AC Fault Ride Control Device and Method for Wind Power Flexible DC Grid-Connected Systems

Abstract

This invention provides an AC fault ride-through control device and method for wind power flexible DC grid-connected systems, belonging to the field of flexible DC transmission and new energy grid connection technology. Addressing the DC-side power redundancy and overvoltage issues caused by AC grid faults at the receiving end when wind power is connected to the grid via a modular multilevel converter (MWC) type HVDC transmission system, two control modes are proposed: First, when the DC voltage exceeds a preset threshold, the WFMMC initiates a step-down control; the energy storage system switches to a constant power control mode to adaptively absorb redundant power, and the wind turbine GSVSC maintains constant DC voltage control. Second, after the WFMMC initiates step-down control, the wind turbine GSVSC switches to a constant power control mode based on the grid connection point voltage deviation, and the energy storage system switches to a constant DC voltage control mode to absorb remaining power. This invention achieves redundant power storage, effectively suppresses DC voltage fluctuations, and improves the dynamic stability and operational reliability of the system during fault ride-through.

Description

Technical Field

[0001] This invention relates to the field of flexible DC power transmission and new energy grid connection technology, and to AC fault ride-through control device and method for wind power flexible DC grid connection system. Specifically, it relates to two ride-through control modes and system design for responding to AC grid faults at the receiving end when wind power is connected to the grid via MMC-HVDC system. Background Technology

[0002] Against the backdrop of building a new power system with new energy sources as the mainstay, the adoption of Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) technology to achieve large-scale wind power grid connection has become an important solution.

[0003] However, when a short circuit or other fault occurs in the AC grid on the receiving end of the MMC-HVDC system, the active power delivered to the grid by the grid-Side Modular Multilevel Converter (GSMMC) will drop sharply due to the AC voltage drop.

[0004] Due to the electrical isolation characteristics of the AC and DC sides of the flexible DC transmission system, the wind turbines located at the sending end cannot detect the fault at the receiving end and continue to maintain the power output before the fault. This causes a large amount of power to accumulate on the DC side, resulting in a rapid increase in DC voltage. This seriously threatens the safety of equipment such as the capacitors of the converter station submodules, and may lead to system protection activation, wind turbine disconnection from the grid, and affect the reliable operation of the power grid.

[0005] To address this challenge, existing technical solutions mainly fall into two categories:

[0006] The first type is the additional energy-consuming resistor scheme, which converts redundant power into heat energy by connecting energy-consuming resistors in parallel or series on the DC or AC side. This mode is simple to control, but it essentially wastes energy, especially when the voltage drops sharply, which reduces the economic efficiency of system operation. In addition, the repeated switching of the resistor will cause large fluctuations in DC voltage and system power.

[0007] The second category is wind farm load shedding schemes, which transmit fault signals to the wind farm through methods such as voltage reduction, frequency increase, or communication, thereby reducing turbine output and eliminating redundant power at the source. However, this type of scheme has problems such as dynamic response hysteresis, complex control, and the possibility of excessively low voltage reduction causing turbine overcurrent. Furthermore, the control effect may be unsatisfactory due to factors such as dispersed turbine parameters and wind speed fluctuations.

[0008] In addition, some studies have proposed using energy storage systems (such as flywheels and supercapacitors) to absorb redundant power during faults. However, existing energy storage applications are mostly designed for specific topologies (such as pseudo-bipolar systems), failing to fully leverage the potential for deep collaborative control between wind farms and distributed energy storage systems installed on the DC bus of wind turbines. Energy storage systems within wind farms are typically used only as independent backup power sources, without being organically integrated with the control of wind turbine converters to form a smooth, efficient, and energy-saving collaborative fault ride-through mechanism.

[0009] Therefore, there is an urgent need for an AC fault ride-through control scheme that can fully utilize the synergistic advantages of wind and energy storage on the DC side, achieve smooth power regulation, small DC voltage fluctuations, and avoid energy waste. Summary of the Invention

[0010] (a) Purpose of the invention

[0011] The purpose of this invention is to overcome the shortcomings of existing technologies and provide one or more AC fault ride-through control methods based on wind power and energy storage DC-side coordination. By coordinating the control of the grid-side voltage source converter (GSVSC) of the wind turbine and the energy storage unit (ESU) connected in parallel to the DC bus of the wind turbine during a fault, redundant power on the DC side is dynamically and smoothly eliminated, thereby effectively suppressing DC overvoltage and improving the economy and reliability of system operation.

[0012] (II) Technical Solution

[0013] An AC fault ride-through control device for a wind power flexible DC grid-connected system includes an MMC-HVDC system, a wind farm, an energy storage system, and a coordination controller;

[0014] The MMC-HVDC system includes a WFMMC that connects to the wind farm and a GSMMC that connects to the receiving-end power grid;

[0015] The wind farm consists of several permanent magnet direct-drive wind turbines, each of which is connected to a common DC bus via a wind turbine GSVSC.

[0016] The energy storage system consists of several ESUs, each of which is connected in parallel to the DC bus of the corresponding wind turbine through a bidirectional DC-DC converter.

