UPFC (Unified Power Factor Correction)-based alternating current / direct current hybrid power grid dual-time scale voltage control method and system

Abstract

According to the UPFC-based AC / DC hybrid power grid dual-time scale voltage control method and system, the reactive power regulation margin of the UPFC is accurately described, the voltage control characteristics of the UPFC are further considered on the basis of considering the OLTC and CB voltage control characteristics, fast and slow time scale control object cooperation is realized through a dual-time scale control architecture, and the UPFC reactive power regulation margin is accurately described. Extra-high voltage direct current drop point reactive power requirements are fully considered, and more sufficient dynamic reactive power reserve is reserved to cope with dynamic regulation requirements of a power grid while the economic requirements of power grid operation are met based on track sensitivity, so that a reasonable and efficient voltage control method is formulated. And the advantages of the double-time-scale voltage control method and system for the alternating-current and direct-current hybrid power grid based on the UPFC can be reflected.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system control, and in particular relates to a dual-time-scale voltage control method and system for an AC / DC hybrid power grid based on UPFC. Background Art

[0002] my country's power system is characterized by hybrid AC / DC power grids and large-scale cross-regional power transmission, so the complexity of the power grid and the difficulty of voltage regulation have increased significantly. In an AC / DC hybrid power grid, AC and DC, the sending end and the receiving end are closely coupled, and the use of different types of voltage regulation equipment affects the operating status of the power grid at different time scales. Traditional power grids control the voltage of the power grid by controlling on-load tap-changing transformers and capacitor banks, but they are discrete adjustments, and the equipment response speed is insufficient, the dynamic reactive power reserve is insufficient, and the coordination of voltage regulation equipment at different time scales is difficult. Although the traditional method has a significant steady-state regulation effect, it is difficult to meet the needs of rapid voltage control.

[0003] In the prior art, for example, the patent application with publication number CN116169686A obtains the initial power flow result and state variable value by calculating the power flow of the AC / DC hybrid system; based on the obtained initial power flow result and state variable value, the active network loss and voltage offset of the AC / DC hybrid system are calculated, and a multi-objective reactive power optimization model with the minimum active network loss and voltage offset as the goal is established; the whale swarm optimization model solving algorithm combining particle swarm and whale algorithm is used to solve the multi-objective reactive power optimization model and obtain the optimization result; based on the obtained optimization result, the reactive power voltage control of the AC / DC hybrid system is realized. Another example is the patent application with publication number CN117595251A, which determines the calculation model according to the operating constraints of the dynamic reactive power reserve evaluation of the AC / DC hybrid power grid; then establishes the optimization model with the minimum dynamic reactive power reserve as the goal; then determines whether the operating constraints are linear, constructs an equivalent linear optimization model for solution if linear, and uses the dynamic constraint coefficient shearing algorithm for conversion and solution if nonlinear, and obtains the optimization result; finally, a voltage stability control strategy is formulated according to the optimization result, and the voltage control of the AC / DC hybrid power grid is executed. However, the existing technology does not introduce UPFC equipment in the process of voltage control, relies on single time scale optimization during control, does not distinguish the response speed differences of different voltage control devices, lacks trajectory sensitivity analysis, and leads to insufficient dynamic reactive power reserve, inaccurate dynamic resource allocation, low efficiency of multi-device coordination, and difficulty in dealing with sudden voltage fluctuations. Therefore, there is an urgent need for a dynamic adjustment method for voltage stability and rapid control in AC / DC hybrid power grids. Summary of the invention

[0004] In order to solve the deficiencies in the prior art, the present invention provides a dual-time-scale voltage control method and system for an AC / DC hybrid power grid based on UPFC, which can accurately describe the reactive power regulation margin of UPFC, and further consider the voltage control characteristics of UPFC on the basis of considering the voltage control characteristics of OLTC and CB. The coordination of fast and slow time-scale control objects is achieved through a dual-time-scale control architecture, and the reactive power demand of the UHV DC landing point is fully considered. Based on trajectory sensitivity, while meeting the economic requirements of power grid operation, more sufficient dynamic reactive reserves are reserved to cope with the dynamic regulation requirements of the power grid, and then a reasonable and efficient voltage control method is formulated, which can better reflect the advantages of the dual-time-scale voltage control method and system for an AC / DC hybrid power grid based on UPFC.

[0005] The present invention adopts the following technical solution.

