A modulation method of an active neutral point clamped inverter

Abstract

The application relates to a modulation method of an active neutral point clamped inverter, and belongs to the technical field of three-level inverters. The method solves the problems of complex algorithm, complicated program, incapability of real-time balancing, large switch loss and the like in the prior art. The method obtains a space vector diagram of the active neutral point clamped inverter, divides the space vector diagram, obtains a plurality of basic vectors, synthesizes a reference vector, allocates the action time of the reference vector, obtains a switch sequence, controls the inverter, uses a voltage control factor to obtain a judgment result of high and low of a neutral point voltage, uses a positive-negative small vector and a corresponding relation between positive and negative of a neutral point current to obtain an action relation of the positive-negative small vector on the neutral point voltage, adjusts the action time of the positive-negative small vector in the action time allocation, and balances the neutral point voltage. The neutral point voltage balancing control strategy adopted by the method uses basic space vectors, so that the algorithm structure complexity of the control strategy is low, the program is simple to realize, the calculation response is fast, and the switch loss is small.

Description

Technical Field

[0001] This invention relates to the field of three-level inverter technology, and more particularly to a modulation method for an active neutral-point clamping inverter. Background Technology

[0002] Traditional three-level NPC (Neutral Point Clamped) inverters use clamping diodes, which cannot generate redundant zero states. Furthermore, some power switching devices have long conduction times and significant losses, resulting in an imbalance of losses among the various power switching devices in the circuit, thus limiting the inverter's capacity and efficiency. Three-level ANPC (Active Neutral Point Clamped) inverters, with their active clamped three-level topology, increase the number of zero-state redundancies to four. This allows for flexible control of the loss distribution balance of the inverter's switching devices by selecting appropriate redundant zero states, thereby improving the inverter's capacity and conversion efficiency. Meanwhile, the three-level ANPC inverter can output 27 space voltage vectors, including 12 small vectors, and half of them, namely 6 redundant small vectors. These small vectors have different effects on the neutral point voltage. If they are not properly allocated and synthesized, it will lead to neutral point voltage imbalance. Neutral point voltage imbalance will lead to an increase in output harmonics, three-phase output asymmetry, and even damage to switching devices. Therefore, it is necessary to optimize the vector pulse width modulation algorithm to achieve the effect of balancing the neutral point voltage.

[0003] In existing three-level inverter research, the virtual space vector method is commonly used to balance the neutral point voltage. This method synthesizes certain basic space vectors into a new virtual space vector. The principle of synthesis is that the basic vectors used to synthesize the vectors can cancel each other out their influence on the neutral voltage. Then, new sectors are divided based on the virtual vector for modulation. One existing method defines a virtual neutral vector, synthesized from two pairs of small vectors and the central vector. Since the small vectors have different effects on the neutral point voltage, the duration of the small vectors is controlled by controlling the neutral point voltage balance coefficient, thereby modulating the neutral point voltage balance. While this method can dynamically adjust the neutral point voltage in real time, it requires redrawing the space vector diagram based on the synthesized virtual space vector. Furthermore, the number of switching sequence segments varies with the reference voltage, and the switching sequence and duration are inconsistent across different sectors of the space vector diagram, increasing computational complexity and program cumbersomeness. Another existing method adjusts the angle space of the upper and lower small vectors, employing a 5-segment switching sequence to achieve neutral point voltage balance control in NPC inverters. Another existing method constructs a thirteen-segment conduction mode containing positive and negative small vectors. When a given space vector is determined to be at a low modulation coefficient, thirteen-segment conduction is used; otherwise, the traditional seven-segment conduction mode is used. While this method can balance the midpoint voltage, it cannot perform balancing in real time. Balancing only occurs when the voltage imbalance reaches a certain level, i.e., the modulation coefficient falls below a certain value. Furthermore, the thirteen-segment balancing mode increases the number of switching transistor operations compared to the seven-segment method, thus increasing switching losses. Another existing method uses positive and negative small vectors to obtain two different sectors, and then obtains the vector action time of the positive and negative small vector sectors respectively, and then synthesizes a virtual vector for modulation. This method also suffers from algorithmic complexity and cumbersome programming.

