A starting device and method of a control loop of a modular multilevel matrix converter

Abstract

The application discloses a starting device and method of a control loop of a modular multilevel matrix converter, and comprises the following steps: locking all control loops of the modular multilevel matrix converter; unlocking and controlling the power frequency system network side control in the control loop of the modular multilevel matrix converter; unlocking the voltage balance control between the bridge arms of the modular multilevel matrix converter, and adjusting the voltage of each sub-converter so that the voltage between and inside each sub-converter is balanced; unlocking and controlling the constant frequency side outlet bus voltage control in the control loop; until all control loops of the modular multilevel matrix converter are completely unlocked; closing the circuit breaker between the frequency division step-down transformer and the frequency division offshore wind power system, and connecting the overall circuit; and increasing the output power of the fan to the rated power. The application can ensure that the impact generated by the system is within the tolerance range, and ensure the safe and stable operation of the system.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology and relates to a starting device and method for a modular multilevel matrix converter control circuit. Background Technology

[0002] With the development of wind power technology, the distance of offshore wind farms from the shore is gradually increasing, and the capacity of a single wind turbine has reached over 10MW. Large-capacity, long-distance offshore wind power will be the future trend of offshore wind power development. How to achieve long-distance transmission and grid connection of large-capacity wind power is a problem of great practical significance and urgently needs to be solved. At the same time, the choice of connection method between wind farms and onshore power grids will become even more important. On the one hand, the performance indicators of the transmission system connecting wind farms and onshore power grids have a significant impact on the efficiency of offshore wind farms; on the other hand, the construction and maintenance costs of the transmission system account for a high proportion of the total cost, generally second only to the investment in the wind farm itself, and have a crucial impact on the economics of the entire wind power system.

[0003] Frequency-divided transmission is a novel power transmission method proposed based on hydropower development. On one hand, because hydropower generators operate at very low speeds, low-frequency transmission reduces the number of pole pairs. On the other hand, frequency-divided transmission systems reduce line reactance, allowing for a significant increase in transmission capacity. For example, when the transmission line frequency is reduced to 50 / 3Hz, the theoretical maximum transmission power increases to three times its original value, essentially reaching the thermal limit of the transmission line. Furthermore, various technical indicators of the line, such as end-of-line voltage, compensation capacity, and voltage fluctuation, are significantly improved. Frequency-divided offshore wind power systems offer the following technical advantages:

[0004] 1) The charging current in AC submarine cables is proportional to the frequency, while the thermal effect of the charging current is proportional to the square of the frequency. Reducing the transmission frequency to 50 / 3Hz can greatly alleviate the charging current problem, improve the transmission capacity and transmission distance of the line, and reduce transmission loss.

[0005] 2) No offshore converter station is required, which significantly reduces the investment and maintenance costs of the power transmission process;

[0006] 3) It does not have the technical bottlenecks of transformers and circuit breakers, has excellent networking performance, and is suitable for building multi-terminal offshore power grids;

[0007] 4) For DFIG, the turbine speed of wind turbines with a capacity of megawatts or above generally does not exceed 20 r / min. The generator and the wind turbine need to be connected through a speed-increasing gearbox, and a three-stage speed-increasing structure is currently commonly used. This increases the size, weight and cost of the wind turbine, and makes installation and maintenance difficult. However, by using frequency division transmission, at least one stage of the gearbox can be eliminated, reducing the purchase and maintenance costs of the wind turbine.

[0008] Since the introduction of frequency-division transmission systems, frequency converters have been key equipment. Modular multilevel matrix converters (MMMCs) are the best technical solution for the frequency conversion devices of next-generation flexible frequency-division transmission systems. However, MMMCs have numerous control loops. During the startup process of the entire frequency-division offshore wind power system, different control loops and the timing and sequence of wind turbine activation will have different impacts on the system's transient characteristics. When an incorrect startup method is used, the resulting inrush current will seriously exceed the system's tolerance, threatening the safe and stable operation of the entire system. Therefore, starting a frequency-division offshore wind power system in a reasonable manner is a critical issue in engineering applications.