[0017] The coordination controller, which is communicatively connected to the WFMMC, wind turbine GSVSC, and ESU, is configured to execute one of the following two control modes when an abnormal increase in DC voltage is detected due to a fault in the receiving-end AC grid:

[0018] Mode 1: Send an instruction to the WFMMC to reduce the grid connection point voltage of the wind farm according to the DC voltage deviation; when the grid connection point voltage is lower than the grid connection point voltage threshold, all ESUs of the energy storage system absorb power in constant power control mode according to the grid connection point voltage deviation, and at the same time control the wind turbine GSVSC to maintain the voltage stability of the DC bus where the wind turbine GSVSC is located.

[0019] Mode 2: Send a command to the WFMMC to reduce the grid connection voltage of the wind farm according to the DC voltage deviation; when the grid connection voltage is lower than the grid connection voltage threshold, control all wind turbines GSVSC to reduce the wind turbine load in constant power control mode according to the grid connection voltage deviation, and at the same time control ESU to switch to constant DC voltage control mode to absorb the redundant power of the DC bus where ESU is located.

[0020] Furthermore, in Mode 1, the reference value of the total power required to be absorbed by the energy storage system is... Calculate according to the following formula:

[0021] (1)

[0022] In the formula, This is a reference value for the total power required to be absorbed by the energy storage system. This represents the number of ESUs in the energy storage system. The power absorption ratio of the energy storage system; The voltage threshold at the grid connection point; This is the actual value of the voltage at the grid connection point;

[0023] Energy storage system power absorption ratio factor The formula for calculation is:

[0024] (2)

[0025] In the formula: Rated power for ESU; This is the minimum voltage limit at the grid connection point;

[0026] Furthermore, in Mode 1, the coordination controller is further configured as follows:

[0027] Reference value of the total power required to be absorbed by the energy storage system Distributed to each ESU;

[0028] Initial power reference value assigned to the i-th ESU Calculate according to the following formula:

[0029] (3)

[0030] In the formula, This represents the real-time state of charge value of the i-th ESU;

[0031] If there exists an initial power reference value for the i-th ESU Greater than the rated power of ESU In the case of i-th ESU, let the initial power reference value be the rated power of the ESU. The power reference values ​​for the remaining ESUs, excluding the i-th ESU, are corrected according to the following formula:

[0032] (4)

[0033] In the formula, The number of ESUs whose initial power reference value is greater than the rated power; This is the corrected reference value for energy storage power;

[0034] Furthermore, in Mode 2, the reference value for the total load shedding power of the wind farm is... Calculate according to the following formula:

[0035] (5)

[0036] In the formula, This represents the output power of the fan before the failure. This refers to the fan load reduction ratio coefficient.

[0037] Fan load reduction ratio coefficient The calculation formula is:

[0038] (6)

[0039] Furthermore, in the second mode, the coordination controller is further configured as follows:

[0040] Reference value of total load reduction of wind farm Distributed to each wind turbine GSVSC;

[0041] The initial active power reference value of the i-th wind turbine GSVSC Calculate according to the following formula:

[0042] (7)

[0043] If there exists an initial active power reference value for the i-th wind turbine GSVSC Greater than the output power before the i-th fan failure In the case where the power reference value of the i-th wind turbine GSVSC is the output power before the wind turbine failure, let it be... The active power reference value of the remaining GSVSCs (excluding the i-th fan GSVSC) is corrected according to the following formula:

[0044] (8)

[0045] In the formula, The corrected active power reference value for the wind turbine GSVSC; This refers to the number of wind turbines whose initial power reference value is greater than their rated power.

[0046] Furthermore, the control method for reducing the grid connection point voltage using the WFMMC is as follows: when the DC voltage... Exceeding the DC voltage threshold At that time, the d-axis voltage reference value of WFMMC Calculate according to the following formula:

[0047] (9)

[0048] In the formula, The d-axis component of the rated voltage on the wind farm side; This is the pressure reduction ratio coefficient;

[0049] Voltage reduction ratio factor The calculation formula is:

[0050] (10)

[0051] In the formula, This is the minimum d-axis voltage limit on the wind farm side; This is the maximum limit for DC voltage.

[0052] The control method for the AC fault ride-through control device of a wind power flexible DC grid-connected system includes the following steps:

[0053] Fault detection steps: Monitor the system DC voltage in real time;

[0054] Step-down startup procedure: When the DC voltage exceeds the preset DC voltage threshold, a fault in the receiving-end AC grid is determined, and the WFMMC is controlled to start the step-down method to reduce the voltage at the wind farm's grid connection point;

[0055] Collaborative control steps: Monitor the grid connection point voltage. When the grid connection point voltage drops to a preset grid connection point voltage threshold, execute one of the following two modes according to the preset control mode:

[0056] Mode 1: Control all ESUs to absorb power in constant power control mode according to the voltage deviation at the grid connection point, while controlling the wind turbine GSVSC to maintain constant DC voltage control;

[0057] Mode 2: Control all wind turbines' GSVSCs to reduce power output in constant power control mode based on the grid connection point voltage deviation, while controlling the ESU to switch to constant DC voltage control.

[0058] Furthermore, when executing Mode 1, a power optimization allocation sub-step is also included: the total power reference value absorbed by the energy storage system is calculated based on the grid connection point voltage and then allocated to each unit according to the SOC of each ESU, giving priority to units with lower SOC to absorb more power.

