Permanent magnet direct drive type medium and high voltage high power wind power generation converter and control method

Abstract

A kind of permanent magnet direct drive type medium-high voltage high-power wind power converter and control method, belong to power electronics technical field.The wind power converter adopts H bridge midpoint clamping type five-level converter topology, wherein wind generator side is five-level midpoint clamping type H bridge converter, grid side is three three-phase three-level midpoint clamping type grid-connected converter, its DC side is connected to wind generator side converter, grid side is connected to 66kV or higher voltage grid through isolated four-winding power frequency step-up transformer.The topology structure can improve wind generator voltage to 8.5kV, and can be used in 20MW and above power level wind power system.Compared with traditional wind power converter, it has the advantages of high rated voltage, less power device, no need for device or converter parallel, large single machine capacity, simple cable twisting, small cable loss, good harmonic characteristic, high fault tolerance, simple structure, easy to control, etc., and has important application prospect in medium-high voltage high-power offshore system.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a permanent magnet direct-drive type medium-voltage high-power wind power converter and its control method. Background Technology

[0002] Over the past 20 years, wind power generation has grown rapidly, and the power rating of wind turbines has continuously increased. Currently, the capacity of a single unit has reached 14MW, and wind turbines with power ratings of 20MW and above are under research and development. The transmission system structure of large wind turbine units is mainly a permanent magnet direct drive structure and a doubly fed + gearbox structure. Among them, the direct drive structure of permanent magnet generator + full-power converter does not require a complex gearbox, has high reliability and is easy to maintain, and has become the preferred choice for large wind turbines. Therefore, for offshore wind turbine units with high stability requirements and power ratings of 10MW and above, most adopt the direct drive structure.

[0003] Traditional wind turbines primarily employ low-voltage two-level back-to-back parallel converter schemes, which suffer from problems such as low rated current, excessive number of parallel units, excessive wind turbine current, and difficulty in cable twisting, making them unsuitable for large-scale wind turbine systems. Increasing the voltage level of wind turbines and adopting medium-voltage wind power converters are important methods to solve these problems. With continuous increases in power ratings, the entire system can use smaller cables, smaller converters, and lighter weight.

[0004] In recent years, medium-voltage wind power converters based on mid-point clamping three-level converters have been developed to increase the voltage level of wind turbine generators to 3.3kV, thereby reducing turbine current and improving overall efficiency. This type of converter topology has significant advantages in wind power systems with a power rating of 7–10MW. However, limited by the power rating of the power devices, the single-unit capacity of mid-point clamping three-level converters is limited. To achieve wind power systems with a power rating of 20MW or higher, parallel connection of devices or converters is still required, leading to complex structures, circulating current and current sharing control issues, which restrict the development of large-scale wind power systems. With the proposal of my country's strategic goals of "carbon neutrality and carbon peaking," the development of wind power systems with a power rating of 20MW or higher is of great significance and promising application prospects for improving the R&D and manufacturing level of wind power equipment and promoting the development of wind power technology in my country. Summary of the Invention

[0005] The purpose of this invention is to provide a permanent magnet direct-drive medium-voltage high-power wind power converter and its control method, which solves the problem that the current mainstream parallel converters have complex structures and cumbersome control when increasing power, and cannot be applied to wind power generation systems with power levels of 20MW and above.

[0006] To achieve the above technical objectives, the present invention provides a topology for a permanent magnet direct-drive medium- and high-voltage wind power converter. The converter topology adopts an H-bridge midpoint clamping five-level converter topology, comprising: an H-bridge converter, a grid-connected converter, and a grid-side step-up transformer.

[0007] The H-bridge converter on the wind turbine side is a five-level midpoint clamped H-bridge converter, and the input terminal of the H-bridge converter is directly connected to the stator terminal of the permanent magnet synchronous generator.

[0008] The grid-connected converter is a three-phase, three-level, neutral-point clamped grid-connected converter on the grid side. The output terminal of the grid-connected converter is connected to the primary side of the step-up transformer on the grid side.

[0009] The grid-side step-up transformer is an isolated four-winding power frequency step-up transformer, and the high-voltage side of the grid-side step-up transformer is directly connected to the power grid.

[0010] The H-bridge converter and the grid-connected converter are interconnected at their DC terminals to form a back-to-back structure, which constitutes a permanent magnet direct-drive medium-voltage high-power wind power converter.

