Cascade control method and system for reheating steam temperature of coal-fired power generating unit

Abstract

The invention belongs to the technical field related to boiler control, and discloses a coal-fired power generation unit reheating steam temperature cascade control method and system. The control method comprises the following steps that the deviation between the preset steam temperature and the actually-measured steam temperature of an outlet of the reheater to be controlled is calculated, and the ideal flue gas temperature corresponding to the minimum deviation is calculated through PID control; calculating the change quantity of the real-time coal quantity to the ideal temperature by utilizing the relationship between the coal quantity and the flue gas temperature, and calculating the adjusted ideal flue gas temperature by utilizing the change quantity; and the optimal flue gas flow corresponding to the minimum difference value between the actually measured flue gas temperature and the adjusted ideal flue gas temperature is calculated through PID control, and the flue gas flow which is sent back to the hearth for circulation again is controlled according to the optimal flue gas flow, so that the steam temperature of the reheater to be controlled is reduced. According to the invention, the problem of poor reheating steam temperature control effect is solved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of boiler control, and more particularly to a reheated steam temperature cascade control method and system for a coal-fired power generating unit. BACKGROUND

[0002] Steam temperature control is a very important control point in the operation of a boiler. Too high a steam temperature will accelerate the creep rate of the metal material of the boiler heating surface and the steam pipe, affecting the service life; and too low a steam temperature will reduce the circulating thermal efficiency of the unit and increase the coal consumption. There has been a problem of difficulty in steam temperature control for coal-fired power generating units. The reheated steam temperature has a large thermal inertia during the variable load process, and the conventional control strategy cannot effectively control the stability of the reheated steam temperature during the variable load process. Spraying a large amount of reheater desuperheating water will also reduce the efficiency of the unit.

[0003] The control of the reheated steam temperature mainly uses flue gas recirculation and flue gas damper control, and two-stage reheater desuperheating water to prevent over-temperature accidents. In the original conventional control logic, the PID directly adjusts the flow of the flue gas recirculation according to the deviation of the set value and the measured value of the reheated steam temperature, and the adjustment is made after the deviation of the reheated steam temperature from the set value is monitored. This adjustment method has a problem of lag, and the reheated steam temperature is prone to over-temperature. At the same time, the method of adjusting the flue gas recirculation flow to control the reheated steam temperature has a large delay and inertia, which makes it difficult to stabilize the subsequent reheated steam temperature.

[0004] Therefore, there is a need for a reheated steam temperature control method to solve the above problems. SUMMARY

[0005] In view of the above defects or improvement needs of the prior art, the present application provides a reheated steam temperature cascade control method and system for a coal-fired power generating unit, which solves the problem of poor control effect of the reheated steam temperature.

[0006] To achieve the above-mentioned purpose, according to one aspect of the present application, a reheated steam temperature cascade control method for a coal-fired power generating unit is provided, which control method comprises the following steps: calculating the deviation between the preset steam temperature and the measured steam temperature of the reheater to be controlled, and calculating the ideal flue gas temperature corresponding to the minimum deviation by using PID control; calculating the change amount of the real-time coal quantity to the ideal temperature by using the relationship between the coal quantity and the flue gas temperature, and calculating the adjusted ideal flue gas temperature by using the change amount; calculating the optimal flue gas flow corresponding to the minimum difference between the measured flue gas temperature and the adjusted ideal flue gas temperature by using PID control, and controlling the flue gas flow circulating back to the furnace according to the optimal flue gas flow, so as to reduce the steam temperature of the reheater to be controlled.

[0007] Further preferably, the calculation method of the change amount is as follows:

[0008] wherein x is the change amount; K is the gain coefficient; τ is the time constant; u b is the real-time coal amount; t is the time.

