Extended discontinuous pulse modulation method of three-level ANPC

Abstract

The invention discloses an extended discontinuous pulse modulation method of a three-level ANPC, and relates to the technical field of converter control. The method comprises the following steps: by taking balance of neutral-point potential and reduction of switching loss as optimization objectives, obtaining an equation of duty ratio of each phase; the duty ratio of each phase and each level is calculated by using the magnitude relation of each phase voltage and different clamping modes and increasing the condition of each phase duty ratio equation, and the most suitable clamping mode is selected for modulation; in order to ensure that the current injected by the neutral point potential needs to be controlled to be zero, a clamping mode with a relatively low power factor and a relatively high modulation degree is selected for optimization; and performing extended discontinuous pulse modulation on the three-level ANPC by using the duty ratio and the clamping mode. According to the invention, the extended discontinuous pulse modulation method is applied to the converter, and the converter is controlled based on the optimal clamping mode, so that the balance of neutral-point potential and the reduction of switching loss are realized, and the operation efficiency of the converter is improved.

Description

Technical Field

[0001] The invention relates to the technical field of converter control, and in particular to an extended discontinuous pulse modulation method of a three-level ANPC. Background Art

[0002] When the active neutral point clamped three-level inverter (ANPC) is working, the switching device will be continuously turned on and off, which will increase the switching loss. The most intuitive way to reduce the switching loss of the device is to reduce the number of switching times of the switching device. The traditional SVPWM uses a seven-segment time allocation to synthesize the reference vector. When synthesizing the reference vector, a pair of redundant vectors is used to increase the number of switching actions in one cycle. One of the traditional control strategies to reduce the switching loss of the device is the discontinuous pulse width modulation strategy (DPWM). The traditional modulation removes the redundant reference vector when synthesizing the reference vector, and uses a five-segment time allocation to reduce the number of switching actions in one cycle, thereby achieving the purpose of reducing the power loss of the switching device. The carrier stacking method, but the midpoint potential oscillation amplitude it produces is very large. For this reason, the DPWM five-segment time allocation method is usually used to suppress the low-frequency ripple of the midpoint voltage, but different DPWM modulation strategies have different effects on the midpoint potential. Therefore, it is necessary to simultaneously achieve midpoint potential balance and switching loss reduction under all operating conditions, but there is an contradictory relationship between midpoint potential balance and switching loss reduction, which is difficult to meet at the same time. Summary of the invention

[0003] The purpose of the present invention is to provide an extended discontinuous pulse modulation method for a three-level ANPC, which controls the converter based on an optimal clamping method, achieves a balance of midpoint potential and a reduction in switching losses, and improves overall operating efficiency.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] An extended discontinuous pulse modulation method of a three-level ANPC, comprising:

[0006] With the optimization goal of balancing the midpoint potential and reducing switching losses, the equations for the duty cycle of each phase are obtained;

[0007] By using the magnitude relationship of each phase voltage and different clamping methods, the conditions of each phase duty cycle equation are added to calculate the duty cycle of each phase and each level, and the most suitable clamping method is selected for modulation;

[0008] In order to ensure that the current injected into the midpoint potential that needs to be controlled is zero, a clamping method with a lower power factor and a higher modulation index is selected for optimization;

[0009] The three-level ANPC is subjected to extended discontinuous pulse modulation by utilizing the duty cycle and the clamping method.

[0010] Optionally, the equation for the duty cycle of each phase is obtained by taking balancing the midpoint potential and reducing the switching loss as the optimization goal, and specifically includes:

[0011] To balance the midpoint potential and reduce switching losses, it is first necessary to obtain a mathematical model of the midpoint current generated by connecting the output points of the three-phase bridge arms of the three-level ANPC topology to the neutral point; then, it is analyzed that in order to meet the requirement of midpoint potential balance under all working conditions, the zero-state duty cycle of the three-phase switch tubes of the three-level ANPC topology needs to be completely equal; based on the principle that the injected current to the midpoint of a switching cycle of the three-level ANPC topology is zero, the zero-state duty cycle of the three-phase switch tube is recalculated; based on the limitation of the duty cycle and the equation condition of volt-second balance, the equations for the duty cycles of each state of the three-phase switch tube are obtained.

