Low-frequency direct-drive wind turbine fault ride-through coordination control method and related device

Abstract

The application discloses a low-frequency direct-drive wind turbine fault ride-through coordination control method and related devices, in the normal operation control mode, the maximum wind energy tracking is realized through the control of the machine side converter, and the DC bus voltage is maintained constant through the control of the grid side converter; in the power grid fault ride-through control mode, after the power grid fault occurs, the DC bus voltage is controlled through the machine side converter, the grid side converter adjusts the grid-connected power factor according to the power grid voltage drop depth, and the maximum reactive power is output to support the power grid voltage recovery; meanwhile, the generator output power is limited, the excess energy of the generator is stored in the generator itself, and low-voltage ride-through is realized. The method and related devices can realize low-voltage ride-through, improve the reactive power support capability and voltage tracking precision during the power grid fault, reduce the system debugging difficulty and transformation cost, guarantee the safe and stable operation of the unit and the power grid, and meet the technical requirements of the large-scale wind power grid connection.

Description

Technical Field

[0001] This invention belongs to the field of wind power generation control technology, and relates to a fault ride-through coordinated control method and related devices for low-frequency direct-drive wind turbines. Background Technology

[0002] Low-frequency AC (LFAC), also known as flexible frequency-division transmission, is a novel power transmission method first proposed by Professor Wang Xifan of Xi'an Jiaotong University in 1994. It reduces the electrical distance of AC transmission lines by lowering the transmission frequency (e.g., from 50Hz to 50 / 3Hz) without increasing the voltage level, thereby increasing the transmission power capacity and reducing the number of transmission loops and outgoing corridors. LFAC is particularly suitable for long-distance, high-capacity power transmission and also provides a new option for wind power grid connection. LFAC combines the advantages of traditional AC and DC transmission methods. By reducing the frequency, it increases the cable current-carrying capacity and reduces capacitive current in the line, significantly improving transmission capacity and distance. Furthermore, the manufacturing and maintenance costs of LFAC converter stations are significantly lower than those of converter stations required for DC transmission. At the same time, LFAC switches are similar to AC transmission switches, making it easier for LFAC systems to form multi-terminal grids than DC transmission systems. In recent years, the application scenarios of LFAC technology have further expanded. For example, the Zhejiang Yuhuan No. 2 offshore low-frequency wind power demonstration project is scheduled to be fully connected to the grid in 2025. This project uses low-frequency wind turbines with a single unit capacity of 16MW, with a total capacity of 304MW. The implementation of these projects has not only promoted the development of the low-frequency technology industry chain, but also provided technical support for future large-scale and long-distance wind power aggregation systems.

[0003] During normal grid operation, the core control mechanism of direct-drive wind turbines is to achieve maximum energy tracking through the turbine-side converter, while the grid-side converter maintains a constant DC bus voltage to ensure efficient and stable power output. However, when a three-phase symmetrical fault occurs in the grid (such as a three-phase voltage drop or a three-phase short circuit), the sudden drop in grid voltage can obstruct the active power transmission of the grid-side converter, causing a large amount of energy to accumulate on the DC bus side. This leads to a sharp rise in the DC bus voltage, and the converter is prone to triggering protection actions due to excessive output current. In severe cases, this can cause the wind turbine to disconnect from the grid, exacerbating the risk of grid instability. To address this issue, existing low-voltage ride-through control for direct-drive wind turbines primarily employs three approaches: First, adding a crowbar circuit on the DC bus. When the DC bus voltage exceeds a threshold, an unloading resistor is activated to dissipate excess energy and maintain voltage stability. This approach is popular due to its simple structure and low cost, but it suffers from significant energy waste, high heat dissipation pressure, and difficulty adapting to prolonged faults. Second, using energy storage units such as supercapacitors and batteries to store excess energy during faults and feeding it back to the grid after fault recovery. While this recovers energy, the energy storage components are expensive and have limited lifespan, resulting in poor economic viability in engineering applications. Third, improving the converter control algorithm to adjust the control objectives of the turbine-side and grid-side converters to adapt to fault conditions. However, traditional coordinated control schemes only achieve basic control objective switching and do not form a mechanism for coordinated control of multiple means, including converter control, energy regulation, and reactive power support. Furthermore, they do not optimize voltage tracking accuracy for harmonic interference, limiting control effectiveness and engineering practicality.

