Optimization control method for load shedding water hammer pressure of pumped storage power station containing fixed-variable speed unit

Abstract

The invention discloses an optimal control method for load shedding water hammer pressure of a pumped storage power station containing a fixed-variable speed unit. The method comprises the following steps: firstly, based on a pump turbine full-characteristic curve, performing variable-speed unit efficiency optimization, and determining the highest efficiency of a variable-speed unit and the corresponding optimal rotating speed; then, establishing a transition process mathematical model of the water delivery and power generation system of the pumped storage power station containing the constant-variable speed unit, and calculating Pareto leading edges taking the maximum pressure of a volute and the minimum pressure of a draft tube as optimization targets under different control schemes by adopting different guide vane closing rules for the constant speed unit and the variable speed unit; and finally, carrying out normalization processing on the maximum pressure of the volute and the minimum pressure of the draft tube, establishing a transient characteristic multi-target evaluation function, and screening out an optimal control scheme from the Pareto frontier. By means of the method, the water hammer pressure extreme value caused by hydraulic interference between the constant-speed unit and the variable-speed unit can be effectively improved, the operation stability and safety of a power station are improved, and good engineering adaptability is achieved.

Description

Technical Field

[0001] The present invention relates to the field of water conservancy and hydropower engineering, and in particular to an optimization control method for load rejection water hammer pressure of a pumped storage power station containing a fixed-variable speed unit. Background Art

[0002] With the rapid development of renewable energy and the growing demand for regulation capabilities in power systems, pumped-storage power stations, as a key component of large-scale energy storage technology, play an irreplaceable role in grid peak-shaving, frequency regulation, and emergency standby. However, traditional fixed-speed pumped-storage units face challenges in complex operating conditions, such as a narrow operating range, low weighted efficiency, and limited regulation capabilities. In recent years, variable-speed pumped-storage technology, with its flexible operating characteristics, higher energy conversion efficiency, and greater adaptability to operating conditions, has demonstrated significant engineering value in improving grid stability, optimizing energy utilization efficiency, and promoting renewable energy consumption. It has become a key research and application area in the field of water conservancy and hydropower engineering.

[0003] Despite the numerous advantages of variable-speed units in terms of operational performance and regulation quality, the high cost of electrical equipment has limited their large-scale application in production practices. Therefore, to optimize investment costs while ensuring regulation performance, some pumped-storage power plants adopt a single-pipe, multi-unit configuration, forming a unit structure composed primarily of conventional fixed-speed units, supplemented by variable-speed units. This configuration retains the lower cost advantage of fixed-speed units while, through the introduction of variable-speed units, enhancing the plant's regulation capabilities and operating efficiency under complex operating conditions. This provides a viable solution for balancing the economic viability and regulation performance of pumped-storage power plants.

[0004] While numerous studies have explored the control optimization of load shedding transients in fixed-speed pumped-storage power plants and small-fluctuation transients in variable-speed units, limited research has focused on the load shedding transients in pumped-storage power plants with a hybrid arrangement of fixed- and variable-speed units. The units in conventional fixed-speed pumped-storage power plants typically operate symmetrically, with the two units maintaining identical operating parameters. However, variable-speed units adjust their steady-state operating speed to maximize efficiency, resulting in differences in operating parameters such as speed, flow rate, and guide vane opening compared to fixed-speed units. These differences complicate the hydraulic coupling between fixed-speed and variable-speed units, exacerbating water hammer pressure fluctuations during transients and threatening the operational safety of pumped-storage power plants.

[0005] In view of this, it is urgent to invent an optimization control method for the water hammer pressure of a pumped storage power station containing fixed-variable speed units under load rejection conditions to provide a basis for the safe and stable operation of the power station. Summary of the Invention

[0006] A method for optimizing the control of water hammer pressure in a load rejection condition of a pumped storage power station containing a fixed-variable speed unit, characterized by comprising the following steps:

[0007] Step 1: Based on the full characteristic curve of the pump-turbine, optimize the efficiency of the variable speed unit to determine the maximum efficiency of the variable speed unit and its corresponding optimal speed;