[0006] The present invention proposes a dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC, comprising: Based on the voltage regulation characteristics of on-load tap changer OLTC, capacitor bank CB and unified power flow controller UPFC, the OLTC dynamic model, UPFC dynamic model and LCC-HVDC dynamic model are obtained; Analyze the dynamic responses of the OLTC dynamic model, the UPFC dynamic model and the CB dynamic model to the grid state, and define the trajectory sensitivity of the AC / DC hybrid grid; Based on trajectory sensitivity, a slow-time-scale voltage control model is built to minimize active loss cost, OLTC and CB operation costs, and UPFC reactive cost, and the slow-time-scale voltage control scheme is solved. Based on the slow time scale voltage control scheme, with the help of real-time updated ultra-short-term load forecast information, a fast time scale voltage control model is built to optimize the reactive power output of UPFC in real time; A dual-time-scale voltage control model is proposed based on the slow-time-scale and fast-time-scale voltage control models, and the optimal voltage control scheme is obtained by solving it.

[0007] Furthermore, the steps for constructing the OLTC dynamic model are: Introduce virtual node j′ and define the transformer ratio as ,node , The voltages are and ,but: ; The voltage step of each tap of the OLTC tap changer is defined as , the tap position of the tap changer is ,but ,So .

[0008] Furthermore, the construction steps of the UPFC dynamic model are as follows: UPFC adjustable reactive power output Calculated by the following formula: ; In the formula, is the UPFC input node voltage, is the parallel converter voltage, is the angular frequency, is the inductance of the parallel converter, is the modulation index of the converter, is the UPFC DC side capacitor voltage; UPFC injects voltage With grid current The expressions of active and reactive power generated by the action are and .

[0009] Furthermore, the steps for constructing the LCC-HVDC dynamic model are: The current flowing into the common connection point of the AC system is defined as , the fundamental voltage phasor of the AC bus is , the fundamental voltage phase angle of the AC bus is , the fundamental voltage phasor output by the converter is , the fundamental voltage phase angle of the converter output is , the equivalent resistance is , the equivalent reactance is ,but: ; ; In the formula, and are the active and reactive powers flowing from the AC system into the converter transformer, respectively; The phase angle difference is defined as The impedance angle is ,but: ; In the formula, and points are the active and reactive powers flowing into the converter, is the equivalent admittance; DC Power With injection converter are equal, so we have: ; In the formula, and They are the voltage and current at the landing point of the UHV DC system of the AC / DC hybrid power grid, respectively.

[0010] Furthermore, the specific steps of defining the trajectory sensitivity of the AC / DC hybrid power grid are as follows: use and They represent the change trajectories of the state variables and algebraic variables of the AC / DC hybrid power grid respectively. The vectors of the state variables and algebraic variables of the AC / DC hybrid power grid at time t are respectively expressed as and , the control variable is expressed as ;Will and exist Taylor series expansion is performed at the position, ignoring the change in the control quantity The higher-order term of the control quantity is The change in state variables caused by the change and the change in algebraic variables It can be approximately expressed as: ; In the formula, and They are and about The trajectory sensitivity of When the value is small enough, the trajectory sensitivity of the AC / DC hybrid power grid is defined as: ; The trajectory sensitivity is calculated for the control variables of different voltage regulation devices.

[0011] Furthermore, the specific steps to solve the slow time scale voltage control scheme are: Considering the active power loss cost of the power grid and the voltage regulation cost of the voltage regulation equipment, the slow time scale voltage control model is as follows: ; In the formula, t is the time period number, T is the optimization period, is a node set, is the set of child nodes of node i, is the phase set, Nodes connected to OLTC The set of child nodes of is the set of nodes connected to CB, Nodes connected to UPFC The set of child nodes of is the marginal price of grid-lost electricity; is the active power loss of the system at time t; , and are the voltage regulation cost weight coefficients of OLTC, CB and UPFC respectively; , and They are OLTC voltage regulation cost, CB voltage regulation cost and UPFC voltage regulation cost respectively; The calculation formula for active power loss of AC / DC hybrid power grid is as follows: ; In the formula, At time t, the phase from node i to node j is The square of the current amplitude, is the time at which nodes i and j are Harmony Resistance between phases; The OLTC and CB operation costs and UPFC reactive power costs are as follows: ; In the formula, It is the unit adjustment cost calculated when OLTC changes one gear. It is the unit adjustment cost calculated when CB switches on and off a group of quantities; and are the gear position of OLTC and the number of switching groups of CB in time period t; c is the reactive power output cost coefficient of UPFC, is the reactive power output by UPFC at node i and node j.

