A load distribution method for security recovery of a stand-alone microgrid system

Abstract

The application discloses a load distribution method for safe recovery of an independent microgrid system, and belongs to the field of power systems and their automation, and aims to solve the technical problems of large calculation redundancy of a recovery process and easy secondary power outage. The method comprises the following steps: determining a single maximum load recovery amount based on steady-state and transient frequency safety constraints of the independent microgrid system; setting a transient voltage safety binary table according to transient voltage safety operation standards of loads and various types of power sources; obtaining motor slip through iteration by combining a motion equation of the motor with a speed-slip relationship equation; calculating a voltage value at the moment according to the motor slip and the single maximum load recovery amount; comparing the voltage value with a voltage set value to determine that the current checked load recovery amount is the single maximum recoverable load amount that meets frequency and voltage safety; and optimizing distribution of the maximum recoverable load amount according to net recovery benefits of loads to be recovered. The application can improve the calculation efficiency, safety and economy of a recovery process.

Description

Technical Field

[0001] This invention relates to a load distribution method for the safe restoration of an independent microgrid system, belonging to the field of power system and automation technology. Background Technology

[0002] The installed capacity of renewable energy sources in the power system has been increasing year by year, with renewable energy accounting for more than 50%-60% in some regions. However, renewable energy sources have insufficient tolerance to low frequencies and low voltages and suffer from low inertia. Extreme weather conditions can easily cause renewable energy sources to disconnect from the grid, leading to large-scale power outages. Furthermore, restoring the power system according to conventional constraints may result in restoration failures and secondary outages. With the integration of a large number of renewable energy sources and improvements in control capabilities, it is urgent to study parallel restoration decision-making methods for independent microgrids containing various power sources, including renewable energy sources, in order to fully leverage the role of renewable energy sources in preemptively restoring load power after grid outages. Currently, domestic and foreign scholars have conducted relevant research on power restoration in independent microgrids. The control modes of independent microgrids can be broadly divided into master-slave control and peer control. Considering that peer control lacks the support and regulation of the main grid, which can easily cause power supply security problems, master-slave control is often the primary approach in actual restoration strategies for independent microgrids. The master-slave independent microgrid recovery problem is essentially a high-dimensional power supply combination optimization problem. It usually aims to maximize the load recovery or continuous power supply, minimize the cost of switching operations, and minimize the recovery time. The above research objectives are studied from three aspects: resilience, security, and speed. In order to ensure safe power restoration, steady-state and transient security verification is required. Among them, transient verification consumes a lot of computation time. Summary of the Invention

[0003] The purpose of this invention is to provide a load allocation method for the safe restoration of an independent microgrid system. By constructing an assessment system for the load restoration capability of an independent microgrid system and optimizing power system restoration, efficient and economical system restoration can be achieved.

[0004] To solve the above-mentioned technical problems, the present invention is implemented using the following technical solution.

[0005] In a first aspect, the present invention provides a load distribution method for secure recovery of an independent microgrid system, comprising: Based on the steady-state and transient frequency security constraints of the independent microgrid system, the maximum allowable single load recovery amount during the recovery process of the independent microgrid system is determined. Based on the load of the independent microgrid system and the transient voltage safety operation standards of various types of power sources, determine the voltage setpoint and its corresponding duration setpoint for transient voltage safety verification. Combining the motion equations and speed-slip relationship equations of the motor in the independent microgrid system, the motor slip at the set duration value is obtained through iterative calculation; Calculate the voltage value at the set duration based on the motor slip and the maximum load recovery amount in a single operation; If the voltage value exceeds the voltage setting value, the maximum single load recovery amount is reduced proportionally, and the voltage safety verification is performed again; if the voltage value is within the voltage setting value range, the currently verified load recovery amount is determined to be the maximum single recoverable load amount that meets frequency and voltage safety requirements. With the goal of maximizing the net recovery benefit of the loads to be restored in the independent microgrid system, the maximum recoverable load amount that meets frequency and voltage safety requirements is optimized and allocated.

[0006] In conjunction with the first aspect, further, the maximum allowable single load recovery amount for the independent microgrid system during the recovery process is determined, including: Based on the maximum permissible deviation of the steady-state frequency of the independent microgrid system, determine the single power adjustment limit to ensure steady-state frequency security. ; Based on the safe operating range of transient frequencies of each power source in the independent microgrid system and the action threshold of the low-frequency load shedding device, the maximum permissible deviation of the transient frequency of the independent microgrid system is determined. ; According to the maximum permissible deviation of the transient frequency Based on the rated capacity of each power source in the independent microgrid system, determine the single power adjustment limit to meet transient frequency safety. ; The single power adjustment limit that satisfies steady-state frequency safety and the single power adjustment limit to meet transient frequency safety The minimum value is used as the maximum single load recovery amount to satisfy both steady-state and transient frequency safety. .

