Output voltage balance control and harmonic suppression method under unbalanced load

Abstract

The invention provides an output voltage balance control and harmonic suppression method under an unbalanced load, and the method comprises the steps: building a mathematical model of an NPC three-level inverter under different coordinate systems, taking the output three-phase voltage as a control target, analyzing the expression form of the output three-phase voltage under the different coordinate systems under an unbalanced working condition, and calculating the output voltage balance of the NPC three-level inverter; and finally performing research and analysis under an alpha-beta coordinate system. Proportional complex integral QPCI control is improved, and a fundamental wave negative sequence component is suppressed. A corresponding repetitive controller is improved, and the anti-disturbance capability to harmonic waves is improved; and based on the control algorithm in the above steps, carrying out improved composite control on a controlled object, and comprehensively realizing tracking of a fundamental frequency positive sequence component, suppression of a negative sequence component and suppression of harmonic waves.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter control, and in particular to a method for output voltage balance control and harmonic suppression under unbalanced load. Background Art

[0002] Compared with the traditional two-level inverter, the three-level inverter has the advantage of lower output harmonic content, which reduces the size of the back-end filter and improves the efficiency and performance of the system. Ideally, when the inverter operates under a three-phase balanced load, the output voltage waveform is of high quality and can provide stable and accurate power.

[0003] In actual industrial and life applications, the load is often unbalanced due to various reasons. This unbalanced load will have a significant impact on the output voltage of the inverter, resulting in the output voltage imbalance failing to meet the 5% standard requirement. In addition, the unbalanced load will cause the total harmonic distortion (THD) value to increase, causing the output voltage waveform to distort, seriously affecting the power supply quality.

[0004] According to the inventors' understanding, the present invention points out that under unbalanced load, negative sequence components and harmonic components appear as AC quantities in the dq coordinate system, and the PI controller cannot achieve zero-error tracking for them, which will cause an increase in THD; at the same time, the modeling of the NPC inverter system is not extremely accurate, and the designed controller may cause deviations in the control target during actual control, destroying the stability of the system and affecting the imbalance of the output voltage. Summary of the invention

[0005] According to the technical problems raised above, a method for output voltage balance control and harmonic suppression under unbalanced load is provided. The present invention establishes a mathematical model of an NPC three-level inverter in different coordinate systems, takes the output three-phase voltage as the control target, and analyzes its expression in different coordinate systems under unbalanced working conditions. The present invention improves the quasi-proportional complex integral (QPCI) control to achieve the effect of suppressing the fundamental negative sequence component; for the harmonic components in the output voltage, according to the internal model principle, an improved composite control combined with a repetitive control algorithm is used to suppress them. The improved QPCI control algorithm can effectively track the fundamental positive sequence voltage and suppress the fundamental negative sequence voltage to ensure the balance of the output voltage. The improved composite control algorithm can effectively suppress harmonics on this basis.

[0006] The technical means adopted by the present invention are as follows:

[0007] A method for output voltage balance control and harmonic suppression under unbalanced load, comprising:

[0008] S1. In different coordinate systems, the mathematical model of the NPC three-level inverter is established, and the output three-phase voltage is used as the control target to analyze its performance in different coordinate systems under unbalanced working conditions;

[0009] S2, improve the quasi-proportional complex integral QPCI control to suppress the fundamental negative sequence component;

[0010] S3. Combine the repetitive control algorithm and design the corresponding repetitive controller to improve the anti-disturbance capability against harmonics.

[0011] S4. Based on the control algorithm in the above steps, an improved composite control is performed on the controlled object to comprehensively realize the tracking of the positive sequence component of the fundamental frequency, the suppression of the negative sequence component and the suppression of the harmonics.