[0011] The electrical energy generated from the stator side of the permanent magnet synchronous generator is connected to the power grid via a wind power converter with matching power rating and a step-up transformer on the grid side.

[0012] The H-bridge converter consists of three midpoint clamped single-phase H-bridge converters with identical structures. Each midpoint clamped single-phase H-bridge converter has two bridge arms, each of which is a three-level midpoint clamped structure consisting of eight power switching devices and eight corresponding anti-parallel diodes. Each bridge arm is connected to two clamping diodes, which are connected to the midpoint of two series-connected DC capacitors on the DC side.

[0013] The grid-connected converter consists of three midpoint clamping converters with identical structures. Each midpoint clamping converter is composed of twelve power switching devices and eighteen corresponding clamping diodes. The midpoint clamping converter has three bridge arms, and each bridge arm has the same device composition structure as the bridge arm of the midpoint clamping single-phase H-bridge converter.

[0014] The grid-side step-up transformer is a four-winding power frequency step-up transformer; the low-voltage side has three windings, and the input terminals of the low-voltage side windings are respectively connected to three grid-connected converters on the grid side; the high-voltage side has one winding, and the output terminal of the high-voltage side winding is connected to the grid.

[0015] The specific control method for permanent magnet direct-drive medium-voltage high-power wind power converter is as follows:

[0016] The permanent magnet synchronous generator has a generating voltage of 8.5kV;

[0017] H-bridge converters and grid-connected converters use 6kV, 6kA IGCT devices. The capacity of a single midpoint clamping converter in the grid-connected converter is 7MW or more, and the capacity of the grid-connected converter is 20MW or more.

[0018] The H-bridge converter uses a vector control strategy to regulate the speed and wind speed of the permanent magnet synchronous generator, achieving maximum power point tracking (MPPT). The three-phase alternating current generated by the permanent magnet synchronous generator is converted into direct current with a fixed voltage value by the H-bridge converter.

[0019] When the grid-connected converter is running, it adopts the traditional voltage and current dual closed-loop control method to control the DC side capacitor voltage to a fixed value and control the midpoint potential balance of each converter in the grid-connected converter. The DC side electrical energy is converted into three-phase stable AC electrical energy at the grid frequency through the grid-connected converter.

[0020] Three-phase stable AC power is fed into the three windings on the low-voltage side of the grid-side step-up transformer. After being stepped up to 66kV or above by the grid-side step-up transformer, it is connected to the power grid through the output terminal of the high-voltage side winding.

[0021] Beneficial effects: Due to the adoption of the above scheme, the input terminal of the H-bridge converter is directly connected to the stator terminal of the permanent magnet synchronous generator, and the output of the grid-connected converter is connected to the medium- and high-voltage power grid through the grid-side step-up transformer, forming a permanent magnet direct-drive type medium- and high-voltage high-power wind power converter. The wind power converter topology achieves wind power generation operation using conventional control methods, and its control and capacitor voltage balance are greatly simplified compared with traditional parallel-type wind power converters.

[0022] Compared to traditional three-level neutral-point clamping converters on the wind turbine side, the H-bridge converter uses a five-level neutral-point clamping H-bridge converter, and the AC voltage amplitude is twice that of traditional three-level neutral-point clamping converters on the wind turbine side. Therefore, this topology can increase the wind turbine voltage to 8.5kV.

[0023] The grid-side step-up transformer is a grid-side isolated four-winding power frequency step-up transformer, which can collect the AC output power of the three neutral-point clamping converters on the grid side and achieve grid connection after being stepped up by the grid-side step-up transformer. The high-voltage side of the grid-side step-up transformer can be connected to the grid with a rated voltage of 66kV or above. The grid-side step-up transformer is an isolated transformer, which eliminates the circulating current effect between the low-voltage side and the three neutral-point clamping converters on the grid side.

[0024] This invention addresses the shortcomings of current mainstream parallel converters, such as complex structure and cumbersome control, which prevent them from being applied to wind power generation systems with power ratings of 20MW and above when increasing power output, thus achieving the objective of this invention.

[0025] Advantages: This invention is suitable for medium and high voltage, high power applications, especially for future offshore wind power systems with a single unit capacity of 20MW or more. Compared with traditional wind power converters, it has the advantages of high rated voltage, fewer power devices, no need for parallel connection of devices or converters, large single unit capacity, simple cable twisting, low cable loss, good harmonic characteristics, high fault tolerance, simple structure, and easy control. It has important application prospects in medium and high voltage, high power offshore systems.

