Offshore wind power flexible low-frequency power transmission system based on dispersed energy consumption type M3C and fault ride-through method

Abstract

The invention provides an offshore wind power flexible low-frequency power transmission system based on a decentralized energy consumption type M3C and a fault ride-through method.The system comprises an offshore wind power plant, a decentralized energy consumption type M3C frequency converter and a power frequency power grid which are connected in sequence, and the decentralized energy consumption type M3C frequency converter comprises three sub-converters; each sub-converter comprises a full-bridge type sub-module and an energy consumption unit; the energy consumption unit is connected in parallel with the full-bridge type sub-module, and the energy consumption unit is configured to be put into use when the capacitor voltage average value of the full-bridge type sub-module exceeds a first threshold value so as to consume accumulated power, and is cut off when the capacitor voltage average value is lower than a second threshold value so as to consume the accumulated power when the capacitor voltage average value is lower than the second threshold value. And switching in turn according to a preset rule when the capacitor voltage average value is lower than a first threshold value and exceeds a second threshold value. According to the invention, the accumulated power of the system can be quickly absorbed, the flexibility and reliability of an accumulated power absorption mechanism under different working conditions are remarkably enhanced, and the continuous grid-connected capability of the system in a transient fault of a power grid is improved.

Description

Technical Field

[0001] The present invention relates to the field of power transmission and transformation control technology, and in particular to an offshore wind power flexible low-frequency power transmission system based on a decentralized energy-consuming M3C and a fault ride-through method. Background Art

[0002] Flexible low-frequency transmission technology can effectively suppress charging power by reducing the transmission frequency, significantly increasing transmission capacity. It also offers flexible control and eliminates the need for offshore converter stations, demonstrating superior technical and economic benefits. It is particularly suitable for grid-connected power transmission of large-capacity, long-distance offshore wind farms. However, fault ride-through is one of the key challenges facing flexible low-frequency offshore wind grid-connected systems. When a fault occurs on the onshore power grid side, the power-frequency voltage drops rapidly, making it difficult for the M3C to output rated power to the grid. However, the offshore wind farm continues to generate active power, resulting in a power imbalance between the power-frequency and low-frequency sides of the M3C. The accumulated power will flow into the M3C submodule capacitors, potentially causing overvoltage in the M3C submodules and seriously threatening equipment safety. Therefore, studying the M3C accumulated power absorption mechanism is of great significance for flexible low-frequency offshore wind transmission. Summary of the Invention

[0003] The purpose of the present invention is to overcome the shortcomings of the prior art and provide an offshore wind power flexible low-frequency transmission system and a fault ride-through method based on a distributed energy-consuming M3C.

[0004] In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions.

[0005] In a first aspect, the present invention provides an offshore wind power flexible low-frequency transmission system based on a distributed energy consumption type M3C, the system comprising an offshore wind farm, a distributed energy consumption type M3C frequency converter, and an industrial frequency power grid connected in sequence, wherein the distributed energy consumption type M3C frequency converter comprises three sub-converters, each of which comprises a full-bridge sub-module and an energy consumption unit;

[0006] The energy consumption unit is connected in parallel with the full-bridge submodule. The energy consumption unit is configured to be put into operation when the average value of the capacitor voltage of the full-bridge submodule exceeds a first threshold to consume accumulated power, and to be cut off when the average value of the capacitor voltage is lower than a second threshold. When the average value of the capacitor voltage is lower than the first threshold and exceeds the second threshold, the energy consumption unit is switched on and off in rotation according to a preset rule.

[0007] Furthermore, the energy consumption unit includes an energy consumption resistor and a controllable switch device connected in series with the energy consumption resistor, and the current flowing through the energy consumption resistor is less than the locking current of the controllable switch device.

[0008] Furthermore, the power dissipated by the energy consumption unit satisfies the following constraints:

[0009]

[0010] Among them, P N is the rated power delivered by the system, N is the total number of energy consuming units, R SM is the resistance of the energy dissipation resistor; U SM is the rated voltage of the full-bridge submodule capacitor.

