Control method of three-level converter, three-level converter, parallel system and power supply equipment

Abstract

The invention provides a control method of a three-level converter, the three-level converter, a parallel system and power supply equipment, and relates to the technical field of power supplies. The method comprises the steps of obtaining a current extreme value of each phase of the three-level converter, a preset midpoint current and a target current of a direct current side midpoint; if the target current is between the maximum value of the current extreme values of the phases and the minimum value of the current extreme values of the phases, determining a first target phase and a first modulation wave offset of the first target phase; the target current is located between the current extreme value of the first target phase and the preset midpoint current; and according to the first modulation wave offset, determining a dual modulation wave of the first target phase, and according to the dual modulation wave of the first target phase, controlling the first target phase. According to the invention, the neutral-point potential balance control of the direct-current side of the three-level converter can be realized, zero-sequence current is not introduced, zero-sequence current control is not needed, the quantity required to be controlled is reduced, and the control complexity is reduced.

Description

Technical Field

[0001] This application relates to the technical field of power supplies, and particularly to a control method for a three-level converter, a three-level converter, a parallel system, and a power supply device. Background Art

[0002] The three-level converter has the advantages of high output voltage waveform quality, low voltage borne by switching devices, and low electromagnetic interference, and is widely used in fields such as uninterruptible power supplies, data center power supplies, new energy power generation, and electric vehicles.

[0003] However, the three-level converter may have the problem of unbalanced DC-side midpoint potential. In related technologies, in order to control the potential balance of the DC-side midpoint, a zero-sequence component is usually injected. However, after injecting the zero-sequence component, there may be a problem of zero-sequence current, and further zero-sequence current suppression is required. There are more quantities to be controlled and the control is relatively complex. Summary of the Invention

[0004] Embodiments of this application provide a control method for a three-level converter, a three-level converter, a parallel system, and a power supply device, so as to solve the problem that when the existing three-level converter performs DC-side midpoint potential balance control, it will bring the problem of zero-sequence current, resulting in the need to simultaneously control the DC-side midpoint potential balance and suppress the zero-sequence current, with more quantities to be controlled and a relatively complex control process.

[0005] In a first aspect, embodiments of this application provide a control method for a three-level converter, including:

[0006] Obtain the current extreme values of each phase of the three-level converter, a preset midpoint current, and a target current of the DC-side midpoint; the current extreme value of each phase is the midpoint current corresponding to the phase under double modulation wave limit modulation;

[0007] If the target current is between the maximum value of the current extreme values of each phase and the minimum value of the current extreme values of each phase, determine a first target phase and a first modulation wave offset of the first target phase; the target current is between the current extreme value of the first target phase and the preset midpoint current;

[0008] Determine the double modulation wave of the first target phase according to the first modulation wave offset, and control the first target phase according to the double modulation wave of the first target phase to achieve midpoint balance control without injecting a zero-sequence component.

[0009] In a possible implementation manner, determining the first modulation wave offset of the first target phase includes:

[0010] According to Calculate the first modulation wave offset Δd1;

[0011] Among them, m x1 is the fundamental wave of the modulation wave of the first target phase; i NPref is the target current; i NP0 is the preset midpoint current; i NPx1 is the current extreme value of the first target phase.

[0012] In a possible implementation manner, the double modulation wave of the first target phase includes a first modulation wave and a second modulation wave;

[0013] Determining the double modulation wave of the first target phase according to the first modulation wave offset includes:

[0014] Obtain the fundamental wave of the modulation wave of the first target phase;

[0015] When the fundamental wave of the modulation wave of the first target phase is greater than or equal to 0, add the first modulation wave offset to the fundamental wave of the modulation wave of the first target phase, and subtract the first modulation wave offset from the preset modulation wave fundamental wave, and when the fundamental wave of the modulation wave of the first target phase is less than 0, subtract the first modulation wave offset from the fundamental wave of the modulation wave of the first target phase, and add the first modulation wave offset to the preset modulation wave fundamental wave;

[0016] Among them, the first modulation wave is the fundamental wave of the modulation wave of the first target phase after offset, and the second modulation wave is the fundamental wave of the preset modulation wave after offset; the preset modulation wave fundamental wave is located between the two carriers of the first target phase.

[0017] In a possible implementation manner, after obtaining the current extreme values, the preset midpoint current and the target current of the DC side midpoint of each phase of the three-level converter, it further includes:

[0018] If the target current is not between the maximum value of the current extreme values of each phase and the minimum value of the current extreme values of each phase, determine the second target phase and the second modulation wave offset of the second target phase; the second target phase is the phase with the smallest absolute value of the difference between the current extreme value and the target current;

[0019] Determine the double modulation wave of the second target phase according to the second modulation wave offset, and control the second target phase according to the double modulation wave of the second target phase.

[0020] In a possible implementation manner, determining the second modulation wave offset of the second target phase includes:

[0021] According to Δd2 = (1 - |m x2 |) / 2, calculate the second modulation wave offset Δd2;

[0022] Among them, m x2Is the fundamental wave of the modulation wave for the second target phase.

[0023] In a possible implementation, obtaining the target current at the midpoint of the DC side of the three-level converter includes:

[0024] According to i NPref =(u Cp -u Cn )×C / T s , obtain the target current i NPref ;

[0025] Where, u Cp Is the positive half-bus voltage of the three-level converter; u Cn Is the negative half-bus voltage of the three-level converter; C is the capacitance value of the half-bus capacitor of the three-level converter; T s Is the switching period of the three-level converter.

[0026] In a possible implementation, obtaining the preset midpoint current of the three-level converter includes:

[0027] According to i NP0 =(1 - m a )×i a +(1 - m b )×i b +(1 - m c )×i c , obtain the preset midpoint current i NP0 ;

[0028] Obtaining the current extreme values of each phase of the three-level converter includes:

[0029] According to i NPA =(1 - m b )×i b +(1 - m c )×i c , obtain the current extreme value i NPA of phase A;

[0030] According to i NPB =(1 - m a )×i a +(1 - m c )×i c , obtain the current extreme value i NPB of phase B;

[0031] According to i NPC =(1 - m b )×i b +(1 - m a )×i a , obtain the current extreme value iNPC ;

[0032] Among them, m a is the fundamental wave of the modulation wave of phase A, and i a is the current of phase A; m b is the fundamental wave of the modulation wave of phase B, and i b is the current of phase B; m c is the fundamental wave of the modulation wave of phase C, and i c is the current of phase C.

