Unbalanced load compensation method for three-phase four-wire three-level converter

Abstract

The invention discloses an unbalanced load compensation method for a three-phase four-wire three-level converter. According to the method, a three-phase four-wire system three-level converter bridge arm voltage mathematical model is established based on sine carrier in-phase laminated modulation, and harmonic content analysis is completed; an SOGI-pro module is established (an error integration link is added at the input front end of a D axis of an SOGI filter, and D and Q axis second-order band-pass filters are transformed into third-order band-pass filters), so that the positive and negative sequence separation precision of three-phase unbalanced load voltage is improved. According to the method, the order of the SOGI filter is improved by reasonably designing a closed-loop transfer function, and the filtering effect of the SOGI filter in high and low frequency bands is enhanced. And a sequence-dividing imbalance compensation control loop is designed based on the three-sequence dq0 coordinate system, so that zero-static-error tracking of a positive-sequence fundamental component and powerful suppression of negative-sequence and zero-sequence imbalance components are realized. According to the invention, the accuracy of traditional positive and negative sequence separation is improved, so that the unbalanced load compensation effect under sequence separation control can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic control, and particularly relates to a method for compensating unbalanced loads of a three-phase four-wire three-level converter. Background Art

[0002] With the rapid development of renewable energy and energy storage technologies, the application of energy storage converters in off-grid operation has received increasing attention. In off-grid power supply systems, load imbalance is a common problem. Especially in microgrids or small power grids in remote areas, due to the diverse types of users and the widespread existence of single-phase loads, the system often faces significant load imbalance problems. Unbalanced loads not only cause waveform distortion of the converter output voltage and current, but also increase system losses, affect the service life of the converter and the load, and even may endanger the stability and power supply quality of the system. Therefore, how to effectively compensate for unbalanced loads has become one of the key problems in the research of off-grid converters.

[0003] The three-phase four-wire neutral-connected energy storage converter has gradually become a research hotspot due to its unique advantages in unbalanced load compensation. Compared with the traditional three-phase three-wire topology, the three-phase four-wire topology can provide independent phase voltage regulation ability under unbalanced load conditions by introducing a neutral line. This structure allows the load current of each phase to be asymmetric without significantly affecting other phases, thereby effectively reducing the interference of load imbalance on the system. Therefore, the three-phase four-wire neutral-connected energy storage converter has significant advantages in off-grid unbalanced load scenarios, and its research is of great significance for improving the reliability and power supply quality of off-grid systems.

[0004] In off-grid power supply systems, reducing the three-phase unbalance degree and improving the unbalanced compensation accuracy are one of the core objectives of the energy storage converter to achieve unbalanced compensation. However, achieving this goal faces multiple challenges. First, the load imbalance situation is highly dynamic and random, requiring the control strategy of the converter to have high precision and fast dynamic response capabilities. Second, due to the possible existence of impact loads, nonlinear loads, harmonic interference and other complex operating conditions in off-grid systems, the control strategy needs to take into account both harmonic suppression and system stability at the same time. Finally, the implementation of advanced control algorithms places higher requirements on the hardware performance and computing resources, and it is necessary to find a balance among cost, efficiency and complexity. Summary of the Invention

[0005] In view of the above problems existing in the prior art, the present invention proposes an unbalanced load compensation method for a three-phase four-wire three-level converter. The invention establishes a mathematical model of the bridge arm voltage of the three-phase four-wire three-level converter based on sinusoidal carrier in-phase stacking modulation and completes the harmonic content analysis. On this basis, by reasonably designing the closed-loop transfer function, the order of the traditional SOGI filter is increased, and the filtering effect on high and low frequency bands is enhanced, which is beneficial to eliminating the harmonic components in the unbalanced bridge arm voltage and improving the positive and negative sequence separation accuracy. Further, the invention designs a sequence unbalanced compensation control loop based on the three-sequence dq0 coordinate system to achieve zero-static error tracking of the positive-sequence fundamental component and strong suppression of the negative-sequence and zero-sequence unbalanced components, and has a better unbalanced compensation control effect than the traditional strategy.