[0008] Step 2: Establish a transient mathematical model of the water transmission and power generation system of a pumped storage power station including fixed-speed and variable-speed units, wherein the mathematical model is composed of a pressurized pipeline system and a pump-turbine unit system;

[0009] Step 3: Based on the numerical simulation model of the transition process in Step 2, by adopting different guide vane closing rules for the fixed-speed unit and the variable-speed unit, calculate the Pareto front with maximum volute pressure and minimum draft tube pressure as the optimization objectives under different control schemes;

[0010] Step 4: Normalize the maximum volute pressure and the minimum draft tube pressure to establish a comprehensive transient characteristic evaluation function;

[0011] Step 5: Substitute the Pareto frontier solution of step 3 into the transient characteristic comprehensive evaluation function of step 4, and determine the optimal control solution by comparing the transient characteristic comprehensive evaluation function values ​​under different Pareto frontier solutions.

[0012] In step 1, the improved Suter transform is used to process the full characteristic curve of the pump-turbine, and the data on the full characteristic curve is expressed by the following formula:

[0013]

[0014] WB=m1′(3)where: Z is the independent variable of the flow function and torque function, q1′ is the relative unit flow, q1′=Q 11 / Q 11r , Q 11 is the unit flow rate, Q 11r is the rated unit flow rate; q 1B ' is the offset coefficient; n1' is the relative unit speed, n1'=N 11 / N 11r , N 11 is the unit speed, N 11r is the rated unit speed; m1′ is the relative unit dynamic torque, WH is the flow function, WB is the torque function, m1′=M 11 / M 11r ;M 11 is the unit moment, M 11r is the rated unit torque; the subscript r indicates the rated value;

[0015] The expressions for unit speed, unit flow and unit torque of a pump-turbine are:

[0016]

[0017]

[0018] Where: n, D1, H T , P, Q T They are the unit's speed, runner diameter, water head, and output flow;

[0019] With the goal of maximizing unit efficiency, an efficiency optimization method based on the full characteristic curve is proposed. The specific steps are as follows:

[0020] (a) Input unit speed n is n min ;

[0021] (b) Assume that the unit flow rate Q is Q assumed ;

[0022] (c) According to the unit flow Q assumed , the water levels of upstream and downstream reservoirs, and the head loss coefficient of the water pipeline, and the unit operating head H is obtained. T ;

[0023] (d) According to the unit's operating head, flow rate, and speed, the unit speed N is calculated using equations (4) and (5). 11 And the unit flow Q 11 ;

[0024] (e) Calculate x using equations (1) and (2) Z and WH, then interpolate the flow function curve to obtain the opening τ;

[0025] (f) x obtained according to step (e) Z and τ, interpolate the torque function curve to get WB, and then get the unit output according to formula (6);

[0026] (g) If the difference between the output and the target output is greater than the allowable error ε, the reference flow Q is re-assumed based on the output deviation. assumed and re-execute step (b); if the difference between the output and the target output is less than ε, then end the loop calculation and proceed to step (h);

[0027] (h) Speed, opening, efficiency, output and flow of the output unit;

[0028] (i) If the unit speed n <n max , then n=n+Δ, and repeat step (a);

[0029] (j) By comparing the efficiency at different speeds, the speed, opening, efficiency, output and flow rate at the optimal efficiency are output.

[0030] In step 2, the dynamic equation and continuity equation of the pressurized pipeline describing the elastic water body are:

[0031]

[0032] Where: Q is the flow rate; A is the cross-sectional area of ​​the pipe; H is the water head of the pressure tube; x is the pipe coordinate; f is the friction coefficient; c is the water hammer wave velocity; and D is the pipe diameter.

[0033] The motion equation of the pump turbine rotor is:

[0034]

[0035] Where: m g is the relative value of the generator electromagnetic torque, m g =M g / M r , M g is the electromagnetic torque of the generator, M r is the rated shaft torque; n is the relative speed; T a is the inertia time constant of the unit; m is the relative value of the turbine shaft torque;

[0036] Formula (10) is the head balance equation of the pump turbine, which represents the head H T and volute pressure measuring tube water head H SC and tailwater pipe pressure gauge head H DT Relationship:

[0037]

[0038] Where: Q T A is the unit operating flow; SC A is the cross-sectional area of ​​the volute; DT is the cross-sectional area of ​​the tailwater pipe, and g is the acceleration due to gravity.

