Midpoint self-balancing three-level rectifier system

Abstract

The invention discloses a midpoint self-balancing three-level rectification system, including a transformer, a filter, and a three-level rectifier, wherein the transformer is used to convert the grid voltage into a three-phase four-wire AC voltage by adopting a star connection in the secondary winding; the filter is connected between the transformer and the three-level rectifier, used for midpoint balance, and filtering out the harmonic current components from the three-level rectifier to reduce the harmonic pollution to the grid; the three-level rectifier is connected to the filter, and is used to convert the three-phase four-wire AC voltage from which the harmonic components are filtered out by the filter into an adjustable stable and balanced three-level DC voltage. The midpoint self-balancing three-level rectification system provided by the invention can balance the midpoint without a complex switch control strategy, has good system robustness, is easy to debug and use, and is applicable to various working conditions; and has a low implementation cost, which is comparable to the cost of a general three-level rectifier.

Description

Technical Field

[0001] The invention relates to the field of power electronics, and in particular to a midpoint self-balancing three-level rectification system. Background Art

[0002] The rectifier system generally consists of three parts: transformer, filter and rectifier. When the rectifier system capacity is small and the incoming line voltage meets the matching requirements of the rectifier system, the transformer can be omitted. Since rectifiers are mostly composed of nonlinear switching devices, they will inevitably bring harmonics. In order to reduce the harmonic pollution to the power grid, it is usually necessary to configure the corresponding filter according to the rectifier. From the principle point of view, rectifiers can be divided into three categories: uncontrolled rectification, semi-controlled rectification and fully controlled rectification.

[0003] Uncontrolled rectification is composed of diodes, and the purpose of rectification is achieved by the natural commutation effect between the phase voltages. Semi-controlled rectification is composed of thyristors, also known as phase-controlled rectification. The rectification output is adjusted by controlling the phase time of conduction. The three-level DC voltage can be adjusted within the range of [0~1.35*incoming line voltage] (when the semi-controlled rectification is 6-pulse rectification). Fully controlled rectification is composed of fully controllable tubes (CMOS or IGBT), and the purpose of rectification is achieved by adjusting the amplitude and phase of the generated PWM modulation wave. Fully controlled rectification can generally be adjusted within the range of [1.35*incoming line voltage~device voltage level]. Its active power and reactive power functions can be easily controlled, which is a relatively superior rectification mode.

[0004] The rectifier is a special application based on the inverter structure. Its hardware structure is basically the same as the inverter, but it has a special configuration application. For three-level inverters, there are two main topologies, diode clamped and flying capacitor.

[0005] Flying capacitor topology, such as Figure 1 As shown in the figure, it was first proposed by TA Meynard and H. Foch. A suspended capacitor is introduced to replace the clamping diode, and the capacitor voltage difference is balanced by flexibly configuring the switch combination to achieve the purpose of bus balancing. This topology has unique advantages: due to the existing switch combinations, these combinations can be used to balance the clamping capacitor voltage balance problem. However, the disadvantages are also obvious: a large number of capacitors are used in this structure, but the capacitor is a device with poor reliability, short life, and large size; the freedom of switch selection is relatively large, and the modulation strategy is relatively complex.

[0006] Midpoint clamp topology, such as Figure 2As shown, it is also called diode clamping, which was first proposed by Nabae et al., a scholar from Nagaoka University of Science and Technology. It is clamped by two series diodes connected across each phase bridge arm. The diode clamp topology can control the power flow in both directions and control the power factor very conveniently. For the rectifier, the main function is to provide a stable DC power supply for inverter modulation. In terms of control algorithms, there are mainly two types: indirect current control and DC current control. Both control the capacitor current injected into the DC side to achieve the purpose of controlling the three-level DC voltage, where:

[0007] In indirect current control, the rectifier AC current does not participate in the control operation, and the purpose of controlling the three-level DC voltage is achieved through the difference between the voltage controlled by itself and the grid voltage. The indirect current control algorithm is simple to implement, but the dynamic response is poor.

[0008] In direct current control, the rectifier current participates in the control, which is more direct and effective. Therefore, compared with indirect current control, it has superior dynamic performance.

[0009] Although three-level rectification has only one more level than two-level rectification, it introduces an inevitable problem, the balance of the two three-level DC voltages. Therefore, the three-level rectifier not only needs to provide a stable and reliable DC power supply for the inverter, but also needs to ensure the balance of the two DC buses.