[0012] Furthermore, step S1 specifically includes:

[0013] S11. In the three-phase stationary coordinate system, the mathematical model is established as follows:

[0014]

[0015] Where X = [V a V b V c i La i Lb i Lc ],U=[U a U b U c i LLa i LLb i LLc ];

[0016]

[0017]

[0018] S12. Perform Clark transformation on the mathematical model in the abc coordinate system to obtain the mathematical model in the αβ coordinate system, as follows:

[0019]

[0020] S13, perform Park transformation to obtain the mathematical model of the controlled system in the two-phase rotating dq coordinate system, as follows:

[0021]

[0022] S14, mathematical model of output fundamental voltage under unbalanced load, output three-phase unbalanced voltage is expressed as follows:

[0023]

[0024] Among them, V m + , is the amplitude and initial phase of the positive sequence component, V m - , is the amplitude and initial phase of the negative sequence component;

[0025] S15. The output three-phase unbalanced voltage of the αβ coordinate system is expressed as follows:

[0026]

[0027] S16. The positive sequence component and negative sequence component of the dq coordinate system are expressed as follows:

[0028]

[0029] S17. The harmonic content in the three-phase output voltage in the αβ coordinate system is expressed as follows:

[0030]

[0031] Among them, V m_(6h±1) is the amplitude corresponding to each harmonic, is its initial phase angle, and the harmonics appear in the form of integer multiples of the fundamental wave (6h+1) in the αβ coordinate system;

[0032] S18. Express the harmonics in the dq coordinate system as follows:

[0033]

[0034] Further, step S2 specifically includes:

[0035] S21. Use the αβ coordinate system to design the transfer function of the PCI controller as follows:

[0036]

[0037] S22, introduce the bandwidth coefficient and convert the PCI controller into a quasi-PCI controller QPCI, as follows:

[0038]

[0039] Among them, ω c is the bandwidth factor, after adjusting K p , K i Meet the required steady-state error requirements;

[0040] S23, improve the structure of the quasi-PCI controller QPCI to obtain a DQPCI controller, so that the DQPCI controller r and -ω r Both have large gains, completing the tracking of positive sequence and the suppression of negative sequence. The improved transfer function is as follows:

[0041]

[0042] Further, step S3 specifically includes:

[0043] S31. Perform amplitude and phase compensation on the controlled object p(z) to ensure the stability of the control system and design the compensator as follows:

[0044] c(z)=K r z k s(z)

[0045] Among them, K r is the gain of the compensator; z k is the leading link of the compensator; s(z) is the filter;

[0046] S32. The continuous domain transfer function p(s) of the three-level inverter of the controlled object is expressed as:

[0047]

[0048] S33, discretize the continuous domain transfer function p(s) of the three-level inverter of the controlled object using a zero-order holder, and obtain:

[0049]

[0050] S34, the s(z) link in the compensator uses a cascaded form of a notch filter and a second-order low-pass filter; the notch filter will not affect other frequency bands; for high-frequency attenuation, a second-order low-pass filter with a suitable cutoff frequency is selected to achieve a perfect attenuation effect, where:

[0051] The transfer function of the Z-domain zero-phase-shift notch filter is as follows:

[0052]

[0053] The expression of the second-order low-pass filter is as follows:

[0054]

[0055] The compensator discrete filter is as follows:

[0056]

[0057] S35. Analyze the stability of the repetitive controller and express the input-output relationship of the repetitive controller as follows:

[0058]

[0059] Where, e(z) = r(z) - u r (z)P(z)-d(z) is the input error of the system. Substituting it into the above formula, we get:

[0060]

[0061] S36. The characteristic equation of the system after adding the improved repetitive controller is obtained as follows:

[0062] 1-Q(z)z -N +C(z)P(z)z -N =0

[0063] Among them, the sufficient condition for the stability of the system is The Nyquist plot of Q(z)-C(z)P(z) is obtained based on the designed repetitive controller, and its amplitude is less than 1, which proves that the parameters of the designed repetitive controller meet the stability requirements;

[0064] Further, step S4 specifically includes:

[0065] S41. Improved compound control. The expression of the closed-loop transfer function of the improved compound control is as follows:

[0066]

[0067] S42. Substitute the expression of the improved repetitive controller into the formula of S41 to obtain:

[0068]

[0069] S43, let z = e jωT s , the sufficient conditions for the stability of the system are as follows:

[0070]

[0071] Compared with the prior art, the present invention has the following advantages:

[0072] 1. The present invention provides a method for output voltage balance control and harmonic suppression under unbalanced load. Aiming at the shortcomings of traditional double closed-loop PI control, the present invention improves it on the basis of quasi-proportional complex integral (QPCI) control to achieve the effect of suppressing the fundamental negative sequence component. When the load is balanced, the traditional PI control method and the improved QPCI algorithm can basically maintain the imbalance below 0.1%, and the PI control effect is the best. However, when the load changes suddenly, the traditional PI control cannot control the output voltage balance. After reaching the steady state, the output power imbalance is large, greater than 5%. The improved QPCI output voltage imbalance can be maintained at about 0.05%, which proves the ability of the method of the present invention to suppress negative sequence components.

