Voltage regulation method for three-phase four-leg three-level inverter

Abstract

The present invention proposes a voltage regulation method for a three-phase four-leg three-level inverter, which can balance the voltage at the midpoint of the inverter. [Solution] The method includes the steps of determining voltage vectors corresponding to three-phase voltages, constructing a spatial polyhedron controllable region based on a three-dimensional coordinate system of ABC and the voltage vectors, positioning predetermined reference vectors in multiple subregions of the spatial polyhedron controllable region to generate reference voltage vectors corresponding to the reference vectors, and using the reference vectors to control the output states of the corresponding four legs, ordering the reference voltage vectors with an adjustment strategy to reduce zero-level output states, and generating switching pulse trains for the legs based on the ordered reference voltage vectors and their corresponding action times to adjust the voltage at the midpoint of the three-phase four-leg three-level inverter.

Description

[Technical Field]

[0001] The present invention relates to the field of electronic technology, and in particular to a voltage regulation method for a three-phase four-leg three-level inverter. [Background technology]

[0002] With the development of electronic technology, multilevel inverters have attracted much attention for medium and high voltage output, and various circuit topologies and inverter control methods have been proposed. Multilevel inverters are gradually entering the practical stage, and there is a relatively large market demand, especially in China. With people's awareness of power saving increasing and driven by market interests, China's potential market demand must be converted into actual market demand, so that medium-voltage high-power inverter products have a broad market prospect.

[0003] The control process of a multilevel inverter always has the following problem: the balance problem at the midpoint of the DC side capacitor. Due to the topology of the multilevel inverter, the diode-clamped PWM inverter has the problem of capacitor voltage imbalance caused by the unbalanced charging and discharging of the DC voltage divider capacitor. The change in the capacitor voltage at the midpoint is mainly affected by the current flowing at the midpoint of each phase. When the capacitor voltage at the midpoint is unbalanced, DC bias and distortion will occur in the output load voltage waveform, so the balance problem at the midpoint must be solved. Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention aims to propose a voltage regulation method for a three-phase four-leg three-level inverter, so as to improve / solve the problem of voltage imbalance at the midpoint of the multilevel inverter in the related art. [Means for solving the problem]

[0005] The present invention provides a voltage regulation method for a three-phase four-leg three-level inverter, comprising the steps of: determining voltage vectors corresponding to three-phase voltages; constructing a spatial polyhedron controllable region based on a three-dimensional coordinate system of ABC and the voltage vectors; positioning predetermined reference vectors in a plurality of sub-regions of the spatial polyhedron controllable region to generate reference voltage vectors corresponding to the reference vectors, and using the reference vectors to control output states of corresponding four legs; ordering the reference voltage vectors with an adjustment strategy to reduce zero-level output states; and generating leg switching pulse trains to regulate voltages at midpoints of the three-phase four-leg three-level inverter based on the ordered reference voltage vectors and their corresponding action times.

[0006] In some embodiments, determining the voltage vectors corresponding to the three-phase voltages includes determining the voltage vectors corresponding to the three-phase voltages according to output states of the legs, the output states including a high level, a zero level, and a low level.

[0007] In some embodiments, the step of determining a voltage vector corresponding to the three-phase voltages according to the output states of the legs includes determining a voltage vector corresponding to the three-phase voltages according to the following equation:

number

number

[0008] In some embodiments, the step of constructing a spatial polyhedron controllable region based on the three-dimensional coordinate system of ABC and the voltage vector includes the steps of normalizing the voltage vector and introducing the normalized voltage vector into the three-dimensional coordinate system of ABC to construct a spatial polyhedron controllable region.

[0009] In some embodiments, the step of normalizing the voltage vector comprises normalizing the voltage vector according to the following formula:

number

number

number

number

[0010] In some embodiments, the spatial polyhedron controllable area is a dodecahedron controllable area, and the dodecahedron controllable area includes three cubic areas of normalized voltage vectors and six fictitious cubic areas attached to the outside of the three cubic areas.

[0011] In some embodiments, the step of positioning a predetermined reference vector in a plurality of sub-regions of the spatial polyhedron controllable region to generate a reference voltage vector corresponding to the reference vector includes the steps of positioning a predetermined reference vector in a cubic region of the plurality of sub-regions of the spatial polyhedron controllable region, positioning a predetermined reference vector in a tetrahedral region of the plurality of sub-regions of the spatial polyhedron controllable region, and generating a reference voltage vector corresponding to the reference vector based on the final positioning result.

