the low-pressure cylinder, so that the risk of unit overspeed is caused.

Therefore, there is a need to provide an improved solution to the above-mentioned deficiencies of the prior art.

SUMMERY OF THE UTILITY MODEL

An object of the utility model is to provide a steam turbine and steam turbine low pressure cylinder cut jar method to solve the pipeline of present steam turbine and shake easily, the control logic of valve module is complicated and frequently move, and when the steam turbine breaks down, steam flows backward through the steam bypass in the pipeline and gets into the low pressure cylinder, leads to the unit to have the problem of overspeed risk.

In order to achieve the above object, the present invention provides the following technical solutions:

the utility model provides a steam turbine, include: the steam communication pipeline is communicated with the intermediate pressure cylinder and the low pressure cylinder, and a communication pipe valve is arranged on the steam communication pipeline; the steam bypass pipeline is communicated with the intermediate pressure cylinder and the low pressure cylinder, a bypass valve is arranged on the steam bypass pipeline, and the communication pipe valve is connected with the bypass valve in parallel; a steam bypass line between the bypass valve and the intermediate pressure cylinder branches off to a steam line to the heat supply network heater.

In the steam turbine as described above, it is preferable that: the steam turbine is a 600MW steam turbine, and the communication pipe valve and the bypass valve are butterfly valves.

In the steam turbine as described above, it is preferable that: the communication pipe valve and the bypass valve have two valve opening control modes as follows: the first mode is that the valve opening of the communicating pipe valve is automatically adjusted according to the middle exhaust pressure; and a second mode in which the valve opening of the communication pipe valve is manually adjusted, wherein when the communication pipe valve is in the first mode, the bypass valve is in a closed state, and when the bypass valve is in the first mode, the communication pipe valve is in the second mode.

In the steam turbine as described above, it is preferable that: the valve opening degree in the first mode is adjusted according to the intermediate discharge pressure, when the intermediate discharge pressure is greater than a given target value, the opening degree of the bypass valve or the communication pipe valve is increased, and when the intermediate discharge pressure is less than the given target value, the opening degree of the bypass valve or the communication pipe valve is decreased; the given target value can be dynamically adjusted.

In the steam turbine as described above, it is preferable that: when the communication pipe valve is in the second mode, the adjustment basis for the given target value is: the sum of the flow in the steam communication pipeline and the flow in the steam bypass pipeline is ensured to be constant.

In the steam turbine as described above, it is preferable that: the given target value is controlled by a PID controller.

In the steam turbine as described above, it is preferable that: the low pressure cylinders are provided with N, N is a positive integer greater than or equal to 2 and is respectively a first low pressure cylinder, a second low pressure cylinder, … … and an Nth low pressure cylinder, the communication pipe valve comprises a first communication pipe valve, a second communication pipe valve, … … and an Nth communication pipe valve, the bypass valve comprises a first bypass valve, a second bypass valve, … … and an Nth bypass valve, the first communication pipe valve and the first bypass valve are connected between the medium pressure cylinder and the first low pressure cylinder in parallel, the second communication pipe valve and the second bypass valve are connected between the medium pressure cylinder and the second low pressure cylinder in parallel, … … is connected between the medium pressure cylinder and the Nth low pressure cylinder in parallel, the Nth communication pipe valve and the Nth bypass valve are connected between the medium pressure cylinder and the Nth low pressure cylinder in parallel, and flow into the first low pressure cylinder through the first communication pipe valve, the second communication pipe valve, … … and the Nth communication pipe valve respectively, The steam flow rates of the second low pressure cylinder, … …, and the nth low pressure cylinder are the same, and the steam flow rates flowing into the first low pressure cylinder, the second low pressure cylinder, … …, and the nth low pressure cylinder through the first bypass valve, the second bypass valve, … …, and the nth bypass valve are the same, respectively.

