A High-Reactive-Power Overload Energy Storage Converter Topology and Its Modulation Method

Abstract

The present invention discloses a low-cost and high-reactive-power overload energy storage converter topology and its modulation strategy, which is used to improve the reactive-power overload capacity of the energy storage converter and belongs to the technical field of AC-DC converters. By adding auxiliary diodes to the traditional ANPC bridge arm, when the converter outputs a high proportion of reactive power, the reverse current of the IGBT module can be shared, so that the reactive-power overload capacity of the converter can be improved at low cost and high efficiency on the basis of making full use of existing equipment. In addition, a modulation strategy for enhancing the reactive-power overload capacity based on third-harmonic injection is proposed for this topology. By injecting third harmonics into the modulation wave, the utilization rate of the auxiliary diodes is improved, and the output capacity of the reactive current is further enhanced. The present invention conducts experimental verification on a 100 kVA prototype in Simulink / Plecs. Based on the traditional ANPC, the device cost of the proposed topology only increases by 5.45%, and the reactive-power overload capacity increases by 22%, which can effectively enhance the grid voltage support capacity of the energy storage converter.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, in particular to a high-reactive overload energy storage converter topology and its modulation method. Background Art

[0002] New energy power generation bases are usually far from load centers, lack synchronous power source support, have relatively weak system grid structures, and exhibit low anti-interference and weak support capabilities. This causes the voltage stability margin and dynamic voltage support capabilities of new energy power generation bases to drop significantly compared to traditional thermal power-dominated systems. In addition, the output of new energy often has volatility, intermittency, and uncertainty, further exacerbating the complexity of the voltage fluctuation problem.

[0003] Therefore, some scholars have proposed using the flexible power regulation ability of grid-forming energy storage to improve the voltage safety and stability level of new energy bases. Grid-forming energy storage acts as a stable and reliable reactive power source during the steady state of the power grid, providing steady-state reactive power support for the power grid; when the grid voltage drops, the energy storage can emit reactive power to provide instantaneous voltage support for the power grid and quickly restore the system voltage.

[0004] In the selection of the energy storage converter topology, the Active Neutral Point Clamped (ANPC) topology has been widely used in the field of medium-voltage energy storage converters by introducing active switches at the midpoint, reconstructing the current loop, optimizing the device loss distribution, significantly reducing the device switching loss, and improving the voltage equalization ability. However, in voltage support scenarios that require overload, it will have the problem of over-temperature limit of the junction temperature, which will trigger over-temperature protection and trip off the grid, making it difficult to fully exert the application potential of grid-forming energy storage.

[0005] To improve the overload capacity of grid-forming energy storage converters, domestic and foreign research mainly focuses on the following aspects:

[0006] 1. Adjust the control strategy: By changing parameters such as the switching frequency and duty cycle, reduce the device junction temperature. This method has a low cost, but the effect is limited, and the change of control parameters during operation may lead to control instability.

[0007] 2. Optimize the heat dissipation structure: Use phase change heat conduction materials to absorb the device power loss and improve the instantaneous over-current ability, but do not consider the long-term reactive power support scenario. Another study uses liquid cooling to reduce the device junction temperature, but this method is difficult to cope with instantaneous temperature changes.

[0008] 3. Hardware over-sizing: By increasing the device rated capacity, improve the system overload capacity, but this will lead to an increase in cost and volume.

[0009] In summary, existing solutions have problems such as a significant increase in cost, an increase in system complexity, or limited effectiveness in improving the reactive overload capacity of energy storage converters. There is an urgent need for a new technical solution to overcome the above deficiencies.

[0010] Glossary:

[0011] IPM: Intelligent Power Module.

[0012] Modulation index: It is an important parameter of the modulated wave, reflecting the degree to which the amplitude, frequency, or phase of the carrier is controlled by the low-frequency modulation signal. Modulation index = amplitude of the modulation wave / amplitude of the carrier; generally in SPWM, the modulation wave = sine wave, and the carrier = triangular wave; the output amplitude is proportional to the modulation index. Summary of the Invention

[0013] The technical problem to be solved by the present invention is, aiming at the deficiencies of the existing technology, to propose a topology and modulation method of a high-reactive overload energy storage converter to improve the reactive power output capacity of the converter, thereby improving the voltage support capacity of the converter without significantly increasing the hardware cost.