[0012] In some embodiments, the step of positioning a predetermined reference vector in a cubic region of a plurality of subregions of the controllable region of the spatial polyhedron includes selecting a cube in the controllable region of the spatial polyhedron where a reference original coordinate point is located as follows, based on a reference original coordinate point obtained by rounding the reference vector to an integer:

number

number

number

number

[0013] In some embodiments, the step of positioning a predetermined reference vector in the tetrahedron region of multiple subregions of the spatial polyhedron controllable region includes: using an original coordinate point corresponding to the reference vector as a reference point for the positioned target cube, dividing it using three 45° cutting planes, calculating the distances of the three cutting planes with the reference original coordinate point as the center, and determining the position of the tetrahedron where the reference vector is located, wherein the three 45° cutting planes include a 45° cutting plane of the AB axis, a 45° cutting plane of the CB axis, and a 45° cutting plane of the AC axis that pass through the original coordinate point.

[0014] In some embodiments, the step of ordering the reference voltage vectors with an adjustment strategy to reduce zero-level output states includes determining a composite order of reference voltage vectors in a spatial polyhedron controllable region and adjusting the order of the reference voltage vectors with the goal of reducing the current of each leg injected into a midpoint.

[0015] In some embodiments, the step of generating a leg switching pulse train based on the ordered reference voltage vectors and their corresponding durations comprises determining pulse duty factors corresponding to the reference voltage vectors according to the following formula:

number

number

[0016] In some embodiments, the step of generating a switching pulse train for the legs based on the ordered reference voltage vectors and their corresponding application times includes: ordering the application times of the reference voltage vectors based on an arrangement pattern of the ordered reference voltage vectors; performing a nine-stage allocation for the ordered application times, where the nine-stage allocation is performed as follows, assigning the same period to the first, fifth, and ninth stages of the nine-stage vector, with the first and ninth stages each occupying ¼, the fifth stage occupying ½, and the remaining six stages symmetrically occupying ½ of their corresponding periods; and determining the states of the four legs based on the voltage vectors, and allocating four corresponding switching states to each state to form a pulse train.

[0017] The voltage regulation method for a three-phase four-leg three-level inverter proposed by the present invention includes the steps of: determining voltage vectors corresponding to three-phase voltages; constructing a spatial polyhedron controllable region based on the three-dimensional coordinate system of ABC and the voltage vectors; positioning predetermined reference vectors in multiple subregions of the spatial polyhedron controllable region to generate reference voltage vectors corresponding to the reference vectors, and using the reference vectors to control the output states of the corresponding four legs; ordering the reference voltage vectors with an adjustment strategy to reduce the zero-level output state; and generating switching pulse trains for the legs based on the ordered reference voltage vectors and their corresponding action times to regulate the voltage at the midpoint of the three-phase four-leg three-level inverter. [Effects of the Invention]

[0018] The present invention has the following beneficial effects: (1) The voltage balance at the midpoint of a three-phase four-leg three-level inverter is achieved without adding a controller. (2) By introducing six virtual cubes, the position of the reference vector can be determined reliably, and the redundant controllable region of the conventional SVPWM adjustment method is reduced, thereby reducing the complexity of the conventional SVPWM adjustment method. (3) A regulation strategy aims to reduce the current flowing into the midpoint in each leg, sequentially modifying the original reference voltage vector to balance the zero-level output condition, which affects the voltage balance at the midpoint. [Brief explanation of the drawings]

[0019] In order to more clearly explain the technical solutions of the embodiments of the present invention, we will now briefly describe the drawings necessary for the embodiments. It should be understood that the following drawings only show specific embodiments of the present invention, and therefore should not be considered as limiting the scope. Those skilled in the art can also obtain other related drawings based on these drawings without paying creative labor. [Figure 1] 1 is a structural schematic diagram of a three-phase four-leg three-level inverter according to the present invention; [Figure 2] 1 is an exemplary flow diagram of a method for regulating voltage of a three-phase four-leg three-level inverter according to the present invention. [Figure 3] 1 is a flow diagram of step 22 as an example. [Figure 4] 1 is a schematic diagram of a spatial polyhedron controllable region according to the present invention; [Figure 5] 1 is a flow diagram of step 23 as an example. [Figure 6] 1 is a process diagram of cube positioning according to the present invention; FIG. [Figure 7] 1 is a process diagram of tetrahedron positioning according to the present invention; FIG. [Figure 8] 1 is a flow diagram of an example of step 25. [Figure 9] FIG. 1 is a schematic diagram of a nine-stage time allocation method in accordance with the present invention. [Figure 10] FIG. 10 is a point location diagram of the intermediate point under balanced load conditions according to the present invention. [Figure 11] FIG. 10 is a point location diagram of the midpoint under balanced load conditions after balancing is performed using the voltage adjustment method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] The technical means in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it goes without saying that the described embodiments are not all of the embodiments but only a part of the embodiments of the present application. Any other embodiments that a person skilled in the art can obtain based on the embodiments of the present application without any creative effort shall all be included in the scope of protection of the present application.