[0009] Defects and shortcomings of existing technology:

[0010] Currently, existing startup strategies for modular multilevel matrix converters (MMBMCs) mainly focus on the capacitor charging stage. However, during the operation of the entire frequency-division offshore wind power system, the MMBMC must comply with all grid technical standards and be able to obey and implement received dispatch commands. The grid-side controller needs to meet various technical indicators proposed by the grid, such as transmitting specified active or reactive power, providing slack nodes for the offshore grid, or ensuring the input and output power balance of the MMBMC. Therefore, a large number of control loops are required during the startup process of the frequency-division offshore wind power system to meet these requirements.

[0011] In the research on the startup of frequency-division offshore wind power systems, no scheme has been proposed regarding the activation sequence of the M3C control loop. For frequency-division transmission, the startup sequence of the modular multilevel matrix converter (MMC) control loop is a crucial factor affecting the safety of the entire frequency-division offshore wind power system. Therefore, filling the research gap on the startup sequence of the MMC control loop is of great significance for improving the safety and stability of the entire offshore frequency-division transmission system. Summary of the Invention

[0012] The purpose of this invention is to solve the problems in the prior art and provide a starting device and method for the control loop of a modular multilevel matrix converter. The starting process is simple, and it can reduce the peak values ​​of bridge arm capacitor voltage, bridge arm capacitor current and each sub-converter. At the same time, it can reduce the peak values ​​of active power and reactive power when the control loop is put into operation, thereby improving the safety margin of the entire system.

[0013] To achieve the above objectives, the present invention employs the following technical solution:

[0014] A starting device for a control circuit of a modular multilevel matrix converter includes: a modular multilevel matrix converter, a frequency-divided step-down transformer, a power frequency step-up transformer, a power frequency grid, and a frequency-divided grid side;

[0015] The frequency-division grid side includes a step-up transformer on the wind turbine side, submarine cables, and a frequency-division offshore wind power system;

[0016] The modular multilevel matrix converter is connected to a frequency-division step-down transformer and a power frequency step-up transformer, respectively; the power frequency step-up transformer is connected to the power frequency grid; the wind turbine-side step-up transformer is connected to the frequency-division offshore wind power system and is connected to the frequency-division step-down transformer via a submarine cable; the wind turbine-side step-up transformer increases the voltage entering the submarine cable.

[0017] A further improvement of the present invention is that:

[0018] A frequency-division offshore wind power system includes: wind turbines and wind turbine converters;

[0019] The fan is connected to a fan converter; the fan converter is connected to a step-up transformer on the fan side; the fan supplies power to the power frequency system through the fan converter.

[0020] A circuit breaker is installed between the step-up transformer on the wind turbine side and the step-down transformer on the frequency division, and the circuit breaker is used to disconnect the connection between the step-up transformer on the wind turbine side and the step-down transformer on the frequency division.

[0021] The modular multilevel matrix converter includes nine bridge arms. The three-phase input on the power frequency side of the modular multilevel matrix converter is connected to the three-phase output on the frequency division side. Each bridge arm consists of a bridge arm reactor and n full-bridge modules connected in series. From the power frequency side, the modular multilevel matrix converter is divided into three sub-converters u, v, and w; from the frequency division side, the modular multilevel matrix converter is divided into three sub-converters a, b, and c.

[0022] A startup method for a modular multilevel matrix converter control loop includes:

[0023] Disconnect the circuit breaker and lock out all control loops of the modular multilevel matrix converter;

[0024] Unlock the grid-side control of the power frequency system in the control loop of the modular multilevel matrix converter, and control the grid-side of the power frequency system.

[0025] Unlock the voltage equalization control between the bridge arms of the modular multilevel matrix converter, and adjust the voltage of each sub-converter to make the voltage between the sub-converters balanced.

