Voltage Emergency Control Method and System for Wind Power Ramping Events in Multi-Infeed AC-DC Power Systems

Abstract

The present invention proposes a voltage emergency control method and system for wind power ramp events in a multi-infeed AC-DC system. The two-level coordinated control based on the tube-based MPC method is used to perform emergency voltage control on the system. After the emergency voltage control, preventive voltage control considering reactive power balance is carried out on the system. It includes: based on the voltage state and wind power prediction information within the partition, when the AC-DC hybrid system fed by wind power enters the emergency state, the nominal system is controlled to obtain the reference trajectory; based on the generated reference trajectory, in the presence of uncertainties, the actual system trajectory is guided to the reference trajectory, and the voltage emergency controller is used for voltage emergency control; based on the voltage state and wind power prediction information within the partition, after the voltage emergency control, the fast reactive power reserve restoration controller is used to perform fast reactive power reserve restoration considering reactive power balance on the system, and the replacement of fast and slow reactive power is carried out.

Description

Technical Field

[0001] The present invention belongs to the technical field of power systems, and particularly relates to a voltage emergency control method and system for a wind power ramp event in a multi-infeed AC-DC system. Background Technique

[0002] The statements in this part merely provide background technical information related to the present invention and do not necessarily constitute prior art.

[0003] With the rapid consumption of global fossil energy, clean and renewable energy represented by wind energy has been attracting increasing attention from the international community. When offshore wind power is transmitted through long-distance AC lines, the charging effect brought by long-distance submarine cables may greatly affect the transmission cost and is not conducive to voltage stability. In this context, flexible DC transmission (VSC-HVDC) has become the best solution for existing offshore wind power grid connection due to its excellent characteristics such as fast active and reactive decoupling control ability, no need for reactive device compensation, and easy expansion into multi-terminal DC (MTDC).

[0004] Wind power is a highly intermittent and uncertain energy source. When a large amount of wind power is fed into the AC grid through VSC-MTDC, the volatility of its output will not only impact the frequency stability of the grid but also cause voltage fluctuations or even over-limit, especially when a wind power ramp event occurs. The significantly changing active power output will seriously affect the voltage stability of the receiving-end grid. Therefore, it is necessary to fully utilize the fast reactive power support ability of the VSC converter station, coordinate the reactive power control resources with different time response characteristics of the AC system, and construct a voltage coordination control method for wind power ramp events.

[0005] In the current research on voltage control of AC-DC systems with high wind power penetration, most studies focus on the wind power grid connection method via long-distance AC lines and the control mainly targets the internal equipment of the wind farm to keep the wind power collection busbar and the fan terminal voltage within the allowable range, ignoring the voltage control resources on the receiving-end AC system side. When a large amount of wind power is connected to the grid through VSC-MTDC, the impact of wind power output fluctuations on the voltage of the fed grid will extend from a single point to multiple points of the AC system. Therefore, it is necessary to coordinate and cooperate with the reactive power control resources of the AC system.

[0006] Among the control methods adopted in the existing research on voltage control under high wind power penetration, model predictive control (MPC) has been widely used. Its characteristics of rolling prediction and optimization endow it with a certain degree of robustness. However, the superiority of MPC performance depends to a large extent on the accuracy of the prediction model. When the system has large uncertainties and its actual trajectory deviates seriously from the predicted trajectory, the performance of traditional MPC control methods will be greatly affected, which may lead to slow performance, overshoot, being overly conservative, or even guiding the system state to the wrong trajectory. When a wind power ramp event occurs in a multi-infeed AC-DC system under high wind power penetration, the voltage fluctuates violently and the prediction error of wind power output is large. At this time, it is necessary to take targeted measures to handle the uncertainty of wind power prediction and improve the robustness of control. Summary of the Invention

[0007] To overcome the deficiencies of the above-mentioned prior art, the present invention provides a voltage emergency control method for a multi-infeed AC-DC system during a wind power ramp event, adopting a tube-based MPC control method to handle the uncertainty of wind power output and improve the voltage stability of the system fed by multiple DCs via VSC-MTDC, especially during a wind power ramp event.

[0008] To achieve the above object, one or more embodiments of the present invention provide the following technical solutions:

[0009] In a first aspect, a voltage emergency control method for a multi-infeed AC-DC system during a wind power ramp event is disclosed, including:

[0010] Based on the voltage state and wind power prediction information within the partition, when the AC-DC hybrid system fed by wind power enters an emergency state, control the nominal system to obtain a reference trajectory;

[0011] Based on the generated reference trajectory, in the presence of uncertainties, guide the actual system trajectory to the reference trajectory and perform voltage emergency control using a voltage emergency controller;

[0012] Based on the voltage state and wind power prediction information within the partition, after voltage emergency control, use a fast reactive power reserve restoration controller to perform fast reactive power reserve restoration considering reactive power balance for the system and perform replacement of fast and slow reactive power.