[0012] Furthermore, the constraints are as follows: OLTC constraints include: voltage ratio constraints on both sides of each ideal transformer; maximum and minimum gear constraints of OLTC tap changers; maximum number of tap changer actions at one time; CB constraints include: the maximum and minimum switching group number constraints of CB; the maximum number of groups that CB can switch at one time; UPFC constraints include: active interactive inequality constraints of converters; voltage and current amplitude constraints of series converters; reactive current constraints on the parallel side; absolute value constraints on voltage deviation and reactive power change of UPFC; The AC power grid constraints include: three-phase active and reactive power balance constraints for each node in each area; voltage drop constraints for each branch of each phase; current amplitude constraints for all branches and tap switches in each phase.

[0013] LCC-HVDC constraints include: active and reactive power flowing into the converter transformer of the AC system, voltage and current at the point of fall of the UHVDC system of the AC / DC hybrid power grid, modulation degree of the converter and size constraints of the phase shift angle; Based on the constraints, a slow time scale voltage control scheme is solved.

[0014] Furthermore, the specific steps of building a fast time scale voltage control model and optimizing the reactive power output of UPFC in real time are as follows: The fast time scale voltage control model aims to minimize the total voltage deviation and voltage fluctuation in adjacent time periods. The objective function is as follows: ; Where n is the total number of branches in the AC / DC hybrid power grid. is the ultra-short-term predicted voltage value of node i at time t, which is calculated based on the trajectory sensitivity of the steady-state node voltage and ultra-short-term real-time changes in the slow time scale voltage control scheme; is the voltage change, according to the trajectory sensitivity, through the reactive power output of UPFC And the load deviation of each node is calculated; is the node voltage slow time scale voltage control reference value; is the actual voltage value of node i in period t-1; The constraints include: AC grid constraints, LCC-HVDC constraints and UPFC constraints.

[0015] Furthermore, the specific steps for obtaining the optimal voltage control solution are as follows: Based on the slow time scale and fast time scale voltage control models, a dual time scale voltage control model of AC / DC hybrid power grid based on UPFC regulation margin and trajectory sensitivity is proposed, which can be expressed as: ; In the formula, is the objective function of slow time scale voltage control and fast time scale voltage control; is an equality constraint, is an inequality constraint; X is all state variables, and u is all control variables.

[0016] Furthermore, a genetic algorithm is used to solve the optimal voltage control scheme; In the iterative process, the probability constraints are combined to solve the problem and the discrete variables are processed into continuous variables to ensure the stability of the optimization path.

[0017] The present invention also provides a dual-time-scale voltage control system for an AC / DC hybrid power grid based on UPFC, comprising: The dynamic model building module obtains the OLTC dynamic model, UPFC dynamic model and LCC-HVDC dynamic model based on the voltage regulation characteristics of the on-load tap changer OLTC, capacitor bank CB and unified power flow controller UPFC; A trajectory sensitivity calculation module is used to analyze the dynamic responses of the OLTC dynamic model, the UPFC dynamic model and the CB dynamic model to the grid state, and define the trajectory sensitivity of the AC / DC hybrid grid; The slow time scale voltage control solution solution module builds a slow time scale voltage control model based on trajectory sensitivity with the goal of minimizing active loss cost, OLTC and CB operation costs, and UPFC reactive cost, and solves the slow time scale voltage control solution; The fast-time-scale voltage control model building module builds a fast-time-scale voltage control model based on the slow-time-scale voltage control scheme and uses the real-time updated ultra-short-term load forecast information to optimize the reactive output of UPFC in real time; The optimal voltage control solution solution module proposes a dual-time scale voltage control model based on the slow time scale and fast time scale voltage control models to solve and obtain the optimal voltage control solution.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention combines the equipment regulation characteristics with the characteristics of the power grid to construct OLTC, UPFC and LCC-HVDC models, which can accurately grasp the operating rules of each device in the power grid. The OLTC model is conducive to analyzing its voltage regulation process, the UPFC model highlights the advantages of rapid voltage regulation, and the LCC-HVDC model clarifies the interaction between AC and DC, providing a clear basis for the subsequent formulation of voltage control strategies and enhancing the accuracy and reliability of voltage regulation.

[0019] 2. This invention defines trajectory sensitivity and analyzes the impact of equipment on grid state variables, and can quantify the control effects of different equipment. Clearly understand how the adjustment of OLTC, CB and UPFC affects voltage and power, and can accurately operate equipment according to actual needs when the grid is running, prevent improper regulation from causing grid fluctuations, and effectively ensure the safety and stability of grid operation.

[0020] 3. The present invention aims to reduce costs by building a slow time scale model to solve the control scheme, taking into account both economy and stability. It reduces active power loss, reduces equipment operation costs, and reserves reactive power reserves by setting weight coefficients. It not only saves the cost of grid operation, but also ensures that the grid can stably regulate voltage under a slow time scale, thereby improving the economy and reliability of long-term operation.