[0010] There are two main methods for three-level rectification in application: dual controller and single controller.

[0011] Dual controller, as the name implies, consists of two two-level rectifier controllers connected in series, such as Figure 3 As shown, each rectifier is responsible for stabilizing a section of direct current, thereby achieving the effect of balancing the current. Since two rectifiers are used, the system integration is not high, and the implementation cost will also be high. In addition, in order to realize the series connection of the two rectifiers, the two rectifiers are electrically isolated, which is usually achieved by using a transformer with double secondary windings, which will also affect the implementation cost.

[0012] In the single-controller method, the three-level rectifier is composed of one controller, which generally adopts a specially designed switch control strategy, similar to the flying capacitor inverter, and uses different switch combinations to balance the voltage of the two DC sections. The single-controller method has a high degree of integration, but it requires detailed switching rules to be formulated during implementation, which makes it difficult to implement.

[0013] Therefore, the high hardware cost or complex software modulation of the existing three-level rectifier is a technical problem that needs to be solved urgently. Summary of the invention

[0014] The invention provides a midpoint self-balancing three-level rectifier system to solve the technical problems of high hardware cost or complex software modulation in the existing three-level rectifier.

[0015] The technical solution adopted by the present invention is as follows:

[0016] The present invention provides a midpoint self-balancing three-level rectification system, comprising a transformer, a filter, and a three-level rectifier, wherein:

[0017] Transformer, used to convert the grid voltage into three-phase four-wire AC voltage using star connection in the secondary winding;

[0018] The filter is connected between the transformer and the three-level rectifier, and is used to balance the three-level midpoint voltage and filter out the harmonic components from the three-level rectifier to reduce harmonic pollution to the power grid;

[0019] The three-level rectifier is connected to the filter and is used to convert the three-phase four-wire AC voltage whose harmonic components are filtered out by the filter into an adjustable stable and balanced three-level DC voltage.

[0020] Furthermore, the three-level rectifier includes a rectifier and a rectifier controller.

[0021] A rectifier, connected to the filter, for converting the three-phase four-wire AC voltage from which harmonic components are filtered out by the filter into a three-level DC voltage;

[0022] The rectifier controller is connected to the rectifier and is used to control the rectifier to output a preset PWM waveform; it is also used to modulate the output PWM waveform according to the bus voltage so that the three-level DC voltage output by the rectifier is stabilized within the set voltage threshold.

[0023] Furthermore, the midpoint self-balancing three-level rectification system further includes a three-level DC voltage sampling module.

[0024] A three-level DC voltage sampling module is connected to the output end of the rectifier and is used to collect the three-level DC voltage output by the rectifier;

[0025] The rectifier controller includes a modulation control module and a voltage control module.

[0026] A modulation control module is connected to the rectifier and is used to generate a PWM modulation signal to control the rectifier to generate a desired voltage modulation signal;

[0027] The voltage control module is connected to the three-level DC voltage sampling module and is used to perform closed-loop control on the three-level DC voltage collected by the three-level DC voltage sampling module so that the three-level DC voltage output by the rectifier converges within a set voltage threshold.

[0028] Furthermore, the modulation control module includes a phase modulation control unit and an amplitude modulation control unit.

[0029] A phase modulation control unit is connected to the rectifier and is used to control the modulation voltage phase of the rectifier to lag behind the grid voltage phase, so that active energy flows from the grid into the rectifier, so that the three-level DC voltage output by the rectifier can be raised; or control the modulation voltage phase of the rectifier to lead the grid voltage phase, so that active energy flows from the rectifier to the grid, so that the three-level DC voltage output by the rectifier can be reduced;

[0030] The amplitude modulation control unit is connected to the rectifier and is used to control the modulation voltage amplitude of the rectifier to be smaller than the grid voltage amplitude, so that the reactive energy flows into the rectifier from the grid and the rectifier is inductive to the outside; or control the modulation voltage amplitude of the rectifier to be larger than the grid voltage amplitude, so that the reactive energy flows out of the rectifier to the grid and the rectifier is capacitive to the outside.

[0031] Furthermore, the midpoint self-balancing three-level rectification system further includes an AC current sampling module.

[0032] An AC current sampling module is connected to the output end of the transformer and is used to collect the three-phase current output by the transformer;

[0033] The rectifier controller also includes a current decoupling module and a current control module.