[0059] Furthermore, when executing Mode 2, it also includes a load reduction optimization allocation sub-step: after the total load reduction power reference value of the wind turbine is calculated based on the grid connection point voltage, it is allocated to each wind turbine according to the SOC of the ESU connected to each wind turbine, giving priority to wind turbines with higher SOC to perform more load reduction.

[0060] A computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the control method described above.

[0061] (III) Beneficial Effects

[0062] Compared with traditional DC energy-consuming resistor solutions, the two wind-storage coordinated control modes proposed in this invention have the following significant advantages:

[0063] 1. High energy efficiency: Redundant electrical energy that cannot be transmitted during a fault is stored in the energy storage system instead of being dissipated as heat through resistors, thus avoiding energy waste and improving the economic efficiency of system operation.

[0064] 2. Smooth control process: Through WFMMC voltage reduction and continuous and smooth power regulation in coordination between wind turbine and energy storage, power imbalance can be eliminated more gradually, resulting in smaller DC voltage fluctuations, lower overshoot, and more stable system dynamic response, which is beneficial to equipment safety and grid stability.

[0065] 3. Energy Storage Utilization Optimization: Based on the SOC-based power consumption and wind turbine load reduction allocation scheme, intelligent optimization management of the energy storage system capacity is achieved. It can automatically reduce the difference in SOC between energy storage units, maximize the utilization of the overall available capacity, prevent individual units from prematurely shutting down due to overcharging or over-discharging, and extend the overall service life and reliability of the energy storage system.

[0066] 4. Enhanced system reliability: By making full use of the existing distributed energy storage resources within the wind farm, and through the intelligent switching and coordination of the control modes of the wind turbine and the energy storage converter, an active and adaptive fault ride-through mechanism is formed, which significantly improves the survivability and operational reliability of the wind power flexible DC grid connection system in the face of severe AC faults. Attached Figure Description

[0067] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0068] Figure 1This is a schematic diagram of the topology of the wind power grid-connected system via MMC-HVDC according to the present invention;

[0069] Figure 2 This diagram shows the installation location and switching control principle of a traditional energy-consuming resistor solution.

[0070] Figure 3 This is the overall control structure diagram of the control mode one of the present invention;

[0071] Figure 4 A timing diagram of the step-down control curve of the grid-side converter in Mode 1;

[0072] Figure 5 This is a timing diagram of the energy storage power control curve in Mode 1;

[0073] Figure 6 This is the overall control structure diagram of control mode two of the present invention;

[0074] Figure 7 A timing diagram of the step-down control curve for the grid-side converter in Mode 2;

[0075] Figure 8 This is a timing diagram of the active power control curve of the grid-side converter of the wind turbine in Mode 2.

[0076] Figure 9 The specific control timing flowcharts for Mode 1 and Mode 2 are shown below.

[0077] Figure 10 Here are the specific control logic flowcharts for Mode 1 and Mode 2;

[0078] Figure 11 The simulated waveform of the grid-side AC voltage when using a traditional energy-consuming resistor scheme is shown.

[0079] Figure 12 The simulated AC voltage waveform on the wind farm side is shown when using a traditional energy-consuming resistor scheme.

[0080] Figure 13 The simulation waveforms of the positive and negative DC voltages when using a traditional energy-consuming resistor scheme are shown.

[0081] Figure 14 The simulation waveforms of power on the grid side and the wind farm side are shown when using the traditional energy-consuming resistor scheme.

[0082] Figure 15 The simulation waveform of the trigger pulse of the energy-consuming resistor when using the traditional energy-consuming resistor scheme is shown.

[0083] Figure 16 The simulation waveform of energy absorption by the energy-consuming resistor when using the traditional energy-consuming resistor scheme is shown.

[0084] Figure 17The simulated waveform of the grid-side AC voltage when using the control mode one of this invention;

[0085] Figure 18 Simulated power waveforms on the grid side and wind farm side when using the control mode one of this invention;

[0086] Figure 19 The simulated AC voltage waveform on the wind farm side when using the control mode one of this invention;

[0087] Figure 20 The simulation waveforms of the positive and negative DC voltages when using the control mode one of this invention are shown.

[0088] Figure 21 The simulation waveforms of the reference and actual voltage values ​​at the grid connection point when using the control mode one of this invention are shown.

[0089] Figure 22 The simulation waveforms of the reference and actual values ​​of the energy storage power when using the control mode one of this invention are shown.

[0090] Figure 23 The simulated waveform of the wind turbine output power when using the control mode 1 of this invention;

[0091] Figure 24 The simulated DC voltage waveform inside the fan is shown when the control mode of this invention is adopted.

[0092] Figure 25 The simulation waveform of the internal energy storage charge state of the wind turbine when using the control mode of the present invention;

[0093] Figure 26 The simulation waveform of the energy storage state of charge variance when using the control mode one of the present invention;

[0094] Figure 27 The simulated waveform of the grid-side AC voltage when using control mode two of this invention;

[0095] Figure 28 The simulation waveforms of power on the grid side and the wind farm side when using the second control mode of this invention are shown.

[0096] Figure 29 The simulated waveform of AC voltage on the wind farm side when using control mode two of this invention;

[0097] Figure 30 The simulated DC voltage waveforms of the positive and negative terminals are shown when the control mode 2 of this invention is adopted.