[0004] Therefore, in response to the problems of low energy utilization efficiency, unsmooth operating condition switching, limited reactive power support, insufficient voltage tracking accuracy, and poor engineering practicality in the existing low voltage ride-through control of direct-drive wind turbines, there is an urgent need to develop a coordinated control strategy with a simple and symmetrical structure, clear control logic, and excellent multi-means synergy. This strategy should not only achieve low voltage ride-through but also improve reactive power support and voltage tracking accuracy during grid faults, reduce system debugging difficulty and modification costs, ensure the safe and stable operation of the turbine and the grid, and meet the technical requirements for large-scale wind power grid connection. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a low-frequency direct-drive wind turbine fault ride-through coordinated control method and related devices. This method and related devices can achieve low voltage ride-through while improving the reactive power support capability and voltage tracking accuracy during grid faults, reducing the difficulty of system commissioning and modification costs, ensuring the safe and stable operation of the unit and the grid, and meeting the technical requirements for large-scale wind power grid connection.

[0006] To achieve the above objectives, this invention discloses a coordinated control method for fault ride-through of low-frequency direct-drive wind turbines, including a normal operation control mode and a power grid fault ride-through control mode. In normal operation control mode, maximum wind energy tracking is achieved by controlling the machine-side converter, and the DC bus voltage is kept constant by controlling the grid-side converter. In the grid fault ride-through control mode, after a grid fault occurs, the DC bus voltage is controlled by the generator-side converter, and the grid-side converter adjusts the grid-connected power factor according to the grid voltage drop depth to maximize the output reactive power to support grid voltage recovery; at the same time, by limiting the generator output power, the excess energy of the generator is stored in the generator itself to achieve low voltage ride-through.

[0007] Furthermore, the grid fault ride-through control mode also includes the control of a DC unloading circuit, which is activated when the DC bus voltage is higher than a set threshold and deactivated when it is lower than the set threshold, in order to consume excess energy accumulated on the DC side.

[0008] Furthermore, the grid fault ride-through control mode also includes pitch control, which limits the kinetic energy captured by the wind turbine and reduces the rate of increase in generator speed by controlling the pitch.

[0009] Furthermore, in the grid fault ride-through control mode, the active and reactive current reference values ​​of the grid-side converter are redistributed according to the grid voltage sag depth. A voltage sag depth coefficient is set. When the grid voltage fluctuates within the normal range, the reactive current reference value is zero, and the converter only outputs active power. When the grid voltage drops to outside the normal range, the reactive current reference value is proportional to the voltage sag depth coefficient and does not exceed the rated current limit of the converter. At this time, active power is stopped from being transmitted to the grid.

[0010] Furthermore, a smooth switching between the normal operation control mode and the grid fault ride-through control mode is achieved through a control power loop.

[0011] Furthermore, when the grid voltage is normal, the reference value of the control power loop is equal to the actual power and changes with the maximum wind energy tracking calculation value; when the grid voltage fails, the reference value of the control power loop is equal to the maximum wind energy tracking calculation value, the power loop is automatically switched off, and the DC voltage closed loop is superimposed on the original motor active current reference value as the active current reference value of the generator under fault conditions.

[0012] Furthermore, positive sequence voltage separation is achieved through DSOGI-PLL, enabling voltage tracking when symmetrical high-order harmonics are injected into the low-frequency power grid due to faults, or when minor asymmetrical faults occur.

[0013] Furthermore, the power grid fault is a three-phase symmetrical fault or a slightly asymmetrical fault.

[0014] The present invention discloses a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the steps of the low-frequency direct-drive wind turbine fault ride-through coordinated control method.