[0034] A current decoupling module is connected to the AC current sampling module and is used to decouple the three-phase current collected by the AC current sampling module to obtain an active DC component and a reactive DC component;

[0035] The current control module is connected to the current decoupling module and is used to perform closed-loop control on the active DC component and reactive DC component decoupled by the current decoupling module, so that the active DC component and reactive DC component converge within a set current threshold.

[0036] Furthermore, the current decoupling module includes an equal-amplitude Clarke transformation unit and an equal-power Park transformation unit.

[0037] The equal-amplitude Clarke transformation unit is connected to the AC current sampling module and is used to transform the three-phase current collected by the AC current sampling module into a two-phase rotating current by using the equal-amplitude Clarke transformation;

[0038] The equal power Park transformation unit is connected to the equal amplitude Clark transformation unit and is used for converting the two-phase rotating current into an active DC component and a reactive DC component by adopting equal power Park transformation.

[0039] Furthermore, the current control module includes an active current control unit and a reactive current control unit.

[0040] An active current control unit is connected to the equal power Park conversion unit and is used to control the active DC component converted by the equal power Park conversion unit to output a stable active current;

[0041] The reactive current control unit is connected to the equal power Park conversion unit and is used to control the reactive DC component converted by the equal power Park conversion unit to output a stable reactive current.

[0042] Furthermore, the rectifier controller also includes a fault protection module.

[0043] The fault protection module is connected to the rectifier and is used to monitor the operating information of the rectifier and compare it with the preset fault protection value so as to block the pulse in time when a fault occurs to protect the equipment.

[0044] Furthermore, the three-level rectifier adopts a four-wire input line and outputs a three-level PWM waveform, wherein the four-wire input line includes an A-phase line, a B-phase line, a C-phase line and a neutral line.

[0045] Furthermore, the midpoint self-balancing three-level rectification system further includes a three-level inverter.

[0046] The three-level inverter is connected to the three-level rectifier and is used for converting the three-level PWM waveform output by the three-level rectifier into a three-level sinusoidal waveform.

[0047] The present invention has the following beneficial effects:

[0048] The midpoint self-balancing three-level rectification system provided by the present invention adopts a transformer, a filter and a three-level rectifier, and outputs a balanced and stable three-level DC voltage by cooperating with each other. The structure is simple, easy to implement and cost-effective. Compared with a general three-level rectifier, the midpoint self-balancing three-level rectification system provided by the present invention can complete the three-level midpoint voltage balance without a complex switch planning strategy, and the system has good robustness, is easy to debug and use, and is applicable to various working conditions; and the implementation cost is low, which is comparable to the cost of a general three-level rectifier.

[0049] In addition to the above-described purposes, features and advantages, the present invention has other purposes, features and advantages. The present invention will be further described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The drawings constituting a part of this application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0051] Figure 1 It is a circuit diagram of a three-level inverter of an existing flying capacitor topology structure;

[0052] Figure 2 It is a circuit diagram of a three-level inverter with an existing midpoint clamped topology structure;

[0053] Figure 3 It is a circuit diagram of an existing three-level rectifier using dual controllers;

[0054] Figure 4 It is a structural block diagram of a first preferred embodiment of a midpoint self-balancing three-level rectifier system of the present invention;

[0055] Figure 5 It is a structural block diagram of a second preferred embodiment of a midpoint self-balancing three-level rectification system of the present invention;

[0056] Figure 6 It is a structural block diagram of a third preferred embodiment of a midpoint self-balancing three-level rectifier system of the present invention;

[0057] Figure 7 is a structural block diagram of a fourth preferred embodiment of a midpoint self-balancing three-level rectifier system of the present invention;

[0058] Figure 8 It is a schematic diagram of the PWM waveform output by the existing two-level rectifier;

[0059] Fig. 9 It is a schematic diagram of the PWM waveform output by the three-level rectifier in the midpoint self-balancing three-level rectification system of the present invention.