[0012] Optionally, by using the magnitude relationship of each phase voltage and different clamping methods, the conditions of the duty cycle equation of each phase are added, the duty cycle of each phase and each level is calculated, and the most suitable clamping method is selected for modulation, specifically including:

[0013] The magnitude relationship of each phase voltage under each mode is analyzed, and combined with different clamping methods, five clamping methods are obtained. At the same time, the equation conditions of the duty cycle of each phase are added under different clamping methods, the current flowing through the midpoint under various clamping methods is calculated, and the most suitable clamping method is selected for modulation.

[0014] Optionally, in order to ensure that the current injected into the midpoint potential that needs to be controlled is zero, a clamping method with a lower power factor and a higher modulation index is selected for optimization, specifically including:

[0015] When the power factor is low and the modulation index is high, the midpoint potential will still fluctuate at a low frequency, and the midpoint potential balance cannot be guaranteed. Therefore, it is necessary to control the current injected into the midpoint potential to zero, and choose to expand when the power factor is low and the modulation index is high. These two expansion modes increase the number of actions in one switching cycle of the voltage intermediate phase to four times, and there are jumps in the three output levels.

[0016] Optionally, using the duty cycle and the clamping method to perform extended discontinuous pulse modulation on the three-level ANPC specifically includes:

[0017] By utilizing the duty cycle and the clamping method, the three-level ANPC is ordered to generate corresponding three-phase voltages at the midpoint current of the corresponding mode to meet different input-output voltage ratios and complete modulation.

[0018] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0019] The present invention discloses an extended discontinuous pulse modulation method for a three-level ANPC, and relates to the technical field of converter control. The method comprises: taking balanced midpoint potential and reduced switching loss as optimization goals, obtaining an equation for the duty cycle of each phase; using the magnitude relationship of each phase voltage and different clamping methods, adding conditions to the duty cycle equation of each phase, calculating the duty cycle of each phase and each level, and selecting the most suitable clamping method for modulation; in order to ensure that the current injected into the midpoint potential that needs to be controlled is zero, selecting a clamping method with a lower power factor and a higher modulation index for optimization; using the duty cycle and the clamping method, performing extended discontinuous pulse modulation on the three-level ANPC. The present invention applies the extended discontinuous pulse modulation method to a converter, controls the converter based on the optimal clamping method, achieves the balance of the midpoint potential and the reduction of the switching loss, and improves the operating efficiency of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0021] Figure 1 It is a flow chart of the extended discontinuous pulse modulation method of the three-level ANPC of the present invention;

[0022] Figure 2 : is a topological diagram of a three-level ANPC in this embodiment;

[0023] Figure 3 This is a schematic diagram of the controller control flow in this embodiment;

[0024] Figure 4 1 is a level action sequence diagram of different clamping modes in this embodiment; wherein, part (a) is a level action sequence diagram of E_DPWM1, part (b) is a level action sequence diagram of E_DPWM2, part (c) is a level action sequence diagram of E_DPWM3, part (d) is a level action sequence diagram of E_DPWM4, part (e) is a level action sequence diagram of E_DPWM5, part (f) is a level action sequence diagram of E_DPWM6, and part (g) is a level action sequence diagram of E_DPWM7;

[0025] Figure 5 : This is the E_DPWM simulation effect diagram in this embodiment; wherein, part (a) is the A-phase voltage in the three-level ANPC, part (b) is the upper and lower capacitor voltages on the DC side, part (c) is the A-phase current in the three-level ANPC, and part (d) is the clamping mode selected by the controller;

[0026] Figure 6 Graphs showing the loss of switching devices under different modulation strategies in this embodiment; wherein, part (a) is a graph showing the loss of SVPWM switching devices, and part (b) is a graph showing the loss of E_DPWM switching devices. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] The purpose of the present invention is to provide an extended discontinuous pulse modulation method for a three-level ANPC, which controls the converter based on an optimal clamping method, achieves a balance of midpoint potential and a reduction in switching losses, and improves overall operating efficiency.

[0029] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] like Figure 1 As shown, the present invention provides an extended discontinuous pulse modulation method of a three-level ANPC, comprising:

[0031] Step 100: Taking balancing the midpoint potential and reducing the switching loss as the optimization goals, the equations for the duty cycle of each phase are obtained, including:

[0032] To balance the midpoint potential and reduce switching losses, it is first necessary to obtain a mathematical model of the midpoint current generated by connecting the output points of the three-phase bridge arms of the three-level ANPC topology to the neutral point; then, it is analyzed that in order to meet the requirement of midpoint potential balance under all working conditions, the zero-state duty cycle of the three-phase switch tubes of the three-level ANPC topology needs to be completely equal; based on the principle that the injected current to the midpoint of a switching cycle of the three-level ANPC topology is zero, the zero-state duty cycle of the three-phase switch tube is recalculated; based on the limitation of the duty cycle and the equation condition of volt-second balance, the equations for the duty cycles of each state of the three-phase switch tube are obtained.