[0006] The present invention provides an unbalanced load compensation method for a three-phase four-wire three-level converter, which includes the following steps:

[0007] 1) Establish a mathematical model of the bridge arm voltage of the three-phase four-wire three-level converter and analyze the harmonic components of the bridge arm voltage;

[0008] 2) Design a SOGI-pro module based on the SOGI filter, specifically: add an error integration link at the front end of the D-axis input of the SOGI filter, and transform the second-order band-pass filter on the D-axis of the SOGI into a third-order band-pass filter and transform the second-order low-pass filter on the Q-axis into a third-order band-pass filter;

[0009] 3) Perform high-precision positive and negative sequence separation on the three-phase unbalanced load voltage in the αβ coordinate system based on the SOGI-pro module;

[0010] 4) Design a sequence unbalanced compensation control loop in the three-sequence dq0 coordinate system to compensate the unbalanced load.

[0011] As a preferred solution of the present invention, the three-phase four-wire three-level converter in step 1) adopts a split-capacitor midpoint reconnection topology, and the output load neutral line is reconnected to the midpoint of the DC bus to provide a loop for the zero-sequence current component in the three-phase output system, thereby improving the converter's management ability for zero-sequence unbalanced components and having significant advantages in the scenario of the energy storage converter in the microgrid system with unbalanced loads.

[0012] As a preferred solution of the present invention, the three-phase four-wire three-level converter in step 1) adopts the sinusoidal carrier in-phase stacking modulation (PD-SPWM, Phase Deposition Sinusoidal Pulse Width Modulation) method, and realizes the improvement of the power transmission efficiency of the output sinusoidal component and the enhancement of the anti-interference ability by multiplexing the carrier stacking of the upper and lower sinusoidal half-cycles to concentrate the signal energy in the time domain or frequency domain. Define the switching functions of the four fully controlled switches of each phase bridge arm as Syx , where y = 1, 2, 3, 4 represents the serial number of the switching device, and x = a, b, c represents the three-phase serial number; = 0 indicates turn-off, = 1 indicates turn-on.

[0013] As a preferred embodiment of the present invention, in step 1), the switching functions S 1,x and S 2,x are further calculated according to the double Fourier analysis, and the specific expressions are as follows:

[0014] ;

[0015] ;

[0016] where A 0,0 and D 0,0 represent the DC component amplitudes of the corresponding switching functions, A 0,n and D 0,n represent the amplitudes of the nth harmonic components of the fundamental frequency of the corresponding switching functions, n is an integer ≥ 1, A m,n and D m,n represent the high-frequency components of the carrier frequency band of the corresponding switching functions, m represents the frequency of the switching frequency components, n represents the frequency of the fundamental frequency components, ω0 represents the fundamental angular frequency, and ω c represents the high-frequency carrier angular frequency, represents the phase angles of the three-phase modulation waves, where phase A = 0°, phase B = 120°, phase C = 240°, and t represents time.

[0017] In the positive half modulation period, the differences between the switching functions S 1,x and S 2,x represent the duty ratios of the three-level converter topology in the "O" level state. After filtering out the high-frequency components of the carrier frequency band, the sum of the switching functions S 2,x - S 1,x contains DC components and baseband harmonic components, and the baseband harmonic components are mainly even harmonics of the second harmonic; after filtering out the high-frequency components of the carrier frequency band, the sum of the switching functions S 2,x + S 1,x is obtained, and its main components are DC components and fundamental frequency components;

[0018] ;

[0019] ;

[0020] where represents the duty ratio of the DC component, represents the duty ratio of the second harmonic component, represents the duty ratio of the fundamental frequency component.

[0021] As a preferred embodiment of the present invention, a mathematical model of the arm voltage of a three-phase four-wire three-level converter is derived based on the periodic switching function expression. Specifically, it includes: the arm voltage of the three-phase four-wire three-level converter The main components of bal include the neutral point balance component u ubl and the neutral point unbalance component u bal为 . The neutral point balance component u ubl为 is the three-phase fundamental frequency component after inverting the DC bus voltage of the bus. The neutral point unbalance component u

[0022] ;

[0023] where U dc is the DC bus capacitor voltage, and u np (t) is the triple-frequency neutral point fluctuation voltage, and the specific expression is as follows:

[0024] ;

[0025] where U np , φ np respectively represent the amplitude and phase of the neutral point voltage fluctuation. According to the above analysis, the neutral point balance component u bal of the arm voltage is obtained, and its main component is the fundamental frequency component, and the specific expression is:

[0026] ;

[0027] is the duty cycle of the fundamental frequency component of the switching function S 1,x , is the duty cycle of the fundamental frequency component of the switching function S 2,x ;

[0028] According to the difference between the switching functions and the neutral point fluctuation voltage u np (t), the neutral point unbalance component u ubl of the arm voltage is obtained, and its main components are the fundamental frequency negative sequence component, the triple-frequency zero sequence component, and the fifth-frequency positive sequence component. The specific expression is

[0029] .

