An optimization method for minimum start-up combinations of thermal power plants considering static voltage stability and safety constraints

Abstract

This invention relates to an optimization method for the minimum operating combination of thermal power units considering static voltage stability and safety constraints, belonging to the field of power system security analysis and optimization scheduling technology. The method includes: acquiring a power grid model and pre-condition boundary conditions; constructing a minimum operating combination model of thermal power units considering safety constraints with the objective of minimizing the operating capacity of thermal power units; performing redundant constraint filtering on the N-1 safety constraints to reduce the computational scale of the optimization problem; solving the minimum operating combination model with safety constraints to obtain the initial minimum unit combination; performing a voltage stability margin scan to calculate the voltage stability margin for fault-free conditions and various anticipated fault conditions; for anticipated faults where the voltage stability margin does not meet the requirements, constructing a set of proposed operating units, and using the constraint that at least one unit in the proposed operating unit set must be operating, constructing and solving the minimum unit combination model considering static voltage stability and safety constraints, determining the minimum unit combination, until the requirements are met.

Description

Technical Field

[0001] This invention belongs to the field of power system security analysis and optimization scheduling technology, specifically involving an optimization method for minimum start-up combinations of thermal power plants that considers static voltage stability and safety constraints. Background Technology

[0002] Clean energy sources, represented by wind power and solar power, are experiencing explosive growth. As the penetration rate of new energy sources continues to increase, large-scale grid connection of these sources is squeezing out the power generation capacity of conventional generating units, leading to reduced operation of conventional units and insufficient system voltage support. Conversely, increasing the operation of conventional units will further encroach on the space for new energy consumption, weakening the capacity for new energy absorption.

[0003] The safe operation of a power system requires that, under normal circumstances, the power flow of all equipment does not exceed its long-term current-carrying capacity, and that it has a certain ability to withstand anticipated fault impacts. That is, when any component is disconnected without fault, the power flow of each device does not exceed its specified fault overload requirements, and a certain voltage stability margin is reserved to avoid cascading interruptions or voltage instability, and to allow sufficient time for dispatching personnel to handle accidents, so as to ensure the safe and reliable operation of the power system.

[0004] Taking into account both the static safety constraints and static voltage stability constraints of the power system, it is of great significance to rationally determine the minimum start-up mode of thermal power units, so as to maximize the consumption of new energy sources and reduce wind and solar curtailment losses while ensuring the safe operation requirements of the power system. Summary of the Invention

[0005] The purpose of this invention is to provide an optimization method for the minimum start-up combination of thermal power plants that considers static voltage stability and safety constraints. By performing a static voltage stability scan on the anticipated fault set, the static voltage stability margin constraint is converted into a start-up constraint based on the scan results. On this basis, a series of minimum start-up models for thermal power plants that comprehensively consider the static safety constraints and static voltage stability constraints of the power system are established. The established model is mathematically a linear mixed integer optimization problem that can be reliably solved using commercial optimization software (such as CPLEX).

[0006] To achieve the above objectives, the technical solution of the present invention is: an optimization method for minimum start-up combinations of thermal power plants considering static voltage stability and safety constraints, comprising the following steps:

[0007] Step S1: Obtain the power grid model and prerequisite boundary conditions, including: predicted output of new energy units, output arrangement of hydropower and nuclear power, minimum reserve capacity requirement of the system, anticipated fault set and voltage stability margin requirements;

[0008] Step S2: With the goal of minimizing the operating capacity of thermal power units, and in accordance with the constraints of power balance at each node of the system, upper and lower limits of unit output, equipment transmission capacity, safety constraints of N-1 anticipated fault states, and minimum reserve capacity requirements of the system, construct a minimum operating combination model for thermal power units that takes into account safety constraints.

[0009] Step S3: Redundancy constraint filtering is performed on the safety constraints of the fault-free state and the N-1 expected fault states to reduce the computational scale of the minimum start-up combination problem of thermal power considering safety constraints and improve the efficiency of solving the optimization problem.

[0010] Step S4: Solve the minimum start-up combination model of thermal power considering safety constraints to obtain the initial minimum unit combination;

[0011] Step S5: Obtain the minimum start-up mode of thermal power based on the minimum unit combination and perform AC power flow calculation;

[0012] Step S6: Perform voltage stability margin scan and calculate the voltage stability margin for the fault-free state and each anticipated fault state.

