Medium-voltage high-capacity high-power-density cascade type wind power generation converter

Abstract

The invention belongs to the technical field of wind power generation, and provides a medium-voltage large-capacity high-power-density cascade type wind power generation converter which is formed by cascading a plurality of power modules, and each power module comprises a three-stage structure, namely a generator side H-bridge rectifier, an intermediate-stage six-port boost isolation type DC / DC converter and a power grid side H-bridge inverter. A cascaded H-bridge rectifier is adopted on the generator side, the voltage grade of the generator is effectively improved, the current amplitude is reduced, and the problem of cable twisting is solved; the six-port boost isolation type DC / DC converter in the middle stage can eliminate low-frequency pulsating power output by the cascaded H-bridge converter in the system and reduce the capacitance value of a voltage stabilization capacitor on the direct current side. The wind power generation converter has the characteristics of high power density, high reliability and low cost, and can be more efficiently and stably applied to the field of wind power generation.

Description

Technical Field

[0001] The present invention belongs to the technical field of wind power generation, and in particular relates to a medium-voltage, large-capacity, and high-power-density cascade wind power converter. Background Art

[0002] In large offshore wind turbines, power converters are a core component for efficient and stable power generation. As offshore wind power technology evolves toward larger turbine capacities, greater offshore distances, and deeper waters, the performance requirements for power converters are becoming increasingly stringent.

[0003] Currently, direct-drive permanent magnet synchronous generator systems with a rated power of 10 MW or above generally use back-to-back three-level neutral-point-clamped converters with voltage levels of 1.14 kV or 3.3 kV. However, as the installed capacity of offshore wind turbines continues to climb toward 20 MW, the voltage limitations of existing converter systems have become increasingly apparent, leading to a series of problems such as excessive system current, increased losses, and difficulty twisting cables. Therefore, the development of converters with higher voltage levels, such as cascaded H-bridge converters, is crucial for the development of next-generation offshore wind turbines.

[0004] Figure 1 The figure shows a typical cascaded H-bridge converter topology of the prior art. Due to its modular structure, high voltage adaptability, and excellent harmonic suppression capability, it shows great application potential in the field of wind power generation. However, during operation, the H-bridge rectifier on the generator side of the converter will generate low-frequency voltage ripple, which needs to be suppressed by a large number of DC bus capacitors. This reduces the operational reliability of the DC side to a certain extent. In addition, the multi-winding power frequency isolation transformer on the grid side is large in size, which greatly compresses the already limited installation space of the offshore platform and reduces the power density of the overall power conversion system. Therefore, there is an urgent need for an innovative technical solution to break through the above bottlenecks in order to promote the further development of the offshore wind power industry. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a medium voltage, large capacity, and high power density cascaded wind power converter to solve the problems in the prior art. The technical solution adopted by the present invention is:

[0006] A medium-voltage, large-capacity, and high-power-density cascaded wind power converter, wherein the converter adopts a modular structure and is constructed by cascading multiple power modules with identical structures;

[0007] Among them, the three-phase cable of the cascade output of the power module on the generator side is connected to the stator end of the medium-voltage permanent magnet synchronous generator, and the three-phase cable of the cascade output of the power module on the grid side is connected to the three-phase medium / high voltage AC power grid in the wind farm through the three-phase filter inductor on the grid side, thereby forming a medium-voltage, large-capacity, and high-power density cascaded wind turbine converter topology.

[0008] Furthermore, the power module comprises a three-stage structure, namely, a generator-side H-bridge rectifier, an intermediate-stage six-port boost isolated DC / DC converter, and a grid-side H-bridge inverter;

[0009] The H-bridge rectifier on the generator side includes three independent standard two-level H-bridge converters. The input ends of the three standard two-level H-bridge converters are cascaded with the input ends of other power modules. The three cascaded AC ports are connected to the stator side of the permanent magnet synchronous generator.