[0004] In summary, existing methods for balancing the neutral point voltage in three-level inverter technology suffer from drawbacks such as complex algorithms, cumbersome programming, inability to perform balancing in real time, and high switching losses. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a modulation method for an active neutral-point clamping inverter to solve the problems of complex algorithms, cumbersome programs, inability to perform real-time balancing, and large switching losses in existing three-level inverter technology for balancing the neutral-point voltage.

[0006] The objective of this invention is mainly achieved through the following technical solutions:

[0007] This invention provides a modulation method for an active midpoint clamped inverter, comprising the following steps:

[0008] Obtain the spatial vector diagram of the active midpoint clamp inverter;

[0009] The spatial vector map is divided into two levels to obtain multiple primary regions and multiple secondary regions included in each primary region;

[0010] Based on each secondary region and its corresponding primary region, multiple basic vectors are obtained, and a reference vector is synthesized using these multiple basic vectors; wherein, among the multiple basic vectors, two basic vectors are a pair of redundant positive and negative small vectors;

[0011] The application time of the reference vector is allocated to obtain a switching sequence;

[0012] Based on the switching sequence, the active midpoint clamping inverter is controlled, and the voltage control factor is used to obtain the judgment result of the midpoint voltage level; the correspondence between the positive and negative small vectors and the positive and negative midpoint current is used to obtain the effect relationship of the positive and negative small vectors on the midpoint voltage.

[0013] Based on the judgment results of the midpoint voltage level and the relationship between the positive and negative small vectors on the midpoint voltage, the action time of the positive and negative small vectors in the action time allocation is adjusted to balance the midpoint voltage.

[0014] Based on a further improvement of the above method, the topology of the active midpoint clamp inverter includes:

[0015] Each phase arm consists of 6 power switching devices T with freewheeling diodes. x1 T x2 T x3 T x4 T x5 T x6 Composition; where x is 1, 2, or 3;

[0016] Each phase bridge arm outputs V dc / 2, 0 and -V dc / 2 There are three voltage values, denoted as P state, O state, and N state respectively; among them, there are four O level states, denoted as OU2, OU1, OL1, and OL2 respectively; among them,

[0017] In states OU2 and OU1, when the current flows out of the midpoint, Dx5 and Tx2 are connected; when the current flows into the midpoint, Tx5 and Dx2 are connected.

[0018] In OL1 and OL2 states, Tx6 and Dx3 are turned on when the current flows out of the midpoint, and Dx6 and Tx3 are turned on when the current flows into the midpoint.

[0019] Based on a further improvement of the above method, the spatial vector map is divided into two levels to obtain multiple primary regions and multiple secondary regions included in each primary region, including:

[0020] The spatial vector diagram of the topology of the active midpoint clamp inverter is divided into 6 hexagonal primary regions;

[0021] Each hexagonal primary region is divided into 6 sector secondary regions.

[0022] Based on a further improvement of the above method, multiple basic vectors are obtained based on each secondary region and its corresponding primary region, and a reference vector is synthesized using these multiple basic vectors, including:

[0023] Based on each secondary region and its corresponding primary region, four basic vectors are determined for the synthetic reference vector, and the reference vector is synthesized using the four basic vectors.

[0024] Based on a further improvement to the above method, the application time of the reference vector is allocated to obtain a switching sequence, including:

[0025] The application time of the reference vector, i.e., the four basic vectors, is allocated in seven segments to obtain the switching sequence.

[0026] Based on further improvements to the above method, a voltage control factor is used to obtain the judgment result of the midpoint voltage level, including:

[0027] When the voltage control factor is greater than zero, the midpoint voltage is judged to be too low;

[0028] When the voltage control factor is less than zero, the midpoint voltage is judged to be too high.

[0029] Based on a further improvement of the above method, the voltage control factor, denoted as f, is expressed as:

[0030] f = (V DC1 -V DC2 ) / V ref

[0031] Among them, V DC1 V DC2 These are the upper and lower capacitor voltages on the DC side, respectively, V ref This refers to the fluctuation range of the midpoint voltage control.