[0073] 2. The present invention provides a method for output voltage balancing control and harmonic suppression under unbalanced load. To address the problem of harmonics, an algorithm combined with repetitive control is proposed, and a repetitive controller is designed. The improved composite control algorithm has a stronger anti-disturbance capability against harmonics than the improved QPCI control algorithm.

[0074] Based on the above reasons, the present invention can be widely promoted in the fields of inverter control and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] 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 or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are 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.

[0076] Figure 1 It is a three-phase three-wire topology structure of the NPC three-level inverter of the present invention.

[0077] Figure 2 It is a structural block diagram of the NPC three-level inverter system in the αβ coordinate system of the present invention.

[0078] Figure 3 It is a structural block diagram of the NPC three-level inverter system in the dq coordinate system of the present invention.

[0079] Figure 4 The improved QPCI controller structure block diagram of the present invention.

[0080] Figure 5 This is a structural block diagram of a control system based on improved repetitive control according to the present invention.

[0081] Figure 6 It is the Nyquist diagram of Q(z)-C(z)P(z) of the present invention.

[0082] Figure 7This is the overall structural block diagram of the improved composite control system of the present invention.

[0083] Figure 8 The figure is a comparison diagram of output voltage imbalance under three different control algorithms of the present invention.

[0084] Fig. 9 This is a comparison chart of FFT analysis of three control algorithms of the present invention.

[0085] Fig.10 This is a comparison diagram of FFT analysis after adding harmonic disturbance to the present invention. DETAILED DESCRIPTION

[0086] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings 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 should fall within the scope of protection of the present invention.

[0087] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0088] The present invention provides a method for output voltage balance control and harmonic suppression under unbalanced load, comprising:

[0089] S1. In different coordinate systems, the mathematical model of the NPC three-level inverter is established, and the output three-phase voltage is used as the control target to analyze its performance in different coordinate systems under unbalanced working conditions;

[0090] S2, improve the quasi-proportional complex integral QPCI control to suppress the fundamental negative sequence component;

[0091] S3. Combine the repetitive control algorithm and design the corresponding repetitive controller to improve the anti-disturbance capability against harmonics.

[0092] In specific implementation, as a preferred embodiment of the present invention, step S1 specifically includes:

[0093] S11. In this embodiment, the three-phase three-wire inverter topology used in the present invention is as follows: Figure 1 As shown, in the three-phase stationary coordinate system, the mathematical model is established as follows:

[0094]

[0095] Where X = [V a V b V c i La i Lb i Lc ],U=[U a U b U c i LLa i LLb i LLc ];

[0096]

[0097] C = [1 1 1 0 0 0];

[0098] S12. Perform Clark transformation on the mathematical model in the abc coordinate system to obtain the mathematical model in the αβ coordinate system, as follows:

[0099]

[0100] like Figure 2 As shown, the system structure block diagram of the inverter in the complex domain (s) can be obtained from the formula.

[0101] S13, perform Park transformation to obtain the mathematical model of the controlled system in the two-phase rotating dq coordinate system, as follows:

[0102]

[0103] The structural block diagram of the inverter in the complex domain (s) can also be derived from the formula in step S13, such as Figure 3 As shown. Figure 3 It can be seen that there is a coupling relationship between the dq axes in the two-phase rotating coordinate system. When designing the controller, it is necessary to first perform feedforward decoupling and then control them separately.

