Dual-time-scale voltage control method and system for AC-DC hybrid power grid based on UPFC

Abstract

Dual-time-scale voltage control method and system for AC / DC hybrid power grid based on UPFC. By accurately describing the reactive power regulation margin of UPFC, on the basis of considering the voltage control characteristics of OLTC and CB, the voltage control characteristics of UPFC are further considered. Through the dual-time-scale control architecture, the coordination of fast and slow time-scale control objects is realized. The reactive power demand at the UHV DC landing point is fully considered. Based on the trajectory sensitivity, while meeting the economic requirements of grid operation, more sufficient dynamic reactive power reserves are reserved to cope with the dynamic regulation demands of the grid. Furthermore, 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 AC / DC hybrid power grid based on UPFC.

Description

Technical Field

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

[0002] China's power system has the characteristics of AC-DC hybrid power grid and large-scale power transmission across regions. Therefore, the complexity of the power grid and the difficulty of voltage regulation have been significantly improved. In an AC-DC hybrid power grid, AC and DC, the sending end and the receiving end are closely coupled. The use of different types of voltage regulation equipment affects the operation state of the power grid on different time scales. Traditional power grids control the voltage of the power grid by controlling on-load tap-changing transformers and capacitor banks. However, they are discrete adjustments, and the equipment has insufficient response speed, insufficient dynamic reactive power reserve, and difficulty in coordinating voltage regulation equipment on different time scales. Although the traditional method has a significant steady-state regulation effect, it is difficult to meet the demand for rapid voltage control.

[0003] In the prior art, for example, the patent application with the publication number CN116169686A calculates the power flow of the AC-DC hybrid system to obtain the initial power flow result and the state variable value; based on the obtained initial power flow result and state variable value, calculates the active power loss and voltage offset of the AC-DC hybrid system, and establishes a multi-objective reactive power optimization model with the minimum of active power loss and voltage offset as the objective; uses a whale swarm optimization model solving algorithm combining particle swarm and whale algorithm to solve the multi-objective reactive power optimization model to obtain the optimization result; based on the obtained optimization result, realizes the reactive power voltage control of the AC-DC hybrid system. Another example is the patent application with the publication number CN117595251A, which determines the calculation model according to the operation constraint conditions of the dynamic reactive power reserve evaluation of the AC-DC hybrid power grid; then establishes an optimization model with the minimum of dynamic reactive power reserve as the objective; then judges whether the operation constraint conditions are linear. When linear, constructs an equivalent linear optimization model to solve. When non-linear, uses the dynamic constraint coefficient shear algorithm to transform and then solve to obtain the optimization result; finally, formulates a voltage stability control strategy according to the optimization result and executes the voltage control of the AC-DC hybrid power grid. However, the prior art 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 regulation equipment, lacks trajectory sensitivity analysis, resulting in insufficient reserved dynamic reactive power reserve, inaccurate allocation of dynamic resources, low multi-device cooperation efficiency, and difficulty in coping with sudden voltage fluctuations. Therefore, there is an urgent need for a dynamic regulation method for voltage stability and rapid control in an AC-DC hybrid power grid. Summary of the Invention

[0004] To address the deficiencies in the existing technologies, the present invention provides a two-time-scale voltage control method and system for AC-DC hybrid power grids 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. Through a two-time-scale control architecture, the coordination of fast- and slow-time-scale control objects is achieved, and the reactive power demand at the UHVDC drop point is fully considered. Based on the trajectory sensitivity, while meeting the economic requirements of grid operation, more sufficient dynamic reactive power reserves are reserved to cope with the dynamic regulation requirements of the grid, and then a reasonable and efficient voltage control method is formulated, which can better reflect the advantages of the two-time-scale voltage control method and system for AC-DC hybrid power grids based on UPFC.

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

[0006] The present invention proposes a two-time-scale voltage control method for AC-DC hybrid power grids based on UPFC, including:

[0007] Based on the voltage regulation characteristics of on-load tap-changer (OLTC), capacitor bank (CB) and unified power flow controller (UPFC), obtain the OLTC dynamic model, UPFC dynamic model, CB dynamic model and LCC-HVDC dynamic model;

[0008] Analyze the dynamic responses of the OLTC dynamic model, UPFC dynamic model and CB dynamic model to the grid state, and define the trajectory sensitivity of the AC-DC hybrid power grid;

[0009] Based on the trajectory sensitivity, build a slow-time-scale voltage control model with the goal of minimizing the active power loss cost, OLTC and CB action costs, and UPFC reactive power cost, and solve the slow-time-scale voltage control scheme;

[0010] Based on the slow-time-scale voltage control scheme, with the help of the real-time updated short-term load forecasting information, build a fast-time-scale voltage control model to optimize the reactive power output of UPFC in real time;

[0011] Based on the slow-time-scale and fast-time-scale voltage control models, propose a two-time-scale voltage control model, and solve to obtain the optimal voltage control scheme.