[0010] The intermediate-stage six-port step-up isolated DC / DC converter includes three H-bridge input converters, a six-winding medium-frequency step-up transformer, and three H-bridge output converters. The three ports on the low-voltage side of the six-winding medium-frequency step-up transformer are connected to the three H-bridge input converters, while the three ports on the high-voltage side are connected to the three H-bridge output converters.

[0011] The grid-side H-bridge inverter includes three independent H-bridge converters. The DC ends of the three H-bridge converters are respectively connected to the three H-bridge converters on the high-voltage side of the six-port boost isolated DC / DC converter output. The AC output of the grid-side H-bridge inverter is cascaded with the AC output of other power modules. The three cascaded AC output ports are connected to the three-phase medium-voltage AC transmission network through the grid-side three-phase filter inductor.

[0012] Furthermore, the three H-bridge converters on the input low-voltage side of the intermediate-stage six-port boost isolated DC / DC converter have the same DC terminal voltage and input power; the three H-bridge converters on the output high-voltage side have the same DC terminal voltage and output power, and the DC terminal voltage of the three H-bridge converters on the input low-voltage side is less than the DC terminal voltage of the three H-bridge converters on the output high-voltage side.

[0013] Furthermore, the six-winding medium-frequency step-up transformer in the intermediate-stage six-port step-up isolated DC / DC converter has the same turns ratio for the three windings on the input low-voltage side and the same turns ratio for the three windings on the output high-voltage side, and the turns ratio of each winding is 1:1:1:n:n:n, where n>1.

[0014] Furthermore, the structure of the three single-phase H-bridge converters on the input low-voltage side of the intermediate-stage six-port boost isolated DC / DC converter is consistent with the structure of the single-phase H-bridge converter in the generator-side cascade H-bridge rectifier, both of which are standard two-level H-bridge topologies, including 4 power switching devices and 4 anti-parallel diodes; the three single-phase H-bridge converters on the output high-voltage side have a topology consistent with the topology of the single-phase H-bridge converter in the grid-side cascade H-bridge inverter, including two optional forms: the first is a standard two-level H-bridge structure; the second is a mid-point clamped three-level H-bridge structure, including 8 power switching devices and 8 anti-parallel diodes, plus 4 clamping diodes; the voltage withstand level of the switching devices in the H-bridge converter on the input low-voltage side of the six-port boost isolated DC / DC converter is lower than the voltage withstand level of the H-bridge converter on the output high-voltage side.

[0015] Furthermore, the cascaded H-bridge rectifier on the generator side uses vector control to adjust the generator speed. The modulation strategy is carrier phase shift modulation or carrier cascade modulation. It rectifies the low-frequency AC power output by the permanent magnet synchronous generator into DC power and implements maximum power point tracking control.

[0016] The intermediate-stage six-port step-up isolated DC / DC converter uses a multi-port coordinated control method to control the bidirectional coordinated transmission of instantaneous power in the low-voltage winding of the six-winding medium-frequency step-up transformer and the low-frequency pulsating power in the high-voltage winding. The coupled magnetic circuit inside the six-winding medium-frequency step-up transformer eliminates the three-phase low-frequency pulsating reactive power in the high- and low-voltage windings, while ensuring the stable transmission of active power from the low-voltage winding of the six-winding medium-frequency step-up transformer to the high-voltage winding.

[0017] The grid-side cascaded H-bridge inverter controls the DC terminal voltage using dual closed-loop voltage and current control. When the single-phase H-bridge converter adopts a mid-point clamped three-level H-bridge structure, mid-point potential balance control is also required. The modulation strategy is carrier phase-shift modulation or carrier cascade modulation to transmit the active power in the converter to the grid.

[0018] The present invention has the following beneficial effects:

[0019] (1) Since the six-port boost isolated DC / DC converter used in the intermediate stage of the converter can gather and eliminate the low-frequency pulsating power output by the cascaded H-bridge rectifier on the generator side and the cascaded H-bridge inverter on the grid side, the capacitance of the DC side stabilizing capacitor can be greatly reduced, thereby significantly improving the reliability of the DC side of the converter.

