Backpressure switching optimization control process applied to condensing steam turbine

Abstract

The invention relates to the technical field of optimal control over circulation of condensing steam turbines in the power generation process of large heat energy such as fire coal, fuel gas and nuclear energy, and discloses a backpressure switching optimal control technology applied to a condensing steam turbine. Comprising the following steps: a step of establishing a cold circulating water temperature control model: establishing a condenser cold circulating water temperature control model used for determining a relationship among a field environment temperature, a field wind speed, a unit load state and cold circulating water opening and closing time; and a backpressure adjustment model establishment step: controlling opening and closing of at least one vacuum pump inlet communication door according to the cold circulating water temperature control model and the blocking backpressure value of the steam turbine, so that the vacuum pressure between the high-pressure-side condenser and the low-pressure-side condenser is kept consistent. From implementation of technical measures of single and double backpressure switching and vacuum pump operation optimization, it is checked that the final-stage blade condition of the low-pressure cylinder of the steam turbine is good, and no obvious water erosion condition exists.

Description

Technical Field

[0001] The present invention relates to the technical field of optimized control of condensing steam turbine cycles in large-scale thermal power generation processes such as coal-fired, gas-fired and nuclear energy, and in particular to a back pressure switching optimized control process applied to condensing steam turbines. Background Art

[0002] Steam turbines are often used in thermal power generation companies to generate electricity or drive compressors. However, the steam that has done work in the turbine needs to be discharged into the condenser to condense into water, and then returned to the boiler for recycling. In recent years, air cooling systems have been the main cooling method for thermal power plants in northern China, including direct air cooling systems and indirect air cooling systems. Indirect air cooling systems are generally configured. Indirect air cooling systems have the advantages of low noise, little impact from ambient wind, and low power consumption. The cooling water of the indirect air cooling system is completely closed during the circulation process, and the water loss is small. In theory, the system water consumption is zero, and the water-saving effect is significant.

[0003] The indirect air-cooling system uses an indirect-cooled reaction-condensing steam turbine, in which the exhaust gas from the low-pressure cylinder enters the A and B condensers respectively, and the circulating water passes through the A and B condensers in series, enters from the low back-pressure condenser A, and the outlet water enters the high back-pressure condenser B. After the heat exchange is completed, it is transported to the indirect cooling tower for cooling through the indirect cooling circulating water pump.

[0004] In winter, due to the gradual decrease in winter ambient temperature in northern regions, the exhaust temperature of the low-pressure condenser in the indirect air cooling system will gradually decrease. When the exhaust temperature of the low-pressure condenser decreases, the back pressure value of the low-pressure condenser will gradually decrease. At this time, the superheated steam in the low-pressure cylinder may be converted into saturated steam or even wet steam after the steam works, causing water erosion of the last-stage blades of the low-pressure cylinder of the turbine. The turbulent steam flow formed behind the last-stage blades may also increase the vibration of the turbine, posing a threat to the safe operation of the turbine.

[0005] In addition, the back pressure value of the turbine condenser is reduced, the thermal efficiency of the cycle is increased, and the steam intake of the turbine is reduced under the same power generation, so that the steam flow rate at the turbine outlet is close to the back pressure at the sonic level at that point, resulting in turbine blocking back pressure, which makes the low-pressure condenser back pressure lower than the corresponding load blocking back pressure value; when the temperature is low in winter, the circulating water temperature can be controlled over a large range, but because the turbine is limited by the condenser blocking back pressure, the operating personnel are required to close the intercooler shutters to increase the circulating water temperature and control the condenser back pressure value. This operation causes the average back pressure of the unit to increase, the coal consumption of the unit to increase, and the economy to decrease. Summary of the invention

[0006] The present invention is intended to provide a back pressure switching optimization control process for condensing steam turbines. The inventor has conducted research and development on the optimization control of the back pressure or vacuum of an indirect air-to-condenser to solve the technical problem that the condenser operates with double back pressure in winter, and the low-pressure condenser operates with a back pressure lower than the corresponding load blocking back pressure value, which will lead to poor economy and bring the risk of water erosion damage to the last stage blades of the low-pressure cylinder.