[0015] The present invention discloses a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the low-frequency direct-drive wind turbine fault-through coordinated control method.

[0016] The present invention has the following beneficial effects: The low-frequency direct-drive wind turbine fault ride-through coordinated control method and related devices described in this invention achieve maximum wind energy tracking by controlling the turbine-side converter and maintain a constant grid bus voltage by controlling the grid-side converter. Furthermore, after a grid fault, DC bus voltage control can be delegated to the turbine-side converter, adjusting the grid power factor according to the grid voltage drop depth to maximize reactive power output from the grid-side converter and support grid voltage recovery. Additionally, by reducing the converter's input energy (i.e., limiting generator output power), excess generator energy can be stored within the generator itself. This coordinated control strategy features highly modular design, precise operating condition switching, excellent multi-method synergy, fast low-voltage ride-through response, and good grid support, making it widely applicable to three-phase symmetrical fault applications. Compared to existing low-voltage ride-through control methods for direct-drive wind turbines, this strategy has a simpler, more symmetrical structure, decoupled control logic, fewer equivalent control links, effectively reducing the difficulty and cost of system hardware and software debugging and modification, improving the stability of turbine operation during faults and the efficiency of grid voltage recovery, and demonstrating strong practicality for engineering implementation.

[0017] Furthermore, this invention uses a DSOGI-PLL in conjunction with it to achieve separation of positive sequence voltage, thereby enabling accurate tracking even when the low-frequency power grid is injected with symmetrical high-order harmonics due to faults, and accurately capturing the degree of voltage drop when a slight asymmetrical fault occurs, further optimizing the overall performance. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A simulation model diagram of a direct-drive wind turbine. Figure 2This is a topology diagram of the grid-side converter; Figure 3 This is a topology diagram of the machine-side converter; Figure 4a This is a structural diagram of a grid-side converter; Figure 4b This is a structural diagram of the control model; Figure 5a A schematic diagram of the control structure of the converter on the permanent magnet synchronous generator side; Figure 5b A schematic diagram of the control structure of the converter on the permanent magnet synchronous generator side; Figure 5c This is a schematic diagram of the current control module for the generator-side converter. Figure 6 The control structure diagram of DSOGI-PLL; Figure 7 This is a system block diagram for a low-voltage ride-through coordinated control strategy based on mode switching. Figure 8 Flowchart for coordinated control of low-frequency direct-drive fans; Figure 9 The DC bus voltage waveforms obtained from the simulation are as follows: under the condition of three-phase symmetrical voltage drop of the grid (dropping to 0.2pu in 1s-1.2s), a coordinated control strategy is adopted for low voltage ride-through. Figure 10 The generator speed diagrams are obtained by simulating low voltage ride-through using a coordinated control strategy under the condition of three-phase symmetrical voltage drop (drop to 0.2pu in 1s-1.2s). Figure 11 The simulation results show the active and reactive power output from the generator to the grid under the condition of three-phase symmetrical voltage drop (dropping to 0.2pu in 1s-1.2s) and low voltage ride-through using a coordinated control strategy. Figure 12 The simulation results show the reactive power diagram and generator electromagnetic power and reactive power diagrams obtained under the condition of three-phase symmetrical voltage drop in the grid (dropping to 0.2pu in 1s-1.2s) using a coordinated control strategy for low voltage ride-through. Figure 13a The waveform of the wind turbine outlet bus current when using DSOGI-PLL is shown in the case of a minor three-phase asymmetrical fault in a low-frequency power grid (phase A drops to 0.7pu in 1s-1.2s). Figure 13b The waveform of the wind turbine outlet bus current is shown in the case of a minor three-phase asymmetrical fault in a low-frequency power grid (phase A drops to 0.7 pu in 1s-1.2s) when using a conventional phase-locked loop. Figure 14aThe diagram shows the active and reactive power delivered by the generator to the grid when a minor three-phase asymmetric fault occurs in a low-frequency power grid (phase A drops to 0.7 pu in 1s-1.2s) using a conventional phase-locked loop. Figure 14b The diagram shows the active and reactive power delivered by the generator to the grid when a minor three-phase asymmetrical fault occurs in a low-frequency power grid (phase A drops to 0.7 pu in 1s-1.2s) and a DSOGI-PLL is used. Figure 15a Frequency tracking diagram when using DSOGI-PLL under the condition that high-order harmonics are injected into the low-frequency power grid (7th harmonic of 2 / 70p.u. injected from 0.6s to 1.6s) and symmetrical voltage drop occurs (drop to 0.2pu from 1s to 1.2s). Figure 15b Frequency tracking diagram when a low-frequency power grid is injected with high-order harmonics (the 7th harmonic of 2 / 70 p.u. is injected from 0.6s to 1.6s) and a symmetrical voltage drop occurs (drops to 0.2 p.u. from 1s to 1.2s). Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] In the description of this invention, it should be understood that the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0022] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0023] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes such combinations. For example, A and / or B can represent three cases: A alone, A and B simultaneously, and B alone. Additionally, the character " / " in this invention generally indicates that the preceding and following objects have an "or" relationship.