[0032] Based on further improvements to the above method, the correspondence between the positive and negative small vectors and the positive and negative values ​​of the midpoint current is obtained by deriving the topology of the active midpoint clamped inverter and testing the midpoint current.

[0033] Based on further improvements to the above method, the relationship between the positive and negative small vectors and the midpoint voltage includes:

[0034] The current flowing out of the inverter is taken as positive;

[0035] According to the current value measured in each sampling period, the influence of each positive and negative small vector on the increase or decrease of the neutral point voltage is determined: when the neutral point current is positive, it has a decreasing effect on the neutral point voltage; when the neutral point current is negative, it has an increasing effect on the neutral point voltage.

[0036] Based on the further improvement of the above method, adjusting the action time of positive and negative small vectors in the action time distribution includes:

[0037] In the 7-segment distribution of the action time of the reference vector, the sum of the action times of the positive and negative small vectors is constructed as T; wherein the action time of the positive small vector voltage is T p , the action time of the negative small vector is T n , then the sum T of the action times of the positive and negative small vectors is T p +T n ;

[0038] Construct reference values t for the two times ref1 , t ref2 , and a flag K;

[0039] Test the current, if the current is positive, mark K=1; if the current is negative, mark K=0;

[0040] When the voltage difference between the upper capacitor and the lower capacitor is greater than V ref , f<-1 or f>1; wherein when f<-1, t ref1 =T, t ref2 =0; when f>1, t ref1 =0, t ref2 =T; when -1<f<1, t ref1 =T×(1-f) / 2, t ref2 =T×(1+f) / 2;

[0041] According to the three-phase current I measured by the current Hall sensor a , I b , I c positive and negative values; if the current is positive, T p =t ref1 , T n =t ref2 ; if the current is negative, T p =t ref2 , T n =t ref1 .

[0042] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0043] 1. This invention proposes a modulation method for an active midpoint clamping inverter and designs a novel midpoint voltage balancing strategy. The three-level vector space is divided into six hexagonal primary regions. For any reference vector falling within any position of each primary region, the basic vector used to synthesize it includes a pair of redundant positive and negative small vectors. These redundant positive and negative small vectors have opposite effects on the midpoint voltage. When the midpoint voltage is too high or too low, the midpoint voltage can be balanced by adjusting the duration of the positive and negative small vectors. During the dynamic operation of the inverter, the midpoint voltage can be monitored in real time. The midpoint voltage can be balanced for any reference vector in each switching cycle. The midpoint voltage balancing control strategy adopted in this invention does not synthesize virtual space vectors but uses basic space vectors, resulting in low algorithm complexity, simple program implementation, and fast computational response.

[0044] 2. This invention proposes a modulation method for an active neutral-point clamping inverter, which can achieve neutral-point voltage balance by using a seven-segment switching sequence, and can reduce switching losses while balancing the neutral-point voltage in real time.

[0045] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0046] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0047] Figure 1 A flowchart of the modulation method for an active midpoint clamped inverter provided by the present invention;

[0048] Figure 2 The topology diagram of the active midpoint clamping inverter provided by the present invention;

[0049] Figure 3 A spatial vector diagram of the topology of the active midpoint clamp inverter provided by this invention;

[0050] Figure 4 The basic space vector diagram for converting three-level signals to two-level signals provided by this invention. Detailed Implementation

[0051] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0052] Example 1

[0053] A specific embodiment of the present invention discloses a modulation method for an active midpoint clamped inverter, the flow of which is as follows: Figure 1 As shown, it includes the following steps:

[0054] S1. Based on the topology of the active midpoint clamp inverter, obtain the spatial vector diagram of the topology of the active midpoint clamp inverter;

[0055] The spatial vector map is divided into two levels to obtain multiple primary regions and multiple secondary regions included in each primary region;

[0056] Based on each secondary region and its corresponding primary region, multiple basic vectors are obtained, and a reference vector is synthesized using these multiple basic vectors; wherein, among the multiple basic vectors, two basic vectors are a pair of redundant positive and negative small vectors;

[0057] The application time of the reference vector is allocated to obtain the switching sequence.