[0012] Furthermore, the construction steps of the OLTC dynamic model are as follows:

[0013] Introduce a virtual node j′, and define the transformer turns ratio as , and the voltages of nodes , are and respectively, then:

[0014] ;

[0015] Define the voltage step of each tap of the OLTC as , and the tap position of the tap changer, i.e., the gear position, is , then , so .

[0016] Furthermore, the construction steps of the UPFC dynamic model are as follows:

[0017] The adjustable reactive power output of the UPFC is calculated by the following formula:

[0018] ;

[0019] In the formula, is the voltage of the input node of the UPFC, is the voltage of the shunt converter, is the angular frequency, is the inductance value of the shunt converter, is the modulation degree of the converter, is the voltage of the DC side capacitor of the UPFC;

[0020] The active and reactive power expressions generated by the UPFC through the injection voltage and the grid current are respectively and .

[0021] Furthermore, the construction steps of the LCC-HVDC dynamic model are as follows:

[0022] Define the current flowing from the AC system into the common connection point 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 output by the converter is , the equivalent resistance is , the equivalent reactance is , then:

[0023] ;

[0024] ;

[0025] In the formula, and are the active and reactive powers flowing from the AC system into the converter transformer respectively;

[0026] Define the phase angle difference as , the impedance angle is , then:

[0027] ;

[0028] In the formula, and are the active and reactive powers flowing into the converter respectively, is the equivalent admittance;

[0029] The DC power is equal to the injected into the converter. Thus, there is:

[0030] ;

[0031] In the formula, and are the voltage and current at the landing point of the UHV DC system in the AC-DC hybrid power grid respectively.

[0032] Furthermore, the specific steps to define the trajectory sensitivity of the AC-DC hybrid power grid are as follows:

[0033] Use and to 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 are represented as and respectively, and the control variable is represented as ; Expand and at , and ignore the high-order terms of the control quantity change . Then, the change in the state variable and the change in the algebraic variable caused by the change in the control quantity at time can be approximately expressed as:

[0034] ;

[0035] In the formula, and are the trajectory sensitivities of and with respect to respectively; When is small enough, the trajectory sensitivity of the AC-DC hybrid power grid is defined as:

[0036] ;

[0037] Calculate the trajectory sensitivity for the control variables of different voltage regulation devices respectively.

[0038] Furthermore, the specific steps to solve the slow-time-scale voltage control scheme are as follows:

[0039] Considering the active power loss cost of the power grid and the voltage regulation cost of voltage regulation devices, the slow-time-scale voltage control model is as follows:

[0040] ;

[0041] In the formula, is the period number, is the optimization period, is the node set, is the node 's child node set, is the phase set, is the node connected to the OLTC 's child node set, is the node set connected to the CB, is the node connected to the UPFC 's child node set; is the marginal price of network loss electric energy; is the moment 's active power loss of the system; , and are the voltage regulation cost weight coefficients of OLTC, CB and UPFC respectively; , and are the OLTC voltage regulation cost, CB voltage regulation cost and UPFC voltage regulation cost respectively;

[0042] The calculation formula for the active power loss of the AC / DC hybrid power grid is as follows:

[0043] ;

[0044] In the formula, is the square of the current amplitude from node to node at moment in phase , is the resistance between node and node at moment in phase and phase;

[0045] The operation costs of OLTC and CB and the reactive power cost of UPFC are as follows:

[0046] ;

[0047] Wherein, is the unit regulation cost calculated when the OLTC changes one gear, is the unit regulation cost calculated when the CB switches one group of quantities; and are respectively the gear position of the OLTC and the number of switching groups of the CB in the time period ; is the UPFC reactive power output cost coefficient, is the node and the node the reactive power output by the UPFC at;

[0048] Furthermore, the constraint conditions are as follows:

[0049] The OLTC constraints include: the voltage ratio relationship constraint on both sides of each ideal transformer; the maximum and minimum gear position constraints of the OLTC tap changer; the maximum number of times the tap changer can act once;

[0050] The CB constraints include: the maximum and minimum number of switching groups constraints of the CB; the maximum number of groups that the CB can switch at one time;

[0051] The UPFC constraints include: the active power interaction inequality constraint of the converter; the voltage and current amplitude constraints of the series converter; the reactive current constraint on the shunt side; the voltage deviation and the absolute value constraint of the reactive power change of the UPFC;

[0052] The AC power grid constraints include: the three-phase active and reactive power balance constraints of each node in each area; the voltage drop constraint of each branch in each phase; the current amplitude constraints of all branches and tap changers in each phase.