The utility model also provides a cut jar method based on as above steam turbine, cut jar method includes following step: a cylinder cutting step, namely, the bypass valve is in a closed state, the opening degree of the communication pipe valve is automatically adjusted by taking the middle exhaust pressure of the intermediate pressure cylinder as a target, and steam enters the low pressure cylinder from the intermediate pressure cylinder only through the communication pipe valve; a transition step of opening the bypass valve to enable the communicating pipe valve to be adjusted to a manual mode, gradually reducing the opening degree of the communicating pipe valve from a first opening degree value to zero, and adjusting the opening degree of the bypass valve to a second opening degree value according to the intermediate pressure of the intermediate pressure cylinder in the process of reducing the opening degree of the communicating pipe valve to zero; and a cylinder cutting step, namely, completely closing the communicating pipe valve, wherein the steam enters the low pressure cylinder from the intermediate pressure cylinder only through a bypass valve, and the opening degree of the bypass valve is dynamically adjusted according to the intermediate discharge pressure.

In the cylinder cutting method of the steam turbine as described above, it is preferable that: in the step of transition, the opening degree of the bypass valve is adjusted according to the intermediate discharge pressure of the intermediate pressure cylinder in the process of reducing the opening degree of the communication pipe valve to zero, after the opening degree of the communication pipe valve is zero and closed, the opening degree of the bypass valve is adjusted to a second opening value, and then the opening degree of the bypass valve is automatically adjusted by taking the intermediate discharge pressure of the intermediate pressure cylinder as a target.

In the cylinder cutting method of the steam turbine as described above, it is preferable that: in the transition step, the steam enters the low pressure cylinder through the communication pipe valve and the bypass valve, and the instantaneous total steam flow entering the low pressure cylinder is equal to the instantaneous steam flow flowing into the low pressure cylinder by the communication pipe valve in the cylinder switching preparation step at the first opening degree.

In the cylinder cutting method of the steam turbine as described above, it is preferable that: when the bypass valve is at the second opening value, the steam flow which instantaneously flows into the low pressure cylinder through the bypass valve is equal to the steam flow which instantaneously flows into the low pressure cylinder when the communication pipe valve is at the first opening in the cylinder cutting preparation step.

In the cylinder cutting method of the steam turbine as described above, it is preferable that: the first opening value is 10% and the second opening value is 30.6%.

Compared with the closest prior art, the utility model discloses use the adjustable steam extraction system of 600MW steam turbine low pressure jar overall process as the example to explain its and cut jar control method, the technical scheme based on this control method has following excellent effect:

1) under the normal motion state of the steam turbine, the bypass valve is in a closed state, and the steam in the pipeline is prevented from flowing backwards through the steam bypass pipeline and entering the low pressure cylinder, so that the unit is prevented from overspeed. Meanwhile, the steam bypass pipeline is prevented from vibrating due to the fact that the flow is too small.

2) The control of the bypass valve and the communicating pipe valve takes the middle exhaust pressure as a target, the control strategy is simple and reliable, and when the communicating pipe valve manually adjusts the opening of the valve, the dynamic compensation of the steam flow of the low-pressure cylinder can be formed when the opening of the bypass valve is controlled by taking the middle exhaust pressure as a target.

3) The opening degree of the bypass valve can be dynamically adjusted, and the air inlet of the low-pressure cylinder can be more flexibly controlled. And in the transition stage of cylinder cutting operation, the stable air input of the low-pressure cylinder is ensured, so that the stable output of the low-pressure cylinder is ensured, and the stable generating capacity of the unit is further ensured.

4) And the PID controller is adopted to control the opening of the bypass valve, so that the cost is low and the failure rate is low.

5) When the communicating pipe valve is completely closed, the opening degree of the bypass valve reaches 30.6%, and the stability of the air input of the low-pressure cylinder can be further ensured, so that the stable output of the low-pressure cylinder is ensured, and the stability of the generating capacity of the unit is further ensured.

Drawings

Fig. 1 is a schematic view of a part of the structure of the steam turbine of the present invention.