[0033] Second, an embodiment of the present application provides a control device for a three-level converter, including:

[0034] An acquisition module, configured to acquire the current extreme values of each phase of the three-level converter, a preset midpoint current, and a target current of the midpoint of the DC side; the current extreme values of each phase are the midpoint currents of the corresponding phase under double modulation wave limit modulation;

[0035] A first determination module, configured to determine a first target phase and a first modulation wave offset of the first target phase if the target current is between the maximum value of the current extreme values of each phase and the minimum value of the current extreme values of each phase; the target current is between the current extreme value of the first target phase and the preset midpoint current;

[0036] A first control module, configured to determine the double modulation wave of the first target phase according to the first modulation wave offset, and control the first target phase according to the double modulation wave of the first target phase to achieve midpoint balance control without injecting zero-sequence components.

[0037] Third, an embodiment of the present application provides a control device, including a memory and a processor. The memory stores a computer program, and when the processor executes the computer program, it implements the control method of the three-level converter in the first aspect or any possible implementation manner of the first aspect above.

[0038] Fourth, an embodiment of the present application provides a three-level converter, including transformation units corresponding to three phases respectively and the control device described in the third aspect; the transformation units are controlled by the control device.

[0039] Fifth, an embodiment of the present application provides a parallel system, including at least two transformation modules connected in parallel. The transformation modules include the three-level converter described in the fourth aspect, and the midpoints of the DC sides of each three-level converter are independent of each other.

[0040] Sixth, an embodiment of the present application provides a power supply device, including the three-level converter described in the fourth aspect, or including the parallel system described in the fifth aspect.

[0041] In a seventh aspect, an embodiment of the present application provides a computer-readable storage medium storing a computer program, which when executed by a processor, implements the control method of the three-level converter in the first aspect above or any possible implementation manner of the first aspect.

[0042] In an eighth aspect, an embodiment of the present application provides a computer program product including a computer program, which when executed by a processor, implements the control method of the three-level converter in the first aspect above or any possible implementation manner of the first aspect.

[0043] In the embodiment of the present application, for the midpoint balance control of the three-level converter without injecting zero-sequence components, specifically, it is determined whether the target current is between the maximum value of the current extremes of each phase and the minimum value of the current extremes of each phase. If so, the first target phase and the first modulation wave offset of the first target phase are determined; the target current is between the current extreme of the first target phase and the preset midpoint current; according to the first modulation wave offset, the double modulation wave of the first target phase is determined, and the first target phase is controlled according to the double modulation wave of the first target phase, so that the midpoint balance control without injecting zero-sequence components can be realized, which can not only realize the balance control of the DC-side midpoint potential of the three-level converter, but also not introduce zero-sequence current, and there is no need to perform zero-sequence current control anymore, reducing the quantity to be controlled and lowering the control complexity. Description of the Drawings

[0044] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0045] Figure 1 It is a flowchart of the implementation of the control method of the three-level converter provided by the embodiment of the present application;

[0046] Figure 2 It is a comparison schematic diagram of two different control signals provided by the embodiment of the present application;

[0047] Figure 3 It is a schematic diagram of the double modulation wave of the first target phase provided by the embodiment of the present application;

[0048] Figure 4 It is a schematic diagram of a parallel system provided by the embodiment of the present application;

[0049] Figure 5 It is a schematic diagram of another parallel system provided by the embodiment of the present application;

[0050] Figure 6 It is a comparison schematic diagram of the midpoint voltage provided by an embodiment of the present application;

[0051] Figure 7 It is a schematic structural diagram of a control device for a three-level converter provided by an embodiment of the present application;

[0052] Figure 8 It is a schematic diagram of a control device provided by an embodiment of the present application. Specific embodiments

[0053] In the following description, for the purpose of illustration rather than limitation, specific details such as specific system structures and technologies are presented to thoroughly understand the embodiments of the present application. However, those skilled in the art should clearly understand that the present application can also be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to avoid unnecessary details from interfering with the description of the present application.

[0054] To make the purpose, technical solutions, and advantages of the present application clearer, the following will be described through specific embodiments in conjunction with the accompanying drawings.

[0055] Refer to Figure 1 , which shows a flowchart for implementing a control method for a three-level converter provided by an embodiment of the present application, and is described in detail as follows:

[0056] In S101, obtain the current extreme values of each phase of the three-level converter, a preset midpoint current, and a target current of the midpoint of the DC side; the current extreme values of each phase are the midpoint currents of the corresponding phase under double modulation wave limit modulation.

[0057] Among them, the current extreme values of each phase of the three-level converter may include the current extreme value of phase A, the current extreme value of phase B, and the current extreme value of phase C of the three-level converter. The current extreme values of each phase being the midpoint currents of the corresponding phase under double modulation wave limit modulation can be understood as the midpoint currents when the time of 0 in the control signal of the corresponding phase is 0 (i.e., there is no time clamped to 0), that is, the midpoint current when the control signal of the corresponding phase only switches between P and N and there is no 0. Exemplarily, refer to Figure 2 , Figure 2 The above control signal is a control signal that switches between P, O, and N under double modulation wave modulation, and the following control signal is a control signal that switches between P and N under double modulation wave limit modulation, and there is no time of 0.

[0058] The preset midpoint current is the current at the midpoint of the DC side when the three-level converter adopts the SPWM (Sinusoidal Pulse Width Modulation) method. The embodiment of the present application improves on the basis of the SPWM method to obtain a dual modulation wave modulation method, and controls the three-level converter through the dual modulation wave modulation method to achieve the DC side midpoint balance control without injecting zero-sequence current.

[0059] The target current of the DC side midpoint is the midpoint current that needs to be injected to achieve the potential balance of the DC side midpoint of the three-level converter.

[0060] The embodiment of the present application does not specifically limit the specific acquisition means for obtaining the current extreme values of each phase, the preset midpoint current, and the target current of the DC side midpoint of the three-level converter, and any achievable means can be used.

[0061] Among them, the three-level converter can be a three-level inverter, a three-level rectifier, or other types of three-level converters, which are not specifically limited here.