[0009] Further preferably, the control method further comprises the following steps: calculating the ideal steam temperature at the inlet of the to-be-controlled reheater corresponding to the minimum deviation between the preset steam temperature and the measured steam temperature at the outlet of the to-be-controlled reheater by using PID control; calculating the ideal desuperheating water flow rate for desuperheating the to-be-controlled reheater corresponding to the minimum deviation between the ideal steam temperature at the inlet of the to-be-controlled reheater and the measured steam temperature at the inlet of the to-be-controlled reheater by using PID control, and controlling the flow rate of the desuperheating water according to the ideal desuperheating water flow rate, so as to further reduce the temperature at the outlet of the to-be-controlled reheater.

[0010] Further preferably, the control method further comprises the following steps: calculating the flue gas flow rate entering each to-be-controlled reheater so as to minimize the difference between the steam temperatures at the outlets of different to-be-controlled reheaters by using PID control, so as to further reduce the steam temperature at the outlet of the to-be-controlled reheater.

[0011] Further preferably, the boiler of the coal-fired power generating unit is a π-type boiler, and the reheating unit is a double-reheating unit.

[0012] According to another aspect of the present application, a reheating steam temperature cascade control system for a coal-fired power generating unit is provided, which comprises a controller, a PID control module and a flue gas recirculation control module, wherein: the controller is configured to set a preset steam temperature at the outlet of a to-be-controlled reheater, calculate the deviation between the preset steam temperature and the measured steam temperature, and calculate the change amount of the ideal temperature with respect to the real-time coal amount by using the relationship between the coal amount and the flue gas temperature, and finally calculate the adjusted ideal flue gas temperature by using the change amount; the PID control module is configured to calculate the ideal flue gas temperature corresponding to the minimum deviation by using PID control, and calculate the optimal flue gas flow rate corresponding to the minimum difference between the measured flue gas temperature and the adjusted ideal flue gas temperature; the flue gas recirculation control module is connected with the PID control module, and is configured to control the flue gas flow rate entering the furnace for recirculation according to the optimal flue gas flow rate calculated by the PID control module.

[0013] Further preferably, the control system further comprises a reheater desuperheating water control module. The PID control module is further configured to calculate, by PID control, an ideal steam temperature at the inlet of the to-be-controlled reheater corresponding to the minimum deviation between the preset steam temperature and the measured steam temperature at the outlet of the to-be-controlled reheater, and calculate, by PID control again, an ideal desuperheating water flow rate corresponding to the minimum deviation between the ideal steam temperature at the inlet of the to-be-controlled reheater and the measured steam temperature, and the to-be-controlled reheater is desuperheated by the ideal desuperheating water flow rate. The reheater desuperheating water control module is connected with the PID control module, and the ideal desuperheating water flow rate obtained by the base calculation is used to control the flow rate of the desuperheating water.

[0014] Further preferably, the control system further comprises a flue gas baffle module. The PID control module is further configured to calculate, by PID control, the flue gas flow rate entering each to-be-controlled reheater so as to minimize the steam temperature difference at the outlet of different to-be-controlled reheaters. The flue gas baffle module is configured to control the opening degree of the flue gas baffle module according to the flue gas flow rate of each to-be-controlled reheater calculated by the PID control module.

[0015] According to still another aspect of the present application, there is provided a coal-fired power generating unit reheated steam temperature cascade control system, which comprises an executor configured to execute the control method described above.

[0016] According to still another aspect of the present application, there is provided a computer storage medium having a computer program stored thereon, the computer program being executed by an executor to implement the control method described above.

[0017] Overall, the above technical solutions conceived by the present application have the following beneficial effects compared with the prior art: 1. The present application realizes stable and rapid control of reheated steam temperature through two-stage PID cascade control. The original control directly adjusts the reheated steam temperature through flue gas recirculation, which has large delay and inertia. After the flue gas flow rate is changed, the flue gas is heated with the wall surface, and the wall surface is heated with the steam, and the heat transfer process takes a long time, resulting in large time lag delay in the change of the flue gas flow rate to the change of the reheated steam temperature, which significantly reduces the control effect. At the same time, the reheated steam temperature is affected by various disturbances such as coal quantity and load, which further reduces the control effect of the control process which already has large delay.