[0033] Step 200: using the magnitude relationship of each phase voltage and different clamping methods, adding conditions to the duty cycle equation of each phase, calculating the duty cycle of each phase and each level, and selecting the most suitable clamping method for modulation, specifically including:

[0034] The magnitude relationship of each phase voltage under each mode is analyzed, and combined with different clamping methods, five clamping methods are obtained. At the same time, the equation conditions of the duty cycle of each phase are added under different clamping methods, the current flowing through the midpoint under various clamping methods is calculated, and the most suitable clamping method is selected for modulation.

[0035] Step 300: In order to ensure that the current injected into the midpoint potential that needs to be controlled is zero, a clamping method with a lower power factor and a higher modulation index is selected for optimization, specifically including:

[0036] When the power factor is low and the modulation index is high, the midpoint potential will still fluctuate at a low frequency, and the midpoint potential balance cannot be guaranteed. Therefore, it is necessary to control the current injected into the midpoint potential to zero, and choose to expand when the power factor is low and the modulation index is high. These two expansion modes increase the number of actions in one switching cycle of the voltage intermediate phase to four times, and there are jumps in the three output levels.

[0037] Step 400: performing extended discontinuous pulse modulation on the three-level ANPC by using the duty cycle and the clamping method, specifically comprising:

[0038] By utilizing the duty cycle and the clamping method, the three-level ANPC is ordered to generate corresponding three-phase voltages at the midpoint current of the corresponding mode to meet different input-output voltage ratios and complete modulation.

[0039] Based on the above technical solution, the following embodiments are provided.

[0040] When the midpoint of the three-level ANPC DC-side capacitor outputs zero level, current will flow out or in, causing the DC-side capacitor to charge or discharge. If the midpoint potential is offset, the three-level inverter will degenerate into a two-level inverter, and may also threaten the DC-side capacitor. At the same time, when the three-level ANPC is working, the switching devices will be continuously turned on and off, which will increase the switching loss.

[0041] Therefore, this embodiment is intended to solve the following problems: 1. The problem of unbalanced midpoint potential of the three-level ANPC; 2. The problem of power loss of the switching device.

[0042] This embodiment discloses a modulation method for solving the midpoint potential imbalance and switching device power loss of an active midpoint clamped inverter. Its topology is composed of two large capacitors connected in series in parallel on a DC bus, each phase bridge arm is composed of two switch tubes and four controllable switch tube anti-parallel diodes, and the two switch tubes are connected to the midpoint of the two capacitors. Such a topology can reduce the voltage stress of the switch tube and improve the flexibility of the converter, so that the converter can be applied to higher voltage level occasions, such as electric vehicles and medium voltage power grids.

[0043] Normally, converters often use DPWM modulation as a control method for converters, but this modulation will cause distortion of the output voltage waveform and large ripples in the output current when the load is large, which seriously increases the loss and reduces the energy transmission efficiency. In order to improve this situation, this embodiment adopts a modulation method of an extended discontinuous pulse width modulation strategy. Compared with traditional modulation methods such as DPWM, the E_DPWM modulation of this embodiment reduces the switching loss while fully considering the influence of the midpoint potential on the system. By adopting the optimal sequence, the midpoint potential is balanced on the basis of achieving the best output current harmonics and obtaining better power quality, thereby improving the efficiency and stability of the converter.

[0044] Firstly, the equations of duty cycle of each phase are obtained with the optimization goals of balancing the midpoint potential and reducing the switching loss; then, the conditions of the duty cycle equation of each phase are increased by using the magnitude relationship of the voltages of each phase and different clamping methods, the duty cycle of each phase and each level is calculated, and the most suitable clamping method is selected for modulation; in order to ensure that the current injected into the midpoint potential that needs to be controlled is zero, a clamping method with a lower power factor and a higher modulation index is selected for optimization; finally, the duty cycle and the clamping method are used to perform extended discontinuous pulse modulation on the three-level ANPC.

