Voltage support control method for permanent magnet direct drive wind turbine based on voltage drop depth grading

Abstract

The application provides a voltage drop depth grading-based permanent magnet direct drive wind turbine voltage support control method and belongs to the technical field of voltage safety and stability. pcc The technical scheme is as follows: S1, establishing a voltage drop depth grading system; S2, detecting the voltage amplitude U of the grid connection point; S3, calculating the grid-side dq-axis current reference instruction, and turning to step S6; S4, when the moderate drop voltage support mode is selected; S5, when the deep drop voltage support mode is selected; S6, performing current closed-loop control on the grid-side converter according to the dq-axis current reference instructions calculated in steps S3, S4 and S5. The application has the beneficial effect that the comprehensive optimization of voltage support strength, converter capacity utilization rate and DC bus voltage stability is achieved, and the low voltage ride-through capability of the system is significantly improved.

Description

Technical Field

[0001] This invention relates to the field of voltage safety and stability technology, and in particular to a voltage support control method for permanent magnet direct-drive wind turbines based on voltage drop depth classification. Background Technology

[0002] Permanent magnet direct-drive wind power systems have attracted widespread attention in the wind power field due to their excellent reliability and low maintenance costs. With the gradual increase in wind power penetration and the continuous expansion of wind power system transmission capacity, when grid faults cause voltage drops at the grid connection point, wind power systems often face problems such as grid-side overcurrent and DC bus voltage exceeding limits. In severe cases, this can even lead to wind turbine disconnection from the grid, posing a serious challenge to the safe and stable operation of the power grid. Experts and scholars have conducted extensive research on voltage support control methods.

[0003] Chinese Patent Publication No. CN116014800A, published on April 25, 2023, discloses a low-voltage ride-through method, circuit, and system for a permanent magnet direct-drive wind power system based on coordinated control. This method achieves low-voltage ride-through by coordinating the pitch angle control on the turbine side, the DC-side Crowbar circuit, and the grid-side converter. However, this method increases the complexity of system control, and the grid-side converter adjusts the grid-side reactive current according to the minimum reactive power compensation criterion, failing to fully utilize the system's ability to support the grid connection voltage.

[0004] Chinese Patent Publication No. CN118100209A, published on May 28, 2024, discloses an active voltage support method for a photovoltaic power station grid-connected system with dual-mode coordinated control. This method includes: dynamically increasing the system's reactive current when a shallow voltage drop occurs until the grid-connected point voltage amplitude reaches the minimum requirement; searching for the reactive current command value required for the system to reach maximum voltage support when a deep voltage drop occurs; and converting any remaining capacity into an active current command value with the aim of maximizing active power output if there is any remaining capacity. This method does not consider the possibility of remaining capacity after the system achieves voltage support and DC bus voltage stabilization during minor faults; it only raises the grid-connected point voltage to the minimum requirement, failing to further utilize the converter capacity to maximize the system's voltage support capability.

[0005] Chinese Patent Publication No. CN113783214A, published on December 10, 2021, discloses a low-voltage ride-through control method for a doubly-fed pumped storage system. This method selects the appropriate operating mode based on voltage dip conditions, enabling the doubly-fed pumped storage system to have good low-voltage ride-through capability under different voltage dip conditions. However, it does not consider the limiting effect of the DC bus outer loop output on active current, which may affect the grid connection voltage support effect in certain scenarios.

[0006] The research by the aforementioned experts and scholars has provided a theoretical basis and technical support for voltage support control methods under grid voltage fault conditions. However, the voltage support control strategies for permanent magnet direct-drive wind power systems still have the following limitations: On the one hand, traditional control strategies support the grid connection point voltage according to the minimum reactive power output standard in the low voltage ride-through specification, failing to fully utilize the voltage regulation potential of the grid-side converter; on the other hand, some improved strategies ignore the limiting effect of the DC bus voltage outer loop on active current, weakening the voltage support effect and low voltage ride-through capability of the system under complex voltage drop scenarios.

[0007] How to solve the above-mentioned technical problems is the challenge facing this invention. Summary of the Invention

[0008] The purpose of this invention is to provide a voltage support control method for permanent magnet direct-drive wind turbines based on voltage dip depth classification. Addressing the shortcomings and improvement needs of existing low-voltage ride-through strategies, this invention proposes a voltage support control method for permanent magnet direct-drive wind turbines based on voltage dip depth classification. Its aim is to enhance the flexible voltage support capability of wind power systems under complex voltage dip scenarios, while maximizing grid-side active power output, thereby improving energy utilization efficiency and grid-connection economy during the wind power system's grid connection process.