[0018] 2. This invention introduces coal quantity into PID control as a feedforward signal. For boilers, coal quantity directly determines the heat entering the system. Fluctuations in coal quantity have a significant impact on the unit's steam temperature. In control systems without coal quantity feedforward, fluctuations in coal quantity affect steam temperature through heat transfer. However, heat transfer takes time, and by the time the PID controller detects a temperature deviation and makes adjustments, it is already too late; the steam temperature has already been affected by the coal quantity fluctuation. In summary, changes in coal quantity directly affect the combustion heat load, thus rapidly altering the ratio of radiative to convective heat transfer within the furnace. Therefore, coal quantity can reflect potential steam temperature fluctuation trends in advance. Using coal quantity as a feedforward signal allows for earlier actuator action. Through feedforward compensation, the system can adjust desuperheating water or flue gas dampers before the steam temperature significantly deviates from the set value, effectively suppressing the lag effect caused by coal quantity fluctuations and reducing the dynamic deviation of the main control loop. This control strategy is particularly suitable for operating conditions with frequent load changes. It can improve the regulation quality of systems with large inertia and large delay, reduce overshoot, improve steam temperature stability, and at the same time reduce the burden on the main controller, making the entire system faster and more stable.

[0019] 3. This invention introduces flue gas temperature as a control variable into PID control. The original control uses single-loop control, adjusting the reheat steam temperature by regulating the recirculated flue gas flow rate. This process is mainly achieved through heat transfer: the flue gas first transfers heat to the wall, and then the wall transfers heat to the steam. This two-stage heat transfer process takes a long time, resulting in poor control of the reheat steam temperature. However, flue gas temperature can respond quickly to changes in the recirculated flue gas flow rate, effectively reflecting whether the recirculated flue gas flow rate is playing a regulatory role. Furthermore, steam temperature changes occur through heat transfer between the flue gas and the wall, and between the wall and the steam, allowing the flue gas temperature to anticipate potential steam temperature fluctuations. Therefore, using flue gas temperature as a control variable improves the control effect of the reheat steam temperature, making the regulation process faster and more stable.

[0020] 4. The cascade control of this invention significantly improves the system's control performance by introducing two control loops: a primary loop and a secondary loop. The secondary loop controls both the desuperheating water flow rate and the flue gas flow rate. Its core advantage lies in the secondary loop's ability to quickly suppress inner-loop disturbances, reducing their impact on the primary controlled variable and thus greatly enhancing the system's anti-interference capability and dynamic response speed. The secondary loop utilizes flue gas temperature as a fast-response variable, enabling timely compensation for high-frequency disturbances such as flue gas flow rate changes, valve fluctuations, and load changes, while the primary loop's reheat steam temperature focuses on setpoint tracking and steady-state accuracy. Furthermore, cascade control decomposes control complexity, allowing independent parameter tuning for both the primary and secondary loops, optimizing the adjustment process, and is particularly suitable for industrial processes with large time lags and multiple disturbances. Compared to single-loop control, cascade systems exhibit significant advantages in stability, robustness, and adjustment accuracy. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating a cascade control method for reheat steam temperature of a coal-fired power generating unit constructed according to a preferred embodiment of the present invention.

[0022] Figure 2 This is a flowchart of hot steam temperature auxiliary control constructed according to a preferred embodiment of the present invention.

[0023] Figure 3 This is a schematic diagram of the process structure of a coal-fired power generation secondary reheat unit constructed according to a preferred embodiment of the present invention.

[0024] Figure 4a This is a comparison chart of the 750 MW-1000 MW variable load process before and after single reheat steam temperature optimization, constructed according to a preferred embodiment of the present invention. Among them, (a) is a variable load process with a variable load command rate of 2% Pe / min, (b) is a variable load process with a variable load command rate of 3% Pe / min, (c) is a variable load process with a variable load command rate of 4% Pe / min, and (d) is a variable load process with a variable load command rate of 5% Pe / min.