[0026] This wind power converter topology has the following significant advantages compared to existing traditional parallel wind power converter topologies:

[0027] 1) Simple structure, few power devices, large overall converter capacity, no need to increase capacity through parallel connection, power level can be increased to over 20MW, high rated voltage level, can increase wind turbine voltage to 8.5kV, and low cable loss.

[0028] 2) Convenient control: Since the capacity is no longer increased by parallel connection and an isolation transformer is used, there are no issues with circulating current and current sharing. Conventional control can be used to achieve grid-connected operation. Attached Figure Description

[0029] Figure 1 This is a topology diagram of the permanent magnet direct-drive medium-voltage high-power wind power converter of the present invention.

[0030] Figure 2 This is a structural diagram of the H-bridge converter of the present invention.

[0031] Figure 3 This is a structural diagram of the single-phase neutral point clamping H-bridge converter on the wind turbine side of the present invention.

[0032] Figure 4 This is a structural diagram of the grid-connected converter of the present invention.

[0033] Figure 5 This is a structural diagram of the grid-side isolated four-winding step-up transformer unit of the present invention.

[0034] Figure 6(a) is a simulation diagram of the three-phase output current on the stator side of the permanent magnet synchronous generator of the present invention.

[0035] Figure 6(b) is a simulation diagram of the AC side output voltage of the five-level midpoint clamping H-bridge converter of the present invention.

[0036] Figure 6(c) is a simulation diagram of the three-phase grid current on the high-voltage side of the grid-side step-up transformer of the present invention.

[0037] Figure 6(d) is a simulation diagram of the DC side voltage of the permanent magnet direct-drive medium-voltage high-power wind power converter of the present invention.

[0038] Figure 1In the system, 1. Permanent magnet synchronous generator; 2. H-bridge converter; 3. Grid-connected converter; 4. Grid-side step-up transformer; 5. Power grid.

[0039] Figure 5 In the middle, 4-1 is the low-pressure side; 4-2 is the high-pressure side. Detailed Implementation

[0040] A topology of a permanent magnet direct-drive medium- and high-voltage wind power converter is disclosed. The converter topology adopts an H-bridge midpoint clamping five-level converter topology, including: an H-bridge converter 2, a grid-connected converter 3, and a grid-side step-up transformer 4.

[0041] H-bridge converter 2 is a five-level midpoint clamping type H-bridge converter on the wind turbine generator side. The input terminal of H-bridge converter 2 is directly connected to the stator terminal of permanent magnet synchronous generator 1.

[0042] The grid-connected converter 3 is a three-phase three-level neutral-point clamping grid-connected converter on the grid side. The output terminal of the grid-connected converter 3 is connected to the primary of the step-up transformer 4 on the grid side.

[0043] The grid-side step-up transformer 4 is an isolated four-winding power frequency step-up transformer, and the high-voltage side of the grid-side step-up transformer 4 is directly connected to the power grid 5.

[0044] H-bridge converter 2 and grid-connected converter 3 are interconnected at their DC terminals to form a back-to-back structure, constituting a permanent magnet direct-drive type medium- and high-voltage high-power wind power converter.

[0045] The electrical energy generated from the stator side of the permanent magnet synchronous generator 1 is connected to the power grid 5 via the grid-side step-up transformer 4 after passing through a wind power converter with matching power level.

[0046] The H-bridge converter 2 is composed of three midpoint clamped single-phase H-bridge converters with identical structures. Each midpoint clamped single-phase H-bridge converter has two bridge arms. Each bridge arm is a three-level midpoint clamped structure, consisting of eight power switching devices and eight corresponding anti-parallel diodes. Each bridge arm is connected to two clamping diodes, and the two clamping diodes are connected to the midpoint of two series-connected DC capacitors on the DC side.

[0047] The grid-connected converter 3 consists of three midpoint clamping converters with identical structures. Each midpoint clamping converter is composed of twelve power switching devices and eighteen corresponding clamping diodes. The midpoint clamping converter has three bridge arms, and each bridge arm has the same device composition structure as the bridge arm of the midpoint clamping single-phase H-bridge converter.