[0021] 4. The present invention builds a fast time scale model based on the slow time scale scheme and load forecasting to optimize the UPFC reactive output in real time. When the load changes rapidly, the voltage is adjusted quickly to reduce voltage deviation and fluctuation and improve the power quality. It ensures the normal operation of voltage-sensitive equipment, reduces equipment failures caused by voltage problems, and improves the overall operation efficiency of the power grid.

[0022] 5. The present invention proposes a dual time scale model and solves it with a genetic algorithm, which can handle the complex problem of power grid voltage control. The genetic algorithm can search for the best control solution under multi-constraint and nonlinear conditions. The discrete variable continuity and elite retention strategy ensure stable and efficient optimization, realize the coordinated work of different devices, and ensure the safe, stable and efficient operation of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the framework of the dual-time-scale voltage control method and system of the AC / DC hybrid power grid based on UPFC of the present invention; Figure 2 is a schematic diagram of an equivalent model of an on-load tap-changing transformer of the present invention; Figure 3 is a schematic diagram of the structure of the unified power flow controller of the present invention; Figure 4 It is a model structure diagram of the receiving-end grid phase-commutated converter type direct current transmission of the present invention; Figure 5 It is the flow chart of genetic algorithm of the present invention. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only embodiments of a part of the present invention, rather than all embodiments. Based on the spirit of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention.

[0025] The present invention proposes a dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC. The overall process of the method is as follows: Figure 1 As shown in Figure 1, each area has a unique local controller to collect its own information, including: ① operation information of OLTC, CB and UPFC; ② predicted power of UPFC, and exchange information with neighboring controllers. They use genetic algorithms to calculate control decisions. Figure 1 In the slow time scale, the control variables of OLTC, CB and UPFC are optimized based on trajectory sensitivity to reserve more dynamic reactive power reserves while minimizing the overall system cost. The fast time scale aims to solve the fast voltage problem by coordinating the reactive power injection of UPFC.

[0026] The specific method steps of the present invention are as follows.

[0027] Based on the voltage regulation characteristics of on-load tap changer OLTC, capacitor bank CB and unified power flow controller UPFC, the OLTC dynamic model, UPFC dynamic model and LCC-HVDC dynamic model are obtained.

[0028] Specifically, the steps for constructing the OLTC dynamic model are: OLTC is essentially a transformer. For the convenience of calculation, a virtual node j′ is introduced. The virtual node divides the transformer into an ideal transformer and an equivalent impedance. Its equivalent model is as follows: Figure 2 As shown. The transformer ratio is defined as ,node , The voltages are and ,but: ; The voltage step of each tap of the OLTC tap changer is defined as , the tap position of the tap changer is ,but ,So .

[0029] Furthermore, the construction steps of the UPFC dynamic model are as follows: UPFC is mainly composed of parallel converter, series converter, DC bus capacitor, parallel transformer, series transformer, etc. Figure 3 First, the voltage balance characteristics of UPFC are analyzed, and its voltage balance equation is shown as follows: ; In the formula, is the UPFC input node voltage, is the injected voltage of UPFC to the grid, is the line impedance at the receiving end, I is the grid current, is the receiving end voltage.

[0030] UPFC uses the reactive power control function of its own parallel converter to provide voltage support for the AC / DC hybrid power grid. UPFC can adjust the reactive power output. Calculated by the following formula: ; In the formula, is the UPFC input node voltage, is the parallel converter voltage, is the angular frequency, is the inductance of the parallel converter, is the modulation index of the converter, is the UPFC DC side capacitor voltage; The adjustable reactive power range of UPFC is at the minimum To the maximum value Changes between , The absolute value of ).

[0031] UPFC injects voltage With grid current The expressions of active and reactive power generated by the action are and .

[0032] Furthermore, the steps for constructing the LCC-HVDC dynamic model are: The present invention studies the interaction between AC and DC systems and provides a mathematical model for LCC-HVDC, the main component of the AC / DC hybrid power grid. Figure 4 The current flowing into the common connection point of the AC system is defined as , the fundamental voltage phasor of the AC bus is , the fundamental voltage phase angle of the AC bus is , the fundamental voltage phasor output by the converter is , the fundamental voltage phase angle of the converter output is , the equivalent resistance is , the equivalent reactance is ,but: ; ; In the formula, and are the active and reactive powers flowing from the AC system into the converter transformer, respectively; The phase angle difference is defined as The impedance angle is ,but: ; In the formula, and points are the active and reactive powers flowing into the converter, is the equivalent admittance, ; DC Power With injection converter are equal, so we have: ; In the formula, and They are the voltage and current at the landing point of the UHV DC system of the AC / DC hybrid power grid, respectively.