[0058] S2. Based on the switching sequence, control the active midpoint clamping inverter, use the voltage control factor to obtain the judgment result of the midpoint voltage level; use the correspondence between the positive and negative small vectors and the positive and negative midpoint current to obtain the effect relationship of the positive and negative small vectors on the midpoint voltage.

[0059] S3. Based on the judgment results of the midpoint voltage level and the relationship between the positive and negative small vectors on the midpoint voltage, adjust the action time of the positive and negative small vectors in the action time allocation to balance the midpoint voltage.

[0060] Example 2

[0061] Based on Example 1, step S1 can be further refined into the following steps:

[0062] S11. Based on the topology of the active midpoint clamp inverter, obtain the spatial vector diagram of the topology of the active midpoint clamp inverter.

[0063] Specifically, the topology of an active midpoint clamp inverter is as follows: Figure 2 As shown.

[0064] Each phase arm consists of 6 power switching devices T with freewheeling diodes. x1 T x2 T x3 Tx4 T x5 T x6 Composition; where x is 1, 2 or 3.

[0065] Each phase bridge arm outputs V dc / 2, 0 and -V dc / 2 There are three voltage values, denoted as P state, O state, and N state respectively; among them, there are four O level states, denoted as OU2, OU1, OL1, and OL2 respectively. For example, the output V of each phase bridge arm is... dc / 2, 0 and -V dc The three voltage values ​​and their corresponding switching states are represented in Table 1. By reasonably allocating the working time and working state of these four redundant zero states of the 0 level, the loss balance of each switching device can be controlled.

[0066] In states OU2 and OU1, when the current flows out of the midpoint, Dx5 and Tx2 are connected, and when the current flows into the midpoint, Tx5 and Dx2 are connected. In states OL1 and OL2, when the current flows out of the midpoint, Tx6 and Dx3 are connected, and when the current flows into the midpoint, Dx6 and Tx3 are connected.

[0067] Table 1. Topology and Switching States of Active Midpoint Clamped Inverters

[0068] <![CDATA[V dc / 2]]> P 1 1 0 0 0 1 0 OU2 0 1 0 0 1 0 0 OU1 0 1 0 1 1 0 0 OL1 1 0 1 0 0 1 0 OL2 0 0 1 0 0 1 <![CDATA[-V dc / 2]]> N 0 0 1 1 1 0

[0069] Considering that different output voltages of the three phases correspond to different space vectors, and since each phase can output three voltage values, the output voltage of the active neutral-point clamped inverter topology has 27 space vectors. The position of each vector in the space vector diagram is as follows: Figure 3 The diagram shows the spatial vector diagram of the topology of an active midpoint clamp inverter. There are 3 zero vectors: PPP, OOO, and NNN; 12 small vectors: POO, ONN, PPO, OON, NON, OPO, OPP, NOO, OOP, NNO, POP, and ONO; 6 medium vectors: PON, OPN, NPO, NOP, ONP, and PNO; and 6 large vectors: PNN, PPN, NPN, NPP, NNP, and PNP.

[0070] S12. Divide the spatial vector diagram of the topology of the active midpoint clamping inverter to obtain multiple primary regions, i.e., large regions; divide each primary region into multiple secondary regions, i.e., small regions.

[0071] It should be noted that converting a three-level signal to a two-level signal simplifies the synthesis of the reference vector and the calculation of the duration. Therefore, the spatial vector region division for converting a three-level signal to a two-level signal is as follows: Figure 4As shown.

[0072] Specifically, the spatial vector diagram of the topology of the active midpoint clamp inverter is divided into 6 large hexagonal regions, each of which contains 6 small sectors.

[0073] Considering that each large hexagonal region is a spatial vector diagram of two levels, the angle of the voltage vector can be used to determine the large hexagonal region in which it is located. Then, the coordinate system is transformed within the large hexagonal region, and vector synthesis is performed according to the traditional two-level algorithm.

