A modular multilevel matrix converter embedded with common-mode voltage cancellation unit and a control method thereof

Abstract

The application discloses a modular multilevel matrix converter embedded with a common-mode voltage offset unit and a control method thereof. Each bridge arm of a main circuit of the modular multilevel matrix converter is formed by cascading a plurality of full-bridge sub-modules and at least one common-mode voltage offset unit. The common-mode voltage offset unit comprises a three-winding transformer, a first group of full-bridge modules, a second group of full-bridge modules and a capacitor C2. Three ports of the three-winding transformer are respectively connected to three first group of full-bridge modules. Each first group of full-bridge module is connected to the capacitor C2 at a direct current side. The capacitor C2 between the direct current sides of the three first group of full-bridge modules forms a three-port power channel through the three-winding transformer, which is used for transferring pulsating power among the three first group of full-bridge modules. The capacitor C2 is connected to the bridge arm through a second group of full-bridge modules at a front stage. The second group of full-bridge modules injects a reverse common-mode voltage into the bridge arm, so that the injected common-mode voltage does not appear at a motor side. The application can effectively eliminate the influence of the common-mode voltage on the operation of the motor.

Description

Technical Field

[0001] This invention relates to the field of power electronic converter technology, and in particular to a modular multilevel matrix converter with embedded common-mode voltage cancellation unit and its control method. Background Technology

[0002] Modular multilevel matrix converters (M3Cs) are widely used in motor drives due to their advantages such as the absence of intermediate DC links, ease of modularization, and bidirectional energy flow. However, when the M3C operates at the same frequency, the power in the M3C bridge arms may contain low-frequency pulsations or even DC current, causing severe voltage fluctuations in the submodule capacitors. If these fluctuations are not suppressed, the system will malfunction. Traditional methods to suppress these voltage fluctuations involve injecting common-mode voltage into the M3C. However, injecting common-mode voltage accelerates motor insulation aging and poses a risk of bearing erosion failure due to the continuous impact of high-frequency shaft currents. Summary of the Invention

[0003] To address the problem that when traditional M3C converters inject common-mode voltage to suppress voltage fluctuations in submodule capacitors under the same frequency operating conditions, the injected common-mode voltage appears on the motor side, accelerating motor insulation aging and increasing the risk of bearing erosion failure, this invention provides a modular multilevel matrix converter with an embedded common-mode voltage cancellation unit and its control method. By embedding a common-mode voltage cancellation unit consisting of a three-winding transformer, two sets of full-bridge modules, and capacitor C2 into the M3C bridge arm, this unit outputs a reverse-injected common-mode voltage, preventing the injected common-mode voltage from appearing on the motor side, thereby eliminating the impact of common-mode voltage on motor operation.

[0004] According to one aspect of the present invention, a modular multilevel matrix converter with an embedded common-mode voltage cancellation unit is provided, comprising a modular multilevel matrix converter main circuit, wherein each bridge arm of the modular multilevel matrix converter main circuit is composed of multiple full-bridge sub-modules and at least one common-mode voltage cancellation unit cascaded together; the common-mode voltage cancellation unit includes a three-winding transformer, a first group of full-bridge modules, a second group of full-bridge modules, and a capacitor C2; the three ports of the three-winding transformer are respectively connected to three of the first group of full-bridge modules, and the turns ratio of the three-winding transformer is 1:1:1; the DC side of each of the first group of full-bridge modules is connected to the capacitor C2, and the three DC-side capacitors C2 of the three first group of full-bridge modules form a three-port power channel through the three-winding transformer for transferring pulsating power between the three first group of full-bridge modules; the front end of the capacitor C2 is connected to the bridge arm through a second group of full-bridge modules.

[0005] As a further technical solution, in the common-mode voltage cancellation unit, the first port of the three-winding transformer is connected to the first full-bridge module corresponding to the first phase, the second port of the three-winding transformer is connected to the first full-bridge module corresponding to the second phase, and the third port of the three-winding transformer is connected to the first full-bridge module corresponding to the third phase; the three second full-bridge modules are respectively connected to the three-phase bridge arms of the modular multilevel matrix converter.