[0098] Figure 31 The simulation waveforms of the reference and actual voltage values ​​at the grid connection point are shown when the second control mode of this invention is adopted.

[0099] Figure 32The simulation waveforms of the wind turbine power reference value and actual value are shown when the control mode 2 of this invention is adopted.

[0100] Figure 33 The simulated DC voltage waveform inside the fan is shown when the control mode 2 of this invention is adopted.

[0101] Figure 34 The simulated waveform of the energy storage absorption power is shown when the second control mode of this invention is adopted.

[0102] Figure 35 The simulation waveform diagram of the internal energy storage charge state of the wind turbine when using the second control mode of the present invention;

[0103] Figure 36 The waveform diagram of the energy storage state of charge variance simulation when using the second control mode of the present invention. Detailed Implementation

[0104] This invention provides an AC fault ride-through control device for a wind power flexible DC grid-connected system, comprising an MMC-HVDC system, a wind farm, an energy storage system, and a coordination controller. The MMC-HVDC system includes a WFMMC connected to the wind farm and a GSMMC connected to the receiving-end grid. The wind farm consists of several permanent magnet direct-drive wind turbines, each connected to a common DC bus via a wind turbine GSVSC. The energy storage system consists of several ESUs, each connected in parallel to the DC bus of its corresponding wind turbine via a bidirectional DC converter. The coordination controller is communicatively connected to the WFMMC, the wind turbine GSVSC, and the ESUs, and is configured to execute one of the following two control modes when an abnormal increase in DC voltage is detected due to a fault in the receiving-end AC grid; Mode Mode 1: Instruct the WFMMC to reduce the grid connection voltage of the wind farm based on the DC voltage deviation. When the grid connection voltage is lower than the grid connection voltage threshold, all ESUs in the energy storage system absorb power in constant power control mode based on the grid connection voltage deviation, while controlling the wind turbine GSVSC to maintain the voltage stability of the DC bus where the wind turbine GSVSC is located. Mode 2: Instruct the WFMMC to reduce the grid connection voltage of the wind farm based on the DC voltage deviation. When the grid connection voltage is lower than the grid connection voltage threshold, control all wind turbine GSVSCs to reduce the wind turbine load in constant power control mode based on the grid connection voltage deviation, while controlling the ESU to switch to constant DC voltage control mode to absorb the redundant power of the DC bus where the ESU is located.

[0105] This invention provides an AC fault ride-through control method for a wind power flexible DC grid-connected system. This method provides two core control modes, which are based on the following: when the DC voltage caused by the AC fault at the receiving end exceeds a preset threshold, the wind farm-side converter (WFMMC) of the flexible DC system first starts step-down control to actively reduce the AC voltage at the wind farm grid connection point, thereby serving as a medium for transmitting fault signals to the wind farm.

[0106] Mode 1: Energy storage with constant power absorption, and wind turbine with constant DC voltage control; such as Figure 3 The diagram shows the overall control structure for control mode one. In this mode, when the grid connection point voltage drops to the grid connection point voltage threshold due to the WFMMC step-down control, the following actions are triggered: The specific schemes for the step-down control and energy storage active power control in control mode one are as follows: Figure 4 , Figure 5 As shown; assuming a fault occurs in the AC system at time t0, the DC voltage begins to rise. When the DC voltage does not exceed the DC voltage threshold... During the t0-t1 time period, the d-axis voltage reference value of WFMMC remains at the normal operating level. =1 runs. When the DC voltage rises to the threshold at time t1. When the WFMMC starts buck control, the voltage reference value is calculated according to equation (9):

[0107] (9)

[0108] In the formula, This is the d-axis voltage reference value for WFMMC; The d-axis component of the rated voltage on the wind farm side; DC voltage threshold; This refers to the voltage reduction ratio coefficient. Wherein, the voltage reduction ratio coefficient... Calculate according to formula (10):

[0109] (10)

[0110] In the formula: This is the minimum d-axis voltage limit on the wind farm side; This is the maximum limit for DC voltage.

[0111] The voltage at the grid connection point of the wind farm at time t1 As the buck control starts to decrease, the voltage drop at the grid connection point does not fall below the threshold. During the time period t1-t2, the ESU power reference value remained constant. =0; when the grid connection point voltage is lower than the grid connection point voltage threshold at time t2. When the ESU begins to absorb power, the total power to be absorbed by the energy storage system should be calculated according to formula (1):

[0112] (1)

[0113] In the formula, This is a reference value for the total power that the energy storage system should absorb; This represents the number of ESUs in the energy storage system. The power absorption ratio of the energy storage system; The voltage threshold at the grid connection point; This represents the actual voltage at the grid connection point. The power absorption ratio coefficient of the energy storage system is also included. Calculate according to formula (2):

[0114] (2)

[0115] In the formula: Rated power for ESU; This is the minimum voltage limit at the grid connection point.