[0048] The grid-side step-up transformer 4 is a four-winding power frequency step-up transformer; the low-voltage side has three windings, and the input terminals of the low-voltage side windings are respectively connected to the three grid-connected converters 3 on the grid side; the high-voltage side has one winding, and the output terminal of the high-voltage side winding is connected to the grid.

[0049] The generating voltage of permanent magnet synchronous generator 1 is 8.5kV;

[0050] H-bridge converter 2 and grid-connected converter 3 use 6kV, 6kA IGCT devices. The capacity of a single midpoint clamping converter in grid-connected converter 3 is 7MW or more, and the capacity of grid-connected converter 3 is 20MW or more.

[0051] H-bridge converter 2 uses a vector control strategy to regulate the speed and wind speed of permanent magnet synchronous generator 1, and achieves maximum power point tracking (MPPT). The three-phase alternating current generated by permanent magnet synchronous generator 1 is converted into direct current with a fixed voltage value by H-bridge converter 2.

[0052] When the grid-connected converter 3 is running in grid-connected mode, it adopts the traditional voltage and current dual closed-loop control method to control the DC side capacitor voltage to a fixed value and control the midpoint potential balance of each converter in the grid-connected converter 3. The DC side electrical energy is converted into three-phase stable AC electrical energy at the grid frequency through the grid-connected converter 3.

[0053] Three-phase stable alternating current is supplied to the three windings on the low-voltage side of the grid-side step-up transformer 4. After being stepped up to 66kV or higher by the grid-side step-up transformer 4, it is connected to the power grid through the output terminal of the high-voltage side winding. The invention will be further described with reference to the accompanying drawings and specific embodiments.

[0054] Example 1: The topology and operation of the power circuit of the present invention are described in detail below.

[0055] The permanent magnet direct-drive medium- and high-voltage high-power wind power converter adopts an H-bridge midpoint clamping five-level converter structure. The input terminal of the H-bridge converter is directly connected to the stator terminal of the permanent magnet synchronous motor. The output of the grid-connected converter is connected to the medium- and high-voltage grid through the grid-side step-up transformer, thus forming a permanent magnet direct-drive medium- and high-voltage high-power wind power converter.

[0056] The topology of the permanent magnet direct-drive medium- and high-voltage wind power converter includes: an H-bridge converter 2, a grid-connected converter 3, and a grid-side step-up transformer 4;

[0057] Among them, H-bridge converter 2 is a five-level midpoint clamping H-bridge converter. The AC side of H-bridge converter 2 is connected to a permanent magnet synchronous generator, and the DC side is connected to three midpoint clamping converters on the grid side.

[0058] The three neutral-point clamping converters on the grid side are three three-phase three-level neutral-point clamping grid-connected converters. The AC side of the three neutral-point clamping converters on the grid side is connected to the three low-voltage windings of the step-up transformer on the grid side, respectively.

[0059] The grid-side step-up transformer is an isolated four-winding power frequency step-up transformer. The high-voltage side of the grid-side step-up transformer is directly connected to the power grid and transmits electrical energy to the power grid.

[0060] The H-bridge converter consists of three midpoint clamped single-phase H-bridge converters. Each arm of the midpoint clamped single-phase H-bridge converter is a three-level midpoint clamped structure. Each midpoint clamped single-phase H-bridge converter requires 8 power switching devices and 8 corresponding anti-parallel diodes. In addition, the midpoint of the two series DC capacitors on the DC side needs to be connected to the two clamping diodes of each arm, so a total of 12 diodes are required.

[0061] Compared with the traditional three-level neutral-point clamping converter on the wind turbine side, the H-bridge converter adopts a five-level neutral-point clamping H-bridge converter, and the AC voltage amplitude is twice that of the traditional three-level neutral-point clamping converter on the wind turbine side. Therefore, this topology can increase the wind turbine voltage to 8.5kV.

[0062] The two voltage-stabilizing capacitors in the DC link of the H-bridge converter have the same capacitance value, which together ensures the balance of the midpoint potential. However, since the H-bridge converter is composed of three midpoint clamped single-phase H-bridge converters, its ripple is affected by the base frequency output power. Therefore, the required capacitance is greater than that of the traditional three-phase three-level midpoint clamped converter on the wind turbine side.