[0007] In conjunction with the first aspect, the expressions for determining the single-power adjustment limit that satisfies steady-state frequency security and the single-power adjustment limit that satisfies transient frequency security are as follows: ; ; in, This indicates the single-time power adjustment limit that satisfies steady-state frequency security. This indicates the maximum allowable steady-state frequency deviation for an independent microgrid system; Indicates the real-time hot standby coefficient; Indicates the equivalent unit of power supply adjustment power; Indicates the load adjustment effect coefficient; This indicates the single power adjustment limit that meets transient frequency safety requirements. This represents the maximum permissible transient frequency safety deviation for an independent microgrid system; k represents the power supply. Indicates the set of operating power supplies; Indicates the rated capacity of the power supply; This indicates the frequency response of the power supply.

[0008] In conjunction with the first aspect, the motion equation and speed-slip relationship equation of the motor in the independent microgrid system are further as follows: ; in, This represents the time corresponding to any slip value; This represents the inertial time constant of the electric motor unit; This represents the reference angular velocity of the electric motor. This represents the slippage value of the upper limit of the integral; This represents the electromagnetic torque of the electric motor; This indicates the mechanical torque of the electric motor; It represents the reciprocal function of the torque difference.

[0009] In conjunction with the first aspect, the motor slip at the set duration value is further obtained through iterative calculation, including: By discretizing the motion equation and speed-slip relationship equation of the electric motor using the composite Simpson formula, a discrete expression for the vertical relationship between calculation time and slip is obtained. Based on the discrete expression relating the calculation time to the vertical slip, iterative calculations are performed by adjusting the upper limit slip value of the integral, so that the calculated time approximates the set duration value. and with the duration set value The corresponding slip value is used as the slip of the motor. .

[0010] In conjunction with the first aspect, the discrete expression for the relationship between calculation time and vertical slip is further as follows: ; in, This represents the time corresponding to any slip value; Indicates a fixed integration step size; Indicates the number of equal divisions within the integration interval; Indicates the iteration node number in the numerical integration process; Indicates slip The reciprocal function value of the torque difference at that time; Indicates slip The reciprocal function value of the torque difference at that time; This represents the slip value corresponding to the i-th discrete node; Indicates slip The value of the reciprocal function of the torque difference at that time.

[0011] In conjunction with the first aspect, the iterative calculation by adjusting the slip value of the integral upper limit further includes: Assume a fixed integration step size. By modifying the number of equal divisions within the integration zone The value of is used to update the slippage value of the upper limit of the points. ; Based on the discrete expression relating the calculation time to the vertical slip, calculate the number of equal divisions within the current integration interval. corresponding time ; Compare the corresponding times With the duration setting value If the difference is less than a preset threshold, then the current integral upper limit slip value is taken as the motor slip. Otherwise, update the number of equal parts within the integration interval. And repeat the above calculation process.

[0012] In conjunction with the first aspect, the expression for calculating the voltage value at the set duration is as follows: ; in, Indicates the duration setting value The voltage value at that moment; This represents the maximum load recovery amount in a single operation that satisfies both steady-state and transient safety requirements. This represents the equivalent resistance of the motor rotor; This represents the equivalent impedance of the motor rotor. Indicates the slip of the electric motor; This represents the number of equal divisions within the integration interval; K represents the motor structural constant.

[0013] In conjunction with the first aspect, further optimization of the allocation of the single maximum recoverable load that satisfies frequency and voltage safety includes: With the goal of maximizing the net recovery benefit of the loads to be restored, the maximum recoverable load is allocated to the loads to be restored in descending order of load importance, while satisfying constraints on the maximum single-time load restoration of the independent microgrid system, the topology constraints of the independent microgrid system, the power balance constraints, the power supply capacity constraints, the branch transmission capacity constraints, the short-circuit capacity ratio constraints, and the system inertia constraints. The recovery process is discretized into multiple time steps, with each time step recovering a portion of the load.