[0060] Description of Figure Numbers:

[0061] 10. Transformer; 20. Filter; 30. Three-level rectifier; 31. Rectifier; 32. Rectifier controller; 40. Three-level DC voltage sampling module; 321. Modulation control module; 322. Voltage control module; 3211. Phase modulation control unit; 3212. Amplitude modulation control unit; 50. AC current sampling module; 323. Current decoupling module; 324. Current control module; 3231. Equal amplitude Clarke transformation unit; 3232. Equal power Park transformation unit; 3241. Active current control unit; 3242. Reactive current control unit; 325. Fault protection module; 60. Three-level inverter. DETAILED DESCRIPTION

[0062] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0063] Reference Figure 4The preferred embodiment of the present invention provides a midpoint self-balancing three-level rectification system, including a transformer 10, a filter 20, and a three-level rectifier 30, wherein the transformer 10 is used to convert the grid voltage into a three-phase four-wire AC voltage by adopting a star connection in the secondary winding; the filter 20 is connected between the transformer 10 and the three-level rectifier 30, and is used to balance the three-level midpoint voltage and filter out the harmonic components from the three-level rectifier 30 to reduce the harmonic pollution to the grid; the three-level rectifier 30 is connected to the filter 20, and is used to convert the three-phase four-wire AC voltage from which the harmonic components are filtered out by the filter 20 into an adjustable stable and balanced three-level DC voltage. Wherein, please see Figure 4 , the transformer 10 is mainly used for voltage matching conversion, the secondary winding adopts a star connection, and the output is a four-wire output line. Its impedance needs to be specially designed according to the rectifier 30. The filter 20 is to filter out the harmonics generated when the three-level rectifier 30 is running, and reduce the harmonic pollution to the power grid. It needs to be designed accordingly with the three-level rectifier 30. The three-level rectifier 30 is a device for converting AC to DC, and is the core device in the rectifier system. In this embodiment, the three-level rectifier 30 adopts a four-wire incoming line, which is respectively an A phase line, a B phase line, a C phase line and a neutral line. There are four IGBT fully controlled devices in each phase of the three-level rectifier 30, and each phase has two clamping diodes connected in series and freewheeling diodes connected in anti-parallel with the four IGBT fully controlled devices. The A phase line, the B phase line, the C phase line and the neutral line are respectively connected between the two clamping diodes connected in series, and the modulation process is realized by controlling the conduction or closing of the IGBT. The intermediate potential of the three-level rectifier 30 is embedded in the DC0 potential.

[0064] When the rectifier 30 is working, it can be considered as a controllable voltage source, and the voltage amplitude and phase of the output can be flexibly adjusted. When the three-level rectifier 30 is connected to the power grid, by reasonably controlling the modulation amplitude and phase of the three-level rectifier 30, the active current or reactive current injected into the three-level rectifier 30 can be controlled, thereby realizing the function of converting AC to DC.

[0065] The midpoint self-balancing three-level rectification system provided in this embodiment adopts a transformer, a filter and a three-level rectifier, and outputs a balanced and stable three-level DC voltage by cooperating with each other. It has a simple structure, is easy to implement and has superior cost. Compared with a general three-level rectifier, the midpoint self-balancing three-level rectification system provided in this embodiment can complete the three-level midpoint voltage balance without a complex switch planning strategy, has good system robustness, is easy to debug and use, and is applicable to various working conditions; and has a low implementation cost, which is comparable to the cost of a general three-level rectifier.

[0066] Preferably, if Figure 5As shown, the midpoint self-balancing three-level rectifier system provided in this embodiment, the three-level rectifier 30 includes a rectifier 31 and a rectifier controller 32. The rectifier 31 is connected to the filter 20 and is used to convert the three-phase four-wire AC voltage from which the filter 20 filters out the harmonic components into a three-level DC voltage; the rectifier controller 32 is connected to the rectifier 31 and is used to control the rectifier 31 to output a preset PWM waveform; it is also used to modulate the output PWM waveform according to the bus voltage so that the three-level DC voltage output by the rectifier 31 is stable within the set voltage threshold. Among them, the rectifier 31 is a diode-clamped three-level controllable rectifier, which is an electric power device that converts AC into DC. The DC neutral point of the rectifier 31 adopts a high-resistance grounding method, which reduces the insulation level of the system and can also effectively suppress the ground fault current.

[0067] Preferably, if Figure 6 As shown, the midpoint self-balancing three-level rectifier system provided in this embodiment also includes a three-level DC voltage sampling module 40, which is connected to the output end of the rectifier 31 and is used to collect the three-level DC voltage output by the rectifier 31; the rectifier controller 32 includes a modulation control module 321 and a voltage control module 322, and the modulation control module 321 is connected to the rectifier 31 and is used to generate a PWM modulation signal to control the rectifier 31 to generate an expected voltage modulation signal. The voltage control module 322 is connected to the three-level DC voltage sampling module 40 and is used to perform closed-loop control on the three-level DC voltage collected by the three-level DC voltage sampling module 40, so that the three-level DC voltage output by the rectifier 31 converges within a set voltage threshold.