[0021] As described herein, the orientations and positional relationships indicated by terms such as "center," "longitudinal," "lateral," "length," "width," "thickness," "top," "bottom," "front," "rear," "left," "right," "vertical," "horizontal," "up," "bottom," "inside," "outside," "clockwise," "counterclockwise," and the like, which refer to orientations and positional relationships shown in the drawings, are merely for the convenience of simplifying the description and explanation of this application, and should not be understood as limiting this application, as they do not expressly or imply that the devices or components referred to must be in a particular orientation, or be configured or operated according to a particular orientation. Moreover, the terms "first" and "second," which are for descriptive purposes only, should not be understood as expressing or implying a relative importance or number of technical features. Thus, features qualified as "first" and "second" may explicitly or implicitly include one or more features. "Plurality" as used in this application means two or more unless expressly and specifically limited otherwise.

[0022] "A and / or B" corresponds to three meanings: A only, B only, and a combination of A and B.

[0023] In the present invention, the terms "suitable for" or "configured with" are intended to be open-ended and do not exclude situations where the term is suitable for or configured with equipment that performs additional tasks or steps. Moreover, the term "based on" is intended to be open-ended because the process, step, calculation, or other situation that is "based on" one or more of the conditions or values ​​may actually be based on additional conditions or exceed the values.

[0024] In the present invention, the term "exemplary" means "serving as an example, instance, or example." In the present invention, any embodiment described as "exemplary" should not necessarily be construed as preferred or advantageous over other embodiments. The following description is provided to enable those skilled in the art to implement and apply the present invention. In the following description, details are enumerated for clarity. It should also be noted that those skilled in the art may recognize that the present invention can be implemented without these specific details. In other embodiments, well-known structures and processes are not detailed to avoid obscuring the description of the present invention with unnecessary details. Therefore, the present invention is not intended to be limited to the embodiments, but is to be accorded the widest possible scope consistent with the principles and features disclosed herein.

[0025] As shown in Figure 1, Figure 1 is a structural schematic diagram of a three-phase four-leg three-level inverter according to the present invention. The topology of the three-phase four-leg three-level inverter includes a DC voltage source, and from left to right, A leg, B leg, C leg, and N leg are provided. Each leg consists of four switches (e.g., IGBTs) and two diodes, and the output point of each leg is ux (x = a, b, c, n). The midpoint of the diode of each leg is connected to the midpoint n of the splitting capacitor. In this embodiment of the present invention, it is necessary to adjust the voltage at the midpoint n of the splitting capacitor.

[0026] As shown in Figure 2, Figure 2 is an exemplary flow chart of a voltage regulation method for a three-phase four-leg three-level inverter according to the present invention, which includes the following steps:

[0027] S21: Determine the voltage vectors corresponding to the three-phase voltages.

[0028] Preferably, in one example, a voltage vector corresponding to the three-phase voltage is determined according to the output states of the legs, where the output states include a high level, a zero level, and a low level.

[0029] First, Sa, Sb, Sc, and Sn are defined as the output states of the A leg, B leg, C leg, and N leg of the three-phase four-leg three-level inverter, respectively.

[0030]

number

number

[0031] In the formula, U AN represents the voltage vector of leg A relative to leg N; U BN represents the voltage vector of leg B relative to leg N; U CN represents the voltage vector of leg C relative to leg N; S a , S b , S c , S n respectively represent the output states of A leg, B leg, C leg and N leg of the three-phase four-leg three-level inverter; D uc represents the DC power supply voltage.

[0032] S22: Construct a spatial polyhedron controllable region based on the ABC three-dimensional coordinate system and voltage vector.