[0013] Step S7: Determine whether the voltage stability margin under the fault-free state and each expected fault state meets the requirements. If it does, the optimization ends and the minimum start-up scheme is obtained; otherwise, proceed to step S8.

[0014] Step S8: For anticipated faults where the voltage stability margin does not meet the requirements, select out-of-operation units at weak voltage stability nodes to form a set of units to be started, and add at least one unit in the set of units to be started as a constraint to the optimization model, and construct a minimum unit combination model that considers static voltage stability and safety constraints.

[0015] Step S9: Solve the minimum unit combination model considering static voltage stability and safety constraints, determine the minimum unit combination, and go to step S5.

[0016] In one embodiment of the present invention, step S2 is specifically implemented as follows:

[0017] Step S21: With the goal of minimizing the operating capacity of thermal power plants, the corresponding objective function can be described as follows:

[0018]

[0019] In the formula, Indicates the maximum technical output of unit i; u Gi This is the start / stop variable for unit i at node; 1 indicates the unit is on, and 0 indicates the unit is off. G This represents the set of generator terminal nodes;

[0020] Step S22: The safe operation of the power grid requires the following constraints to be met:

[0021] 1) Node power balance constraints under normal operating conditions

[0022]

[0023] In the formula, the superscript 0 indicates the normal operating state, i.e., the fault-free state; P represents the active power output of unit i; Li B represents the active load of node i; ij This represents the element in the i-th row and j-th column of the nodal susceptance matrix; The voltage phase angle at node j; S Ni Represents node i and the set of its neighboring nodes;

[0024] 2) Static safety constraints under normal operating conditions

[0025] During normal operation of a power system, the active power flow of each branch is required to not exceed the long-term current carrying capacity of the equipment, i.e.

[0026]

[0027] In the formula, This represents the active power flow value of branch ij flowing into branch i from node i; S represents the permissible long-term current-carrying capacity of branch ij; A Represents the set of all branches;

[0028] For DC power flow, the branch power flow is described as follows:

[0029]

[0030] In the formula, x ij Indicates the reactance of branch ij;

[0031] 3) Static safety constraints under anticipated fault conditions

[0032] A anticipated fault scan is performed on the anticipated fault set, and the power flow after a anticipated fault is described as a linear function of the fault-free power flow using the branch interruption distribution factor.

[0033]

[0034] In the formula, k represents the expected fault number; S K Represents the set of anticipated faults; L represents the active power flow from node i into branch ij; ij-mn This represents the distribution factor of branch ij when branch mn is disconnected; This represents the set of branches that are operating normally under the anticipated fault k. This represents the set of branches that are expected to be interrupted by fault k.

[0035] To ensure the safe operation of the power system, the power flow after a fault should not exceed the short-term overload capacity of the equipment.

[0036]

[0037] In the formula, This indicates the short-time allowable current-carrying capacity of branch ij;

[0038] Substituting equation (5) into equation (6) yields

[0039]

[0040] 4) Generator output constraints

[0041]

[0042] In the formula, P Gi This represents the minimum technical output of unit i at node i;

[0043] 5) System minimum standby capacity requirement constraints

[0044]

[0045] In the formula, P is the upper limit of active power output of node i unit; res This is the spinning reserve capacity required by the system.

[0046] In one embodiment of the present invention, step S3 is specifically implemented as follows:

[0047] Step S31: The number of static safety constraints formed by equations (3) and (7) is extremely large, but many of them are redundant and have no effect on the solution of the optimization problem. Eliminating these redundant constraints helps to reduce the computational scale of the optimization problem and improve the optimization solution speed.

[0048] Step S32: Using equations (3) and (7) as constraints, find the maximum and minimum values ​​of each constraint function. If the minimum value of the constraint function in the optimization result is greater than the lower limit of the constraint and the maximum value of the constraint function is less than the upper limit of the constraint, it indicates that the constraint is a redundant constraint. Excluding this type of constraint will not affect the optimization result.

[0049] In one embodiment of the present invention, step S4 is specifically implemented as follows:

[0050] Solve the minimum start-up combination model of thermal power considering safety constraints, which consists of equations (1) to (4) and equations (7) to (9), to obtain the initial minimum unit combination; the minimum unit combination model is mathematically a mixed integer linear optimization model, which is solved by the branch-cut plane method combined with the simplex method, or by commercial optimization software.

