Steam turbine and method for cutting cylinder of low-pressure cylinder of steam turbine

Abstract

The present invention provides a steam turbine and a cylinder cutting control method thereof, characterized in that the steam turbine comprises: a steam connecting pipeline, the steam connecting pipeline connects the intermediate pressure cylinder and the low pressure cylinder, a connecting pipe valve is arranged on the steam connecting pipeline; a steam bypass pipeline, the steam bypass pipeline connects the intermediate pressure cylinder and the low pressure cylinder, a bypass valve is arranged on the steam bypass pipeline, the connecting pipe valve and the bypass valve are connected in parallel; a steam pipeline is branched off from the steam bypass pipeline between the bypass valve and the intermediate pressure cylinder to go to the heat network heater.

Description

Technical Field

[0001] The invention relates to the field of steam turbine power generation, and in particular to a steam turbine and a method for cutting a low-pressure cylinder of the steam turbine. Background Art

[0002] The steam turbine in the prior art includes an intermediate pressure cylinder, a heat network heater and a low pressure cylinder. The output pipeline of the intermediate pressure cylinder is divided into two paths, one of which is a heating pipeline connected to the heat network heater. The other is a steam connecting pipeline, which is connected to the low pressure cylinder. A connecting pipe valve is provided on the steam connecting pipeline. A steam bypass is provided between the intermediate pressure cylinder and the low pressure cylinder. A bypass valve is provided on the steam bypass. The bypass valve is connected in parallel with the connecting pipe valve.

[0003] In the prior art, the opening of the connecting pipe valve is controlled by the intermediate pressure, and the opening of the bypass valve is controlled by the flow rate of the steam bypass. When the intermediate pressure is reduced to the pressure corresponding to the minimum allowable opening of the connecting pipe valve, the steam flowing from the intermediate pressure cylinder to the low pressure cylinder passes completely through the steam bypass. When the intermediate pressure is low, the specific volume of the steam is large, resulting in a large volume flow in the steam bypass, which exceeds the design value of the steam bypass and causes pipeline vibration. The control of the bypass valve and the connecting pipe valve is based on different parameters and the control logic is complex. There is no compensation between them, which easily leads to frequent valve movements. The bypass valve is always in the open state. When the turbine fails, there is a risk that the steam in the pipeline will flow back into the low pressure cylinder through the steam bypass, causing the unit to overspeed.

[0004] Therefore, it is necessary to provide an improved technical solution to address the above-mentioned deficiencies in the prior art. Summary of the invention

[0005] The purpose of the present invention is to provide a steam turbine and a method for cutting the low-pressure cylinder of a steam turbine, so as to solve the problems that the pipeline of the current steam turbine is easy to vibrate, the control logic of the valve assembly is complex and frequently operates, and when a fault occurs in the steam turbine, the steam in the pipeline flows back into the low-pressure cylinder through the steam bypass, resulting in the risk of overspeed of the unit.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] The present invention provides a steam turbine, comprising: a steam connecting pipeline, the steam connecting pipeline connecting an intermediate pressure cylinder and a low pressure cylinder, the steam connecting pipeline being provided with a connecting pipe valve; a steam bypass pipeline, the steam bypass pipeline connecting the intermediate pressure cylinder and the low pressure cylinder, the steam bypass pipeline being provided with a bypass valve, the connecting pipe valve and the bypass valve being connected in parallel; a steam pipeline is branched off from the steam bypass pipeline between the bypass valve and the intermediate pressure cylinder to go to a heat network heater.

[0008] In the steam turbine as described above, preferably: the steam turbine is a 600MW steam turbine, and the connecting pipe valve and the bypass valve are butterfly valves.

[0009] In the steam turbine as described above, it is preferred that: the connecting pipe valve and the bypass valve both have the following two valve opening control modes: a first mode, automatically adjusting the valve opening of the connecting pipe valve according to the middle row pressure; and a second mode, manually adjusting the valve opening of the connecting pipe valve, when the connecting 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 connecting pipe valve is in the second mode.