[0026] Unlock the voltage control of the output bus on the fixed frequency division side in the control loop, and control the grid side of the frequency division system; until all control loops of the modular multilevel matrix converter are completely unlocked;

[0027] The circuit breaker between the frequency-division step-down transformer and the frequency-division offshore wind power system is closed to connect the overall circuit.

[0028] Start the fan and increase its output power to the rated power.

[0029] The entire control loop of the modular multilevel matrix converter is locked out, specifically as follows: First, the voltage / power outer loop control in the control loop of the modular multilevel matrix converter is locked out; then, the current inner loop corresponding to the voltage / power outer loop control in the control loop of the modular multilevel matrix converter is locked out; finally, the modulation part of the control loop of the modular multilevel matrix converter is locked out, that is, the output is 0.

[0030] Unlock the grid-side control of the power frequency system in the control loop of the modular multilevel matrix converter, and then control the grid-side control of the power frequency system, specifically as follows:

[0031] First, unlock the modulation loop in the control loop; then unlock the outer loop control of the reactive power control on the power frequency side in the control loop, give a reactive power reference value, and unlock its corresponding inner loop current control; finally, unlock the outer loop control of the voltage equalization control of the bridge arm capacitors on the power frequency side in the control loop, give a reference value for the voltage of the modular multilevel matrix converter bridge arm capacitors, and unlock its corresponding inner loop current control.

[0032] The voltage equalization control between the sub-converter arms in the control loop of the modular multilevel matrix converter 1 is unlocked, and the voltage between the sub-converter arms of the modular multilevel matrix converter 1 is adjusted to balance the voltage between the sub-converters. Specifically:

[0033] First, unlock the horizontal voltage equalization control in the control loop to balance the voltages of the sub-converters a, b, and c on the frequency division side; then unlock the vertical voltage equalization control in the control loop to balance the voltages of the sub-converters u, v, and w on the power frequency side; finally, unlock the diagonal voltage equalization control in the control loop to balance the arm voltages of each sub-converter.

[0034] Unlock the voltage control of the output bus on the frequency division side in the control loop, and control the grid side of the frequency division system, specifically:

[0035] Unlock the outer loop control of the frequency division side output bus voltage control in the control loop, give the reference value of the frequency division output side voltage of the modular multilevel matrix converter, and unlock its corresponding inner loop current control.

[0036] Start the fan and increase its output power to the rated power. Specifically, start the fan so that its active power output increases to the rated value within 0.1s, and deliver active power to the power grid.

[0037] Starting with the entire control loop of the lockout modular multilevel matrix converter;

[0038] When the system control loop is locked for 2 seconds, the power frequency system grid-side control and the voltage equalization control between each sub-converter arm in the control loop of the modular multilevel matrix converter are unlocked.

[0039] When the system control loop is locked for 4 seconds, the voltage control of the output bus on the fixed frequency division side in the control loop is unlocked.

[0040] When the system control loop is locked for 5 seconds, after all control loops are unlocked, the circuit breaker between the frequency-division step-down transformer and the frequency-division offshore wind power system is closed to connect the overall circuit.

[0041] When the system control loop is locked for 6 seconds, the fan is started and its output power is increased to the rated power.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] This invention uses a modular multilevel matrix converter connected to a power frequency step-up transformer to boost the voltage of the power frequency circuit. The modular multilevel matrix converter is then locked out, disconnecting the entire circuit. The power frequency system grid side is unlocked sequentially, followed by the unlocking of voltage control between each sub-converter arm and the unlocking of voltage control on the fixed-frequency-division side output bus in the control loop. This minimizes the peak impacts of the arm capacitor current waveform, arm capacitor voltage waveform, and active power waveform. During the startup of the frequency-division offshore wind power system, this ensures that the impact generated by the entire system remains within the equipment's tolerance range, maximizing the safe and stable operation of the entire system and enhancing its safety margin. Attached Figure Description

[0044] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0045] Figure 1 This is an overall structural diagram of the system in this invention;

[0046] Figure 2 This is a circuit topology diagram of a modular multilevel matrix converter;

[0047] Figure 3 The startup flowchart of the modular multilevel matrix converter and its system of the present invention;

[0048] Figure 4 This outlines the overall framework for a modular multilevel matrix converter control system.