[0053] The LCC-HVDC constraints include: the constraints on the active and reactive power flowing into the converter transformer from the AC system, the voltage and current at the landing point of the UHVDC system in the AC-DC hybrid power grid, the modulation degree and phase shift angle of the converter;

[0054] Based on the above constraint conditions, a slow-time-scale voltage control scheme is solved.

[0055] Furthermore, a fast-time-scale voltage control model is built, and the specific steps for real-time optimizing the reactive power output of the UPFC are as follows:

[0056] The fast-time-scale voltage control model aims to minimize the total voltage deviation and the voltage fluctuation in the adjacent time period, and the objective function is as follows:

[0057] ;

[0058] Wherein, is the total number of branches in the AC / DC hybrid power grid, is the node at time ultra-short-term predicted voltage value, calculated based on the steady-state node voltage and the trajectory sensitivity of ultra-short-term real-time changes in the slow time-scale voltage control scheme; is the voltage change amount, calculated based on the trajectory sensitivity through the reactive power output of the UPFC and the load deviation of each node; is the slow time-scale voltage control reference value of the node voltage; is the node during the time period actual voltage value;

[0059] The constraint conditions include: AC power grid constraints, LCC-HVDC constraints, and UPFC constraints.

[0060] Furthermore, the specific steps to obtain the optimal voltage control scheme are as follows:

[0061] Based on the slow time-scale and fast time-scale voltage control models, a two-time-scale voltage control model for the AC / DC hybrid power grid based on the regulation margin and trajectory sensitivity of the UPFC is proposed, which can be expressed as:

[0062] ;

[0063] In the formula, is the objective function of slow time-scale voltage control and fast time-scale voltage control; is the equality constraint, is the inequality constraint; is all state variables, is all control variables.

[0064] Furthermore, a genetic algorithm is used to solve the optimal voltage control scheme;

[0065] During the iteration process, the solution is carried out in combination with probability constraints, and the discrete variables are continuously processed to ensure the stable progress of the optimization path.

[0066] The present invention also proposes a two-time-scale voltage control system for an AC / DC hybrid power grid based on the UPFC, including:

[0067] A dynamic model construction module, based on the voltage regulation characteristics of the on-load tap-changer OLTC, capacitor bank CB, and unified power flow controller UPFC, obtains the OLTC dynamic model, UPFC dynamic model, CB dynamic model, and LCC-HVDC dynamic model;

[0068] The trajectory sensitivity calculation module analyzes the dynamic responses of the OLTC dynamic model, UPFC dynamic model, and CB dynamic model to the grid state, and defines the trajectory sensitivity of the AC-DC hybrid grid.

[0069] The slow-time-scale voltage control scheme solving module builds a slow-time-scale voltage control model with the goal of minimizing the active power loss cost, OLTC and CB operation costs, and UPFC reactive power cost based on the trajectory sensitivity, and solves the slow-time-scale voltage control scheme.

[0070] 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, with the help of the real-time updated ultra-short-term load prediction information, and optimizes the reactive power output of the UPFC in real time.

[0071] The optimal voltage control scheme solving module proposes a dual-time-scale voltage control model based on the slow-time-scale and fast-time-scale voltage control models, and solves to obtain the optimal voltage control scheme.

[0072] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0073] 1. The present invention constructs OLTC, UPFC, and LCC-HVDC models by combining the equipment regulation characteristics and grid characteristics, and can accurately grasp the operation laws of each equipment in the grid. The OLTC model is conducive to analyzing its voltage regulation process, the UPFC model highlights the fast voltage regulation advantage, and the LCC-HVDC model clarifies the AC-DC interaction, providing a clear basis for formulating subsequent voltage control strategies and enhancing the accuracy and reliability of voltage regulation.

[0074] 2. The present invention defines the trajectory sensitivity and analyzes the influence of equipment on grid state variables, and can quantify the regulation effects of different equipment. Clearly understanding how the adjustments of OLTC, CB, and UPFC affect voltage and power can accurately operate the equipment according to actual needs during grid operation, prevent improper regulation from causing grid fluctuations, and effectively ensure the safety and stability of grid operation.