[0116] Considering the differences in State of Charge (SOC) among the ESUs within each wind turbine, simply distributing redundant power equally may cause ESUs with higher SOCs to easily reach capacity limits, while ESUs with lower SOCs may still have considerable unused storage space. Therefore, a scheme is proposed to distribute the total power absorbed by the energy storage system according to the SOC of the energy storage units. First, the initial power reference value allocated to each ESU is calculated according to equation (3):

[0117] (3)

[0118] In the formula, Let be the real-time state of charge value of the i-th ESU.

[0119] If there exists an initial power reference value for the i-th ESU Greater than the rated power of ESU In the case where the initial power reference value of the i-th ESU is the rated power of the i-th ESU, then let the initial power reference value of the i-th ESU be the rated power of the i-th ESU. The power reference values ​​of the remaining ESUs, excluding the i-th ESU, are corrected according to equation (4):

[0120] (4)

[0121] In the formula, The number of ESUs whose initial power reference value is greater than the rated power; This is the corrected reference value for energy storage power.

[0122] After each ESU absorbs power according to formula (4), the DC voltage stops rising and stabilizes within the limit. After the fault ends at time t3, the input power of the GSMMC increases. As the DC voltage decreases, the grid connection point voltage rises, and the energy storage power absorbed decreases synchronously. When the grid connection point voltage rises to the threshold, the energy storage power reference value drops to zero; when the DC voltage drops to the threshold, the step-down control exits, the grid connection point voltage returns to the rated value, the system returns to the stable state before the fault, and the AC fault ride-through is completed.

[0123] The parameter values ​​for control mode one are shown in Table 1. Among them, due to the entire control process... Set to 0, therefore it can be considered and The values ​​are the same.

[0124] Table 1. Parameter values ​​for control mode 1

[0125]

[0126] Mode 2: Wind turbine constant power load reduction, energy storage constant DC voltage control

[0127] like Figure 6 This describes the overall control structure for control mode two. In this mode, when the grid connection point voltage drops to a threshold, the following actions are triggered:

[0128] In Mode 2, the buck control of WFMMC is the same as in Mode 1. However, when the wind turbine GSVSC detects that the grid connection point voltage is lower than the threshold, it switches from constant DC voltage control to constant power control to reduce the power delivered by the wind turbine to the system. At the same time, the energy storage system switches from constant power control to constant DC voltage control to ensure the stability of the wind turbine DC bus voltage.

[0129] The specific control schemes for the step-down control and the active power control of the wind turbine GSVSC in control mode two are as follows: Figure 7 , Figure 8 As shown. Assume a fault occurs in the AC system at time t0, and the DC voltage rises to the threshold voltage at time t1. When WFMMC enables buck control, the WFMMC voltage reference value is calculated using the same formulas (9) and (10). When the grid connection point voltage... Drop to threshold When the wind turbine GSVSC switches to active power control mode to reduce load, the total active power reference value is calculated according to formula (5):

[0130] (5)

[0131] In the formula, This is a reference value for the total active power of the wind farm. This represents the output power of the fan before the failure. This represents the fan load reduction ratio coefficient. Wherein, the fan load reduction ratio coefficient... Calculate according to formula (6):

[0132] (6)

[0133] In order to fully utilize the absorption capacity of the energy storage system and reduce the difference in remaining capacity among the various ESUs, the total active power of the wind farm is also allocated according to the SOC of the energy storage units.

[0134] The initial active power reference value of the i-th wind turbine GSVSC Calculate according to equation (7):

[0135] (7)

[0136] If there exists an initial active power reference value for the i-th wind turbine GSVSC Greater than the output power before the i-th fan failure In the case of the i-th wind turbine GSVSC, the initial active power reference value is set to the output power of the i-th wind turbine before the failure. The active power reference value of the remaining wind turbines GSVSC, excluding the i-th wind turbine GSVSC, is corrected according to equation (8):

[0137] (8)

[0138] In the formula, The corrected active power reference value for the wind turbine GSVSC; This refers to the number of wind turbines whose initial active power reference value is greater than their output power before the turbine failure.

[0139] Each wind turbine GSVSC generates power according to the power reference value calculated by formula (8), and at the same time, the energy storage system control mode is switched from constant power control to constant DC voltage control.

[0140] The fault was cleared at time t3, according to... Figure 7 The voltage drop curve shown indicates that the voltage at the wind farm's grid connection point gradually increases, until the DC voltage decreases to the threshold value. At that time, the grid connection point voltage returns to its rated value. During this period, the grid connection point voltage rises to the threshold value. At this time, the wind turbine's GSVSC control mode switches to constant DC voltage control, and the energy storage system's control mode switches to constant power control. The wind farm continues to transmit power to the flexible DC system normally, completing the AC fault ride-through process.

[0141] The specific control timing and control logic of control mode one and control mode two are as follows: Figure 9 , Figure 10 As shown, the parameter values ​​for control mode two are basically the same as those for control mode one.

[0142] The following describes how to build a simulation platform on PSCAD / EMTDC. Figure 1 The wind power system shown is connected to the grid via MMC-HVDC, and the effectiveness of the proposed schemes in the above sections is verified. The system control parameters and system simulation parameters are shown in Tables 2 and 3, respectively.

[0143] Table 2 System Control Parameters

[0144]

[0145] Table 3 System Simulation Parameters

[0146]

[0147] Set the receiving-end AC power grid at t=5s. Figure 8 A three-phase short-circuit fault occurred at the location shown, and the voltage dropped to 0.2 pu. The fault lasted for 625 ms.