[0049] Figure 5 The waveform of the average voltage of the bridge arm capacitor under the startup scheme of this invention is shown.

[0050] Figure 6 The active power waveform diagram is shown under the startup scheme of this invention;

[0051] Figure 7 The reactive power waveform diagram is shown under the starting scheme of this invention;

[0052] Figure 8 The waveform diagram of the three-phase voltage on the power frequency side under the starting scheme of the present invention;

[0053] Figure 9 The waveform diagram of the three-phase current on the power frequency side under the starting scheme of the present invention;

[0054] Figure 10 shows the waveform of the bridge arm capacitor current flowing through the sub-converter under the startup scheme of the present invention, where (a) is the sub-converter u bridge arm capacitor current; (b) is the sub-converter v bridge arm capacitor current; and (c) is the sub-converter w bridge arm capacitor current.

[0055] Among them, 1-modular multilevel matrix converter; 2-power frequency step-up transformer; 3-frequency step-down transformer; 4-power frequency grid; 5-wind turbine side step-up transformer; 6-wind turbine converter; 7-wind turbine. Detailed Implementation

[0056] 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 embodiments of the present invention, and not all embodiments. 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.

[0057] Therefore, the following detailed description of the embodiments of the 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 invention without inventive effort are within the scope of protection of the invention.

[0058] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0059] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper," "lower," "horizontal," or "inner" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is in use, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. Furthermore, terms such as "first" and "second" are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0060] Furthermore, the use of the term "horizontal" does not imply that the component must be absolutely horizontal, but rather that it can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0061] In the description of the embodiments of the present invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the present invention according to the specific circumstances.

[0062] The present invention will now be described in further detail with reference to the accompanying drawings:

[0063] See Figure 1 This invention discloses a starting device for a control loop of a modular multilevel matrix converter, comprising: a modular multilevel matrix converter 1, a frequency-division step-down transformer 3, a power frequency step-up transformer 2, a power frequency grid 4, and a frequency-division grid side;

[0064] The frequency-division grid side includes the wind turbine-side step-up transformer 5 and the frequency-division offshore wind power system;

[0065] The modular multilevel matrix converter 1 is connected to the frequency-divided step-down transformer 3 and the power frequency step-up transformer 2 respectively; the power frequency step-up transformer 2 is connected to the power frequency grid 4; the wind turbine side step-up transformer 5 is connected to the frequency-divided offshore wind power system and is connected to the frequency-divided step-down transformer 3 through a submarine cable; the wind turbine side step-up transformer 5 increases the voltage entering the submarine cable.

[0066] The frequency-division offshore wind power system includes: wind turbine 7 and wind turbine converter 6;

[0067] The fan 7 is connected to the fan converter 6; the fan converter 6 is connected to the fan-side step-up transformer 5, and the fan 7 supplies power to the power frequency system through the fan converter 6.

[0068] A circuit breaker is installed between the wind turbine side step-up transformer 5 and the frequency divider transformer 3. The circuit breaker is used to disconnect the wind turbine side step-up transformer 5 and the frequency divider transformer 3.

[0069] See Figure 2 The modular multilevel matrix converter 1 includes nine bridge arms. The three-phase input on the power frequency side of the modular multilevel matrix converter 1 is connected to the three-phase output on the frequency division side. Each bridge arm consists of a bridge arm reactor and n full-bridge modules connected in series. From the power frequency side, the modular multilevel matrix converter 1 is divided into three sub-converters u, v, and w; from the frequency division side, the modular multilevel matrix converter 1 is divided into three sub-converters a, b, and c.