[0009] The inventive concept of this invention is as follows: This invention provides a voltage support method for permanent magnet direct-drive wind turbines based on voltage sag depth classification, comprising: establishing a voltage sag depth classification system, using the maximum voltage support capacity of the wind turbine to raise the grid connection point voltage to the normal voltage of 0.9pu as the dividing line, and setting the critical voltage between deep sag and mild to moderate sag as U. L Using the constraint effect of the outer loop of the DC bus voltage on the d-axis active current of the grid side as the dividing line, a critical voltage U is set for mild voltage dips and moderate voltage dips. H The drop depth at the grid connection point is classified as mild [0.9pu, U]. H ], moderate [U H U L ] and depth [U L Three levels: [0.2pu]; based on the voltage drop amplitude U at the grid connection point. pcc Determine the drop level and switch to the corresponding control mode; if U pcc >0.9 PU, the fan remains in normal operating mode; U pcc <0.2 PU, the fan disconnects from the grid; if 0.9 PU pcc H Select the mild voltage drop support mode; U H pcc L Select the moderate voltage drop support mode; if U L pcc ​​​​​For voltage dips <0.2 pu, a deep voltage dip support mode is selected; the dq-axis current reference command is solved according to the constraints and control objectives to control the grid-side converter current. The voltage support control method based on voltage dip depth classification proposed in this invention fully considers the diversity of complex voltage dip scenarios, achieving comprehensive optimization of voltage support strength, converter capacity utilization, and DC bus voltage stability, significantly improving the system's low-voltage ride-through capability.

[0010] To achieve the aforementioned objectives, the present invention employs the following technical solution: a voltage support control method for a permanent magnet direct-drive wind turbine based on voltage drop depth grading, comprising the following steps:

[0011] S1: Establish a voltage sag depth classification system, using the maximum voltage support capacity of the wind turbine to raise the grid connection voltage to 0.9 pu of the normal voltage as the dividing line, and set the critical voltage between deep sag and mild to moderate sag as U. L Using the constraint effect of the outer loop of the DC bus voltage on the d-axis active current of the grid side as the dividing line, a critical voltage U is set for mild voltage dips and moderate voltage dips. H Therefore, the voltage drop depth at the grid connection point is divided into 1) slight drop [U H ,0.9pu]、2) Moderate drop [U L U H ]、3) Deep drop [0.2pu,U L Three levels;

[0012] S2: Detect the voltage amplitude U at the grid connection point pcc , when U pcc When U > 0.9 pu, the wind turbine maintains normal operating mode, and the grid-side converter operates at unity power factor; when U pcc When 0.2pu < 0.2pu, the wind turbine system disconnects the grid-side converter from the grid connection point, and the wind turbine operates offline. When 0.2pu ≤ U pcc When the voltage drop is ≤0.9pu, the voltage sag depth is determined according to step S1, and the grid-side converter switches to the corresponding voltage support mode: if U H ≤U pcc ≤0.9pu, select the mild drop voltage support mode and proceed to step S3; if U L ≤U pcc ≤U H Select the moderate voltage drop support mode and proceed to step S4; if 0.2pu≤U pcc ≤U L Select the deep drop voltage support mode and proceed to step S5;

[0013] S3: When the mild voltage drop support mode is selected, the grid-side converter aims to achieve maximum active power output on the basis of raising the grid connection point voltage. Taking into account the DC bus voltage outer loop constraint, grid-side converter capacity limit and low voltage ride-through reactive power support constraint, the grid-side dq axis current reference command is calculated accordingly, and then proceed to step S6.

[0014] S4: When the moderate voltage drop support mode is selected, the grid-side converter still aims to raise the grid connection point voltage while achieving maximum active power output. However, the DC bus voltage outer loop constraint is not considered. The grid-side dq axis current reference command is calculated only based on the grid-side converter capacity limit and the low voltage ride-through reactive power support criterion constraint. Proceed to step S6.

[0015] S5: When the deep voltage drop support mode is selected, the system prioritizes the grid connection point voltage support effect. The grid-side converter only takes the maximum support of the grid connection point voltage as the control target. The grid-side dq axis current reference command is calculated according to the grid-side converter capacity limit and the low voltage ride reactive power support criterion constraint. Proceed to step S6.

[0016] S6: Perform current closed-loop control on the grid-side converter based on the dq-axis current reference command calculated in steps S3 / S4 / S5 to obtain the drive signals for each power device of the grid-side converter.

[0017] In step S1, a voltage drop depth classification system is established, as follows:

[0018] S1.1: The voltage drop at the grid connection point is set based on the maximum voltage support capacity of the wind turbine, which raises the grid connection point voltage to 0.9pu of normal voltage. L This serves as the critical voltage between deep voltage drop and mild to moderate voltage drop. If the system voltage support capacity reaches its upper limit but still cannot raise the grid connection point voltage to 0.9 pu, it is defined as a deep voltage drop [0.2 pu, U]. L If the system can raise the grid connection point voltage to above 0.9 pu, it is defined as a mild to moderate voltage drop [U]. L ,0.9pu].

[0019] Under the constraints of converter capacity limitations and low-voltage ride-through reactive power support criteria, the system's maximum voltage support capability can only raise the grid connection point voltage to 0.9 pu (0.9 pu), i.e., maxU pcc (I d ,I q Given 0.9pu, solve for the critical voltage U. L The details are as follows:

[0020]

[0021] In the formula, Id I q These represent the active current along the d-axis and the reactive current along the q-axis on the grid side, respectively; I max The maximum safe current limit for the grid-side converter is set to 1.2 pu; I N This is the rated current of the grid-side converter.

[0022] S1.2: The constraint effect of the outer loop of the DC bus voltage on the active current of the grid-side d-axis is used as the boundary, based on the fault drop voltage U at the grid connection point at this time. H Mild to moderate voltage sags are further subdivided into mild voltage sags and moderate voltage sags. A moderate voltage sag is defined as the maximum voltage that the system can provide to raise the grid connection point voltage to above 0.9 pu, without considering the outer loop constraint of the DC bus voltage. L U H The system can provide a maximum voltage support to raise the grid connection point voltage to above 0.9 pu, while considering the outer loop constraint of the DC bus voltage, which is defined as a slight voltage drop [U]. H ,0.9pu].