[0013] As a further technical solution, before controlling the nominal system to obtain a reference trajectory, judge whether the two-level control is started according to the converter station-level control and system-level control start conditions. When the two-level control is triggered simultaneously, establish communication between the two-level controls;

[0014] Based on the trigger action instructions of the two-level coordinated control, control the nominal system based on the current system voltage state and wind power prediction information to obtain a reference trajectory for control.

[0015] As a further technical solution, it further includes: solving the optimization problem of the voltage emergency controller and the fast reactive power standby restoration controller to obtain the corresponding optimal control sequence.

[0016] As a further technical solution, the converter station-level control uses the VSC converter stations at each wind power connection point to perform real-time autonomous control on the wind power connection point voltage;

[0017] The system-level control uses the reactive power resources of the AC system to control the voltage of the central nodes within the partition;

[0018] After the system-level control is started, the VSC converter station preferentially participates in the system-level control. Reasonable start thresholds should be set for the two-level control to achieve better coordination. Both the converter station-level control and the system-level control adopt the tube-based MPC controller.

[0019] As a further technical solution, the tube-based MPC controller consists of two cascaded controllers. First, a nominal MPC controller controls the nominal system that ignores uncertainties and tightens constraints to obtain a reference trajectory, and then an auxiliary MPC controller controls the actual system to direct the state of the actual system to the reference trajectory.

[0020] As a further technical solution, it further includes: constructing a voltage prediction model based on trajectory sensitivity;

[0021] The specific steps for constructing the model are as follows:

[0022] Based on the online synchronous measurement data provided by the wide-area measurement system, obtain the initial value information of the AC / DC hybrid system, including the voltage vectors and power injection vectors of each load node, and calculate the system admittance matrix and the initial value information of each state variable;

[0023] Obtain the trajectory sensitivity of each control variable of the AC / DC hybrid system to the load bus voltage through the Jacobian matrix obtained in the time-domain simulation;

[0024] Linearize the system model through the trajectory sensitivity, and transform the original problem into a quadratic programming problem of a linear system.

[0025] As a further technical solution, the nominal system is a system in which the wind power output has no prediction error and changes in real time according to the wind power output prediction curve.

[0026] As a further technical solution, by solving the optimization problem of the nominal MPC controller before each control moment, the optimal control sequence is obtained, and the first control variable of the control sequence is applied to the nominal system, and then the reference trajectory required by the auxiliary MPC controller can be obtained.

[0027] As a further technical solution, the auxiliary MPC controller solves the optimization problem before the start of each control moment, obtains the control sequence and applies the first control quantity to the system to track the reference trajectory. Continuously rolling forward for control, guiding the actual system trajectory to the reference trajectory, and guiding the actual system state to the reference trajectory in the presence of wind power output prediction errors, so as to achieve voltage emergency control.

[0028] As a further technical solution, a rapid reactive power reserve restoration considering reactive power balance is performed on the system, specifically:

[0029] Based on the current voltage state of the system and wind power prediction information, it is judged whether to start the rapid reactive power reserve restoration control. If the adjustment is not satisfied, priority should be given to ensuring that the voltage emergency control mode is in the triggered state;

[0030] If it is judged to start the rapid reactive power reserve restoration control, first a voltage prediction model is constructed based on trajectory sensitivity;

[0031] According to the system voltage state, rapid reactive power margin situation and voltage prediction model, a rapid reactive power restoration MPC controller is constructed;

[0032] Model the optimization problem solved by the rapid reactive power restoration MPC controller, solve the quadratic programming model, and obtain the control change amounts of the generator and rapid reactive power equipment for control.

[0033] In the second aspect, a voltage emergency control system for a multi - HVDC fed system is disclosed, including:

[0034] A reference trajectory acquisition module, configured to: based on the voltage state and wind power prediction information within the partition, when the AC - DC hybrid system fed by wind power enters the emergency state, control the nominal system to obtain the reference trajectory;

[0035] A voltage emergency control module, configured to: based on the generated reference trajectory, in the presence of uncertainties, guide the actual system trajectory to the reference trajectory and perform voltage emergency control using the voltage emergency controller;

[0036] A reactive power reserve restoration module, configured to: based on the voltage state and wind power prediction information within the partition, after voltage emergency control, use the rapid reactive power reserve restoration controller to perform rapid reactive power reserve restoration considering reactive power balance on the system and perform the replacement of fast and slow reactive power.