[0020] (2) Since the volume of the intermediate-stage six-port boost isolated DC / DC converter is much smaller than that of the traditional multi-winding power frequency transformer, the power density of the entire power conversion system is greatly improved.

[0021] (3) When connected to a medium-voltage AC transmission network with a higher voltage level, the grid-side cascaded H-bridge inverter of the converter adopts a mid-point clamped three-level H-bridge structure, which can greatly reduce the withstand voltage level of the H-bridge converter switching devices.

[0022] (4) The present invention is applicable to wind power generation scenarios, especially medium-voltage, large-capacity permanent magnet direct-drive offshore wind power generation systems. Compared with existing cascaded H-bridge converters, the present invention effectively solves the problem of low DC reliability of existing cascaded H-bridge converters. Moreover, by removing the bulky multi-winding power frequency transformer, the power density of the cascaded wind power generation system is greatly improved. Therefore, the present invention has broad application prospects in medium-voltage, high-power offshore wind power generation systems that have a strong demand for high reliability and high power density. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a topology circuit diagram of a cascaded H-bridge wind power converter in the prior art;

[0024] Figure 2 This is a topology diagram of a medium voltage, large capacity, and high power density cascade wind power converter according to the present invention;

[0025] Figure 3 Two topological circuit diagrams of the grid-side single-phase H-bridge converter of the present invention are shown, where (a) is a standard two-level H-bridge converter; (b) is a midpoint-clamped three-level H-bridge converter;

[0026] Figure 4 These are simulation diagrams according to embodiments of the present invention, wherein (a) is a simulation waveform diagram of the generator-side cascaded H-bridge rectifier of the present invention; (b) is a simulation waveform diagram of the intermediate-stage six-port boost isolated DC / DC converter of the present invention; and (c) is a simulation waveform diagram of the grid-side cascaded H-bridge inverter of the present invention. DETAILED DESCRIPTION

[0027] The following is a combination of the embodiments of the present invention Figures 1-4 , the technical solutions in the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0028] A medium-voltage, high-capacity, high-power-density cascaded wind turbine converter consists of multiple identically structured power modules connected in cascade. The three-phase cables output by the generator-side power modules are connected to the stator of the medium-voltage permanent magnet synchronous generator. The three-phase cables output by the grid-side power modules are connected to the three-phase medium / high-voltage AC grid via a three-phase grid-side filter inductor, forming the topology of a medium-voltage, high-capacity, high-power-density cascaded wind turbine converter.

[0029] The power module consists of a three-stage structure, namely the generator-side H-bridge rectifier, the intermediate-stage six-port boost isolated DC / DC converter, and the grid-side H-bridge inverter.

[0030] The H-bridge rectifier on the generator side consists of three independent standard two-level H-bridge converters. The input ends of the three standard two-level H-bridge converters are cascaded with the input ends of other power modules. The three cascaded AC ports are connected to the stator side of the permanent magnet synchronous generator.

[0031] The intermediate-stage six-port step-up isolated DC / DC converter consists of three H-bridge input converters, a six-winding medium-frequency step-up transformer, and three H-bridge output converters. The three ports on the low-voltage side of the six-winding medium-frequency step-up transformer are connected to the three H-bridge input converters, and the three ports on the high-voltage side are connected to the three H-bridge output converters.

[0032] The grid-side H-bridge inverter consists of three independent H-bridge converters. The DC ends of the three H-bridge converters are connected to the three H-bridge converters on the high-voltage side of the intermediate-stage six-port boost isolated DC / DC converter output. The AC output of the grid-side H-bridge inverter is cascaded with the AC outputs of other power modules. The three cascaded AC output ports are connected to the three-phase medium / high-voltage AC grid through the grid-side three-phase filter inductor.