[0024] It should be understood that although terms such as first, second, third, etc., may be used in the embodiments of the present invention to describe the preset range, these preset ranges should not be limited to these terms. These terms are only used to distinguish the preset ranges from one another. For example, without departing from the scope of the embodiments of the present invention, the first preset range may also be referred to as the second preset range, and similarly, the second preset range may also be referred to as the first preset range.

[0025] Depending on the context, the word "if" as used here can be interpreted as "when," "when," "in response to determination," or "in response to detection." Similarly, depending on the context, the phrase "if determination" or "if detection (of the stated condition or event)" can be interpreted as "when determination," "in response to determination," "when detection (of the stated condition or event)," or "in response to detection (of the stated condition or event)."

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0027] The accompanying drawings illustrate various structural schematic diagrams according to embodiments disclosed in this invention. These drawings are not to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0028] Example 1 refer to Figure 7 The low-frequency direct-drive wind turbine fault ride-through coordinated control method of the present invention includes the following steps: 1) Construct the converter and its control model; The control objective of the grid-side converter for wind turbines is to maintain a constant DC bus voltage while controlling the power factor of the wind turbine connected to the grid, achieving decoupled control of active and reactive power in grid-connected wind power. The topology of the grid-side converter is as follows: Figure 2 As shown, where, This is the three-phase grid voltage. L For the input AC side filter inductor, This represents the total equivalent resistance, including switching losses and the resistance in series with the filter inductor. Assuming the IGBT device is an ideal switch, its switching state can be described by a switching function. C For output DC side filter capacitor, If it is the DC bus voltage, then from Figure 2 The mathematical model of the grid-side converter in the three-phase stationary coordinate system can be obtained as follows: (1) in, These are the switching functions for each bridge arm. This indicates that the upper bridge arm is open and the lower bridge arm is closed. On the contrary.

[0029] Under normal circumstances, the three-phase voltages of the power grid are balanced and it is a three-wire system, then: (2) Substituting equation (2) into equation (1) and adding the three equations together, we get: (3) Substituting equation (3) into equation (1), we obtain the mathematical model of the grid-side converter in the abc coordinate system as follows: (4) It can be seen that the AC side of the converter model consists of time-varying AC quantities, making it impossible to effectively design its control system. Therefore, it is necessary to use coordinate transformation to convert the mathematical model to the dq coordinate system that rotates synchronously with the fundamental frequency of the power grid, where the d-axis is in the same direction as the composite vector of the three-phase voltage of the power grid.

[0030] Performing a dq rotation transformation on the mathematical model, the final mathematical model of the grid-side converter in a two-phase synchronous rotating coordinate system is as follows: (5) As can be seen from equation (5), Dynamic switch function In addition to control, it is also affected by cross-coupling. and The impact, at the same time, when Fluctuations or sudden changes can also affect the dq-axis current. Therefore, in order to achieve... Completely independent control is required. Currently, typical grid-side converter control schemes all adopt a feedforward decoupling control strategy based on grid voltage orientation.