[0023] The maximum active current remaining after the grid-side converter capacity meets the low-voltage ride-through reactive power support criterion. Reference to the outer loop output active current of the DC bus voltage ( P g The active power output of the grid-side converter before the fault is equal to that before the fault, so the critical voltage U can be calculated. H ;

[0024] The method for calculating the voltage drop amplitude at the grid connection point in step S2 is as follows:

[0025]

[0026] In the formula, U pccf U represents the voltage drop amplitude at the grid connection point. pccN U is the rated voltage at the grid connection point. pccα U pccβ These are the α and β axis components of the grid connection point voltage in a two-phase stationary coordinate system.

[0027] The specific constraints and objective function for the mild voltage drop support mode in step S3 are as follows:

[0028] S3.1: Taking into account the constraints of the DC bus voltage outer loop, the capacity limit of the grid-side converter, and the low voltage ride-through reactive power support criterion, the grid-side converter fully utilizes its active power output potential while increasing the grid connection point voltage to achieve maximum active power output.

[0029] To avoid excessively raising the grid connection voltage, the voltage increase should be limited to the normal operating voltage range, i.e., between 0.9 pu and 1.1 pu; simultaneously, given the decoupling of active and reactive power on the grid side, the active current I should be controlled. d This transforms the problem of adjusting and maximizing active power output into solving for the maximum value of the active current along the d-axis. Specifically:

[0030]

[0031] Based on the above constraints and control objectives, solve for the reference command for the dq-axis current on the network side;

[0032] S3.2: Based on the d-axis active current reference value obtained in step S3.1, determine whether it is consistent with the outer loop output of the DC bus voltage. If equal, proceed to step S6; if equal, proceed to the secondary control objective: maintain DC bus voltage stability while meeting the conditions of converter capacity limits, low-voltage ride-through reactive power support criteria, and grid connection point voltage amplitude limits. Simultaneously, it achieves maximum voltage support at the grid connection point. Specifically:

[0033]

[0034] Based on the above constraints and control objectives, re-solve the reference command for the dq-axis current on the network side, and proceed to step S6;

[0035] The constraints and objective function for the moderate voltage drop support mode in step S4 are as follows:

[0036] Under the premise of meeting the capacity limitations of the grid-side converter and the reactive power support criteria for low voltage ride-through, the grid-side converter fully utilizes its active power output potential while increasing the grid connection point voltage to achieve maximum active power output:

[0037]

[0038] Based on the above constraints and control objectives, solve for the reference command for the dq-axis current on the network side, and proceed to step S6;

[0039] The specific constraints and objective function for the deep drop voltage support mode in step S5 are as follows:

[0040] At this point, the voltage drop at the grid connection point is relatively deep. The system prioritizes ensuring the grid connection point voltage support effect. Under the premise of meeting the converter capacity limit and the low voltage ride-through reactive power support criterion, the grid-side converter only aims to achieve the maximum support of the grid connection point voltage, maximizing the rise of the grid connection point voltage:

[0041]

[0042] Based on the above constraints and control objectives, solve for the reference command for the dq-axis current on the network side, and proceed to step S6;

[0043] In step S6, current closed-loop control of the grid-side converter is performed according to the dq-axis current reference command to obtain the drive signals for each power device of the grid-side converter. Specifically:

[0044] Closed-loop control is performed on the dq-axis current of the grid-side converter. The difference between the dq-axis current reference command and the actual dq-axis current is input into the proportional-integral regulator for decoupling control. The resulting modulation command voltage is used to generate the switching signal of the power device of the grid-side converter through space vector pulse width modulation (SVPWM).

[0045] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0046] 1. This invention proposes a voltage support control method for permanent magnet direct-drive wind turbines based on voltage sag depth classification. It classifies voltage sag levels by combining the maximum voltage support capacity of the wind turbine system with the limiting effect of the DC bus voltage outer loop on active current. It proposes corresponding voltage support control strategies for different voltage sag scenarios, improves the flexible voltage support capability of the wind power system under complex voltage sag scenarios, maximizes the active power output on the grid side, and improves the energy utilization efficiency and grid connection economy of the wind power system during grid connection.

[0047] 2. This invention proposes a voltage support control method for permanent magnet direct-drive wind turbines based on voltage sag depth classification. It uses the maximum voltage support capacity of the wind power system and the DC bus voltage outer loop constraint as the dividing criteria to classify voltage sag levels. Corresponding voltage support control strategies are proposed according to different voltage sag levels, ensuring that the wind power system has flexible voltage support capabilities to cope with complex voltage sag scenarios. At the same time, it fully taps the system's active power output potential, providing a certain reference value for improving the grid connection stability and low-voltage ride-through performance of permanent magnet direct-drive wind power systems. Attached Figure Description

[0048] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0049] Figure 1 This is a structural diagram of the permanent magnet direct drive wind turbine system of the present invention.

[0050] Figure 2 This is a schematic diagram illustrating the voltage drop scenario division principle of the present invention.

[0051] Figure 3 This is a flowchart illustrating the calculation of current reference values ​​under different voltage drop scenarios according to the present invention.