[0011] In a second aspect, the present invention further provides a fault ride-through method, which is implemented based on the offshore wind power flexible low-frequency transmission system based on the decentralized energy-consuming M3C provided in the first aspect, and the method comprises:

[0012] Detect the average value of the capacitor voltage of the full-bridge submodule;

[0013] When the average value of the capacitor voltage exceeds a first threshold, all energy-consuming units are put into operation;

[0014] When the average value of the capacitor voltage is lower than a second threshold, cutting off all energy-consuming units;

[0015] When the average value of the capacitor voltage is lower than a first threshold value and exceeds a second threshold value, the energy consumption units are switched on and off in turn according to a preset rule.

[0016] Furthermore, the step of switching the energy-consuming units in rotation according to a preset rule includes:

[0017] Calculating the duty cycle of the energy consumption unit put into PWM, and at the same time, determining the starting value of the energy consumption unit to be put into use in the current cycle according to the starting value of the energy consumption unit in the previous cycle and the number of energy consumption units put into use;

[0018] The energy-consuming units are switched on and off in rotation according to the duty cycle and the starting value of the energy-consuming units that need to be put into use in the current cycle.

[0019] Furthermore, the method further comprises:

[0020] The power frequency circulating current is controlled according to the DC component control equation of the power frequency circulating current between the sub-converters to achieve dynamic energy balance between the low-frequency sub-converters:

[0021]

[0022] in, are the power frequency circulating current amplitudes of sub-converters a, b, and c, respectively. is the reference value of the total capacitor voltage between M3C sub-converters, u a,sum 、u b,sum are the sum of the capacitor voltages of the bridge arms a and b of the low-frequency side sub-converter, k P 、k I are the proportional and integral control parameters of the PI controller respectively.

[0023] Furthermore, the method further comprises:

[0024] Low-frequency circulating current control is performed based on the unbalanced active power of the bridge arm within the sub-converter, the d-axis positive sequence component of the low-frequency side voltage, and the low-frequency circulating current amplitude of the bridge arm to achieve dynamic energy balance between the bridge arms within the low-frequency sub-converter:

[0025]

[0026] Where ΔP ua , ΔP va are the unbalanced active powers of the corresponding bridge arms respectively; is the d-axis positive sequence component of the low-frequency side voltage; (x=u, v, w; y=a, b, c) is the low-frequency circulating current amplitude of the corresponding bridge arm.

[0027] Furthermore, the method further comprises:

[0028] The circulating current control instruction of the bridge arm is generated according to the power frequency circulating current instantaneous value reference instruction and the low frequency circulating current instantaneous value instruction of the bridge arm.

[0029] The present invention provides an offshore wind power flexible low-frequency transmission system based on a distributed energy-consuming M3C and a fault crossing method, wherein the offshore wind power flexible low-frequency transmission system based on a distributed energy-consuming M3C forms a distributed energy-consuming M3C by installing an energy-consuming unit composed of a switch device and an energy-consuming resistor in the full-bridge sub-module of the three symmetrical bridge arms of the M3C, making full use of the symmetrical structure of the topology to absorb the accumulated power. By establishing a capacitor voltage control mechanism between sub-converters and an unbalanced power compensation mechanism within the sub-converter, the coordinated regulation of the power frequency circulating current and the low-frequency circulating current is achieved to maintain the dynamic balance of energy between the bridge arms. Based on the bridge arm energy-consuming unit rotation switching strategy, the system accumulated power is quickly absorbed, significantly enhancing the flexibility and reliability of the accumulated power absorption mechanism under different working conditions, effectively suppressing the sub-module capacitor voltage dynamically within 1.1pu, suppressing the overvoltage of the M3C sub-module, and improving the system's continuous grid-connected capability during transient faults in the power grid.

[0030] Additional aspects and advantages of the present invention will be set forth in part in the following description, will become apparent from the following description, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0032] Figure 1 A schematic structural diagram of an offshore wind power flexible low-frequency transmission system based on a decentralized energy-consuming M3C according to an embodiment of the present invention;

[0033] Figure 2 A topological diagram of a distributed energy-consuming M3C provided in an embodiment of the present invention;

[0034] Figure 3 A schematic diagram of bucket subtraction between tree nodes provided by an embodiment of the present invention;

[0035] Figure 4 One of the flowcharts of the fault ride-through method provided in an embodiment of the present invention;

[0036] Figure 5 A second flowchart of the fault ride-through method provided in an embodiment of the present invention;

[0037] Figure 6 Response diagram of the three-phase short circuit M3C on the power frequency side of the offshore wind power flexible low-frequency transmission fault system provided by the embodiment of the present invention;

[0038] Figure 7 This is a response diagram of the BC phase grounding short circuit fault M3C on the power frequency side of the offshore wind power flexible low-frequency transmission fault system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0039] The embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention and are not to be construed as limiting the present invention.