[0062] In S102, if the target current is between the maximum value of the current extreme values of each phase and the minimum value of the current extreme values of each phase, then determine the first target phase and the first modulation wave offset of the first target phase; the target current is between the current extreme value of the first target phase and the preset midpoint current.

[0063] The embodiment of the present application determines whether the DC side midpoint balance can be achieved through the dual modulation wave modulation method provided by the embodiment of the present application by determining whether the target current is between the maximum value of the current extreme values of each phase and the minimum value of the current extreme values of each phase. If the target current is between the maximum value of the current extreme values of each phase and the minimum value of the current extreme values of each phase, that is, min[i NPA , i NPB , i NPC ≤ i NPref ≤ max[i NPA , i NPB , i NPC , then it is determined that the DC side midpoint balance can be achieved through the dual modulation wave modulation method provided by the embodiment of the present application. At this time, the first target phase can be determined based on the target current and the preset midpoint current, and the first modulation wave offset of the first target phase can be determined at the same time.

[0064] Among them, i NPref is the target current; i NPA is the current extreme value of phase A; i NPB is the current extreme value of phase B; i NPC is the current extreme value of phase C.

[0065] The first target phase is the A phase, B phase, or C phase, which is the phase that needs to be controlled using the dual modulation wave modulation method. The target current is between the current extreme value of the first target phase and the preset midpoint current, that is, the phase in which the target current is between the current extreme value and the preset midpoint current is the first target phase.

[0066] Exemplarily, the current extreme values of the A, B, and C phases can be substituted in sequence to determine whether the target current is between the current extreme value of the A phase and the preset midpoint current. If so, the first target phase is the A phase. If not, it is determined whether the target current is between the current extreme value of the B phase and the preset midpoint current. If so, the first target phase is the B phase. If not, the first target phase is the C phase. There must be one phase such that the target current is between the current extreme value of that phase and the preset midpoint current.

[0067] The first modulation wave offset of the first target phase refers to the magnitude value of moving the modulation wave of the first target phase upward and / or downward, that is, the specific value of moving the modulation wave of the first target phase when using the SPWM method. Exemplarily, the first modulation wave offset can be calculated based on the fundamental wave of the modulation wave of the first target phase, the target current, the preset midpoint current, and the current extreme value of the first target phase.

[0068] It should be noted that in the dual modulation wave modulation method of the embodiments of the present application, only one phase (i.e., the above-mentioned first target phase or the second target phase mentioned later) is controlled using this method, and the other two phases still use the SPWM method for control.

[0069] In S103, according to the first modulation wave offset, the dual modulation wave of the first target phase is determined, and based on the dual modulation wave of the first target phase, the first target phase is controlled to achieve midpoint balance control without injecting zero-sequence components.

[0070] Through the first modulation wave offset of the first target phase in the present application, the modulation wave when controlling the first target phase using the SPWM method can be offset, thereby obtaining the dual modulation wave of the first target phase. Then, based on the dual modulation wave of the first target phase, the first target phase is controlled, so that midpoint balance control without injecting zero-sequence components can be achieved.

[0071] As mentioned above, the other two phases except the first target phase still use the SPWM method for control.

[0072] The embodiments of the present application do not specifically limit the method for determining the first modulation wave offset, the means for determining the dual modulation wave of the first target phase according to the first modulation wave offset, and the means for controlling the first target phase based on the dual modulation wave of the first target phase. Any means that can achieve midpoint balance control without injecting zero-sequence components are acceptable.

[0073] In the embodiments of the present application, for the neutral point balance control of the three-level converter without injecting zero-sequence components, specifically, it is determined whether the target current is between the maximum value among the current extreme values of each phase and the minimum value among the current extreme values of each phase. If so, the first target phase and the first modulation wave offset of the first target phase are determined; the target current is between the current extreme value of the first target phase and the preset neutral point current; according to the first modulation wave offset, the double modulation waves of the first target phase are determined, and the first target phase is controlled according to the double modulation waves of the first target phase, so that the neutral point balance control without injecting zero-sequence components can be realized, the neutral point potential balance control of the three-level converter can be realized, and zero-sequence current will not be introduced, so there is no need to perform zero-sequence current control, reducing the quantity to be controlled and lowering the control complexity.

[0074] The foregoing embodiments introduce the overall implementation process of the control method of the three-level converter. Next, the implementation methods of some of the steps will be introduced in detail. First, how to determine the first modulation wave offset of the first target phase will be introduced.

[0075] In some embodiments, determining the first modulation wave offset of the first target phase includes:

[0076] According to Calculate the first modulation wave offset Δd1;

[0077] where m x1 is the fundamental wave of the modulation wave of the first target phase; i NPref is the target current; i NP0 is the preset neutral point current; i NPx1 is the current extreme value of the first target phase.

[0078] In the embodiments of the present application, through the absolute value of the fundamental wave of the modulation wave of the first target phase, the target current, the preset neutral point current, the current extreme value of the first target phase, and the above formula, the first modulation wave offset can be calculated. The first modulation wave offset is used to generate the double modulation waves of the first target phase, so that based on the double modulation waves of the first target phase for modulation, the neutral point balance control without injecting zero-sequence components can be realized.

[0079] Next, how to determine the double modulation waves of the first target phase according to the first modulation wave offset will be introduced.

[0080] In some embodiments, the double modulation waves of the first target phase include a first modulation wave and a second modulation wave;

[0081] Determining the double modulation waves of the first target phase according to the first modulation wave offset includes:

[0082] Obtain the fundamental wave of the modulation wave of the first target phase;

[0083] When the fundamental wave of the modulation wave of the first target phase is greater than or equal to 0, add the first modulation wave offset to the fundamental wave of the modulation wave of the first target phase, and subtract the first modulation wave offset from the preset modulation wave fundamental wave; when the fundamental wave of the modulation wave of the first target phase is less than 0, subtract the first modulation wave offset from the fundamental wave of the modulation wave of the first target phase, and add the first modulation wave offset to the preset modulation wave fundamental wave;

[0084] Wherein, the first modulation wave is the fundamental wave of the modulation wave of the offset first target phase, and the second modulation wave is the fundamental wave of the offset preset modulation wave; the preset modulation wave fundamental wave is located between the two carrier waves of the first target phase.