[0039] In step 3, the maximum volute pressure is the smaller the better, and the minimum tailwater pressure is the larger the better. Taking a pumped storage power station with two fixed-speed units and one variable-speed unit as an example, the volute objective function and the tailwater pressure objective function are as follows:

[0040] MinF1=max{H Smax-1#FSU ,H Smax-2#FSU ,H Smax-VSU} (11)

[0041] MaxF2=min{H Dmin-1#FSU1 ,H Dmin-2#FSU ,HDmin-VSU} (12) Where: H SMAX Indicates the maximum pressure of the volute; H Dmin Indicates the minimum pressure of the draft tube; the subscripts 1#FSU, 2#FSU and VSU represent the 1# fixed speed unit, 2# fixed speed unit and variable speed unit respectively; F1 represents the maximum pressure of the volute, and F2 represents the minimum pressure of the draft tube;

[0042] According to the mathematical model of the transition process in step 2, the maximum volute pressure and the minimum draft tube pressure of the unit under different control schemes are calculated. Then, combining equations (11) and (12), the Pareto frontier scheme with the maximum volute pressure and the minimum draft tube pressure as the optimization objectives is obtained.

[0043] In step 4, the maximum-minimum normalization method is used to normalize the maximum volute pressure and the minimum draft tube pressure:

[0044]

[0045] Finally, the transient characteristic comprehensive evaluation function is established as follows:

[0046] maxF object =w1F1′+w2F2′ (15)

[0047] Where: w1 and w2 are weight coefficients, F1′ is the maximum pressure of the volute after normalization, and F2′ is the minimum pressure of the draft tube after normalization; F 1-max and F 1-min Respectively represent the maximum and minimum volute pressures in different control schemes; F 2-max and F 2-min Respectively represent the maximum and minimum tailwater pipe minimum pressures in different control schemes; F object Represents the comprehensive evaluation function of transient characteristics.

[0048] In step 5, the Pareto frontier solution calculated in step 3 is substituted into formula (15), and F is obtained. object , select the largest F object The corresponding control scheme is the optimal control scheme.

[0049] Beneficial Effects: This invention addresses the adverse hydraulic coupling between fixed-speed and variable-speed units under load rejection conditions and provides a method for optimizing the control of water hammer pressure during load rejection in pumped-storage power plants containing fixed- and variable-speed units. This method effectively reduces the extreme water hammer pressure caused by hydraulic interference between the fixed- and variable-speed units, enhancing the operational stability and safety of the power plant. The method is simple to implement, has good engineering adaptability, and broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 Flow chart of the method of the present invention;

[0051] Figure 2 This is a schematic diagram of the layout of the water transmission and power generation system of a pumped storage power station containing fixed-variable speed units according to an embodiment;

[0052] Figure 3 is the Pareto front of the maximum volute pressure and the minimum tailwater pressure under different control schemes in this embodiment;

[0053] Figure 4 The fluctuation process of the volute pressure and the draft tube pressure under different typical control schemes in the embodiment is shown in FIG. Figure 4 (a) is Scheme 1 in the embodiment; (b) is Scheme 20 in the embodiment; (c) is Scheme 11 in the embodiment. DETAILED DESCRIPTION

[0054] The present invention will be further described below with reference to the accompanying drawings.

[0055] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present 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 only used to explain the present invention and are not intended to limit the present invention.

[0056] The present invention proposes a method for optimizing the control of water hammer pressure in a load rejection condition of a pumped storage power station with a fixed-variable speed unit. Figure 1 As shown, it mainly includes the following 5 steps.