[0104] S14, output fundamental voltage mathematical model under unbalanced load, the present invention adopts a three-phase three-wire topology structure, the zero-sequence component has no circulation loop, and only considers positive sequence and negative sequence. The inverter output three-phase positive sequence voltage is set as V x + (x=a,b,c), the negative sequence component is Vx - (x=a,b,c), the output three-phase unbalanced voltage is expressed as follows:

[0105]

[0106] Among them, V m + , is the amplitude and initial phase of the positive sequence component, V m - , is the amplitude and initial phase of the negative sequence component;

[0107] S15. The output three-phase unbalanced voltage of the αβ coordinate system is expressed as follows:

[0108]

[0109] S16. From the formula in S15, it can be seen that in the αβ coordinate system, the positive sequence component and the negative sequence component are the alternating quantities of the fundamental angular frequency ω and -ω respectively; the positive sequence component and the negative sequence component of the three-phase output voltage on the dq axis can be obtained by performing positive sequence Park transformation and negative sequence Park transformation respectively, and the expressions are as follows:

[0110]

[0111] S17. Mathematical model of output voltage harmonic voltage under unbalanced load. Clark transformation of the harmonic content in the three-phase output voltage can obtain its expression in the αβ coordinate system. The harmonic content in the three-phase output voltage in the αβ coordinate system is expressed as follows:

[0112]

[0113] Among them, V m_(6h±1) is the amplitude corresponding to each harmonic, is its initial phase angle, and the harmonics appear in the form of integer multiples of the fundamental wave (6h+1) in the αβ coordinate system;

[0114] S18. Perform Park transformation to obtain the expression of harmonics on the dq axis as follows:

[0115]

[0116] It can be seen from the above formula that the 5th and 7th harmonics on the dq coordinate axis will appear in the form of 6 times the fundamental frequency AC quantity, and the 11th and 13th harmonics will appear in the form of 12 times the fundamental frequency AC quantity.

[0117] In specific implementation, as a preferred embodiment of the present invention, step S2 specifically includes:

[0118] S21. Use the αβ coordinate system to design the transfer function of the PCI controller as follows:

[0119]

[0120] S22. Excessive gain will cause insufficient system stability. The bandwidth factor is introduced to convert the PCI controller to a quasi-PCI controller QPCI, as follows:

[0121]

[0122] Among them, ω c is the bandwidth factor. The above stability problem has been solved. By adjusting K p , K i Meet the required steady-state error requirements, in addition ω c The intervention can improve the anti-disturbance performance of the system.

[0123] S23, PCI and QPCI controllers are not capable of suppressing negative sequence components. The present invention improves the structure of the quasi-PCI controller QPCI to obtain a DQPCI controller, so that the DQPCI controller r and -ω r Both have large gains, completing the tracking of positive sequence and the suppression of negative sequence. The improved transfer function is as follows:

[0124]

[0125] According to its transfer function formula, its control structure block diagram can be obtained, such as Figure 4 shown.

[0126] In specific implementation, as a preferred embodiment of the present invention, step S3 includes: improving the design of the repetitive controller. Since the modeling of the inverter system is not extremely accurate, the controller designed may cause deviations in the control target during actual control and destroy the stability of the system. The discrete domain structure block diagram of the repetitive control system after improving the internal model structure is as follows: Figure 5 In this embodiment, step S3 specifically includes:

[0127] S31. Design of compensator In the design of repetitive controller, the compensation link is the core element, which mainly performs amplitude and phase compensation on the controlled object p(z) to ensure the stability of the control system. The compensator is designed as follows:

[0128] c(z)=K r z k s(z)

[0129] Among them, K ris the gain of the compensator. The larger its value, the faster the system responds, but it will weaken its stability. Therefore, it is necessary to comprehensively consider the system stability to select the appropriate gain value; k It is the compensator advance link, which is mainly used to compensate the phase lag caused by the controlled object and the compensator, so that the phase shift of the controlled object c(z)p(z) after compensation is 0 in the medium and low frequency bands. s(z) is a filter, whose purpose is to correct the amplitude of the controlled object p(z) so that its amplitude is 0dB in the medium and low frequency bands, and to attenuate the high frequency to stabilize the system.