[0085] The fundamental wave of the modulation wave of the first target phase can be the modulation wave of the first target phase when controlling the three-level converter by the SPWM method. The preset modulation wave fundamental wave is the modulation wave fundamental wave located between the two carrier waves of the first target phase. The intersection between the preset modulation wave fundamental wave and the two carrier waves of the first target phase only includes the minimum value of the upper carrier wave (positive carrier wave) and the maximum value of the lower carrier wave (load wave). That is, the preset modulation wave fundamental wave is the modulation wave fundamental wave located between the two carrier waves of the first target phase, and the intersection between the preset modulation wave fundamental wave and the two carrier waves of the first target phase only includes the minimum value of the upper carrier wave and the maximum value of the lower carrier wave. Exemplarily, the preset modulation wave fundamental wave can be a straight line of y = 0.

[0086] For a three-level converter, there are two carrier waves for each phase, and both of the two carrier waves can be triangular carrier waves. The frequency of the carrier wave is greater than the frequency of the modulation wave of the corresponding phase. The carrier waves of each phase in the method of the embodiment of the present application can be the same as the carrier waves of each phase when using the SPWM method.

[0087] In the embodiment of the present application, when the fundamental wave of the modulation wave of the first target phase is greater than or equal to 0, add the first modulation wave offset to the fundamental wave of the modulation wave of the first target phase to obtain the first modulation wave, and at the same time subtract the first modulation wave offset from the preset modulation wave fundamental wave to obtain the second modulation wave; when the fundamental wave of the modulation wave of the first target phase is less than 0, subtract the first modulation wave offset from the fundamental wave of the modulation wave of the first target phase to obtain the first modulation wave, and at the same time add the first modulation wave offset to the preset modulation wave fundamental wave to obtain the second modulation wave.

[0088] That is to say, the first modulation wave includes a partial modulation wave obtained by adding the first modulation wave offset to the fundamental wave of the modulation wave of the first target phase when the fundamental wave of the modulation wave of the first target phase is greater than or equal to 0, and a remaining partial modulation wave obtained by subtracting the first modulation wave offset from the fundamental wave of the modulation wave of the first target phase when the fundamental wave of the modulation wave of the first target phase is less than 0. The second modulation wave includes a partial modulation wave obtained by subtracting the first modulation wave offset from the preset modulation wave fundamental when the fundamental wave of the modulation wave of the first target phase is greater than or equal to 0, and a remaining partial modulation wave obtained by adding the first modulation wave offset to the preset modulation wave fundamental when the fundamental wave of the modulation wave of the first target phase is less than 0.

[0089] The above-mentioned first modulation wave is the fundamental wave of the modulation wave of the first target phase after offset, and the second modulation wave is the fundamental wave of the preset modulation wave after offset. The offset here can be understood as adding the first modulation wave offset and / or subtracting the first modulation wave offset.

[0090] After determining the double modulation waves of the first target phase, that is, the above-mentioned first modulation wave and second modulation wave, comparing the first modulation wave and the second modulation wave with the two carrier waves of the first target phase can obtain the control signal of the first target phase.

[0091] Exemplarily, refer to Figure 3 , Figure 3 shows the first modulation wave and the second modulation wave when the fundamental wave of the modulation wave of the first target phase is greater than or equal to 0. The fundamental wave of the modulation wave of the first target phase is as shown in Figure 3 31, the preset modulation wave fundamental is as shown in Figure 3 32, Δd1 is the first modulation wave offset, the first modulation wave is as shown in Figure 3 33, the second modulation wave is as shown in Figure 3 34, and the two carrier waves of the first target phase are as shown in Figure 3 35. Figure 3 36 in it is the first control signal of the first target phase obtained by comparing the fundamental wave of the modulation wave of the first target phase with the two carrier waves of the first target phase using the SPWM method. The first control signal only contains P and O levels; Figure 3 37 in it is the second control signal of the first target phase obtained by comparing the first modulation wave and the second modulation wave with the two carrier waves of the first target phase using the double modulation wave modulation method provided in the embodiments of the present application. The second control signal contains P, O, and N levels.

[0092] Compared with the first control signal, the second control signal significantly increases the N level and the time of the P level, and reduces the time of O, which is equivalent to reducing the time of injecting the midpoint. Therefore, midpoint regulation can be performed to balance the midpoint. And in the second control signal, the time of the N level is equal to the increased time of the P level, and the action effects of the N level and the increased P level can cancel each other out without affecting the modulation result.

[0093] It should be noted that the specific implementation manners of determining the first modulation wave offset of the first target phase and determining the dual modulation waves of the first target phase according to the first modulation wave offset are given in the above embodiments. However, the specific implementation manners of determining the first modulation wave offset of the first target phase and determining the dual modulation waves of the first target phase according to the first modulation wave offset are not limited to the above embodiments. In related technologies, any applicable implementation manner can implement the above method.

[0094] In the foregoing embodiments, it is mainly introduced how to achieve the DC side midpoint balance without injecting zero-sequence components of the three-level converter when the target current is between the maximum value of the current extremes of each phase and the minimum value of the current extremes of each phase. Next, it continues to introduce how to process to achieve the DC side midpoint balance without injecting zero-sequence components or approximate balance of the three-level converter when the target current is not between the maximum value of the current extremes of each phase and the minimum value of the current extremes of each phase.

[0095] In some embodiments, after obtaining the current extremes of each phase of the three-level converter, the preset midpoint current, and the target current of the DC side midpoint, it further includes:

[0096] If the target current is not between the maximum value of the current extremes of each phase and the minimum value of the current extremes of each phase, then determine the second target phase and the second modulation wave offset of the second target phase; the second target phase is the phase with the smallest absolute value of the difference between the current extreme and the target current;

[0097] Determine the dual modulation waves of the second target phase according to the second modulation wave offset, and control the second target phase according to the dual modulation waves of the second target phase.