[0033] Furthermore, the dynamic responses of the OLTC dynamic model, the UPFC dynamic model and the CB dynamic model to the grid state are analyzed, and the trajectory sensitivity of the AC / DC hybrid grid is defined.

[0034] Specifically, the dynamic responses of the above-mentioned different voltage control devices to the grid state are studied, and the specific steps of defining the trajectory sensitivity of the AC / DC hybrid grid are as follows: use and They represent the changing trajectories of the state variables and algebraic variables of the AC / DC hybrid power grid, namely: ; In the formula, and are respectively represented as the vectors of the state variables and algebraic variables of the AC / DC hybrid power grid at time t, u is represented as the control variable, ;Will and exist Taylor series expansion is performed at the position, ignoring the change in the control quantity The higher-order term of the control quantity at time t is The change in state variables caused by the change and the change in algebraic variables It can be approximately expressed as: ; In the formula, and They are and about The trajectory sensitivity of When the value is small enough, the trajectory sensitivity of the AC / DC hybrid power grid is defined as: ; The trajectory sensitivity of the control variables of different voltage control devices is calculated respectively, and the grid state variable is represented by the node voltage, that is: ; To avoid dimensional differences, the above formula is normalized: ; In the formula, is the reference value of the control variable (maximum gear position of OLTC, maximum number of switching groups of CB, reactive power reference value of UPFC); is the voltage reference value (1.0 pu).

[0035] Furthermore, based on trajectory sensitivity, a slow-time-scale voltage control model is built with the goal of minimizing active loss cost, OLTC and CB operation costs, and UPFC reactive cost, and the slow-time-scale voltage control scheme is solved.

[0036] Specifically, the specific steps to solve the slow time scale voltage control scheme are: Combined with the LCC-HVDC dynamic model, considering the active power loss cost of the power grid and the voltage regulation cost of the voltage control equipment, the slow time scale voltage control model is actually an optimization problem with the following objective function: ; In the formula, t is the time period number, T is the optimization period, is a node set, is the set of child nodes of node i, is the phase set, Nodes connected to OLTC The set of child nodes of is the set of nodes connected to CB, Nodes connected to UPFC The set of child nodes of is the marginal price of grid-lost electricity; is the active power loss of the system at time t; , and are the voltage regulation cost weight coefficients of OLTC, CB and UPFC respectively; , and They are OLTC voltage regulation cost, CB voltage regulation cost and UPFC voltage regulation cost respectively.

[0037] In order to avoid excessive use of UPFC's dynamic reactive resources in the slow time scale voltage control process, thereby affecting the demand for subsequent fast time scale voltage control, the voltage regulation cost weight coefficient of different voltage control devices is set proportional to the trajectory sensitivity, so as to ensure that the slow time scale voltage control scheme can reserve more sufficient dynamic reactive reserves. The weight coefficient is set to ,Before each slow time scale voltage control, the trajectory sensitivity needs to be updated according to the current grid operation status, so as to update the weight coefficient.

[0038] The calculation formula for active power loss of AC / DC hybrid power grid is as follows: ; In the formula, At time t, the phase from node i to node j is The square of the current amplitude, is the time at which nodes i and j are Harmony Resistance between phases; The OLTC and CB operation costs and UPFC reactive power costs are as follows: ; In the formula, It is the unit adjustment cost calculated when OLTC changes one gear. It is the unit adjustment cost calculated when CB switches on and off a group of quantities; and are the gear position of OLTC and the number of switching groups of CB in time period t; c is the reactive power output cost coefficient of UPFC, is the reactive power output by UPFC at node i and node j.

[0039] Furthermore, the constraints are as follows: (1) OLTC constraints include: The voltage ratio relationship constraints on both sides of each ideal transformer are: ; In the formula, , is the square of the voltage amplitude at nodes i and j, For the region The node set of .

[0040] Maximum and minimum gear constraints of OLTC tap changer: ; In the formula, , It is the minimum and maximum tap position of the tap changer.

[0041] Constraints on the maximum number of tap changer operations: ; In the formula, the maximum number of times the switch can be operated at one time is approximately the upper limit of the number of times an action can be performed at one time.

[0042] (2) CB constraints include: The maximum and minimum switching group number constraints of CB: ; In the formula, The maximum number of switching groups The maximum number of groups that can be switched on and off at one time by CB; ; (3) UPFC constraints include: Active interaction inequality constraints of converter: ; In the formula, , are the output voltage and current phasors of the series converter, The maximum value of the converter active power exchanged.