[0037] The above - mentioned one or more technical solutions have the following beneficial effects:

[0038] The present invention is directed to an AC system with a high wind power penetration connected to the grid via VSC-MTDC. The tube-based MPC method is adopted to coordinate the voltage control by dealing with the uncertainty of wind power output, which can improve the voltage stability of the multi-infeed AC / DC system, especially during wind power output fluctuations, particularly wind power ramp events.

[0039] The voltage control method of the present invention is aimed at a large-scale wind power integrated grid-connected system via VSC-MTDC, focusing on the reactive power control resources on the system side where wind power is fed in. The tube-based MPC controller designed in the voltage emergency control can well handle the uncertainty of wind power prediction and obtain better performance than the conventional MPC method.

[0040] The two-level coordinated control at the converter station level and the system level in the voltage emergency control of the present invention gives full play to the fast reactive power control ability of the VSC converter station, improving the voltage stability of the AC system, especially during wind power fluctuations, particularly wind power ramp events.

[0041] The fast reactive power reserve restoration control of the present invention uses slow dynamic reactive power resources to replace fast dynamic reactive power resources, so as to ensure that once a disturbance occurs, the fast reactive power device can play a fast support role, realizing preventive control and improving the transient voltage stability of the system.

[0042] Advantages of additional aspects of the present invention will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The specification drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0044] Figure 1 is the flowchart of the coordinated robust voltage control of the multi-infeed AC / DC system under high wind power penetration proposed by the present invention;

[0045] Figure 2 is the schematic diagram of the principle of the tube-based MPC method adopted in the voltage emergency control of the present invention;

[0046] Figure 3 is the implementation flowchart of the voltage emergency control method provided by the present invention;

[0047] Figure 4 is the structure diagram and partition of the modified IEEE Nordic multi-infeed AC / DC hybrid system provided by the present invention;

[0048] Figure 5 is the curve of the actual wind power output and the predicted wind power output during the wind power ramp event in the example simulation provided by the present invention.

[0049] Figure 6 This is the voltage control effect of the emergency voltage control in the method of the present invention after the occurrence of the wind power upward ramp event, where busbars 4043 and 4044 are the central area central nodes and the wind power feeding points;

[0050] Figure 7 This is the comparison of the voltage control effects between the tube-based MPC method and the conventional MPC method adopted in the voltage emergency control provided by the present invention;

[0051] Figure 8 This is the voltage recovery effect of the system when a fault occurs after the fast reactive power reserve recovery control is adopted after the occurrence of a line break fault on line 4042 - 4043 after the wind power downward ramp event provided by the present invention. Detailed implementation manners

[0052] It should be noted that the following detailed descriptions are all exemplary and are intended to provide further illustration of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0053] It should be noted that the terms used herein are only for describing the specific implementation manners and are not intended to limit the exemplary embodiments according to the present invention.

[0054] Without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0055] Embodiment 1

[0056] This embodiment discloses a voltage emergency control method for a multi-fed AC-DC system wind power ramp event. As Figure 1 shown, this embodiment provides a voltage emergency control and reactive power reserve recovery method for a multi-DC-fed system under a wind power ramp event, including the following steps:

[0057] Step 1) Based on the voltage state and wind power prediction information within the partition, when the AC system fed with wind power enters the emergency state, use the nominal MPC controller to control the nominal system to obtain the reference trajectory;

[0058] Step 2) Based on the generated reference trajectory, in the presence of uncertainties, use the designed auxiliary MPC controller to direct the system trajectory to the reference trajectory to achieve voltage emergency control.

[0059] Step 3) Based on the voltage state and wind power prediction information within the partition, after voltage emergency control, perform fast reactive power reserve recovery considering reactive power balance for the system to achieve preventive control.

[0060] In the appendix Figure 1In it, the voltage information of each regional node is obtained, and it is judged whether the voltage deviation within the partition exceeds the threshold. If so, it is judged whether the dynamic reactive power reserve capacity within the region meets the requirements. If so, emergency voltage control is entered. If not, discrete voltage regulating equipment is incorporated. It is judged whether the voltage deviation within the partition exceeds the threshold. If not, it is judged whether the fast reactive power reserve capacity within the region meets the requirements. If so, it is judged whether the predicted wind power output is stable. If so, preventive voltage control is performed.