[0052] Figure 4 This is a block diagram of voltage support control for the permanent magnet direct-drive wind turbine system under grid fault conditions in this invention.

[0053] Figure 5 This is a waveform diagram of the dq-axis current of the grid-side converter under three control strategies in the case of a slight voltage drop, as described in this invention.

[0054] Figure 6 The figures show the simulation results of the grid connection point voltage, DC bus voltage, and grid-side active power output under three control strategies in the case of a slight voltage drop in this invention.

[0055] Figure 7 This is a bar chart comparing the performance of three control strategies in terms of voltage support effect, grid-side active power output, and grid-side converter capacity utilization under mild voltage drop conditions in this invention.

[0056] Figure 8 This is a waveform diagram of the dq-axis current of the grid-side converter under three control strategies under the condition of moderate voltage drop in this invention.

[0057] Figure 9 The figures show the simulation results of grid connection point voltage, DC bus voltage and grid-side active power output under three control strategies in the case of moderate voltage drop according to the present invention.

[0058] Figure 10 This is a bar chart comparing the results of three control strategies under moderate voltage drop conditions according to the present invention.

[0059] Figure 11 This is a waveform diagram of the dq-axis current of the grid-side converter under two control strategies in the case of deep voltage drop in this invention.

[0060] Figure 12 The figures show the simulation results of grid connection point voltage, DC bus voltage and grid-side active power output under two control strategies in the case of deep voltage drop at the grid connection point in this invention.

[0061] Figure 13 This is a bar chart comparing strategy 1 and strategy 3 under deep voltage drop conditions in this invention. Detailed Implementation

[0062] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. Of course, the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0063] Example 1

[0064] See Figure 1 The technical solution provided in this embodiment is as follows: a structural block diagram of a permanent magnet direct-drive fan system is shown in the figure. Figure 1 As shown, the system includes: a wind turbine, a permanent magnet synchronous generator (PMSG), a turbine-side converter, a DC bus capacitor, and a back-to-back converter (comprising a grid-side converter), filters, a transformer, and grid connection lines. The wind turbine is directly connected to the rotor of the PMSG, and the stator of the PMSG is connected to the grid via the back-to-back converter and a transformer. The voltage sag classification principle diagram is shown below. Figure 2 As shown. The flowchart for calculating the dq-axis current reference command of the grid-side converter is as follows. Figure 3 As shown in the figure. The voltage support control block diagram of the permanent magnet direct-drive wind turbine based on voltage sag depth classification is as follows. Figure 4 As shown. The method of the present invention specifically includes the following steps:

[0065] S1: Establish a voltage drop depth classification system, such as Figure 2 As shown, the threshold voltage for deep voltage drop and mild to moderate voltage drop is set as U, based on the wind turbine's maximum voltage support capacity raising the grid connection point voltage to 0.9 pu of normal voltage. L Using the constraint effect of the outer loop of the DC bus voltage on the d-axis active current of the grid side as the dividing line, a critical voltage U is set for mild voltage dips and moderate voltage dips. H Therefore, the voltage drop depth at the grid connection point is divided into 1) slight drop [U H ,0.9pu]、2) Moderate drop [U L U H ]、3) Deep drop [0.2pu,U L Three levels. Details are as follows:

[0066] S1.1: The voltage drop at the grid connection point is set based on the maximum voltage support capacity of the wind turbine, which raises the grid connection point voltage to 0.9pu of the normal voltage. L This serves as the critical voltage between deep voltage drop and mild to moderate voltage drop. If the system voltage support capacity reaches its upper limit but still cannot raise the grid connection point voltage to 0.9 pu, it is defined as a deep voltage drop [0.2 pu, U]. L If the system can raise the grid connection point voltage to above 0.9 pu, it is defined as a mild to moderate voltage drop [U]. L ,0.9pu].

[0067] Under the constraints of converter capacity limitations and low-voltage ride-through reactive power support criteria, the system's maximum voltage support capability can only raise the grid connection point voltage to 0.9 pu (0.9 pu), i.e., maxU pcc (I d ,I q Given 0.9pu, solve for the critical voltage U. L ;

[0068]

[0069] In the formula, I d I q These represent the active current along the d-axis and the reactive current along the q-axis on the grid side, respectively; I max The maximum safe current limit for the grid-side converter is set to 1.2 pu; I N This is the rated current of the grid-side converter.

[0070] S1.2: The constraint effect of the outer loop of the DC bus voltage on the active current of the grid-side d-axis is used as the boundary, based on the fault drop voltage U at the grid connection point at this time. H Mild to moderate voltage sags are further subdivided into mild voltage sags and moderate voltage sags. A moderate voltage sag is defined as the maximum voltage that the system can provide to raise the grid connection point voltage to above 0.9 pu, without considering the outer loop constraint of the DC bus voltage. L U H The system can provide a maximum voltage support to raise the grid connection point voltage to above 0.9 pu, while considering the outer loop constraint of the DC bus voltage, which is defined as a slight voltage drop [U]. H ,0.9pu].