[0045] In the above scheme, the converter is controlled based on the duty cycle, which can more accurately describe and control the operating state of the converter and select the most appropriate control strategy according to actual needs. At the same time, the converter is controlled based on the clamping method to reduce the output current harmonics of the converter and reduce the switching loss of the switch tube, thereby improving the overall operating efficiency while ensuring safe operation.

[0046] In addition, as the controller control flow shown, the steps are:

[0047] S1: With the optimization goal of balancing the midpoint potential and reducing the switching loss, the equations for the duty cycle of each phase are obtained;

[0048] S2: Using the magnitude relationship of each phase voltage and different clamping methods, add conditions to the duty cycle equation of each phase, calculate the duty cycle of each phase and each level, and select the most suitable clamping method for modulation;

[0049] S3: In order to ensure that the current injected into the midpoint potential that needs to be controlled is zero, a clamping method with a lower power factor and a higher modulation index is selected for optimization;

[0050] S4: performing extended discontinuous pulse modulation on the three-level ANPC by using the duty cycle and the clamping method.

[0051] The control method proposed in this example expands the traditional discontinuous pulse modulation method. Under this premise, the above scheme selects different clamping methods for the converter to deal with the problems of midpoint potential imbalance and switch tube power loss; and derives the duty cycle of each phase level under different clamping methods; selects a clamping method with a lower power factor and a higher modulation degree for optimization; and finally, under the full power range, selects the duty cycle and the clamping method that meet the conditions to modulate the three-level ANPC.

[0052] In this example, the extended discontinuous pulse modulation method of the three-level ANPC can effectively achieve the midpoint potential balance and optimize the loss of the switching device, reduce the loss in the converter, and improve the overall stability and operating efficiency. In addition, in this implementation patent, by using the magnitude relationship of each phase voltage and different clamping methods, and selecting the most suitable clamping method for modulation, the operating state of the converter can be more accurately described and controlled, and the most suitable clamping method can be selected according to actual needs; at the same time, the clamping method with a lower power factor and a higher modulation index is selected for optimization, which can reduce the loss of the switching device and improve the efficiency of the system.

[0053] The extended discontinuous pulse modulation method of three-level ANPC specifically comprises the following steps:

[0054] The first step is to balance the midpoint potential and reduce switching losses as the optimization goals, and obtain the equations for the duty cycle of each phase.

[0055] The equations for the duty cycle of each phase in this example are:

[0056] Going back to the source, the balanced midpoint potential is to control the time of the output current of the inverter capacitor midpoint, and the ultimate goal of forming a modulation strategy is to generate a PWM waveform, that is, to obtain the duty cycle of each level of the three phases, thus 3*3=9 duty cycle variables d can be obtained xp (x = a, b, c; p = 0, 1, 2), where the duty cycle of the zero level is the time for the output current at the midpoint of the DC side capacitor. To solve nine variables, nine independent equations are required. It is known that the following equation can be obtained based on the duty cycle limit and volt-second balance:

[0057]

[0058] There are five equations above, which cannot solve the nine three-phase level duty cycle variables.

[0059] The unique condition of the modulation strategy is that the injection current to the midpoint in one switching cycle is zero. For a three-phase inverter, the injection current inp to the midpoint in one switching cycle is:

[0060] i np =i a da1 +i b d b1 +i c d c1 (2)

[0061] d a1 =d b1 =d c1 (3)

[0062] When equation (3) holds true, since the sum of the three-phase currents is zero, then:

[0063] i np =i a d a1 +i b d b1 +i c d c1 =d a1 (i a +i b +i c )=0 (4)

[0064] Therefore, with the unique conditional equation (3), a total of seven equations cannot solve the nine unknown variables, and more equations need to be created to solve the duty cycle variable. max 、Intermediate phase mid and minimum phase u min , the duty cycle variable of each phase becomes d mp (m = max, mid, min; p = 0, 1, 2), then according to the output duty cycle limit and the equation condition of volt-second balance, it can be rewritten as:

[0065] Gd=H (5)

[0066] in:

[0067]

[0068] d=[d max2 , d max1 , d max0 , d mid2 , d mid1 , d mid0 , d min2 , d min1 , d min0 ] T

[0069] H=[u max -u mid ,u mid -u min ,1,1,1] T

[0070] At the same time, according to the three-phase reference voltage, u max >0 and u min <0, and in order to reduce the switching loss, the following conditional formula can be obtained:

[0071]

[0072] The constraint that the current injected into the midpoint is zero within one cycle is rewritten as:

[0073] d max1 =d mid1 =d min1 (7)

[0074] From this, we can complete nine equations and solve the duty ratio of each phase and level as follows:

[0075]

[0076] The second step is to use the magnitude relationship of each phase voltage and different clamping methods to increase the conditions of each phase duty cycle equation, calculate the duty cycle of each phase and each level, and select the most suitable clamping method for modulation.