[0025] Figure 4b The figures show a comparison of the secondary reheat steam temperature before and after optimization under different variable load rates in a 750 MW-1000 MW variable load process, constructed according to a preferred embodiment of the present invention. (a) is a variable load process with a variable load command rate of 2% Pe / min, (b) is a variable load process with a variable load command rate of 3% Pe / min, (c) is a variable load process with a variable load command rate of 4% Pe / min, and (d) is a variable load process with a variable load command rate of 5% Pe / min.

[0026] Figure 5a The figures are comparisons of reheat steam temperature before and after optimization for a 500 MW-750 MW variable load process under different variable load rates, constructed according to a preferred embodiment of the present invention. (a) is a variable load process with a variable load command rate of 2% Pe / min, (b) is a variable load process with a variable load command rate of 3% Pe / min, (c) is a variable load process with a variable load command rate of 4% Pe / min, and (d) is a variable load process with a variable load command rate of 5% Pe / min.

[0027] Figure 5bThe figures are comparisons of secondary reheat steam temperature before and after optimization under different variable load rates in the 500 MW-750 MW variable load process constructed according to the preferred embodiment of the present invention. Among them, (a) is the variable load process with a variable load command rate of 2% Pe / min, (b) is the variable load process with a variable load command rate of 3% Pe / min, (c) is the variable load process with a variable load command rate of 4% Pe / min, and (d) is the variable load process with a variable load command rate of 5% Pe / min.

[0028] Figure 6a The figures show a comparison of the reheat steam temperature before and after optimization for a 1000 MW-750 MW variable load process under different variable load rates, constructed according to a preferred embodiment of the present invention. (a) is a variable load process with a variable load command rate of 2% Pe / min, (b) is a variable load process with a variable load command rate of 3% Pe / min, (c) is a variable load process with a variable load command rate of 4% Pe / min, and (d) is a variable load process with a variable load command rate of 5% Pe / min.

[0029] Figure 6b The figures show a comparison of the secondary reheat steam temperature before and after optimization under different variable load rates in the 1000 MW-750 MW variable load process according to the preferred embodiment of the present invention. Among them, (a) is the variable load process with a variable load command rate of 2% Pe / min, (b) is the variable load process with a variable load command rate of 3% Pe / min, (c) is the variable load process with a variable load command rate of 4% Pe / min, and (d) is the variable load process with a variable load command rate of 5% Pe / min.

[0030] Figure 7a The figures show a comparison of the reheat steam temperature before and after optimization for a 750 MW-500 MW variable load process under different variable load rates, constructed according to a preferred embodiment of the present invention. (a) is a variable load process with a variable load command rate of 2% Pe / min, (b) is a variable load process with a variable load command rate of 3% Pe / min, (c) is a variable load process with a variable load command rate of 4% Pe / min, and (d) is a variable load process with a variable load command rate of 5% Pe / min.

[0031] Figure 7bThe figures show a comparison of the secondary reheat steam temperature before and after optimization under different variable load rates in the 750 MW-500 MW variable load process according to the preferred embodiment of the present invention. Among them, (a) is the variable load process with a variable load command rate of 2% Pe / min, (b) is the variable load process with a variable load command rate of 3% Pe / min, (c) is the variable load process with a variable load command rate of 4% Pe / min, and (d) is the variable load process with a variable load command rate of 5% Pe / min. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0033] like Figure 1 As shown, this invention proposes a cascade control method for reheat steam temperature in a coal-fired power generation unit. The control method includes a main regulation mode and an auxiliary regulation mode. The main regulation mode is cascade control of reheat steam temperature and flue gas temperature, with the flue gas recirculation pump as the main regulator. The auxiliary regulation mode uses a secondary regulator, which includes flue gas dampers and two-stage reheater desuperheating water.

[0034] In the cascade control structure, the inner loop controls the flue gas temperature, the outer loop controls the reheat steam temperature, and the change in coal quantity serves as the feedforward signal. The setpoint for the reheat steam temperature is given by the steady-state steam temperature function corresponding to the load. The setpoint for the flue gas temperature is given by the PID output value of the outer loop. The flue gas temperature signal is taken from the inlet of the low-pressure, low-temperature reheater (at the turning chamber). The feedforward signal is taken from the filtered differential signal of the coal quantity. The flue gas damper adjusts the deviation between the two-stage reheat steam temperatures, and the desuperheating water in the two-stage reheaters prevents overheating accidents.