[0033] The three H-bridge converters on the input low-voltage side of the intermediate-stage six-port boost isolated DC / DC converter have the same DC terminal voltage and input power; the three H-bridge converters on the output high-voltage side have the same DC terminal voltage and output power, and the DC terminal voltage of the three H-bridge converters on the input low-voltage side is lower than the DC terminal voltage of the three H-bridge converters on the output high-voltage side;

[0034] The six-winding medium-frequency step-up transformer in the intermediate-stage six-port step-up isolated DC / DC converter has the same turn ratio of the three windings on the input low-voltage side and the same turn ratio of the three windings on the output high-voltage side, and the turn ratio of each winding is 1:1:1:n:n:n, where n>1;

[0035] The models of the three single-phase H-bridge converters on the input low-voltage side of the intermediate-stage six-port boost isolated DC / DC converter are consistent with those of the H-bridge rectifier on the generator side, and are standard two-level H-bridge topology, including four power switching devices and four anti-parallel diodes. The topology of the three single-phase H-bridge converters on the output high-voltage side is consistent with the topology of the single-phase H-bridge converter in the grid-side cascade H-bridge inverter, and has two optional forms: the first is a standard two-level H-bridge topology; the second is a mid-point clamped three-level H-bridge topology, including eight power switching devices and eight anti-parallel diodes, plus four clamping diodes. The voltage withstand rating of the switching devices in the H-bridge converter on the input low-voltage side of the six-port boost isolated DC / DC converter is lower than that of the H-bridge converter on the output high-voltage side.

[0036] The control method and modulation strategy of the wind power converter are as follows:

[0037] The generator-side cascaded H-bridge rectifier uses vector control to adjust the generator speed. The modulation strategy uses carrier phase-shift modulation or carrier cascade modulation. It rectifies the low-frequency AC power output of the permanent magnet synchronous generator into DC power and implements maximum power point tracking (MPPT).

[0038] The intermediate-stage six-port step-up isolated DC / DC converter uses a multi-port coordinated control method to control the bidirectional coordinated transmission of instantaneous power in the low-voltage winding of the six-winding medium-frequency step-up transformer and the low-frequency pulsating power in the high-voltage winding. The coupled magnetic circuit inside the six-winding medium-frequency step-up transformer eliminates the three-phase low-frequency pulsating reactive power in the high- and low-voltage windings, while ensuring the stable transmission of active power from the low-voltage winding of the six-winding medium-frequency step-up transformer to the high-voltage winding.

[0039] The grid-side cascaded H-bridge inverter controls the DC terminal voltage using dual closed-loop voltage and current control. When the single-phase H-bridge converter adopts a neutral-point-clamped three-level H-bridge topology, neutral-point potential balance control is also required. The modulation strategy remains carrier phase-shift modulation or carrier stacking modulation to transmit the active power in the converter to the grid.

[0040] The present invention will be further described with reference to the accompanying drawings and specific embodiments.

[0041] Example 1: The topological structure and working process of the power circuit of the present invention are described in detail below.

[0042] (1) Overall circuit structure

[0043] like Figure 2As shown, the present invention has an overall modular structure, comprising a cascade of multiple identical power modules. The three-phase cable 1 output from the generator-side cascaded power modules is connected to the stator of the medium-voltage permanent magnet synchronous generator. The three-phase cable output from the grid-side cascaded power modules is connected to the three-phase medium / high voltage AC grid 5 (33kV or 66kV) via a three-phase grid-side filter inductor, thus forming a medium-voltage, high-capacity, and high-power-density cascaded wind turbine converter topology.

[0044] The power module consists of a three-stage structure, namely the generator-side H-bridge rectifier 2, the intermediate-stage six-port boost isolated DC / DC converter 3, and the grid-side H-bridge inverter 4. The details are as follows:

[0045] The generator-side H-bridge rectifier 2 consists of three independent standard two-level H-bridge converters. The input terminals of the three standard two-level H-bridge converters are cascaded with the input terminals of other power modules. The three cascaded AC ports are connected to the stator side of the permanent magnet synchronous generator. When the generator-side H-bridge rectifier uses IGBT devices with a withstand voltage rating of 1700V and the number of power modules cascaded is 9, the stator terminal voltage of the permanent magnet synchronous generator can be increased to 10kV, and the power output can reach more than 20MW.