[0033] Preferably, in one example, the spatial polyhedron controllable region includes a plurality of cubic subregions and a plurality of tetrahedron subregions.

[0034] Preferably, as shown in Figure 3, Figure 3 is a flow chart of an example of step 22. The process includes the following steps:

[0035] S221: Normalize the voltage vector.

[0036] Specifically, the voltage vector is normalized according to the following formula:

[0037]

number

number

number

number

[0038] S222: The normalized voltage vector is introduced into the three-dimensional coordinate system of ABC to construct the spatial polyhedron controllable region.

[0039] As shown in Figure 4, Figure 4 is a schematic diagram of the spatial polyhedron controllable area according to the present invention. After normalizing the voltage vector, the normalized voltage vector is introduced into the three-dimensional coordinate system of ABC to construct the dodecahedron controllable area.

[0040] In one example, the controllable region of this spatial polyhedron includes three large cubes and six virtual cubes, which are controllable regions with an adjustment ratio in the range of 0.866 to 1. The controllable region is shifted to the positive direction of the ABC coordinate system for the convenience of integrated analysis and calculation.

[0041] S23: In a plurality of sub-regions of the spatial polyhedron controllable region, a predetermined reference vector is positioned to generate a reference voltage vector corresponding to the reference vector, and the reference vector is used to control the output states of the corresponding four legs.

[0042] In one example, as shown in Figure 5, Figure 5 is a flow chart of an example of step 23. The process includes the following steps:

[0043] S231: A predetermined reference vector is positioned in a cubic region of a plurality of subregions of the spatial polyhedron controllable region.

[0044] Among them, a cube in which the reference original coordinate point is located in the controllable region of the spatial polyhedron is selected based on the reference original coordinate point obtained by rounding the reference vector to an integer.

[0045] Among them, the reference coordinate points are as follows:

[0046]

number

number

number

number

[0047] As shown in FIG. 6, FIG. 6 is a process diagram of the positioning of the cube according to the present invention.

[0048] In a specific embodiment, the reference original coordinate point (a, b, c) is calculated using the above formula, and then a decision is made according to the logical sequence of FIG.

[0049] a<0.5, b<0.5, c<0.5, select the cube as cube1; a<0.5, b<0.5, c≥0.5, select the cube as cube4; a<0.5, 0.5≦b<1.5, c<0.5, select the cube as cube3; a<0.5, 0.5≦b<1.5, c≧0.5, select the cube as cube6; a<0.5, b≥1.5, select the cube as cube27; 0.5≦a<1.5, b<0.5, c<0.5, select the cube as cube2; 0.5≦a<1.5, b<0.5, c≧0.5, select the cube as cube5; 0.5≦a<1.5, 0.5≦b<1.5, c<0.5, select the cube as cube7; 0.5≦a<1.5, 0.5≦b<1.5, 0.5≦c<1.5, select the cube as cube8; 0.5≦a<1.5, 0.5≦b<1.5, 1.5≦c<2.5, select the cube as cube11; 0.5≦a<1.5, 0.5≦b<1.5, c≧2.5, select the cube as cube26; 0.5≦a<1.5, 1.5≦b<2.5, c<1.5, select the cube as cube10; 0.5≦a<1.5, 1.5≦b<2.5, c≧1.5, select the cube as cube13; 0.5≦a<1.5, b≧2.5, select the cube as cube28; 1.5≦a<2.5, b<0.5, select the cube as cube25; 1.5≦a<2.5, 0.5≦b<1.5, c<1.5, select the cube as cube9; 1.5≦a<2.5, 0.5≦b<1.5, c≧1.5, select the cube as cube12; 1.5≦a<2.5, 1.5≦b<2.5, c<0.5, select the cube as cube23; 1.5≦a<2.5, 1.5≦b<2.5, 0.5≦c<1.5, select the cube as cube14; 1.5≦a<2.5, 1.5≦b<2.5, 1.5≦c<2.5, select the cube as cube15; 1.5≦a<2.5, 1.5≦b<2.5, c≧2.5, select the cube as cube18; 1.5≦a<2.5, b≧2.5, c<2.5, select the cube as cube17; 1.5≦a<2.5, b≧2.5, c≧2.5, select the cube as cube20; a≧2.5, b<1.5, select the cube as cube24; a≧2.5, 1.5≦b<2.5, c<1.5, select the cube as cube16; a≧2.5, 1.5≦b<2.5, c≧1.5, select the cube as cube19; a ≥ 2.5, b ≥ 2.5, c < 2.5, select the cube as cube21; a≧2.5, b≧2.5, c≧2.5, and select the cube as cube22.