[0057] Step 1: Based on the full characteristic curve of the pump-turbine, optimize the efficiency of the variable speed unit to determine the maximum efficiency of the variable speed unit and its corresponding optimal speed;

[0058] Step 2: Establish a transient mathematical model of the water transmission and power generation system of a pumped storage power station including fixed-speed and variable-speed units, wherein the mathematical model is composed of a pressurized pipeline system and a pump-turbine unit system;

[0059] Step 3: Based on the numerical simulation model of the transition process in Step 2, by adopting different guide vane closing rules for the fixed-speed unit and the variable-speed unit, calculate the Pareto front with the maximum volute pressure and the minimum draft tube pressure as the optimization objectives under different control schemes;

[0060] Step 4: Normalize the maximum volute pressure and the minimum draft tube pressure to construct a transient characteristic comprehensive evaluation function;

[0061] Step 5: Substitute the Pareto frontier solution of step 3 into the transient characteristic comprehensive evaluation function of step 4, and determine the optimal control solution by comparing the transient characteristic comprehensive evaluation function values ​​under different Pareto frontier solutions.

[0062] In step 1, the improved Suter transform is used to process the full characteristic curve of the pump-turbine, and the data on the full characteristic curve is expressed by the following formula:

[0063]

[0064] WB=m1′ (3) Where: x Z is the independent variable of the flow function and torque function, q1′ is the relative unit flow, q1′=Q 11 / Q 11r , Q 11 is the unit flow rate, Q 11r is the rated unit flow rate; q 1B ' is the offset coefficient; n1' is the relative unit speed, n1'=N 11 / N 11r , N 11 is the unit speed, N 11r is the rated unit speed; m1′ is the relative unit dynamic torque, WH is the flow function, WB is the torque function, m1′=M 11 / M 11r ;M 11 is the unit moment, M 11r is the rated unit torque; the subscript r indicates the rated value;

[0065] The expressions for unit speed, unit flow and unit torque of a pump-turbine are:

[0066]

[0067]

[0068] Where: n, D1, H T , P, Q T They are the unit's speed, runner diameter, water head, and output flow;

[0069] With the goal of maximizing unit efficiency, an efficiency optimization method based on the full characteristic curve is proposed. The specific steps are as follows:

[0070] (a) Input unit speed n is n min ;

[0071] (b) Assume that the unit flow rate Q is Q assumed ;

[0072] (c) According to the unit flow Q assumed , the water levels of upstream and downstream reservoirs, and the head loss coefficient of the water pipeline, and the unit operating head H is obtained. T ;

[0073] (d) According to the unit's operating head, flow rate, and speed, the unit speed N is calculated using equations (4) and (5). 11 And the unit flow Q 11 ;

[0074] (e) Calculate x using equations (1) and (2) Z and WH, then interpolate the flow function curve to obtain the opening τ;

[0075] (f) x obtained according to step (e) Z and τ, interpolate the torque function curve to get WB, and then get the unit output according to formula (6);

[0076] (g) If the difference between the output and the target output is greater than the allowable error ε, the reference flow Q is re-assumed based on the output deviation. assumed and re-execute step (b); if the difference between the output and the target output is less than ε, then end the loop calculation and proceed to step (h);

[0077] (h) Speed, opening, efficiency, output and flow of the output unit;

[0078] (i) If the unit speed n <n max , then n=n+Δ, and repeat step (a);

[0079] (j) By comparing the efficiency at different speeds, the speed, opening, efficiency, output and flow rate at the optimal efficiency are output.

[0080] Specifically, the following is an example of a pumped storage power station with fixed-variable speed units to further illustrate this method. Figure 2 As shown, the rated speed is 500r / min, the variable speed operating range is 500±8%r / min, the rated head is 665m, and the rated output is 408.16MW.

[0081] According to the efficiency optimization method proposed above, it can be obtained that the maximum operating efficiency of the power station when operating at rated head and rated output is 90.81%, and the corresponding optimal speed is 464.20r / min.

[0082] In step 2, the dynamic equation and continuity equation of the pressurized pipeline describing the elastic water body are:

[0083]

[0084]

[0085] Where: Q is the flow rate; A is the cross-sectional area of ​​the pipe; H is the water head of the pressure tube; x is the pipe coordinate; f is the friction coefficient; c is the water hammer wave velocity; and D is the pipe diameter.