[0006] As a further technical solution, the full-bridge submodule, the first group of full-bridge modules, and the second group of full-bridge modules are all composed of four power switching transistors.

[0007] As a further technical solution, the common-mode voltage cancellation unit injects a reverse common-mode voltage into the bridge arm during operation, so that the common-mode voltage injected by the modular multilevel matrix converter under the same frequency condition does not appear on the motor side.

[0008] As a further technical solution, the three-port power channel is controlled by single-phase-shift modulation, and the direction and magnitude of power transmission are achieved by controlling the phase shift angle between the voltages of each port.

[0009] According to one aspect of the present invention, a control method for the modular multilevel matrix converter with the embedded common-mode voltage cancellation unit is provided, comprising: stabilizing the voltage of capacitor C2 in the common-mode voltage cancellation unit by subtracting the voltage of capacitor C2 from a given value, multiplying the difference signal by the bridge arm current, adjusting it by a proportional controller, superimposing it with a reverse-injected common-mode voltage, and generating a drive signal after modulation to maintain the voltage of capacitor C2 stable at a given value and generate a reverse common-mode voltage signal injected into the bridge arm via the second group of full-bridge modules; and transferring the pulsating power between the DC-side capacitors C2 of the three first group of full-bridge modules by using the voltage of one phase capacitor as a reference and adjusting the voltage deviation between the other two phases and the reference phase to control the three-port power channel to transfer power between the ports.

[0010] As a further technical solution, the three-port power channel is equivalent to two dual-port dual active bridges, which control the phase shift angle between the first port and the second port, and the phase shift angle between the first port and the third port, respectively. The second port and the third port automatically achieve power balance.

[0011] As a further technical solution, the control of the main circuit of the modular multilevel matrix converter includes total input energy control, bridge arm capacitor voltage equalization control, equal frequency operating condition pulsation suppression control, motor side voltage control, input and output current and circulating current control, internal voltage equalization and PWM modulation.

[0012] As a further technical solution, the common-mode voltage injected in reverse by the common-mode voltage cancellation unit is equal in amplitude and opposite in direction to the common-mode voltage injected by the main circuit of the modular multilevel matrix converter.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The modular multilevel matrix converter with embedded common-mode voltage cancellation unit of the present invention can eliminate the common-mode voltage injected when suppressing the voltage fluctuation of the sub-module capacitor by traditional methods, so that it does not appear on the motor side, thereby greatly reducing the impact of common-mode voltage on motor operation;

[0014] 2. Regarding the voltage fluctuation of capacitor C2 in the common-mode voltage cancellation unit, considering the symmetrical voltage pulsation of the capacitor in the corresponding position of the three-phase bridge arm under the same frequency condition, a three-port power channel is added between the three phases to realize the pulsation transfer of the three-phase capacitor; the difference between the voltage of capacitor C2 and the given value of the capacitor voltage is calculated, and then multiplied by the bridge arm current. After passing through the proportional P controller, it is superimposed with the inverted common-mode voltage, and finally modulated to generate a PWM signal, which can maintain the voltage of capacitor C2 at the given value and generate the inverted common-mode voltage signal. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This invention relates to an improved M3C topology with an embedded common-mode voltage cancellation unit; Figure 2 This is a structural diagram of the SM module in the improved M3C proposed in this invention; Figure 3 This is a structural diagram of the New SM module in the improved M3C proposed in this invention; Figure 4 This invention relates to the control of the SM module in the improved M3C. Figure 5 This is the equivalent circuit model of the three-port power channel of the New SM module of this invention; Figure 6 This is the equivalent circuit diagram of the two-port DAB in this invention; Figure 7 This is a typical working waveform of the dual-port DAB in this invention; Figure 8 This is a control block diagram of the New SM module in this invention; Figure 9 This is the capacitor voltage waveform in the improved M3C of this invention; Figure 10 This is a comparison of the common-mode voltage of the improved M3C and the traditional M3C in this invention; Figure 11 This is a comparison of the AR bridge arm voltages of the improved M3C and the traditional M3C in this invention; Figure 12 This is the port voltage waveform of the three-port power channel in the improved M3C of this invention. Detailed Implementation

[0017] The present invention proposes an improved M3C topology with an embedded common-mode voltage cancellation unit. The common-mode voltage cancellation unit, consisting of a three-winding transformer, two sets of full-bridge modules and capacitor C2, is embedded in the M3C bridge arm. This unit emits a reverse-injected common-mode voltage, so that the injected common-mode voltage does not appear on the motor side, thereby eliminating the influence of common-mode voltage on motor operation.