[0035] The flue gas temperature is taken from the turning chamber, where it is convenient to set up a flue gas temperature measuring point in the actual unit.

[0036] The feedforward signal of coal quantity change is first differentiated to obtain the rate of change of coal quantity. The signal is then amplified by multiplying it by the corresponding gain coefficient, and finally filtered to obtain the feedforward signal of coal quantity change.

[0037] The relationship between coal quantity and flue gas temperature is used to calculate the change in ideal temperature due to real-time coal quantity, and this change is used to calculate the adjusted ideal flue gas temperature. The method for calculating the change is as follows:

[0038] Where x is the change; K is the gain coefficient; τ is the time constant; ub The unit is real-time coal quantity, expressed in kg / s; t represents time, expressed in seconds.

[0039] The desuperheating water flow control in the auxiliary regulation method also adopts cascade control. The inner loop controls the high reheat inlet steam temperature, and the outer loop controls the high reheat outlet steam temperature. The setpoint of the high reheat inlet steam temperature is given by the output value of the PID controller in the outer loop, while the setpoint of the high reheat outlet steam temperature is the steady-state steam temperature corresponding to the load plus 5°C. Its advantage is that the steam temperature overheating temperature in power plants is generally 8°C above the steady-state value. The setting method of adding 5°C to the steady-state steam temperature can prevent reheat steam temperature overheating while minimizing the amount of reheating water sprayed, thus reducing its impact on unit efficiency.

[0040] The distribution of flue gas to each reheater in the auxiliary regulation method is as follows: PID control is used to calculate the flue gas flow rate entering each reheater when the steam temperature difference at the outlet of the different reheaters to be controlled is minimized. In the embodiment of this invention, two reheaters are used: a primary reheater and a secondary reheater.

[0041] like Figure 3 The diagram shows the process structure of a coal-fired power plant with secondary reheat. The blue lines represent the steam flow direction, and the black lines represent the flue gas flow direction. Other equipment is omitted in the diagram, and only the equipment related to this invention is shown.

[0042] The present invention will be further described below with reference to specific embodiments.

[0043] The specific implementation plan focuses on a simulation model of a 1000 MW double reheat unit. The unit's boiler is a π-type boiler with a rated steam flow rate of 2764 t / h. The temperatures / pressures of the main steam, primary reheat steam, and secondary reheat steam are 605℃ / 33.5 MPa, 623℃ / 10.82 MPa, and 623℃ / 3.32 MPa, respectively. The turbine is an ultra-supercritical, double intermediate reheat, single-shaft, five-cylinder, four-exhaust, double back-pressure condensing turbine (N1000~31 / 600 / 620 / 620). The turbine's flow path consists of four parts: ultra-high pressure, high pressure, intermediate pressure, and low pressure. The regenerative system includes 10 stages of regenerative extraction, comprising 4 stages of high-pressure heaters, 5 stages of low-pressure heaters, and a deaerator.

[0044] The specific implementation steps of the reheat steam temperature cascade control method are as follows: Step 1: Set the reheat steam temperature function corresponding to the load to obtain the setpoint value of reheat steam temperature for different load segments. Real-time acquisition of the primary reheat steam temperature, secondary reheat steam temperature, and flue gas temperature at the turning chamber is achieved through temperature measuring points. Real-time acquisition of the coal feed rate is also obtained.

[0045] Step 2: Calculate the deviation between the measured reheat steam temperature and the setpoint in real time. Use a PID controller to control this deviation, outputting the preliminary setpoint for the flue gas temperature, with an output range of 500℃ to 1100℃. Differentiate the coal quantity signal to obtain the rate of change of coal quantity, then multiply it by -2 to -10. This value serves as the feedforward compensation. The feedforward compensation is then superimposed with the preliminary setpoint for the flue gas temperature to achieve the real-time setpoint for the flue gas temperature.