[0040] It will be understood by those skilled in the art that, unless expressly stated otherwise, the singular forms "a", "an" and "the" used herein may also include the plural forms. It should be further understood that the term "comprising" used in the specification of the present invention refers to the presence of features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof. It should be understood that when we refer to an element as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or there may be intermediate elements. In addition, "connected" or "coupled" as used herein may include wireless connections or couplings. The term "and / or" used herein includes any unit and all combinations of one or more associated listed items.

[0041] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art in the art to which the present invention pertains. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art and, unless defined as such herein, will not be interpreted in an idealized or overly formal sense.

[0042] To facilitate understanding of the embodiments of the present invention, several specific embodiments will be further explained below with reference to the accompanying drawings, and each embodiment does not constitute a limitation on the embodiments of the present invention.

[0043] Example 1

[0044] See also Figure 1 A flexible low-frequency transmission system for offshore wind power based on a distributed energy-consuming M3C system is described. The system comprises an offshore wind farm, a distributed energy-consuming M3C inverter, and a power grid. Specifically, the offshore wind farm and the distributed energy-consuming M3C inverter are connected via a submarine cable. The system also includes a step-down transformer, a step-up transformer, and a distributed energy-consuming M3C inverter. The offshore wind farm is connected to the power grid via the step-down transformer, submarine cable, step-up transformer, and distributed energy-consuming M3C inverter.

[0045] like Figure 2 As shown, the distributed energy consumption type M3C inverter includes sub-inverters a, b, and c. Each sub-inverter includes a full-bridge sub-module (i.e., SM module) and an energy consumption unit (i.e., DED-SM module). The energy consumption unit can meet the symmetry of the sub-inverter structure on both sides of the industrial and low-frequency and the balance of the bridge arm current between the sub-inverters. The full-bridge sub-module contains 4 IGBTs (i.e., Figure 2 D1-D4 in) and 1 energy storage capacitor (i.e. Figure 2 C in the figure) forms an H-bridge structure through four IGBTs and one energy storage capacitor, and realizes positive, negative and zero-level outputs through switch control.

[0046] The energy consumption unit is connected in parallel with the full-bridge submodule and is configured to be activated when the average capacitor voltage of the full-bridge submodule exceeds a first threshold to consume accumulated power. That is, after a symmetrical or asymmetrical fault occurs on the power frequency side, the energy consumption unit in the distributed energy consumption M3C is activated to absorb accumulated power and achieve system fault ride-through. The energy consumption unit is removed when the average capacitor voltage falls below a second threshold. That is, when the system is operating normally, the energy consumption unit does not operate and the M3C operates according to normal control logic. When the average capacitor voltage falls below the first threshold and exceeds the second threshold, the energy consumption unit is switched on and off according to preset rules.

[0047] The energy consumption unit includes an energy consumption resistor and a controllable switch device connected in series with the energy consumption resistor, and the controllable switch device is an IGBT, such as Figure 2 As shown, the energy consumption unit specifically includes 5 IGBTs (i.e., D1-D5), a capacitor C, and an energy consumption resistor R SM .

[0048] The current I flowing through the energy dissipation resistor Rmax is less than the locking current of the controllable switch device, that is:

[0049]

[0050] Among them, K r is the current margin flowing through the energy dissipation resistor; I block is the IGBT latching current, which is generally twice its rated current; R SM is the energy dissipation resistor; U SM is the rated voltage of the submodule capacitor.

[0051] The energy dissipation unit can prevent M3C from overcurrent locking and achieve maximum accumulated power dissipation.

[0052] The power dissipated by the energy consumption unit satisfies the following constraints:

[0053]

[0054] Among them, P N is the rated power delivered by the system, N is the total number of energy consuming units, R SM is the resistance of the energy dissipation resistor; U SM is the rated voltage of the full-bridge submodule capacitor. Based on this, the system can still achieve fault ride-through under the most severe three-phase short-circuit fault.