[0061] For the AC system adopting the VSC-MTDC grid connection mode, the wind power access mode gradually develops from the original single-point access to multi-point access, which will make it difficult for the original centralized control to meet the requirements of unified coordinated control of multi-regional and multi-objective voltages. At the same time, the three-level voltage control also requires zoning control of the system reactive power-voltage to achieve local balance of reactive power, reduce the dependence on the communication system, and improve the voltage stability and control flexibility of the area. Therefore, the method for emergency voltage control and reactive power reserve recovery of a multi-DC feed-in system under a wind power ramp event proposed by the present invention adopts decentralized control after zoning the reactive power-voltage of a large-scale wind power through a VSC-MTDC grid-connected system.

[0062] In order to make full use of the fast reactive power support ability of the VSC converter station, the emergency voltage control is designed as a two-level coordinated control of converter station-level control and system-level control. The converter station-level control uses the VSC converter stations at each wind power connection point to perform real-time autonomous control on the voltage of the wind power connection point, reducing the impact of wind power fluctuations on the voltage of the feed-in system. The system-level control uses the reactive power resources of the AC system to control the voltage of the central nodes within the partition. When the system-level control is started, the VSC converter station preferentially participates in the system-level control. Reasonable starting thresholds should be set for the two-level control to achieve better coordination. Both the converter station-level control and the system-level control adopt the same designed tube-based MPC controller.

[0063] As one or more embodiments, step 1) is based on the voltage state and wind power prediction information within the partition. When the system enters the emergency state, the nominal MPC controller is used to control the nominal system to obtain the reference trajectory. The control principle of the tube-based MPC controller is as Figure 2 shown. It consists of two cascaded controllers. First, a nominal MPC controller controls the nominal system that ignores uncertainties and tightens the constraints to obtain the reference trajectory. Then, an auxiliary MPC controller is used to control the actual system to direct the state of the actual system to the reference trajectory.

[0064] The above-mentioned nominal system is an AC system with wind power feed-in that ignores the uncertainties of the feed-in wind power.

[0065] Step 1-1) Construct a voltage prediction model based on trajectory sensitivity. Based on the online synchronous measurement data such as the voltage vectors and power injection vectors of each load node provided by the wide-area measurement system, obtain the initial value information of the AC / DC hybrid system, and calculate the system admittance matrix and the initial value information of each state variable.

[0066] Model the AC / DC hybrid system. Combine the multi-infeed AC / DC hybrid system model (i.e., the differential-algebraic equations describing the power system), apply the implicit trapezoidal integration method and the Newton-Raphson method for time-domain simulation, and combine the predicted output data of wind power generation to predict the system voltage output trajectory. Obtain the trajectory sensitivity of each control variable of the AC / DC hybrid system to the load bus voltage through the Jacobian matrix obtained in the time-domain simulation. Linearize the system model through the trajectory sensitivity, and transform the original problem into a quadratic programming problem of a linear system. The linearized relationship between the input and output in voltage prediction control is the voltage prediction model, which is expressed as:

[0067] V′ k =V k +S v-u Δu i

[0068] In the formula, the subscript k represents the value of the corresponding variable at time t k ; V′ k is the voltage prediction trajectory, V k is the voltage before control, obtained through time-domain simulation according to the predicted wind power output; Δu i is the change of the i-th input control variable; is the trajectory sensitivity of the voltage amplitude to .

[0069] This voltage prediction model predicts the output based on the input at the k-th moment and constructs an optimization problem. Each subsequent MPC controller has a voltage prediction model part.

[0070] Step 1-2) Design a nominal MPC controller to control the nominal system that ignores uncertainties. The control objective is the same as the original problem, that is, to minimize the voltage deviation and the control cost. In a high wind power penetration system, the nominal system is a system in which the wind power output has no prediction error and changes in real time according to the wind power output prediction curve. The optimization problem solved by the nominal MPC controller is modeled as:

[0071]

[0072]

[0073] ρ2u i,min ≤v i ≤ρ2u i,max

[0074] ρ1V min ≤V′ N ≤ρ1V max

[0075] ρ2Δu i,min ≤Δv k,i ≤ρ2Δu i,max

[0076]

[0077] In the formula, the objective function is divided into two parts: voltage deviation and control cost, ω v ,ω u are the weight coefficients of voltage deviation and control variables respectively; k represents the kth control period, N p ,N c are the prediction period and the number of control periods respectively; is the nominal system voltage control reference value, V N,k ,V′ N,k are the nominal system voltage amplitudes before and after control at the kth moment respectively; is the sensitivity of the ith control quantity to the voltage of the control quantity u at the kth moment; v i is the ith nominal system control quantity, Δv k,i is the change of the ith nominal system control quantity at the kth moment; u i,max ,u i,min are the upper and lower limits of the actual system control variables, Δu i,max ,Δu i,min are the upper and lower limits of the single change of the actual system control variables; P vsc ,Q vsc ,S vsc represent the active power, reactive power and capacity of the VSC converter station respectively; ρ1 and ρ2 are the state constraint convergence parameters and control quantity constraint convergence parameters of the nominal controller respectively.