[0018] Considering that the SM module in the improved M3C topology contains low-frequency pulsations or even DC current in the submodule capacitor voltage under the same frequency operating conditions, the traditional injection method, while suppressing the submodule capacitor voltage fluctuation, will generate the same common-mode voltage on the motor side. Over long-term operation, this will accelerate the aging of the motor insulation and lead to the risk of bearing erosion failure due to the continuous impact of high-frequency shaft current. A further improvement of the present invention is that the full-bridge module before capacitor C2 generates a reverse-injected common-mode voltage, preventing the injected common-mode voltage from appearing on the motor side, thereby greatly reducing the impact of common-mode voltage on motor operation.

[0019] A further improvement of the technical solution of the present invention is as follows: For the voltage fluctuation of the common-mode voltage cancellation unit's own capacitor C2, considering the symmetrical characteristics of the voltage pulsation of the module capacitors at the same position of each phase under the same frequency operating condition, a three-port power channel is added between the three phases to realize the pulsation transfer of the three-phase capacitors; for the control of the three-port power channel, single-phase-shift modulation (SPS) is adopted, taking the voltage of a certain phase capacitor as a reference, and adjusting the voltage deviation of the other two phases from it to achieve voltage balance of the three-port power channel.

[0020] A further improvement to the technical solution of this invention lies in the following: Since the three-port power channel only serves to transfer pulsating energy and does not generate energy to maintain the capacitor voltage at a given value, a control loop needs to be designed to keep the voltage of capacitor C2 stable. The difference between the voltage of capacitor C2 and the given capacitor voltage is calculated, multiplied by the bridge arm current, and then superimposed on the inverted common-mode voltage after passing through a proportional P controller. Finally, after modulation, a PWM signal is generated, which can maintain the voltage of capacitor C2 stable at the given value and generate the inverted common-mode voltage signal.

[0021] It should be noted that: The terms “comprising” and “having”, and any variations thereof, in the specification, claims, and accompanying drawings of this invention are intended to cover a non-exclusive inclusion, such as a process, method, system, product, or apparatus that includes a series of steps or units, not necessarily limited to those explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0022] The block diagrams shown in the accompanying drawings are merely functional entities and do not necessarily correspond to physically independent entities. That is, these functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices. The flowcharts shown in the accompanying drawings are merely illustrative and do not necessarily include all content and operations / steps, nor do they necessarily have to be performed in the described order. For example, some operations / steps can be decomposed, while others can be combined or partially combined; therefore, the actual execution order may change depending on the specific circumstances.

[0023] 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, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. In addition, the technical features of the various embodiments or individual embodiments provided by the present invention can be arbitrarily combined to form new technical solutions. Such combinations are not bound by the order of steps and / or structural composition patterns, but must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0024] This invention proposes a modular multilevel matrix converter (i.e., an improved M3C topology) with embedded common-mode voltage cancellation unit, such as... Figure 1 As shown. Where u a u b u c For grid-side voltage; i a i b i c For grid-side current; u r u s u t For motor-side voltage; i r is i t i is the motor-side current; xy (x = a, b, c; y = r, s, t) represents the bridge arm current; L b SM is the bridge arm inductor; O and N are the midpoints of the three-phase systems on the grid side and motor side, respectively; SM is the full-bridge module; New SM is the common-mode voltage cancellation unit.

[0025] The SM module structure in the improved M3C topology proposed in this invention is as follows: Figure 2 As shown, the SM module is a full-bridge structure consisting of four switching transistors and capacitor C1.