[0055] The embodiment of the present invention provides an offshore wind power flexible low-frequency transmission system based on a distributed energy-consuming M3C. By installing energy-consuming units consisting of switching devices and energy-consuming resistors in the full-bridge sub-modules of the three symmetrical bridge arms of the M3C, a distributed energy-consuming M3C is formed, which makes full use of the symmetrical structure of the topology to absorb the accumulated power. By establishing a capacitor voltage control mechanism between sub-converters and an unbalanced power compensation mechanism within the sub-converter, the coordinated regulation of the power frequency circulating current and the low-frequency circulating current is achieved to maintain the dynamic balance of energy between the bridge arms. Based on the bridge arm energy-consuming unit rotation switching strategy, the system accumulated power is quickly absorbed, significantly enhancing the flexibility and reliability of the accumulated power absorption mechanism under different working conditions, effectively suppressing the sub-module capacitor voltage dynamically within 1.1pu, suppressing the overvoltage of the M3C sub-module, and improving the system's continuous grid-connected capability during transient grid faults.

[0056] Example 2

[0057] On the basis of Example 1, this Example 2 provides a fault ride-through method, which is implemented based on the above-mentioned offshore wind power flexible low-frequency transmission system based on the decentralized energy consumption type M3C. Figure 3 as well as Figure 4 As shown, the fault ride-through method includes the following steps:

[0058] S101 , detecting an average capacitor voltage Udc(pu) of a full-bridge submodule.

[0059] S102: When the average value of the capacitor voltage exceeds a first threshold, all energy-consuming units are switched on. Schematically, the first threshold may be 1.1 pu.

[0060] S103: When the average capacitor voltage falls below a second threshold, all energy-consuming units are disconnected. It will be appreciated that when the average capacitor voltage falls below the second threshold, the system operates normally, the energy-consuming units are inactive, and the M3C operates according to normal control logic. The full-bridge submodules and energy-consuming units are controlled independently and do not affect each other, requiring no coordinated operation. Illustratively, the second threshold may be 0.9 pu. The first and second thresholds can be adjusted based on actual needs and are not limited thereto.

[0061] S104 , when the average value of the capacitor voltage is lower than a first threshold value and exceeds a second threshold value, switching the energy consumption units on and off in rotation according to a preset rule.

[0062] It should be understood that there is no particular order in which steps S102 , S103 and S104 are executed, and only any one of them may be executed, without limitation.

[0063] In some embodiments of the present invention, the switching of the energy consuming units according to a preset rule includes:

[0064] The duty cycle of the energy consumption unit put into PWM is calculated, and at the same time, the starting value of the energy consumption unit to be put into use in the current cycle is determined according to the starting value of the energy consumption unit in the previous cycle and the number of energy consumption units put into use.

[0065] The duty cycle D is calculated using the following formula:

[0066]

[0067] Where, P wind is the active power transmitted from the wind farm to M3C, P g is the active power output on the power frequency side, R SM is the energy dissipation resistor; U SM is the rated voltage of the submodule capacitor, N in is the number of energy consumption units, Tin , T are the duration of the energy consumption unit input phase and the action cycle of the energy consumption unit respectively.

[0068] The number of energy-consuming units N required in the current cycle in Calculated by the following formula:

[0069]

[0070] Among them, P g is the active power output on the power frequency side, P N is the rated power delivered to the system; N is the total number of energy consumption units, R SM is the energy dissipation resistor; U SM is the rated voltage of the submodule capacitor.

[0071] The energy-consuming units are switched on and off in rotation according to the duty cycle and the starting value of the energy-consuming units that need to be put into use in the current cycle.

[0072] In some embodiments of the present invention, Figure 5 As shown, the method further includes:

[0073] The power frequency circulating current is controlled according to the DC component control equation of the power frequency circulating current between the sub-converters to achieve dynamic energy balance between the low-frequency sub-converters:

[0074]

[0075] in, are the power frequency circulating current amplitudes of sub-converters a, b, and c, respectively. is the reference value of the total capacitor voltage between M3C sub-converters, u a,sum 、u b,sum are the sum of the capacitor voltages of the bridge arms a and b of the low-frequency side sub-converter, k P 、k I are the proportional and integral control parameters of the PI controller respectively.