[0075] 3. The present invention builds a slow-time-scale model to solve the control scheme with the goal of reducing costs, taking both economy and stability into account. It reduces the active power loss, lowers the equipment operation cost, and reserves reactive power through setting weight coefficients. It not only saves the grid operation cost but also ensures that the grid can regulate voltage stably on the slow-time scale, improving the economy and reliability of long-term operation.

[0076] 4. The present invention constructs a fast time-scale model based on a slow time-scale scheme and load forecasting to optimize the reactive power output of the UPFC in real time. When the load changes rapidly, the voltage is quickly adjusted to reduce voltage deviation and fluctuation, improve power quality, ensure the normal operation of voltage-sensitive equipment, reduce equipment failures caused by voltage problems, and improve the overall operation efficiency of the power grid.

[0077] 5. The present invention proposes a dual time-scale model and solves it using a genetic algorithm, which can handle complex problems in power grid voltage control. The genetic algorithm can search for the optimal control scheme under multi-constraint and non-linear conditions. The discretization of discrete variables and the elite retention strategy ensure stable and efficient optimization, enable different devices to work together, and ensure the safe, stable, and efficient operation of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 is a schematic framework diagram of the dual time-scale voltage control method and system for an AC-DC hybrid power grid based on UPFC of the present invention;

[0079] Figure 2 is a schematic diagram of the equivalent model of the on-load tap-changer transformer of the present invention;

[0080] Figure 3 is a schematic structural diagram of the unified power flow controller of the present invention;

[0081] Figure 4 is a model structure diagram of the line-commutated converter-based HVDC transmission in the receiving-end power grid of the present invention;

[0082] Figure 5 is a flow chart of the genetic algorithm of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0083] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only a part of the embodiments of the present invention, rather than all embodiments. Based on the spirit of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0084] 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 this method is as Figure 1 shown. Each area has a unique local controller to collect its own information, including: ① the operation information of the OLTC, CB, and UPFC; ② the predicted power of the UPFC, and exchange information with adjacent controllers. They use a genetic algorithm to calculate control decisions. Figure 1Among them, in the slow time scale, based on the trajectory sensitivity, the control variables of OLTC, CB, and UPFC are optimized to reserve more dynamic reactive power reserves while minimizing the comprehensive cost of the system. The fast time scale aims to solve the fast voltage problem by coordinating the reactive power injection of UPFC.

[0085] The specific method steps of the present invention are as follows.

[0086] Based on the voltage regulation characteristics of the on-load tap-changing transformer OLTC, capacitor bank CB, and unified power flow controller UPFC, the dynamic models of OLTC, UPFC, and LCC-HVDC are obtained.

[0087] Specifically, the construction steps of the OLTC dynamic model are as follows:

[0088] 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, and its equivalent model is as Figure 2 shown. Define the transformer turns ratio as , and the voltages of nodes , are and respectively, then:

[0089] ;

[0090] Define the voltage step of each tap of the OLTC tap changer as , and the tap position of the tap changer, that is, the gear position, is , then , so .

[0091] Furthermore, the construction steps of the UPFC dynamic model are as follows:

[0092] UPFC is mainly composed of a shunt converter, a series converter, a DC bus capacitor, a shunt transformer, a series transformer, etc., as Figure 3 shown. First, analyze the voltage balance characteristics of UPFC, and its voltage balance equation is shown as follows:

[0093] ;

[0094] In the formula, is the voltage of the input node of UPFC, is the voltage injected by UPFC into the power grid, is the receiving-end line impedance, is the power grid current, is the receiving-end voltage.

[0095] The UPFC uses the reactive power control function of its shunt converter to provide voltage support for the AC-DC hybrid power grid. The adjustable reactive power output of the UPFC is calculated by the following formula:

[0096] ;

[0097] In the formula, is the node voltage at the input end of the UPFC, is the shunt converter voltage, is the angular frequency, is the inductance value of the shunt converter, is the modulation degree of the converter, is the DC-side capacitor voltage of the UPFC;

[0098] The adjustable reactive power range of the UPFC varies between the minimum value and the maximum value ( , The absolute value of is equal to ).

[0099] The active and reactive power expressions generated by the UPFC through the injection voltage and the grid current are respectively and .