[0050] S232: Position predetermined reference vectors in the tetrahedron regions of the plurality of subregions of the spatial polyhedron controllable region.

[0051] The original coordinate point corresponding to the reference vector is used as the reference point for the positioned target cube, and the cube is divided using three 45° cut planes. The distances of the three cut planes are calculated with the reference original coordinate point as the center, and the position of the tetrahedron where the reference vector is located is determined. The three 45° cut planes include a 45° cut plane of the AB axis, a 45° cut plane of the CB axis, and a 45° cut plane of the AC axis that pass through the original coordinate point.

[0052] As shown in FIG. 7, FIG. 7 is a process diagram of the positioning of the tetrahedron according to the present invention.

[0053] In a specific embodiment, the above formula is used to calculate the reference original coordinate point (a, b, c) and the reference vector (u an ,u bn ,u cn ) and then make a decision according to the logic sequence in FIG.

[0054] u cn -c≧u bn -b, u an -a≧u cn -c, select the first tetrahedron; u cn -c≧u bn -b, u an -a cn -c, u an -a≧u bn -b, select the second tetrahedron; u cn -c≧u bn -b, u an -a cn -c, u an ​​-a bn -b, select the third tetrahedron; u cn -c bn -b, u cn -c≧u an -a, select the fourth tetrahedron; u cn -c bn -b, u cn -c an -a, u bn -b≧u an -a, select the fifth tetrahedron; u cn -c bn -b, u cn -c an -a, u bn -b an -a, select the sixth tetrahedron.

[0055] S233: Based on the final positioning result, a reference voltage vector corresponding to the reference vector is generated.

[0056] S24: Order the reference voltage vectors with a regulation strategy that reduces the zero-level output state.

[0057] Therein, S24 may include a step of determining a synthesis order of reference voltage vectors in the spatial polyhedron controllable region, and a step of adjusting the order of the reference voltage vectors with the aim of reducing the current of each leg flowing into the midpoint.

[0058] Among all voltage vectors, only the zero level, i.e., the zero state, can affect the midpoint voltage, so if the influence of the zero state is minimized as much as possible, a new arrangement method of the reference voltage vectors is adopted to balance with the zero state. The new arrangement method is as follows (V1-V4 represent the four reference voltage vectors of a certain tetrahedron):

[0059] The order of the reference voltage vectors of the tetrahedrons in the 1st-11th cubes is: V4, V3, V2, V1; ​​​​​​​12-22nd cube reference voltage vector order: V1, V2, V3, V4; The order of the reference voltage vectors of the tetrahedrons in the 23rd, 25th and 27th virtual cubes: V2, V3, V4, V1; The order of the reference voltage vectors of the tetrahedrons in the 24th, 26th and 28th virtual cubes is: V3, V2, V1, V4.

[0060] S25: Generate a leg switching pulse train based on the ordered reference voltage vectors and their corresponding action times to adjust the voltage at the midpoint of the three-phase four-leg three-level inverter.

[0061] As shown in FIG. 8, FIG. 8 is a flow diagram of step 25 as an example.

[0062] S251: Determine the action time corresponding to the reference voltage vector.

[0063] Determine the pulse duty factor corresponding to the reference voltage vector according to the following formula:

[0064]

number

[0065] among them,

number

[0066] In this case, if the voltage vector is V1-V4, the corresponding action time is T1-T4, and the order of the action time is consistent with the order of the reference voltage vectors.

[0067] S252: Generate a leg switching pulse train based on the ordered reference voltage vectors and their corresponding durations.

[0068] According to the arrangement method of the ordered reference voltage vectors, the operation time of the reference voltage vectors is ordered; the ordered operation time is allocated to 9 stages. The 9-stage allocation is performed as follows: the first, fifth, and ninth stages of the 9-stage vector are assigned the same period, the first and ninth stages each occupy 1 / 4, the fifth stage occupies 1 / 2, and the remaining six stages are symmetrically allocated to 1 / 2 of the corresponding period (i.e., T x1 / 4, T x2 / 2, T x3 / 2, T x4 / 2, T x1 / 2, T x4 / 2, T x3 / 2, T x2 / 2, T x1 / 4). The states of the four legs are determined according to the voltage vector, and four corresponding switching states are assigned to each state to form a pulse train. As shown in Figure 9, Figure 9 is a schematic diagram of the nine-stage time allocation method according to the present invention.