[0068] The midpoint self-balancing three-level rectifier system provided in this embodiment adopts a modulation control module and a voltage control module. The voltage control module performs closed-loop control on the three-level DC voltage collected by the three-level DC voltage sampling module, so that the three-level DC voltage output by the rectifier converges within the set voltage threshold; the modulation control module generates a PWM modulation signal to control the rectifier 31 to generate an expected voltage modulation signal, so that the rectifier outputs a balanced and stable three-level DC voltage. The midpoint self-balancing three-level rectifier system provided in this embodiment has a simple structure, easy implementation, and superior cost. Compared with general three-level rectifiers, the midpoint voltage balance can be achieved without complex switch planning strategies, the system has good robustness, convenient debugging and use, and is applicable to various working conditions; and the implementation cost is low, which is comparable to the cost of general three-level rectifiers.

[0069] Preferably, if Figure 6As shown, in the midpoint self-balancing three-level rectifier system provided in this embodiment, the rectifier 31 is a phase voltage modulation rectifier. Different from the general rectifier, the rectifier 31 in this embodiment adopts a four-wire system. Due to the introduction of the neutral line, the modulation of the rectifier 31 adopts a phase voltage modulation method. Specifically, the modulation control module 321 includes a phase modulation control unit 3211 and an amplitude modulation control unit 3212. The phase modulation control unit 3211 is connected to the rectifier 31 and is used to control the modulation voltage phase of the rectifier 31 to lag behind the grid voltage phase, so that the active energy flows from the grid into the rectifier 31, so that the three-level DC voltage output by the rectifier 31 can be raised; or control the modulation voltage phase of the rectifier 31 to be ahead of the grid voltage phase, so that the active energy flows from the rectifier 31 to the grid, so that the three-level DC voltage output by the rectifier 31 can be reduced; the amplitude modulation control unit 3212 is connected to the rectifier 31 and is used to control the modulation voltage amplitude of the rectifier 31 to be smaller than the grid voltage amplitude, so that the reactive energy flows from the grid into the rectifier 31, so that the rectifier 31 is inductive to the outside; or control the modulation voltage amplitude of the rectifier 31 to be larger than the grid voltage amplitude, so that the reactive energy flows from the rectifier 31 to the grid, so that the rectifier 31 is capacitive to the outside.

[0070] In this embodiment, by reasonably controlling the amplitude and phase of the rectifier modulation voltage through the rectifier controller, the DC voltage value can be flexibly modulated, and even part of the reactive power of the power grid can be compensated.

[0071] Preferably, if Figure 6As shown, the midpoint self-balancing three-level rectification system provided in this embodiment also includes an AC current sampling module 50, which is connected to the output end of the transformer 10 and is used to collect the three-phase current output by the transformer 10; the rectifier controller 32 also includes a current decoupling module 323 and a current control module 324. The current decoupling module 323 is connected to the AC current sampling module 50 and is used to decouple the three-phase current collected by the AC current sampling module 50 to obtain an active DC component and a reactive DC component; the current control module 324 is connected to the current decoupling module 323 and is used to perform closed-loop control on the active DC component and the reactive DC component decoupled by the current decoupling module 323, so that the active DC component and the reactive DC component converge within a set current threshold. Specifically, the current decoupling module 323 includes an equal-amplitude Clarke transformation unit 3231 and an equal-power Park transformation unit 3232. The equal-amplitude Clarke transformation unit 3231 is connected to the AC current sampling module 50 and is used to use Clarke (equal-amplitude Clarke) transformation to convert the three-phase current collected by the AC current sampling module 50 into a two-phase rotating current; the equal-power Park transformation unit 3232 is connected to the equal-amplitude Clarke transformation unit 3231 and is used to use Park (equal-power Park) transformation to convert the two-phase rotating current into an active DC component and a reactive DC component. The current control module 324 includes an active current control unit 3241 and a reactive current control unit 3242. The active current control unit 3241 is connected to the equal-power Park transformation unit 3232 and is used to control the active DC component converted by the equal-power Park transformation unit 3232 to output a stable active current; the reactive current control unit 3242 is connected to the equal-power Park transformation unit 3232 and is used to control the reactive DC component converted by the equal-power Park transformation unit 3232 to output a stable reactive current.