[0010] In the steam turbine as described above, it is preferred that: 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 of the bypass valve or the connecting pipe valve increases, and when the intermediate discharge pressure is less than a given target value, the opening of the bypass valve or the connecting pipe valve decreases; the given target value can be dynamically adjusted.

[0011] In the steam turbine as described above, preferably, when the connecting pipe valve is in the second mode, the given target value is adjusted based on ensuring that the sum of flow rates in the steam connecting pipe and the steam bypass pipe is constant.

[0012] In the steam turbine as described above, preferably, the given target value is controlled by a PID controller.

[0013] In the steam turbine as described above, preferably: the low-pressure cylinders have N, N is a positive integer greater than or equal to 2, and are respectively the first low-pressure cylinder, the second low-pressure cylinder, . . . , and the Nth low-pressure cylinder; the connecting pipe valves include the first connecting pipe valve, the second connecting pipe valve, . . . , and the Nth connecting pipe valve; the bypass valves include the first bypass valve, the second bypass valve, . . . , and the Nth bypass valve; the first connecting pipe valve and the first bypass valve are connected in parallel between the intermediate-pressure cylinder and the first low-pressure cylinder; and the connecting pipe valve and the first bypass valve are connected in parallel between the intermediate-pressure cylinder and the second low-pressure cylinder. There are the second connecting pipe valve and the second bypass valve, ..., the N connecting pipe valve and the N bypass valve are connected in parallel between the medium-pressure cylinder and the N low-pressure cylinder, and the steam flow rates flowing into the first low-pressure cylinder, the second low-pressure cylinder, ..., and the N low-pressure cylinder through the first connecting pipe valve, the second connecting pipe valve, ..., and the N connecting pipe valve respectively are the same, and the steam flow rates flowing into the first low-pressure cylinder, the second low-pressure cylinder, ..., and the N low-pressure cylinder through the first bypass valve, the second bypass valve, ..., and the N bypass valve respectively are the same.

[0014] The present invention also provides a cylinder cutting method based on the steam turbine as described above, and the cylinder cutting method comprises the following steps: a pre-cylinder cutting step, wherein the bypass valve is in a closed state, and the opening of the connecting pipe valve is automatically adjusted with the middle row pressure of the medium pressure cylinder as the target, and steam enters the low pressure cylinder from the medium pressure cylinder only through the connecting pipe valve; a transition step, wherein the bypass valve is opened, so that the connecting pipe valve is adjusted to a manual mode, and the opening of the connecting pipe valve is gradually reduced from a first opening value to zero, and in the process of reducing the opening of the connecting pipe valve to zero, the opening of the bypass valve is adjusted to a second opening value according to the middle row pressure of the medium pressure cylinder; a cylinder cutting step, wherein the connecting pipe valve is completely closed, and steam enters the low pressure cylinder from the medium pressure cylinder only through the bypass valve, and the opening of the bypass valve is dynamically adjusted according to the middle row pressure.

[0015] In the turbine cylinder cutting method as described above, it is preferred that: in the pre-cylinder cutting step, the opening of the connecting pipe valve is dynamically adjusted according to the middle row pressure of the medium pressure cylinder to ensure that the opening of the connecting pipe valve is always greater than the first opening value; in the transition step, in the process of reducing the opening of the connecting pipe valve to zero, the opening of the bypass valve is adjusted according to the middle row pressure of the medium pressure cylinder; after the opening of the connecting pipe valve is zero and closed, the opening of the bypass valve is adjusted to the second opening value; thereafter, the opening of the bypass valve is automatically adjusted with the middle row pressure of the medium pressure cylinder as the target.

[0016] In the turbine cylinder cutting method as described above, it is preferred that: in the transition step, steam enters the low-pressure cylinder through the connecting 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 when the connecting pipe valve is at the first opening in the cylinder cutting preparation step.

[0017] In the turbine cylinder cutting method as described above, it is preferred that: when the bypass valve is at the second opening value, the steam flow rate instantly flowing into the low-pressure cylinder through the bypass valve is equal to the steam flow rate instantly flowing into the low-pressure cylinder when the connecting pipe valve is at the first opening value in the cylinder cutting preparation step.