[0077] It should be noted that the traditional use of DPWM to balance the midpoint potential requires the selection of DPWMs with different effects on the midpoint potential for switching. However, since the implementation of a DPWM requires rearranging the order of action of the vectors, the implementation code for switching DPWM is complex, and only two DPWM modulation strategies for alternating the balanced midpoint potential have appeared. However, the code for implementing DPWM using duty cycle is not complex, so this implementation method can select the DPWM that best balances the midpoint potential for modulation, thereby using DPWM to achieve the balance of the midpoint potential. Compared with the SVPWM modulation strategy, DPWM reduces 1 / 3 of the switching in one switching cycle, so it reduces the loss of the switching device. By using DPWM to switch and balance the midpoint potential, the midpoint potential and the switching loss can be optimized at the same time. The specific implementation method is described as follows.

[0078] The three-phase voltage and current and their expressions after separation by size, let i max ,i mid ,i min are the phase currents corresponding to the maximum voltage phase, the middle phase, and the minimum voltage phase respectively. Similarly, the variable to be solved is the duty cycle variable d of each phase mp (m=max,mid,min;p=0,1,2) The angle ωt of the reference vector will be swapped every time it passes through π / 3. The relationship is shown in Table 3-2.

[0079] Formula (5) has given five equations, combined with u max>0 and u min >0, 7 equations about the duty cycle of each phase can be listed. Five clamping methods can be enumerated based on the magnitude relationship of each phase voltage and different clamping methods, such as Figure 3 As shown, these five clamping modes are called E_MODE1, E_MODE2, ..., E_MODE5. Through different clamping modes, the equation conditions can be added to calculate the duty cycle of each phase and each level. Taking E_MODE1 as an example, according to this clamping mode, the equation conditions can be added. max2 =1,d max1 =0,d max0 =0,d mid0 =0 and d min2 =0, and substituting it into formula (5) can solve the equation.

[0080] After finding the duty cycle of each phase, the most appropriate clamping method needs to be selected for modulation, so the current flowing through the midpoint under various clamping methods needs to be calculated. Current is injected into the midpoint of the DC side capacitor only when the output is zero level, so the average current injected into the midpoint during a switching cycle is calculated as follows:

[0081] i np =d max1 i max +d mid1 i mid +d min1 i min (9)

[0082] The duty cycle of the five clamping methods and the calculated results of the current injected into the midpoint are shown in Table 4-2. The calculated duty cycle must meet the following conditions:

[0083] 0≤d mp ≤1 (10)

[0084] where d mp (m=max, min, mid; p=0, 1, 2). Only when equation (10) is satisfied can this clamping method be used.

[0085] Table 1 Duty cycle and midpoint current of E_MODE clamping method

[0086]

[0087] where u L1 =u max -u min ,u L2 =u max -u mid ,u L3 =u mid -umin .

[0088] In the third step, in order to ensure that the current injected into the midpoint potential that needs to be controlled is zero, a clamping method with lower power factor and higher modulation index is selected for optimization.

[0089] It should be noted that at a certain moment, the most suitable clamping method is selected to control the midpoint voltage. However, when the power factor is low and the modulation index is high, the midpoint potential will still have low-frequency fluctuations, and the balance of the midpoint potential cannot be guaranteed. Therefore, it is necessary to control the current injected into the midpoint potential to zero, and select E_MODE1 and E_MODE5, which are the most selected methods when the power factor is low and the modulation index is high, for expansion to obtain E_MODE6 and E_MODE7. These two expansion modes increase the number of actions in one switching cycle of the intermediate phase of the voltage to four times, and there are jumps in the three output levels. The level switching of the intermediate phase is as follows Figure 3 shown.

[0090] On the basis of equation (5), add equation (11) and E_MODE1 clamp constraint equation (12):

[0091] d max1 i max +d mid1 i mid +d min1 i min =0 (11)

[0092] d max1 =d max0 =d min2 =0 (12)

[0093] The duty cycle of E_MODE1 can be calculated as:

[0094]

[0095] For E_MODE5, the additional constraints brought by its clamping method are:

[0096] d min2 =d min1 =d max0 =0 (14)

[0097]

[0098] The fourth step is to perform extended discontinuous pulse modulation on the three-level ANPC by using the duty cycle and the clamping method. Figure 2 The control flow chart for the controller to execute the above method.