[0086] The motion equation of the pump turbine rotor is:

[0087]

[0088] Where: m g is the relative value of the generator electromagnetic torque, m g =M g / M r , M g is the electromagnetic torque of the generator, M r is the rated shaft torque; n is the relative speed; T a is the inertia time constant of the unit; m is the relative value of the turbine shaft torque;

[0089] Formula (10) is the head balance equation of the pump turbine, which represents the head H T and volute pressure measuring tube water head H SC and tailwater pipe pressure gauge head H DT Relationship:

[0090]

[0091] Where: Q T A is the unit operating flow; SC A is the cross-sectional area of ​​the volute; DT is the cross-sectional area of ​​the tailwater pipe, and g is the acceleration due to gravity.

[0092] In step 3, the maximum volute pressure is the smaller the better, and the minimum tailwater pressure is the larger the better. Taking a pumped storage power station with two fixed-speed units and one variable-speed unit as an example, the volute objective function and the tailwater pressure objective function are as follows:

[0093] MinF1=max{H Smax-1#FSU ,H Smax-2#FSU ,H Smax-VSU} (11)

[0094] MaxF2=min{H Dmin-1#FSU1 ,H Dmin-2#FSU ,H Dmin-VSU} (12) Where: H SMAX Indicates the maximum pressure of the volute; H Dmin Indicates the minimum pressure of the draft tube; the subscripts 1#FSU, 2#FSU and VSU represent the 1# fixed speed unit, 2# fixed speed unit and variable speed unit respectively; F1 indicates the maximum pressure of the volute, and F2 indicates the minimum pressure of the draft tube.

[0095] Based on the transient process simulation model in step 2, the maximum volute pressure and the minimum draft tube pressure of the unit under different control schemes are calculated. Then, combining equations (11) and (12), the Pareto front with the maximum volute pressure and the minimum draft tube pressure as the optimization targets can be obtained.

[0096] Specifically, based on the actual operation of the power station, the guide vane closing time T of the fixed speed unit is selected. F The guide vane closing time of the variable speed unit is 20s-40s. V The step length is 20s-40s, and the step length is 1s. The Pareto fronts of the maximum pressure of the volute and the minimum pressure of the draft tube under different control schemes are calculated through numerical simulation, as shown in Figure 3 and as shown in Table 1.

[0097] Table 1 Pareto fronts of maximum volute pressure and minimum draft tube pressure under different control schemes

[0098]

[0099]

[0100] Depend on Figure 3 As shown in Table 1, numerical simulations of 441 control methods yielded 20 Pareto solutions. Theoretically, all of these solutions are applicable in practice. However, in practice, power plant operators may struggle to accurately determine the superiority of a particular solution over others due to their limited understanding of operational strategies when evaluating candidate solutions. Therefore, it is necessary to further develop a comprehensive transient characteristics evaluation function to assist engineers in selecting the optimal control strategy.

[0101] In step 4, the maximum-minimum normalization method is used to normalize the maximum volute pressure and the minimum draft tube pressure:

[0102]

[0103] Finally, the transient characteristic multi-objective function is established as follows:

[0104] maxF object =w1F1′+w2F2′ (15)

[0105] In step 4, the maximum-minimum normalization method is used to normalize the maximum volute pressure and the minimum draft tube pressure:

[0106]

[0107] Finally, the transient characteristic comprehensive evaluation function is established as follows:

[0108] maxF object =w1F1′+w2F2′ (15)

[0109] Where: w1 and w2 are weight coefficients, and this power station takes and F1′ is the maximum pressure of the volute after normalization, and F2′ is the minimum pressure of the draft tube after normalization; F 1-max and F 1-min Respectively represent the maximum and minimum volute pressures in different control schemes; F 2-max and F 2-min Respectively represent the maximum and minimum tailwater pipe minimum pressures in different control schemes; F object Represents the comprehensive evaluation function of transient characteristics.

[0110] In step 5, the Pareto frontier solution calculated in step 3 is substituted into formula (15), and F is obtained. object , select the largest F object The corresponding control scheme is the optimal control scheme.

[0111] Specifically, the Pareto frontier solution calculated in step 3 is substituted into equation (15) to sort the Pareto solution set. Among all the Pareto frontier solutions, the objective function F of solution 11 (i.e., the fixed-speed unit adopts a 40s guide vane closing time and the variable-speed unit adopts a 31s guide vane closing time) is object Therefore, this control scheme is selected as the optimal control scheme for load shedding in this power station.