In the figure: 1. an intermediate pressure cylinder; 2. a heat supply network heater; 3. a first low pressure cylinder; 4. a second low pressure cylinder; 5. a first bypass valve; 6. a second bypass valve; 7. a first communication pipe valve; 8. a second communication pipe valve; 9. heating the pipeline; 10. a steam communication pipeline; 11. a first branch; 12. a second branch circuit; 13. a first steam bypass line; 14. a second steam bypass line.

Detailed Description

The present invention will be described in detail below with reference to the accompanying drawings in conjunction with embodiments. It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict.

In the description of the present invention, the terms "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only for convenience of description of the present invention and do not require that the present invention must be constructed and operated in a particular orientation, and therefore, should not be construed as limiting the present invention. The terms "connected" and "connected" used in the present invention should be understood in a broad sense, and may be, for example, either fixed or detachable; they may be directly connected or indirectly connected through intermediate members, and specific meanings of the above terms will be understood by those skilled in the art as appropriate.

The accompanying drawings, which form a part of the present application, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention and not to limit the invention. Wherein:

the utility model discloses use 600MW steam turbine as the example, simply explain the concrete structure of steam turbine: as shown in fig. 1, the steam turbine includes an intermediate pressure cylinder 1, a low pressure cylinder and a heat supply network heater 2, the intermediate pressure cylinder 1 is communicated with the heat supply network heater 2 through a heating pipeline 9, and the heat supply network heater 2 belongs to a shell-and-tube heat exchanger and is used for transferring heat in steam extracted from a steam communicating pipe to heat supply network circulating water. Actually, one path is extracted from the steam communicating pipe to the heat supply network heater, a branch pipe is taken out on the path to be used as a bypass pipeline, and the function of the bypass pipeline is to take out a part of the extracted steam from the heat supply network heater to be used for cooling the low pressure cylinder.

The low pressure cylinder may be one low pressure cylinder or two low pressure cylinders, such as a first low pressure cylinder 3 and a second low pressure cylinder 4, and the first low pressure cylinder 3 and the second low pressure cylinder 4 are connected in parallel in the pipeline. The steam turbine further comprises a steam communication pipeline 10, the steam communication pipeline 10 comprises a first branch 11 and a second branch 12 which are respectively communicated with the first low pressure cylinder 3 and the second low pressure cylinder 4, and the intermediate pressure cylinder 1 conveys steam to the first low pressure cylinder 3 and the second low pressure cylinder 4 through the steam communication pipeline 10. The first branch 11 is provided with a first communicating pipe valve 7, and the second branch 12 is provided with a second communicating pipe valve 8. The first communication pipe valve 7 and the second communication pipe valve 8 are collectively called communication pipe valves and are both butterfly valves.

The steam turbine further comprises a steam bypass line comprising a first steam bypass line 13 and a second steam bypass line 14. The first steam bypass pipeline 13 is communicated with the intermediate pressure cylinder 1 and the first low pressure cylinder 3 and is connected with the first branch 11 in parallel, a first bypass valve 5 is arranged on the first steam bypass pipeline 13, and the first bypass valve 5 is a butterfly valve. The second steam bypass pipeline 14 is communicated with the intermediate pressure cylinder 1 and the second low pressure cylinder 4 and is connected with a second branch in parallel, a second bypass valve 6 is arranged on the second steam bypass pipeline 14, the second bypass valve 6 is a butterfly valve, and the first bypass valve 5 and the second bypass valve 6 are collectively called bypass valves.

The opening degree of the communicating pipe valve and the opening degree of the bypass valve can be dynamically adjusted according to the medium discharge pressure, and specifically comprises the following steps: the communication pipe valve has two modes, the first mode automatically adjusts the valve opening according to the middle discharge pressure so as to ensure that the middle discharge pressure is as close to the target value as possible. The valve opening is manually adjusted in a second mode, which is generally used in the transition phase before cylinder switching, and the bypass valve is in a closed state when the communication pipe valve is in the first mode.