[0007] In order to achieve the above object, the present invention adopts the following technical scheme:

[0008] 1) A back pressure switching optimization control process applied to a condensing steam turbine includes the following steps:

[0009] The steps of establishing the cold circulating water temperature control model are as follows: establishing a condenser cold circulating water temperature control model for determining the relationship between the on-site ambient temperature, on-site wind speed, unit load state and the cold circulating water start and stop time;

[0010] Back pressure adjustment model establishment steps: According to the cold circulating water temperature control model and combined with the turbine blocking back pressure value to control the opening and closing of at least one vacuum pump inlet communication door, so that the vacuum pressure between the high-pressure side condenser and the low-pressure side condenser remains consistent.

[0011] The present invention realizes on-site temperature control of cold circulating water by establishing a cold circulating water opening and closing model, and controls the cold circulating water sausage temperature to control the condenser back pressure value, thereby realizing real-time adjustment of the condenser back pressure value according to the cold circulating water temperature control model, and synchronously opening and closing the vacuum pump inlet communication door between the high-pressure side condenser and the low-pressure side condenser in combination with the turbine blocking back pressure value. After opening and closing, the vacuum pressure between the high-pressure side condenser and the low-pressure side condenser is kept consistent, and finally reaches a balanced state, and changes to single back pressure operation. The low-pressure condenser back pressure rises, and the high-pressure condenser back pressure decreases. Under the condition of not lower than the blocking back pressure, the circulating water temperature can be further reduced, and the average back pressure of the unit can be reduced to an economic value closer to the blocking back pressure.

[0012] 2) The back pressure switching optimization control process for a condensing steam turbine according to 1), wherein:

[0013] The cold circulating water temperature control model includes: monitoring the on-site ambient temperature in real time through a temperature sensor, and calculating the cooling capacity of the cold circulating water when the ambient temperature is greater than or equal to 2°C:

[0014] Q=Cp×r×Vs×ΔT

[0015] Where: Q is the heat load in the circulating water connecting pipe between the high-pressure side condenser and the low-pressure side condenser (KW);

[0016] Cp is the specific heat at constant pressure (KJ / kg.℃); r is the specific gravity (Kg / m 3 ); Vs is the water flow rate (m3 / h); ΔT is the water temperature difference (°C), the temperature value of the cold circulating water outlet of the high-pressure side condenser, and the temperature value of the cold circulating water inlet of the low-pressure side condenser.

[0017] 3) The back pressure switching optimization control process for a condensing steam turbine according to 1), wherein:

[0018] The cold circulating water temperature control model includes: monitoring the on-site environmental wind speed in real time through a wind speed measuring instrument, adjusting the indirect cooling louvers according to the wind speed, the indirect cooling louvers are connected to a servo motor for synchronous rotation, and the servo motor drives the spacing between all blades at the outlet of the last stage blade of the turbine to adjust the air intake.

[0019] The wind speed measuring instrument designed in the present invention is used to monitor the wind speed conditions of the on-site environment to automatically adjust the intercooling louvers. The rotation of the blades at the outlet of the last-stage blades of the steam turbine is controlled by a servo motor, so that the spacing between the blades can be flexibly changed, the air intake volume of the air inlet can be changed, and turbulent steam flow can be prevented from being generated inside the air inlet.

[0020] 4) The back pressure switching optimization control process for a condensing steam turbine according to 1), wherein:

[0021] The unit load state in the cold circulating water temperature control model includes the following steps:

[0022] S1. Establish a flow pressure model for determining the relationship between the cooling medium flow rate of the air-cooling fan, the fan inlet pressure, the fan pressure head and the condenser heat exchanger resistance, that is, the condenser aerodynamic model;

[0023] S2, establishing a fan energy consumption model for determining the relationship between the power consumption of the air cooling fan and the rotation speed and the cooling medium flow rate;

[0024] S3, establishing a fan aerodynamic model for determining the relationship between the air cooling fan pressure head and the rotation speed and the cooling medium flow rate;

[0025] S4. The flow pressure model, the fan energy consumption model and the fan aerodynamic model together constitute a fan model.