[0014] To solve the above technical problems, the technical solution of the present invention is as follows:

[0015] A low-cost high-reactive overload energy storage converter topology, comprising: three-phase output lines, three-phase converter bridge arms, three-phase auxiliary diodes, a data acquisition and calculation unit, and a drive signal generation unit;

[0016] The three-phase output lines include an A-phase line, a B-phase line, and a C-phase line, and their input ends are electrically connected to the auxiliary diode group and the output ends of the converter bridge arms;

[0017] Each phase of the three-phase auxiliary diode group includes an upper diode and a lower diode , the positive electrode of the upper diode and the negative electrode of the lower diode are commonly electrically connected to the output end of the converter bridge arm circuit, the negative electrode of the upper diode is electrically connected to the positive pole of the DC bus , the positive electrode of the lower diode is electrically connected to the negative pole of the DC bus ; , corresponding to the A-phase line, the B-phase line, and the C-phase line respectively;

[0018] The A-phase line, B-phase line, and C-phase line are also electrically connected to a data acquisition and calculation unit, and the data acquisition and calculation unit is communicatively connected to a control system; the control system generates a modulation signal according to the current and voltage signals of the A-phase line, B-phase line, and C-phase line and according to a control algorithm, and the drive signal generation unit generates a drive signal according to the modulation signal to control the on / off of the switching devices in the converter arm.

[0019] Optionally, the converter arm is an ANPC arm, an NPC arm, or a two-level arm.

[0020] Optionally, the converter arm circuit is an ANPC arm, and the ANPC arm includes six switching devices, which are represented by letters S Each switching device contains a switching tube and an antiparallel diode, where the switching tube is represented by the letter T and the corresponding antiparallel diode is represented by the letter D Taking the A phase as an example, the six switching devices are the first switching device , the second switching device , the third switching device , the fourth switching device , the fifth switching device and the sixth switching device . The collector of the first switching device is electrically connected to the positive pole of the DC bus , and the emitter is electrically connected to the collector of the second switching device and the collector of the fifth switching device ; the emitter of the second switching device is electrically connected to the collector of the third switching device and the output end of the converter; the emitter of the fifth switching device is electrically connected to the collector of the sixth switching device and the neutral point O ; the emitters of the third switching device and the sixth switching device are electrically connected to the collector of the fourth switching device , and the emitter of the fourth switching device is electrically connected to the negative pole of the DC bus ; the neutral point O is electrically connected to one end of the upper capacitor and one end of the lower capacitor . The other end of the upper capacitor is electrically connected to the positive pole of the DC bus , and the other end of the lower capacitor is electrically connected to the negative pole of the DC bus .

[0021] Optionally, in the ANPC bridge arm, the first switching device and the fifth switching device are placed in the same IPM, and the second switching device and the third switching device are placed in the same IPM, and the fourth switching device and the sixth switching device are placed in the same IPM. The two switching devices in each IPM are complementary in on and off states.

[0022] Optionally, the switching tube is an IGBT or a MOS tube; the way of anti-parallel diode of the switching tube is that the positive pole of the diode is electrically connected to the emitter of the switching tube, and the negative pole of the diode is electrically connected to the collector of the switching tube.

[0023] Optionally, the three IPMs of the same ANPC bridge arm are placed on the same heat dissipation module.

[0024] Optionally, the upper diode and the lower diode are both Schottky diodes or fast recovery diodes; the three auxiliary diode groups are placed on the same heat dissipation module together.

[0025] A modulation strategy for a low-cost and high-reactive-power overload energy storage converter topology. The low-cost and high-reactive-power overload energy storage converter topology is as described above. The modulation strategy of the ANPC bridge arm corresponding to each phase output line includes the following steps:

[0026] Step 1: Generate a modulation signal, and the modulation signal is a modulation wave ;

[0027] Step 2: When the modulation wave is greater than 0, if the modulation wave is greater than the triangular carrier wave, output the P state; if the modulation wave is less than the carrier wave, output the state; when the modulation wave is less than 0, if the modulation wave is greater than the triangular carrier wave, output the state; if the modulation wave is less than the carrier wave, output the N state;

[0028] When outputting the P state 、 and are turned on;

[0029] When outputting the O+ state 、 and are turned on;

[0030] When outputting the O- state 、 and are turned on;

[0031] When outputting the N state 、 and conduct.