[0071] The maximum active current remaining after the grid-side converter capacity meets the low-voltage ride-through reactive power support criterion. Reference to the outer loop output active current of the DC bus voltage ( P g The active power output of the grid-side converter before the fault is equal to that before the fault, so the critical voltage U can be calculated. H ;

[0072] S2: Detect the voltage amplitude U at the grid connection point pcc , when U pcc When U > 0.9 pu, the wind turbine maintains normal operating mode, and the grid-side converter operates at unity power factor; when U pcc When 0.2pu < 0.2pu, the wind turbine system disconnects the grid-side converter from the grid connection point, and the wind turbine operates offline. When 0.2pu ≤ U pcc When the voltage drop is ≤0.9pu, the voltage sag depth is determined according to step S1, and the grid-side converter switches to the corresponding voltage support mode: if U H ≤U pcc ≤0.9pu, select the mild drop voltage support mode and proceed to step S3; if U L ≤U pcc ≤U H Select the moderate voltage drop support mode and proceed to step S4; if 0.2pu≤U pcc ≤U LSelect the deep voltage drop support mode and proceed to step S5. The calculation method for the voltage drop amplitude at the grid connection point is as follows:

[0073]

[0074] In the formula, U pccf U represents the voltage drop amplitude at the grid connection point. pccN U is the rated voltage at the grid connection point. pccα U pccβ These are the α and β axis components of the grid connection point voltage in a two-phase stationary coordinate system.

[0075] S3: When the mild voltage drop support mode is selected, the grid-side converter aims to achieve maximum active power output while raising the grid connection point voltage. Taking into account the DC bus voltage outer loop constraint, grid-side converter capacity limitation, and low voltage ride-through reactive power support constraint, the grid-side dq axis current reference command is calculated accordingly. Proceed to step S6, as follows:

[0076] S3.1: Taking into account the constraints of the DC bus voltage outer loop, the capacity limit of the grid-side converter, and the low voltage ride-through reactive power support criterion, the grid-side converter fully utilizes its active power output potential while increasing the grid connection point voltage to achieve maximum active power output.

[0077] To avoid excessively raising the grid connection voltage, the voltage increase should be limited to the normal operating voltage range, i.e., between 0.9 pu and 1.1 pu; simultaneously, given the decoupling of active and reactive power on the grid side, the active current I should be controlled. d The problem of adjusting and maximizing active power output is thus transformed into solving for the maximum value of the active current along the d-axis. The specific control objectives and constraints are as follows:

[0078]

[0079] Based on the above constraints and control objectives, solve for the reference command for the dq-axis current on the network side;

[0080] S3.2: Based on the d-axis active current reference value obtained in step S3.1, determine whether it is consistent with the outer loop output of the DC bus voltage. If equal, proceed to step S6; if equal, proceed to the secondary control objective: maintain DC bus voltage stability while meeting the conditions of converter capacity limits, low-voltage ride-through reactive power support criteria, and grid connection point voltage amplitude limits. Simultaneously, it achieves the maximum support for the grid connection point voltage. The corresponding control objectives and constraints are as follows:

[0081]

[0082] Resolve the reference command for the dq-axis current on the grid side, and proceed to step S6;

[0083] S4: When the moderate voltage drop support mode is selected, the grid-side converter still aims to raise the grid connection point voltage while achieving maximum active power output. However, the DC bus voltage outer loop constraint does not need to be considered. The grid-side dq axis current reference command is calculated only based on the grid-side converter capacity limit and the low voltage ride-through reactive power support criterion constraint. Proceed to step S6. The specific control objectives and constraints are as follows:

[0084]

[0085] S5: When the deep voltage drop support mode is selected, the system prioritizes ensuring the grid connection point voltage support effect. The grid-side converter only aims to achieve the maximum support of the grid connection point voltage. Based on the grid-side converter capacity limit and the low voltage ride-through reactive power support criterion constraints, the grid-side dq axis current reference command is calculated, and the process proceeds to step S6. The specific control objectives and constraints are as follows:

[0086]

[0087] S6: Based on the dq-axis current reference command calculated in steps S3 / S4 / S5, perform current closed-loop control on the grid-side converter to obtain the drive signals for each power device of the grid-side converter, such as... Figure 4 As shown. Specifically:

[0088] Closed-loop control is performed on the dq-axis current of the grid-side converter. The difference between the dq-axis current reference command and the actual dq-axis current is input into the proportional-integral regulator for decoupling control. The resulting modulation command voltage is used to generate the switching signal of the power device of the grid-side converter through space vector pulse width modulation (SVPWM).

[0089] Through the above steps S1-S6, a voltage support control method for permanent magnet direct-drive wind turbines based on voltage sag depth classification can be realized. The voltage sag levels are classified according to the maximum voltage support capacity of the wind turbine system and the limiting effect of the DC bus voltage outer loop on the active current. Corresponding voltage support control strategies are proposed for different voltage sag scenarios, which improves the flexible voltage support capability of the wind power system in complex voltage sag scenarios, maximizes the active power output on the grid side, and improves the energy utilization efficiency and grid connection economy of the wind power system during grid connection.