[0043] Voltage and current amplitude constraints of series converter: ; In the formula, , It is the upper limit of the output voltage and current amplitude of the series converter.

[0044] Reactive current constraint on the parallel side: ; In the formula, is the current flowing through the parallel converter, is the maximum value of the current flowing through the parallel converters.

[0045] Absolute value constraints of voltage deviation and reactive power variation of UPFC: ; ; In the formula, is the absolute value of the voltage deviation of UPFC at node i, is the voltage at node i, is the voltage reference value (usually 1.0 pu), is the absolute value of UPFC reactive power change.

[0046] (4) AC grid constraints include: Three-phase active and reactive power balance constraints for each node in each area: ; ; In the formula, is the active power of UPFC. is the parent node set of node i. , is the active and reactive power flow from node j to i, , are the active and reactive power flows from node i to j. The phase from node i to node j The square of the current amplitude, For nodes i and j in Harmony The reactance between the phases, , is the active and reactive power demand at node i. Reactive power injection of CB Can be Calculate, where It is the reactive capacity of each capacitor switched at node i.

[0047] Voltage drop constraints for each branch of each phase: ; In the formula, , For nodes i and j in Harmony Equivalent resistance and equivalent reactance between phases, For nodes i and j in Harmony Impedance between phases. is the set of child nodes of node i connected by branches.

[0048] Current amplitude constraints for all branches and tap changers in each phase: ; LCC-HVDC constraints include: ; In the formula, is the phase shift angle.

[0049] Furthermore, the fast-time-scale voltage control is executed with reference to the slow-time-scale voltage control scheme. Combined with the LCC-HVDC dynamic model, with the help of real-time updated ultra-short-term load forecast information, the reactive output of UPFC is optimized in real time to achieve the goal of reducing voltage deviation and voltage fluctuation, improving the power quality of the system and realizing efficient operation of the power grid.

[0050] Specifically, the fast time scale voltage control model aims to minimize the total voltage deviation and voltage fluctuation in adjacent time periods. The objective function is as follows: ; Where n is the total number of branches in the AC / DC hybrid power grid. is the ultra-short-term predicted voltage value of node i at time t, which is calculated based on the trajectory sensitivity of the steady-state node voltage and ultra-short-term real-time changes in the slow time scale voltage control scheme; is the voltage change, according to the trajectory sensitivity, through the reactive power output of UPFC And the load deviation of each node is calculated; is the node voltage slow time scale voltage control reference value; is the actual voltage value of node i in period t-1; The constraints include: AC grid constraints, LCC-HVDC constraints and UPFC constraints, which have been pointed out in the slow time scale voltage control model and will not be repeated here.

[0051] Furthermore, a dual-time-scale voltage control model is proposed based on the slow-time-scale and fast-time-scale voltage control models to obtain the optimal voltage control scheme.

[0052] Specifically, based on the slow time scale and fast time scale voltage control models, a dual time scale voltage control model of AC / DC hybrid power grid based on UPFC regulation margin and trajectory sensitivity is proposed, which can be expressed as: ; In the formula, is the objective function of slow time scale voltage control and fast time scale voltage control; is an equality constraint, is an inequality constraint; X is all state variables, and u is all control variables.

[0053] The voltage control problem of AC / DC hybrid power grid has the characteristics of nonlinearity, multiple constraints and significant differences in the characteristics of different voltage control devices. Figure 5 The genetic algorithm in the algorithm solves the optimal voltage control solution. In the iterative process, the probability constraints are combined to solve the problem, and the discrete variables such as the gear position of OLTC and the number of switching groups of CB are processed continuously to ensure the stability of the optimization path. At the same time, the elite retention strategy is adopted to avoid the randomness of operations such as crossover and mutation from destroying the high-quality chromosomes.

[0054] The present invention also proposes a dual-time-scale voltage control system for an AC / DC hybrid power grid based on UPFC, comprising: The dynamic model building module obtains the OLTC dynamic model, UPFC dynamic model and LCC-HVDC dynamic model based on the voltage regulation characteristics of the on-load tap changer OLTC, capacitor bank CB and unified power flow controller UPFC; A trajectory sensitivity calculation module is used to analyze the dynamic responses of the OLTC dynamic model, the UPFC dynamic model and the CB dynamic model to the grid state, and define the trajectory sensitivity of the AC / DC hybrid grid; The slow time scale voltage control solution solution module builds a slow time scale voltage control model based on trajectory sensitivity with the goal of minimizing active loss cost, OLTC and CB operation costs, and UPFC reactive cost, and solves the slow time scale voltage control solution; The fast-time-scale voltage control model building module builds a fast-time-scale voltage control model based on the slow-time-scale voltage control scheme and uses the real-time updated ultra-short-term load forecast information to optimize the reactive output of UPFC in real time; The optimal voltage control solution solution module proposes a dual-time scale voltage control model based on the slow time scale and fast time scale voltage control models to solve and obtain the optimal voltage control solution.