[0032] Optionally, a SPWM modulation method with third-harmonic injection is adopted to generate a modulation signal, and the modulation signal is:

[0033]

[0034] is the modulation wave, is the modulation degree, is the angular frequency, is the time.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] 1) The present invention improves the topology and modulation method of the traditional ANPC converter, and concentrates the reactive current loss in the auxiliary diode as much as possible, thereby reducing the junction temperature of the ANPC bridge arm and effectively improving the reactive overload capacity of the three-level topology converter. Through reactive overload, the converter can better provide voltage support ability when a transient fault occurs in the power grid; a large amount of reactive power also helps the power grid voltage to recover during the fault process, ensuring the safe and stable operation of the power grid.

[0037] 2) Compared with the method of using an oversized converter to achieve overload capacity in engineering applications, the topology proposed by the present invention only needs to add diodes to the traditional ANPC topology, and can greatly reduce the device cost of the converter under the same overload capacity, which is one of the best topology choices for grid-forming energy storage.

[0038] 3) The present invention verifies the reactive overload capacity of the proposed topology and modulation method through the analysis of the device junction temperature of the converter, and at the same time provides a basis for quantifying the overload capacity of the converter. Description of the Drawings

[0039] Figure 1 is the topology structure of a low-cost and high-reactive overload energy storage converter according to an embodiment of the present invention.

[0040] Figure 2 is the schematic diagram of the heat dissipation structure according to an embodiment of the present invention.

[0041] Figure 3 is the main circuit structure according to an embodiment of the present invention.

[0042] Figure 4 is the driving signals of different devices within a modulation period according to an embodiment of the present invention.

[0043] Figure 5Voltage and current waveform diagram when only reactive power is output in an embodiment of the present invention.

[0044] Figure 6 Schematic diagram of the current loop in different output states in an embodiment of the present invention, where (a) is the circuit diagram in the output P state; (b) is the circuit diagram in the output O+ state; (c) is the circuit diagram in the output O- state; (d) is the circuit diagram in the output N state.

[0045] Figure 7 Device power loss comparison diagram in an embodiment of the present invention, where (a) is the power loss diagram of the traditional ANPC topology; (b) is the power loss diagram of the low-cost and high-reactive overload energy storage converter of the present invention.

[0046] Figure 8 Distribution diagram of current in different devices when only reactive power is output in an embodiment of the present invention.

[0047] Figure 9 Comparison diagram of the junction temperature of the ANPC bridge arm devices when only reactive power is output in an embodiment of the present invention.

[0048] Figure 10 Comparison diagram of the junction temperature before and after overload in an embodiment of the present invention.

[0049] Figure 11 Circuit diagram of the converter bridge arm circuit as an NPC bridge arm in an embodiment of the present invention.

[0050] Figure 12 Circuit diagram of the converter bridge arm circuit as a two-level bridge arm in an embodiment of the present invention. Detailed implementation manners

[0051] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0052] The output capacity of the converter depends on the rated value of the device operation, which is actually the thermal constraint of the device. Therefore, if the converter outputs more power at a certain junction temperature, it can be considered that the converter has overload capacity.

[0053] For the ANPC topology converter, when its output power factor is not 1, a phase difference is generated between the output voltage and current , at this time, part of the current will flow through the anti-parallel diode, generating power loss on the diode. If this part of the loss can be reduced, the junction temperature of the devices in the ANPC bridge arm can be effectively reduced when outputting reactive power, and its reactive overload capacity can be increased. Based on this idea, the present invention provides a low-cost and high-reactive-overload energy storage converter topology, the structure of which includes three-phase output lines, three-phase converter bridge arms, three-phase auxiliary diodes, a data acquisition and calculation unit, and a drive signal generation unit;

[0054] See the appendix Figure 1 , the DC-side bus capacitor of the low-cost and high-reactive-overload energy storage converter includes an upper capacitor and a lower capacitor , which is used to provide the midpoint potential and stabilize the DC voltage. After the two capacitors are connected in series, they are jointly connected in parallel to the positive and negative ends of the DC bus, and the middle series connection point is the neutral point O .