[0112] To further validate the effectiveness of the proposed optimization control method, three representative control schemes were selected for comparative analysis. Scheme 1, corresponding to the maximum minimum draft tube pressure, requires the fixed-speed unit to shut down within 36 seconds and the variable-speed unit to shut down within 24 seconds. Scheme 20, corresponding to the minimum maximum volute pressure, requires the guide vanes of both the fixed-speed and variable-speed units to close within 40 seconds. Scheme 11 represents the optimized control strategy proposed in this paper. Figure 4 Table 2 shows the transient characteristics of the unit under three control schemes.

[0113] Table 2 Maximum volute pressure and minimum draft tube pressure under three typical control schemes

[0114]

[0115] Depend on Figure 4As shown in Table 2, while Scheme 1 increases the minimum draft tube pressure by 3.28 m / s compared to Scheme 11, it also increases the maximum volute pressure by 9.42 m / s, indicating that its transient performance is slightly inferior to that of Scheme 11. Furthermore, while Scheme 20 reduces the maximum volute pressure by 1.24 m / s compared to Scheme 11, the minimum draft tube pressure drops by 27.04 m / s. Due to the excessively low minimum draft tube pressure in Scheme 20, its transient performance is significantly inferior to that of Scheme 11.

[0116] It will be easily understood by those skilled in the art 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 in the scope of protection of the present invention.

Claims

1. A method for optimizing the control of water hammer pressure during load rejection in a pumped storage power station with fixed-variable speed units, characterized in that: The following steps are involved: Step 1: Based on the full characteristic curve of the pump-turbine, optimize the efficiency of the variable speed unit to determine the maximum efficiency of the variable speed unit and its corresponding optimal speed; Step 2: Establish a transient mathematical model of the water transmission and power generation system of a pumped storage power station including fixed-speed and variable-speed units, wherein the mathematical model is composed of a pressurized pipeline system and a pump-turbine unit system; Step 3: Based on the transition process mathematical model from Step 2, by adopting different guide vane closing rules for the fixed-speed unit and the variable-speed unit, calculate the Pareto frontier solutions with maximum volute pressure and minimum draft tube pressure as the optimization objectives under different control schemes; Step 4: Normalize the maximum volute pressure and the minimum draft tube pressure to establish a comprehensive transient characteristic evaluation function; Step 5: Substitute the Pareto frontier solution of step 3 into the transient characteristic comprehensive evaluation function of step 4, and determine the optimal control solution by comparing the transient characteristic comprehensive evaluation function values ​​under different Pareto frontier solutions.

2. The method for optimizing the control of load rejection water hammer pressure in a pumped storage power station with fixed-variable speed units according to claim 1, characterized in that: The specific method of step one is to use the improved Suter transform to process the full characteristic curve of the pump-turbine, and express the data on the full characteristic curve of the pump-turbine with the following formula: WB=m1′(3)where: Z is the independent variable of the flow function and torque function, q1′ is the relative unit flow, q1′=Q 11 / Q 11r , Q 11 is the unit flow rate, Q 11r is the rated unit flow rate; q 1B ' is the offset coefficient; n1' is the relative unit speed, n1'=N 11 / N 11r , N 11 is the unit speed, N 11r is the rated unit speed; m1′ is the relative unit dynamic torque, WH is the flow function, WB is the torque function, m1′=M 11 / M 11r ;M 11 is the unit moment, M 11r is the rated unit torque; The subscript r indicates the rated value; Unit speed of pump turbine N 11 , unit flow Q 11 and unit moment M 11 The expression is: Where: n, D1, H T , P, Q T They are the unit's speed, runner diameter, water head, and output flow; With the goal of maximizing unit efficiency, an efficiency optimization method based on the full characteristic curve is proposed. The specific steps are as follows: (a) Input unit speed n is n min ; (b) Assume that the unit flow rate Q is Q assumed ; (c) According to the unit flow Q assumed , the water levels of upstream and downstream reservoirs, and the head loss coefficient of the water pipeline, and the unit operating head H is obtained. T ; (d) According to the unit's operating head, flow rate, and speed, the unit speed N is calculated using equations (4) and (5). 11 And the unit flow Q 11 ; (e) Calculate x using equations (1) and (2) Z and WH, then interpolate the flow function curve to obtain the opening τ; (f) x obtained according to step (e) Z and τ, interpolate the torque function curve to get WB, and then get the unit output according to formula (6); (g) If the difference between the output and the target output is greater than the allowable error ε, the reference flow Q is re-assumed based on the output deviation. assumed and re-execute step (b); if the difference between the output and the target output is less than ε, then end the loop calculation and proceed to step (h); (h) Speed, opening, efficiency, output and flow of the output unit; (i) If the unit speed n <n max , then n=n+Δ, and repeat step (a); (j) By comparing the efficiency at different speeds, the speed, opening, efficiency, output and flow rate at the optimal efficiency are output.