[0018] In the above-mentioned method for cutting cylinders in a steam turbine, preferably, the first opening value is 10% and the second opening value is 30.6%.

[0019] Compared with the closest prior art, the present invention takes the adjustable steam extraction system of the whole process of the low-pressure cylinder of a 600MW steam turbine as an example to illustrate its cylinder cutting control method. The technical solution based on the control method has the following excellent effects:

[0020] 1) When the turbine is in normal operation, the bypass valve is closed to prevent the steam in the pipeline from flowing back into the low-pressure cylinder through the steam bypass pipeline, thereby avoiding overspeed of the unit. At the same time, it prevents the steam bypass pipeline from vibrating due to too small flow.

[0021] 2) The control of the bypass valve and the connecting pipe valve are both based on the middle row pressure. The control strategy is simple and reliable. When the connecting pipe valve is manually adjusted to adjust the valve opening, the opening of the bypass valve is controlled based on the middle row pressure to form a dynamic compensation for the steam flow of the low-pressure cylinder.

[0022] 3) The opening of the bypass valve can be adjusted dynamically, which can more flexibly control the air intake of the low-pressure cylinder. During the transition phase of the cylinder cutting operation, the air intake of the low-pressure cylinder is kept stable, thereby ensuring the stable output of the low-pressure cylinder and the stable power generation of the unit.

[0023] 4) The PID controller is used to control the opening of the bypass valve, which has low cost and low failure rate.

[0024] 5) When the connecting pipe valve is completely closed, the opening of the bypass valve reaches 30.6%, which can further ensure the stability of the air intake of the low-pressure cylinder, thereby ensuring the stable output of the low-pressure cylinder and further ensuring the stability of the power generation of the unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a partial structural schematic diagram of the steam turbine of the present invention.

[0026] In the figure: 1, medium-pressure cylinder; 2, heating network heater; 3, first low-pressure cylinder; 4, second low-pressure cylinder; 5, first bypass valve; 6, second bypass valve; 7, first connecting pipe valve; 8, second connecting pipe valve; 9, heating pipeline; 10, steam connecting pipeline; 11, first branch; 12, second branch; 13, first steam bypass pipeline; 14, second steam bypass pipeline. DETAILED DESCRIPTION

[0027] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments. It should be noted that the embodiments and features in the embodiments of the present application can be combined with each other without conflict.

[0028] In the description of the present invention, the terms "longitudinal", "lateral", "upper", "lower", "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 accompanying drawings. They are only for the convenience of describing the present invention and do not require that the present invention must be constructed and operated in a specific orientation. Therefore, they should not be understood as limitations on the present invention. The terms "connected" and "connected" used in the present invention should be understood in a broad sense. For example, they can be fixed connections or detachable connections; they can be directly connected or indirectly connected through intermediate components. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0029] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. Among them:

[0030] The present invention takes a 600MW steam turbine as an example to briefly explain the specific structure of the steam turbine: Figure 1 As shown, the steam turbine includes an intermediate pressure cylinder 1, a low pressure cylinder and a heating network heater 2. The intermediate pressure cylinder 1 is connected to the heating network heater 2 through a heating pipeline 9. The heating network heater 2 is a shell and tube heat exchanger, and its function is to transfer the heat in the steam extracted from the steam connecting pipe to the heating network circulating water. In fact, a path is extracted from the steam connecting pipe to the heating network heater, and a branch pipe is taken out from this path as a bypass pipeline. The function of the bypass pipeline is to take out a part of the steam extracted from the heating network heater for cooling the low pressure cylinder.

[0031] The low-pressure cylinder can be one low-pressure cylinder or two low-pressure cylinders, such as the first low-pressure cylinder 3 and the second low-pressure cylinder 4, which are connected in parallel in the pipeline. The steam turbine also includes a steam connecting pipeline 10, which includes a first branch 11 and a second branch 12 respectively connected to the first low-pressure cylinder 3 and the second low-pressure cylinder 4, and the medium-pressure cylinder 1 transports steam to the first low-pressure cylinder 3 and the second low-pressure cylinder 4 through the steam connecting pipeline 10. The first branch 11 is provided with a first connecting pipe valve 7, and the second branch 12 is provided with a second connecting pipe valve 8. The first connecting pipe valve 7 and the second connecting pipe valve 8 are collectively referred to as connecting pipe valves, and both are butterfly valves.