[0046] Step 3: Calculate the deviation between the measured flue gas temperature and the set value in real time, and control the deviation through a PID controller. The output value is the opening degree of the flue gas recirculation pump, and the output range is 0 to 100%.

[0047] Step 4: Calculate the deviation between the primary reheat steam temperature and the secondary reheat steam temperature in real time, and control the deviation through a PID controller. The output value is the flue gas damper opening, and the output value ranges from 0.2 to 1.

[0048] Step 5: Set the reheat steam temperature setpoint of the reheater desuperheating water circuit to 3-5°C above the steady-state temperature. Calculate the deviation between the measured and setpoint reheat steam temperature in real time, and control this deviation using a PID controller. The output value is the setpoint for the high reheat inlet steam temperature, ranging from 400 to 600°C. Calculate the deviation between the measured and setpoint high reheat inlet steam temperature in real time, and control this deviation using a PID controller. The output value is the opening degree of the desuperheating water valve, ranging from 0 to 1. The above six steps constitute the complete reheat steam temperature control strategy. The steam temperature control effect of this invention is demonstrated below with specific embodiments.

[0049] To demonstrate that the steam temperature control effect of the present invention has good steam temperature control effect under various operating conditions, four embodiments are set up: Embodiment 1 is a load increase process of 750 MW to 1000 MW; Embodiment 2 is a load increase process of 500 MW to 750 MW; Embodiment 3 is a load decrease process of 1000 MW to 750 MW; and Embodiment 4 is a load decrease process of 750 MW to 500 MW.

[0050] In the variable load process of the embodiment, four different variable load rates are set as 2% Pe / min, 3% Pe / min, 4% Pe / min and 5% Pe / min.

[0051] To demonstrate the control effect of this invention on reheat steam temperature, the control effect of conventional control logic was also statistically analyzed. Conventional control logic also includes two-stage reheater desuperheating water and flue gas damper control; the difference is that the flue gas recirculation flow rate is controlled by a PID controller that directly adjusts the opening of the flue gas recirculation pump based on the deviation between the setpoint and measured reheat steam temperature.

[0052] Taking Example 1 as an example, after the model stabilized at 750 MW, the load command was set to 1000 MW and the variable load rate command was set to 2% Pe / min. The simulation process was started, and the model was considered to have completed one operating condition after running for 1 hour. The reheat steam temperature per second during this process was recorded and plotted as a corresponding graph.

[0053] Figures 4a to 7b Four embodiments are shown, illustrating the reheat steam temperature variation curves of the conventional control method and the control method of the present invention. As can be seen from the figures, the reheat steam temperature variation curves of the optimized method of the present invention closely match the setpoint curves, with significantly reduced steam temperature deviation and a significantly shorter stabilization time. For 750 MW to 1000 MW, the steam temperature deviations for primary and secondary reheat are maintained within ±2.2℃ and ±3.8℃ of the setpoints, respectively. For 500 MW to 750 MW, the steam temperature deviations for primary and secondary reheat are maintained within ±1.7℃ and ±4.1℃ of the setpoints, respectively. For 1000 MW to 750 MW, the steam temperature deviations for primary and secondary reheat are maintained within ±1.3℃ and ±2.1℃ of the setpoints, respectively. For 750 MW to 500 MW, the steam temperature deviations for primary and secondary reheat are maintained within ±3.6℃ and ±4.7℃ of the setpoints, respectively.

[0054] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for cascade control of reheat steam temperature in a coal-fired power generating unit, characterized in that, The control method includes the following steps: Calculate the deviation between the preset steam temperature and the measured steam temperature at the outlet of the reheater to be controlled, and use PID control to calculate the ideal flue gas temperature corresponding to the minimum deviation. The relationship between coal quantity and flue gas temperature is used to calculate the change in ideal temperature due to real-time coal quantity, and this change is used to calculate the adjusted ideal flue gas temperature. The optimal flue gas flow rate is calculated using PID control to minimize the difference between the measured flue gas temperature and the adjusted ideal flue gas temperature. The flow rate of the flue gas recirculated back into the furnace is then controlled according to this optimal flow rate, thereby reducing the steam temperature of the reheater to be controlled.