[0099] Finally from Figure 5It can be seen that (a) and (c) are the A-phase voltage and current output by the three-phase inverter, respectively, and (b) is the upper and lower capacitor voltages UC1 and UC2 on the DC side, which fluctuate between ±3V. Compared with the DPWM modulation strategy, the ability to balance the midpoint potential is further improved. (d) is the clamping mode at this time. When it is 6, it means that the clamping mode at this time is E_DPWM6, and when it is 7, the clamping mode at this time is E_DPWM7. When the expanded clamping mode is used, the current injected into the midpoint potential is zero, and there is no effect on the balance of the midpoint potential. Figure 6 It can be seen that the power consumed by the three switching devices under E_DPWM modulation is the smallest. Therefore, the loss of the switching devices of E_DPWM is reduced compared with SVPWM, so the loss of the switching devices is optimized.

[0100] It should be noted that this step can achieve the system steady-state control requirements while ensuring the system robustness and dynamic response speed.

[0101] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0102] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. An extended discontinuous pulse modulation method for three-level ANPC, characterized in that: include: With the optimization goal of balancing the midpoint potential and reducing switching losses, the equations for the duty cycle of each phase are obtained; By using the magnitude relationship of each phase voltage and different clamping methods, the conditions of each phase duty cycle equation are added to calculate the duty cycle of each phase and each level, and the most suitable clamping method is selected for modulation; In order to ensure that the current injected into the midpoint potential that needs to be controlled is zero, a clamping method with a lower power factor and a higher modulation index is selected for optimization; The three-level ANPC is subjected to extended discontinuous pulse modulation by utilizing the duty cycle and the clamping method.

2. The extended discontinuous pulse modulation method of three-level ANPC according to claim 1, characterized in that: The equations for the duty cycle of each phase are obtained by taking balancing the midpoint potential and reducing the switching loss as the optimization goals, specifically including: To balance the midpoint potential and reduce switching losses, it is first necessary to obtain a mathematical model of the midpoint current generated by connecting the output points of the three-phase bridge arms of the three-level ANPC topology to the neutral point; then, it is analyzed that in order to meet the requirement of midpoint potential balance under all working conditions, the zero-state duty cycle of the three-phase switch tubes of the three-level ANPC topology needs to be completely equal; based on the principle that the injected current to the midpoint of a switching cycle of the three-level ANPC topology is zero, the zero-state duty cycle of the three-phase switch tube is recalculated; based on the limitation of the duty cycle and the equation condition of volt-second balance, the equations for the duty cycles of each state of the three-phase switch tube are obtained.

3. The extended discontinuous pulse modulation method of three-level ANPC according to claim 1, characterized in that: By using the magnitude relationship of each phase voltage and different clamping methods, the conditions of each phase duty cycle equation are added, the duty cycle of each phase and each level is calculated, and the most suitable clamping method is selected for modulation, including: The magnitude relationship of each phase voltage under each mode is analyzed, and combined with different clamping methods, five clamping methods are obtained. At the same time, the equation conditions of the duty cycle of each phase are added under different clamping methods, the current flowing through the midpoint under various clamping methods is calculated, and the most suitable clamping method is selected for modulation.

4. The extended discontinuous pulse modulation method of the three-level ANPC according to claim 1, in order to ensure that the current injected into the midpoint potential that needs to be controlled is zero, a clamping mode with a lower power factor and a higher modulation index is selected for optimization, specifically comprising: When the power factor is low and the modulation index is high, the midpoint potential will still fluctuate at a low frequency, and the midpoint potential balance cannot be guaranteed. Therefore, it is necessary to control the current injected into the midpoint potential to zero, and choose to expand when the power factor is low and the modulation index is high. These two expansion modes increase the number of actions in one switching cycle of the voltage intermediate phase to four times, and there are jumps in the three output levels.

5. The extended discontinuous pulse modulation method of three-level ANPC according to claim 1, characterized in that: Using the duty cycle and the clamping method, the three-level ANPC is subjected to extended discontinuous pulse modulation, specifically comprising: By utilizing the duty cycle and the clamping method, the three-level ANPC is ordered to generate corresponding three-phase voltages at the midpoint current of the corresponding mode to meet different input-output voltage ratios and complete modulation.

Images