[0100] Furthermore, the construction steps of the LCC-HVDC dynamic model are as follows:

[0101] The present invention studies the interaction between the AC and DC systems, and gives a mathematical model for the main component LCC-HVDC of the AC-DC hybrid power grid, as Figure 4 shown. Define the current flowing from the AC system into the common connection point as , the fundamental voltage phasor of the AC bus as , the fundamental voltage phase angle of the AC bus as , the fundamental voltage phasor output by the converter as , the fundamental voltage phase angle output by the converter as , the equivalent resistance as , the equivalent reactance as , then:

[0102] ;

[0103] ;

[0104] In the formula, and are respectively the active and reactive power flowing from the AC system into the converter transformer;

[0105] Define the phase angle difference as , and the impedance angle as , then:

[0106] ;

[0107] In the formula, and are the active and reactive powers flowing into the converter respectively, is the equivalent admittance, ;

[0108] The DC power is equal to the injected into the converter, so there is:

[0109] ;

[0110] In the formula, and are the voltage and current at the connection point of the UHVDC system in the AC / DC hybrid power grid respectively.

[0111] Furthermore, analyze the dynamic responses of the OLTC dynamic model, UPFC dynamic model, and CB dynamic model to the power grid state, and define the trajectory sensitivity of the AC / DC hybrid power grid.

[0112] Specifically, the specific steps for studying the dynamic responses of the above different voltage control devices to the power grid state and defining the trajectory sensitivity of the AC / DC hybrid power grid are as follows:

[0113] Use and to represent the change trajectories of the state variables and algebraic variables of the AC / DC hybrid power grid respectively, that is:

[0114] ;

[0115] In the formula, and represent the vectors of the state variables and algebraic variables of the AC / DC hybrid power grid at time respectively, represents the control variable, ; Substitute and into the Taylor series expansion at , and ignore the high-order terms of the control variable change amount , then the change amounts of the state variables and the change amounts of the algebraic variables caused by the change of the control variable change amount at time

[0116] ;

[0117] Wherein, and are respectively and the trajectory sensitivities with respect to ; when is small enough, the trajectory sensitivity of the AC / DC hybrid power grid is defined as:

[0118] ;

[0119] Calculate the trajectory sensitivities for the control variables of different voltage regulation devices respectively. The grid state variables are represented by the node voltages, i.e.:

[0120] ;

[0121] To avoid dimensional differences, normalize the above formula:

[0122] ;

[0123] Wherein, is the reference value of the control variable (the maximum tap position of the OLTC, the maximum switching group number of the CB, the reactive power reference value of the UPFC); is the voltage reference value (1.0 p.u.).

[0124] Furthermore, based on the trajectory sensitivity, build a slow-time-scale voltage control model with the goal of minimizing the active power loss cost, the regulation cost of the OLTC and the CB, and the reactive power cost of the UPFC, and solve the slow-time-scale voltage control scheme.

[0125] Specifically, the specific steps to solve the slow-time-scale voltage control scheme are as follows:

[0126] 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 regulation equipment, the slow-time-scale voltage control model is actually an optimization problem with the following objective function:

[0127] ;

[0128] Wherein, is the time period number, is the optimization period, is the node set, is the sub-node set of node , is the phase set, is the sub-node set of the node connected by the OLTC, is the node set connected by the CB, Nodes connected by UPFC The subset of child nodes; The marginal price of network loss power; For the time The active power loss of the system; , and The voltage regulation cost weight coefficients of OLTC, CB, and UPFC are respectively; , and The voltage regulation cost of OLTC, the voltage regulation cost of CB, and the voltage regulation cost of UPFC are respectively.

[0129] To avoid overusing the dynamic reactive power resources of UPFC during the slow-time-scale voltage control process, which may affect the subsequent fast-time-scale voltage control requirements, the voltage regulation cost weight coefficients of different voltage regulation devices are set to be proportional to the trajectory sensitivity here, so as to ensure that the slow-time-scale voltage control scheme can reserve more sufficient dynamic reactive power reserves. The weight coefficients are set to Before each slow-time-scale voltage control, the trajectory sensitivity needs to be updated according to the current power grid operation state, so as to update the weight coefficients.

[0130] The calculation formula for the active power loss of the AC / DC hybrid power grid is as follows:

[0131] ;

[0132] In the formula, For the time Node To node The square of the current amplitude at phase ; For the time Node And node At Phase and The resistance between phases;

[0133] The operation costs of OLTC and CB and the reactive power cost of UPFC are as follows:

[0134] ;

[0135] In the formula, The unit regulation cost calculated when OLTC changes one gear, The unit regulation cost calculated when CB switches one group; And Are respectively the gear of OLTC and the number of switching groups of CB during the time period ; is the reactive power output cost coefficient of the UPFC, is the node and the node where the reactive power output by the UPFC is located.

[0136] Furthermore, the constraint conditions are as follows:

[0137] (1) The OLTC constraints include:

[0138] The voltage ratio relationship constraint on both sides of each ideal transformer:

[0139] ;

[0140] In the formula, , are the squares of the voltage amplitudes at nodes , , and is the set of nodes belonging to the region .