[0070] See Figure 3 and Figure 4 This invention discloses a startup method for a modular multilevel matrix converter control loop, comprising:

[0071] S101, disconnect the circuit breaker and lock out all control loops of the modular multilevel matrix converter 1.

[0072] The entire control loop of the modular multilevel matrix converter 1 is locked out, specifically as follows: First, the voltage / power outer loop control in the control loop of the modular multilevel matrix converter 1 is locked out; then, the current inner loop corresponding to the voltage / power outer loop control in the control loop of the modular multilevel matrix converter 1 is locked out; finally, the modulation part of the control loop of the modular multilevel matrix converter 1 is locked out, that is, the output is 0.

[0073] S102 unlocks the grid-side control of the power frequency system in the control loop of the modular multilevel matrix converter 1, and controls the grid-side control of the power frequency system.

[0074] First, unlock the modulation loop in the control loop; then unlock the outer loop control of the reactive power control on the power frequency side in the control loop, give a reactive power reference value, and unlock its corresponding inner loop current control; finally, unlock the outer loop control of the voltage equalization control of the bridge arm capacitors on the power frequency side in the control loop, give a reference value for the voltage of the modular multilevel matrix converter bridge arm capacitors, and unlock its corresponding inner loop current control.

[0075] S103 unlocks the voltage equalization control between each sub-converter arm in the control loop of the modular multilevel matrix converter 1, and adjusts the voltage between each sub-converter arm of the modular multilevel matrix converter 1 to make the voltage between each sub-converter balanced.

[0076] First, unlock the horizontal voltage equalization control in the control loop to balance the voltages of the sub-converters a, b, and c on the frequency division side; then unlock the vertical voltage equalization control in the control loop to balance the voltages of the sub-converters u, v, and w on the power frequency side; finally, unlock the diagonal voltage equalization control in the control loop to balance the arm voltages of each sub-converter.

[0077] S104 unlocks the voltage control of the output bus on the fixed frequency division side in the control loop and controls the grid side of the frequency division system until all control loops of the modular multilevel matrix converter 1 are completely unlocked.

[0078] Unlock the frequency division side output bus voltage control in the control loop, give the reference value of the frequency division output side voltage of the modular multilevel matrix converter, and unlock its corresponding current inner loop control.

[0079] S105, a circuit breaker that closes the frequency-divided step-down transformer 3 and the frequency-divided offshore wind power system, connecting the overall circuit;

[0080] S106, start fan 7 and increase the output power of fan 7 to the rated power.

[0081] Start the wind turbine 7, so that the active power output of the wind turbine 7 rises to the rated value within 0.1s, and transmits active power to the power frequency grid 4.

[0082] Meanwhile, the entire control loop of the lockout modular multilevel matrix converter 1 is taken as the starting point;

[0083] When the system control loop is locked for 2 seconds, the power frequency system grid-side control and the voltage equalization control between each sub-converter arm in the control loop of modular multilevel matrix converter 1 are unlocked.

[0084] When the system control loop is locked for 4 seconds, the voltage control of the output bus on the fixed frequency division side in the control loop is unlocked.

[0085] When the system control loop is locked for 5 seconds, after all control loops are unlocked, the circuit breaker between the frequency-division step-down transformer 3 and the frequency-division offshore wind power system is closed to connect the overall circuit.

[0086] When the system control loop is locked for 6 seconds, start fan 7 and increase the output power of fan 7 to the rated power.

[0087] refer to Figure 5As the control loop is engaged starting at 2 seconds, the average value of the bridge arm capacitor voltage reaches its peak. Before 6 seconds, the highest value of the bridge arm capacitor voltage is 1.0100 pu and the lowest is 0.9856 pu. After 6 seconds, as the inverter load power increases, the bridge arm capacitor voltage experiences a surge, with a peak value of 1.1097 pu and a duration of approximately 0.012 seconds. All surge values ​​are limited to a reasonable range.