3. The method for optimizing the control of load rejection water hammer pressure in a pumped storage power station with fixed-variable speed units according to claim 2, characterized in that: The specific method of step 2 is as follows: The dynamic equation and continuity equation for a pressurized pipe describing elastic water are: Where: Q is the flow rate; A is the cross-sectional area of ​​the pipe; H is the water head of the pressure tube; x is the pipe coordinate; f is the friction coefficient; c is the water hammer wave velocity; and D is the pipe diameter. The motion equation of the pump turbine rotor is: Where: m g is the relative value of the generator electromagnetic torque, m g =M g / M r , M g is the electromagnetic torque of the generator, M r is the rated shaft torque; n is the relative speed; T a is the inertia time constant of the unit; m is the relative value of the turbine shaft torque; Formula (10) is the head balance equation of the pump turbine, which represents the head H T and volute pressure measuring tube water head H SC and tailwater pipe pressure gauge head H DT Relationship: Where: Q T A is the unit operating flow; SC A is the cross-sectional area of ​​the volute; DT is the cross-sectional area of ​​the tailwater pipe, and g is the acceleration due to gravity.

4. The method for optimizing the control of load rejection water hammer pressure in a pumped storage power station with fixed-variable speed units according to claim 3, characterized in that: The specific method for step three is as follows: the smaller the maximum volute pressure, the better the performance, and the larger the minimum tailwater pressure, the better the performance. Taking a pumped storage power station with two fixed-speed units and one variable-speed unit as an example, the volute objective function and tailwater pressure objective function are as follows: MinF1=max{H Smax-1#FSU ,H Smax-2#FSU ,H Smax-VSU } (11) MaxF2=min{H Dmin-1#FSU1 ,H Dmin-2#FSU ,H Dmin-VSU } (12) Where: H SMAX Indicates the maximum pressure of the volute; H Dmin Indicates the minimum pressure of the draft tube; the subscripts 1#FSU, 2#FSU and VSU represent the 1# fixed speed unit, 2# fixed speed unit and variable speed unit respectively; F1 represents the maximum pressure of the volute, and F2 represents the minimum pressure of the draft tube; According to the mathematical model of the transition process in step 2, the maximum volute pressure and the minimum draft tube pressure of the unit under different control schemes are calculated. Then, combining equations (11) and (12), the Pareto frontier scheme with the maximum volute pressure and the minimum draft tube pressure as the optimization objectives is obtained.

5. The method for optimizing the control of load rejection water hammer pressure in a pumped storage power station with fixed-variable speed units according to claim 4, characterized in that: In step 4, the maximum-minimum normalization method is used to normalize the maximum volute pressure and the minimum draft tube pressure: Finally, the transient characteristic comprehensive evaluation function is established as follows: maxF object =w1F1′+w2F2′ (15) Where: w1 and w2 are weight coefficients, F1′ is the maximum pressure of the volute after normalization, and F2′ is the minimum pressure of the draft tube after normalization; F 1-max and F 1-min Respectively represent the maximum and minimum volute pressures in different control schemes; F 2-max and F 2-min Respectively represent the maximum and minimum tailwater pipe minimum pressures in different control schemes; F object Represents the comprehensive evaluation function of transient characteristics.

6. The method for optimizing the control of load rejection water hammer pressure in a pumped storage power station with fixed-variable speed units according to claim 5, characterized in that: In step 5, the Pareto frontier solution calculated in step 3 is substituted into formula (15), and F is obtained. object , select the largest F object The corresponding control scheme is the optimal control scheme.

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