[0026] The main problem with air cooling systems is that climatic conditions such as wind force and direction directly affect the flow and temperature of the cooling medium, including the operating status of the fan. However, these effects cannot be effectively tracked and measured. Since it is impossible to determine the flow and temperature of the cooling medium under the influence of environmental conditions, the problem cannot be solved.

[0027] The present invention establishes a complete fan model to monitor the load status of the unit, and uses each fan as a working machine and a measuring device at the same time. The power of the fan can be calculated by current, voltage and power factor. At a certain speed, the flow rate and power of the fan have a certain relationship. The status of each fan in the air cooling system is used to obtain the difficult-to-measure time-varying factors of the environmental conditions, including the fan inlet pressure, air flow and inlet average temperature. This method solves the measurement problem of key data such as total air volume and inlet average temperature in the air cooling system.

[0028] 5) The back pressure switching optimization control process for a condensing steam turbine according to 4), wherein:

[0029] The fan energy consumption model of the air cooling fan power consumption and the relationship between the speed and the cooling medium flow is established, which specifically includes the following steps:

[0030] The fan power is equal to the product of the power function of the fan speed and the power function of the air flow rate.

[0031] 6) The back pressure switching optimization control process for a condensing steam turbine according to 4), wherein:

[0032] The fan aerodynamic model of the relationship between the air cooling fan pressure head and the speed and cooling medium flow rate is established, which specifically includes the following steps:

[0033] The fan pressure head is equal to the sum of the power function of the fan speed and the power function of the air flow rate.

[0034] 7) The back pressure switching optimization control process for a condensing steam turbine according to 1), wherein:

[0035] The back pressure adjustment model includes the following steps:

[0036] S1. Calculate the cooling capacity of the cold circulating water through the cold circulating water temperature control model, adjust the air intake of the intercooling louvers, and the load status of the unit, and adjust the steam flow of the high-pressure side condenser and the low-pressure side condenser according to the above data;

[0037] S2, through the steam flow combined with the turbine blocking back pressure value, issues instructions to open and close the electric valve set at the vacuum pump inlet contact door;

[0038] S3. Open the electric valve of at least one vacuum pump inlet communication door to connect the high-pressure condenser and the low-pressure condenser to each other, so that the high-pressure condenser and the low-pressure condenser are under the same vacuum pressure, and keep the back pressure values ​​of the high-pressure condenser and the low-pressure condenser not lower than the blocking back pressure value of the unit load state.

[0039] The present invention starts and stops the intercooling circulating water pump according to the on-site ambient temperature, wind speed and unit load status, opens and closes at least one vacuum pump inlet communication door by referring to the blocking back pressure value and combining the change trend of the above three parameters, stops one vacuum pump and switches to a single back pressure operation mode or starts one vacuum pump and switches to a double back pressure operation mode. The single and double back pressure switching of the condenser not only meets the purpose of deeply reducing the back pressure, but also avoids the low-pressure cylinder turbine from operating under the blocking back pressure, ensuring the unit's economy while also ensuring the safe operation of the turbine's low-pressure cylinder last-stage blades.