[0069] Conversion from voltage vector to switching signal: Before generating the driving pulse, the states of the four legs are determined according to the voltage vector, and four corresponding switching states are assigned to each state, and a switching pulse train of exactly 16 courses can be obtained according to 4*4.

[0070] It may be understood that by adopting the nine-stage time allocation method, multiple vectors among the reference voltage vectors can be made to have the same action time, thereby achieving the dual purpose of controlling the three-level inverter and balancing the midpoint potential.

[0071] In contrast to the prior art, the voltage regulation method for a three-phase four-leg three-level inverter proposed by the present invention includes the steps of: determining voltage vectors corresponding to three-phase voltages; constructing a spatial polyhedron controllable region based on the three-dimensional coordinate system of ABC and the voltage vectors; positioning predetermined reference vectors in multiple subregions of the spatial polyhedron controllable region to generate reference voltage vectors corresponding to the reference vectors, and using the reference vectors to control the output states of the corresponding four legs; ordering the reference voltage vectors with an adjustment strategy to reduce the zero-level output state; and generating switching pulse trains for the legs based on the ordered reference voltage vectors and their corresponding action times to regulate the voltage at the midpoint of the three-phase four-leg three-level inverter. The present invention has the following beneficial effects:

[0072] (1) The voltage balance at the midpoint of a three-phase four-leg three-level inverter is achieved without adding a controller. (2) By introducing six virtual cubes, the position of the reference vector can be determined reliably, and the redundant controllable region of the conventional SVPWM adjustment method is reduced, thereby reducing the complexity of the conventional SVPWM adjustment method. (3) A regulation strategy aims to reduce the current flowing into the midpoint in each leg, sequentially modifying the original reference voltage vector to balance the zero-level output condition, which affects the voltage balance at the midpoint.

[0073] To verify the effectiveness of the midpoint voltage balancing method for the three-phase four-leg three-level inverter, a simulation experiment of space vector regulation is carried out for the three-phase four-leg three-level inverter. The inverter parameters are shown in the following table:

[0074] [Table 1]

[0075] As shown in Figures 10-11, Figure 10 is a diagram of midpoint location under balanced load conditions according to the present invention. Figure 11 is a diagram of midpoint location under balanced load conditions after balancing is achieved using the voltage regulation method of the present invention. The horizontal axis represents time and the vertical axis represents voltage. The inverter output in Figure 10 is 5 kW, while the inverter output in Figure 11 is 100 kW. It can be seen from the diagrams that the method of this embodiment not only maintains midpoint voltage balance well, but also operates very stably even after the time axis is extended.

[0076] The above content describes in detail the methods provided by the embodiments of the present application, and discusses the principles and implementation states of the present application with specific examples. However, the description of the above embodiments is only used to help understand the method and spirit of the present application. Furthermore, those skilled in the art will understand that changes will occur in the specific embodiments and application scope according to the spirit of the present application. In summary, the content of this specification should not be interpreted as a limitation on the present application.

Claims

1. determining a voltage vector corresponding to the three-phase voltages; constructing a spatial polyhedron controllable region based on the ABC three-dimensional coordinate system and the voltage vector; positioning predetermined reference vectors in a plurality of sub-regions of the spatial polyhedron controllable region to generate reference voltage vectors corresponding to the reference vectors, and controlling the output states of the corresponding four legs using the reference vectors; sequencing said reference voltage vectors with a regulation strategy that reduces zero level output states; generating a leg switching pulse train based on the ordered reference voltage vectors and their corresponding durations to adjust a voltage at a midpoint of the three-phase four-leg three-level inverter; A voltage regulation method for a three-phase four-leg three-level inverter, comprising:

2. the step of determining a voltage vector corresponding to the three-phase voltages includes determining a voltage vector corresponding to the three-phase voltages according to output states of legs, the output states including a high level, a zero level, and a low level; 2. The method of claim 1 .