[0051] In one embodiment of the present invention, step S5 is specifically implemented as follows:

[0052] AC power flow calculations are performed based on the minimum start-up mode of thermal power plants obtained. During the calculation, the operating units are set according to PV nodes, the active power output is taken from the optimized result of the minimum start-up mode, and the generator terminal voltage setpoint is taken as 1.0 per unit.

[0053] In one embodiment of the present invention, step S6 is specifically implemented as follows:

[0054] Static voltage stability analysis refers to the distance from the system operating point along a predetermined load level to the voltage collapse critical point. The formula for calculating the relative margin of static voltage stability is as follows:

[0055]

[0056] In the formula, λ (k) This represents the voltage stability margin under the anticipated fault k. P represents the system's maximum power under the anticipated fault k. ∑ Indicates the total system load;

[0057] The anticipated fault set includes the system's normal state and N-1 fault states. After calculation, the actual voltage stability margin of the system under normal and anticipated fault states is obtained.

[0058] In one embodiment of the present invention, the voltage stability margin calculation method adopts the continuous power flow method.

[0059] In one embodiment of the present invention, step S7 is specifically implemented as follows:

[0060] Based on the voltage stability margin scan results, by comparing the actual voltage stability margin with the expected voltage stability margin, if the voltage stability margin under no-fault conditions and under all anticipated fault conditions is greater than the expected voltage stability margin, then the calculated minimum unit combination meets the voltage stability requirements, the optimization ends, and the minimum start-up scheme is obtained; otherwise, proceed to step S8.

[0061] In one embodiment of the present invention, step S8 is specifically implemented as follows:

[0062] Step S81: The sensitivity index of voltage amplitude to voltage stability margin can effectively reflect the impact of unit start-up on the static voltage stability of the system. Its calculation formula is as follows:

[0063]

[0064] In the formula, This represents the set of generator terminal nodes for units that are out of service. This indicates the sensitivity of the voltage amplitude at the terminal node g to the voltage stability margin under the anticipated fault state k. This represents the voltage amplitude at the terminal node g under the anticipated fault state k.

[0065] Step S82: Select units whose voltage amplitude sensitivity to voltage stability margin is negative and whose sensitivity value is less than 0.5 times the minimum sensitivity to include them in the candidate start-up set, and add constraints to the candidate start-up set:

[0066]

[0067] In the formula, This represents the set of candidate units to be started under the voltage stability constraint of the expected fault k. This constraint means that at least one unit in the candidate unit to be started under fault k needs to be started.

[0068] In one embodiment of the present invention, step S9 is specifically implemented as follows:

[0069] The minimum unit combination model considering static voltage stability and safety constraints is mathematically a mixed integer optimization model, which can be solved using the branch-cut plane method combined with the simplex method, or by using commercial optimization software.

[0070] Compared with existing technologies, this invention has the following advantages: Under the condition that the predicted output of new energy units, the output arrangement of hydropower and nuclear power, and the minimum reserve capacity requirement of the system are determined, this invention constructs a minimum start-up combination model of thermal power units considering static voltage stability and safety constraints, based on the set expected fault set and voltage stability margin requirements, with the goal of minimizing the start-up capacity of thermal power units. It satisfies the constraints of power balance at each node of the system, upper and lower limits of unit output, equipment transmission capacity constraints, N-1 safety constraints, and minimum reserve capacity requirements of the system. A solution method is also provided. By filtering the fault-free power flow and N-1 safety constraints for redundancy, the computational scale of the optimization problem is reduced, and the solution efficiency is improved. The minimum start-up method of thermal power units can maximize the consumption of new energy and reduce wind and solar curtailment losses while ensuring the safe operation requirements of the power system. Attached Figure Description

[0071] Figure 1 This is a schematic diagram of the overall process of an optimization method for minimum start-up combinations of thermal power plants that takes into account static voltage stability and safety constraints, as provided in this invention. Detailed Implementation

[0072] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0073] like Figure 1 As shown, this invention presents an optimization method for the minimum start-up combination of thermal power plants, considering static voltage stability and safety constraints. This method maximizes the absorption of new energy sources while ensuring the safe operation of the power system, including:

[0074] Step S1: Obtain the power grid model and prerequisite boundary conditions, including: predicted output of new energy units, output arrangement of hydropower and nuclear power, minimum reserve capacity requirement of the system, anticipated fault set and voltage stability margin requirements, etc.