[0130] S32. The continuous domain transfer function p(s) of the three-level inverter of the controlled object is expressed as:

[0131]

[0132] S33, discretize the continuous domain transfer function p(s) of the three-level inverter of the controlled object using a zero-order holder, and obtain:

[0133]

[0134] S34, the s(z) link in the compensator uses a cascaded form of a notch filter and a second-order low-pass filter; the notch filter will not affect other frequency bands; for high-frequency attenuation, a second-order low-pass filter with a suitable cutoff frequency is selected to achieve a perfect attenuation effect, where:

[0135] The transfer function of the Z-domain zero-phase-shift notch filter is as follows:

[0136]

[0137] The expression of the second-order low-pass filter is as follows:

[0138]

[0139] The compensator discrete filter is as follows:

[0140]

[0141] S35. Analyze the stability of the repetitive controller and express the input-output relationship of the repetitive controller as follows:

[0142]

[0143] Where, e(z) = r(z) - u r (z)P(z)-d(z) is the input error of the system. Substituting it into the above formula, we get:

[0144]

[0145] S36. The characteristic equation of the system after adding the improved repetitive controller is obtained as follows:

[0146] 1-Q(z)z -N +C(z)P(z)z -N =0

[0147] Among them, the sufficient condition for the stability of the system is According to the designed repetitive controller, the Nyquist diagram of Q(z)-C(z)P(z) is obtained, as shown in Figure 6 As shown, their amplitudes are all less than 1, proving that the parameters of the designed repetitive controller meet the stability requirements;

[0148] In specific implementation, as a preferred embodiment of the present invention, step S4 specifically includes:

[0149] S41, improved composite control, the repetitive control algorithm mentioned in step S36 has a better suppression effect on the fundamental wave and its harmonics of integer multiples of frequency due to the use of an improved internal model structure, but in order to ensure the stability of the system, 5% of the steady-state error is sacrificed, and the suppression effect on the negative sequence component is insufficient. At the same time, due to the delay link z -200 The existence of makes the system response almost delayed by one cycle. The improved QPCI control algorithm mentioned in step S31 can achieve better tracking of the fundamental wave, and at the same time has better suppression ability for the negative sequence component, and the current loop proportional control can speed up the dynamic response speed of the system. Combining these two control algorithms to improve the composite control of the controlled object can comprehensively achieve tracking of the fundamental frequency positive sequence component, suppression of the negative sequence component and suppression of harmonics. The improved overall control is carried out in the αβ coordinate system. The structural block diagram of the system is as follows: Figure 7 As shown. The expression of the improved composite control closed-loop transfer function is as follows:

[0150]

[0151] S42. Substitute the expression of the improved repetitive controller into the formula of S41 to obtain:

[0152]

[0153] S43, Order The sufficient conditions for the stability of the system are as follows:

[0154]

[0155] Example

[0156] The simulation results verify the output voltage imbalance under three different control algorithms, such as Figure 8 shown. Figure 8 (a) Figure 8 (b) Figure 8 (c) represents the voltage imbalance of PI control, improved QPCI control and improved composite control respectively. It can be seen that the first three control methods can basically maintain the imbalance below 0.1% at 0.15s, and PI control has the best effect. When the load suddenly changes at 0.15s, the traditional PI control cannot control the output voltage balance. After reaching the steady state, the output voltage imbalance fluctuates at about 8.3%. The output voltage imbalance of improved QPCI and improved composite control is not much different, and can be maintained at about 0.05%, which proves the ability of its algorithm to suppress negative sequence components.

[0157] Fourier analysis comparison of phase A voltage of three control algorithms, such as Fig. 9 As shown, Fig. 9 (a) Fig. 9 (b) Fig. 9 (c) FFT analysis of traditional PI control, improved QPCI control and improved composite control, respectively. The latter two are not much different.

[0158] In order to verify the effect of improved composite control on harmonic suppression, fifth and seventh harmonic disturbances with an amplitude of 20V are added to the output voltage, and the output waveform effects of the two are compared and analyzed. The FFT analysis of the improved QPCI and improved composite control after adding disturbances is shown in the figure below. Fig.10 As shown, Fig.10 (a) Fig.10 (b) FFT analysis of improved QPCI control and improved composite control, respectively. It can be seen that the improved composite control reduces THD from 3.57% to 1.34%, and the harmonic suppression effect is more obvious.

[0159] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for output voltage balance control and harmonic suppression under unbalanced load, characterized in that: include: S1. In different coordinate systems, the mathematical model of the NPC three-level inverter is established, and the output three-phase voltage is used as the control target to analyze its performance in different coordinate systems under unbalanced working conditions; S2, improve the quasi-proportional complex integral QPCI control to suppress the fundamental negative sequence component; S3. Combine the repetitive control algorithm and design the corresponding repetitive controller to improve the anti-disturbance capability against harmonics; S4. Based on the control algorithm in the above steps, an improved composite control is performed on the controlled object to comprehensively realize the tracking of the positive sequence component of the fundamental frequency, the suppression of the negative sequence component and the suppression of the harmonics.