[0074] Based on each small region and its corresponding large region, four basic vectors are obtained, and a reference vector is synthesized using the four basic vectors. Among the four basic vectors, two are a pair of redundant positive and negative small vectors, and the positive and negative small vectors have opposite effects on the increase or decrease of the midpoint voltage.

[0075] S13. Allocate the application time of the reference vector to obtain the switching sequence.

[0076] It is worth noting that all 12 small vectors in the large hexagonal regions are converted into the zero vectors of the corresponding large hexagonal regions. In the traditional two-level SVPWM seven-segment allocation, since the zero vectors are redundant, the two redundant zero vectors are usually used as the transition of the seven-segment switching state.

[0077] In each large hexagonal region, the application time of the four basic vectors, i.e., the reference vectors, is allocated in seven segments to obtain the switching sequence.

[0078] Taking the first large hexagonal region as an example, the switching sequence after the first small sector is allocated in 7 segments is ONN-PNN-PON-POO-PON-PNN-ONN; where ONN and POO are a pair of redundant positive and negative small vectors.

[0079] Specifically, the switching sequences of each large hexagonal region and its corresponding small region are given in Table 2.

[0080] Table 2. Switching sequences for each hexagonal large region and its corresponding small region.

[0081]

[0082]

[0083] Preferably, step S2 can be further refined into the following steps:

[0084] S21. Construct a voltage control factor, determine the midpoint voltage level, and obtain the determination result of the midpoint voltage level.

[0085] Specifically, the voltage control factor f is constructed as follows:

[0086] f = (V DC1 -V DC2 ) / V ref

[0087] Among them, V DC1 V DC2 These are the upper and lower capacitor voltages on the DC side, respectively, V ref V represents the fluctuation amplitude of the midpoint voltage control. ref The value of f should be set according to the user's actual needs or the relevant parameters of the capacitor; when f>0, it indicates that the midpoint potential is too low, that is, the midpoint voltage is too low; when f<0, it indicates that the midpoint potential is too high, that is, the midpoint voltage is too high.

[0088] S22. Construct the correspondence between the positive and negative small vectors and the positive and negative signs of the midpoint current to obtain the relationship between the positive and negative small vectors and the effect of the midpoint voltage.

[0089] The current flowing out of the inverter is defined as positive. Based on the current value measured in each sampling period, the influence of each positive and negative small vector on the increase or decrease of the midpoint voltage is determined. When the midpoint current is positive, it has a decreasing effect on the midpoint voltage, and when the midpoint current is negative, it has an increasing effect on the midpoint voltage.

[0090] For example, if the current flowing out of the midpoint is defined as positive, then the midpoint current in the POO state is -I. a The midpoint current in the ONN state is I. a If at some moment I a If the value is positive, the midpoint voltage increases in the POO state, while it decreases in the ONN state.

[0091] It is not difficult to see that the two small vectors have completely opposite effects on the midpoint voltage. Therefore, the midpoint voltage can be monitored in real time, and the midpoint voltage can be balanced by adjusting the action time of the two small vectors, without affecting the synthesis of the space vector.

[0092] Therefore, by measuring the midpoint current under the action of each positive and negative small vector, the correspondence between the positive and negative small vectors and the positive and negative midpoint currents is constructed as shown in Table 3; then, based on this correspondence, the effect of the positive and negative small vectors on the midpoint voltage is obtained.

[0093] Table 3. Correspondence between positive and negative small vectors and the sign of the midpoint current.

[0094] ONN <![CDATA[I a ]]> POO <![CDATA[-I a ]]> PPO <![CDATA[I c ]]> OON <![CDATA[-I c ]]> NON <![CDATA[I b ]]> OPO <![CDATA[-I b ]]> OPP <![CDATA[I a ]]> NOO <![CDATA[-I a ]]> NNO <![CDATA[I c ]]> OOP <![CDATA[-I c ]]> POP <![CDATA[I b ]]> ONO <![CDATA[-I b ]]>

[0095] Preferably, step S3 can be further refined into the following steps:

[0096] S31. Based on the judgment result of the neutral point voltage level and the action relationship of positive and negative small vectors on the neutral point voltage, adjust the action durations of positive and negative small vectors in the action duration allocation, thereby balancing the neutral point voltage.