[0075] Step S2: With the goal of minimizing the operating capacity of thermal power units, and while satisfying constraints such as power balance at each node of the system, upper and lower limits of unit output, equipment transmission capacity, N-1 safety constraints, and minimum reserve capacity requirements of the system, construct a minimum operating combination model for thermal power units that takes into account safety constraints.

[0076] Step S3: Redundancy constraint filtering is performed on the safety constraints of the fault-free state and the N-1 expected fault states to reduce the computational scale of the minimum start-up combination problem of thermal power considering safety constraints and improve the efficiency of solving the optimization problem.

[0077] Step S4: Solve the minimum start-up combination model with safety constraints to obtain the initial minimum unit combination;

[0078] Step S5: Calculate AC power flow based on the obtained minimum start-up mode of thermal power plants;

[0079] Step S6: Perform voltage stability margin scan and calculate the voltage stability margin for the fault-free state and each anticipated fault state.

[0080] Step S7: Determine whether the voltage stability margin under the fault-free state and each expected fault state meets the requirements. If it does, the optimization ends and the minimum start-up scheme is obtained; otherwise, proceed to step S8.

[0081] Step S8: For anticipated faults where the voltage stability margin does not meet the requirements, select out-of-operation units at weak voltage stability nodes to form a set of units to be started, and add at least one unit in the set of units to be started as a constraint to the optimization model, and construct a minimum unit combination model that considers static voltage stability and safety constraints.

[0082] Step S9: Solve the minimum unit combination model considering static voltage stability and safety constraints, determine the minimum unit combination, and go to step S5.

[0083] This invention constructs a minimum operating combination model for thermal power plants considering safety constraints. The objective is to minimize the operating capacity of thermal power plants. Furthermore, the safe operation of the power grid also requires meeting the following constraints: node power balance constraints under normal operating conditions, static safety constraints under normal and fault operating conditions, generator output constraints, and system reserve capacity demand constraints. The mathematical expressions for its objective function and constraints are as follows:

[0084]

[0085] The static safety constraints for the fault-free state and the N-1 anticipated fault state contain many redundant constraints. For each of these constraint functions, we calculate its maximum and minimum values. If the minimum value of the constraint function in the optimization result is greater than the lower constraint limit, and the maximum value is less than the upper constraint limit, then the constraint is considered redundant. Eliminating these redundant constraints helps reduce the computational scale of the optimization problem and improves the optimization solution speed.

[0086] The minimum start-up combination model under the above safety constraints is calculated to obtain the initial minimum unit combination; AC power flow calculation is performed based on the obtained minimum start-up mode of thermal power. In the calculation, the operating units are set according to PV nodes, the active power output is taken from the optimization result of the minimum start-up mode, and the terminal voltage setpoint is taken as 1.0 per unit.

[0087] A continuous power flow method is used to perform voltage stability margin scanning, calculating the voltage stability margin under fault-free conditions and for each anticipated fault condition. Static voltage stability analysis refers to the distance from the system operating point along a certain load level to the voltage collapse critical point. The formula for calculating the static voltage stability relative margin is as follows:

[0088]

[0089] In the formula, λ (k) This represents the voltage stability margin under the anticipated fault k. P represents the system's maximum power under the anticipated fault k. ∑ This indicates the total system load.

[0090] After calculation, the actual voltage stability margins under normal system conditions and anticipated fault conditions are obtained. Based on the voltage stability scan results, by comparing the actual voltage stability margins with the expected voltage stability margins, if the voltage stability margins under fault-free conditions and all anticipated fault conditions are greater than the expected voltage stability margins, then the calculated minimum unit combination meets the voltage stability requirements, the optimization ends, and the minimum start-up scheme is obtained.

[0091] Otherwise, for anticipated faults where the voltage stability margin does not meet the requirements, the set of units to be started is formed by selecting the out-of-operation units at the weak voltage stability nodes, and the requirement that at least one unit in the set of units to be started is added to the optimization model as a constraint. The minimum unit combination model considering static voltage stability and safety constraints is constructed, and the operation steps are as follows.