[0040] Compared with the prior art, the present invention also has the following technical effects:

[0041] The present invention uses the single and double back pressure switching of the condenser as an operation technical measure, without consuming equipment consumables or increasing energy consumption. During the single and double back pressure switching process, the low-pressure condenser back pressure rises, and the high-pressure condenser back pressure drops, and finally reaches a corresponding equilibrium state, without affecting the safe operation. The operation is simple and applicable to all double back pressure condenser units. Since the implementation of the single and double back pressure switching and vacuum pump operation optimization technical measures, it has been checked that the last stage blades of the low-pressure cylinder of the turbine are in good condition and there is no obvious water erosion. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a flow chart of the back pressure switching optimization control process applied to the condensing steam turbine of the present invention;

[0043] Figure 2 It is a structural schematic diagram of the back pressure switching optimization control process applied to a condensing steam turbine according to the present invention. DETAILED DESCRIPTION

[0044] The following is further described in detail through specific implementation methods:

[0045] The reference numerals in the drawings of the specification include: a high-pressure side condenser 1, a low-pressure side condenser 2, a first electric valve 3, a second electric valve 4, an IA vacuum pump 5, an IB vacuum pump 6, and an IC vacuum pump 7.

[0046] Now will be referred to in detail the embodiments disclosed in the present invention, although the present disclosure will be described in conjunction with the embodiments and / or examples, they are not intended to limit the present disclosure to these embodiments and / or examples. On the contrary, the present disclosure covers alternatives, modifications and equivalents.

[0047] Example 1

[0048] See also Figure 1 As shown, the back pressure switching optimization control process applied to the condensing steam turbine in this embodiment includes the following steps:

[0049] The steps of establishing the cold circulating water temperature control model are as follows: establishing a condenser cold circulating water temperature control model for determining the relationship between the on-site ambient temperature, on-site wind speed, unit load state and the cold circulating water start and stop time;

[0050] Back pressure adjustment model establishment steps: According to the cold circulating water temperature control model and combined with the turbine blocking back pressure value, at least one vacuum pump inlet communication door is controlled to open and close so that the vacuum pressure between the high-pressure side condenser 1 and the low-pressure side condenser 2 remains consistent.

[0051] By establishing a cold circulating water opening and closing model, the on-site temperature control of the cold circulating water is realized. The cold circulating water sausage temperature control is used to control the condenser back pressure value, so as to adjust the condenser back pressure value in real time according to the cold circulating water temperature control model, and in combination with the turbine blocking back pressure value, the vacuum pump inlet communication door between the high-pressure side condenser 1 and the low-pressure side condenser 2 is opened and closed synchronously. After opening and closing, the vacuum pressure between the high-pressure side condenser 1 and the low-pressure side condenser 2 is kept consistent, and finally reaches a balanced state, changing to single back pressure operation. The low-pressure condenser back pressure rises and the high-pressure condenser back pressure decreases. Under the condition of not lower than the blocking back pressure, the circulating water temperature can be further reduced to reduce the average back pressure of the unit to an economic value closer to the blocking back pressure.

[0052] In the above cold circulating water temperature control model, the temperature sensor is used to monitor the on-site ambient temperature in real time. When the ambient temperature is greater than or equal to 2°C, the cooling capacity of the cold circulating water is calculated:

[0053] Q=Cp×r×Vs×ΔT

[0054] Where: Q is the heat load in the circulating water connecting pipe between the high-pressure side condenser 1 and the low-pressure side condenser 2 (KW);

[0055] Cp is the specific heat at constant pressure (KJ / kg.℃); r is the specific gravity (Kg / m 3 ); Vs is the water flow rate (m 3 / h); ΔT is the water temperature difference (°C), the temperature value of the cold circulating water outlet of the high-pressure side condenser 1, and the temperature value of the cold circulating water inlet of the low-pressure side condenser 2.

[0056] At the same time, in this embodiment, a wind speed measuring instrument is used to monitor the on-site environmental wind speed in real time, and the indirect cooling louvers are adjusted according to the wind speed. The indirect cooling louvers are connected to a servo motor for synchronous rotation, and the servo motor drives the spacing between all blades at the outlet of the last stage blades of the turbine to adjust the air intake.