[0088] refer to Figure 6 The active power surged at 2s and 6s during the entire process, with the surge reaching its maximum value of 2.047 pu after 6s and lasting for approximately 0.0016s, which is within a reasonable range.

[0089] refer to Figure 7 The reactive power surged at 2s and 6s during the entire process, with the surge reaching its maximum value of 1.323 pu after 6s, lasting for approximately 0.0016s.

[0090] refer to Figure 8 As can be seen from the three-phase voltage waveform diagram on the power frequency side, the three-phase voltage increases slightly at 2s and decreases slightly starting at 4s, while the system continues to operate normally.

[0091] refer to Figure 9 During the entire process, the three-phase current at power frequency experienced surges at 2 seconds and 6 seconds. The amplitude of the 2-second surge was 0.1457 pu, and its duration was approximately 0.0065 seconds. After the inverter power began to increase, the surge reached its maximum value of 1.2624 pu after 6 seconds, lasting approximately 0.02 seconds. After the surge, the peak value of the three-phase current waveform was as high as 1.019 pu, and for most of the time the peak value was within 1 p.u, which is within a reasonable range.

[0092] Referring to Figure 10, the arm capacitor current surges at 2s and 6s during the entire process. The peak values ​​of the arm capacitor currents flowing through the U, V, and W phases at 2s are 0.108 pu, 0.123 pu, and 0.132 pu, respectively. After the inverter power begins to increase, the surge reaches its maximum value after 6s, with peak values ​​of 2.143 pu, 1.972 pu, and 1.710 pu, respectively. The reference values ​​are the RMS values ​​of the rated arm current.

[0093] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A starting device for a modular multilevel matrix converter control loop, characterized in that, include: Modular multilevel matrix converter (1), frequency divider step-down transformer (3), power frequency step-up transformer (2), power frequency grid (4), frequency divider grid side; The frequency-division grid side includes a wind turbine-side step-up transformer (5), submarine cables, and a frequency-division offshore wind power system; The modular multilevel matrix converter (1) is connected to the frequency step-down transformer (3) and the power frequency step-up transformer (2) respectively; the power frequency step-up transformer (2) is connected to the power frequency grid (4); the wind turbine side step-up transformer (5) is connected to the frequency-divided offshore wind power system and is connected to the frequency step-down transformer (3) through a submarine cable; the wind turbine side step-up transformer (5) increases the voltage entering the submarine cable; The starting method of the starting device is as follows: Disconnect the circuit breaker and lock out all control loops of the modular multilevel matrix converter (1); Unlock the grid-side control of the power frequency system in the control loop of the modular multilevel matrix converter (1), and control the grid-side control of the power frequency system; Unlock the voltage equalization control between each sub-converter arm in the control loop of the modular multilevel matrix converter (1), and adjust the voltage between each sub-converter arm of the modular multilevel matrix converter (1) so that the voltage between each sub-converter is balanced. Unlock the voltage control of the output bus on the fixed frequency division side in the control loop, and control the grid side of the frequency division system until all control loops of the modular multilevel matrix converter (1) are completely unlocked; Close the circuit breaker between the frequency-division step-down transformer (3) and the frequency-division offshore wind power system to connect the overall circuit; Start the fan (7) and increase the output power of the fan (7) to the rated power.

2. The starting device for the control circuit of the modular multilevel matrix converter according to claim 1, characterized in that, The frequency-division offshore wind power system includes: a wind turbine (7) and a wind turbine converter (6); The fan (7) is connected to the fan converter (6); the fan converter (6) is connected to the fan-side step-up transformer (5); the fan (7) supplies power to the power frequency system through the fan converter (6); A circuit breaker is provided between the wind turbine side step-up transformer (5) and the frequency divider transformer (3), and the circuit breaker is used to disconnect the connection between the wind turbine side step-up transformer (5) and the frequency divider transformer (3).