[0026] The improved M3C topology common-mode voltage cancellation unit (i.e., the New SM module in the figure) proposed in this invention has the following structure: Figure 3 As shown. The common-mode voltage cancellation unit includes a three-winding transformer, a first set of full-bridge modules, a second set of full-bridge modules, and a capacitor C2. The three ports of the three-winding transformer are respectively connected to three of the first set of full-bridge modules. The DC side of each first set of full-bridge modules is connected to the capacitor C2. The front end of the capacitor C2 is connected to the bridge arm via a second set of full-bridge modules. The turns ratio of the three-winding transformer is 1:1:1. u1, u2, and u3 are the port voltages of the three-winding transformer, and i1, i2, and i3 are the port currents of the three-winding transformer.

[0027] For the control of the SM module in the improved M3C topology, such as Figure 4 As shown, since it is a known technology, it will not be described in detail in this application. The SM module control includes total input energy control, bridge arm capacitor voltage equalization, constant frequency operating condition pulsation suppression, motor side voltage control, input and output current and circulating current control, internal voltage equalization and PWM modulation.

[0028] v Cxy (x = a, b, c; y = r, s, t) represents the capacitor voltage for each bridge arm. A double αβ transformation is performed on the matrix form of the capacitor voltage. , , and This represents the degree of voltage imbalance between the capacitors in the bridge arms within each sub-converter. and This represents the degree of capacitor voltage imbalance between the input sub-converters; and This represents the degree of capacitor voltage imbalance between the output sub-converters; It represents the degree of imbalance in the sum of the overall capacitor voltages. Used for total input energy control, obtaining the d-axis reference value of the grid-side current. ; This is the reference value for the q-axis current on the grid side, which is 0 under the condition of unity power factor in grid connection. , , and The circulating current reference value is generated after equalization of the bridge arm capacitor voltage and suppression of pulsation under constant frequency operating conditions; motor side voltage u r u s and u t v is generated after dq transformation md and v mq The reference value of motor side current is generated by controlling the motor side voltage. and The aforementioned generated current reference value, after being controlled by input / output current and circulating current, is combined with the common-mode voltage v. N The injected energy, after undergoing internal voltage equalization and PWM modulation, is applied to the SM module of the improved M3C.

[0029] Figure 3 The equivalent circuit of the three-port power channel in the New SM module is as follows: Figure 5 As shown in (a) above, the power transmission path is as follows: Figure 5 As shown in (b) in the figure, where L s It is the leakage inductance of the transformer, the equivalent leakage inductance L 12 = L 13 = L 23 = 3L s Assume the pulsating power of the capacitor voltages in the three submodules are p1, p2, and p3, respectively; and the power transferred between the three ports is p1, p2, and p3, respectively. 12 p 13 and p 23 The above power requirements are met:

[0030] (1)

[0031] To prevent circulating power from being introduced between the three ports, the following must also be satisfied:

[0032] (2)

[0033] The control of the three-port power channel can be based on the voltage of phase A capacitor. By adjusting the voltage deviation between the other two phases and phase A, voltage balance of the three-port power channel can be achieved. In actual control, it can be equivalent to two two-port DABs, which can ensure that the capacitor voltages of the three sub-modules are consistent.

[0034] The equivalent circuit of a two-port DAB (two-port dual active bridge) is as follows: Figure 6As shown, V1 and V2 are the amplitudes of port voltages u1 and u2; the dual-port DAB selects single-phase-shift modulation (SPS), and the direction and magnitude of the transmitted power can be controlled by adjusting the phase shift angle between voltages u1 and u2. A typical operating waveform is shown below. Figure 7 As shown; where i 12 For flow through equivalent leakage inductance L 12 Current on; φ 12 Let be the phase shift angle between ports 1 and 2. Under ideal operating conditions, the current expression for each time period is:

[0035] (3)

[0036] In single-phase-shift modulation, the voltage applied across the inductor is completely symmetrical within one cycle, and the inductor current waveform also exhibits symmetry, i.e.:

[0037] (4)

[0038] In the formula, f h Given the switching frequency of the power channel, the transmitted power p can be obtained. 12 :

[0039] (5)

[0040] In the formula, T s The switching cycle.