[0141] The maximum and minimum tap positions constraints of the OLTC tap changer:

[0142] ;

[0143] In the formula, , are the minimum and maximum tap positions of the tap changer.

[0144] The maximum number of times the tap changer can act in one operation constraint:

[0145] ;

[0146] In the formula, is the upper limit number of times of one operation.

[0147] (2) The CB constraints include:

[0148] The maximum and minimum switching groups constraints of the CB:

[0149] ;

[0150] In the formula, is the maximum number of switching groups

[0151] The maximum number of switching groups that the CB can switch in one operation constraint;

[0152] ;

[0153] (3) The UPFC constraints include:

[0154] The active power interaction inequality constraint of the converter:

[0155] ;

[0156] Wherein, and are respectively the output voltage and current phasors of the series converter, is the maximum value of the active power exchanged by the converter.

[0157] Constraints on the voltage and current amplitudes of the series converter:

[0158] ;

[0159] Wherein, and are the upper limit values of the output voltage and current amplitudes of the series converter.

[0160] Constraints on the reactive current of the parallel side:

[0161] ;

[0162] Wherein, is the current flowing through the parallel converter, is the maximum value of the current flowing through the parallel converter.

[0163] Constraints on the absolute values of the voltage deviation and reactive power change of the UPFC:

[0164] ;

[0165] ;

[0166] Wherein, is the absolute value of the voltage deviation of the UPFC at node , is the voltage at node , is the voltage reference value (usually 1.0 p.u.), is the absolute value of the reactive power change of the UPFC.

[0167] (4) AC grid constraints include:

[0168] Three-phase active and reactive power balance constraints for each node in each area:

[0169] ;

[0170] ;

[0171] Wherein, is the active power of the UPFC. is the set of parent nodes of node . , For the active and reactive power flows from node to , , For the active and reactive power flows from node to . is the square of the current amplitude from node to node at phase . is the reactance between node and at phase and phase. , are the active and reactive power demands at node . The reactive power injection of CB can be calculated by , where is the reactive power capacity of each switched capacitor at node .

[0172] Voltage drop constraint for each branch in each phase:

[0173] ;

[0174] In the formula, , are the equivalent resistance and equivalent reactance between node and at phase and phase, is the impedance between node and at phase and phase. is the set of child nodes of node connected by the branch.

[0175] Current amplitude constraint for all branches and tap switches in each phase:

[0176] ;

[0177] The LCC-HVDC constraints include::

[0178] ;

[0179] In the formula, is the phase shift angle.

[0180] Furthermore, the fast-time-scale voltage control is executed with reference to the slow-time-scale voltage control scheme. Combining with the LCC-HVDC dynamic model and leveraging the ultra-short-term load prediction information updated in real time, the reactive power output of the 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 the efficient operation of the power grid.

[0181] 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:

[0182] ;

[0183] In the formula, is the total number of branches in the AC-DC hybrid power grid, is the node ultra-short-term predicted voltage value at time , which is calculated based on the steady-state node voltage and the trajectory sensitivity of the ultra-short-term real-time change in the slow-time-scale voltage control scheme; is the voltage change amount, which is calculated based on the trajectory sensitivity through the reactive power output of the UPFC and the load deviation of each node; is the slow-time-scale voltage control reference value of the node voltage; is the node actual voltage value at time ;

[0184] The constraint conditions include: AC power grid constraints, LCC-HVDC constraints, and UPFC constraints, which have all been pointed out in the slow-time-scale voltage control model and will not be elaborated here.

[0185] 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.

[0186] Specifically, based on the slow-time-scale and fast-time-scale voltage control models, a dual-time-scale voltage control model for the AC-DC hybrid power grid based on the regulation margin and trajectory sensitivity of the UPFC is proposed, which can be expressed as:

[0187] ;

[0188] In the formula, are the objective functions of the slow-time-scale voltage control and the fast-time-scale voltage control; is the equality constraint, is the inequality constraint; are all state variables, are all control variables.

[0189] The problem of voltage control in AC-DC hybrid power grids has characteristics such as non-linearity, multiple constraints, and significant differences in the characteristics of different voltage regulation devices. Therefore, the genetic algorithm in Figure 5 is used to solve the optimal voltage control scheme. During the iteration process, the solution is carried out in combination with probabilistic constraints, and discrete variables such as the tap positions of OLTC and the switching groups of CB are continuously processed to ensure the stable progress 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 high-quality chromosomes.