[0076] In some embodiments of the present invention, Figure 5 As shown, the method further includes:

[0077] Low-frequency circulating current control is performed based on the unbalanced active power of the bridge arm within the sub-converter, the d-axis positive sequence component of the low-frequency side voltage, and the low-frequency circulating current amplitude of the bridge arm to achieve dynamic energy balance between the bridge arms within the low-frequency sub-converter:

[0078]

[0079] Where ΔP ua , ΔP va are the unbalanced active powers of the corresponding bridge arms respectively; is the d-axis positive sequence component of the low-frequency side voltage; (x=u, v, w; y=a, b, c) is the low-frequency circulating current amplitude of the corresponding bridge arm.

[0080] Schematically, taking sub-converter a as an example, the low-frequency circulating current realizes the power balance inside the bridge arm and satisfies that the DC components of the three bridge arm active powers ua, va, and wa are 0, and the amplitudes of the low-frequency circulating current inside the bridge arms ua, va, and wa are obtained.

[0081] In some embodiments of the present invention, Figure 5 As shown, the method further includes:

[0082] The circulating current control instruction of the bridge arm is generated according to the instantaneous value reference instruction of the power frequency circulating current of the bridge arm and the instantaneous value instruction of the low-frequency circulating current, namely: Where: is the circulation of bridge arm xy; Reference instruction for the instantaneous value of power frequency circulating current; Instantaneous value command of low-frequency circulating current.

[0083] The fault ride-through method based on a distributed energy-consuming M3C (M3C) system, provided in an embodiment of the present invention, involves switching on all energy-consuming units when the average capacitor voltage exceeds a first threshold; switching off all energy-consuming units when the average capacitor voltage falls below a second threshold; and rotating the switching on and off of the energy-consuming units according to preset rules when the average capacitor voltage falls below the first threshold and exceeds the second threshold. This method, based on a rotating switching strategy for the energy-consuming units in the bridge arms, rapidly absorbs system backlog power, significantly enhancing the flexibility and reliability of the backlog power absorption mechanism under different operating conditions. It effectively suppresses the submodule capacitor voltage to within 1.1 pu, suppresses overvoltage in the M3C submodules, and improves the system's ability to maintain continuous grid connection during transient faults. Furthermore, by installing energy-consuming units consisting of switching devices and energy-consuming resistors in the full-bridge submodules of the three symmetrical M3C arms, a distributed energy-consuming M3C is formed, fully utilizing the topological symmetry to absorb backlog power. By establishing a capacitor voltage control mechanism between sub-converters and an unbalanced power compensation mechanism within the sub-converters, coordinated regulation of power-frequency circulating current and low-frequency circulating current is achieved, maintaining dynamic energy balance between the bridge arms.

[0084] The offshore wind power flexible low-frequency transmission system based on the decentralized energy-consuming M3C and the fault ride-through method provided in the above embodiment are used to conduct experiments on three-phase symmetrical faults and BC two-phase grounding asymmetrical faults.

[0085] in, Figure 6The demonstration demonstrated how a three-phase symmetrical fault on the power frequency side of an offshore wind power flexible low-frequency transmission system could be handled. The circulating current control strategy and energy consumption unit switching strategy were used to dissipate accumulated power, achieving dynamic energy balance between the power frequency and low-frequency sub-converters and across all nine bridge arms. Furthermore, when the capacitor voltage of a submodule within the M3C exceeds 1.1 pu, the energy is dissipated by the energy consumption units in the three symmetrical bridge arms of the distributed energy-dissipating M3C, preventing overvoltage in the M3C submodules.

[0086] Figure 7 The results show that when a BC two-phase grounding asymmetric fault occurs on the power frequency side of the offshore wind power flexible low-frequency transmission system, the accumulated power can be absorbed according to the circulating current control strategy and the energy consumption unit switching strategy, and the energy between the sub-converters on the power frequency side and the low-frequency side can still be dynamically balanced. The dynamic energy balance between the 9 bridge arms is achieved, ensuring that when the capacitor voltage of the sub-module in the M3C exceeds 1.1pu, the energy is consumed by the energy consumption units in the symmetrical 3 bridge arms of the distributed energy-consuming M3C, preventing overvoltage of the M3C sub-module.