Although two low pressure cylinders are provided in the structure shown in fig. 1, it is also possible to have only one low pressure cylinder, such as a steam turbine, including one intermediate pressure cylinder 1, one low pressure cylinder (such as low pressure cylinder 3), and a heat network heater 2, wherein one communication pipe valve is provided on a communication pipe from the intermediate pressure cylinder 1 to the one low pressure cylinder, one bypass pipe valve (such as bypass pipe valve 5) is provided on a bypass pipe from the intermediate pressure cylinder to the one low pressure cylinder, and the heat network heater 2 is connected to the bypass pipe, that is, the one bypass pipe valve is provided between the heat network heater 2 and the one bypass pipe valve, and the one communication pipe valve and the one butterfly valve are both butterfly valves.

The valve opening of the communicating pipe valve has two control modes, the first control mode is to automatically control the valve opening of the communicating pipe valve according to the middle exhaust pressure of the intermediate pressure cylinder, and can be realized by a PID controller, the PID controller monitors the deviation of the middle exhaust pressure actual value and the target value, and transmits the deviation to a valve actuator after proportional, integral and differential processing is carried out on the deviation, so as to control the valve opening; the second control mode is to manually adjust the valve opening of the communicating pipe valve, and the manual mode is only used in the cylinder cutting transition stage. Also, generally, the bypass valve is closed when the communication tube valve is in the first mode.

In fact, the bypass valve can also be controlled in both modes, except that in the present application, the bypass valve is used only in the first control mode.

In the above configuration, it is preferable that the bore of the bypass line is designed to have a maximum volume flow rate corresponding to a minimum target value of the intermediate discharge pressure, in order to further ensure safety of the bypass line. In general, the diameter of the communication line is larger than the diameter of the bypass line, for example, the former is twice or more, and in the structure according to the present application, the diameter of the communication line is 1300mm and the diameter of the bypass line is 500 mm.

Taking the operation of cutting a single cylinder (for example, only for the first low pressure cylinder 3 and the first communication pipe valve 7 and the first bypass valve 5) as an example, the description of the cylinder cutting method of the low pressure cylinder of the steam turbine of the present invention is as follows: the cylinder cutting method for the low-pressure cylinder of the steam turbine comprises the following steps:

in step 1 (preparation stage before cylinder cutting), the opening degree of the first communicating pipe valve 7 is dynamically adjusted according to the medium discharge pressure, and the opening degree of the first communicating pipe valve 7 is always larger than a first opening degree value, which can be any value of 8-12%, such as 8%, 9%, 10%, 11%, and the like, and is preferably 10%, so that in the process, the first bypass valve 5 is in a completely closed state, steam completely enters the first low-pressure cylinder from the first communicating pipe valve, the first low-pressure cylinder stably works, that is, the first communicating pipe valve is controlled in a first mode, and the first bypass valve is in a closed state. The reason why the opening degree of the communicating pipe valve cannot be lower than the first opening degree is that the valve is washed when the opening degree of the communicating pipe valve is lower than the critical value, vibration is increased, and reliability of the valve is reduced.

Step 2 [ transition phase ], in which, as more steam needs to be input into the heating network heater to cope with more and more heating loads, the control mode of the first communication pipe valve is changed from the automatic control of the previous phase to the manual control, and the opening degree of the first communication pipe valve is gradually reduced from the first opening value to 0% (i.e. completely closed), in the process, as the intermediate discharge pressure rises, when the opening degree of the first communication pipe valve is reduced to the first opening value, for example, 10% by the manual control mode, the first bypass valve is opened, and the opening degree of the first bypass valve is automatically controlled according to the intermediate discharge pressure until the opening degree of the first bypass valve is increased from 0% to the second opening value, for example, 28 to 32.6%, preferably 30.6%, in the process, the steam enters the first low pressure cylinder 3 through the first branch 11 and the steam bypass pipe 13, respectively, the steam flow passing through the first branch 11 is continuously reduced to zero due to the gradual reduction of the valve opening, the steam flow passing through the first steam bypass pipeline 13 is increased due to the continuous increase of the valve opening until the valve opening is increased to 30.6%, in this stage, preferably, according to the condition that the sum of the steam instantaneous flow passing through the bypass valve and the steam instantaneous flow of the communication pipe valve is equal to the instantaneous steam flow when only the first communication pipe valve is opened and the opening is 10%, but the total steam flow into the low pressure cylinder remains unchanged [ i.e., two times are arbitrarily selected, such as t1 and t2, the flow rate of steam flowing into the low pressure cylinder through the communication pipe valve and the bypass valve at the instant of time t1 is equal to the flow rate of steam flowing into the low pressure cylinder through the communication pipe valve and the bypass valve at the instant of time t2 c, this step is a non-transient step in order to avoid steam impingement on the piping, low pressure cylinder and intermediate pressure cylinder. In this stage, it is preferable to slowly decrease the opening degree of the first communication pipe valve while automatically adjusting the opening degree of the first bypass valve so that the intermediate discharge pressure approaches the target value as close as possible with the target of the stable intermediate discharge pressure.