[0041] The capacitor voltage at each port is determined by power balance, as follows:

[0042] (6)

[0043] Define voltage difference Δu 12 =u1-u2, from equation (6) we can get:

[0044] (7)

[0045] According to equation (5), we can obtain :

[0046] (8)

[0047] The control block diagram between port 1 and port 2 is as follows: Figure 8As shown, the closed-loop output of the capacitor voltage difference is the instantaneous power that the DAB needs to transfer. Adding 1 / (3p1-3p2) as a feedforward term to the control loop improves the system's dynamic response. Ports 1 and 3 use the same control strategy; ports 2 and 3 do not require independent control and can be automatically balanced. Since the three-port power channel only transfers pulsating energy and does not generate energy to maintain the capacitor voltage at a given value, a control loop needs to be designed to keep the voltage of capacitor C2 stable. The voltage v of capacitor C2 is... C2 With the given value of capacitor voltage v C2_ref The difference is calculated, then multiplied by the bridge arm current, and after passing through a proportional P controller, it is combined with the inverted injected common-mode voltage (-V). N The superposition of these signals, after modulation, generates a PWM signal, which can maintain the voltage of capacitor C2 at a given value and generate an inverted common-mode voltage signal.

[0048] Under the aforementioned basic control method, the capacitor voltage waveform of the improved M3C topology is obtained as follows: Figure 9 As shown, the capacitor voltages in both the SM module and the New SM module remain stable. The common-mode voltage waveforms of the improved M3C and the traditional M3C are compared as follows: Figure 10 As shown, for the improved M3C, due to the presence of the common-mode voltage cancellation unit, the injected common-mode voltage under the same frequency operating condition does not appear on the motor side, only containing the common-mode voltage caused by modulation. However, for the traditional M3C, the injected common-mode voltage appears on the motor side, damaging the insulation of the motor windings. A comparison of the AR bridge arm voltages of the improved M3C and the traditional M3C is shown below. Figure 11 As shown, in traditional M3C, the arm voltage exhibits a stepped waveform containing high-frequency components due to the injection of common-mode voltage. In contrast, in the improved M3C, the injected common-mode voltage is canceled out, resulting in a regular stepped waveform for the arm voltage, significantly reducing arm voltage stress and harmonic content. The port voltage waveforms of the three-port power channel in the improved M3C are shown below. Figure 12 As shown, a phase-shifting operation is employed to transfer the pulsating energy between the three capacitors. The topology in this embodiment of the invention can eliminate the common-mode voltage injected when suppressing capacitor voltage fluctuations using traditional methods, preventing it from appearing on the motor side, thereby greatly reducing the impact of common-mode voltage on motor operation.

[0049] In summary, this invention discloses an improved modular multilevel matrix converter (M3C) topology with an embedded common-mode voltage cancellation unit, belonging to the field of power electronic converters. This topology consists of a modular multilevel matrix converter main circuit and a common-mode voltage cancellation unit. The common-mode voltage cancellation unit comprises a three-winding transformer and two sets of full-bridge modules. The three ports of the three-winding transformer are respectively connected to three full-bridge modules. A capacitor C2 is connected to the DC side of each full-bridge module, and the front end of capacitor C2 is connected to the M3C bridge arm via the full-bridge module. The common-mode voltage cancellation unit outputs a reverse-injected common-mode voltage, preventing the injected common-mode voltage from appearing on the motor side. The improved M3C with an embedded common-mode voltage cancellation unit in this invention can prevent the injected common-mode voltage from appearing on the motor side when conventional methods are used to suppress capacitor voltage fluctuations, thereby eliminating the influence of common-mode voltage on motor operation.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the technical solutions of the embodiments of the present invention.