[0057] The output shaft of the servo motor is fixedly connected to the side of the last-stage blade of the steam turbine, driving the last-stage blade of the steam turbine to flip around the radial direction of the air inlet of the steam turbine to adjust the inclination angle of the last-stage blade of the steam turbine. In addition, the wind speed conditions of the on-site environment are monitored by an anemometer to automatically adjust the intercooling shutters. The rotation of the blades at the outlet of the last-stage blade of the steam turbine is controlled by the servo motor, which can flexibly change the spacing between the blades, change the air intake volume of the air inlet, and prevent turbulent steam flow inside the air inlet.

[0058] In addition, the unit load state in the cold circulating water temperature control model in this embodiment includes the following steps:

[0059] S1. Establish a flow pressure model for determining the relationship between the cooling medium flow rate of the air-cooling fan, the fan inlet pressure, the fan pressure head and the condenser heat exchanger resistance, namely, the condenser aerodynamic model; wherein the fan inlet pressure is equal to the difference between the heat exchanger pressure loss and the fan pressure head.

[0060] S2. Establishing a fan energy consumption model for determining the relationship between the power consumption of the air-cooling fan and the rotation speed and the cooling medium flow rate; wherein the fan energy consumption model specifically includes the following steps:

[0061] The fan power is equal to the product of the power function of the fan speed and the power function of the air flow rate; wherein the exponent of the power function of the fan speed ranges from 2.3 to 2.6; and the exponent of the power function of the air flow rate ranges from 1.7 to 2.0.

[0062] S3, establishing a fan aerodynamic model for determining the relationship between the air cooling fan pressure head and the rotation speed and the cooling medium flow rate; wherein the fan aerodynamic model specifically includes the following steps:

[0063] The fan pressure head is equal to the sum of the power function of the fan speed and the power function of the air flow rate; wherein the exponent of the power function of the fan speed ranges from 1.7 to 2.0; the exponent of the power function of the air flow rate ranges from 1.7 to 2.0, and the power function has a coefficient that is proportional to the speed.

[0064] S4. The above-mentioned flow pressure model, fan energy consumption model and fan aerodynamic model are combined to form a fan model.

[0065] The main problem with air cooling systems is that climatic conditions such as wind force and direction directly affect the flow and temperature of the cooling medium, including the operating status of the fan. However, these effects cannot be effectively tracked and measured. Since it is impossible to determine the flow and temperature of the cooling medium under the influence of environmental conditions, the problem cannot be solved.

[0066] The present invention establishes a complete fan model to monitor the load status of the unit, and uses each fan as a working machine and a measuring device at the same time. The power of the fan can be calculated by current, voltage and power factor. At a certain speed, the flow rate and power of the fan have a certain relationship. The status of each fan in the air cooling system is used to obtain the difficult-to-measure time-varying factors of the environmental conditions, including the fan inlet pressure, air flow and inlet average temperature. This method solves the measurement problem of key data such as total air volume and inlet average temperature in the air cooling system.

[0067] Example 2

[0068] See also Figure 2 As shown, on the basis of Example 1, the back pressure adjustment model in this embodiment includes a first electric valve 3 and a second electric valve 4 arranged on the circulating water connecting pipe, and includes an IA vacuum pump 5, an IB vacuum pump 6 and an IC vacuum pump 7, wherein the first electric valve 3 corresponds to the IA vacuum pump 5, and the second electric valve 4 corresponds to the IB vacuum pump 6. When normally used, the IC vacuum pump 7 is connected to the high-pressure side condenser 1 and the low-pressure side condenser 2 through the circulating water connecting pipe, respectively, in a double back pressure state; when the first electric valve 3 is opened, the IA vacuum pump 5 is connected to the IC vacuum pump 7 and acts on the high-pressure side condenser 1 and the low-pressure side condenser 2, so that the high-pressure side condenser 1 and the low-pressure side condenser 2 maintain a consistent vacuum pressure; or the second electric valve 4 is opened, so that the IB vacuum pump 6 is connected to the IC vacuum pump 7 and acts on the high-pressure side condenser 1 and the low-pressure side condenser 2, so that the high-pressure side condenser 1 and the low-pressure side condenser 2 maintain a consistent vacuum pressure.