[0055] Each phase of the ANPC bridge arm is composed of 6 IGBTs with anti-parallel diodes. The anti-parallel structures formed by the anti-parallel connection of IGBTs and diodes are respectively denoted as corresponding to the A-phase line, B-phase line, and C-phase line respectively), respectively adopt complementary on-off intelligent power modules (Intelligent Power Module, IPM). The IGBTs in the module are denoted by T , and the diodes are denoted by D . The IPMs in one phase are placed on the same heat dissipation module. See the appendix Figure 1 . The midpoints of are connected to the neutral point O , The midpoints of are led out as the output terminals of the converter.

[0056] The auxiliary diodes of each phase include an upper tube and a lower tube , which adopt Schottky diodes or fast-recovery diodes, have smaller conduction resistance, and better switching performance. The auxiliary diodes of the three phases A, B, and C are jointly placed on the same heat dissipation module, and the two ends are respectively connected in parallel to the output terminals of the converter and the positive and negative poles of the DC bus to provide an additional reactive current channel for the converter;

[0057] The three-phase voltage and current data are sent to STM32F28335 for calculation after being collected by the acquisition unit to generate a modulation wave; the input end of the drive signal generation unit is connected to the EPWM pin of STM32F28335, receives the PWM drive signal, and the output end is connected to the IPM drive pin in the ANPC bridge arm to control the action of the IGBT and realize the power transmission of the converter.

[0058] The three-phase main circuit structure is shown in the appendix Figure 3 , where the DC side of the converter is directly connected in parallel with the energy storage battery, and the voltage of the energy storage battery is . The AC side is connected to the point of common coupling (PCC) through a filter inductor , and then accesses the power grid through the line impedance. is the grid voltage, and the effective value of its phase voltage is denoted as is the voltage at the outlet of the converter, and the effective value of its phase voltage is denoted as is the voltage at the PCC point, and the effective value of its phase voltage is denoted as is the output current of the inverter, and its effective value is denoted as I . By changing the amplitude and phase of the output voltage of the converter port, the active power and reactive power transmitted to the power grid can be controlled. Ignoring the line resistance, the active power and reactive power transmitted by the converter to the power grid are expressed as follows:

[0059] (1);

[0060] In the topology proposed in the present invention, the auxiliary diode provides an additional current path for the ANPC bridge arm when the converter outputs reactive current. As a passive device, it does not need to be applied with a driving signal. Therefore, the modulation method of the ANPC topology is also applicable to the low-cost and high-reactive-overload energy storage converter, and different modulation methods will not affect the shunt characteristics of the auxiliary diode.

[0061] It can be seen from Equation (1) that when the grid voltage is constant, the reactive power transmitted by the converter is limited by and . If sinusoidal modulation is adopted, when the modulation index is the maximum value of 1 . To further improve the reactive power output ability of the converter, the SPWM modulation method with third-harmonic injection is adopted in this paper. After injecting the third harmonic, the maximum value of the modulation index is 1.15, and at this time . Its modulation function expression is as follows.

[0062] (2);

[0063] At the same time, since respectively adopt complementary on-off IGBT modules, four switching states (P, O+, O-, N) in Table 1 can be generated through different combinations of driving signals.

[0064]

[0065] Taking phase A as an example, the IGBT drive signals within one modulation period after injecting the third harmonic are shown in the appendix Figure 4 .

[0066] Specifically, the working principle of the low-cost high-reactive overload energy storage converter topology and modulation strategy is described as follows:

[0067] Taking the reactive power output by the converter as an example, the power factor angle , see the appendix Figure 5 . At this time, the output modes of the converter can be divided into four types: (1) positive voltage and negative current; (2) positive voltage and positive current; (3) negative voltage and positive current; (4) negative voltage and negative current.

[0068] When in mode 1, the switching state of the circuit switches between P and O+. As Figure 6 shown in (a) and (b) of , for the P state, since the conduction voltage drop of the auxiliary diode is less than the threshold voltage of the two IGBT anti-parallel diodes, the current will only flow through the auxiliary diode; for the O+ state, the current flows through . Therefore, during mode 1, switching losses and conduction losses are generated, only conduction losses. Compared with the traditional ANPC topology, part of the ANPC bridge arm losses in mode 1 are transferred to the auxiliary diode, thus optimizing the loss distribution.