[0072] The midpoint self-balancing three-level rectification system provided in this embodiment is as follows: Figure 6 As shown, its working principle is as follows:

[0073] The rectifier controller 32 controls the rectifier 31 to work stably according to the bus voltage and the incoming current, and provides a reliable DC power supply. The three-level DC voltage sampling module 40 collects the three-level DC voltage output by the rectifier 31; the voltage control module 322 compares the three-level DC voltage collected by the three-level DC voltage sampling module 40 with the preset voltage threshold. If the collected three-level DC voltage is less than the set voltage threshold, the modulation control module 321 is controlled to adjust the output amplitude and phase, and the active current control unit 3241 increases the active current entering the rectifier 31, and the DC voltage will increase. At the same time, the reactive current of the reactive current control unit 3242 is stabilized at the set value.

[0074] Preferably, if Figure 6 As shown, in the midpoint self-balancing three-level rectifier system provided in this embodiment, the rectifier controller 32 also includes a fault protection module 325, which is connected to the rectifier 31 and is used to monitor the operation information of the rectifier 31 and compare it with the preset fault protection value to block the pulse in time when a fault occurs to protect the device. The fault protection module 325 has various protection functions, such as overvoltage, overcurrent and overtemperature protection.

[0075] The midpoint self-balancing three-level rectifier system provided in this embodiment monitors the operating information of the rectifier through a fault protection module and compares it with a preset fault protection value to block the pulse in time when a fault occurs to protect the equipment, thereby improving the safety of the equipment.

[0076] Preferably, if Figure 7 As shown, in the midpoint self-balancing three-level rectification system provided by the present embodiment, the three-level rectifier 30 adopts a four-wire input line and outputs a three-level PWM waveform, wherein the four-wire input line includes an A-phase line, a B-phase line, a C-phase line and a neutral line. The midpoint self-balancing three-level rectification system also includes a three-level inverter 60, which is connected to the three-level rectifier 30 and is used to convert the three-level PWM waveform output by the three-level rectifier 30 into a three-level sinusoidal waveform.

[0077] The midpoint self-balancing three-level rectifier system provided in this embodiment has an additional level in the three-level rectifier 30 compared to the two-level rectifier, and the waveform quality is better. Figure 8 and Fig. 9 As shown, under the same conditions, the PWM waveforms output by the two-level rectifier and the three-level rectifier 30 are shown. It can be seen that the modulation voltage in the PWM waveform output by the three-level rectifier 30 is more stepped, closer to a sine wave, and can obtain a better waveform quality.