[0078] By solving the optimization problem of the nominal MPC controller before the start of each control moment, the optimal control sequence is obtained. The control sequence is the value of each control quantity obtained by solving the optimization problem at each control moment within the prediction time domain, and the first control quantity of the control sequence is applied to the nominal system, and the reference trajectory required by the auxiliary MPC controller in step 2) can be obtained.

[0079] As one or more embodiments, step 2) is to design an auxiliary MPC controller based on the generated reference trajectory to guide the system trajectory to the reference trajectory in the presence of uncertainties, so as to achieve voltage emergency control. The cooperation relationship between the auxiliary MPC controller and the nominal MPC controller is as Figure 3As shown, the auxiliary MPC controller controls the actual system. Using the trajectory sensitivity-based prediction model in step 1-1), the optimization problem solved by the controller is modeled as:

[0080]

[0081]

[0082] u i,min ≤u i ≤u i,max

[0083] V min ≤V′ k ≤V max

[0084] Δu i,min ≤Δu i ≤Δu i,max

[0085]

[0086] In the formula, the objective function is divided into two parts: the voltage trajectory deviation and the control quantity trajectory deviation. μ v , μ u are the weight coefficients of the voltage trajectory deviation and the control quantity trajectory deviation respectively; V N,k is the target voltage value of the voltage reference trajectory at time k, V k , V′ k are the actual system voltage values before and after control at time k respectively; u i is the control quantity of the i-th actual system, and Δu k,i is the change in the control quantity of the i-th actual system.

[0087] Before each control moment, the auxiliary MPC controller solves the above optimization problem, obtains the control sequence, and applies the first control quantity to the system to track the reference trajectory. Continuously rolling forward the control, guiding the actual system trajectory to the reference trajectory, and guiding the actual system state to the reference trajectory in the presence of wind power output prediction errors, to achieve voltage emergency control.

[0088] As one or more embodiments, step 3) is to perform a rapid reactive power reserve restoration considering reactive power balance on the system after voltage emergency control based on the voltage state within the partition and wind power prediction information, to achieve preventive control.

[0089] Considering the time response characteristics of different reactive power resources, the reactive power control device includes generators, transformers, capacitors, static var compensators / static synchronous compensators (SVC / STATCOM), and flexible DC transmission devices such as VSC converter stations. As a traditional reactive power compensation device, a generator can continuously generate or absorb reactive power by controlling the excitation system, but its response speed is relatively slow compared to SVC and VSC converter stations. The response speeds of DC excitation and AC excitation systems are generally above 100 ms. Flexible DC devices with power electronics at their core combine the rapidity and continuous smoothness of control, and can significantly improve the voltage stability of the system.

[0090] Fast reactive power reserve restoration is the replacement of fast and slow reactive power under the condition of qualified voltage and small fluctuations in wind power predicted output. The reactive power of generators and shunt capacitors is used to replace the fast reactive power of VSC converter stations and SVCs, so that when a disturbance occurs again, the fast reactive power compensation equipment can play the voltage support role to the greatest extent, improve the voltage recovery effect after the disturbance, and achieve preventive control of voltage stability. After voltage emergency control, it is necessary to perform fast reactive power reserve restoration considering reactive power balance for the system through step 3). The specific implementation content of step 3) includes:

[0091] Step 3-1): Based on the current voltage state of the system and wind power prediction information, judge whether to start fast reactive power reserve restoration control. Fast reactive power reserve restoration control should be carried out on the premise that the current voltage of the system is qualified and the fluctuations in wind power predicted output are small. If the adjustment conditions are not met, priority should be given to ensuring that the voltage emergency control mode is in the triggered state. If it is judged to start fast reactive power reserve restoration control, first construct a voltage prediction model based on trajectory sensitivity. Based on the online synchronous measurement data such as the voltage vectors and power injection vectors of each load node provided by the wide-area measurement system, obtain the initial value information of the AC-DC hybrid system, calculate the system admittance matrix and the initial value information of each state quantity, and the calculation method is the same as that in step 1-1).