[0046] The intermediate-stage six-port step-up isolated DC / DC converter 3 consists of three H-bridge input converters, a six-winding intermediate-frequency step-up transformer, and three H-bridge output converters. The three ports on the low-voltage side of the six-winding intermediate-frequency step-up transformer are connected to the three H-bridge input converters, while the three ports on the high-voltage side are connected to the three H-bridge output converters.

[0047] The grid-side H-bridge inverter 4 includes three independent H-bridge converters. The DC ends of the three H-bridge converters are respectively connected to the three H-bridge converters on the high-voltage output side of the six-port boost isolated DC / DC converter 3. The AC output of the grid-side H-bridge inverter is cascaded with the AC outputs of other power modules. The three cascaded AC output ports are connected to the three-phase medium / high-voltage AC grid 5 through the grid-side three-phase filter inductor.

[0048] (2) Intermediate-stage six-port boost isolated DC / DC converter 3

[0049] The three standard two-level H-bridge converters on the low-voltage input side of the converter maintain consistent DC voltage and input power. The three H-bridge converters on the high-voltage output side also maintain consistent DC voltage and output power, with the DC voltage of the three standard two-level H-bridge converters on the low-voltage input side lower than the DC voltage of the three H-bridge converters on the high-voltage output side. Furthermore, the three windings on the low-voltage input side of the six-winding medium-frequency step-up transformer have the same turns ratio, as do the three windings on the high-voltage output side, with a turns ratio of 1:1:1:n:n:n (n>1). For example, when the generator stator terminal voltage is 10kV and the grid voltage is 66kV, the turns ratio of the windings in the six-winding medium-frequency step-up transformer is 1:1:1:7:7:7.

[0050] The models of the three single-phase H-bridge converters on the low-voltage side of the intermediate-stage six-port boost isolated DC / DC converter 3 input are consistent with those of the H-bridge rectifier on the generator side. They are standard two-level H-bridge topologies, consisting of four power switching devices and four anti-parallel diodes. Figure 3 (a) shows the topology of the three single-phase H-bridge converters on the output high-voltage side. The topology is consistent with that of the single-phase H-bridge converter in the grid-side cascaded H-bridge inverter. There are two optional forms: the first is a standard two-level H-bridge topology; the second is a midpoint clamped three-level H-bridge topology, as shown in Figure 1. Figure 3 As shown in (b), it includes 8 power switching devices and 8 anti-parallel diodes, plus 4 clamping diodes. The voltage withstand level of the switching devices in the input low-voltage side H-bridge converter of the six-port boost isolated DC / DC converter is lower than the voltage withstand level of the output high-voltage side H-bridge converter.

[0051] (3) Control method

[0052] The cascaded H-bridge rectifier 2 on the generator side uses vector control to adjust the generator speed. The modulation strategy is carrier phase-shift modulation or carrier cascade modulation. It rectifies the low-frequency AC power output of the permanent magnet synchronous generator into DC power and implements maximum power point tracking (MPPT).

[0053] The intermediate-stage six-port step-up isolated DC / DC converter uses a multi-port coordinated control method to control the bidirectional coordinated transmission of instantaneous power in the low-voltage winding of the six-winding medium-frequency step-up transformer and the low-frequency pulsating power in the high-voltage winding. The coupled magnetic circuit inside the six-winding medium-frequency step-up transformer eliminates the three-phase low-frequency pulsating reactive power in the high- and low-voltage windings, while ensuring the stable transmission of active power from the low-voltage winding of the six-winding medium-frequency step-up transformer to the high-voltage winding.

[0054] The grid-side cascaded H-bridge inverter 4 controls the DC terminal voltage using dual closed-loop voltage and current control. When the single-phase H-bridge converter adopts a neutral-point-clamped three-level H-bridge topology, neutral-point potential balance control is also required. The modulation strategy remains carrier phase-shift modulation or carrier stacking modulation, transmitting the active power in the converter to the three-phase medium / high-voltage AC grid 5.