[0014] In conjunction with the first aspect, the expressions for the single maximum load recovery constraint, the topology constraint, the power balance constraint, the power supply capacity constraint, the branch transmission capacity constraint, the short-circuit capacity ratio constraint, and the inertia constraint of the independent microgrid system are as follows: Maximum load recovery constraint for standalone microgrid systems in a single operation: ; in, This represents the load recovery amount of the nth independent microgrid system at time step t in the m-th time step; This represents the single maximum load recovery constraint at time t; Topology constraints of standalone microgrid systems: , ; in, This indicates the main power node controlled by V / f; This indicates the first slave power supply in a standalone microgrid system to adopt PQ control; This represents the k-th slave power supply in a standalone microgrid system that uses PQ control; This indicates the bus node where the main power supply is connected; This represents the intermediate connecting bus node within an independent microgrid; Indicates the first slave power source in a standalone microgrid system The connected bus node; Represents the k-th slave power source in a standalone microgrid system. The connected bus node; This refers to the collection of grid-type power sources in a standalone microgrid system; This represents a set of independent microgrid system topologies consisting of a main power source and several slave power sources. Power balance constraints: ; in, This represents the active power injected by the power source at node i in the nth independent microgrid at time t; This represents the active power of the load at time t of node i, which is assigned to the nth independent microgrid. represents the voltage value of node i, which is assigned to the nth independent microgrid, at time t; j represents the power node index value in the independent microgrid system; This represents the set of power nodes connected to an independent microgrid system. This represents the voltage value of node j, which has been assigned to the nth independent microgrid, at time t. Let represent the real part of the admittance of the line ij assigned to the nth independent microgrid; Let represent the imaginary part of the admittance of the line ij assigned to the nth independent microgrid; This represents the voltage phase difference between nodes i and j, which are assigned to the nth independent microgrid, at time t. This represents the reactive power injected by the power source at node i in the nth independent microgrid at time t; This represents the reactive power of node i, which is assigned to the nth independent microgrid, at time t; i represents the node index value in the independent microgrid system n. This represents the set of energized nodes within the power supply range of the nth independent microgrid system. Power capacity constraints: ; in, This represents the lower limit of the power supply capacity at node i in an independent microgrid system; This represents the upper limit of the power supply capacity at node i in an independent microgrid system; Tributary transmission capacity constraints: ; Where, represents the transmission capacity of line ij assigned to the nth independent microgrid at time t; represents the upper limit of the transmission capacity of line ij assigned to the nth independent microgrid at time t; represents the set of all transmission branches included in the independent microgrid system; Short-circuit capacity ratio constraint: ; Where K represents the motor structure constant; This represents the rated capacity of the power supply at node i; This represents the total capacity of asynchronous motors already in operation within the independent microgrid system; This represents the capacity of the asynchronous motor to be restored at node i at the current moment; Inertia constraint: ; ; in, Indicates the upper limit of the rate of change of frequency; Indicates the lower limit of the rate of change of frequency; Indicates the maximum load recovery amount in a single instance; This represents the equivalent system inertial time constant after the independent microgrid system has recovered. Represents the equivalent inertial time constant; Indicates the number of power sources; k represents the number of power sources; Indicates the rated capacity of the power supply; This represents the inertial time constant of the power source.

[0015] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: (1) High computational efficiency: This paper uses independent microgrid pre-decision to screen feasible solutions in advance, avoiding computational redundancy in real-time verification during the recovery process, and greatly improving the solution efficiency; (2) High safety: Taking into account multiple safety constraints such as frequency, voltage, and inertia, the risk of secondary power outages is effectively avoided; (3) Good economic efficiency: With the goal of maximizing net recovery benefits, the system takes into account load recovery benefits, power supply costs and recovery risks, and achieves optimal economic efficiency. (4) Wide applicability: It is applicable to power systems with a high proportion of new energy sources and can give full play to the role of new energy sources in system recovery. Attached Figure Description

[0016] Figure 1 The diagram shows a flowchart of a load distribution method for secure recovery of an independent microgrid system provided by an embodiment of the present invention. Detailed Implementation

[0017] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solution of the present invention, rather than limitations thereof. In the absence of conflict, the embodiments of the present invention and the technical features in the embodiments can be combined with each other. Example 1

[0018] See Figure 1 This embodiment introduces a load allocation method for the safe restoration of an independent microgrid system. This method, by constructing a pre-decision model (including load restoration capability assessment and constraints), screens feasible load restoration schemes before power restoration, thereby significantly improving the safety and economy of the restoration process. Specifically, it includes: First, based on the steady-state and transient frequency security constraints of the stand-alone microgrid system, the maximum allowable single load restoration amount during the recovery process of the stand-alone microgrid system is determined. Ensure that the frequency is within a safe range.

[0019] Next, based on the load of the independent microgrid system and the transient voltage safety operation standards of various types of power sources, a binary table describing transient voltage safety is established. This binary table includes voltage setpoints. and its corresponding duration setting value This provides a basis for voltage safety verification; Based on this, and combining the motion equations of the motors and the speed-slip relationship equations in the independent microgrid system, the set value for the duration is obtained through iterative calculation. Motor slip Furthermore, based on the slip of the motor With the maximum load recovery amount in a single instance Calculate the voltage value at this moment. If the calculated voltage value Exceeding the voltage setting value in the binary table Then the maximum single load recovery amount will be... Reduce the voltage proportionally and re-verify the voltage safety; if the voltage value in the binary table... At this voltage setting value If the load recovery amount is within the specified range, then the current load recovery amount is determined to be the maximum recoverable load amount that satisfies frequency and voltage safety in a single operation.

[0020] Finally, based on the net recovery benefit of the load to be restored in the standalone microgrid system. Taking into account the benefits of load recovery Electricity supply costs With recovery risk The maximum recoverable load for a single operation that meets frequency and voltage safety requirements is optimally allocated, thereby maximizing the economy of the recovery process.