3. The starting device for the control circuit of the modular multilevel matrix converter according to claim 2, characterized in that, The modular multilevel matrix converter (1) includes nine bridge arms. The three-phase output on the power frequency side and the three-phase input on the frequency division side of the modular multilevel matrix converter (1) are connected through the modular multilevel matrix converter. Each bridge arm consists of a bridge arm reactor and n full-bridge modules connected in series. From the power frequency side, the modular multilevel matrix converter (1) is divided into three sub-converters u, v, w; from the frequency division side, the modular multilevel matrix converter (1) is divided into three sub-converters a, b, c.

4. The starting device for the control circuit of the modular multilevel matrix converter according to claim 1, characterized in that, The entire control loop of the modular multilevel matrix converter (1) is locked in the following ways: first, the voltage / power outer loop control in the control loop of the modular multilevel matrix converter (1) is locked in; then, the current inner loop corresponding to the voltage / power outer loop control in the control loop of the modular multilevel matrix converter (1) is locked in; finally, the modulation part of the control loop of the modular multilevel matrix converter (1) is locked in, that is, the output is 0.

5. The starting device for the control circuit of the modular multilevel matrix converter according to claim 4, characterized in that, The unlocking of the power frequency system grid-side control in the control loop of the modular multilevel matrix converter (1) and the control of the power frequency system grid side are specifically as follows: First, unlock the modulation loop in the control loop; then unlock the outer loop control of the reactive power control on the power frequency side in the control loop, give a reactive power reference value, and unlock its corresponding inner loop current control; finally, unlock the outer loop control of the voltage equalization control of the bridge arm capacitors on the power frequency side in the control loop, give a reference value for the voltage of the modular multilevel matrix converter bridge arm capacitors, and unlock its corresponding inner loop current control.

6. The starting device for the control circuit of the modular multilevel matrix converter according to claim 5, characterized in that, The process of unlocking the inter-arm voltage equalization control in the control loop of the modular multilevel matrix converter (1) and adjusting the voltage of each sub-converter between the arms to achieve voltage balance between the sub-converters is as follows: First, unlock the horizontal voltage equalization control in the control loop to balance the voltages of the sub-converters a, b, and c on the frequency division side; then unlock the vertical voltage equalization control in the control loop to balance the voltages of the sub-converters u, v, and w on the power frequency side; finally, unlock the diagonal voltage equalization control in the control loop to balance the arm voltages of each sub-converter.

7. The starting device for the control loop of the modular multilevel matrix converter according to claim 6, characterized in that, The unlocking of the frequency division side output bus voltage control in the control loop and the control of the frequency division system grid side are specifically as follows: Unlock the outer loop control of the frequency division output bus voltage control in the control loop, give the reference value of the frequency division output voltage of the modular multilevel matrix converter, and unlock its corresponding inner loop current control.

8. The starting device for the control circuit of the modular multilevel matrix converter according to claim 7, characterized in that, The start-up of the fan (7) and the increase of the output power of the fan (7) to the rated power are specifically: start the fan (7) so that the active power output of the fan (7) increases to the rated value within 0.1s and transmits active power to the power frequency grid (4).

9. The starting device for the control circuit of the modular multilevel matrix converter according to claim 8, characterized in that, Starting with the entire control loop of the lockout modular multilevel matrix converter (1); When the system control loop is locked for 2 seconds, the power frequency system grid-side control and the voltage equalization control between each sub-converter arm in the control loop of the modular multilevel matrix converter (1) are unlocked. When the system control loop is locked for 4 seconds, the voltage control of the output bus on the fixed frequency division side in the control loop is unlocked. When the system control loop is locked for 5 seconds, after all control loops are unlocked, the circuit breaker between the frequency-divided step-down transformer (3) and the frequency-divided offshore wind power system is closed to connect the overall circuit. When the system control loop is locked for 6 seconds, the fan (7) is started and the output power of the fan (7) is increased to the rated power.

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