[0055] The present disclosure may be a system, a method and / or a computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC, characterized in that: include: Based on the voltage regulation characteristics of on-load tap changer OLTC, capacitor bank CB and unified power flow controller UPFC, the OLTC dynamic model, UPFC dynamic model and LCC-HVDC dynamic model are obtained; Analyze the dynamic responses of the OLTC dynamic model, the UPFC dynamic model and the CB dynamic model to the grid state, and define the trajectory sensitivity of the AC / DC hybrid grid; Based on trajectory sensitivity, a slow-time-scale voltage control model is built to minimize active loss cost, OLTC and CB operation costs, and UPFC reactive cost, and the slow-time-scale voltage control scheme is solved. Based on the slow time scale voltage control scheme, with the help of real-time updated ultra-short-term load forecast information, a fast time scale voltage control model is built to optimize the reactive power output of UPFC in real time; A dual-time-scale voltage control model is proposed based on the slow-time-scale and fast-time-scale voltage control models, and the optimal voltage control scheme is obtained by solving it.

2. The dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC according to claim 1, characterized in that: The steps to construct the OLTC dynamic model are: Introduce virtual node j′ and define the transformer ratio as ,node , The voltages are and ,but: ； The voltage step of each tap of the OLTC tap changer is defined as , the tap position of the tap changer is ,but ,So .

3. The dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC according to claim 1, characterized in that: The steps for constructing the UPFC dynamic model are: UPFC adjustable reactive power output Calculated by the following formula: ； In the formula, is the UPFC input node voltage, is the parallel converter voltage, is the angular frequency, is the inductance of the parallel converter, is the modulation index of the converter, is the UPFC DC side capacitor voltage; UPFC injects voltage With grid current The expressions of active and reactive power generated by the action are and .

4. The dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC according to claim 1, characterized in that: The steps for constructing the LCC-HVDC dynamic model are: The current flowing into the common connection point of the AC system is defined as , the fundamental voltage phasor of the AC bus is , the fundamental voltage phase angle of the AC bus is , the fundamental voltage phasor output by the converter is , the fundamental voltage phase angle of the converter output is , the equivalent resistance is , the equivalent reactance is ,but: ； ； In the formula, and are the active and reactive powers flowing from the AC system into the converter transformer, respectively; The phase angle difference is defined as The impedance angle is ,but: ； In the formula, and points are the active and reactive powers flowing into the converter, is the equivalent admittance; DC Power With injection converter are equal, so we have: ； In the formula, and They are the voltage and current at the landing point of the UHV DC system of the AC / DC hybrid power grid, respectively.

5. The dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC according to claim 1, characterized in that: The specific steps to define the trajectory sensitivity of the AC / DC hybrid power grid are: use and They represent the change trajectories of the state variables and algebraic variables of the AC / DC hybrid power grid respectively. The vectors of the state variables and algebraic variables of the AC / DC hybrid power grid at time t are respectively expressed as and , the control variable is expressed as ;Will and exist Taylor series expansion is performed at the position, ignoring the change in the control quantity The higher-order term of the control quantity is The change in state variables caused by the change and the change in algebraic variables It can be approximately expressed as: ； In the formula, and They are and about The trajectory sensitivity of When the value is small enough, the trajectory sensitivity of the AC / DC hybrid power grid is defined as: ； The trajectory sensitivity is calculated for the control variables of different voltage regulation devices.