[0092] Step 3-2): According to the system voltage state and fast reactive power margin, construct a fast reactive power restoration MPC controller. The optimization problem solved by the fast reactive power restoration MPC controller is modeled as:

[0093]

[0094]

[0095]

[0096] V min ≤V′ k ≤V max

[0097] u i,min ≤ui ≤ u i,max

[0098] ΔU min ≤ |ΔU k | ≤ ΔU max

[0099] In the formula, the fast reactive power reserve restoration control objective function is divided into three parts. The first term in the objective function represents the deviation between the voltage of the central node before and after control and the set value. The second part is to maximize the reactive power margin of the fast reactive power equipment. The third term is to minimize the control quantity, λ v , λ r , λ u are the weights of the three terms respectively; θ i is the reactive power margin of the i-th fast reactive power equipment, Q i , ΔQ i are its current reactive power output and reactive power change respectively, Q min,i , Q max,i are the minimum and maximum values of its reactive power output respectively; Q ref is the optimal dynamic reactive power reserve set value of the i-th fast reactive power equipment. Solve this quadratic programming model to obtain the control change amounts of the generator and the fast reactive power equipment for control.

[0100] Whether the above step 3-2) is started needs to be based on the judgment of step 3-1). The voltage prediction model in the MPC controller of step 3-2) is obtained based on the trajectory sensitivity in step 3-1).

[0101] In this embodiment, after the voltage emergency control, the fast reactive power reserve restoration method is used. According to the system voltage and reactive power margin status, the wind power prediction information is added to the startup criterion, and the fast reactive power reserve restoration considering the reactive power balance of the system is carried out to improve the transient stability of the system voltage.

[0102] In this embodiment, considering the uncertainty of wind power prediction, especially the large prediction error during the wind power ramp process, the voltage emergency control adopts a control method based on tube MPC to cope with the influence of wind power prediction error on the control performance during the control process, and improves the robustness of the control. Among them, the nominal MPC controller controls the nominal system whose wind power output changes in real time according to the predicted output and tightens the constraints to obtain the reference trajectory of the actual control. The auxiliary MPC controller controls the actual system according to the reference trajectory given by the nominal MPC controller to lead the actual system state to the reference trajectory.

[0103] In this embodiment, decentralized control is performed within each partition, and the fast reactive power reserve is restored after voltage emergency control. The voltage emergency control is designed as a two-level coordinated control method of converter station-level control and system-level control to make full use of the fast reactive power support capability of the VSC converter station.

[0104] Simulation example

[0105] Figure 4 A modified IEEE Nordic multi-infeed AC-DC hybrid system is given for simulation verification of the proposed method. The reactive power-voltage partition results obtained according to step 1) are shown in the figure.

[0106] In the appendix Figure 4 The nominal system refers to the actual system ignoring uncertainties. Specifically, in the AC system with wind power feeding to be controlled, the wind power output has no prediction error and changes in real time according to the wind power output prediction curve.

[0107] In the expression for tightening constraints in the nominal controller optimization problem of step 1-2, the actual state constraints and control quantity constraints are tightened through two parameters ρ1 and ρ2.

[0108] The constraints of the auxiliary controller are described in the auxiliary controller optimization problem of step 2, which are the constraints that the actual system should satisfy, including voltage amplitude constraints, control quantity constraints, and control quantity change constraints.

[0109] In Figure 5 According to the measured and predicted wind power output data when large-scale up-ramp and down-ramp events occur in the wind power in the BPA region of the United States as shown, the voltage control effect obtained by the method proposed in the present invention is as Figure 6 、 Figure 7 and Figure 8 shown. Figure 6 It shows that the voltage emergency control method proposed in the present invention can maintain the stability of the AC system voltage through two-level coordinated control when the wind power output fluctuates greatly. Figure 7 It shows that the tube-based MPC method adopted in the voltage emergency control of the present invention has good robustness. Compared with the traditional MPC controller, it can better overcome the influence of wind power prediction uncertainty on the control performance and control the central node voltage to the target value of 1.03 p.u. Figure 8 It shows that the fast reactive power reserve restoration method proposed by this method can increase the fast reactive power margin of the system. When the system is disturbed again, the fast reactive power equipment can quickly support the system voltage and improve the system voltage stability.

[0110] This embodiment is aimed at a large-scale wind power integrated into the grid system via VSC-MTDC, focusing on the reactive power control resources on the system side of the wind power feed-in. The tube-based MPC method is introduced to overcome the high uncertainty during wind power ramp events. Based on the voltage states and wind power prediction information in each partition of the wind power feed-in system, when the system enters an emergency state, the nominal MPC controller is first used to control the nominal system to obtain the reference trajectory. Based on the generated reference trajectory, in the presence of uncertainties, the designed auxiliary MPC controller is used to guide the system trajectory to the reference trajectory to achieve voltage emergency control. After voltage emergency control, a rapid reactive power reserve recovery considering reactive power balance is performed on the system to maximize the rapid reactive power margin, thereby ensuring that when a disturbance occurs again, the transient voltage stability of the system is improved.