[0063] The three midpoint clamping converters on the grid side are three converters of the same specifications. Each half-bridge arm of the converter has the same device composition as the half-bridge arm of the H-bridge converter. A single midpoint clamping converter on the grid side requires 12 power switching devices and 18 corresponding diodes.

[0064] The grid-side step-up transformer is a grid-side isolated four-winding power frequency step-up transformer, which can collect the AC output power of the three grid-side neutral-point clamping converters and achieve grid connection after being stepped up by the grid-side step-up transformer. The high-voltage side of the grid-side step-up transformer can be connected to the grid with a rated voltage of 66kV or above. Since an isolated transformer is used, the circulating current problem between the three grid-side neutral-point clamping converters connected to the low-voltage side is solved.

[0065] The specific control method for the permanent magnet direct-drive type medium-voltage high-power wind power converter is as follows:

[0066] (1) Overall circuit structure

[0067] like Figure 1As shown: This invention provides a permanent magnet direct-drive medium-high voltage high-power wind power converter. This topology adopts an H-bridge midpoint clamping five-level converter structure, including an H-bridge converter 2 and a grid-connected converter 3. To achieve grid-connected energy transmission, a grid-side step-up transformer 4 is also required.

[0068] Among them, H-bridge converter 2 is a five-level midpoint clamping H-bridge converter. The AC side of the H-bridge converter is connected to a permanent magnet synchronous generator, and the DC side is connected to three midpoint clamping converters 3 on the grid side.

[0069] The three neutral-point clamping converters 3 on the grid side are three three-phase three-level neutral-point clamping grid-connected converters. The AC side of the three neutral-point clamping converters on the grid side is connected to the three low-voltage windings of the step-up transformer 4 on the grid side, respectively.

[0070] The grid-side step-up transformer 4 is an isolated four-winding power frequency step-up transformer. The high-voltage side of the grid-side step-up transformer is directly connected to the power grid 5 and transmits electrical energy to the power grid.

[0071] (2) H-bridge converter

[0072] H-bridge converters are used to rectify low-frequency AC power generated from the stator side of permanent magnet synchronous generators into DC power with a fixed voltage value. The topology is as follows: Figure 2 As shown, the H-bridge converter consists of three midpoint-clamped single-phase H-bridge converters, each of which is as follows: Figure 3 As shown, each arm of the midpoint clamped single-phase H-bridge converter is a three-level midpoint clamped structure. Each midpoint clamped single-phase H-bridge converter requires 8 power switching devices (such as 6kV, 6kAIGCT) and 8 corresponding anti-parallel diodes. The midpoint of the two series DC capacitors on the DC side needs to be connected to the two clamping diodes of each arm, so a total of 12 diodes are required.

[0073] The AC side of the H-bridge converter is connected to the stator side of the permanent magnet synchronous generator. Because the H-bridge converter uses a five-level neutral-point clamped type, the AC voltage amplitude is twice that of a traditional three-level neutral-point clamped type converter on the wind turbine side. Therefore, this topology can increase the wind turbine voltage to 8.5kV; for example... Figure 2 When using 6kV, 6kA IGCT devices, the AC side voltage can reach 8.5kV, and the power can be increased to over 20MW.

[0074] (3) Grid-connected converter

[0075] Three midpoint clamping converters on the grid side are used to convert the DC power, tuned to a fixed voltage value by the H-bridge converter, into AC power that conforms to the grid frequency. The topology is as follows: Figure 4As shown, the three grid-side midpoint clamping converters are three converters of the same specifications. Each arm of the converter has the same component composition as the arm of the generator-side midpoint clamping H-bridge converter. A single grid-side midpoint clamping converter requires 12 power switching devices and 18 corresponding diodes. Figure 4 The grid-connected converter adopts the same as Figure 2 When using 6kV, 6kA IGCT devices of the same specifications for the H-bridge converter, the capacity of a single converter can reach more than 7MW, and the capacity of three midpoint clamping converters on the grid side can reach more than 20MW.

[0076] (4) Grid-side step-up transformer

[0077] The low-voltage AC side of the grid-side step-up transformer needs to be connected to the AC outputs of three neutral-point clamping converters on the grid side, with the topology as follows: Figure 5 As shown, the low-voltage side has a three-winding input and a single-winding high-voltage output. The grid-side isolated four-winding power frequency step-up transformer can boost the output voltage of the three grid-side neutral-point clamped grid-connected converters to 66kV and above, and can achieve electrical isolation between the windings to avoid the circulating current problem between the three grid-side neutral-point clamped converters. When the capacity of the three grid-side neutral-point clamped converters reaches 20MW or more, its rated power also needs to reach 20MW or more.