[0098] If the target current is not between the maximum value of the current extremes of each phase and the minimum value of the current extremes of each phase, that is, i NPref >max[i NPA , i NPB , i NPC , or, i NPref <min[i NPA , i NPB , i NPC, when determining the target phase and the corresponding modulation wave offset by the method when the target current is between the maximum value among the current extremes of each phase and the minimum value among the current extremes of each phase, and then obtaining the corresponding double modulation waves for control, it may not be possible to achieve the midpoint balance of the DC side. Therefore, the embodiments of the present application adopt different methods to determine the target phase and the corresponding modulation wave offset, that is, determine the second target phase based on the target current, and at the same time, based on the fundamental wave of the modulation wave of the second target phase, determine the second modulation wave offset of the second target phase.

[0099] Exemplarily, calculate the absolute value of the difference between the current extreme value of each phase and the target current respectively, and take the phase with the smallest absolute value as the second target phase.

[0100] The second modulation wave offset of the second target phase refers to the magnitude value of moving the modulation wave of the second target phase upward and / or downward, that is, the specific value of moving the modulation wave of the second target phase when using the SPWM method.

[0101] Through the second modulation wave offset of the second target phase, the present application can offset the modulation wave when controlling the second target phase using the SPWM method, so as to obtain the double modulation waves of the second target phase, and then control the second target phase according to the double modulation waves of the second target phase to achieve or approximate the midpoint balance of the DC side.

[0102] When the target current is not between the maximum value among the current extremes of each phase and the minimum value among the current extremes of each phase, the other two phases except the second target phase still use the SPWM method for control.

[0103] The following further details each step when the target current is not between the maximum value among the current extremes of each phase and the minimum value among the current extremes of each phase.

[0104] In some embodiments, determining the second modulation wave offset of the second target phase includes:

[0105] According to Δd2 = (1 - |m x2 |) / 2, calculate the second modulation wave offset Δd2;

[0106] where m x2 is the fundamental wave of the modulation wave of the second target phase.

[0107] Through the absolute value of the fundamental wave of the modulation wave of the second target phase and the above formula, the embodiments of the present application can calculate the second modulation wave offset.

[0108] In some possible implementation manners, the double modulation waves of the second target phase include a third modulation wave and a fourth modulation wave;

[0109] Determine the dual modulation waves of the second target phase according to the second modulation wave offset, including:

[0110] Obtain the fundamental wave of the modulation wave of the second target phase;

[0111] When the fundamental wave of the modulation wave of the second target phase is greater than or equal to 0, add the second modulation wave offset to the fundamental wave of the modulation wave of the second target phase, subtract the second modulation wave offset from the preset modulation wave fundamental wave, and when the fundamental wave of the modulation wave of the second target phase is less than 0, subtract the second modulation wave offset from the fundamental wave of the modulation wave of the second target phase, and add the second modulation wave offset to the preset modulation wave fundamental wave;

[0112] Wherein, the third modulation wave is the fundamental wave of the modulation wave of the second target phase after offset, and the fourth modulation wave is the preset modulation wave fundamental wave after offset; the preset modulation wave fundamental wave is located between the two carrier waves of the second target phase.

[0113] The fundamental wave of the modulation wave of the second target phase can be the modulation wave of the second target phase when controlling a three-level converter using the SPWM method. The preset modulation wave fundamental wave is the modulation wave fundamental wave located between the two carrier waves of the second target phase. The intersection between the preset modulation wave fundamental wave and the two carrier waves of the second target phase only has the minimum value of the upper carrier wave (positive carrier wave) and the maximum value of the lower carrier wave (load wave). That is, the preset modulation wave fundamental wave is located between the two carrier waves of the second target phase, and the intersection between the preset modulation wave fundamental wave and the two carrier waves of the second target phase only has the minimum value of the upper carrier wave and the maximum value of the lower carrier wave.

[0114] For a three-level converter, there are two carrier waves for each phase, and the two carrier waves can both be triangular carrier waves. The frequency of the carrier wave is greater than the frequency of the modulation wave of the corresponding phase. The carrier waves of each phase in the method of the embodiment of the present application can be the same as the carrier waves of each phase when using the SPWM method.

[0115] In the embodiment of the present application, when the fundamental wave of the modulation wave of the second target phase is greater than or equal to 0, add the second modulation wave offset to the fundamental wave of the modulation wave of the second target phase to obtain the third modulation wave, and at the same time subtract the second modulation wave offset from the preset modulation wave fundamental wave to obtain the fourth modulation wave; and when the fundamental wave of the modulation wave of the second target phase is less than 0, subtract the second modulation wave offset from the fundamental wave of the modulation wave of the second target phase to obtain the third modulation wave, and at the same time add the second modulation wave offset to the preset modulation wave fundamental wave to obtain the fourth modulation wave.

[0116] That is to say, the third modulation wave includes a partial modulation wave obtained by adding the second modulation wave offset to the fundamental wave of the modulation wave of the second target phase when the fundamental wave of the modulation wave of the second target phase is greater than or equal to 0, and a remaining partial modulation wave obtained by subtracting the first modulation wave offset from the fundamental wave of the modulation wave of the second target phase when the fundamental wave of the modulation wave of the second target phase is less than 0. The fourth modulation wave includes a partial modulation wave obtained by subtracting the second modulation wave offset from the preset modulation wave fundamental when the fundamental wave of the modulation wave of the second target phase is greater than or equal to 0, and a remaining partial modulation wave of the modulation wave obtained by adding the second modulation wave offset to the preset modulation wave fundamental when the fundamental wave of the modulation wave of the second target phase is less than 0.

[0117] The above-mentioned third modulation wave is the fundamental wave of the modulation wave of the second target phase after offset, and the fourth modulation wave is the fundamental wave of the preset modulation wave after offset. The offset here can be understood as adding the second modulation wave offset and / or subtracting the second modulation wave offset.

[0118] After determining the double modulation waves of the second target phase, that is, the above-mentioned third modulation wave and fourth modulation wave, comparing the third modulation wave and the fourth modulation wave with the two carrier waves of the second target phase can obtain the control signal of the second target phase.

[0119] The specific implementation manner of determining the double modulation waves of the second target phase according to the second modulation wave offset in the embodiments of this application is similar to the specific implementation manner of determining the double modulation waves of the first target phase according to the first modulation wave offset in the foregoing embodiments. For the corresponding content, reference can be made to the relevant descriptions in the foregoing embodiments.