[0097] Specifically, in the 7-segment allocation of the action duration of the reference vector, construct that the sum of the action durations of positive and negative small vectors is T, wherein the action duration of the positive small vector voltage is T p , the action duration of the negative small vector is T n , then the sum T of the durations of positive and negative small vectors is T p +T n .

[0098] Construct reference values t ref1 and t ref2 , as well as a flag K.

[0099] Test the current, if the current is positive, mark K=1, and if the current is negative, mark K=0.

[0100] By way of example, according to engineering experience, take V ref as 30, then when the voltage difference between the upper capacitor and the lower capacitor is greater than 30V, f<-1 or f>1; wherein, when f<-1, t ref1 =T, t ref2 =0; when f>1, t ref1 =0, t ref2 =T; when -1<f<1, t ref1 =T×(1-f) / 2, t ref2 =T×(1+f) / 2.

[0101] According to the positive and negative values of three-phase currents I a , I b and I c measured by the current Hall sensor, if the current is a positive value, T p =t ref1 , T n =t ref2 ; if the current is a negative value, T p =t ref2 , T n =t ref1 .

[0102] It can be seen that the switching sequence remains unchanged, and the sum of the durations of positive and negative small vectors also remains unchanged. Based on obtaining the judgment result of the neutral point voltage level and the action relationship of positive and negative small vectors on the neutral point voltage, it is only necessary to adjust the action durations of positive and negative small vectors in the action duration allocation to balance the neutral point voltage.

[0103] Compared with existing technologies, this invention proposes a modulation method for an active midpoint clamping inverter and designs a novel midpoint voltage balancing strategy. The three-level vector space is divided into six large hexagonal regions. For any reference vector falling within any position of each region, the basic vector used to synthesize it includes a pair of redundant positive and negative small vectors. These redundant positive and negative small vectors have opposite effects on the midpoint voltage. When the midpoint voltage is too high or too low, the midpoint voltage can be balanced by adjusting the duration of the positive and negative small vectors. During the dynamic operation of the inverter, the midpoint voltage can be monitored in real time. The midpoint voltage can be balanced for any reference vector in each switching cycle. The midpoint voltage balancing control strategy adopted in this invention does not synthesize virtual space vectors but uses basic space vectors, resulting in low algorithm complexity, simple program implementation, fast computational response, and low switching losses.

[0104] Those skilled in the art will understand that all or part of the processes of the methods described in the above embodiments can be implemented by a computer program instructing related hardware, and the program can be stored in a computer-readable storage medium. The computer-readable storage medium may be a disk, optical disk, read-only memory, or random access memory, etc.

[0105] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A modulation method for an active midpoint clamped inverter, characterized in that, Includes the following steps: Obtain the spatial vector diagram of the active midpoint clamp inverter; The spatial vector map is divided into two levels to obtain multiple primary regions and multiple secondary regions included in each primary region; Based on each secondary region and its corresponding primary region, multiple basic vectors are obtained, and a reference vector is synthesized using these multiple basic vectors; wherein, among the multiple basic vectors, two basic vectors are a pair of redundant positive and negative small vectors; The application time of the reference vector is allocated to obtain a switching sequence; Based on the switching sequence, the active midpoint clamping inverter is controlled, and the voltage control factor is used to obtain the judgment result of the midpoint voltage level; the correspondence between the positive and negative small vectors and the positive and negative midpoint current is used to obtain the effect relationship of the positive and negative small vectors on the midpoint voltage. Based on the judgment results of the midpoint voltage level and the relationship between the positive and negative small vectors on the midpoint voltage, the action time of the positive and negative small vectors in the action time allocation is adjusted to balance the midpoint voltage. The adjustment of the action time allocation for positive and negative small vectors includes: In the seven-segment allocation of the reference vector's action time, the sum of the action times of the positive and negative small vectors is T; among which, the action time of the positive small vector voltage is T. p The duration of action of the negative small vector is T. n Then the sum of the positive and negative small vector times is T = T p +T n ; Construct two time reference values ​​t ref1 t ref2 and a symbol K; Test the current. If the current is positive, mark K=1; if the current is negative, mark K=0. When the voltage difference between the upper capacitor and the lower capacitor is greater than V ref , f<-1 or f>1; wherein when f<-1, t ref1 =T, t ref2 =0; when f>1, t ref1 =0, t ref2 =T; when -1<f<1, t ref1 =T×(1-f) / 2, t ref2 =T×(1+f) / 2; Among them, V ref Here, f represents the fluctuation range of the midpoint voltage control, and f is the voltage control factor. The three-phase current I measured by the current Hall effect. a I b I c The sign of the current; if the current is positive, then T p =t ref1 T n =t ref2 If the current is negative, then T p =t ref2 T n =t ref1 .