[0069] When in mode 2, the switching state of the circuit still switches between P and O+; but due to the forward flowing current, its commutation loop has changed. As Figure 6 shown in (c) and (d) of , for the P state, the current flows through and ; for the O+ state, the current flows through and . Therefore, during mode 2, switching losses and conduction losses are generated, only conduction losses, conduction losses and recovery losses are generated, and all losses are generated in the ANPC bridge arm. Before and after injecting the third harmonic, the action time

[0070] of the auxiliary diode in one modulation period is as shown in Equation (3):

[0071] where, is the modulation degree, is the switching period, is the carrier ratio, represents the One switching period. It can be seen from Equation (3) that the third-harmonic injection can not only improve the reactive power output ability of the converter, but also effectively increase the operation time of the auxiliary diode and improve the current sharing effect.

[0072] The effectiveness and advancement of the control method proposed in the embodiment of the present invention are verified by Simulink / Plecs software.

[0073] See Appendix Figures 1-3 for the circuit structure therein. A circuit model is built in the simulation software, and the device selection and parameters are shown in Table 2.

[0074]

[0075] See Appendix Figure 7 for (a) and (b) therein. Still taking Phase A as an example, the power losses of each device when the traditional ANPC topology and the low-cost high-reactive-power overload energy storage converter topology simultaneously output 100 kVar of reactive power are given in the figure. It can be seen that the auxiliary diode in the low-cost high-reactive-power overload energy storage converter topology effectively undertakes the reactive current and reduces the power loss of the tubes in the ANPC bridge arm. and The current waveforms in one cycle are shown in Appendix Figure 8 .

[0076] See Appendix Figure 9 . Compared with the traditional ANPC topology, since the overall power loss of the low-cost high-reactive-power overload energy storage converter is reduced, the average junction temperature is reduced by 9.35 °C when 100 kW of reactive power is generated, and the maximum junction temperature is transferred from to .

[0077] To increase the reactive power output of the low-cost high-reactive-power overload energy storage converter and keep the maximum junction temperature unchanged, it is considered that the junction temperature in the ANPC bridge arm of the low-cost high-reactive-power overload energy storage converter reaches the maximum reactive power output when it is the same as the junction temperature in the traditional ANPC topology. The temperature change curves of the two tubes are shown in Appendix Figure 10 . At this time, the reactive power output by the traditional ANPC topology is 100 kVar, and the reactive power output by the low-cost high-reactive-power overload energy storage converter is 122 kVar. That is, the low-cost high-reactive-power overload energy storage converter can improve the reactive power overload capacity by 22% compared with the traditional ANPC topology converter. The simulation waveforms verify the effectiveness of the low-cost high-reactive-power overload energy storage converter topology and the proposed modulation method.

[0078] The embodiments of the present invention do not significantly increase the converter device cost while improving the reactive power overload capacity by 22%. Taking the device prices on the website of LCSC (www.szlcsc.com) as an example, the parameters and prices of each device are shown in Table 3.

[0079]

[0080] IGBT drive is part of the switch device cost and is thus considered in the table. The traditional ANPC topology requires 9 IPMs, with a total device cost of 9323.73 yuan. After adding three auxiliary diodes to the low-cost high-reactive-power overload energy storage converter topology proposed in the present invention, the total device cost is 9831.96 yuan, and the device cost only increases by 5.45%, while the reactive power overload capacity increases by 22%.

[0081] Although the preferred embodiments of the present application have been described, those skilled in the art can make additional changes and modifications to these embodiments once they learn the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications falling within the scope of the present application.

[0082] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalent technologies, the present application also intends to include these changes and modifications.