[0055] (4) Simulation verification

[0056] In order to rigorously and fully verify the practical feasibility of the medium voltage, large capacity, high power density cascade wind power converter topology, a permanent magnet direct drive offshore wind power generation simulation platform with a voltage level of 10kV and a rated power of 20MW was built with the help of MATLAB / Simulink simulation software. The simulation results are as follows: Figure 4 shown. Figure 4 (a) is the simulation waveform of the generator-side cascaded H-bridge rectifier 2. It can be clearly observed from the figure that when the generator-side cascaded H-bridge rectifier adopts the vector control strategy, its input three-phase current fluctuates symmetrically, and the current amplitude is only 1632A. At the same time, the generator-side cascaded H-bridge rectifier 2 can stably output a port voltage with a level of 19. It is worth noting that through the power aggregation effect of the intermediate-stage six-port boost isolated DC / DC converter 3, the low-frequency pulsating power output by the generator-side cascaded H-bridge rectifier 2 is eliminated, thereby effectively suppressing the low-frequency ripple in the DC voltage output by the cascaded H-bridge rectifier 2. Figure 4 (b) is a simulation waveform diagram of the intermediate-stage six-port boost-type isolated DC / DC converter 3. As can be seen from the figure, due to the use of multi-port coordinated control, the instantaneous power in the low-voltage winding and the pulsating power in the high-voltage winding of the intermediate-stage six-port boost-type isolated DC / DC converter 3 simultaneously flow into the transformer's internal magnetic circuit. The low-voltage winding current and the high-voltage winding current are affected by the transmission of low-frequency pulsating power, and both exhibit envelope oscillation. Furthermore, the midpoint-clamped three-level H-bridge converter on the output high-voltage side uses midpoint potential balance control to ensure that the voltages of the upper and lower capacitors on the DC side remain essentially consistent. Figure 4 (c) is the simulation waveform of the grid-side cascade H-bridge inverter 4. As can be seen from the figure, when the grid-side cascade H-bridge inverter 4 of the above-mentioned medium-voltage, large-capacity, high-power-density cascade wind power converter is directly connected to the 66kV high-voltage AC grid, the output current of the grid-side cascade H-bridge inverter 4 can maintain a stable state, and each H-bridge converter can output a five-level port voltage. The above simulation results fully demonstrate that the medium-voltage, large-capacity, high-power-density cascade wind power converter topology has good adaptability and can adapt to medium-voltage offshore wind power generation systems with power levels above 20MW, providing a reliable theoretical and simulation basis for practical applications.

[0057] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various deformations, modifications, and substitutions made to the technical solutions of the present invention by ordinary technicians in this field should fall within the scope of protection determined by the claims of the present invention.

Claims

1. A medium voltage, large capacity, high power density cascade type wind power converter, characterized in that: The converter adopts a modular structure and is formed by cascading multiple power modules with exactly the same structure; Among them, the three-phase cable of the cascade output of the power module on the generator side is connected to the stator end of the medium-voltage permanent magnet synchronous generator, and the three-phase cable of the cascade output of the power module on the grid side is connected to the three-phase medium / high voltage AC power grid in the wind farm through the three-phase filter inductor on the grid side, thereby forming a medium-voltage, large-capacity, and high-power density cascaded wind turbine converter topology.