2. The modulation method for an active midpoint clamped inverter according to claim 1, characterized in that, The topology of the active midpoint clamp inverter includes: Each phase arm consists of 6 power switching devices T with freewheeling diodes. x1 T x2 T x3 T x4 T x5 T x6 Composition; where x is 1, 2, or 3; Each phase bridge arm outputs V dc / 2, 0 and -V dc / 2 represents three voltage values, denoted as P state, O state, and N state respectively; among them, there are four O-level states, denoted as OU2, OU1, OL1, and OL2 respectively; among them, In states OU2 and OU1, when the current flows out of the midpoint, Dx5 and Tx2 are connected; when the current flows into the midpoint, Tx5 and Dx2 are connected. In OL1 and OL2 states, Tx6 and Dx3 are turned on when the current flows out of the midpoint, and Dx6 and Tx3 are turned on when the current flows into the midpoint.

3. The modulation method for an active midpoint clamped inverter according to claim 2, characterized in that, The spatial vector map is divided into two levels to obtain multiple primary regions and multiple secondary regions included in each primary region, including: The spatial vector diagram of the topology of the active midpoint clamp inverter is divided into 6 hexagonal primary regions; Each hexagonal primary region is divided into 6 sector secondary regions.

4. The modulation method for an active midpoint clamped inverter according to claim 3, characterized in that, Based on each of the secondary regions and its corresponding primary region, multiple basic vectors are obtained, and a reference vector is synthesized using these multiple basic vectors, including: Based on each secondary region and its corresponding primary region, four basic vectors are determined for the synthetic reference vector, and the reference vector is synthesized using the four basic vectors.

5. The modulation method for an active midpoint clamped inverter according to claim 4, characterized in that, The application time of the reference vector is allocated to obtain a switching sequence, including: The application time of the reference vector, i.e., the four basic vectors, is allocated in seven segments to obtain the switching sequence.

6. The modulation method for an active midpoint clamped inverter according to claim 5, characterized in that, Using the voltage control factor, the determination result of the midpoint voltage level is obtained, including: When the voltage control factor is greater than zero, the midpoint voltage is judged to be too low; When the voltage control factor is less than zero, the midpoint voltage is judged to be too high.

7. The modulation method for an active midpoint clamped inverter according to claim 6, characterized in that, The voltage control factor f is expressed as: f=(V DC1 -V DC2 ) / V ref Among them, V DC1 V DC2 These are the upper capacitor voltage and lower capacitor voltage on the DC side, respectively.

8. The modulation method for an active midpoint clamped inverter according to claim 7, characterized in that, By deriving the topology of the active midpoint clamping inverter and testing the midpoint current, the correspondence between the positive and negative small vectors and the positive and negative midpoint currents is obtained.

9. The modulation method for an active midpoint clamped inverter according to claim 8, characterized in that, The relationship between the positive and negative small vectors and the midpoint voltage includes: The current flowing out of the inverter is taken as positive; Based on the current values ​​measured in each sampling period, the influence of each positive and negative small vector on the increase or decrease of the midpoint voltage is determined: when the midpoint current is positive, it has a decreasing effect on the midpoint voltage; when the midpoint current is negative, it has an increasing effect on the midpoint voltage.

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