[0111] The trajectory in this embodiment is the value of the voltage and control quantity during the control process, corresponding to the voltage and control quantity. Giving an input of a control quantity to the system will result in an output of the corresponding voltage. The trajectory can be understood as the change of input and output during the entire control process.

[0112] Embodiment 2

[0113] This embodiment provides a computer-readable storage medium with a computer program stored thereon. When the program is executed by a processor, it implements the steps in the method in the above-mentioned Embodiment 1.

[0114] Embodiment 3

[0115] This embodiment provides a computer device, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the program, the following steps are implemented:

[0116] Based on the initial value information of the AC-DC hybrid system fed by wind power, the trajectory sensitivity of each bus voltage to the control quantity is calculated by combining time-domain simulation to construct a prediction model of the MPC controller;

[0117] According to the start conditions of converter station-level control and system-level control, it is judged whether the two-level control is started. When the two-level control is triggered simultaneously, communication between the two-level control is established.

[0118] According to the trigger action instruction of the two-level coordinated control, the nominal system is controlled based on the current system voltage state and wind power prediction information to obtain the reference trajectory of the control;

[0119] Based on the reference trajectory formed by the nominal controller module, voltage emergency control is performed on the actual system to guide the actual system to the reference trajectory;

[0120] Based on the current system voltage status and fast reactive power margin, combined with wind power prediction information, it is judged whether to start the fast reactive power reserve recovery control, and after starting, the replacement of fast and slow reactive power resources is realized through the fast reactive power recovery MPC controller;

[0121] Solve the optimization problem constructed by the voltage emergency controller and the fast reactive power reserve recovery controller to obtain the corresponding optimal control sequence.

[0122] The embodiments of the present invention take targeted measures to handle the uncertainty of wind power prediction and improve the robustness of control.

[0123] Embodiment 4

[0124] The purpose of this embodiment is to provide a voltage emergency control system for a multi - HVDC feeding system, including:

[0125] A reference trajectory acquisition module, configured to: based on the voltage status and wind power prediction information within the partition, when the AC - DC hybrid system fed by wind power enters an emergency state, control the nominal system to obtain a reference trajectory;

[0126] A voltage emergency control module, configured to: based on the generated reference trajectory, in the presence of uncertainties, direct the actual system trajectory to the reference trajectory and perform voltage emergency control using a voltage emergency controller;

[0127] A reactive power reserve recovery module, configured to: based on the voltage status and wind power prediction information within the partition, after voltage emergency control, use a fast reactive power reserve recovery controller to perform fast reactive power reserve recovery considering reactive power balance for the system and perform the replacement of fast and slow reactive power.

[0128] After voltage emergency control, perform fast reactive power reserve recovery considering reactive power balance for the system to maximize the fast reactive power margin, thereby ensuring that when a disturbance occurs again, the transient voltage stability of the system is improved.

[0129] The steps involved in the devices in the above Embodiments 2, 3, and 4 correspond to those in Method Embodiment 1. For specific implementation manners, reference can be made to the relevant description part of Embodiment 1. The term "computer - readable storage medium" should be understood to include a single medium or multiple media including one or more instruction sets; it should also be understood to include any medium that can store, encode, or carry an instruction set for execution by a processor and cause the processor to execute any method in the present invention.

[0130] Those skilled in the art should understand that the various modules or steps of the present invention described above can be implemented by a general-purpose computer device. Optionally, they can be implemented by program codes executable by a computing device, so that they can be stored in a storage device and executed by the computing device, or they can be separately fabricated into individual integrated circuit modules, or multiple modules or steps among them can be fabricated into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0131] Although the specific implementation manners of the present invention have been described above in conjunction with the accompanying drawings, it is not a limitation on the protection scope of the present invention. Those skilled in the art should understand that various modifications or deformations that can be made without creative efforts by those skilled in the art on the basis of the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. Voltage emergency control method for wind power ramping events in multi-infeed AC-DC systems, characterized in that Including: Based on the voltage state within the partition and wind power prediction information, when the AC-DC hybrid system fed by wind power enters an emergency state, the nominal system is controlled to obtain a reference trajectory; Based on the generated reference trajectory, in the presence of uncertainties, the actual system trajectory is guided towards the reference trajectory, and a voltage emergency controller is used for voltage emergency control; Based on the voltage state within the partition and wind power prediction information, after voltage emergency control, a fast reactive power reserve restoration controller is used to perform fast reactive power reserve restoration considering reactive power balance for the system, and fast and slow reactive power are replaced; By solving the optimization problem of the nominal MPC controller before the start of each control moment, an optimal control sequence is obtained, and the first control quantity of the control sequence is applied to the nominal system to obtain the reference trajectory required by the auxiliary MPC controller; The auxiliary MPC controller solves the optimization problem before the start of each control moment, obtains the control sequence and applies the first control quantity to the system, tracks the reference trajectory, continuously rolls forward for control, guides the actual system trajectory towards the reference trajectory, and guides the actual system state towards the reference trajectory in the presence of wind power output prediction errors to achieve voltage emergency control; Wherein, The auxiliary MPC controller controls the actual system, adopts a prediction model based on trajectory sensitivity, and the optimization problem solved by the controller is modeled as: u i,min ≤ u i ≤ u i,max V min ≤V k ′≤V max Δu i,min ≤Δu i ≤Δu i,max In the formula, the objective function is divided into two parts: the voltage trajectory deviation and the control quantity trajectory deviation, μ v , μ u are the weight coefficients of the voltage trajectory deviation and the control quantity trajectory deviation respectively; V N,k is the target voltage value of the voltage reference trajectory at time k, V k , V k ′ are the actual system voltage values before and after control at time k respectively; u i is the i-th actual system control variable, Δu k,i is the change in the i-th actual system control variable.