[0031] Applying the Laplace transform to (5) yields: (6) To achieve decoupling, the ultimate control objective is: (7) in, The result obtained using PI control is: (8) Combining with equation (6), we get: (9) Therefore, the controller output is: (10) in, This is a feedforward control term for the grid voltage. This is a decoupling control term.

[0032] Equations (8) and (10) constitute a typical voltage and current dual closed-loop control model for a wind power grid-side converter, and its system control structure diagram is shown below. Figure 4a As shown, by Figure 4a As can be seen, It is completely decoupled in the inner current loop and can be independently implemented. This control mechanism enables power factor control after the wind turbine is connected to the grid. Simultaneously, the DC voltage outer loop ensures a constant DC bus voltage, thus achieving active power control after the wind turbine is connected to the grid.

[0033] The control objective of the wind turbine generator-side converter is to control the active power output of the permanent magnet direct-drive wind turbine, achieving maximum wind energy capture control below the rated wind speed and constant power control above the rated wind speed. The topology of the generator-side converter is as follows: Figure 3 As shown. For ease of analysis, the voltage equation and flux linkage equation of the permanent magnet synchronous generator in the dq synchronous rotating coordinate system are rewritten as follows: (11) (12) Control quantity: (13) It can be generated through closed-loop control and Based on the zero-pole cancellation principle of linear system correction, the controller parameters can be designed as follows: (14) The overall controller output is: (15) This controller also incorporates decoupling terms to overcome the mutual influence between the dynamics of the d-axis and q-axis currents, achieving independent control of the d-axis currents. When rotor field-oriented zero-current control of the d-axis is used, then: (16) The instantaneous active and reactive power output from the generator stator side is as follows: (17) Therefore, a closed-loop control structure for active power can be constructed, and the active power output of the PMSG can be controlled by controlling the stator d-axis current to zero, thereby realizing the functions of maximum power tracking control and constant power control of the wind turbine. It should be noted that, as shown in equation (17), the stator d-axis zero current control method can ensure that the generator does not produce armature reaction, but it cannot achieve direct control of reactive power. Equations (13) and (15) constitute a typical machine-side converter closed-loop control model for permanent magnet synchronous generators. Figure 5a The control principle diagram of the permanent magnet direct-drive generator-side converter is given.

[0034] 2) DSOGI-PLL control principle; In the orthogonal generator of the second-order generalized integrator, SOGI is implemented by constructing an adaptive filter based on the internal model principle, where the transfer function of SOGI is: (18) (19) The functions corresponding to equations (18) and (19) in the time domain are respectively , And the phase difference Construct a two-phase orthogonal coordinate system. For input signals containing harmonics... In terms of output signal It can only track the input fundamental signal, while its output two-phase signals are orthogonal to each other, thus forming a second-order generalized integrator-quadrature signal generator (SOGI-QSG). Currently, most adopt an improved version of SOGI-QSG, with the following transfer function: (20) (twenty one) The system amplitude and phase frequency characteristics in vector form can be calculated from equations (20) and (21): (twenty two) (twenty three) From equations (22) and (23), it can be seen that in steady state... ,but It can be seen that Figures 3-8 The system shown can also achieve zero steady-state error regulation, and it can be seen from equation (21) that... Always more Lag and with , , Irrelevant, indicating output signal and Orthogonal. Therefore, positive sequence voltage components can be generated through DSOGI-QSC. , ,go through Clarke After transformation, we get , The components are transferred via a phase-locked loop. q Axial components The angular frequency is obtained after PID control and error tuning. and angular frequency The feedback is fed back to the second-order generalized integrator, and the resonant frequency is used to achieve adaptive changes. The control structure block diagram of the DSOGI-PLL is shown below. Figure 6 As shown, by separating the positive sequence voltage for tracking, the DSOGI-PLL can more accurately track the positive sequence voltage compared to ordinary phase-locked loops under harmonic influences and asymmetrical faults, further improving its phase-locking capability.