[0030] As a preferred embodiment of the present invention, an SOGI-pro module is designed by adding an error integration link at the input front end of the SOGI filter. The SOGI-pro module is an improved third-order filter SOGI-pro, which is used to enhance the ability to suppress harmonics in the entire frequency band. The design of the SOGI-pro module is specifically as follows: adding positive feedback of error integration at the input front end of the SOGI filter, transforming the D-axis second-order band-pass filter of the traditional SOGI into a third-order band-pass filter, and transforming the Q-axis second-order low-pass filter into a third-order band-pass filter, maintaining the gain of the input reference frequency unchanged, reducing the closed-loop gain of other frequencies, improving the ability to suppress harmonics in the entire frequency band, and achieving adaptive tracking of the reference frequency. The transfer functions of the transformed D-axis and Q-axis filters are respectively expressed as

[0031]

[0032]

[0033] where, represents the D-axis output of the SOGI-pro module; represents the Q-axis output of the SOGI-pro module; represents the input of the SOGI-pro module; represents the reference frequency; k represents the gain coefficient; s represents the Laplace operator.

[0034] As a preferred embodiment of the present invention, the present invention realizes high-precision positive and negative sequence separation of unbalanced voltages based on the above improved SOGI filter. Specifically: constructing a pair of orthogonal components, and completing the extraction of positive and negative sequence components based on their linear combination. The specific mathematical implementation method of the linear combination is:

[0035] ;

[0036] ;

[0037] where, and are respectively the positive and negative sequence components of the bridge arm voltage in the αβ coordinate system, and j is the symbol of orthogonal transformation, which is equivalent to a phase shift of 90° ahead.

[0038] The SOGI-pro module realizes high-accurate positive and negative sequence separation by improving the harmonic suppression performance of the orthogonal D-axis and Q-axis filters in the entire frequency band, generating orthogonal signals of fundamental frequency components while filtering out harmonic interference of other frequencies of the three-phase unbalanced bridge arm voltage. Therefore, the zero-sequence component is equivalent to the common-mode component of the three-phase output, and the zero-sequence component is extracted by solving the average value of the three-phase bridge arm voltage. The specific conversion equation is:

[0039] ;

[0040] Among them, is the zero-sequence component of the arm voltage, is the three-phase arm voltage.

[0041] As a preferred solution of the present invention, a sequence unbalance compensation control loop is designed in the three-sequence dq0 coordinate system. Specifically: the positive and negative sequence components in the αβ coordinate system extracted in step 3) are transformed to the dq0 coordinate system through the Park transformation, and the specific transformation equations are:

[0042] ;

[0043] ;

[0044] Among them, , , are the positive-sequence component, negative-sequence component and zero-sequence component of the arm voltage in the dq0 coordinate system respectively; is the Park transformation equation, is the Park inverse transformation equation;

[0045] A sequence unbalance compensation control loop is designed in the three-sequence dq0 coordinate system. The sequence unbalance compensation control loop adopts positive and negative sequence PI controllers and a zero-sequence PR controller to suppress the unbalanced negative-sequence and zero-sequence components of the three-phase output voltage, and at the same time achieve static-error-free tracking of the positive-sequence fundamental component, thereby realizing high-precision three-phase unbalanced arm voltage compensation control.

[0046] Compared with the prior art, the beneficial effects of the present invention are:

[0047] The mathematical model of the arm voltage of the three-phase four-wire three-level converter proposed by the present invention quantitatively depicts the composition of its harmonic components at each frequency. Compared with the traditional qualitative modeling, it more accurately solves the positive and negative sequence information and proportion of the harmonic components at each frequency, providing a more reliable theoretical basis for the design of the compensation control loop for the three-phase unbalanced arm voltage.

[0048] The improved SOGI-pro module proposed by the present invention improves the harmonic suppression performance of the D and Q-axis filters in the full frequency band by increasing the order of the orthogonal signal generator. Compared with the traditional SOGI filtering module, it more accurately extracts the positive and negative sequence components in the three-phase unbalanced voltage, which is beneficial to improving the control effect of unbalance compensation.