[0148] (I) Simulation verification of energy-consuming resistor control scheme:

[0149] Figures 11 to 16 The waveforms are from a system simulation using a power-dissipating resistor scheme. For example... Figure 11 , Figure 12 As shown, at t=5s, the AC voltage at the receiving end drops to 0.2pu, while the voltage at the PCC of the wind farm remains basically stable. At this time, as... Figure 14 The wind farm shown continues to output about 400MW of active power, while the active power input of the GSMMC drops sharply to about 110MW, causing the DC voltage of the positive and negative terminals to gradually increase.

[0150] like Figure 13 , Figure 15 As shown, when the DC voltage rises to 1.05 pu, the power-dissipating resistor trigger pulse goes high, the switching transistor turns on, and the resistor is switched on to dissipate power, causing the DC voltage to drop. When the voltage drops to 0.95 pu, the trigger pulse goes low, the switching transistor turns off, the resistor is switched off, and the DC voltage rises again. By repeatedly switching the resistor on and off, the DC voltage is limited to the range of 0.95-1.05 pu.

[0151] Throughout the entire failure period, such as Figure 16 The approximately 110 MJ of energy absorbed by the energy-consuming resistor is ultimately dissipated as heat. After the fault is cleared at t=5.625s, the GSMMC transmission capability is restored, the DC voltage stabilizes at the rated value, and the energy-consuming resistor is taken out of service.

[0152] (II) Simulation verification of control mode one:

[0153] Figures 17 to 26 The system simulation waveforms are shown below the control mode. For example... Figure 17 , Figure 18 As shown, a system fault occurs at t=5s, and the grid-side AC voltage drops to 0.2pu. Before control is started, the active power input of the GSMMC suddenly drops to 110MW, while the wind farm continues to output 400MW, causing the DC voltage to rise continuously due to redundant power. Figure 20 As shown. Figure 19 As shown, the AC voltage on the wind farm side drops to approximately 0.6 pu. Figure 21 As shown, when the DC voltage rises to 1.05pu, WFMMC reduces the grid connection point voltage according to equation (9), and the actual value can track the reference value well.

[0154] like Figure 22 As shown, when the grid connection point voltage is lower than 0.95pu, the energy storage system absorbs active power according to the reference values ​​given by equations (1)-(4). Among them, WG1 energy storage has a larger SOC and distributes about 2.7MW of active power, while WG2 energy storage has a smaller SOC and distributes about 4MW of active power.

[0155] like Figure 23 , Figure 24 As shown, after the energy storage absorbs power, the DC voltage gradually stabilizes at around 1.08 pu, and does not exceed the limit of 1.1 pu. When the DC voltage inside the wind turbine decreases due to the energy storage absorbing power, the wind turbine's GSVSC plays a constant DC voltage control role, reducing the wind turbine's output power, thereby maintaining the internal DC voltage within the range of 0.9-1.1 pu, achieving load shedding for the wind farm during fault periods. Figure 25 , Figure 26 As shown, according to the allocation schemes of equations (3) and (4), the SOC variance of energy storage in WG1 and WG2 decreases, indicating that the difference in their remaining capacity has narrowed.

[0156] After the fault is cleared at t=5.625s, if Figure 18 As shown, the GSMMC input power gradually recovers, as... Figure 20 The DC voltage shown subsequently decreases. For example... Figure 21 As shown, when the DC voltage drops to 1.05 pu, the grid connection point voltage returns to its rated value. Figure 22 During the voltage recovery period shown, when the grid connection point voltage rises to 0.95 pu, the reference power of the energy storage system returns to zero and stops absorbing power.

[0157] like Figure 24 As shown, after the energy storage stops absorbing energy, the DC voltage inside the wind turbine begins to rise, and the wind turbine gradually increases its output power. Once all parameters return to their rated values, the system resumes normal operation, and the fault ride-through process ends.

[0158] (III) Simulation verification of control mode two:

[0159] Figures 27 to 36The waveforms are from the system simulation under control mode two. For example... Figure 27 , Figure 28 As shown, at t=5s, the receiving-end AC voltage drops to 0.2pu, the GSMMC input active power decreases to 110MW, while the wind farm input remains at 400MW, causing the DC voltage to begin to rise. Figure 30 As shown. Figure 29 As shown, the AC voltage on the wind farm side drops to approximately 0.6 pu. Figure 31 As shown, when the DC voltage rises to 1.05 pu, the WFMMC starts buck control, and the grid connection point voltage drops to about 0.65 pu.

[0160] When the grid connection point voltage drops to 0.95 pu, the wind turbine GSVSC switches to constant power mode and reduces load according to the active power reference values ​​calculated by equations (5)-(8). Figure 32 As shown, WG1 has a higher internal energy storage SOC, resulting in an output active power of 0.35 pu; WG2 has a lower internal energy storage SOC, resulting in an output power of 0.25 pu. The energy storage absorbs more redundant power, and the wind turbine output active power tracks the reference value better, thus reducing the total power input to the flexible DC system. Figure 30 As shown, as redundant power decreases, the DC voltage gradually stabilizes at around 1.08 pu, which is below the limit of 1.1 pu, thus avoiding the risk of overvoltage.