2. A medium voltage, large capacity, high power density cascaded wind power converter according to claim 1, characterized in that: The power module comprises a three-stage structure, namely a generator-side H-bridge rectifier, an intermediate-stage six-port boost isolated DC / DC converter, and a grid-side H-bridge inverter. The H-bridge rectifier on the generator side includes three independent standard two-level H-bridge converters. The input ends of the three standard two-level H-bridge converters are cascaded with the input ends of other power modules. The three cascaded AC ports are connected to the stator side of the permanent magnet synchronous generator. The intermediate-stage six-port step-up isolated DC / DC converter includes three H-bridge input converters, a six-winding medium-frequency step-up transformer, and three H-bridge output converters. The three ports on the low-voltage side of the six-winding medium-frequency step-up transformer are connected to the three H-bridge input converters, while the three ports on the high-voltage side are connected to the three H-bridge output converters. The grid-side H-bridge inverter includes three independent H-bridge converters. The DC ends of the three H-bridge converters are respectively connected to the three H-bridge converters on the high-voltage side of the six-port boost isolated DC / DC converter output. The AC output of the grid-side H-bridge inverter is cascaded with the AC output of other power modules. The three cascaded AC output ports are connected to the three-phase medium-voltage AC transmission network through the grid-side three-phase filter inductor.

3. A medium voltage, large capacity, high power density cascaded wind power converter according to claim 2, characterized in that: The three H-bridge converters on the input low-voltage side of the intermediate-stage six-port boost isolated DC / DC converter have the same DC terminal voltage and input power; the three H-bridge converters on the output high-voltage side have the same DC terminal voltage and output power, and the DC terminal voltage of the three H-bridge converters on the input low-voltage side is lower than the DC terminal voltage of the three H-bridge converters on the output high-voltage side.

4. The medium voltage, large capacity, and high power density cascaded wind power converter according to claim 2, characterized in that: The six-winding medium-frequency step-up transformer in the intermediate-stage six-port step-up isolated DC / DC converter has the same turns ratio for the three windings on the input low-voltage side and the same turns ratio for the three windings on the output high-voltage side. The turns ratio of each winding is 1:1:1:n:n:n, where n>1.

5. The medium voltage, large capacity, and high power density cascaded wind power converter according to claim 2, characterized in that: The structure of the three single-phase H-bridge converters on the input low-voltage side of the intermediate-stage six-port boost isolated DC / DC converter is consistent with the structure of the single-phase H-bridge converter in the generator-side cascade H-bridge rectifier, both of which are standard two-level H-bridge topologies, including four power switching devices and four anti-parallel diodes; the topology of the three single-phase H-bridge converters on the output high-voltage side is consistent with the topology of the single-phase H-bridge converter in the grid-side cascade H-bridge inverter, including two optional forms: the first is a standard two-level H-bridge structure; the second is a mid-point clamped three-level H-bridge structure, including eight power switching devices and eight anti-parallel diodes, plus four clamping diodes. The voltage withstand level of the switching devices in the H-bridge converter on the input low-voltage side of the six-port boost isolated DC / DC converter is lower than the voltage withstand level of the H-bridge converter on the output high-voltage side.

6. The medium voltage, large capacity, and high power density cascaded wind power converter according to claim 2, characterized in that: The cascaded H-bridge rectifier on the generator side uses vector control to adjust the generator speed. The modulation strategy is carrier phase-shift modulation or carrier cascade modulation. It rectifies the low-frequency AC power output of the permanent magnet synchronous generator into DC power and implements maximum power point tracking control. The intermediate-stage six-port step-up isolated DC / DC converter uses a multi-port coordinated control method to control the bidirectional coordinated transmission of instantaneous power in the low-voltage winding of the six-winding medium-frequency step-up transformer and the low-frequency pulsating power in the high-voltage winding. The coupled magnetic circuit inside the six-winding medium-frequency step-up transformer eliminates the three-phase low-frequency pulsating reactive power in the high- and low-voltage windings, while ensuring the stable transmission of active power from the low-voltage winding of the six-winding medium-frequency step-up transformer to the high-voltage winding. The grid-side cascaded H-bridge inverter controls the DC terminal voltage using dual closed-loop voltage and current control. When the single-phase H-bridge converter adopts a mid-point clamped three-level H-bridge structure, mid-point potential balance control is also required. The modulation strategy is carrier phase-shift modulation or carrier cascade modulation to transmit the active power in the converter to the grid.

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