[0021] In summary, this method effectively avoids the burden of real-time security verification during the recovery process through a pre-decision mechanism, improves computational efficiency and recovery reliability, and is particularly suitable for power system recovery scenarios with a high proportion of renewable energy. Example 2

[0022] This embodiment constructs an independent microgrid simulation system that includes a master-slave control power supply architecture and multi-load characteristics. The system consists of a large-capacity grid-type energy storage master power supply with V / f control, a wind and solar grid-type slave power supply with PQ control, a load side including graded static loads and a large-capacity asynchronous motor for transient voltage verification, and network nodes equipped with low-frequency load shedding protection devices.

[0023] The purpose of this system is to simulate how to safely and orderly restore power to an independent microgrid after a power outage. When establishing an independent microgrid to restore power, all loads cannot be put into operation at once; otherwise, sudden power surges could cause motor frequency and voltage overruns, leading to another system collapse. Therefore, a phased and orderly load restoration approach is necessary, with each step restoring loads within a safe limit. The primary consideration in determining this limit is the system's frequency stability. The specific steps are as follows: Step 1: Identify the load limits to be restored for the steady-state and transient frequency security of the standalone microgrid system during the recovery process. Specifically: Step S11: Determine the single power adjustment limit that meets the steady-state frequency security requirement based on the maximum permissible deviation of the steady-state frequency of the independent microgrid system. ; The expression for determining the single power adjustment limit that satisfies steady-state frequency security is as follows: (1) in, This indicates the single-time power adjustment limit that satisfies steady-state frequency security. This indicates the maximum allowable steady-state frequency deviation for an independent microgrid system; Indicates the real-time hot standby coefficient; Indicates the equivalent unit of power supply adjustment power; Indicates the load adjustment effect coefficient; This indicates the comprehensive regulation capability of an independent microgrid system.

[0024] Step S12: Determine the maximum permissible transient frequency deviation of the independent microgrid system based on the safe operating range of the transient frequency of each power source in the independent microgrid system and the action threshold of the low-frequency load shedding device. ; Considering the inconsistency of transient frequency ranges among power sources in an independent microgrid system, in order to avoid the transient frequency of the independent microgrid system exceeding the limit and causing normally operating or restored units in the independent microgrid system to be shut down due to frequency exceeding the limit, the maximum allowable deviation of transient frequency is determined based on the safe operating range of the transient frequency of the already operating power sources in the independent microgrid system to prevent the low-frequency load shedding device installed in the independent microgrid system from malfunctioning.

[0025] The maximum allowable deviation of the set transient frequency The expression is: (2) in, Indicates the maximum permissible deviation of transient frequency; This indicates the maximum permissible frequency deviation for normal operation of the power supply in an independent microgrid system; This indicates the maximum operating threshold of the low-frequency load shedding device installed within the independent microgrid system; represents the set of operating power supplies; k represents the power supply.

[0026] Step S13: Based on the allowable deviation of the maximum transient frequency Based on the rated capacity of each power source in the independent microgrid system, determine the single power regulation limit to meet transient frequency safety. ; The determination of the single power adjustment limit that satisfies transient frequency safety The expression is: (3) in, This indicates the single power adjustment limit that meets transient frequency safety requirements. Indicates the rated capacity of the power supply; This indicates the frequency response of the power supply.

[0027] Step S14: Set the single power adjustment limit to meet steady-state frequency security. and the single power adjustment limit to meet transient frequency safety The minimum value is used as the maximum single load recovery amount to satisfy both steady-state and transient frequency safety. Its expression is: (4) in, This indicates the maximum load recovery amount in a single instance.

[0028] Step 2: Security assessment during the recovery process of the standalone microgrid system.

[0029] The current step considers the system's safety and stability. Since motor load restoration can significantly impact the transient voltage of the power system, and the transient voltage response is generally faster than the frequency response, it may lead to a situation where the transient voltage drops drastically while the frequency still meets requirements. Therefore, step 2 needs to further determine the maximum single load restoration amount that satisfies both steady-state and transient frequency safety, as determined in step 1. Whether it will cause transient voltage exceedance, the details are as follows: Step S21: Based on the load of the independent microgrid system and the transient voltage safety operation standards of various types of power sources, set voltage setpoints. and its corresponding duration setting value Transient voltage safety binary meter; Specifically, taking into account the permissible transient voltage drop standards for various types of power sources, a binary table describing voltage safety during load recovery is conservatively set based on the permissible voltage thresholds for load and photovoltaic power, as well as the threshold for duration exceeding the threshold.

[0030] Step S22: Combining the motion equation of the motor and the speed-slip relationship equation of the motor in the independent microgrid system, the motor slip at the set duration value is obtained by iterative calculation; Combining the equation of motion of the electric motor and the equation relating the motor speed to slip, for any integral upper limit slip value... and the corresponding time As shown in the following formula (5): (5) in, This represents the time corresponding to any slip value; This represents the inertial time constant of the electric motor unit; This represents the reference angular velocity of the electric motor. This represents the slippage value of the upper limit of the integral; This represents the electromagnetic torque of the electric motor; This indicates the mechanical torque of the electric motor; It represents the reciprocal function of the torque difference.