[0092] The sensitivity index of voltage amplitude to voltage stability margin can effectively reflect the impact of unit start-up on the static voltage stability of the system. Its calculation formula is as follows:

[0093]

[0094] In the formula, This represents the set of generator terminal nodes for units that are out of service. This indicates the sensitivity of the voltage amplitude at the terminal node g to the voltage stability margin under the anticipated fault state k. This represents the voltage amplitude at the terminal node g under the expected fault state k.

[0095] Units with negative sensitivity to voltage stability margin and sensitivity values ​​less than 0.5 times the minimum sensitivity are selected for inclusion in the candidate start-up set, and constraints are added to the candidate start-up set:

[0096]

[0097] In the formula, This represents the set of candidate units to be started under the voltage stability constraint of the anticipated fault k. This constraint means that at least one unit in the candidate unit to be started under fault k.

[0098] The minimum unit combination model considering static voltage stability and safety constraints is mathematically a mixed integer optimization model, which can be solved by combining the branch-cut plane method with the simplex method, or by using commercial optimization software (such as CPLEX).

[0099] To verify the effectiveness and feasibility of the proposed optimization method for minimum start-up combinations of thermal power plants that considers static voltage stability and safety constraints, a simulation was conducted on the software for the winter high-temperature operation mode of a certain power grid in 2030.

[0100] First, the power output arrangements for hydropower and nuclear power are determined: due to its high efficiency, nuclear power will generate electricity at maximum output; hydropower output will be based on the projected output. To maximize the absorption of new energy sources, new energy units will operate at the projected output; the minimum reserve capacity requirement for the system is 10% of the maximum load capacity. The total capacity of thermal power units is estimated at 39,940 MW.

[0101] The set of anticipated faults includes normal conditions, line N-1 disconnection (500kV), and main transformer N-1 disconnection (500kV). The voltage stability margin requirement under normal conditions is 8%, and the voltage stability margin requirement under fault conditions is 9%.

[0102] Based on equation (1), a minimum start-up combination model for thermal power plants considering safety constraints is constructed. For the redundant static safety constraints of the fault-free state and N-1 anticipated fault states in the equation, the optimization result value of each constraint function is calculated and judged to be between the upper and lower limits of the constraint. If so, it indicates that the constraint is a redundant constraint, and this type of constraint is excluded.

[0103] By solving the minimum start-up combination model with safety constraints, the initial minimum unit combination is obtained, as shown in Table 1.

[0104] Table 1 Summary of Initial Unit Combination

[0105]

[0106] The minimum start-up mode of the thermal power plant was obtained for AC power flow calculation, and the active power output was taken from the optimization result of the minimum start-up mode. Subsequently, voltage stability margin scanning was performed, and the static voltage stability margin under normal conditions and various anticipated fault conditions was calculated according to equation (2). Table 2 shows the actual voltage stability margin of some anticipated faults before and after adding static stability constraints.

[0107] Table 2. Voltage stability margins for some anticipated faults before and after adding static stability constraints.

[0108]

[0109] The voltage stability margins in Table 2 before adding constraints are the results of a voltage stability margin scan of the initial unit combination. By comparing the actual voltage stability margin with the expected voltage stability margin, the following anticipated faults are identified: AC line tripping occurs on line A 51525 when FuZHM1525 > 51525, and transformer tripping occurs on line B 51525.0 -> B Z11.0.

[0110] Based on equation (3), the units selected with large voltage stability margins and sensitivity, and currently in a shutdown state, constitute the proposed operating unit set: the pithead G6 unit and the combined heat and power (CHP) unit G1. The requirement of having at least one unit in the proposed operating unit set is added to the optimization model as a constraint. The minimum unit combination model considering static voltage stability constraints and safety constraints is calculated. Table 3 shows the adjustment measures for some unit combinations before and after incorporating static voltage stability constraints.

[0111] Table 3. Adjustment measures for some unit combinations before and after adding static voltage stability constraints.