[0087] Those skilled in the art will appreciate that the accompanying drawings are merely schematic diagrams of an embodiment, and the modules or processes in the accompanying drawings are not necessarily required to implement the present invention.

[0088] Each embodiment in this specification is described in a progressive manner. The same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments. In particular, for method or system embodiments, since they are basically similar to method embodiments, the description is relatively simple. For relevant parts, refer to the partial description of the method embodiment. The method and system embodiments described above are merely illustrative. The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of this embodiment. Ordinary technicians in this field can understand and implement it without expending creative work.

[0089] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A flexible low-frequency transmission system for offshore wind power based on a distributed energy-consuming M3C, characterized in that: The system includes an offshore wind farm, a distributed energy-consuming M3C inverter, and an industrial frequency power grid connected in sequence, wherein the distributed energy-consuming M3C inverter includes three sub-inverters, each of which includes a full-bridge sub-module and an energy-consuming unit; The energy consumption unit is connected in parallel with the full-bridge submodule. The energy consumption unit is configured to be put into operation when the average value of the capacitor voltage of the full-bridge submodule exceeds a first threshold to consume accumulated power, and to be cut off when the average value of the capacitor voltage is lower than a second threshold. When the average value of the capacitor voltage is lower than the first threshold and exceeds the second threshold, the energy consumption unit is switched on and off in rotation according to a preset rule.

2. The system according to claim 1, wherein: The energy consumption unit includes an energy consumption resistor and a controllable switch device connected in series with the energy consumption resistor, and the current flowing through the energy consumption resistor is less than the locking current of the controllable switch device.

3. The system according to claim 2, characterized in that The power dissipated by the energy consumption unit satisfies the following constraints: Among them, P N is the rated power delivered by the system, N is the total number of energy consuming units, R SM is the resistance of the energy dissipation resistor; U SM is the rated voltage of the full-bridge submodule capacitor.

4. A fault ride-through method for an offshore wind power flexible low-frequency transmission system based on a decentralized energy-consuming M3C according to any one of claims 1 to 3, characterized in that: The method comprises: Detect the average value of the capacitor voltage of the full-bridge submodule; When the average value of the capacitor voltage exceeds a first threshold, all energy-consuming units are put into operation; When the average value of the capacitor voltage is lower than a second threshold, cutting off all energy-consuming units; When the average value of the capacitor voltage is lower than a first threshold value and exceeds a second threshold value, the energy consumption units are switched on and off in turn according to a preset rule.

5. The method according to claim 4, characterized in that The step of switching the energy consuming units in rotation according to a preset rule includes: Calculating the duty cycle of the energy consumption unit put into PWM, and at the same time, determining the starting value of the energy consumption unit to be put into use in the current cycle according to the starting value of the energy consumption unit in the previous cycle and the number of energy consumption units put into use; The energy-consuming units are switched on and off in rotation according to the duty cycle and the starting value of the energy-consuming units that need to be put into use in the current cycle.

6. The method according to claim 4, characterized in that The method further comprises: The power frequency circulating current is controlled according to the DC component control equation of the power frequency circulating current between the sub-converters to achieve dynamic energy balance between the low-frequency sub-converters: in, are the power frequency circulating current amplitudes of sub-converters a, b, and c, respectively. is the reference value of the total capacitor voltage between M3C sub-converters, u a,sum 、u b,sum are the sum of the capacitor voltages of the bridge arms a and b of the low-frequency side sub-converter, k P 、k I are the proportional and integral control parameters of the PI controller respectively.

7. The method according to claim 4, characterized in that The method further comprises: Low-frequency circulating current control is performed based on the unbalanced active power of the bridge arm within the sub-converter, the d-axis positive sequence component of the low-frequency side voltage, and the low-frequency circulating current amplitude of the bridge arm to achieve dynamic energy balance between the bridge arms within the low-frequency sub-converter: Where ΔP ua , ΔP va are the unbalanced active powers of the corresponding bridge arms respectively; is the d-axis positive sequence component of the low-frequency side voltage; (x=u, v, w; y=a, b, c) is the low-frequency circulating current amplitude of the corresponding bridge arm.

8. The method according to claim 4, characterized in that The method further comprises: The circulating current control instruction of the bridge arm is generated according to the power frequency circulating current instantaneous value reference instruction and the low frequency circulating current instantaneous value instruction of the bridge arm.

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