[0090] Example 2

[0091] Calculations show that the presence or absence of a DC bus voltage outer loop constraint is used as the criterion for distinguishing between mild and moderate voltage sags, with the dividing voltage value being U. sThe maximum voltage that the generating unit can provide just meets the lower limit of the grid connection point voltage support is used as the dividing line between deep voltage dips and mild / moderate voltage dips. This dividing voltage value is set as U. m Set the boundary voltage U s =0.836 pu, U m =0.66pu. Three fault conditions—mild voltage dip, moderate voltage dip, and deep voltage dip—were set to verify the grid connection point voltage support effect of the following three control strategies: Control Strategy 1: Reactive power compensation strategy based on LVRT criterion; Control Strategy 2: Maximum voltage support strategy based on line impedance ratio distribution of active and reactive currents; Control Strategy 3: Voltage support based on voltage dip depth classification. To verify the effectiveness of the fuzzy adaptive PI control strategy, after the fault was cleared, the unit switched from low voltage ride-through mode to normal operation mode. The DC bus voltage outer loop of Strategy 1 and Strategy 2 still used traditional PI control; the DC bus voltage outer loop of Strategy 3 adopted a fuzzy adaptive PI control strategy.

[0092] The simulation duration is 1 second. From 0s to 0.4s, the unit is in normal operation; from 0.4s to 0.8s, a three-phase symmetrical voltage drop occurs in the power grid, and the unit will switch from normal operation mode to low voltage ride-through mode to provide voltage support at the grid connection point; from 0.8s to 1s, the grid voltage returns to normal.

[0093] a) The voltage at the grid connection point drops to 0.85 pu, which is considered a slight voltage drop. Figure 5 The grid-side converter d-axis and q-axis currents under three control strategies are calculated as follows: d-axis currents under control strategy 1 are 2436A and -162A, respectively; d-axis currents under control strategy 2 are 1308A and -2245A, respectively; and grid-side d-axis and q-axis currents under control strategy 3 are 2102A and -1527A, respectively. The three-phase current amplitudes under strategies 2 and 3 both reach 1.2 pu, while the three-phase current amplitude under strategy 1 is 1.1 pu, indicating that the converter has remaining capacity.

[0094] Figure 6 Simulation results for grid connection point voltage, DC bus voltage, and grid-side active power output under three control strategies are further presented. During a fault, strategy 1 raises the grid connection point voltage from 0.85 pu to 0.88 pu, maintains the rated active power output on the grid side, and keeps the DC bus voltage at 1 p.u., achieving power balance between the generator side and the grid side, but its voltage support capability is limited.

[0095] Strategy 2 achieved maximum voltage support at the grid connection point, raising the grid connection point voltage from 0.85 pu to 1.07 pu. However, the DC-side unloading resistor bore 0.35 pu of unbalanced power, resulting in significant energy loss. After the fault was cleared, the DC bus voltage unloading circuit control switched to DC voltage outer loop control. However, the traditional PI control of the DC voltage outer loop failed to adapt to the change in operating conditions, causing the DC bus voltage to oscillate around 0.88 pu and fail to return to a steady state, resulting in fluctuations in the dq axis current and active power output of the grid-side converter.

[0096] Strategy 3 raises the grid connection point voltage from 0.85 pu to 1.03 pu while maintaining the rated active power output on the grid side, thereby keeping the DC bus voltage at 1 p.u. This strategy effectively raises the grid connection point voltage while achieving power balance between the converters on both sides of the DC bus. After fault clearance in 0.8 seconds, the grid-side fuzzy PI control dynamically adjusts the PI parameters to stabilize the DC bus voltage at 1 p.u., significantly improving the dynamic response characteristics and stability of the DC bus voltage.

[0097] Figure 7 The performance of three control strategies was compared in terms of voltage support effect, grid-side active power output, and grid-side converter capacity utilization. Strategy 1 has a limited voltage support effect, increasing it by only 3.53%. Although it can achieve power balance between the generator side and the grid side, the grid-side converter has residual capacity, failing to fully utilize the voltage support potential of the unit. Strategy 2 has the most significant voltage boost effect, but its active power output is the lowest, with approximately 35% of the unbalanced power being consumed through the unloading resistor. In contrast, the strategy 3 proposed in this invention fully utilizes the converter capacity, effectively raising the grid connection point voltage while enhancing the active power output capability of the grid-side converter, achieving power balance between the converters on both sides of the DC bus, and improving the system energy utilization efficiency.

[0098] b) The voltage at the grid connection point drops to 0.7 pu, which is considered a moderate voltage drop. Figure 8 The grid-side converter d-axis currents under three control strategies are as follows: under control strategy 1, the d-axis currents are 2516A and -650A, respectively; under control strategy 2, the d-axis currents are 1308A and -2245A, respectively; and under control strategy 3, the grid-side d-axis currents are calculated to be 2041A and -1610A, respectively. All three control strategies achieve the maximum safe current limit.

[0099] Figure 9Simulation results for grid connection point voltage, DC bus voltage, and grid-side active power output under three control strategies are further presented. Simulation results show that during a fault, Strategy 1 raises the grid connection point voltage from 0.7 pu to 0.81 pu, with a grid-side active power output of 0.94 pu, exhibiting the weakest voltage support effect. Strategy 2 achieves maximum grid connection point voltage support, raising the grid connection point voltage from 0.7 pu to 0.93 pu, but its active power output is only 0.57 pu, indicating a significant DC-side power imbalance. Strategy 3 raises the grid connection point voltage to 0.9 pu, with a grid-side active power output between the first two strategies at 0.85 pu. While meeting the minimum grid connection point voltage support requirement, it further explores the active power output capability of the grid-side converter, effectively alleviating the power imbalance between the converters on both sides of the DC bus. After the fault is cleared, the DC bus voltage unloading circuit control switches to DC voltage outer loop control. In strategies 1 and 2, the DC voltage outer loop uses traditional PI control, which fails to adapt to changes in operating conditions, causing DC bus voltage oscillations that fail to return to steady state, resulting in fluctuations in the dq-axis current and active power output of the grid-side converter. Strategy 3 uses a fuzzy adaptive PI control strategy to adjust the PI parameters, suppressing DC bus voltage oscillations and ultimately stabilizing at 1p.u.