2. The method for cascade control of reheat steam temperature in a coal-fired power generating unit as described in claim 1, characterized in that, The method for calculating the change is as follows: Where x is the change; K is the gain coefficient; τ is the time constant; u b t represents the real-time coal quantity; t represents time.

3. The method for cascade control of reheat steam temperature in a coal-fired power generating unit as described in claim 1, characterized in that, The control method further includes the following steps: The ideal steam temperature at the inlet of the reheater to be controlled is calculated when the deviation between the preset steam temperature at the outlet of the reheater to be controlled and the measured steam temperature is minimized using PID control. The ideal desuperheating water flow rate for desuperheating the reheater is calculated again using PID control when the deviation between the ideal steam temperature and the measured steam temperature at the inlet of the reheater is minimized. The flow rate of the desuperheating water is then controlled according to this ideal desuperheating water flow rate to further reduce the outlet temperature of the reheater.

4. A method for cascade control of reheat steam temperature in a coal-fired power generating unit as described in claim 1 or 3, characterized in that, The control method further includes the following steps: By using PID control to calculate the minimum steam temperature difference at the outlet of different reheaters to be controlled, the flue gas flow rate entering each reheater to be controlled is minimized, thereby further reducing the steam temperature at the outlet of the reheater to be controlled.

5. The method for cascade control of reheat steam temperature in a coal-fired power generating unit as described in claim 1, characterized in that, The boiler of the coal-fired power generation unit is a π-type boiler, a double reheat unit.

6. A cascade control system for reheat steam temperature in a coal-fired power generating unit, characterized in that, The control system includes a controller, a PID control module, and a flue gas recirculation control module, wherein: The controller is used to set the preset steam temperature at the outlet of the reheater to be controlled, and to calculate the deviation between the preset steam temperature and the measured steam temperature; and to calculate the change in the real-time coal quantity to the ideal temperature using the relationship between the coal quantity and the flue gas temperature, and finally to calculate the adjusted ideal flue gas temperature using the change. The PID control module is used to calculate the ideal flue gas temperature corresponding to the minimum deviation using PID control, and to calculate the optimal flue gas flow rate corresponding to the minimum difference between the measured flue gas temperature and the adjusted ideal flue gas temperature. The flue gas recirculation control module is connected to the PID control module and is used to control the flow rate of flue gas entering the furnace for recirculation based on the optimal flue gas flow rate calculated by the PID control module.

7. The reheat steam temperature cascade control system for a coal-fired power generator unit as described in claim 6, characterized in that, The control system also includes a reheater desuperheating water control module; The PID control module is also used to calculate the ideal steam temperature at the inlet of the reheater to be controlled when the deviation between the preset steam temperature and the measured steam temperature at the outlet of the reheater to be controlled is minimized. And then, using PID control again, calculate the ideal desuperheating water flow rate for desuperheating the reheater under control when the deviation between the ideal steam temperature and the measured steam temperature at the inlet of the reheater under control is minimized. The reheater desuperheating water control module is connected to the PID control module, and the ideal desuperheating water flow rate obtained by the basic calculation controls the flow rate of the desuperheating water.

8. A reheat steam temperature cascade control system for a coal-fired power generator unit as described in claim 6 or 7, characterized in that, The control system also includes a flue gas damper module; The PID control module is also used to calculate the flue gas flow rate entering each reheater when the steam temperature difference at the outlet of the different reheaters to be controlled is minimized. The flue gas damper module is used to control the opening degree of the flue gas damper module based on the flue gas flow rate of each reheater to be controlled calculated by the PID control module.

9. A cascade control system for reheat steam temperature in a coal-fired power generating unit, characterized in that, The system includes an actuator for performing the control method according to any one of claims 1-5.

10. A computer storage medium having a computer program stored thereon, characterized in that, When executed by the executor, the computer program is used to implement the control method according to any one of claims 1-5.

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