A method for compensating unbalanced load of three-phase four-wire three-level converter

Abstract

The present invention discloses a method for compensating unbalanced load of a three-phase four-wire three-level converter. The present invention establishes a mathematical model of the bridge arm voltage of a three-phase four-wire three-level converter based on sinusoidal carrier co-phase stacked modulation and completes harmonic content analysis, establishes a SOGI-pro module (in the SOGI filter D Add an error integration link to the front end of the axis input to D、Q The invention improves the accuracy of separation of positive and negative sequence voltage of three-phase unbalanced load by transforming the second-order bandpass filter of the axis into a third-order bandpass filter. The invention improves the order of SOGI filter by reasonably designing the closed-loop transfer function, and enhances the filtering effect of high and low frequency band of SOGI filter. dq0 The coordinate system is designed to design a split-sequence unbalance compensation control loop, achieving zero-error tracking of the positive-sequence fundamental component and strong suppression of the negative-sequence and zero-sequence unbalance components. This invention improves the accuracy of traditional positive-negative sequence separation, thereby improving the unbalanced load compensation effect under split-sequence control.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic control, and in particular relates to a method for compensating an unbalanced load 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 inverters in off-grid operation is gaining increasing attention. Load imbalance is a common problem in off-grid power supply systems, particularly in microgrids or small grids in remote areas. Due to the diverse user types and the widespread presence of single-phase loads, the system often faces significant load imbalance. Unbalanced loads not only cause waveform distortion in the inverter's output voltage and current, but also increase system losses, shorten the service life of the inverter and load, and even endanger system stability and power supply quality. Therefore, how to effectively compensate for unbalanced loads has become a key challenge in off-grid inverter research.

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

[0004] In off-grid power supply systems, reducing three-phase imbalance and improving the accuracy of imbalance compensation are key objectives of energy storage converters for imbalance compensation. However, achieving this goal faces multiple challenges. First, load imbalance is highly dynamic and random, requiring the converter's control strategy to possess high precision and rapid dynamic response capabilities. Second, because off-grid systems may experience complex operating conditions such as impact loads, nonlinear loads, and harmonic interference, the control strategy must balance harmonic suppression and system stability. Finally, the implementation of advanced control algorithms places higher demands on hardware performance and computing resources, requiring a balance between cost, efficiency, and complexity. Summary of the Invention

[0005] In response to the above-mentioned problems existing in the prior art, the present invention proposes a method for compensating for unbalanced loads of a three-phase four-wire three-level converter. The invention establishes a mathematical model of the bridge arm voltage of a three-phase four-wire three-level converter based on sinusoidal carrier in-phase stacked modulation and completes 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 conducive to eliminating the harmonic components in the three-phase unbalanced bridge arm voltage and improving the accuracy of positive and negative sequence separation. Furthermore, 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 better unbalanced compensation control effect than traditional strategies.

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

[0007] 1) Establish a 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;

[0008] 2) Designing the SOGI-pro module based on the SOGI filter. Specifically, adding an error integration circuit to the front end of the SOGI filter's D-axis input, transforming the SOGI's D-axis second-order bandpass filter into a third-order bandpass filter, and transforming the Q-axis second-order low-pass filter into a third-order bandpass filter.

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

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

[0011] As a preferred embodiment of the present invention, the three-phase four-wire three-level converter in step 1) adopts a split-capacitor midpoint tie-back topology, which is connected to the DC bus midpoint through the output load midline, providing a loop for the zero-sequence current component in the three-phase output system, thereby improving the converter's ability to manage zero-sequence unbalanced components, and has significant advantages in microgrid system energy storage converter scenarios with unbalanced loads.

[0012] As a preferred embodiment of the present invention, the three-phase four-wire three-level converter in step 1) adopts the sinusoidal carrier co-phase stacking modulation (PD-SPWM, Phase Deposition Sinusoidal Pulse Width Modulation) method, which concentrates the signal energy in the time domain or frequency domain by stacking and multiplexing the upper and lower sinusoidal half-cycles of the carrier, thereby improving the power transmission efficiency and anti-interference ability of the output sinusoidal component. The switching function of the four fully controlled switches in each phase bridge arm is defined as Syx , where y = 1, 2, 3, 4 represents the serial number of the switch device, and x = a, b, c represents the serial number of the three phases; =0 means off, =1 means conduction.