[0032] The steam turbine further includes a steam bypass pipeline, which includes a first steam bypass pipeline 13 and a second steam bypass pipeline 14. The first steam bypass pipeline 13 connects the intermediate pressure cylinder 1 and the first low pressure cylinder 3, and is connected in parallel with the first branch 11. The first steam bypass pipeline 13 is provided with a first bypass valve 5, which is a butterfly valve. The second steam bypass pipeline 14 connects the intermediate pressure cylinder 1 and the second low pressure cylinder 4, and is connected in parallel with the second branch. The second steam bypass pipeline 14 is provided with a second bypass valve 6, which is a butterfly valve. The first bypass valve 5 and the second bypass valve 6 are collectively referred to as bypass valves.

[0033] The opening of the connecting pipe valve and the bypass valve can be dynamically adjusted according to the middle row pressure. Specifically: the connecting pipe valve has two modes. The first mode automatically adjusts the valve opening according to the middle row pressure to ensure that the middle row pressure is as close to the target value as possible. The second mode manually adjusts the valve opening. This mode is generally used in the transition stage before cutting the cylinder. When the connecting pipe valve is in the first mode, the bypass valve is in a closed state.

[0034] Despite Figure 1There are two low-pressure cylinders in the structure shown, but there may actually be only one low-pressure cylinder, for example, the steam turbine includes an intermediate-pressure cylinder 1, a low-pressure cylinder (such as low-pressure cylinder 3) and a heat network heater 2, wherein a connecting pipe valve is arranged on the connecting pipe from the intermediate-pressure cylinder 1 to the one low-pressure cylinder, a bypass pipe valve (such as bypass pipe valve 5) is arranged on the 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 arranged between the heat network heater 2 and the one bypass pipe valve, and the one connecting pipe valve and the one bypass valve are both butterfly valves.

[0035] The valve opening of the connecting pipe valve has two control modes. The first control mode is to automatically control the valve opening of the connecting pipe valve according to the middle pressure of the medium pressure cylinder. This can be achieved through a PID controller. The PID controller monitors the deviation between the actual value of the middle pressure and the target value, and transmits the deviation to the valve actuator after proportional, integral and differential processing to control the valve opening. The second control mode is to manually adjust the valve opening of the connecting pipe valve. The manual mode is only used in the transition stage of cylinder cutting. Moreover, generally speaking, when the connecting pipe valve is in the first mode, the bypass valve should be in a closed state.

[0036] In fact, the bypass valve can also be controlled in these two modes, but in this application, the bypass valve only uses the first control mode.

[0037] Moreover, in the above structure, the caliber of the bypass pipeline is preferably designed to be the maximum volume flow rate corresponding to the lowest target value of the middle row pressure, so as to further ensure the safety of the bypass pipeline. In general, the caliber of the connecting pipeline is larger than the caliber of the bypass pipeline, for example, the former is more than twice the latter. In the structure involved in this application, the caliber of the connecting pipeline is 1300mm, and the caliber of the bypass pipeline is 500mm.

[0038] Taking the operation of cutting a single cylinder (for example, only the first low-pressure cylinder 3, the first connecting pipe valve 7 and the first bypass valve 5) as an example, the method for cutting the cylinder of the low-pressure cylinder of the steam turbine of the present invention is described: The method for cutting the cylinder of the low-pressure cylinder of the steam turbine comprises the following steps:

[0039] Step 1 [Preparation stage before cutting the cylinder], the opening of the first connecting pipe valve 7 is dynamically adjusted according to the middle row pressure, and the opening of the first connecting pipe valve 7 is always greater than the first opening value. The first opening value can be any value between 8% and 12%, such as 8%, 9%, 10%, 11%, etc., preferably 10%. In this process, the first bypass valve 5 is in a completely closed state, and the steam completely enters the first low-pressure cylinder through the first connecting pipe valve. The first low-pressure cylinder works stably, that is, the first connecting pipe valve adopts the first mode control, and the first bypass valve is in a closed state. The reason why the opening of the connecting pipe valve is required not to be lower than the first opening value is that when it is lower than the critical value, the valve will be scour, the vibration will increase, and the reliability of the valve will be reduced.