[0100] according to Figure 10 Analysis of the comparison results of the three control strategies shows that Strategy 1 maximizes the active power output on the grid side, but limits its voltage support effect; Strategy 2 aims for maximum voltage support and has the most significant effect on improving the grid connection point voltage, but approximately 43% of the unbalanced power is consumed through the unloading resistor. In contrast, Strategy 3 proposed in this invention raises the grid connection point voltage to the normal voltage, significantly enhancing the voltage support effect, while fully utilizing the active power output capability of the grid-side converter, effectively alleviating the power imbalance between the converters on both sides of the DC bus, and improving the system's energy utilization and economy.

[0101] c) A voltage drop to the grid connection point of 0.5 pu is considered a deep voltage drop. The active and reactive currents calculated by Strategy 3 are consistent with those of Strategy 2. Therefore, only Strategy 1 and Strategy 3 are simulated and verified here.

[0102] Figure 11 The d-axis and q-axis currents of the grid-side converter under two control strategies are as follows: under control strategy 1, the d-axis and q-axis currents are 2516A and -650A, respectively; under control strategy 3, the d-axis and q-axis currents are 1308A and -2245A, respectively. The currents of both control strategies reach the maximum safe current limit.

[0103] Figure 12Simulation results are presented for the grid connection point voltage, DC bus voltage, and grid-side active power output under two control strategies during a deep voltage dip at the grid connection point. During the fault, Strategy 1 raises the grid connection point voltage from 0.5 pu to 0.685 pu, an increase of 0.185 pu, with a grid-side active power output of 0.73 pu. Strategy 3 raises the voltage to 0.75 pu, an increase of 0.25 pu, with an active power output of 0.46 pu. Due to the significant voltage dip, both strategies utilize unloading resistors on the DC side to dissipate excess power, thereby stabilizing the DC bus voltage at 1.1 pu. Strategy 3 increases the active power loss on the unloading resistor by 0.27 pu compared to Strategy 1, but its voltage support capability is stronger, maximizing the unit's non-disconnection operation time and contributing to improved low-voltage ride-through capability of permanent magnet direct-drive wind power systems. After the fault was cleared, the DC bus voltage oscillated in Strategy 1 due to the switching of DC bus voltage control and failed to recover to a steady state. Strategy 3 adopted a fuzzy adaptive PI control strategy for the outer loop of DC voltage, which effectively suppressed the DC bus voltage oscillation and finally stabilized at 1p.u.

[0104] Figure 13 The comparison results between Strategy 1 and Strategy 3 show that, in the deep voltage drop scenario, Strategy 3 meets the engineering requirements for grid connection point voltage recovery and support, maximizes the unit's non-disconnection operation time, helps prevent further voltage collapse at the grid connection point, and improves the low voltage ride-through capability of permanent magnet direct drive wind power systems.

[0105] Simulation results show that the low voltage ride-through control strategy proposed in this invention is superior to traditional strategies in terms of voltage support effect, energy utilization rate and DC bus stability, and can effectively enhance the low voltage ride-through capability of wind turbines in complex voltage drop scenarios.