[0031] Furthermore, the motor slip at the set duration value is obtained through iterative calculation, including: Step S221: Discretize the motion equation and speed-slip relationship equation of the electric motor by using the composite Simpson formula to obtain a discrete expression for the vertical relationship between calculation time and slip, as follows: (6) in, Indicates a fixed integration step size; Indicates the number of equal divisions within the integration interval; Indicates the iteration node number in the numerical integration process; Indicates slip The reciprocal function value of the torque difference at that time; Indicates slip The reciprocal function value of the torque difference at that time; This represents the slip value corresponding to the i-th discrete node; Indicates slip The value of the reciprocal function of the torque difference at that time.

[0032] Step S222: Based on the discrete expression relating computation time and vertical slip, adjust the upper limit of the integral slip value. Perform iterative calculations to make the calculated time approximate the set duration value. and set the duration value The corresponding slip value is used as the slip of the motor. .

[0033] Specifically, set a fixed integration step size. By modifying the number of equal divisions within the integration zone The value of is used to update the slippage value of the upper limit of the points. ; Based on the discrete expression relating computation time to the vertical slip, calculate the number of equal divisions within the current integration interval. corresponding time ; Compare the corresponding times With the duration setting value If the difference is less than a preset threshold, then the current integral upper limit slip value is taken as the slip of the motor. Otherwise, update the number of equal parts within the integration interval. And repeat the above calculation process.

[0034] Step S23: Based on the motor slip and maximum load recovery in a single operation Calculate the duration set value Voltage value at time .

[0035] The expression for calculating the voltage value at the specified duration setting is as follows: (7) in, Indicates the duration setting value The voltage value at that moment; This represents the equivalent resistance of the motor rotor; This represents the equivalent impedance of the motor rotor. represents the slip of the motor; represents the number of equal divisions within the integral interval; K represents the motor structural constant.

[0036] Step 3: Compare voltage values With the voltage setting value in the binary table If the voltage value Exceeding the voltage setting value Then the maximum single load recovery amount will be... Reduce the voltage proportionally and re-verify its safety; if the voltage value At the voltage setting value Within this range, the current load recovery amount is determined to be the maximum recoverable load amount for a single operation that satisfies frequency and voltage safety. .

[0037] Specifically, such as in the duration setting value Voltage value at time Still at the voltage setting value In addition, determine the maximum recoverable load in a single instance. This may cause transient voltage exceeding the limit. In this embodiment of the invention, the maximum recoverable load per instance is... Restore to 95% of the original value and return to step 2 to determine if the transient voltage exceeds the limit. If it is within the duration set value... Voltage value at time At voltage setting value Within, then determined This represents the maximum single load restoration amount required to meet transient frequency and voltage safety requirements for current independent microgrid systems.

[0038] Step 4: Optimize the allocation of the maximum recoverable load per cycle that meets frequency and voltage safety requirements, with the net recovery benefit of the load to be restored in the independent microgrid system as the objective.

[0039] Specifically, with the objective of maximizing the net recovery benefit of loads to be restored in an independent microgrid system, the maximum recoverable load is allocated sequentially to the loads to be restored according to the loads' importance from high to low, while satisfying constraints on the maximum single-time load restoration of the independent microgrid system, topology constraints, power balance constraints, power supply capacity constraints, branch transmission capacity constraints, short-circuit capacity ratio constraints, and system inertia constraints. The expected net recovery benefit of the load in the nth independent microgrid system is... The expression is as follows: (8) in, This represents the expected net recovery benefit of the load in the nth independent microgrid system; This represents the load recovery benefit of the nth independent microgrid system; This represents the power supply cost of the nth independent microgrid system; This represents the recovery risk of the nth independent microgrid system.

[0040] It should be noted that load recovery benefits With power supply cost The specific calculation method for the assessment period is as follows: Independent microgrid systems are based on their continuous power supply level. Loads are restored in time-sharing steps, in descending order of importance. Corresponding to this process, the load restoration benefit... and power supply costs Defined by the following integrals respectively: (9) (10) in, This represents the total number of steps in the recovery plan for the nth independent microgrid system; m represents the time step. Indicates the preset end time of the evaluation; This represents the load recovery scheme time of the nth independent microgrid system at time step m; This represents the unit load recovery benefit of the nth independent microgrid system at time step m t; This represents the load recovery amount of the nth independent microgrid system at time step t in the m-th time step; This represents the unit power generation cost of the nth independent microgrid system at time step m t.

[0041] Furthermore, the process of restoring load in a time-sharing manner faces two types of risks: branch circuit failure risk. Uncertainty risks associated with new energy power output Among them, the risk of branch circuit failure. This reflects the potential losses caused by the changing probability of faults in each branch over time during the restoration process, leading to the failure of independent microgrid restoration and the re-loss of restored loads; and the uncertainty risk of renewable energy output. This stems from errors in forecasting the output of stochastic power sources such as wind and solar power, which may lead to insufficient actual power supply capacity and thus load loss. These two types of risks together constitute recovery risk. Its expression is as follows: (11) in, This indicates two types of recovery risks; This indicates the risk of failure of an independent microgrid system due to the time-varying probability of branch failures; This indicates the risk of load loss caused by the uncertainty of random power output in an independent microgrid system.