[0120] It should be noted that the above embodiments give the specific implementation manners of determining the second modulation wave offset of the second target phase and determining the double modulation waves of the second target phase according to the second modulation wave offset. However, the specific implementation manners of determining the second modulation wave offset of the second target phase and determining the double modulation waves of the second target phase according to the second modulation wave offset are not limited to the above implementation manners. In related technologies, any applicable implementation manner can implement the above method.

[0121] Next, the steps in S101 will be further introduced in detail.

[0122] In some embodiments, obtaining the target current at the midpoint of the DC side of the three-level converter includes:

[0123] According to i NPref =(u Cp -u Cn )×C / T s , obtaining the target current i NPref ;

[0124] Among them, u Cpis the positive half-bus voltage of the three-level converter; u Cn is the negative half-bus voltage of the three-level converter; C is the capacitance value of the half-bus capacitor of the three-level converter; T s is the switching period of the three-level converter.

[0125] u Cp is the positive half-bus voltage of the three-level converter, that is, the voltage across the first capacitor between the positive DC bus and the midpoint of the DC side of the three-level converter. u Cn is the negative half-bus voltage of the three-level converter, that is, the voltage across the second capacitor between the midpoint of the DC side and the negative DC bus of the three-level converter. C is the capacitance value of the half-bus capacitor of the three-level converter, that is, the capacitance value of the above-mentioned first capacitor or the above-mentioned second capacitor, and the capacitance values of the first capacitor and the second capacitor are equal.

[0126] In some embodiments, obtaining the preset midpoint current of the three-level converter includes:

[0127] According to i NP0 =(1 - m a )×i a +(1 - m b )×i b +(1 - m c )×i c , obtain the preset midpoint current i NP0 ;

[0128] Obtaining the current extreme values of each phase of the three-level converter includes:

[0129] According to i NPA =(1 - m b )×i b +(1 - m c )×i c , obtain the current extreme value i NPA of phase A;

[0130] According to i NPB =(1 - m a )×i a +(1 - m c )×i c , obtain the current extreme value i NPB of phase B;

[0131] According to i NPC =(1 - m b )×i b +(1 - m a )×i a , obtain the current extreme value i NPC of phase C;

[0132] Wherein, ma is the fundamental wave of the modulation wave for phase A, and i a is the current for phase A; m b is the fundamental wave of the modulation wave for phase B, and i b is the current for phase B; m c is the fundamental wave of the modulation wave for phase C, and i c is the current for phase C.

[0133] It should be noted that the control method of the three-level converter provided by the embodiments of the present application can be applied to a single three-level converter to achieve the potential balance of the midpoint of the DC side without injecting zero-sequence components; it can also be applied to a parallel system. In the parallel system, the midpoints of the DC sides of the three-level converters are independent of each other, and each three-level converter adopts the above control method of the three-level converter to achieve the potential balance of the midpoint of the DC side without injecting zero-sequence components for each three-level converter.

[0134] Exemplarily, referring to Figure 4 and Figure 5 , the parallel system may include at least two transformation modules 41 connected in parallel. The transformation module 41 includes a three-level converter 42, and the midpoints of the DC sides of the three-level converters 42 are independent of each other.

[0135] In one implementation, referring to Figure 4 , the transformation module 41 only includes a three-level converter 42, and the midpoints of the DC sides of the three-level converters 42 are independent of each other and are not connected together.

[0136] In another implementation, referring to Figure 5 , the transformation module 41 includes a DC-DC unit 43 and a three-level converter 42. The first side of the DC-DC unit 43 serves as the DC side of the transformation module 41, the second side of the DC-DC unit 43 is connected to the DC side of the three-level converter 42, and the AC side of the three-level converter 42 serves as the AC side of the transformation module 41. The midpoints of the DC sides of the three-level converters 42 are independent of each other.

[0137] Figure 4 and Figure 5 In and , P represents the positive DC bus, and N represents the negative DC bus.

[0138] Assume that the parallel system includes n conversion modules 41, that is, it includes n three-level converters 42. If the method of injecting zero-sequence components is used to control the neutral point balance of each three-level converter 42, since the DC-side neutral points of the three-level converters 42 are independent of each other, therefore, it is necessary to control the neutral points of n three-level converters 42 and the zero-sequence current of n - 1 three-level converters 42, and 2n - 1 quantities need to be controlled. The control process is too complex. However, by using the control method without injecting zero-sequence components provided in the embodiments of the present application, only the neutral points of n three-level converters 42 need to be controlled, which can greatly reduce the control complexity and simplify the control method.

[0139] Figure 6 The figure shows the phase-A current, zero-sequence current, and neutral point voltage of the three-level converter corresponding to the method provided in the embodiments of the present application and the SPWM method respectively. Among them, Figure 6 On the left, from top to bottom are the phase-A current, zero-sequence current, and neutral point voltage of the three-level converter when the three-level converter is controlled by the SPWM method; Figure 6 On the right, from top to bottom are the phase-A current, zero-sequence current, and neutral point voltage of the three-level converter when the control method of the three-level converter provided in the embodiments of the present application is used. It can be Figure 6 clearly seen that the control method of the three-level converter provided in the embodiments of the present application can achieve neutral point balance without injecting zero-sequence components.

[0140] It should be understood that the magnitudes of the sequence numbers of the steps in the above embodiments do not mean the order of execution. The order of execution of each process should be determined according to its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0141] The following is the device embodiment of the present application. For the details not described in detail, reference can be made to the corresponding method embodiments above.

[0142] Figure 7 The figure shows a schematic structural diagram of the control device of the three-level converter provided in the embodiments of the present application. For the convenience of description, only the parts related to the embodiments of the present application are shown and are described in detail as follows:

[0143] As Figure 7 shown, the control device 70 of the three-level converter includes: an acquisition module 71, a first determination module 72, and a first control module 73.

[0144] Among them, the acquisition module 71 is used to acquire the current extreme values of each phase of the three-level converter, the preset neutral point current, and the target current of the DC-side neutral point; the current extreme values of each phase are the neutral point currents corresponding to the phase under the limit modulation of the double modulation wave;

[0145] The first determination module 72 is configured to determine a first target phase and a first modulation wave offset of the first target phase if the target current is between the maximum value among the current extremes of each phase and the minimum value among the current extremes of each phase; the target current is between the current extreme of the first target phase and a preset neutral point current;

[0146] The first control module 73 is configured to determine a double modulation wave of the first target phase according to the first modulation wave offset, and control the first target phase according to the double modulation wave of the first target phase to achieve neutral point balance control without injecting zero sequence components.