2. The voltage emergency control method for wind power ramp events in a multi-infeed AC-DC system according to claim 1, characterized in that Before controlling the nominal system to obtain a reference trajectory, according to the converter station-level control and system-level control startup conditions, it is judged whether the two-level control is started. When the two-level control is triggered simultaneously, communication between the two-level controls is established; Based on the trigger action instruction of the two-level coordinated control, the nominal system is controlled based on the current system voltage state and wind power prediction information to obtain the reference trajectory of the control.

3. The voltage emergency control method for the wind power ramp event in the multi-infeed AC-DC system according to claim 1, characterized in that it further Including: The optimization problems constructed by the voltage emergency controller and the fast reactive power reserve restoration controller are solved to obtain the corresponding optimal control sequences.

4. The voltage emergency control method for wind power ramp events in a multi-infeed AC-DC system as claimed in claim 1, wherein The converter station-level control uses the VSC converter stations at each wind power connection point to perform real-time autonomous control on the wind power connection point voltage; The system-level control uses the reactive power resources of the AC system to control the voltage of the central nodes within the partition; After the system-level control is started, the VSC converter stations give priority to participating in the system-level control. Reasonable startup thresholds should be set for the two-level controls to achieve better cooperation. Both the converter station-level control and the system-level control adopt the MPC controller based on tube.

5. The voltage emergency control method for the wind power ramp event in the multi-infeed AC-DC system according to claim 4, wherein The MPC controller based on tube consists of two cascaded controllers. First, a nominal MPC controller controls the nominal system that ignores uncertainties and tightens the constraints to obtain a reference trajectory, and then an auxiliary MPC controller controls the actual system to guide the state of the actual system towards the reference trajectory.

6. The voltage emergency control method for wind power ramp events in a multi-infeed AC-DC system according to claim 1, characterized in that For the system, fast reactive power reserve restoration considering reactive power balance is specifically: Based on the current voltage state of the system and wind power prediction information, it is judged whether to start fast reactive power reserve restoration control. If the adjustment is not satisfied, priority should be given to ensuring that the voltage emergency control mode is in the triggered state; If it is judged to start fast reactive power reserve restoration control, first a voltage prediction model is constructed based on trajectory sensitivity; Construct a fast reactive power recovery MPC controller according to the system voltage state, fast reactive power margin, and voltage prediction model; Model the optimization problem solved by the fast reactive power recovery MPC controller, solve the quadratic programming model, and obtain the control change amounts of generators and fast reactive power equipment for control.

7. The voltage emergency control system for a multi - DC feeding system adopts the voltage emergency control method for a wind power ramp event in a multi - fed AC - DC system as described in any one of claims 1 - 6, and is characterized in that Including: A reference trajectory acquisition module, configured to: based on the voltage state and wind power prediction information within the partition, when the AC-DC hybrid system fed by wind power enters an emergency state, control the nominal system to obtain a reference trajectory; A voltage emergency control module, configured to: based on the generated reference trajectory, in the presence of uncertainties, direct the actual system trajectory to the reference trajectory, and use a voltage emergency controller for voltage emergency control; A reactive power reserve recovery module, configured to: based on the voltage state and wind power prediction information within the partition, after voltage emergency control, use a fast reactive power reserve recovery controller to perform fast reactive power reserve recovery considering reactive power balance for the system, and perform replacement of fast and slow reactive power.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by a processor, it implements the steps in the method according to any one of claims 1-6 above.

9. A computer device, comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, When the processor executes the program, it implements the steps in the method according to any one of claims 1-6.

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