2. The method for output voltage balance control and harmonic suppression under unbalanced load according to claim 1, characterized in that: Step S1 specifically includes: S11. In the three-phase stationary coordinate system, the mathematical model is established as follows: where X = [V a V b V c i La i Lb i Lc , U = [U a U b U c i LLa i LLb i LLc ; C＝[1 1 1 0 0 0]； S12. Perform Clark transformation on the mathematical model in the abc coordinate system to obtain the mathematical model in the αβ coordinate system, as follows: S13, perform Park transformation to obtain the mathematical model of the controlled system in the two-phase rotating dq coordinate system, as follows: S14, mathematical model of output fundamental voltage under unbalanced load, output three-phase unbalanced voltage is expressed as follows: Among them, V m + , is the amplitude and initial phase of the positive sequence component, V m - , is the amplitude and initial phase of the negative sequence component; S15. The output three-phase unbalanced voltage of the αβ coordinate system is expressed as follows: S16. The positive sequence component and negative sequence component of the dq coordinate system are expressed as follows: S17. The harmonic content in the three-phase output voltage in the αβ coordinate system is expressed as follows: Among them, V m_(6h±1) is the amplitude corresponding to each harmonic, is its initial phase angle, and the harmonics appear in the form of integer multiples of the fundamental wave (6h+1) in the αβ coordinate system; S18. Express the harmonics in the dq coordinate system as follows:

3. The method for output voltage balance control and harmonic suppression under unbalanced load according to claim 1, characterized in that: Step S2 specifically includes: S21. Use the αβ coordinate system to design the transfer function of the PCI controller as follows: S22, introduce the bandwidth coefficient and convert the PCI controller into a quasi-PCI controller QPCI, as follows: Among them, ω c is the bandwidth factor, after adjusting K p , K i Meet the required steady-state error requirements; S23, improve the structure of the quasi-PCI controller QPCI to obtain a DQPCI controller, so that the DQPCI controller r and -ω r Both have large gains, completing the tracking of positive sequence and the suppression of negative sequence. The improved transfer function is as follows:

4. The method for output voltage balance control and harmonic suppression under unbalanced load according to claim 1, characterized in that: Step S3 specifically includes: S31. Perform amplitude and phase compensation on the controlled object p(z) to ensure the stability of the control system and design the compensator as follows: c(z)=K r z k s(z) Among them, K r is the gain of the compensator; z k is the leading link of the compensator; s(z) is the filter; S32. The continuous domain transfer function p(s) of the three-level inverter of the controlled object is expressed as: S33, discretize the continuous domain transfer function p(s) of the three-level inverter of the controlled object using a zero-order holder, and obtain: S34, the s(z) link in the compensator uses a cascaded form of a notch filter and a second-order low-pass filter; the notch filter will not affect other frequency bands; for high-frequency attenuation, a second-order low-pass filter with a suitable cutoff frequency is selected to achieve a perfect attenuation effect, where: The transfer function of the Z-domain zero-phase-shift notch filter is as follows: The expression of the second-order low-pass filter is as follows: The compensator discrete filter is as follows: S35. Analyze the stability of the repetitive controller and express the input-output relationship of the repetitive controller as follows: Where, e(z) = r(z) - u r (z)P(z)-d(z) is the input error of the system. Substituting it into the above formula, we get: S36. The characteristic equation of the system after adding the improved repetitive controller is obtained as follows: 1-Q(z)z -N +C(z)P(z)z -N =0 Among them, the sufficient condition for the stability of the system is The Nyquist plot of Q(z)-C(z)P(z) is obtained based on the designed repetitive controller, and its amplitude is less than 1, which proves that the parameters of the designed repetitive controller meet the stability requirements; 5. The method for output voltage balance control and harmonic suppression under unbalanced load according to claim 1, characterized in that: Step S4 specifically includes: S41. Improved compound control. The expression of the closed-loop transfer function of the improved compound control is as follows: S42. Substitute the expression of the improved repetitive controller into the formula of S41 to obtain: S43, Order The sufficient conditions for the stability of the system are as follows:

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