3. the step of determining a voltage vector corresponding to the three-phase voltages according to the output states of the legs includes determining a voltage vector corresponding to the three-phase voltages according to the following equation:

3. The method of claim 2. [Equation 1] (U ＡＮ represents the voltage vector of leg A relative to leg N; U ＢＮ represents the voltage vector of leg B relative to leg N; U ＣＮ represents the voltage vector of leg C relative to leg N; [Equation 2] respectively represent the output states of the A leg, B leg, C leg, and N leg of the three-phase four-leg three-level inverter; U ｄｃ indicates the DC power supply voltage).

4. the step of constructing a spatial polyhedron controllable region based on an ABC three-dimensional coordinate system and the voltage vector, normalizing the voltage vector; and introducing the normalized voltage vector into a three-dimensional coordinate system of ABC to construct a spatial polyhedron controllable region.

2. The method of claim 1 .

5. Normalizing the voltage vectors normalizing the voltage vector according to the formula:

5. The method of claim 4. [Equation 3] represents the normalized voltage vector of leg A relative to leg N; [Equation 4] represents the normalized voltage vector of leg B relative to leg N; [Equation 5] represents the normalized voltage vector of leg C relative to leg N)

6. The spatial polyhedron controllable region is a dodecahedron controllable region, and the dodecahedron controllable region includes three cubic regions formed by normalized voltage vectors and six fictitious cubic regions attached to the outside of the three cubic regions.

2. The method of claim 1 .

7. said step of positioning a predetermined reference vector to generate a reference voltage vector corresponding to said reference vector in a plurality of sub-regions of said spatial polyhedron controllable region, positioning predetermined reference vectors in cubic regions of a plurality of subregions of the spatial polyhedron controllable region; positioning predetermined reference vectors in tetrahedral regions of a plurality of subregions of the spatial polyhedron controllable region; generating a reference voltage vector corresponding to the reference vector based on a final positioning result; 2. The method of claim 1 .

8. the step of positioning predetermined reference vectors in cubic regions of a plurality of subregions of the spatial polyhedron controllable region; The method includes selecting a cube in which a reference original coordinate point is located as shown in the following formula in the controllable region of the spatial polyhedron based on the reference original coordinate point obtained by rounding the reference vector to an integer.

8. The method of claim 7. [Equation 6] represents the a-axis coordinate of the reference vector; [Equation 7] represents the b-axis coordinate of the reference vector; [Equation 8] represents the c-axis coordinate of the reference vector)

9. the step of positioning predetermined reference vectors in tetrahedral regions of a plurality of subregions of the spatial polyhedron controllable region comprises: the original coordinate point corresponding to the reference vector is used as a reference point for the positioned target cube, and the cube is divided using three 45° cut planes; distances of the three cut planes are calculated with the reference original coordinate point as a center; and the position of the tetrahedron where the reference vector is located is determined, wherein the three 45° cut planes include a 45° cut plane of the A-B axis, a 45° cut plane of the C-B axis, and a 45° cut plane of the A-C axis, which pass through the original coordinate point; 8. The method of claim 7.

10. said step of sequencing said reference voltage vectors with a regulation strategy that reduces zero level output states; determining a synthesis order of reference voltage vectors in a controllable region of a spatial polyhedron; and adjusting the order of the reference voltage vectors to reduce the current of each leg flowing into a midpoint; 2. The method of claim 1 .

11. generating a leg switching pulse train based on the ordered reference voltage vectors and their corresponding durations, determining a pulse duty factor corresponding to the reference voltage vector according to the following formula: [Equation 9] respectively represent the fixed point coordinates of the tetrahedron where the reference voltage vector is located; d1-4 represent the pulse duty cycle of the reference voltage vector) multiplying the pulse duty cycle by a switching period Ts to obtain an action time corresponding to the reference voltage vector; generating a leg switching pulse train based on the ordered reference voltage vectors and their corresponding durations; 2. The method of claim 1 .

12. generating a leg switching pulse train based on the ordered reference voltage vectors and their corresponding durations, Sequencing the application times of the reference voltage vectors based on an arrangement scheme of the ordered reference voltage vectors; A nine-step allocation is performed for the ordered action time, and the nine-step allocation is performed as follows, assigning the same period to the first, fifth, and ninth steps of the nine-step vector, with the first and ninth steps each occupying 1 / 4, the fifth step occupying 1 / 2, and the remaining six steps occupying 1 / 2 of their corresponding periods due to symmetry; determining states of the four legs based on the voltage vector and assigning four corresponding switching states to each state to form a pulse train; 12. The method of claim 11 .

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