[0035] 3) Voltage ride-through control strategy for direct-drive wind turbine generators; In traditional low voltage ride-through schemes, the converter control strategy is not adjusted during a fault, and the converter is protected only by unloading resistors. This not only causes energy loss and increases the difficulty of heat dissipation design, but also greatly increases the size of the converter. Furthermore, it lacks flexibility in active and reactive power control, cannot adjust the grid power factor according to the grid voltage drop depth, and cannot support grid recovery, making it difficult to obtain good control performance.

[0036] To address the energy imbalance between the input and output of a converter during grid faults, besides discharging excess energy, another approach is to reduce the input energy to the converter, i.e., limit the generator's output power, storing the excess energy within the generator itself. This will increase the generator rotor speed, but since grid faults are generally short-lived, the rate of increase in speed due to excess energy will not exceed permissible limits. (Using the generator's inertial time constant...) Taking 5 seconds as an example, when the grid voltage drops to zero and the fault lasts for 200ms, it can be calculated that the generator speed increases by only 4% after the fault is cleared. When the fault lasts for a longer period of time, pitch control can be used to adjust the speed, thereby limiting the kinetic energy captured by the wind turbine to reduce the increase in generator speed. Therefore, it is feasible to use a scheme that limits the generator output power for low voltage ride-through.

[0037] Based on this idea, the basic scheme of the low voltage ride-through control strategy proposed in this invention is as follows: under normal grid conditions, maximum wind energy tracking is achieved by controlling the generator-side converter, and the grid-side converter is controlled to maintain the bus grid constant; when a grid fault occurs, the DC bus voltage control will be implemented by the generator-side converter, and the grid-side converter will generate reactive power to the maximum extent possible to support the grid voltage recovery according to the grid voltage drop depth, so as to realize the low voltage ride-through control of the direct-drive wind turbine.

[0038] For grid-side converters, under normal grid operation, their control mode is unity power factor control. The active power reference value is determined by the DC-side closed loop, and the output power is equal to the input power of the generator-side converter to maintain a constant bus voltage. The reactive power reference value is zero. When a grid fault occurs, the control objective of the grid-side converter changes. It needs to provide as much reactive power as possible to the grid to support grid voltage recovery. Active power is generated provided that the actual current does not exceed the rated current of the converter. At this time, DC bus voltage control is handed over to the generator-side converter. Therefore, when a grid fault occurs, the active and reactive current reference values ​​need to be recalculated and redistributed according to the voltage drop depth.

[0039] Assume a three-phase symmetrical voltage drop across the grid, with the drop depth being... k The rated voltage of the power grid is The actual value of the positive sequence voltage amplitude separated by the DSOGI-PLL is The feedback value is Since the allowable range for grid voltage amplitude fluctuation is 10%, the relationship between the feedback value and the actual value is as follows: (twenty four) Let voltage drop depth coefficient be set Because positive sequence voltage is used as the calculation value for voltage drop, three-phase symmetrical reactive power support can be maintained even in the event of slight three-phase asymmetric voltage drop.

[0040] When the grid voltage drops below the lower limit of normal fluctuation, the grid-side converter begins to inject reactive power into the grid. The magnitude of this reactive power is proportional to the depth of the voltage drop. Furthermore, due to the limited capacity of the frequency converter, measures are taken to ensure that the current flowing through the converter does not exceed its maximum rated value. Then the reference current value of the grid-side converter satisfy: (25) Also limited by converter capacity, the maximum active current that can be generated during a fault is: (26) To ensure a smooth switching between the control model under normal and fault conditions, this invention employs a control power loop. When the grid voltage is normal, With actual power P Equal to the maximum wind energy tracking calculation value Changes; when the grid voltage fails, Equal to the maximum wind energy tracking calculation value The power loop automatically switches out, and its expression is as follows: (27) To ensure DC bus voltage stability during fault conditions, bus voltage control is performed by the generator-side frequency converter. With the power loop being switched out, the DC voltage closed-loop should be superimposed on the original generator active current reference value. When the grid is normal, this superposition is zero; during grid faults, it is directly used as the generator active current reference value, expressed as: (28) In summary, the system block diagram of the PMSG low-voltage ride-through coordinated control adopted in this invention is as follows: Figure 7 As shown. Without adding a DC unloading circuit, it can effectively control reactive power, limit the rise of DC bus voltage, and the system can smoothly switch between different modes. The overall control concept is as follows. Figure 8As shown. To ensure the stability of the converter DC voltage during a fault, reduce voltage fluctuations caused by transient energy, and limit the increase in generator speed during a fault, this low-voltage ride-through integrated control strategy can also incorporate a DC unloading control circuit and pitch control to further improve its overall performance.