[0147] In a possible implementation manner, in the first determination module 72, determining the first modulation wave offset of the first target phase includes:

[0148] According to calculate the first modulation wave offset Δd1;

[0149] where m x1 is the fundamental wave of the modulation wave of the first target phase; i NPref is the target current; i NP0 is the preset neutral point current; i NPx1 is the current extreme of the first target phase.

[0150] In a possible implementation manner, the double modulation wave of the first target phase includes a first modulation wave and a second modulation wave;

[0151] In the first determination module 72, determining the double modulation wave of the first target phase according to the first modulation wave offset includes:

[0152] Obtain the fundamental wave of the modulation wave of the first target phase;

[0153] When the fundamental wave of the modulation wave of the first target phase is greater than or equal to 0, add the first modulation wave offset to the fundamental wave of the modulation wave of the first target phase, and subtract the first modulation wave offset from the preset modulation wave fundamental wave, and when the fundamental wave of the modulation wave of the first target phase is less than 0, subtract the first modulation wave offset from the fundamental wave of the modulation wave of the first target phase, and add the first modulation wave offset to the preset modulation wave fundamental wave;

[0154] where the first modulation wave is the offset fundamental wave of the modulation wave of the first target phase, and the second modulation wave is the offset preset modulation wave fundamental wave; the preset modulation wave fundamental wave is between the two carrier waves of the first target phase.

[0155] In a possible implementation manner, the control device 70 of the three-level converter further includes: a second determination module and a second control module.

[0156] A second determination module, configured to determine a second target phase and a second modulation wave offset of the second target phase after obtaining the current extreme values of each phase of the three-level converter, a preset midpoint current, and a target current of the DC side midpoint; the second target phase is the phase with the smallest absolute value of the difference between the current extreme value and the target current.

[0157] A second control module, configured to determine a dual modulation wave of the second target phase according to the second modulation wave offset, and control the second target phase according to the dual modulation wave of the second target phase.

[0158] In a possible implementation manner, in the second determination module, determining the second modulation wave offset of the second target phase includes:

[0159] Calculating the second modulation wave offset Δd2 according to Δd2 = (1 - |m x2 ) / 2;

[0160] where m x2 is the fundamental wave of the modulation wave of the second target phase.

[0161] In a possible implementation manner, in the acquisition module 71, acquiring the target current of the DC side midpoint of the three-level converter includes:

[0162] Acquiring the target current i NPref according to i = (u Cp -u Cn ) × C / T s ; NPref where u

[0163] is the positive half-bus voltage of the three-level converter; u Cp is the negative half-bus voltage of the three-level converter; C is the capacitance value of the half-bus capacitor of the three-level converter; T Cn is the switching period of the three-level converter. s

[0164] In a possible implementation manner, in the acquisition module 71, acquiring the preset midpoint current of the three-level converter includes:

[0165] Acquiring the preset midpoint current i NP0 according to i = (1 - m a ) × i a + (1 - m b ) × i b + (1 - m c ) × i c ; NP0

[0166] Obtain the current extreme values of each phase of the three-level converter, including:

[0167] According to i NPA =(1 - m b )×i b +(1 - m c )×i c , obtain the current extreme value i NPA of phase A;

[0168] According to i NPB =(1 - m a )×i a +(1 - m c )×i c , obtain the current extreme value i NPB of phase B;

[0169] According to i NPC =(1 - m b )×i b +(1 - m a )×i a , obtain the current extreme value i NPC of phase C;

[0170] wherein, m a is the fundamental wave of the modulation wave of phase A, and i a is the current of phase A; m b is the fundamental wave of the modulation wave of phase B, and i b is the current of phase B; m c is the fundamental wave of the modulation wave of phase C, and i c is the current of phase C.

[0171] Figure 8 is a schematic diagram of the control device provided by an embodiment of the present application. As Figure 8 shown, the control device 8 of this embodiment includes: a processor 80 and a memory 81. The memory 81 stores a computer program 82. When the processor 80 executes the computer program 82, the steps in the above-mentioned various method embodiments are implemented. Alternatively, when the processor 80 executes the computer program 82, the functions of each module / unit in the above-mentioned various device embodiments are implemented.

[0172] Exemplarily, the computer program 82 can be divided into one or more modules / units, and the one or more modules / units are stored in the memory 81 and executed by the processor 80 to complete the present application. The one or more modules / units can be a series of computer program instruction segments capable of performing specific functions, and the instruction segments are used to describe the execution process of the computer program 82 in the control device 8.

[0173] The control device 8 may include, but is not limited to, a processor 80 and a memory 81. Those skilled in the art can understand,Figure 8 This is only an example of the control device 8, which does not constitute a limitation on the control device 8. It may include more or fewer components than those shown in the figure, or combine certain components, or different components. For example, the control device 8 may also include input / output devices, network access devices, buses, etc.

[0174] The processor 80 may be a central processing unit (CPU), or may also be other general-purpose processors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or the processor may also be any conventional processor, etc.

[0175] The memory 81 may be an internal storage unit of the control device 8, such as the hard disk or memory of the control device 8. The memory 81 may also be an external storage device of the control device 8, such as a plug-in hard disk, smart media card (SMC), secure digital (SD) card, flash card, etc. equipped on the control device 8. Further, the memory 81 may also include both the internal storage unit and the external storage device of the control device 8. The memory 81 is used to store the computer program 82 and other programs and data required by the control device 8. The memory 81 may also be used to temporarily store the data that has been output or will be output.

[0176] For the convenience and simplicity of description, only the above division of each functional module / unit is used as an example. In practical applications, the above functions may be allocated to different functional modules / units according to needs. The above modules / units may be implemented in the form of hardware, or in the form of software, or in the form of a combination of hardware and software.

[0177] Corresponding to the above control device, an embodiment of the present application provides a three-level converter, including conversion units corresponding to three phases respectively and the control device as described above, and the conversion units are controlled by the control device.