[0069] The backpressure adjustment model establishment includes the following steps:

[0070] S1. Calculate the cooling capacity of the cold circulating water through the cold circulating water temperature control model, adjust the air intake of the intercooling louvers, and the load status of the unit, and adjust the steam flow of the high-pressure side condenser 1 and the low-pressure side condenser 2 according to the above data;

[0071] S2, through the steam flow combined with the turbine blocking back pressure value, issues instructions to open and close the electric valve set at the vacuum pump inlet contact door;

[0072] Our company provides the following table of blocking back pressure values ​​under various load conditions:

[0073] Steam flow-blocking back pressure correspondence table

[0074] Steam flow 1200 1300 1400 1500 1600 1700 Blocking backpressure 4.6 4.65 4.75 4.8 4.9 5 Steam flow 1800 1900 2000 2100 2200 2300 Blocking backpressure 5.1 5.2 5.25 5.35 5.45 5.55 Steam flow 2400 2500 2600 2700 2800 2900 Blocking backpressure 5.65 5.75 5.85 5.9 5.95 6.1

[0075] S3. Open at least one electric valve of the vacuum pump inlet communication door to connect the high-pressure side condenser 1 and the low-pressure side condenser 2 to each other, so that the high-pressure side condenser 1 and the low-pressure side condenser 2 are under the same vacuum pressure, and the back pressure values ​​of the high-pressure side condenser 1 and the low-pressure side condenser 2 are maintained not lower than the blocking back pressure value of the unit load state.

[0076] The intercooling circulating water pump is started and stopped according to the on-site ambient temperature, wind speed and unit load status. At least one vacuum pump inlet contact door is opened and closed in accordance with the blocking back pressure value and the change trend of the above three parameters. A vacuum pump is shut down and switched to single back pressure operation mode or a vacuum pump is started and switched to double back pressure operation mode. The single and double back pressure switching of the condenser not only meets the purpose of deeply reducing the back pressure, but also avoids the low-pressure cylinder turbine from operating under blocking back pressure, ensuring the unit's economy while also ensuring the safe operation of the turbine's low-pressure cylinder last-stage blades.

[0077] In this regard, the coal-saving benefits of this invention after being applied to our company are:

[0078] When the condenser back pressure is lower than the blocking value, it switches to single back pressure operation, and the average back pressure of a single unit drops by 0.5kPa. According to the coal consumption impact value of the unit performance test, when the back pressure is below 8.5kPa, each 1kPa of back pressure affects the coal consumption by 0.751g / kWh. According to the winter working conditions, the daily electricity consumption is 40,000MWh for 5 months. The measure is used for 6 hours every day. The power generation of the two units is 1.5 billion kWh, and the annual standard coal saving is 563.25 tons. The fuel cost is saved by about 4.16805 million yuan at the standard coal price of 740 yuan / ton.

[0079] In terms of carbon emissions, based on the calculation of 2.6 tons of carbon emissions per ton of standard coal, carbon emissions can be reduced by approximately 1,464.45 tons.

[0080] Power saving benefits after single and double back pressure switching combined with vacuum pump optimization:

[0081] By shutting down one vacuum pump during single back-pressure operation, the two units can save about 155,430 kWh of factory electricity each year. Based on the on-grid electricity price of 0.3 yuan / kWh, the five-month winter operation period can save about 466,920 yuan in factory electricity costs.