[0049] The decoupled control of positive sequence components, negative sequence components, and zero sequence components is achieved in the dq0 coordinate system designed by the present invention. By reasonably designing the controller parameters, the static error-free tracking of the positive sequence fundamental component is completed, and the strong suppression of the negative sequence and zero sequence unbalanced components is realized, so as to achieve high-precision three-phase unbalanced arm voltage compensation control. Brief Description of the Drawings

[0050] Figure 1 It is a schematic diagram of a three-phase four-wire three-level converter off-grid with an unbalanced load circuit.

[0051] Figure 2 It is a schematic diagram of the PD-SPWM switching modulation waveform.

[0052] Figure 3 It is a block diagram of the transfer function of the SOGI-pro module.

[0053] Figure 4 It is the Bode plot of the d-axis filter of the SOGI-pro module.

[0054] Figure 5 It is the Bode plot of the q-axis filter of the SOGI-pro module.

[0055] Figure 6 It is a block diagram of the sequence unbalance compensation control strategy.

[0056] Figure 7 It is the simulation waveform of the positive and negative sequence components of the arm voltage in the dq0 coordinate system.

[0057] Figure 8 It is the simulation waveform of the unbalance compensation control. Detailed Embodiment

[0058] In order to describe the method proposed by the present invention more specifically and clearly, the technical implementation solutions of the present invention will be described in detail below with reference to the drawings and actual cases.

[0059] The present invention provides an unbalanced load compensation method based on an improved three-phase four-wire converter. The three-phase four-wire converter off-grid with an unbalanced load circuit described in the present invention is as Figure 1As shown, the positive and negative DC buses are respectively connected to the upper and lower bridge arms. The three-phase bridge arm outputs of the converter are connected to the three-phase load after passing through the LCL filter. The neutral line of the load is connected back to the midpoint of the DC bus capacitor, providing an output zero-sequence current loop to transfer the unbalanced zero-sequence energy on the output side to be borne by the DC bus capacitor, significantly improving the zero-sequence control performance of the inverter and having significant advantages in the scenario of the energy storage converter in the microgrid system with unbalanced loads. The DC bus capacitor is composed of electrolytic capacitors with appropriate capacitance values connected in series and parallel. To improve the ability of the DC side to store zero-sequence energy, the capacitance value of the bus capacitor is generally designed to be large. The method of the present invention is applicable not only to the NPC three-level topology but also to other traditional three-level topologies, such as the T-type three-level topology, the ANPC three-level topology, etc.

[0060] The unbalanced load compensation method of the three-phase four-wire converter based on the improved SOGI filter of the present invention has the following specific process:

[0061] 1) Establish the mathematical model of the bridge arm voltage of the three-phase four-wire three-level converter and analyze the harmonic components of the bridge arm voltage;

[0062] 2) Design the SOGI-pro module based on the SOGI filter. Specifically, an error integration link is added to the front end of the D-axis input of the SOGI filter, transforming the D-axis second-order band-pass filter of the SOGI into a third-order band-pass filter and transforming the Q-axis second-order low-pass filter into a third-order band-pass filter;

[0063] 3) Perform high-precision positive and negative sequence separation on the three-phase unbalanced load voltage in the αβ coordinate system based on the SOGI-pro module;

[0064] 4) Design a sequence unbalanced compensation control loop in the three-sequence dq0 coordinate system to compensate for the unbalanced load.

[0065] In a specific embodiment of the present invention, the off-grid scheme of the three-phase four-wire three-level converter with unbalanced loads is used for illustration. As Figure 2 shown, the three-level converter adopts the PD-SPWM modulation method. The upper and lower sine half-cycles are modulated using in-phase stacked triangular carriers. The in-phase superposition concentrates the signal energy in the frequency / time domain, which helps to improve the power transmission efficiency. The multi-carrier stacking has a more flexible signal control ability, and the reliable transmission of other carriers is ensured by turning off the disturbed sub-carrier layer. Since the switching function has periodic characteristics, the switching function expressions S 1,x \S 2,x of the upper and lower bridge arms can be further calculated according to the double Fourier analysis as:

[0066]

[0067] where A 0,0 、D 0,0Represents the amplitude of the DC component of the corresponding switching function, A 0,n , D 0,n Represents the amplitude of the nth harmonic component of the fundamental frequency of the corresponding switching function, where n is an integer ≥ 1, A m,n , D m,n Represents the high-frequency component of the carrier frequency band of the corresponding switching function, ω0 represents the fundamental angular frequency, ω c Represents the high-frequency carrier angular frequency, m represents the frequency component order of the switching frequency, n represents the fundamental frequency component order, Represents the phase angle of the three-phase modulation wave, where phase A = 0°, phase B = 120°, phase C = 240°, and t represents time.