6. The dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC according to claim 1, characterized in that: The specific steps to solve the slow time scale voltage control scheme are: Considering the active power loss cost of the power grid and the voltage regulation cost of the voltage regulation equipment, the slow time scale voltage control model is as follows: ； In the formula, t is the time period number, T is the optimization period, is a node set, is the set of child nodes of node i, is the phase set, Nodes connected to OLTC The set of child nodes of is the set of nodes connected to CB, Nodes connected to UPFC The set of child nodes of is the marginal price of grid-lost electricity; is the active power loss of the system at time t; , and are the voltage regulation cost weight coefficients of OLTC, CB and UPFC respectively; , and They are OLTC voltage regulation cost, CB voltage regulation cost and UPFC voltage regulation cost respectively; The calculation formula for active power loss of AC / DC hybrid power grid is as follows: ； In the formula, At time t, the phase from node i to node j is The square of the current amplitude, is the time at which nodes i and j are Harmony Resistance between phases; The OLTC and CB operation costs and UPFC reactive power costs are as follows: ； In the formula, It is the unit adjustment cost calculated when OLTC changes one gear. It is the unit adjustment cost calculated when CB switches on and off a group of quantities; and are the gear position of OLTC and the number of switching groups of CB in time period t; c is the reactive power output cost coefficient of UPFC, is the reactive power output by UPFC at node i and node j.

7. The dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC according to claim 6, characterized in that: The constraints are as follows: OLTC constraints include: voltage ratio constraints on both sides of each ideal transformer; maximum and minimum gear constraints of OLTC tap changers; maximum number of tap changer actions at one time; CB constraints include: the maximum and minimum switching group number constraints of CB; the maximum number of groups that CB can switch at one time; UPFC constraints include: active interactive inequality constraints of converters; voltage and current amplitude constraints of series converters; reactive current constraints on the parallel side; absolute value constraints on voltage deviation and reactive power change of UPFC; The AC grid constraints include: three-phase active and reactive power balance constraints for each node in each area; voltage drop constraints for each branch of each phase; current amplitude constraints for all branches and tap switches in each phase; LCC-HVDC constraints include: active and reactive power flowing into the converter transformer of the AC system, voltage and current at the point of fall of the UHVDC system of the AC / DC hybrid power grid, modulation degree of the converter and size constraints of the phase shift angle; Based on the constraints, a slow time scale voltage control scheme is solved.

8. The dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC according to claim 1, characterized in that: The specific steps to build a fast time scale voltage control model and optimize the reactive power output of UPFC in real time are as follows: The fast time scale voltage control model aims to minimize the total voltage deviation and voltage fluctuation in adjacent time periods. The objective function is as follows: ； Where n is the total number of branches in the AC / DC hybrid power grid. is the ultra-short-term predicted voltage value of node i at time t, which is calculated based on the trajectory sensitivity of the steady-state node voltage and ultra-short-term real-time changes in the slow time scale voltage control scheme; is the voltage change, according to the trajectory sensitivity, through the reactive power output of UPFC And the load deviation of each node is calculated; is the node voltage slow time scale voltage control reference value; is the actual voltage value of node i in period t-1; The constraints include: AC grid constraints, LCC-HVDC constraints and UPFC constraints.

9. The dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC according to claim 1, characterized in that: The specific steps to obtain the optimal voltage control solution are as follows: Based on the slow time scale and fast time scale voltage control models, a dual time scale voltage control model of AC / DC hybrid power grid based on UPFC regulation margin and trajectory sensitivity is proposed, which can be expressed as: ； In the formula, is the objective function of slow time scale voltage control and fast time scale voltage control; is an equality constraint, is an inequality constraint; X is all state variables, and u is all control variables.

10. The dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC according to claim 9, characterized in that: Genetic algorithm is used to solve the optimal voltage control scheme; In the iterative process, the probability constraints are combined to solve the problem and the discrete variables are processed into continuous variables to ensure the stability of the optimization path.

11. A dual-time-scale voltage control system for an AC / DC hybrid power grid based on UPFC, running the dual-time-scale voltage control method for an AC / DC hybrid power grid based on UPFC as claimed in any one of claims 1 to 10, characterized in that: include: The dynamic model building module obtains the OLTC dynamic model, UPFC dynamic model and LCC-HVDC dynamic model based on the voltage regulation characteristics of the on-load tap changer OLTC, capacitor bank CB and unified power flow controller UPFC; A trajectory sensitivity calculation module is used to analyze the dynamic responses of the OLTC dynamic model, the UPFC dynamic model and the CB dynamic model to the grid state, and define the trajectory sensitivity of the AC / DC hybrid grid; The slow time scale voltage control solution solution module builds a slow time scale voltage control model based on trajectory sensitivity with the goal of minimizing active loss cost, OLTC and CB operation costs, and UPFC reactive cost, and solves the slow time scale voltage control solution; The fast-time-scale voltage control model building module builds a fast-time-scale voltage control model based on the slow-time-scale voltage control scheme and uses the real-time updated ultra-short-term load forecast information to optimize the reactive output of UPFC in real time; The optimal voltage control solution solution module proposes a dual-time scale voltage control model based on the slow time scale and fast time scale voltage control models to solve and obtain the optimal voltage control solution.

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