[0161] Simultaneously, to maintain stable DC voltage within the wind turbine, energy storage switches from constant power mode to constant DC voltage mode, absorbing redundant power between the grid and the WFMMC. For example... Figure 33 , Figure 34 As shown, during the initial switching phase, the DC voltage inside the wind turbine rises due to redundant power. The energy storage then plays a stabilizing role, absorbing approximately 3MW and 3.5MW of power respectively during the fault period, ultimately stabilizing the voltage within the allowable range of 0.9-1.1 pu, ensuring the safe operation of the wind turbine. Figure 35 , Figure 36 As shown, according to the proposed power allocation scheme, the variance of the energy storage SOC between WG1 and WG2 gradually decreases during the fault period, indicating that the mode can effectively balance the remaining capacity of each ESU.

[0162] After the fault is cleared at t=5.625s, if Figure 28 , Figure 30 , Figure 31 As shown, the input power of the GSMMC gradually recovers, the system DC voltage decreases accordingly, and the voltage at the wind farm grid connection point rises according to equation (9), returning to its rated value when the DC voltage drops to 1.05 pu. During this period, the active power output of the wind turbine GSVSC increases according to equation (8) as follows. Figure 32As shown. When the grid connection point voltage rises to 0.95 pu, the control modes of the wind turbine GSVSC and the energy storage system switch to constant DC voltage control and constant power control, respectively. For example... Figure 33 , Figure 34 As shown, after switching control modes, the DC voltage inside the fan fluctuates briefly before stabilizing at the rated value. The energy storage stops absorbing power, the system resumes normal operation, and the fault ride-through process ends.

[0163] Simulation results show that both control modes proposed in this invention can effectively achieve AC fault ride-through and stabilize the DC voltage within safe limits. Compared with traditional solutions, the proposed solution has significant advantages: smaller DC voltage fluctuations, smoother control process, and complete avoidance of unnecessary energy dissipation. Furthermore, it improves the overall utilization efficiency of the energy storage system through SOC optimization allocation, verifying the effectiveness and advancement of the proposed mode.

[0164] The present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the AC fault ride-through control method for a wind power flexible DC grid-connected system provided in the embodiments of the present invention.

[0165] The computer storage medium of this invention can be any combination of one or more computer-readable media. A computer-readable medium can be a computer-readable signal medium or a computer-readable storage medium. For example, a computer-readable storage medium can be, but is not limited to, an electrical, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any combination thereof. More specific examples (a non-exhaustive list) of computer-readable storage media include: an electrical connection having one or more wires, a portable computer disk, a hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fiber, portable compact disk read-only memory (CD-ROM), optical storage device, magnetic storage device, or any suitable combination thereof. In this document, a computer-readable storage medium can be any tangible medium that contains or stores a program that can be used by or in conjunction with an instruction execution system, apparatus, or device.

[0166] Computer-readable signal media may include data signals propagated in baseband or as part of a carrier wave, carrying computer-readable program code. Such propagated data signals may take various forms, including but not limited to electromagnetic signals, optical signals, or any suitable combination thereof. Computer-readable signal media may also be any computer-readable medium other than computer-readable storage media, capable of sending, propagating, or transmitting programs for use by or in connection with an instruction execution system, apparatus, or device.

[0167] The program code contained on a computer-readable medium may be transmitted using any suitable medium, including, but not limited to, wireless, wire, optical fiber, RF, etc., or any suitable combination thereof.

[0168] Computer program code for performing the operations of this invention can be written in one or more programming languages ​​or a combination thereof. Programming languages ​​include object-oriented programming languages ​​such as Java, Smalltalk, and C++, as well as conventional procedural programming languages ​​such as C or similar languages. The program code can be executed entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving remote computers, the remote computer can be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or it can be connected to an external computer (e.g., via the Internet using an Internet service provider).

[0169] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A fault ride-through control device for a wind power flexible DC grid-connected system, characterized in that, include: The MMC-HVDC system includes a WFMMC that connects to the wind farm and a GSMMC that connects to the receiving-end power grid; The wind farm consists of several permanent magnet direct-drive wind turbines, each of which is connected to a common DC bus via a wind turbine GSVSC. The energy storage system consists of several energy storage units (ESUs), each of which is connected in parallel to the DC bus of the corresponding wind turbine through a bidirectional DC-DC converter. The coordination controller, which is communicatively connected to the WFMMC, the wind turbine GSVSC, and the energy storage unit ESU, is configured to execute one of the following two control modes when an abnormal increase in DC voltage is detected due to an AC grid fault at the receiving end: Mode 1: Send a command to the WFMMC to reduce the grid connection point voltage of the wind farm according to the DC voltage deviation; when the grid connection point voltage is lower than the grid connection point voltage threshold, all energy storage units (ESU) of the energy storage system absorb power in constant power control mode according to the grid connection point voltage deviation, and at the same time control the wind turbine GSVSC to maintain the voltage stability of the DC bus where the wind turbine GSVSC is located. Mode 2: Send a command to the WFMMC to reduce the grid connection voltage of the wind farm according to the DC voltage deviation; when the grid connection voltage is lower than the grid connection voltage threshold, control all wind turbines GSVSC to reduce the wind turbine load in constant power control mode according to the grid connection voltage deviation, and at the same time control the energy storage unit ESU to switch to constant DC voltage control mode to absorb the redundant power of the DC bus where the energy storage unit ESU is located.