The above-mentioned instantaneous steam flow rate is the steam flow rate per unit time flowing into the low pressure cylinder through the valve.

And 3, in the cylinder cutting stage, after the opening degree of the first communicating pipe valve is completely closed, the opening degree of the first bypass valve reaches 30.6%, then the opening degree of the first bypass valve is continuously dynamically adjusted according to the intermediate discharge pressure, and the target value of the intermediate discharge pressure is kept unchanged, so that the stability of the air inflow of the first low-pressure cylinder is ensured, namely in the stage, the opening degree of the first bypass valve is automatically adjusted by taking the intermediate discharge pressure as a target.

In this embodiment, taking a double low pressure cylinder as an example, in implementation, the first bypass valve 5 and the second bypass valve 6 are collected into one bypass main valve, the first communicating pipe valve 7 and the second communicating pipe valve 8 are collected into another communicating pipe main valve, that is, the first bypass valve 5 and the second bypass valve 6 have the same structure, the first bypass valve 5 and the second bypass valve 6 are controlled by the same signal, and the amounts of instant steam passing through the first bypass valve 5 and the second bypass valve 6 are equal, that is, the amounts of steam distributed to the two low pressure cylinders are 50% and 50% respectively. The first communicating pipe valve 7 and the second communicating pipe valve 8 are identical in structure, and the first communicating pipe valve 7 and the second communicating pipe valve 8 are controlled by the same signal, and the amounts of instant steam passing through the first communicating pipe valve 7 and the second communicating pipe valve 8 are equal, i.e., the amounts of steam distributed to the two low pressure cylinders are 50% and 50% respectively. Before the cylinder cutting method is implemented, namely when the steam turbine is in a normal working state, the opening degree of the communicating pipe valve is automatically and dynamically controlled according to the middle exhaust pressure, and the stability of the air input of the low-pressure cylinder is ensured.

Namely, when the double-cylinder cutting operation is carried out, two communicating pipe valves (a first communicating pipe valve and a second communicating pipe valve) are regarded as one valve, two bypass valves (a first bypass valve and a second bypass valve) are regarded as one valve, and then the operation is carried out according to the strategy.

To sum up, the utility model discloses a steam turbine and steam turbine low pressure jar cut jar method compares prior art and has following technological effect:

2) The control of the bypass valve and the communicating pipe valve is based on the medium exhaust pressure, the control strategy is simple and reliable, and dynamic compensation is formed between the communicating pipe valve and the bypass valve.

3) The given target value of the bypass valve can be dynamically adjusted, and the air intake of the low pressure cylinder can be more flexibly controlled. And in the transition stage of cylinder cutting operation, the stable air input of the low-pressure cylinder is ensured, so that the stable output of the low-pressure cylinder is ensured, and the stable generating capacity of the unit is further ensured.

4) The PID controller is adopted to control the given target value of the bypass valve, so that the cost is low and the failure rate is low.