Claims

1. A modular multilevel matrix converter with embedded common-mode voltage cancellation unit, characterized in that, It includes a modular multilevel matrix converter main circuit, wherein each bridge arm of the modular multilevel matrix converter main circuit is composed of multiple full-bridge sub-modules and at least one common-mode voltage cancellation unit cascaded together; The common-mode voltage cancellation unit includes a three-winding transformer, a first full-bridge module, a second full-bridge module, and a capacitor C2; The three ports of the three-winding transformer are respectively connected to three of the first group of full-bridge modules, and the turns ratio of the three-winding transformer is 1:1:1; Each of the first group full-bridge modules is connected to the DC side of the capacitor C2. The three DC-side capacitors C2 of the first group full-bridge modules form a three-port power channel through a three-winding transformer, which is used to transfer pulsating power between the three first group full-bridge modules. The front end of capacitor C2 is connected to the bridge arm via a second set of full-bridge modules.

2. A modular multilevel matrix converter with an embedded common-mode voltage cancellation unit according to claim 1, characterized in that, In the common-mode voltage cancellation unit, the first port of the three-winding transformer is connected to the first full-bridge module corresponding to the first phase, the second port of the three-winding transformer is connected to the first full-bridge module corresponding to the second phase, and the third port of the three-winding transformer is connected to the first full-bridge module corresponding to the third phase; the three second full-bridge modules are respectively connected to the three-phase bridge arms of the modular multilevel matrix converter.

3. A modular multilevel matrix converter with an embedded common-mode voltage cancellation unit according to claim 1, characterized in that, The full-bridge submodule, the first group of full-bridge modules, and the second group of full-bridge modules are all composed of four power switching transistors.

4. A modular multilevel matrix converter with an embedded common-mode voltage cancellation unit according to claim 1, characterized in that, When the common-mode voltage cancellation unit is working, it injects a reverse common-mode voltage into the bridge arm through the second set of full-bridge modules, so that the common-mode voltage injected by the modular multilevel matrix converter under the same frequency condition does not appear on the motor side.

5. A modular multilevel matrix converter with an embedded common-mode voltage cancellation unit according to claim 1, characterized in that, The three-port power channel is controlled by single-phase-shift modulation, and the direction and magnitude of power transmission are achieved by controlling the phase shift angle between the voltages of each port.

6. A control method for a modular multilevel matrix converter with an embedded common-mode voltage cancellation unit as described in any one of claims 1 to 5, characterized in that, include: The voltage of capacitor C2 in the common-mode voltage cancellation unit is stabilized by: taking the difference between the voltage of capacitor C2 and the given value of the capacitor voltage, multiplying the difference signal by the bridge arm current, adjusting it by the proportional controller, superimposing it with the reverse-injected common-mode voltage, and generating a drive signal after modulation, so as to maintain the voltage of capacitor C2 at the given value and generate a reverse common-mode voltage signal injected into the bridge arm through the second group of full-bridge modules. The pulsating power transfer between the DC-side capacitors C2 of the three first group full-bridge modules is controlled by adjusting the voltage deviation between the other two phases and the reference phase, using the voltage of one phase capacitor as a reference, to control the power transfer between the three-port power channels.

7. The control method for a modular multilevel matrix converter with an embedded common-mode voltage cancellation unit according to claim 6, characterized in that, The three-port power channel is equivalent to two dual-port dual active bridges, which control the phase shift angle between the first port and the second port, and the phase shift angle between the first port and the third port, respectively. The second port and the third port automatically achieve power balance.

8. The control method for a modular multilevel matrix converter with an embedded common-mode voltage cancellation unit according to claim 6, characterized in that, The control of the main circuit of the modular multilevel matrix converter includes total input energy control, bridge arm capacitor voltage equalization control, equal frequency operating condition pulsation suppression control, motor side voltage control, input and output current and circulating current control, internal voltage equalization and PWM modulation.

9. The control method for a modular multilevel matrix converter with an embedded common-mode voltage cancellation unit according to claim 8, characterized in that, The common-mode voltage injected in reverse by the common-mode voltage cancellation unit has the same amplitude but opposite direction as the common-mode voltage injected by the main circuit of the modular multilevel matrix converter.

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