[0078] (5) Control method

[0079] The H-bridge converter uses a vector control strategy to regulate the speed and wind speed of the permanent magnet synchronous generator, achieving maximum power point tracking (MPPT). The three-phase alternating current generated by the permanent magnet synchronous generator is converted into direct current with a fixed voltage value by the H-bridge converter.

[0080] When the three midpoint clamping converters on the grid side are running in parallel, the traditional voltage and current dual closed-loop control method is adopted to control the DC side capacitor voltage to a fixed value. The DC side electrical energy can be converted into three-phase stable AC electrical energy at the grid frequency through the three midpoint clamping converters on the grid side. When controlling the three midpoint clamping converters on the grid side, it is also necessary to ensure the balance of the midpoint potential of each converter.

[0081] (6) Working principle

[0082] The permanent magnet direct-drive medium-voltage high-power wind power converter topology can be applied in medium-voltage and high-power applications. Specifically, the AC input terminal of the H-bridge converter is connected to the stator terminal of the permanent magnet synchronous generator. Because the H-bridge converter adopts a five-level neutral-point clamping type, the AC side voltage amplitude is increased to twice that of the traditional three-level neutral-point clamping type converter. When using 6kV and 6kA... When using IGCT devices, this topology can increase the permanent magnet synchronous generator (PMSG) voltage to 8.5kV. The H-bridge converter employs a vector control strategy to regulate the PSG speed and wind speed, achieving maximum power point tracking (MPPT). The continuously changing three-phase AC power generated by the PSG is converted into DC power with a fixed voltage value by the H-bridge converter. The DC terminals of the three grid-side neutral-point clamping converters are connected to the DC side of the H-bridge converter to form a back-to-back structure. The three-phase H-bridge converter needs to be connected to three grid-side neutral-point clamping converters. When the capacity of a single grid-side neutral-point clamping converter reaches 7MW or more, the capacity of the three grid-side neutral-point clamping converters can reach 20MW or more. When the three grid-side neutral-point clamping converters are running in grid-connected mode, a traditional voltage and current dual closed-loop control method is used to control the DC power. With a fixed voltage, the DC power is converted into stable three-phase AC power at the grid frequency by three grid-side neutral-point clamping converters. When controlling the three grid-side neutral-point clamping converters, it is also necessary to ensure the neutral-point potential balance of each converter. The grid-side step-up transformer is a grid-side isolated four-winding step-up transformer, which can collect the AC output power of the three grid-side neutral-point clamping converters and achieve grid connection after stepping up the voltage. The high-voltage side of the grid-side step-up transformer is connected to the grid with a rated voltage of 66kV or above. When the capacity of the three grid-side neutral-point clamping converters reaches 20MW or above, its rated power also needs to reach 20MW or above. Since an isolated transformer is used, the circulating current problem between the three grid-side neutral-point clamping converters connected to the low-voltage side is solved.

[0083] (7) Simulation verification

[0084] The operation of a permanent magnet direct-drive medium-voltage high-power wind turbine converter at a power level of 20MW was simulated using MATLAB / Simulink simulation software. The power switching device used was a 6kV IGCT, the DC-side capacitor of the permanent magnet direct-drive medium-voltage high-power wind turbine converter was 30mF, and the stator-side output of the permanent magnet synchronous generator was a three-phase AC voltage with a frequency of 15Hz and an amplitude of 8.5kV. The simulation results are shown in Figure 6. Figure 6(a) shows the three-phase output current on the stator side of the permanent magnet synchronous generator, demonstrating that the H-bridge converter with vector control can control the permanent magnet synchronous generator to output a three-phase symmetrical sinusoidal AC current. Figure 6(b) shows the nine-level line (relative phase) voltage output on the AC side of the five-level neutral-point clamped H-bridge converter on the generator side. Figure 6(c) shows the three-phase grid current input to the grid (66kV) through the grid-side step-up transformer. The three-phase grid current maintains three-phase symmetry and has high sinusoidal intensity with low harmonic content. Figure 6(d) shows the DC voltage (6500V) of the three neutral-point clamped converters on the grid side using voltage and current dual closed-loop control. The DC side voltage of the three neutral-point clamped converters on the grid side using voltage and current dual closed-loop control is stable. The DC link is affected by the fundamental frequency output power and has obvious low-frequency fluctuations, with the fluctuation frequency (30Hz) being twice the generator frequency (15Hz). Simulation results show that the five-level neutral-point clamped H-bridge converter on the wind turbine generator side, using vector control, and the three three-phase three-level neutral-point clamped grid-connected converters on the grid side, using voltage and current dual closed-loop control, can operate well in a wind power system with a power level of 20MW and a permanent magnet synchronous generator stator output voltage of 8.5kV.