[0068] The switching function includes a DC component, a fundamental frequency component, a fundamental frequency multiple component, and a high-frequency component of the carrier frequency band. The amplitude expressions of each frequency component are as follows:

[0069]

[0070]

[0071]

[0072]

[0073] In the positive half modulation period, S 1,x , S 2,x The difference between the switching functions can represent the duty cycle of the NPC topology in the "O" level state. By obtaining the difference between the switching functions S 2,x -S 1,x It mainly includes a DC component and a fundamental frequency multiple component, and the fundamental frequency multiple component is mainly the even harmonic of the second harmonic. After filtering out the high-frequency component of the carrier frequency band, the sum of the switching functions S 2,x +S 1,x can be obtained. Its main components are a DC component and a fundamental frequency component, and their expressions are as follows

[0074] ;

[0075] ;

[0076] Where represents the duty cycle of the DC component, represents the duty cycle of the second harmonic component, represents the duty cycle of the fundamental frequency component.

[0077] Based on the periodic switching function expression, the mathematical model of the arm voltage of a three-level converter with a three-phase four-wire system can be deduced. The output voltage of the converter is the convolution of the switching function and each voltage component on the DC side. When the upper arm is conducting or the lower arm is conducting, the switching function S 2,x +S1,x The values of –1 are 1 and -1 respectively. After multiplying by half of the DC bus voltage, the output voltage amplitudes are represented as U dc / 2 and -U dc / 2 respectively. By considering the duty cycle of the switching function within the modulation period, a three-phase symmetric fundamental frequency output voltage u bal can be obtained. When the bridge arm is in the freewheeling state, it is connected to the midpoint of the DC bus capacitor. At this time, the switching function S 2,x - S 1,x has a value of 1. After multiplying by the triple-frequency voltage fluctuation at the midpoint of the bus capacitor, a midpoint unbalance component u ubl is superimposed on the output side. The specific expression is as follows:

[0078] ;

[0079] Among them, U dc is the DC bus capacitor voltage, and u np (t) is the triple-frequency midpoint fluctuation voltage. The specific expression is as follows:

[0080]

[0081] Among them, U np and φ np represent the amplitude and phase of the midpoint voltage fluctuation respectively. According to the above analysis, the midpoint balance component u bal of the bridge arm voltage can be obtained. Its main component is the fundamental frequency component, and the specific expression is:

[0082]

[0083] Among them, is the duty cycle of the fundamental frequency component of the switching function S 1,x , is the duty cycle of the fundamental frequency component of the switching function S 2,x ;

[0084] According to the difference between the switching functions S 2,x - S 1,x and the midpoint fluctuation voltage u np (t), the midpoint unbalance component u ubl of the bridge arm voltage can be obtained. Its main components are the fundamental frequency negative sequence component, the triple-frequency zero sequence component, and the fifth-frequency positive sequence component. The specific expression is:

[0085] .

[0086] Establishing the bridge arm voltage model of a three-phase four-wire three-level converter accurately depicts the harmonic components of the output side voltage, which helps to better design filters to suppress the interference of each harmonic component under the condition of driving unbalanced loads, and achieve high-precision positive and negative sequence separation and unbalance component compensation.

[0087] The SOGI-pro module described in the present invention adds an error integration link at the input front end of the SOGI filter, and its transfer function block diagram is as shown in Figure 3 shown. Specifically, it can be described as adding positive feedback of error integration at the input front end of the SOGI filter, transforming the D-axis second-order band-pass filter of the traditional SOGI into a third-order band-pass filter, and transforming the Q-axis second-order low-pass filter into a third-order band-pass filter. The transfer functions of the transformed D-axis and Q-axis filters are respectively expressed as:

[0088]

[0089]

[0090] where, represents the D-axis output of the SOGI-pro module; represents the Q-axis output of the SOGI-pro module; represents the input of the SOGI-pro module; represents the reference frequency; k represents the gain coefficient; s represents the Laplace operator.