[0178] Corresponding to the above three-level converter, an embodiment of the present application provides a parallel system, including at least two conversion modules connected in parallel. The conversion module includes the three-level converter as described above, and the midpoints of the DC sides of each three-level converter are independent of each other.

[0179] For the specific description of the parallel system in the embodiments of the present application, reference may be made to the relevant descriptions in the foregoing embodiments, and details are not repeated herein.

[0180] Corresponding to the above three-level converter and parallel system, the embodiments of the present application provide a power supply device, which includes the above-mentioned three-level converter, or includes the above-mentioned parallel system.

[0181] The embodiments of the present application further provide a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, the methods in the above method embodiments are implemented.

[0182] The embodiments of the present application further provide a computer program product including a computer program. When the computer program is executed by a processor, the methods in the above method embodiments are implemented.

[0183] Among them, the computer program includes computer program code, and the computer program code can be in the form of source code, object code, executable file or some intermediate form, etc. The computer-readable medium may include: any entity or device capable of carrying the computer program code, recording medium, USB flash drive, mobile hard disk, magnetic disk, optical disc, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signal, telecommunication signal, and software distribution medium, etc.

[0184] In the above embodiments, the descriptions of the respective embodiments have their own emphases. For parts not described in detail or recorded in a certain embodiment, reference may be made to the relevant descriptions in other embodiments. Without special instructions and logical conflicts, the terms and / or descriptions among different embodiments are consistent and can be referenced to each other, and the technical features in different embodiments can be combined to form new embodiments according to their inherent logical relationships.

[0185] The above-described embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A control method for a three-level converter, characterized in that Including: Obtaining the current extreme values of each phase of the three-level converter, a preset midpoint current, and a target current of the DC-side midpoint; the current extreme value of each phase is the midpoint current of the corresponding phase under double modulation wave limit modulation. If the target current is between the maximum value among the current extreme values of each phase and the minimum value among the current extreme values of each phase, determining a first target phase and a first modulation wave offset of the first target phase. The target current is between the current extreme value of the first target phase and the preset midpoint current. Determining the double modulation wave of the first target phase according to the first modulation wave offset, and controlling the first target phase according to the double modulation wave of the first target phase to achieve midpoint balance control without injecting zero-sequence components.

2. The control method of the three-level converter according to claim 1, characterized in that, Determining the first modulation wave offset of the first target phase includes: According to calculate the first modulation wave offset Δd1; Among them, m x1 is the fundamental wave of the modulation wave of the first target phase; i NPref is the target current; i NP0 is the preset midpoint current; i NPx1 is the current extreme value of the first target phase.

3. The control method of the three-level converter according to claim 1, wherein The double modulation wave of the first target phase includes a first modulation wave and a second modulation wave. Determining the double modulation wave of the first target phase according to the first modulation wave offset includes: Obtaining the fundamental wave of the modulation wave of the first target phase. When the fundamental wave of the modulation wave of the first target phase is greater than or equal to 0, adding the first modulation wave offset to the fundamental wave of the modulation wave of the first target phase, and subtracting the first modulation wave offset from the preset modulation wave fundamental; when the fundamental wave of the modulation wave of the first target phase is less than 0, subtracting the first modulation wave offset from the fundamental wave of the modulation wave of the first target phase, and adding the first modulation wave offset to the preset modulation wave fundamental. Wherein, the first modulation wave is the fundamental wave of the modulation wave of the first target phase after offset, and the second modulation wave is the preset modulation wave fundamental after offset; the preset modulation wave fundamental is between the two carriers of the first target phase.

4. The control method of the three-level converter according to claim 1, characterized in that After obtaining the current extreme values of each phase of the three-level converter, the preset midpoint current, and the target current of the DC-side midpoint, it further includes: If the target current is not between the maximum value among the current extreme values of each phase and the minimum value among the current extreme values of each phase, determining a second target phase and a second modulation wave offset of the second target phase; the second target phase is the phase with the smallest absolute value of the difference between the current extreme value and the target current. Determining the double modulation wave of the second target phase according to the second modulation wave offset, and controlling the second target phase according to the double modulation wave of the second target phase.

5. The control method of the three-level converter according to claim 4, characterized in that, Determining the second modulation wave offset of the second target phase includes: According to Δd2 = (1 - |m x2 |) / 2, calculate the second modulation wave offset Δd2; where m x2 is the fundamental wave of the modulation wave of the second target phase.

6. The control method of the three-level converter according to any one of claims 1 to 5, characterized in that, Obtaining the target current of the DC-side midpoint of the three-level converter includes: According to i nPref = (u Cp - u Cn ) × C / T s , obtain the target current i NPref ; where, u Cp is the positive half-bus voltage of the three-level converter; u Cn is the negative half-bus voltage of the three-level converter; C is the capacitance value of the half-bus capacitor of the three-level converter; T s is the switching period of the three-level converter.

7. The control method of the three-level converter according to any one of claims 1 to 5, characterized in that, Obtaining the preset midpoint current of the three-level converter includes: According to i NP0 =(1 - m a )×i a +(1 - m b )×i b +(1 - m c )×i c , obtain the preset midpoint current i NP0 ; Obtaining the current extreme values of each phase of the three-level converter includes: According to i NPA =(1 - m b )×i b +(1 - m c )×i c , obtain the extreme value of the current of phase A, i NPA ; According to i NPB = (1 - m a ) × i a + (1 - m c ) × i c , obtain the extreme value i NPB of the current in phase B; According to i NPC =(1 - m b )×i b +(1 - m a )×i a , obtain the extreme value i NPC of the current in phase C; where m a is the fundamental wave of the modulation wave of phase A, and i a is the current of phase A; m b is the fundamental wave of the modulation wave of phase B, and i b is the current of phase B; m c is the fundamental wave of the modulation wave of phase C, and i c is the current of phase C.

8. A three-level converter, characterized in that, Including transformation units and control devices corresponding to three phases respectively; the control device is used to execute the control method of the three-level converter according to any one of claims 1 to 7; the transformation unit is controlled by the control device.

9. A parallel system, characterized in that, Including at least two transformation modules connected in parallel, the transformation module includes the three-level converter according to claim 8, and the DC-side midpoints of each three-level converter are independent of each other.

10. A power supply device, characterized in that, Including the three-level converter according to claim 8, or including the parallel system according to claim 9.

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