[0040] Step 2 [Transition stage], during this period, since more steam needs to be input into the heat network heater to cope with the increasing heating load, the control mode of the first connecting pipe valve is changed from the automatic control of the previous stage to manual control, and the opening of the first connecting pipe valve begins to gradually decrease from the first opening value to 0% (i.e., completely closed). In this process, due to the increase in the middle row pressure, when the opening of the first connecting pipe valve is reduced to the first opening value, such as 10%, through the manual control mode, the first bypass valve is opened. At this time, the opening of the first bypass valve is automatically controlled according to the middle row pressure until the opening of the first bypass valve increases from 0% to the second opening value, such as 28-32.6%, preferably 30.6%. In this process, steam enters the first low-pressure cylinder 3 from the first branch 11 and the steam bypass pipeline 13 respectively, and the first low-pressure cylinder 3 is opened. The steam flow through the branch 11 decreases gradually until it reaches zero due to the gradual decrease of its valve opening, and the steam flow through the first steam bypass line 13 increases due to the continuous increase of its valve opening until its valve opening increases to 30.6%. In this stage, it is preferred that the sum of the instantaneous steam flow through the bypass valve and the instantaneous steam flow through the connecting pipe valve is equal to the instantaneous steam flow when only the first connecting pipe valve is opened and its opening is 10%, but the total steam flow entering the low-pressure cylinder remains unchanged [that is, any two moments are selected, such as t1 and t2, the steam flow through the connecting pipe valve and the bypass valve at the moment t1 is equal to the steam flow through the connecting pipe valve and the bypass valve at the moment t2]. In order to avoid the impact of steam on the pipeline, the low-pressure cylinder and the medium-pressure cylinder, this step is a non-instantaneous step. In this stage, it is preferred to slowly reduce the opening of the first connecting pipe valve, and at the same time, with the goal of stabilizing the middle-pressure pressure, automatically adjust the opening of the first bypass valve so that the middle-pressure pressure is as close to the target value as possible.

[0041] The instantaneous steam flow mentioned above refers to the steam flow that flows into the low-pressure cylinder through the valve per unit time.

[0042] In step 3 [cylinder cutting stage], after the opening of the first connecting pipe valve is completely closed, the opening of the first bypass valve reaches 30.6%. Thereafter, the opening of the first bypass valve continues to be dynamically adjusted according to the middle row pressure, and the target value of the middle row pressure remains unchanged, thereby ensuring the stability of the air intake volume of the first low-pressure cylinder. That is, in this stage, the opening of the first bypass valve is automatically adjusted based on the middle row pressure.

[0043] This embodiment takes the double low-pressure cylinder cutting as an example. During the implementation, the first bypass valve 5 and the second bypass valve 6 are combined into a bypass main valve, and the first connecting pipe valve 7 and the second connecting pipe valve 8 are combined into another connecting 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 amount of instantaneous steam passing through the first bypass valve 5 and the second bypass valve 6 is equal, that is, the amount of steam distributed to the two low-pressure cylinders is 50% and 50% respectively. The first connecting pipe valve 7 and the second connecting pipe valve 8 have the same structure, and the first connecting pipe valve 7 and the second connecting pipe valve 8 are controlled by the same signal, and the amount of instantaneous steam passing through the first connecting pipe valve 7 and the second connecting pipe valve 8 is equal, that is, the amount of steam distributed to the two low-pressure cylinders is 50% and 50% respectively. Before the cylinder cutting method is implemented, that is, when the steam turbine is in normal working state, the connecting pipe valve automatically and dynamically controls the opening according to the middle row pressure to ensure the stability of the low-pressure cylinder intake.

[0044] That is, when performing a double-cylinder cutting operation, it is necessary to regard the two connecting pipe valves (the first and second connecting pipe valves) as one valve, and the two bypass valves (the first and second bypass valves) as one valve, and then operate according to the above strategy.