[0041] Simulation Experiment A model of a direct-drive wind turbine generator low-frequency grid-connected system was built on the MATLAB / SIMULINK platform. The main parameters of the system are shown in Table 1.

[0042] Table 1

[0043] Figures 9 to 12 The simulation results are as follows: DC bus voltage waveform, generator speed diagram, active and reactive power output from the generator to the grid, and electromagnetic power and reactive power output from the generator are obtained under the condition of three-phase symmetrical voltage drop (dropping to 0.2pu in 1s-1.2s) and low voltage ride-through using a coordinated control strategy.

[0044] Figures 13a to 14b The figures show the waveforms of the wind turbine outlet bus current and the active and reactive power transmitted from the generator to the grid under the condition of a slight three-phase asymmetric fault in the low-frequency power grid (phase A drops to 0.7 pu in 1s-1.2s), using DSOGI-PLL and ordinary phase-locked loop respectively.

[0045] Figure 15a and Figure 15b The frequency tracking performance of DSOGI-PLL versus ordinary phase-locked loop is compared when a low-frequency power grid is injected with high-order harmonics (the 7th harmonic of 2 / 70 p.u. is injected from 0.6s to 1.6s) and a symmetrical voltage drop occurs (drops to 0.2 p.u. from 1s to 1.2s).

[0046] Example 2 A computer device includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the steps of the low-frequency direct-drive wind turbine fault ride-through coordinated control method, including, for example, a normal operation control mode and a grid fault ride-through control mode. In the normal operation control mode, maximum wind energy tracking is achieved by controlling the generator-side converter, and the DC bus voltage is maintained constant by controlling the grid-side converter. In the grid fault ride-through control mode, after a grid fault occurs, the DC bus voltage is controlled by the generator-side converter, and the grid-side converter adjusts the grid-connected power factor according to the grid voltage drop depth to maximize reactive power output and support grid voltage recovery. Simultaneously, by limiting the generator output power, excess energy from the generator is stored within the generator itself to achieve low-voltage ride-through. The memory may include main memory, such as high-speed random access memory, and may also include non-volatile memory, such as at least one disk storage device. The processor, network interface, and memory are interconnected via an internal bus, which can be an industry-standard architecture bus, a peripheral component interconnection standard bus, an extended industry-standard architecture bus, etc. The bus can be categorized as an address bus, a data bus, a control bus, etc. The memory is used to store programs; specifically, the program may include program code, which includes computer operation instructions. The memory may include main memory and non-volatile memory, and provides instructions and data to the processor.

[0047] Example 3 A computer-readable storage medium stores a computer program that, when executed by a processor, implements the steps of the low-frequency direct-drive wind turbine fault ride-through coordinated control method. For example, it includes a normal operation control mode and a grid fault ride-through control mode. In the normal operation control mode, maximum wind energy tracking is achieved by controlling the generator-side converter, and the DC bus voltage is maintained constant by controlling the grid-side converter. In the grid fault ride-through control mode, after a grid fault occurs, the DC bus voltage is controlled by the generator-side converter, and the grid-side converter adjusts the grid-connected power factor according to the grid voltage drop depth to maximize reactive power output and support grid voltage recovery. Simultaneously, by limiting the generator output power, excess energy from the generator is stored within the generator itself to achieve low-voltage ride-through. Specifically, the computer-readable storage medium includes, but is not limited to, volatile memory and / or non-volatile memory. The volatile memory may include random access memory (RAM) and / or cache memory, etc. The non-volatile memory may include read-only memory (ROM), hard disk, flash memory, optical disk, magnetic disk, etc.