[0091] The Bode plots of the transformed D-axis and Q-axis filters are respectively as shown in Figure 4 , Figure 5 shown. While maintaining the gain of the input reference frequency, both of them reduce the closed-loop gain of other frequencies, improve the harmonic suppression ability for the entire frequency band, and achieve adaptive tracking of the reference frequency. Based on the above improved SOGI-pro filter, high-precision positive and negative sequence separation of unbalanced voltage is realized in the αβ coordinate system, and the positive and negative sequence components are extracted through the linear combination of orthogonal components. The specific conversion equations can be expressed as

[0092]

[0093]

[0094] where, , are respectively the positive sequence component and the negative sequence component of the arm voltage in the αβ coordinate system, j is the symbol of orthogonal transformation, which is equivalent to a phase shift of 90° ahead.

[0095] The SOGI-pro module realizes high-precision positive and negative sequence separation by improving the harmonic suppression performance of the orthogonal D-axis and Q-axis filters in the entire frequency band, generating orthogonal signals of the fundamental frequency component while filtering out harmonic interference of other frequencies of the three-phase unbalanced arm voltage. Therefore, the zero sequence component is equivalent to the common mode component of the three-phase output, and the zero sequence component is extracted by solving the average value of the three-phase arm voltage. The specific conversion equation is:

[0096] ;

[0097] Among them, is the zero-sequence component of the bridge-arm voltage, is the three-phase bridge-arm voltage. The control block diagram of the sequence unbalance compensation strategy in the dq0 coordinate system described in the present invention is specifically as shown in Figure 6 . Among them, U pdref , U pqref respectively represent the reference values of the d-axis and q-axis of the positive-sequence voltage loop; U Cpd , U Cpq respectively represent the d-axis and q-axis components of the positive-sequence bridge-arm voltage; i pd , i pq respectively represent the d-axis and q-axis components of the positive-sequence inductor current; U Cnd , U Cnq respectively represent the d-axis and q-axis components of the negative-sequence bridge-arm voltage; i nd , i nq respectively represent the d-axis and q-axis components of the negative-sequence inductor current; U C0 represents the zero-sequence bridge-arm voltage; i0 represents the zero-sequence inductor current; U pdm , U pqm , U ndm , U nqm , U 0m respectively represent the modulation waves of the positive-sequence d-axis component, positive-sequence q-axis component, negative-sequence d-axis component, negative-sequence q-axis component and zero-sequence component; U aref , U bref , U cref respectively represent the three-phase modulation waves converted to the abc coordinate system; V pulse represents the pulse waveform after SPWM modulation. The positive and negative sequence components in the αβ coordinate system extracted by the improved SOGI-pro module are transformed to the dq0 coordinate system through Park transformation. The specific transformation equations are:

[0098]

[0099]

[0100] Among them, , , are respectively the positive-sequence component, negative-sequence component and zero-sequence component of the bridge-arm voltage in the dq0 coordinate system; is the Park transformation equation, is the Park inverse transformation equation;

[0101] Furthermore, a sequence unbalance compensation control loop is designed in the three - sequence dq0 coordinate system. The sequence unbalance compensation control loop adopts positive - and negative - sequence PI controllers and a zero - sequence PR controller to suppress the unbalanced negative - sequence and zero - sequence components of the three - phase output voltage, and at the same time achieve a static - error - free tracking of the positive - sequence fundamental component, thereby realizing a high - precision three - phase unbalanced leg voltage compensation control.

[0102] To demonstrate the implementation effect of the unbalanced load compensation method for the three - phase four - wire three - level converter of the present invention, a simulation model of the three - phase four - wire three - level converter with an off - grid unbalanced load is built based on the MATLAB / Simulink simulation tool for verification. The main parameters of the simulation model are as follows: the DC input is 850V, the effective value of the output AC phase voltage is 220V, the switching frequency is 30kHz, the DC - side capacitor is 2800μF, the inductance parameter of the LC filter is 130μH, the capacitance parameter of the filter capacitor is 40μF, the loads of phases A and B are 10kW resistive loads, and phase C is open - circuit. The 100% unbalanced condition of phase C being open - circuit and phases A and B being loaded with 10kW resistive loads is selected for testing, and the unbalanced compensation control effects based on the positive - and negative - sequence separation of the SOGI module and the SOGI - pro module are verified respectively. Figure 7 In (a) and (b) respectively give the positive - and negative - sequence components of the leg voltage after the positive - and negative - sequence separation based on the SOG module and the SOGII - pro module and converted to the dq0 coordinate system. Compared with Figure 7 in (a), Figure 7 in (b), the second - harmonic components of the d - axis positive - sequence component U pd , q - axis positive - sequence component U pq , d - axis negative - sequence component U nd , q - axis negative - sequence component U nq are significantly suppressed, verifying the effectiveness of the SOGI - pro module in improving the accuracy of positive - and negative - sequence separation. Figure 8 In (a) and (b) respectively give the unbalanced compensation control effects based on the positive - and negative - sequence separation of the SOGI module and the SOGI - pro module. Figure 8 In (a), it shows that after starting the unbalanced control based on the SOGI module, the maximum difference in the effective values of the three - phase leg voltages is reduced from 11.4V to 1.7V, and the unbalanced compensation degree reaches 85.1%; while Figure 8 in (b), it shows that after starting the unbalanced control based on the SOGI - pro module, the maximum difference in the effective values of the three - phase leg voltages is reduced from 11.4V to 0.7V, and the unbalanced compensation degree reaches 93.9%. This shows that the improvement in the accuracy of positive - and negative - sequence separation based on the SOGI - pro module can improve the effect of unbalanced compensation control. The simulation results verify the effectiveness of the proposed unbalanced load compensation method for a three - phase four - wire three - level converter.