In short, the opening of the butterfly valve of the communicating pipe of the existing system takes the middle exhaust pressure as a control target, the butterfly valve of the bypass pipe takes the steam flow rate of the bypass pipe not to exceed a certain value as a target, so that most of the control is not problematic, but the butterfly valve of the communicating pipe needs to be directly closed when the opening of the butterfly valve of the communicating pipe is smaller than a certain value, because the small opening has adverse effect on the safe operation of the valve, usually the opening is 10%, in the process that the butterfly valve of the communicating pipe is directly closed from 10% to 0, in order to keep the steam inlet amount of the low-pressure cylinder unchanged, the butterfly valve of the bypass pipe needs to be opened, the flow rate of the bypass pipe is increased, namely, the steam flow rate of the bypass pipe is overspeed, if the steam flow rate still reaches the target value at the moment, the pipeline vibration caused by overspeed is easy to occur, and through the cylinder cutting control strategy of the present application, the control, the opening degree of the bypass pipe valve is automatically adjusted, so that the middle discharge pressure is close to the target value as much as possible, and the flow rate value is ensured to be constant.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A steam turbine, comprising:

the steam communication pipeline is communicated with the intermediate pressure cylinder and the low pressure cylinder, and a communication pipe valve is arranged on the steam communication pipeline;

the steam bypass pipeline is communicated with the intermediate pressure cylinder and the low pressure cylinder, a bypass valve is arranged on the steam bypass pipeline, and the communication pipe valve is connected with the bypass valve in parallel;

a steam bypass line between the bypass valve and the intermediate pressure cylinder branches off to a steam line to the heat supply network heater.

2. The steam turbine of claim 1, wherein:

the steam turbine is a 600MW steam turbine,

the communication pipe valve and the bypass valve are butterfly valves.

3. The steam turbine of claim 1, wherein:

the communication pipe valve and the bypass valve have two valve opening control modes as follows:

the first mode is that the valve opening of the communicating pipe valve is automatically adjusted according to the middle exhaust pressure; and

in the second mode, the valve opening degree of the communicating pipe valve is manually adjusted,

when the communication pipe valve is in the first mode, the bypass valve is in a closed state,

the communication pipe valve is in a second mode when the bypass valve is in the first mode.

4. The steam turbine of claim 3, wherein:

the valve opening in the first mode is adjusted according to the intermediate discharge pressure,

when the intermediate discharge pressure is greater than a given target value, the opening degree of the bypass valve or the communication pipe valve is increased,

when the intermediate discharge pressure is less than a given target value, the opening degree of the bypass valve or the communication pipe valve is decreased;

the given target value can be dynamically adjusted.

5. The steam turbine of claim 4,

when the communication pipe valve is in the second mode, the adjustment basis for the given target value is: the sum of the flow in the steam communication pipeline and the flow in the steam bypass pipeline is ensured to be constant.

6. The steam turbine of claim 4,

the given target value is controlled by a PID controller.

7. The steam turbine of claim 1,

the low pressure cylinders are provided with N, N is a positive integer which is more than or equal to 2 and is respectively a first low pressure cylinder, a second low pressure cylinder, and an Nth low pressure cylinder,

the communication pipe valve comprises a first communication pipe valve, a second communication pipe valve, and an Nth communication pipe valve,

the bypass valve comprises a first bypass valve, a second bypass valve, an Nth bypass valve,

the first communication pipe valve and the first bypass valve are connected in parallel between the intermediate pressure cylinder and the first low pressure cylinder,

the second communication pipe valve and the second bypass valve are connected in parallel between the intermediate pressure cylinder and the second low pressure cylinder,

······，

the Nth communication pipe valve and the Nth bypass valve are connected in parallel between the intermediate pressure cylinder and the Nth low pressure cylinder,

the steam flows into the first low pressure cylinder, the second low pressure cylinder and the Nth low pressure cylinder respectively through the first communication pipe valve, the second communication pipe valve,

the steam flows into the first low pressure cylinder, the second low pressure cylinder and the Nth low pressure cylinder through the first bypass valve, the second bypass valve and the Nth bypass valve respectively, and the steam flows into the first low pressure cylinder, the second low pressure cylinder and the Nth low pressure cylinder are the same.