[0045] In summary, the present invention adopts a steam turbine and a method for cutting a low-pressure cylinder of a steam turbine, which has the following technical effects compared with the prior art:

[0046] 1) When the turbine is in normal operation, the bypass valve is closed to prevent the steam in the pipeline from flowing back into the low-pressure cylinder through the steam bypass pipeline, thereby avoiding overspeed of the unit. At the same time, it prevents the steam bypass pipeline from vibrating due to too small flow.

[0047] 2) The control of the bypass valve and the connecting pipe valve is based on the middle row pressure. The control strategy is simple and reliable, and dynamic compensation is formed between the connecting pipe valve and the bypass valve.

[0048] 3) The given target value of the bypass valve can be adjusted dynamically, which can more flexibly control the air intake of the low-pressure cylinder. In the transition stage of the cylinder cutting operation, the air intake of the low-pressure cylinder is guaranteed to be stable, thereby ensuring the stable output of the low-pressure cylinder and thus ensuring the stability of the power generation of the unit.

[0049] 4) The PID controller is used to control the given target value of the bypass valve, which has low cost and low failure rate.

[0050] 5) When the connecting pipe valve is completely closed, the opening of the bypass valve reaches 30.6%, which can further ensure the stability of the air intake of the low-pressure cylinder, thereby ensuring the stable output of the low-pressure cylinder and further ensuring the stability of the power generation of the unit.

[0051] In summary, the opening of the connecting pipe butterfly valve of the existing system is controlled by the middle row pressure, and the bypass pipe butterfly valve is controlled by the steam flow rate of the bypass pipe not exceeding a certain value. There is no problem with this control most of the time, but when the opening of the connecting pipe butterfly valve is less than a certain value, it needs to be fully closed directly, because a small opening has an adverse effect on the safe operation of the valve. Usually, this opening is 10%. In the process of the connecting pipe butterfly valve being directly closed from 10% to 0, in order to keep the steam intake of the low-pressure cylinder unchanged, the bypass pipe butterfly valve needs to be opened, and the bypass pipe flow rate is increased, that is, compared with the previous moment, the steam flow rate of the bypass pipe is overspeeding. If the steam flow rate is still controlled to reach the target value at this time, pipeline vibration caused by overspeed is likely to occur. Through the cylinder cutting control strategy of the present application, in the transition stage and the subsequent cylinder cutting stage, the control mode of the bypass pipe valve is changed to take the middle row pressure as the target, and the opening of the bypass pipe valve is automatically adjusted, so that the middle row pressure is as close to the target value as possible, and the flow value is also ensured to be unchanged.

[0052] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for cutting a cylinder of a steam turbine, characterized in that: The cylinder cutting method comprises the following steps: Before the cylinder cutting step, the bypass valve is in a closed state, and the opening of the connecting pipe valve is automatically adjusted based on the middle pressure of the medium pressure cylinder. Steam enters the low pressure cylinder from the medium pressure cylinder only through the connecting pipe valve; a transition step of opening the bypass valve so that the connecting pipe valve is adjusted to a manual mode, gradually reducing the opening of the connecting pipe valve from a first opening value to zero, and in the process of reducing the opening of the connecting pipe valve to zero, adjusting the opening of the bypass valve to a second opening value according to the middle discharge pressure of the middle pressure cylinder; Cylinder cutting step: completely close the connecting pipe valve, steam only enters the low-pressure cylinder from the medium-pressure cylinder through the bypass valve, and the opening of the bypass valve is dynamically adjusted according to the middle-discharge pressure; In the pre-cylinder cutting step, the opening of the connecting pipe valve is dynamically adjusted according to the middle pressure of the medium pressure cylinder to ensure that the opening of the connecting pipe valve is always greater than the first opening value. In the transition step, in the process of reducing the opening of the connecting pipe valve to zero, the opening of the bypass valve is adjusted according to the middle pressure of the medium pressure cylinder. After the opening of the connecting pipe valve is zero and closed, the opening of the bypass valve is adjusted to a second opening value. Thereafter, the opening of the bypass valve is automatically adjusted with the middle pressure of the medium pressure cylinder as the target. The steam turbine comprises: A steam connecting pipeline, the steam connecting pipeline connecting the medium-pressure cylinder and the low-pressure cylinder, and a connecting pipe valve is provided on the steam connecting pipeline; A steam bypass pipeline, the steam bypass pipeline is connected with the medium-pressure cylinder and the low-pressure cylinder, a bypass valve is provided on the steam bypass pipeline, and the connecting pipe valve and the bypass valve are connected in parallel; A steam pipeline is branched off from the steam bypass pipeline between the bypass valve and the intermediate pressure cylinder to the heating network heater; The connecting pipe valve and the bypass valve both have the following two valve opening control modes: In the first mode, the valve opening of the connecting pipe valve is automatically adjusted according to the middle row pressure; in the second mode, the valve opening of the connecting pipe valve is manually adjusted. When the connecting pipe valve is in the first mode, the bypass valve is in a closed state. When the bypass valve is in the first mode, the connecting pipe valve is in the second mode.