Claims

1. A permanent magnet direct drive type medium-high voltage high-power wind power generation converter, characterized in that: The topology of the converter adopts a H-bridge neutral-point-clamped five-level converter topology, comprising: an H-bridge converter, a grid-connected converter and a grid-side step-up transformer; The H-bridge converter is on the wind generator side, and is a five-level neutral-point-clamped H-bridge converter, with the input end of the H-bridge converter being directly connected to the stator end of the permanent magnet synchronous generator; The grid-connected converter is on the grid side, and is a three-phase three-level neutral-point-clamped grid-connected converter, with the output end of the grid-connected converter being connected to the primary side of the grid-side step-up transformer; The grid-side step-up transformer is an isolated four-winding power frequency step-up transformer, with the high-voltage side of the grid-side step-up transformer being directly connected to the grid; The DC ends of the H-bridge converter and the grid-connected converter are connected to each other to form a back-to-back structure, thereby forming a permanent magnet direct drive type medium-high voltage high-power wind power converter; The electrical energy from the stator side of the permanent magnet synchronous generator is connected to the grid through the wind power converter with power level matching and the grid-side step-up transformer; The input end of the H-bridge converter is directly connected to the stator end of the permanent magnet synchronous generator, the output of the grid-connected converter is connected to the medium-high voltage grid through the grid-side step-up transformer, thereby forming a permanent magnet direct drive type medium-high voltage high-power wind power converter; The H-bridge converter is composed of three neutral-point-clamped single-phase H-bridge converters, which have the same structure; the neutral-point-clamped single-phase H-bridge converter has two bridge arms, each of which is a three-level neutral-point-clamped structure and is composed of eight power switching devices and eight corresponding anti-parallel diodes; each bridge arm is connected with two clamping diodes, and the two clamping diodes are connected to the midpoint of the two series DC capacitors on the DC side; The grid-connected converter is composed of three neutral-point-clamped converters, which have the same structure; one neutral-point-clamped converter is composed of twelve power switching devices and eighteen corresponding clamping diodes; the neutral-point-clamped converter has three bridge arms, each of which has the same structure as the bridge arm of the neutral-point-clamped single-phase H-bridge converter; The grid-side step-up transformer is a four-winding power frequency step-up transformer; the low-voltage side has three windings, the input ends of the low-voltage side windings are connected to the three grid-connected converters on the grid side, and the high-voltage side has one winding, the output end of the high-voltage side winding is connected to the grid.

2. The control method of the permanent magnet direct drive type medium and high voltage high power wind power converter according to claim 1, characterized in that: The specific control method is as follows: The generation voltage of the permanent magnet synchronous generator is 8.5kV; The H-bridge converter and the grid-connected converter use 6kV, 6kA IGCT devices; the capacity of a single neutral-point-clamped converter of the grid-connected converter is 7MW or more, and the capacity of the grid-connected converter is 20MW or more; The H-bridge converter adopts a vector control strategy to realize the speed regulation of the permanent magnet synchronous generator and the wind speed regulation, and to realize maximum power point tracking (MPPT); the three-phase alternating current with varying voltage generated by the permanent magnet synchronous generator is converted into direct current with a fixed voltage value through the H-bridge converter; When the grid-connected converter is in grid-connected operation, a traditional voltage and current double-loop control method is adopted to control the DC side capacitor voltage to be a fixed value, to control the neutral point potential balance of each converter of the grid-connected converter, and to convert the DC side electrical energy into three-phase stable alternating current energy at the grid frequency through the grid-connected converter. Three-phase stable AC power is input into three windings of a low-voltage side of a grid-side step-up transformer, and is stepped up to 66 kV or above by the grid-side step-up transformer, and then is connected to a power grid through an output end of a high-voltage side winding.

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