[0190] The present invention also proposes a two-time-scale voltage control system for AC-DC hybrid power grids based on UPFC, including:

[0191] A dynamic model construction module, based on the voltage regulation characteristics of on-load tap-changing transformers (OLTCs), capacitor banks (CBs), and unified power flow controllers (UPFCs), obtains the OLTC dynamic model, UPFC dynamic model, and LCC-HVDC dynamic model;

[0192] A trajectory sensitivity calculation module, analyzes the dynamic responses of the OLTC dynamic model, UPFC dynamic model, and CB dynamic model to the power grid state, and defines the trajectory sensitivity of the AC-DC hybrid power grid;

[0193] A slow-time-scale voltage control scheme solving module, based on the trajectory sensitivity, constructs a slow-time-scale voltage control model with the goal of minimizing the active power loss cost, the action costs of OLTC and CB, and the reactive power cost of UPFC, and solves the slow-time-scale voltage control scheme;

[0194] A fast-time-scale voltage control model construction module, based on the slow-time-scale voltage control scheme, with the help of real-time updated ultra-short-term load prediction information, constructs a fast-time-scale voltage control model to optimize the reactive power output of UPFC in real time;

[0195] An optimal voltage control scheme solving module, based on the slow-time-scale and fast-time-scale voltage control models, proposes a two-time-scale voltage control model and solves to obtain the optimal voltage control scheme.

[0196] This disclosure may be a system, method, and / or computer program product. The computer program product may include a computer-readable storage medium having thereon computer-readable program instructions for causing a processor to implement various aspects of this disclosure.

[0197] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications or equivalent replacements can still be made to the specific embodiments of the present invention, and any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the protection scope of the claims of the present invention.

Claims

1. A dual-time-scale voltage control method for AC / DC hybrid power grids based on UPFC, characterized in that Including: Based on the voltage regulation characteristics of the on-load tap-changing transformer (OLTC), capacitor bank (CB), and unified power flow controller (UPFC), obtain the OLTC dynamic model, UPFC dynamic model, CB dynamic model, and LCC-HVDC dynamic model; Analyze the dynamic responses of the OLTC dynamic model, UPFC dynamic model, and CB dynamic model to the grid state, and define the trajectory sensitivity of the AC-DC hybrid power grid; Based on the trajectory sensitivity, build a slow-time-scale voltage control model with the goal of minimizing the active power loss cost, OLTC and CB operation costs, and UPFC reactive power cost, and solve the slow-time-scale voltage control scheme; Based on the slow-time-scale voltage control scheme, with the help of the load ultra-short-term prediction information updated in real time, build a fast-time-scale voltage control model to optimize the reactive power output of the UPFC in real time; Propose a dual-time-scale voltage control model based on the slow-time-scale and fast-time-scale voltage control models, and solve to obtain the optimal voltage control scheme.

2. The dual-time-scale voltage control method for AC-DC hybrid power grid based on UPFC according to claim 1, wherein The construction steps of the OLTC dynamic model are as follows: Introduce a virtual node j′ and define the transformer turns ratio as , the voltages of nodes and are respectively and , then: ； Define the voltage step of each tap of the OLTC tap changer as , and the tap position of the tap changer, i.e., the gear position, is , then , so .

3. The two-time-scale voltage control method for AC-DC hybrid power grid based on UPFC according to claim 1, wherein The construction steps of the UPFC dynamic model are as follows: Reactive power output adjustable by UPFC Calculated by the following formula: ； In the formula, is the voltage of the input terminal node of the UPFC, is the voltage of the shunt converter, is the angular frequency, is the inductance value of the shunt converter, is the modulation degree of the converter, is the DC-side capacitor voltage of the UPFC; The UPFC injects voltage and the grid current The expressions for the active and reactive powers generated by the interaction are respectively and .

4. The dual-time-scale voltage control method for AC-DC hybrid power grid based on UPFC according to claim 1, characterized in that The construction steps of the LCC-HVDC dynamic model are as follows: Define the current flowing into the common connection point of the AC system 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 output by the converter is , the equivalent resistance is , the equivalent reactance is , then: ； ； In the formula, and are the active and reactive powers flowing from the AC system into the converter transformer, respectively; Define the phase angle difference as , and the impedance angle as , then: ； Wherein, and are the active and reactive powers flowing into the converter respectively, is the equivalent admittance; DC power is equal to that injected into the converter, so we have: ​ ； In the formula, and are the voltage and current at the connection point of the UHV DC system in the AC / DC hybrid power grid respectively.