[0078] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A midpoint self-balancing three-level rectification system, characterized in that: It comprises a transformer (10), a filter (20), and a three-level rectifier (30), wherein: The transformer (10) is used to convert the grid voltage into a three-phase four-wire AC voltage by adopting a star connection method in the secondary winding; The filter (20) is connected between the transformer (10) and the three-level rectifier (30), and is used to balance the three-level midpoint voltage and filter out harmonic components from the three-level rectifier (30) to reduce harmonic pollution to the power grid; The three-level rectifier (30) is connected to the filter (20) and is used to convert the three-phase four-wire AC voltage from which the harmonic components are filtered out by the filter (20) into an adjustable stable and balanced three-level DC voltage; the three-level rectifier (30) adopts a four-wire input line and outputs a three-level PWM waveform, wherein the four-wire input line includes an A-phase line, a B-phase line, a C-phase line and a neutral line; each phase of the three-level rectifier (30) has four IGBT fully-controlled devices, each phase has two clamping diodes connected in series and freewheeling diodes respectively connected in anti-parallel with the four IGBT fully-controlled devices; the A-phase line, the B-phase line, the C-phase line and the neutral line are respectively connected between the two clamping diodes connected in series; the modulation process is realized by controlling the conduction or shutdown of the IGBT; and the intermediate potential of the three-level rectifier (30) is embedded in the DC0 potential; The three-level rectifier (30) comprises a rectifier (31) and a rectifier controller (32); the rectifier (31) is connected to the filter (20) and is used to convert the three-phase four-wire AC voltage from which the harmonic components are filtered out by the filter (20) into a three-level DC voltage; the rectifier controller (32) is connected to the rectifier (31) and is used to control the rectifier (31) to output a preset PWM waveform; and is also used to modulate the output PWM waveform according to the bus voltage so that the three-level DC voltage output by the rectifier (31) is stabilized within a set voltage threshold; The midpoint self-balancing three-level rectification system further comprises a three-level DC voltage sampling module (40), the three-level DC voltage sampling module (40) being connected to the output end of the rectifier (31) and being used to collect the three-level DC voltage output by the rectifier (31); the rectification controller (32) comprising a modulation control module (321) and a voltage control module (322), the modulation control module (321) being connected to the rectifier (31) and being used to generate a PWM modulation signal to control the rectifier (31) to generate a preset voltage modulation signal; the voltage control module (322) being connected to the three-level DC voltage sampling module (40) and being used to perform closed-loop control on the three-level DC voltage collected by the three-level DC voltage sampling module (40) so that the three-level DC voltage output by the rectifier (31) converges within a set voltage threshold; The modulation control module (321) comprises a phase modulation control unit (3211) and an amplitude modulation control unit (3212). The phase modulation control unit (3211) is connected to the rectifier (31) and is used to control the modulation voltage phase of the rectifier (31) to lag behind the grid voltage phase, so that active energy flows from the grid into the rectifier (31), so that the three-level DC voltage output by the rectifier (31) can be raised; or control the modulation voltage phase of the rectifier (31) to lead the grid voltage phase, so that active energy flows from the rectifier (31) to the grid voltage phase. 1) flows out to the grid so that the three-level DC voltage output by the rectifier (31) can be reduced; the amplitude modulation control unit (3212) is connected to the rectifier (31) and is used to control the modulation voltage amplitude of the rectifier (31) to be smaller than the grid voltage amplitude, so that the reactive energy flows from the grid into the rectifier (31), so that the rectifier (31) is inductive to the outside; or control the modulation voltage amplitude of the rectifier (31) to be larger than the grid voltage amplitude, so that the reactive energy flows out from the rectifier (31) to the grid, so that the rectifier (31) is capacitive to the outside; The midpoint self-balancing three-level rectification system further comprises an alternating current sampling module (50), the alternating current sampling module (50) being connected to the output end of the transformer (10) and being used to collect the three-phase current output by the transformer (10); the rectification controller (32) further comprises a current decoupling module (323) and a current control module (324), the current decoupling module (323) being connected to the alternating current sampling module (50) and being used to decouple the three-phase current collected by the alternating current sampling module (50) to obtain an active direct current component and a reactive direct current component; the current control module (324) being connected to the current decoupling module (323) and being used to perform closed-loop control on the active direct current component and the reactive direct current component obtained by decoupling the current decoupling module (323) so that the active direct current component and the reactive direct current component converge within a set current threshold.

2. The midpoint self-balancing three-level rectification system according to claim 1, characterized in that: The current decoupling module (323) comprises an equal-amplitude Clarke transformation unit (3231) and an equal-power Park transformation unit (3232). The equal-amplitude Clarke transformation unit (3231) is connected to the AC current sampling module (50) and is used to transform the three-phase current collected by the AC current sampling module (50) into a two-phase rotating current using an equal-amplitude Clarke transformation; The equal-power Park transformation unit (3232) is connected to the equal-amplitude Clarke transformation unit (3231) and is used to transform the two-phase rotating current into an active DC component and a reactive DC component by using equal-power Park transformation.

3. The midpoint self-balancing three-level rectification system according to claim 2, characterized in that: The current control module (324) comprises an active current control unit (3241) and a reactive current control unit (3242). The active current control unit (3241) is connected to the equal power Park transformation unit (3232) and is used to control the active DC component transformed by the equal power Park transformation unit (3232) to output a stable active current; The reactive current control unit (3242) is connected to the equal power Park transformation unit (3232) and is used to control the reactive DC component transformed by the equal power Park transformation unit (3232) to output a stable reactive current.

4. The midpoint self-balancing three-level rectification system according to claim 1, characterized in that: The rectifier controller (32) further includes a fault protection module (325). The fault protection module (325) is connected to the rectifier (31) and is used to monitor the operation information of the rectifier (31) and compare it with a preset fault protection value, so as to timely block the pulse when a fault occurs to protect the device.

5. The midpoint self-balancing three-level rectification system according to claim 1, characterized in that: The midpoint self-balancing three-level rectification system further comprises a three-level inverter (60), The three-level inverter (60) is connected to the three-level rectifier (30) and is used to convert the three-level PWM waveform output by the three-level rectifier (30) into a three-level sinusoidal waveform.

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