[0048] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0049] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0050] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0051] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0052] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and disclosure of the invention. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

[0053] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

[0054] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention. Any simple modifications, alterations, or equivalent structural changes made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A coordinated control method for fault ride-through of a low-frequency direct-drive wind turbine, characterized in that, This includes normal operation control mode and grid fault ride-through control mode; In normal operation control mode, maximum wind energy tracking is achieved by controlling the machine-side converter, and the DC bus voltage is kept constant by controlling the grid-side converter. In the grid fault ride-through control mode, after a grid fault occurs, the DC bus voltage is controlled by the generator-side converter, and the grid-side converter adjusts the grid-connected power factor according to the grid voltage drop depth to maximize the output reactive power to support grid voltage recovery; at the same time, by limiting the generator output power, the excess energy of the generator is stored in the generator itself to achieve low voltage ride-through.

2. The low-frequency direct-drive wind turbine fault ride-through coordinated control method according to claim 1, characterized in that, The grid fault ride-through control mode also includes the control of a DC unloading circuit. The DC unloading circuit is activated when the DC bus voltage is higher than a set threshold and disconnected when it is lower than the set threshold, so as to consume the excess energy accumulated on the DC side.

3. The low-frequency direct-drive wind turbine fault ride-through coordinated control method according to claim 1, characterized in that, The grid fault ride-through control mode also includes pitch control, which limits the kinetic energy captured by the wind turbine and reduces the rate of increase in generator speed by controlling the pitch.

4. The low-frequency direct-drive wind turbine fault ride-through coordinated control method according to claim 1, characterized in that, In the grid fault ride-through control mode, the active and reactive current reference values ​​of the grid-side converter are redistributed according to the grid voltage sag depth. A voltage sag depth coefficient is set. When the grid voltage fluctuates within the normal range, the reactive current reference value is zero, and the converter only outputs active power. When the grid voltage drops to outside the normal range, the reactive current reference value is proportional to the voltage sag depth coefficient and does not exceed the rated current limit of the converter. At this time, the active power is stopped from being transmitted to the grid.

5. The low-frequency direct-drive wind turbine fault ride-through coordinated control method according to claim 1, characterized in that, Smooth switching between normal operation control mode and grid fault ride-through control mode is achieved through a control power loop.

6. The low-frequency direct-drive wind turbine fault ride-through coordinated control method according to claim 5, characterized in that, When the grid voltage is normal, the reference value of the control power loop is equal to the actual power and changes with the maximum wind energy tracking calculation value. When the grid voltage fails, the reference value of the control power loop is equal to the maximum wind energy tracking calculation value, the power loop is automatically switched off, and the DC voltage closed loop is superimposed on the original motor active current reference value as the active current reference value of the generator under fault conditions.

7. The low-frequency direct-drive wind turbine fault ride-through coordinated control method according to claim 1, characterized in that, The DSOGI-PLL enables the separation of positive sequence voltage, allowing for voltage tracking when a low-frequency power grid is injected with symmetrical high-order harmonics due to a fault, or when a minor asymmetrical fault occurs.

8. The low-frequency direct-drive wind turbine fault ride-through coordinated control method according to claim 1, characterized in that, The power grid fault is a three-phase symmetrical fault or a slightly asymmetrical fault.

9. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the steps of the low-frequency direct-drive wind turbine fault ride-through coordinated control method as described in any one of claims 1-8.

10. A computer-readable storage medium storing a computer program, characterized in that, When the computer program is executed by the processor, it implements the steps of the low-frequency direct-drive wind turbine fault ride-through coordinated control method as described in any one of claims 1-8.

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