[0082] The above are only embodiments of the present invention, and the common knowledge such as the known specific technical solutions and / or characteristics in the solution are not described in detail here. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the technical solution of the present invention, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A back pressure switching optimization control process applied to a condensing steam turbine, characterized in that: The following steps are involved: The steps of establishing the cold circulating water temperature control model are as follows: establishing a condenser cold circulating water temperature control model for determining the relationship between the on-site ambient temperature, on-site wind speed, unit load state and the cold circulating water start and stop time; Back pressure adjustment model establishment steps: According to the cold circulating water temperature control model and combined with the turbine blocking back pressure value to control the opening and closing of at least one vacuum pump inlet communication door, so that the vacuum pressure between the high-pressure side condenser and the low-pressure side condenser remains consistent.

2. The back pressure switching optimization control process for a condensing steam turbine according to claim 1, characterized in that: The cold circulating water temperature control model includes: monitoring the on-site ambient temperature in real time through a temperature sensor, and calculating the cooling capacity of the cold circulating water when the ambient temperature is greater than or equal to 2°C: Q=Cp×r×Vs×ΔT Where: Q is the heat load in the circulating water connecting pipe between the high-pressure side condenser and the low-pressure side condenser (KW); Cp is the specific heat at constant pressure (KJ / kg.℃); r is the specific gravity (Kg / m 3 ); Vs is the water flow rate (m 3 / h); ΔT is the water temperature difference (°C), the temperature value of the cold circulating water outlet of the high-pressure side condenser, and the temperature value of the cold circulating water inlet of the low-pressure side condenser.

3. The back pressure switching optimization control process for a condensing steam turbine according to claim 1, characterized in that: The cold circulating water temperature control model includes: monitoring the on-site environmental wind speed in real time through a wind speed measuring instrument, adjusting the indirect cooling louvers according to the wind speed, the indirect cooling louvers are connected to a servo motor for synchronous rotation, and the servo motor drives the spacing between all blades at the outlet of the last stage blade of the turbine to adjust the air intake.

4. The back pressure switching optimization control process for a condensing steam turbine according to claim 1, characterized in that: The unit load state in the cold circulating water temperature control model includes the following steps: S1. Establish a flow pressure model for determining the relationship between the cooling medium flow rate of the air-cooling fan, the fan inlet pressure, the fan pressure head and the condenser heat exchanger resistance, that is, the condenser aerodynamic model; S2, establishing a fan energy consumption model for determining the relationship between the power consumption of the air cooling fan and the rotation speed and the cooling medium flow rate; S3, establishing a fan aerodynamic model for determining the relationship between the air cooling fan pressure head and the rotation speed and the cooling medium flow rate; S4. The flow pressure model, the fan energy consumption model and the fan aerodynamic model together constitute a fan model.

5. The back pressure switching optimization control process for a condensing steam turbine according to claim 4, characterized in that: The fan energy consumption model of the air cooling fan power consumption and the relationship between the speed and the cooling medium flow is established, which specifically includes the following steps: The fan power is equal to the product of the power function of the fan speed and the power function of the air flow rate.

6. The back pressure switching optimization control process for a condensing steam turbine according to claim 4, characterized in that: The fan aerodynamic model of the relationship between the air cooling fan pressure head and the speed and cooling medium flow rate is established, which specifically includes the following steps: The fan pressure head is equal to the sum of the power function of the fan speed and the power function of the air flow rate.

7. The back pressure switching optimization control process for a condensing steam turbine according to claim 1, characterized in that: The back pressure adjustment model includes the following steps: S1. Calculate the cooling capacity of the cold circulating water through the cold circulating water temperature control model, adjust the air intake of the intercooling louvers, and the load status of the unit, and adjust the steam flow of the high-pressure side condenser and the low-pressure side condenser according to the above data; S2, through the steam flow combined with the turbine blocking back pressure value, issues instructions to open and close the electric valve set at the vacuum pump inlet contact door; S3. Open the electric valve of at least one vacuum pump inlet communication door to connect the high-pressure condenser and the low-pressure condenser to each other, so that the high-pressure condenser and the low-pressure condenser are under the same vacuum pressure, and keep the back pressure values ​​of the high-pressure condenser and the low-pressure condenser not lower than the blocking back pressure value of the unit load state.

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