2. The method for cutting a cylinder of a steam turbine according to claim 1, characterized in that: The steam turbine is a 600MW steam turbine. The connecting pipe valve and the bypass valve are butterfly valves.

3. The method for cutting a cylinder of a steam turbine according to claim 1, characterized in that: The valve opening in the first mode is adjusted according to the middle row pressure. When the intermediate discharge pressure is greater than a given target value, the opening of the bypass valve or the connecting pipe valve increases. When the intermediate discharge pressure is less than a given target value, the opening of the bypass valve or the connecting pipe valve is reduced; The given target value can be adjusted dynamically.

4. The method for cutting a cylinder of a steam turbine according to claim 3, characterized in that: When the connecting pipe valve is in the second mode, the adjustment basis of the given target value is: Ensure that the sum of the flow rates in the steam connecting pipeline and the steam bypass pipeline is constant.

5. The method for cutting a cylinder of a steam turbine according to claim 3, characterized in that: The given target value is controlled by a PID controller.

6. The method for cutting a cylinder of a steam turbine according to claim 1, characterized in that: The low-pressure cylinders have N number, where N is a positive integer greater than or equal to 2, and are respectively a first low-pressure cylinder, a second low-pressure cylinder, ..., an Nth low-pressure cylinder, The connecting pipe valve includes a first connecting pipe valve, a second connecting pipe valve, ..., an Nth connecting pipe valve, The bypass valve includes a first bypass valve, a second bypass valve, ..., an Nth bypass valve, The first connecting pipe valve and the first bypass valve are connected in parallel between the medium-pressure cylinder and the first low-pressure cylinder. The second connecting pipe valve and the second bypass valve are connected in parallel between the medium-pressure cylinder and the second low-pressure cylinder, ..., The Nth connecting pipe valve and the Nth bypass valve are connected in parallel between the medium-pressure cylinder and the Nth low-pressure cylinder. The steam flow rates flowing into the first low-pressure cylinder, the second low-pressure cylinder, ..., the Nth low-pressure cylinder through the first communicating pipe valve, the second communicating pipe valve, ..., the Nth communicating pipe valve, respectively, are the same. The steam flow rates flowing into the first low-pressure cylinder, the second low-pressure cylinder, ..., and the Nth low-pressure cylinder respectively through the first bypass valve, the second bypass valve, ..., and the Nth bypass valve are the same.

7. The method for cutting a cylinder of a steam turbine according to claim 1, characterized in that: In the transition step, steam enters the low-pressure cylinder through the connecting 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 when the connecting pipe valve is at the first opening in the cylinder cutting preparation step.

8. The method for cutting a cylinder of a steam turbine according to claim 1, characterized in that: When the bypass valve is at the second opening value, the steam flow rate instantly flowing into the low-pressure cylinder through the bypass valve is equal to the steam flow rate instantly flowing into the low-pressure cylinder when the connecting pipe valve is at the first opening in the cylinder cutting preparation step.

9. The method for cutting a cylinder of a steam turbine according to claim 1, characterized in that: The first opening value is 10%, The second opening value is 30.6%.

Images