[0112]

[0113] Table 3 shows the unit output values ​​before adding constraints, representing the initial unit combination. After adding voltage stability constraints, the unit output was adjusted. As shown in Table 3, after adding voltage stability constraints, units G6 at location b and G1 at location a were started. The resulting minimum unit combination was then scanned again for voltage stability margin. As shown in Table 2, the voltage stability margins after adding constraints were improved for two previously unmet expected stability margin requirements. The actual voltage stability margins under normal conditions and all expected fault conditions reached the expected voltage stability margins. The optimization was completed, and the calculated minimum unit combination met the voltage stability requirements, representing the minimum start-up scheme. The final start-up combination is the minimum unit combination that satisfies both economic and safety requirements.

[0114] The complete solution process in this example involves two voltage stability margin scans. If the required static voltage stability margin of the power grid is high, multiple rounds of iterative calculations using the minimum unit combination model that considers static voltage stability and safety constraints are required.

[0115] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., 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 optimizing the minimum start-up combination of thermal power plants considering static voltage stability and safety constraints, characterized in that, Includes the following steps: Step S1: Obtain the power grid model and prerequisite boundary conditions, including: predicted output of new energy units, output arrangement of hydropower and nuclear power, minimum reserve capacity requirement of the system, anticipated fault set and voltage stability margin requirements; Step S2: With the goal of minimizing the operating capacity of thermal power units, and in accordance with the constraints of power balance at each node of the system, upper and lower limits of unit output, equipment transmission capacity, safety constraints of N-1 anticipated fault states, and minimum reserve capacity requirements of the system, construct a minimum operating combination model for thermal power units that takes into account safety constraints. Step S3: Redundancy constraint filtering is performed on the safety constraints of the fault-free state and the N-1 expected fault states to reduce the computational scale of the minimum start-up combination problem of thermal power considering safety constraints and improve the efficiency of solving the optimization problem. Step S4: Solve the minimum start-up combination model of thermal power considering safety constraints to obtain the initial minimum unit combination; Step S5: Obtain the minimum start-up mode of thermal power based on the minimum unit combination and perform AC power flow calculation; Step S6: Perform voltage stability margin scan and calculate the voltage stability margin for the fault-free state and each anticipated fault state. Step S7: Determine whether the voltage stability margin under the fault-free state and each expected fault state meets the requirements. If it does, the optimization ends and the minimum start-up scheme is obtained; otherwise, proceed to step S8. Step S8: For anticipated faults where the voltage stability margin does not meet the requirements, select out-of-operation units at weak voltage stability nodes to form a set of units to be started, and add at least one unit in the set of units to be started as a constraint to the optimization model, and construct a minimum unit combination model that considers static voltage stability and safety constraints. Step S9: Solve the minimum unit combination model considering static voltage stability and safety constraints, determine the minimum unit combination, and go to step S5; Step S2 is implemented as follows: Step S21: With the goal of minimizing the operating capacity of thermal power plants, the corresponding objective function can be described as follows: In the formula, Indicates the maximum technical output of unit i; u Gi This is the start / stop variable for unit i at node; 1 indicates the unit is on, and 0 indicates the unit is off. G This represents the set of generator terminal nodes; Step S22: The safe operation of the power grid requires the following constraints to be met: 1) Node power balance constraints under normal operating conditions In the formula, the superscript 0 indicates the normal operating state, i.e., the fault-free state; P represents the active power output of unit i; Li B represents the active load of node i; ij This represents the element in the i-th row and j-th column of the nodal susceptance matrix; S represents the voltage phase angle at node j; Ni Represents node i and the set of its neighboring nodes; 2) Static safety constraints under normal operating conditions During normal operation of a power system, the active power flow of each branch is required to not exceed the long-term current carrying capacity of the equipment, i.e. In the formula, This represents the active power flow value of branch ij flowing into branch i from node i; S represents the permissible long-term current-carrying capacity of branch ij; A Represents the set of all branches; For DC power flow, the branch power flow is described as follows: In the formula, x ij Indicates the reactance of branch ij; 3) Static safety constraints under anticipated fault conditions A anticipated fault scan is performed on the anticipated fault set, and the power flow after a anticipated fault is described as a linear function of the fault-free power flow using the branch interruption distribution factor. In the formula, k represents the expected fault number; S K Represents the set of anticipated faults; L represents the active power flow from node i into branch ij; ij-mn This represents the distribution factor of branch ij when branch mn is disconnected; This represents the set of branches that are operating normally under the anticipated fault k. This represents the set of branches that are expected to be interrupted by fault k. To ensure the safe operation of the power system, the power flow after a fault should not exceed the short-term overload capacity of the equipment. In the formula, This indicates the short-time allowable current-carrying capacity of branch ij; Substituting equation (5) into equation (6) yields 4) Generator output constraints In the formula, P Gi This represents the minimum technical output of unit i at node i; 5) System minimum standby capacity requirement constraints In the formula, P res This is the spinning reserve capacity required by the system.