[0042] Specifically, the risk of failure in standalone microgrid systems due to the time-varying probability of branch failures. Estimate using the following formula: (12) (13) in, This represents the failure probability of the l-th branch of the independent microgrid system at time step k. Indicates a branch road; Represents the set of branches of the independent microgrid system in the sub-scheme; This represents the failure probability of the i-th branch in an independent microgrid system at time t; This represents the load recovery scheme time of the nth independent microgrid system at time step m-1; This represents the failure probability of the nth independent microgrid system at time step k.

[0043] Uncertainty risk of new energy output It can be represented as: (14) in, This indicates the processing of prediction error sub-intervals; Let represent the set of sub-intervals of output prediction error of renewable energy sources at the m-th time step in the n-th independent microgrid system; This represents the probability that the output prediction error of the renewable energy source at time step t in the nth independent microgrid system is located within the output prediction error sub-interval. This represents the prediction error of the power output prediction error sub-interval at the m-th time step t in the n-th independent microgrid system. The value is the median of several prediction error values ​​within the power output prediction error sub-interval r.

[0044] In addition, based on net income When optimizing load allocation to maximize the objective, the generated recovery plan must not only meet the aforementioned safety limits, but also strictly adhere to the following physical and operational constraints to ensure the feasibility of the recovery process and system stability, as follows: Maximum load recovery constraint for standalone microgrid systems in a single operation: (15) in, This represents the load recovery amount of the nth independent microgrid system at time step t in the m-th time step; This represents the single maximum load recovery constraint at time t; Topology constraints of standalone microgrid systems: , (16) in, This indicates the main power node controlled by V / f; This indicates the first slave power supply in a standalone microgrid system to adopt PQ control; This represents the k-th slave power supply in a standalone microgrid system that uses PQ control; This indicates the bus node where the main power supply is connected; This represents the intermediate connecting bus node within an independent microgrid; Indicates the first slave power source in a standalone microgrid system The connected bus node; Represents the k-th slave power source in a standalone microgrid system. The connected bus node; This refers to the collection of grid-type power sources in a standalone microgrid system; This represents a set of independent microgrid system topologies consisting of a main power source and several slave power sources. Power balance constraints: (17) in, This represents the active power injected by the power source at node i in the nth independent microgrid at time t; This represents the active power of the load at time t of node i, which is assigned to the nth independent microgrid. represents the voltage value of node i, which is assigned to the nth independent microgrid, at time t; j represents the power node index value in the independent microgrid system; This represents the set of power nodes connected to an independent microgrid system. This represents the voltage value of node j, which has been assigned to the nth independent microgrid, at time t. Let represent the real part of the admittance of the line ij assigned to the nth independent microgrid; Let represent the imaginary part of the admittance of the line ij assigned to the nth independent microgrid; This represents the voltage phase difference between nodes i and j, which are assigned to the nth independent microgrid, at time t. This represents the reactive power injected by the power source at node i in the nth independent microgrid at time t; This represents the reactive power of node i, which is assigned to the nth independent microgrid, at time t; i represents the node index value in the independent microgrid system n. This represents the set of energized nodes within the power supply range of the nth independent microgrid system. Power capacity constraints: (18) in, This represents the lower limit of the power supply capacity at node i in an independent microgrid system; This represents the upper limit of the power supply capacity at node i in an independent microgrid system; Tributary transmission capacity constraints: (19) in, This represents the transmission capacity of line ij, which is assigned to the nth independent micronet, at time t. This represents the maximum transmission capacity of line ij, which is assigned to the nth independent micronet, at time t. This represents the set of all transmission branches included in an independent microgrid system; Short-circuit capacity ratio constraint: (20) Where K represents the motor structure constant; This represents the rated capacity of the power supply at node i; This represents the total capacity of asynchronous motors already in operation within the independent microgrid system; This represents the capacity of the asynchronous motor to be restored at node i at the current moment; Inertia constraint: ;(twenty one) ;(twenty two) in, Indicates the upper limit of the rate of change of frequency; This indicates the lower limit of the rate of change of frequency, typically -0.5 Hz / s; Indicates the maximum load recovery amount in a single instance; This represents the equivalent system inertial time constant after the independent microgrid system has recovered. Represents the equivalent inertial time constant; Indicates the number of power sources; k represents the number of power sources; Indicates the rated capacity of the power supply; This represents the inertial time constant of the power source.