5. The dual-time-scale voltage control method for 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 as follows: Adopt and respectively represent the change trajectories of the state variables and algebraic variables of the AC / DC hybrid power grid. The vectors of the state variables and algebraic variables of the AC / DC hybrid power grid at time are respectively represented as and , and the control variable is represented as ; Expand and at by Taylor series, and ignore the high-order terms of the control quantity change . Then, the change of the state variables and the change of the algebraic variables caused by the change of the control quantity at time can be approximately expressed as: ； In the formula, and are respectively and the trajectory sensitivities with respect to ; when is small enough, the trajectory sensitivity of the AC / DC hybrid power grid is defined as: ； Calculate the trajectory sensitivity for the control variables of different voltage regulation devices respectively.

6. The two-time-scale voltage control method for 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 as follows: Considering the active power loss cost of the power grid and the voltage regulation cost of voltage regulation devices, the slow-time-scale voltage control model is as follows: ； In the formula, is the period number, is the optimization period, is the node set, is the node 's child node set, is the phase set, is the node connected by OLTC 's child node set, is the node set connected by CB, is the node connected by UPFC 's child node set; is the marginal price of network loss electric energy; is the time 's active power loss of the system; , and are the voltage regulation cost weight coefficients of OLTC, CB and UPFC respectively; , and are the OLTC voltage regulation cost, CB voltage regulation cost and UPFC voltage regulation cost respectively; The calculation formula for the active power loss of the AC-DC hybrid power grid is as follows: ； In the formula, is the moment node to node at the phase the square of the current amplitude, is the moment node and node at phase and the resistance between phases; The operation costs of the OLTC and CB and the reactive power cost of the UPFC are as follows: ； In the formula, is the unit regulation cost calculated when the OLTC changes one gear, is the unit regulation cost calculated when the CB switches one group of quantities; and are the gear position of the OLTC and the switching times of the CB at time period respectively; is the reactive power output cost coefficient of the UPFC, is the node and node where the UPFC outputs reactive power.

7. The dual-time-scale voltage control method for AC-DC hybrid power grid based on UPFC according to claim 6, wherein The constraint conditions are as follows: OLTC constraints include: the voltage ratio relationship constraints on both sides of each ideal transformer; the maximum and minimum tap positions of the OLTC tap-changer; the maximum number of times of a single tap-changer operation; CB constraints include: the maximum and minimum switching groups of the CB; the maximum number of switching groups in a single CB switching; UPFC constraints include: the active power interaction inequality constraints of the converter; the voltage and current amplitude constraints of the series converter; the reactive current constraints on the shunt side; the absolute value constraints on the voltage deviation and reactive power change of the UPFC; AC power grid constraints include: the three-phase active and reactive power balance constraints of each node in each area; the voltage drop constraints of each branch in each phase; the current amplitude constraints of all branches and tap-changers in each phase; LCC-HVDC constraints include: the constraints on the active and reactive power flowing into the converter transformer from the AC system, the voltage and current at the landing point of the UHVDC system in the AC-DC hybrid power grid, and the modulation degree and phase shift angle of the converter; Based on the above constraint conditions, solve to obtain the slow-time-scale voltage control scheme.

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

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

10. The dual-time-scale voltage control method for AC / DC hybrid power grid based on UPFC according to claim 9, characterized in that, The genetic algorithm is used to solve the optimal voltage control scheme; During the iteration process, the solution is carried out in combination with probability constraints, and the discrete variables are continuously processed to ensure the stable progress of the optimization path.

11. A dual-time-scale voltage control system for an AC-DC hybrid power grid based on UPFC, operating according to the dual-time-scale voltage control method for an AC-DC hybrid power grid based on UPFC as described in any one of claims 1-10, characterized in that, It includes: A dynamic model construction module that obtains the OLTC dynamic model, UPFC dynamic model, CB 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 that analyzes the dynamic responses of the OLTC dynamic model, UPFC dynamic model, and CB dynamic model to the grid state and defines the trajectory sensitivity of the AC / DC hybrid power grid; A slow-time-scale voltage control scheme solving module that builds a slow-time-scale voltage control model with the goal of minimizing the active power loss cost, OLTC and CB operation costs, and UPFC reactive power cost based on the trajectory sensitivity, and solves the slow-time-scale voltage control scheme; A fast-time-scale voltage control model building module that builds a fast-time-scale voltage control model based on the slow-time-scale voltage control scheme and with the help of real-time updated short-term load forecasting information to optimize the reactive power output of the UPFC in real time; An optimal voltage control scheme solving module that proposes a dual-time-scale voltage control model based on the slow-time-scale and fast-time-scale voltage control models and solves to obtain the optimal voltage control scheme.

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