[0106] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A voltage support control method for permanent magnet direct-drive wind turbines based on voltage drop depth classification, comprising the following steps: S1: Establish a voltage sag depth classification system, using the maximum voltage support capacity of the wind turbine to raise the grid connection point voltage to 0.9 pu of normal voltage as the dividing line, and set the critical voltage between deep sag and mild to moderate sag as... U L ; Using the outer loop of DC bus voltage to the grid side d The constraint effect of shaft active current is used as the dividing line to set the critical voltage for mild voltage drop and moderate voltage drop. U H The voltage drop depth at the grid connection point is divided into: 1) Mild fall [ U H [0.9pu]; 2) Moderate drop [ U L , U H ]; 3) Deep drop [0.2 pu, U L Three levels; The step S1, which establishes a voltage drop depth classification system, includes the following steps: S1.1: The voltage drop at the grid connection point is set based on the maximum voltage support capacity of the wind turbine to raise the grid connection point voltage to 0.9 pu of the normal voltage. U L As the critical voltage between deep voltage drop and mild to moderate voltage drop, if the system voltage support capacity reaches its upper limit but still cannot raise the grid connection point voltage to 0.9 pu, it is defined as a deep voltage drop [0.2 pu, U L ]; If the system can raise the grid connection point voltage to above 0.9 pu, it is defined as a mild to moderate voltage drop. U L [0.9pu]; Under the constraints of converter capacity limitations and low-voltage ride-through reactive power support criteria, the system's maximum voltage support capability raises the grid connection point voltage to 0.9 pu (normal voltage), i.e. Solve for the critical voltage U L The constraints and functions are as follows: ； In the formula, I d 、I q These are the grid-side d-axis active current and the grid-side q-axis reactive current, respectively. I max This is the maximum safe current limit for the grid-side converter; I N This refers to the rated current of the grid-side converter. S1.2: Using the outer loop of the DC bus voltage to the grid side d The constraint effect of the active current on the shaft serves as the boundary, based on the fault drop voltage at the grid connection point at this time. U H Mild to moderate voltage sags are further subdivided into mild and moderate voltage sags. A moderate voltage sag is defined as the maximum voltage the system can provide to raise the grid connection point voltage to above 0.9 pu, without considering the outer loop constraint of the DC bus voltage. U L , U H The system can provide a maximum voltage support to raise the grid connection point voltage to above 0.9 pu, with the DC bus voltage outer loop constraint defined as a slight voltage drop. U H [0.9pu]; After meeting the low-voltage ride-through reactive power support criterion, the maximum active current remaining in the grid-side converter capacity is equal to the reference active current of the outer loop output of the DC bus voltage. The maximum active current... DC bus voltage outer loop output = P g / U H , P g Given the active power output of the grid-side converter before the fault, calculate the critical voltage based on this. U H ; S2: Detect the voltage amplitude at the grid connection point U pcc ,when U pcc When the power factor is >0.9 PU, the wind turbine maintains normal operating mode, and the grid-side converter operates normally with unity power factor; when U pcc When 0.2pu < 0.2pu, the wind turbine system disconnects the grid-side converter from the grid connection point, and the wind turbine is disconnected from the grid; when 0.2pu ≤ U pcc When the voltage drop is ≤0.9 pu, the voltage drop depth is determined according to step S1, and the grid-side converter switches to the corresponding voltage support mode: if U H ≤ U pcc ≤0.9pu, select the mild drop voltage support mode and proceed to step S3; if U L ≤ U pcc ≤ U H Select the moderate voltage drop support mode and proceed to step S4; if 0.2pu≤ U pcc ≤ U L Select the deep drop voltage support mode and proceed to step S5; S3: When selecting the mild voltage drop support mode, calculate the grid-side... dq Shaft current reference command, proceed to step S6; S4: When the moderate voltage drop support mode is selected, the grid-side converter raises the grid connection point voltage, and the DC bus voltage is controlled by the unloading circuit. Based on the grid-side converter capacity limitations and the low-voltage ride-through reactive power support criterion constraints, calculate the grid-side... dq Shaft current reference command, proceed to step S6; S5: When the deep voltage drop support mode is selected, calculate the grid-side converter capacity limit and low voltage ride-through reactive power support criteria based on the grid-side converter capacity limit and low voltage ride-through reactive power support criteria. dq Shaft current reference command, proceed to step S6; S6: Calculated based on steps S3, S4, and S5 dq The shaft current reference command performs current closed-loop control on the grid-side converter to obtain the drive signals for each power device of the grid-side converter.

2. The voltage support control method for permanent magnet direct-drive wind turbines based on voltage drop depth gradation according to claim 1, characterized in that, The method for calculating the voltage drop amplitude at the grid connection point in step S2 is as follows: ； In the formula, U pccf This refers to the voltage drop at the grid connection point. U pccN The rated voltage at the grid connection point, U pccα , U pccβ The grid connection point voltage in a two-phase stationary coordinate system α , β Axial components.

3. The voltage support control method for permanent magnet direct-drive wind turbines based on voltage drop depth gradation according to claim 1, characterized in that, The mild voltage drop support mode constraint conditions and objective function in step S3 include the following steps: S3.1 Controlling active current I d The problem of adjusting and maximizing active power output is transformed into solving... d Maximum active current of the shaft; ； Based on the above constraints and control objectives, solve the network-side... dq Shaft current reference command; S3.2: Obtained from step S3.1 d The shaft active current reference value is used to determine whether it is related to the outer loop output of the DC bus voltage. If they are equal, or if they are not equal, proceed to step S6; If they are equal, switch to the secondary control objective: maintain DC bus voltage stability while meeting the conditions of converter capacity limits, low-voltage ride-through reactive power support criteria, and grid connection point voltage amplitude limits. To achieve maximum voltage support at the grid connection point; ； Based on the above constraints and control objectives, the network side solution is recalculated. dq Shaft current reference command, proceed to step S6.

4. The voltage support control method for permanent magnet direct-drive wind turbines based on voltage drop depth gradation according to claim 1, characterized in that, The constraints and objective function for the moderate voltage drop support mode in step S4 are as follows: Maximum active power output of grid-side converter: ； Based on the above constraints and control objectives, solve the network-side... dq Shaft current reference command, proceed to step S6.

5. The voltage support control method for permanent magnet direct-drive wind turbines based on voltage drop depth gradation according to claim 1, characterized in that, The constraints and objective function for the deep drop voltage support mode in step S5 are as follows: The grid-side converter maximizes the voltage at the grid connection point: ； Based on the above constraints and control objectives, solve the network-side... dq Shaft current reference command, proceed to step S6.

6. The voltage support control method for permanent magnet direct-drive wind turbines based on voltage drop depth classification according to claim 1, characterized in that, In step S6, the results calculated based on steps S3, S4, and S5 are as follows: dq The shaft current reference command performs current closed-loop control on the grid-side converter, resulting in the following drive signals for each power device in the grid-side converter: Grid-side converter dq The shaft current is controlled in a closed loop. dq Shaft current reference command and actual dq The difference in shaft current is input to the proportional-integral regulator for decoupling control. The resulting modulation command voltage is then used for space vector pulse width modulation to generate the switching signals for the power devices of the grid-side converter.

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