[0045] In summary, this invention addresses the issues of high computational redundancy and susceptibility to secondary power outages in high-renewable-proportion power systems by proposing a power system restoration strategy that considers the pre-decision-making of independent microgrids. The strategy proposes a method for assessing the recoverability limits of single-load power sources, oriented towards frequency and voltage security, providing a quantitative basis for system restoration. It establishes a master-slave independent microgrid pre-decision-making model, significantly reducing computational complexity by pre-screening feasible independent microgrid schemes before restoration. Finally, it constructs a power system restoration optimization model that comprehensively considers multiple objectives, including economy and security, to select the optimal restoration scheme.

[0046] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A load distribution method for secure recovery of an independent microgrid system, characterized in that, include: Based on the steady-state and transient frequency security constraints of the independent microgrid system, the maximum allowable single load recovery amount during the recovery process of the independent microgrid system is determined. Based on the load of the independent microgrid system and the transient voltage safety operation standards of various types of power sources, determine the voltage setpoint and its corresponding duration setpoint. Combining the motion equations and speed-slip relationship equations of the motor in the independent microgrid system, the motor slip at the set duration value is obtained through iterative calculation; Calculate the voltage value at the set duration based on the motor slip and the maximum load recovery amount in a single operation; If the voltage value exceeds the voltage setting value, the maximum single load recovery amount will be reduced proportionally, and the voltage safety verification will be performed again. If the voltage value is within the voltage setting range, then the current load recovery amount is determined to be the maximum recoverable load amount that satisfies frequency and voltage safety in a single operation. With the goal of maximizing the net recovery benefit of the loads to be restored in the independent microgrid system, the maximum recoverable load amount that meets frequency and voltage safety requirements is optimized and allocated.

2. The load distribution method for secure recovery of an independent microgrid system according to claim 1, characterized in that, Determining the maximum allowable single load recovery amount for the independent microgrid system during the recovery process includes: Based on the maximum permissible deviation of the steady-state frequency of the independent microgrid system, determine the single power adjustment limit to ensure steady-state frequency security. ; Based on the safe operating range of transient frequencies of each power source in the independent microgrid system and the action threshold of the low-frequency load shedding device, the maximum permissible deviation of the transient frequency of the independent microgrid system is determined. ; According to the maximum permissible deviation of the transient frequency Based on the rated capacity of each power source in the independent microgrid system, determine the single power adjustment limit to meet transient frequency safety. ; The single power adjustment limit that satisfies steady-state frequency safety and the single power adjustment limit to meet transient frequency safety The minimum value is used as the maximum single load recovery amount to satisfy both steady-state and transient frequency safety. .

3. The load distribution method for secure recovery of an independent microgrid system according to claim 2, characterized in that, The expressions for determining the single-power adjustment limit that satisfies steady-state frequency security and the single-power adjustment limit that satisfies transient frequency security are as follows: ； ； in, This indicates the single-time power adjustment limit that satisfies steady-state frequency security. This indicates the maximum allowable steady-state frequency deviation for an independent microgrid system; Indicates the real-time hot standby coefficient; Indicates the equivalent unit of power supply adjustment power; Indicates the load adjustment effect coefficient; This indicates the single power adjustment limit that meets transient frequency safety requirements. This represents the maximum permissible transient frequency safety deviation for an independent microgrid system; k represents the power supply. Indicates the set of operating power supplies; Indicates the rated capacity of the power supply; This indicates the frequency response of the power supply.

4. The load distribution method for secure recovery of an independent microgrid system according to claim 1, characterized in that, The equation of motion and the speed-slip relationship equation of the motor in the independent microgrid system are as follows: ； in, This represents the time corresponding to any slip value; This represents the inertial time constant of the electric motor unit; This represents the reference angular velocity of the electric motor. This represents the slippage value of the upper limit of the integral; This represents the electromagnetic torque of the electric motor; This indicates the mechanical torque of the electric motor; It represents the reciprocal function of the torque difference.

5. The load distribution method for secure recovery of an independent microgrid system according to claim 4, characterized in that, The motor slip at the set duration value is obtained through iterative calculation, including: By discretizing the motion equation and speed-slip relationship equation of the electric motor using the composite Simpson formula, a discrete expression for the vertical relationship between calculation time and slip is obtained. Based on the discrete expression relating the calculation time to the vertical slip, iterative calculations are performed by adjusting the upper limit slip value of the integral, so that the calculated time approximates the set duration value. and with the duration set value The corresponding slip value is used as the slip of the motor. .

6. The load distribution method for secure recovery of an independent microgrid system according to claim 5, characterized in that, The discrete expression for the relationship between calculation time and vertical slip is: ； in, This represents the time corresponding to any slip value; Indicates a fixed integration step size; Indicates the number of equal divisions within the integration interval; Indicates the iteration node number in the numerical integration process; Indicates slip The reciprocal function value of the torque difference at that time; Indicates slip The reciprocal function value of the torque difference at that time; This represents the slip value corresponding to the i-th discrete node; Indicates slip The value of the reciprocal function of the torque difference at that time.