[0103] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent for the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several variations and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the appended claims.

Claims

1. A method for compensating unbalanced loads of a three-phase four-wire three-level converter, characterized in that, It includes the following steps: 1) Establish the mathematical model of the bridge arm voltage of a three-phase four-wire three-level converter and analyze the harmonic components of the bridge arm voltage; 2) Design the SOGI-pro module based on the SOGI filter. Specifically, an error integration link is added to the front end of the D-axis input of the SOGI filter, transforming the D-axis second-order band-pass filter of the SOGI into a third-order band-pass filter and the Q-axis second-order low-pass filter into a third-order band-pass filter; 3) Based on the SOGI-pro module, perform high-precision positive and negative sequence separation on the three-phase unbalanced load voltage in the αβ coordinate system; 4) Design a sequence unbalanced compensation control loop in the three-sequence dq0 coordinate system to compensate for the unbalanced load.

2. The unbalanced load compensation method for a three-phase four-wire three-level converter according to claim 1, characterized in that: The three-phase four-wire three-level converter adopts a split-capacitor midpoint-returned topology, and the output load neutral line is returned to the midpoint of the DC bus, providing a loop for the zero-sequence current component in the off-grid load-bearing system of the three-phase four-wire three-level converter, thereby improving the converter's management ability for zero-sequence unbalanced components.

3. The unbalanced load compensation method for a three-phase four-wire three-level converter according to claim 1, characterized in that: In the said step 1), the three-phase four-wire three-level converter adopts the sinusoidal carrier in-phase stacking modulation method, and through the stacking and multiplexing of the upper and lower sinusoidal half-cycle carriers, the signal energy in the concentrated time domain or frequency domain is achieved, so as to improve the power transmission efficiency of the output sinusoidal component and enhance the anti-interference ability; the switching functions of the four fully controlled switches of each phase bridge arm are defined as , where y = 1, 2, 3, 4 represents the serial number of the switching device, and x = a, b, c represents the serial number of the three phases; = 0 indicates turning off, = 1 indicates turning on.

4. The unbalanced load compensation method for a three-phase four-wire three-level converter according to claim 3, characterized in that: The switching functions S 1,x and S 2,x are further calculated according to the double Fourier analysis, and the specific expressions are as follows: ； ； Among them, A 0,0 , D 0,0 represent the DC component amplitudes of the corresponding switching functions, A 0,n , D 0,n represent the amplitudes of the nth harmonic components of the fundamental frequency of the corresponding switching functions, where n is an integer ≥ 1, A m,n , D m,n represent the high-frequency components of the carrier frequency band of the corresponding switching functions, m represents the frequency component frequency of the switching frequency, n represents the frequency of the fundamental frequency component, ω0 represents the fundamental angular frequency, ω c represents the high-frequency carrier angular frequency, represents the phase angles of the three-phase modulation waves, where Phase A = 0°, Phase B = 120°, Phase C = 240°, and t represents time; During the positive half modulation period, the difference between the switching functions S 1,x , S 2,x represents the duty cycle when the three-level converter topology is in the "O" level state. The difference between the switching functions S 2,x - S 1,x contains a DC component, a baseband double-frequency component, and the baseband double-frequency component is mainly even harmonics of the double frequency. After filtering out the high-frequency components in the carrier frequency band, the sum of the switching functions S 2,x + S 1,x is obtained, and its main components are the DC component and the fundamental frequency component; ； ； wherein represents the duty cycle of the DC component, represents the duty cycle of the double-frequency component, represents the duty cycle of the fundamental frequency component.