2. The AC fault ride-through control device for a wind power flexible DC grid-connected system according to claim 1, characterized in that, In Mode 1, the reference value of the total power required to be absorbed by the energy storage system Calculate according to the following formula: (1) In the formula, This is a reference value for the total power required to be absorbed by the energy storage system. This refers to the number of energy storage units (ESUs) in the energy storage system. The power absorption ratio of the energy storage system; The voltage threshold at the grid connection point; This is the actual value of the voltage at the grid connection point; Energy storage system power absorption ratio factor The formula for calculation is: (2) In the formula: Rated power of the energy storage unit (ESU); This is the minimum voltage limit at the grid connection point.

3. The AC fault ride-through control device for a wind power flexible DC grid-connected system according to claim 2, characterized in that, In Mode 1, the coordination controller is further configured as follows: Reference value of the total power required to be absorbed by the energy storage system Distributed to each energy storage unit (ESU); Initial power reference value allocated to the i-th energy storage unit ESU Calculate according to the following formula: (3) In the formula, This represents the real-time state of charge value of the i-th energy storage unit (ESU). If there exists an initial power reference value for the i-th energy storage unit ESU Greater than the rated power of the energy storage unit ESU In this case, let the initial power reference value of the i-th energy storage unit ESU be the rated power of the energy storage unit ESU. The power reference values ​​of the remaining energy storage units ESU, excluding the i-th energy storage unit ESU, are corrected according to the following formula: (4) In the formula, The number of energy storage units (ESUs) whose initial power reference value is greater than the rated power; This is the corrected reference value for energy storage power.

4. The AC fault ride-through control device for a wind power flexible DC grid-connected system according to claim 1, characterized in that, In Mode 2, the reference value for the total load shedding power of the wind farm Calculate according to the following formula: (5) In the formula, This represents the output power of the fan before the failure. This refers to the fan load reduction ratio coefficient. Fan load reduction ratio coefficient The calculation formula is: (6)。 5. The AC fault ride-through control device for a wind power flexible DC grid-connected system according to claim 4, characterized in that, In mode two, the coordination controller is further configured as follows: Reference value of total load reduction of wind farm Distributed to each wind turbine GSVSC; The initial active power reference value of the i-th wind turbine GSVSC Calculate according to the following formula: (7) If there exists an initial active power reference value for the i-th wind turbine GSVSC Greater than the output power before the i-th fan failure In the case where the power reference value of the i-th wind turbine GSVSC is the output power before the fault, then let the power reference value be the output power before the fault. The active power reference value of the remaining GSVSCs (excluding the i-th fan GSVSC) is corrected according to the following formula: (8) In the formula, The corrected active power reference value for the wind turbine GSVSC; This refers to the number of wind turbines whose initial power reference value is greater than their rated power.

6. The AC fault ride-through control device for a wind power flexible DC grid-connected system according to claim 1, characterized in that, The control method for reducing the grid connection point voltage using WFMMC is as follows: when the DC voltage... Exceeding the DC voltage threshold At that time, the d-axis voltage reference value of WFMMC Calculate according to the following formula: (9) In the formula, The d-axis component of the rated voltage on the wind farm side; This is the pressure reduction ratio coefficient; Voltage reduction ratio factor The calculation formula is: (10) In the formula, This is the minimum d-axis voltage limit on the wind farm side; This is the maximum limit for DC voltage.

7. The control method of the AC fault ride-through control device for a wind power flexible DC grid-connected system as described in any one of claims 1 to 6, characterized in that, Includes the following steps: Fault detection steps: Monitor the system DC voltage in real time; Step-down startup procedure: When the DC voltage exceeds the preset DC voltage threshold, a fault in the receiving-end AC grid is determined, and the WFMMC is controlled to start the step-down method to reduce the voltage at the wind farm's grid connection point; Collaborative control steps: Monitor the grid connection point voltage. When the grid connection point voltage drops to a preset grid connection point voltage threshold, execute one of the following two modes according to the preset control mode: Mode 1: Control all energy storage units (ESU) to absorb power in constant power control mode according to the voltage deviation at the grid connection point, while controlling the wind turbine (GSVSC) to maintain constant DC voltage control. Mode 2: Control all wind turbines GSVSC to reduce power output in constant power control mode according to the voltage deviation at the grid connection point, and at the same time control the energy storage unit ESU to switch to constant DC voltage control.

8. The control method according to claim 7, characterized in that, When executing Mode 1, a power optimization allocation sub-step is also included: the total power reference value absorbed by the energy storage system is calculated based on the grid connection point voltage, and then allocated to each unit according to the SOC of each energy storage unit ESU, giving priority to units with lower SOC to absorb more power.

9. The control method according to claim 7, characterized in that, When executing Mode 2, the method also includes a load reduction optimization allocation sub-step: the total load reduction power reference value of the wind turbine is calculated based on the grid connection point voltage, and then allocated to each wind turbine according to the SOC of the energy storage unit ESU connected to each wind turbine, giving priority to wind turbines with higher SOC to perform more load reduction.

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