7. The load distribution method for secure recovery of an independent microgrid system according to claim 6, characterized in that, The iterative calculation by adjusting the slip value of the integral upper limit includes: Set a fixed integration step size By modifying the number of equal divisions within the integration zone The value of is used to update the slippage value of the upper limit of the points. ; Based on the discrete expression relating the calculation time to the vertical slip, calculate the number of equal divisions within the current integration interval. corresponding time ; Compare the corresponding times With the duration setting value If the difference is less than a preset threshold, then the current integral upper limit slip value is taken as the motor slip. Otherwise, update the number of equal parts within the integration interval. And repeat the above calculation process.

8. The load distribution method for secure recovery of an independent microgrid system according to claim 1, characterized in that, The expression for calculating the voltage value at the specified duration setting is as follows: ； in, Indicates the duration setting value The voltage value at that moment; This represents the maximum load recovery amount in a single operation that satisfies both steady-state and transient safety requirements. This represents the equivalent resistance of the motor rotor; This represents the equivalent impedance of the motor rotor. Indicates the slip of the electric motor; This represents the number of equal divisions within the integration interval; K represents the motor structural constant.

9. The load distribution method for secure recovery of an independent microgrid system according to claim 1, characterized in that, Optimize the allocation of the maximum recoverable single load that meets frequency and voltage safety requirements, including: With the goal of maximizing the net recovery benefit of the loads to be restored, the maximum recoverable load is allocated to the loads to be restored in descending order of load importance, while satisfying constraints on the maximum single-time load restoration of the independent microgrid system, the topology constraints of the independent microgrid system, the power balance constraints, the power supply capacity constraints, the branch transmission capacity constraints, the short-circuit capacity ratio constraints, and the system inertia constraints. The recovery process is discretized into multiple time steps, with each time step recovering a portion of the load.

10. The load distribution method for secure recovery of an independent microgrid system according to claim 9, characterized in that, The expressions for the single maximum load recovery constraint, topology constraint, power balance constraint, power supply capacity constraint, branch transmission capacity constraint, short-circuit capacity ratio constraint, and inertia constraint of the independent microgrid system are as follows: Maximum load recovery constraint for standalone microgrid systems in a single operation: ； in, This represents the load recovery amount of the nth independent microgrid system at time step t in the m-th time step; This represents the single maximum load recovery constraint at time t; Topology constraints of standalone microgrid systems: ， ； in, This indicates the main power node controlled by V / f; This indicates the first slave power supply in a standalone microgrid system to adopt PQ control; This represents the k-th slave power supply in a standalone microgrid system that uses PQ control; This indicates the bus node where the main power supply is connected; This represents the intermediate connecting bus node within an independent microgrid; Indicates the first slave power source in a standalone microgrid system The connected bus node; Represents the k-th slave power source in a standalone microgrid system. The connected bus node; This refers to the collection of grid-type power sources in a standalone microgrid system; This represents a set of independent microgrid system topologies consisting of a main power source and several slave power sources. Power balance constraints: ； in, This represents the active power injected by the power source at node i in the nth independent microgrid at time t; This represents the active power of the load at time t of node i, which is assigned to the nth independent microgrid. represents the voltage value of node i, which is assigned to the nth independent microgrid, at time t; j represents the power node index value in the independent microgrid system; This represents the set of power nodes connected to an independent microgrid system. This represents the voltage value of node j, which has been assigned to the nth independent microgrid, at time t. Let represent the real part of the admittance of the line ij assigned to the nth independent microgrid; Let represent the imaginary part of the admittance of the line ij assigned to the nth independent microgrid; This represents the voltage phase difference between nodes i and j, which are assigned to the nth independent microgrid, at time t. This represents the reactive power injected by the power source at node i in the nth independent microgrid at time t; This represents the reactive power of node i, which is assigned to the nth independent microgrid, at time t; i represents the node index value in the independent microgrid system n. This represents the set of energized nodes within the power supply range of the nth independent microgrid system. Power capacity constraints: ； in, This represents the lower limit of the power supply capacity at node i in an independent microgrid system; This represents the upper limit of the power supply capacity at node i in an independent microgrid system; Tributary transmission capacity constraints: ； in, This represents the transmission capacity of line ij, which is assigned to the nth independent micronet, at time t. This represents the maximum transmission capacity of line ij, which is assigned to the nth independent micronet, at time t. This represents the set of all transmission branches included in an independent microgrid system; Short-circuit capacity ratio constraint: ； Where K represents the motor structure constant; This represents the rated capacity of the power supply at node i; This represents the total capacity of asynchronous motors already in operation within the independent microgrid system; This represents the capacity of the asynchronous motor to be restored at node i at the current moment; Inertia constraint: ； ； in, Indicates the upper limit of the rate of change of frequency; Indicates the lower limit of the rate of change of frequency; Indicates the maximum load recovery amount in a single instance; This represents the equivalent system inertial time constant after the independent microgrid system has recovered. Represents the equivalent inertial time constant; Indicates the number of power sources; k represents the number of power sources; Indicates the rated capacity of the power supply; This represents the inertial time constant of the power source.

Images