5. The unbalanced load compensation method for a three-phase four-wire three-level converter according to claim 1, characterized in that: Bridge-arm voltage of three-phase four-wire three-level converter Its main components include the neutral-point balance component u bal and the neutral-point unbalance component u ubl . The neutral-point balance component u bal is the three-phase fundamental frequency component after the inversion of the bus DC voltage, and the neutral-point unbalance component u ubl is a variety of frequency multiplication components generated by multiplying the triple-frequency neutral-point fluctuation voltage by the switching function; the expression is as follows: ； Among them, U dc is the DC bus capacitor voltage, u np (t) is the triple-frequency midpoint fluctuation voltage, and the specific expression is as follows: ； Among them, U np , φ np respectively represent the amplitude and phase of the midpoint voltage fluctuation; according to the above analysis, the midpoint balance component u bal of the arm voltage is obtained, and its main component is the fundamental frequency component. The specific expression is as follows: ； is the duty cycle of the fundamental frequency component of the switching function S 1,x , is the duty cycle of the fundamental frequency component of the switching function S 2,x ; According to the difference between the switching functions and the midpoint fluctuating voltage u np (t), the midpoint unbalance component u ubl of the bridge arm voltage is obtained. Its main components are the fundamental negative sequence component, the triple-frequency zero sequence component, and the fifth-frequency positive sequence component. The specific expression is 。 6. The unbalanced load compensation method for a three-phase four-wire three-level converter according to claim 1, characterized in that: The SOGI-pro module in step 2) maintains the gain of the input reference frequency unchanged, reduces the closed-loop gain of other frequencies, improves the suppression ability of harmonics in the entire frequency band, realizes the adaptive tracking of the reference frequency, and the transfer function of the transformed D-axis filter , the transfer function of the Q-axis filter are respectively expressed as: ； ； Among them, represents the D-axis output of the SOGI-pro module; represents the Q-axis output of the SOGI-pro module; represents the input of the SOGI-pro module; represents the reference frequency; k represents the gain coefficient; s represents the Laplace operator.

7. The unbalanced load compensation method of the three-phase four-wire three-level converter according to claim 1, characterized in that: In step 3), the high-precision positive and negative sequence separation in the αβ coordinate system is specifically as follows: Construct a pair of orthogonal components and complete the extraction of positive and negative sequence components based on their linear combination. The specific mathematical implementation method of the linear combination is: ； ； Among them, and are the positive-sequence component and the negative-sequence component of the arm voltage in the αβ coordinate system respectively. j is the symbol of orthogonal transformation, which is equivalent to a phase shift of 90° ahead; The SOGI-pro module realizes high-accurate positive and negative sequence separation by improving the harmonic suppression performance of the orthogonal D and Q-axis filters in the full frequency band, generating orthogonal signals of the fundamental frequency component while filtering out harmonic interference of other frequencies of the three-phase unbalanced bridge arm voltage. The zero-sequence component is equivalent to the common-mode component of the three-phase output. Therefore, the zero-sequence component is extracted by solving the average value of the three-phase bridge arm voltage. The specific conversion equation is: ； Among them, is the zero-sequence component of the bridge arm voltage, is the three-phase bridge arm voltage.

8. The unbalanced load compensation method for a three-phase four-wire three-level converter according to claim 1, characterized in that: Step 4) includes: transforming the positive and negative sequence components in the αβ coordinate system extracted in step 3) to the dq0 coordinate system through the Park transformation. The specific conversion equation is: ； ； Among them, , , are respectively the positive-sequence component, negative-sequence component, and zero-sequence component of the bridge arm voltage in the dq0 coordinate system; is the Park transformation equation, is the inverse Park transformation equation; Design a sequence unbalanced compensation control loop in the three-sequence dq0 coordinate system. The sequence unbalanced compensation control loop adopts positive and negative sequence PI controllers and a zero-sequence PR controller to suppress the unbalanced negative and zero-sequence components of the three-phase output voltage, and at the same time achieve a static-error-free tracking of the positive-sequence fundamental frequency component, thereby realizing high-precision compensation control of the three-phase unbalanced bridge arm voltage.

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