2. The method for optimizing the minimum start-up combination of thermal power plants considering static voltage stability and safety constraints according to claim 1, characterized in that, Step S3 is implemented as follows: Using equations (3) and (7) as constraints, calculate the maximum and minimum values ​​of each constraint function. If the minimum value of the constraint function in the optimization result is greater than the lower limit of the constraint and the maximum value of the constraint function is less than the upper limit of the constraint, it indicates that the constraint is a redundant constraint. Excluding such constraints will not affect the optimization result.

3. The method for optimizing the minimum start-up combination of thermal power plants considering static voltage stability and safety constraints according to claim 1, characterized in that, Step S4 is implemented as follows: Solve the minimum start-up combination model of thermal power considering safety constraints, which consists of equations (1) to (4) and equations (7) to (9), to obtain the initial minimum unit combination; the minimum unit combination model is mathematically a mixed integer linear optimization model, which is solved by the branch-cut plane method combined with the simplex method, or by commercial optimization software.

4. The method for optimizing the minimum start-up combination of thermal power plants considering static voltage stability and safety constraints according to claim 1, characterized in that, Step S5 is implemented as follows: AC power flow calculations are performed based on the minimum start-up mode of thermal power plants obtained. During the calculation, the operating units are set according to PV nodes, the active power output is taken from the optimized result of the minimum start-up mode, and the generator terminal voltage setpoint is taken as 1.0 per unit.

5. The method for optimizing the minimum start-up combination of thermal power plants considering static voltage stability and safety constraints according to claim 1, characterized in that, Step S6 is implemented as follows: Static voltage stability analysis refers to the distance from the system operating point along a predetermined load level to the voltage collapse critical point. The formula for calculating the relative margin of static voltage stability is as follows: In the formula, λ (k) This represents the voltage stability margin under the anticipated fault k. P represents the system's maximum power under the anticipated fault k. ∑ Indicates the total system load; The anticipated fault set includes the system's normal state and N-1 fault states. After calculation, the actual voltage stability margin of the system under normal and anticipated fault states is obtained.

6. The method for optimizing the minimum start-up combination of thermal power plants considering static voltage stability and safety constraints according to claim 5, characterized in that, The voltage stability margin is calculated using the continuous power flow method.

7. The method for optimizing the minimum start-up combination of thermal power plants considering static voltage stability and safety constraints according to claim 6, characterized in that, Step S7 is implemented as follows: Based on the voltage stability margin scan results, by comparing the actual voltage stability margin with the expected voltage stability margin, if the voltage stability margin under no-fault conditions and under all anticipated fault conditions is greater than the expected voltage stability margin, then the calculated minimum unit combination meets the voltage stability requirements, the optimization ends, and the minimum start-up scheme is obtained; otherwise, proceed to step S8.

8. The method for optimizing the minimum start-up combination of thermal power plants considering static voltage stability and safety constraints according to claim 1, characterized in that, Step S8 is implemented as follows: Step S81: The sensitivity index of voltage amplitude to voltage stability margin can effectively reflect the impact of unit start-up on the static voltage stability of the system. Its calculation formula is as follows: In the formula, This represents the set of generator terminal nodes for units that are out of service. This indicates the sensitivity of the voltage amplitude at the terminal node g to the voltage stability margin under the anticipated fault state k. This represents the voltage amplitude at the terminal node g under the anticipated fault state k. Step S82: Select units whose voltage amplitude sensitivity to voltage stability margin is negative and whose sensitivity value is less than 0.5 times the minimum sensitivity to include them in the candidate start-up set, and add constraints to the candidate start-up set: In the formula, This represents the set of candidate units to be started under the voltage stability constraint of the expected fault k. This constraint means that at least one unit in the candidate unit to be started under fault k needs to be started.

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