Claims

1. A high reactive power overload energy storage converter, characterized in that Including: Three-phase output lines, three-phase converter bridge arms, three-phase auxiliary diodes, a data acquisition and calculation unit, and a drive signal generation unit; The three-phase output lines include an A-phase line, a B-phase line, and a C-phase line. The input ends of the A-phase line, the B-phase line, and the C-phase line are electrically connected to the auxiliary diode group and the output ends of the converter bridge arms respectively; Each phase of the three-phase auxiliary diode group includes an upper diode and a lower diode . The positive electrode of the upper diode and the negative electrode of the lower diode are commonly electrically connected to the output end of the converter bridge arm circuit. The negative electrode of the upper diode is electrically connected to the positive pole of the DC bus , and the positive electrode of the lower diode is electrically connected to the negative pole of the DC bus ; The A-phase line, the B-phase line, and the C-phase line are also electrically connected to a data acquisition and calculation unit. The data acquisition and calculation unit is communicatively connected to a control system; the control system generates a modulation signal according to the current and voltage signals of the A-phase line, the B-phase line, and the C-phase line, and the drive signal generation unit generates a drive signal according to the modulation signal to control the on / off of the switching devices in the converter bridge arms; The converter leg circuit is an ANPC leg, and the ANPC leg includes six switching devices. Each switching device contains a switching tube, and a diode is anti-parallel connected to the switching tube. The six switching devices are the first switching device S a1 , the second switching device S a2 , the third switching device S a3 , the fourth switching device S a4 , the fifth switching device S a5 and the sixth switching device S a6 . The collector of the first switching device S a1 is electrically connected to the positive pole of the DC bus , and the emitter is electrically connected to the collector of the second switching device S a2 and the collector of the fifth switching device S a5 . The emitter of the second switching device S a2 is electrically connected to the collector of the third switching device S a3 and the output terminal of the converter; the emitter of the fifth switching device S a5 is electrically connected to the collector of the sixth switching device S a6 and the neutral point O ; the emitters of the third switching device S a3 and the sixth switching device S a6 are electrically connected to the collector of the fourth switching device S a4 . The emitter of the fourth switching device S a4 is electrically connected to the negative pole of the DC bus ; the neutral point O is electrically connected to one end of the upper capacitor and one end of the lower capacitor . The other end of the upper capacitor is electrically connected to the positive pole of the DC bus , and the other end of the lower capacitor is electrically connected to the negative pole of the DC bus ; The SPWM modulation method with third-harmonic injection for the ANPC bridge arm, the conduction time of the auxiliary diode before and after the third-harmonic injection is as follows: ； Among them, is the switching period, is the carrier ratio, represents the th switching period, is the angular frequency, is the modulation index.

2. The high-reactive overload energy storage converter according to claim 1, wherein In the ANPC bridge arm, the first switching device S a1 and the fifth switching device S a5 are placed in the same IPM. The second switching device S a2 and the third switching device S a3 are placed in the same IPM. The fourth switching device S a4 and the sixth switching device S a6 are placed in the same IPM. The two switching devices in each IPM are complementary in on / off states.

3. The high-reactive overload energy storage converter according to claim 1, characterized in that The switching tube is an IGBT or a MOS tube; the reverse-parallel diode of the switching tube is connected such that the positive electrode of the diode is electrically connected to the emitter of the switching tube and the negative electrode of the diode is electrically connected to the collector of the switching tube.

4. The high-reactive overload energy storage converter according to claim 1, characterized in that The three IPMs of the same ANPC bridge arm are placed on the same heat dissipation module.

5. The high-reactive overload energy storage converter according to claim 1, wherein The upper diode and the lower diode are both Schottky diodes or fast recovery diodes.

6. The high-reactive overload energy storage converter according to claim 1, characterized in that The three-phase auxiliary diode group is placed on the same heat dissipation module.

7. A modulation method for a high-reactive overload energy storage converter, characterized in that, For the high-reactive-power overload energy storage converter as described in any one of claims 1-6, the modulation method of the ANPC bridge arm corresponding to each phase of the three-phase output line includes the following steps: Step 1: Adopt the SPWM modulation method with third-harmonic injection. The modulation function expression is as follows: ； is the modulation wave, is the modulation degree, is the angular frequency, is the time; Auxiliary Diode Conduction Time Before and After Injecting the Third Harmonic is as follows: ； is the switching period, is the carrier ratio, represents the th switching period; Step 2: Modulation wave When it is greater than 0, if the modulation wave is greater than the triangular carrier wave, output the P state; If the modulation wave is less than the carrier wave, the output is in the O+ state; when the modulation wave is less than 0, if the modulation wave is greater than the triangular carrier wave, the output is in the O- state; If the modulation wave is less than the carrier wave, output the N state; Among them, when the output is in the P state S a1 , S a2 and S a6 conduct; When the output is in the O+ state S a2 , S a5 and S a6 conduct; When outputting the O-state S a3 , S a5 , S a6 conduct; When outputting the N state S a3 , S a4 and S a5 are turned on.

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