[0013] As a preferred solution of the present invention, in step 1), the switching function S is further calculated based on the double Fourier analysis. 1,x 、S 2,x The expression is:

[0014] ;

[0015] ;

[0016] Among them A 0,0 、D 0,0 Represents the DC component amplitude of the corresponding switching function, A 0,n 、D 0,n A represents the amplitude of the n-fold frequency component of the fundamental frequency of the corresponding switching function, where n is an integer ≥ 1. m,n 、D m,n Represents the high-frequency component of the carrier frequency band of the corresponding switching function, m represents the switching frequency component frequency, n represents the fundamental frequency component frequency, ω0 represents the fundamental frequency angular frequency, ω c represents the high-frequency carrier angular frequency, represents the phase angle of the three-phase modulation wave, where phase A = 0°, phase B = 120°, phase C = 240°, and t represents time.

[0017] In the positive half modulation cycle, the difference between the switching functions S 1,x 、S 2,x The duty cycle of the three-level converter topology in the "0" state is expressed as follows: 2,x -S 1,x It contains DC component and baseband frequency multiplication component, and the baseband frequency multiplication component is mainly composed of double frequency even harmonics; after filtering out the high frequency component of the carrier band, the sum of the switching functions S is obtained 2,x +S 1,x , its main components are DC component and fundamental frequency component;

[0018] ;

[0019] ;

[0020] in Indicates the duty cycle of the DC component, Indicates the duty cycle of the twice frequency component, Indicates the duty cycle of the fundamental frequency component.

[0021] As a preferred solution of the present invention, the present invention derives a mathematical model of the bridge arm voltage of a three-phase four-wire three-level converter based on a periodic switching function expression. Specifically, it includes: the bridge arm voltage of a three-phase four-wire three-level converter The main components include the midpoint balance component u bal and midpoint unbalanced component u ubl , the midpoint balance component u bal为 The three-phase fundamental frequency component after the bus DC voltage is inverted, and the midpoint unbalanced component u ubl为 The tripled frequency midpoint ripple voltage is multiplied by the switching function to generate multiple frequency components; the expression is as follows:

[0022] ;

[0023] 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:

[0024] ;

[0025] Among them, U np 、φ np Respectively represent the midpoint voltage fluctuation amplitude and phase; According to the above analysis, the midpoint balance component u of the bridge arm voltage is obtained bal , its main component is the fundamental frequency component, and the specific expression is:

[0026] ;

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

[0028] According to the difference between the switching function and the midpoint ripple voltage u np (t) Obtain the unbalanced component u of the bridge arm voltage midpoint ubl , its main components are the fundamental frequency negative sequence component, the triple frequency zero sequence component, and the quintuple frequency positive sequence component. The specific expression is

[0029] .

[0030] As a preferred embodiment of the present invention, the present invention designs a SOGI-pro module by adding an error integration link to the front end of the SOGI filter input. The SOGI-pro module is an improved third-order filter SOGI-pro, which is used to improve 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 to the front end of the SOGI filter input, transforming the traditional SOGI D-axis second-order bandpass filter into a third-order bandpass filter, and transforming the Q-axis second-order low-pass filter into a third-order bandpass 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 realizing adaptive tracking of the reference frequency. The transfer functions of the modified D-axis and Q-axis filters are respectively expressed as follows:

[0031]

[0032]

[0033] in, Indicates the D-axis output of the SOGI-pro module; Indicates the Q-axis output of the SOGI-pro module; Represents the SOGI-pro module input; 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 voltage based on the improved SOGI filter. Specifically, a pair of orthogonal components is constructed, and the positive and negative sequence components are extracted based on their linear combination. The specific mathematical implementation of the linear combination is as follows:

[0035] ;

[0036] ;

[0037] in, 、 They are the bridge arm voltages in the αβ coordinate system The positive and negative sequence components of the signal are represented by j, which is the sign of the orthogonal transformation, equivalent to a 90° leading phase shift.

[0038] The SOGI-pro module improves the harmonic suppression performance of the orthogonal D and Q axis filters across the entire frequency range, generating an orthogonal signal for the fundamental frequency component while filtering out harmonic interference at other frequencies of the three-phase unbalanced bridge arm voltage, thereby achieving highly accurate positive and negative sequence separation. The zero-sequence component is equivalent to the common-mode component of the three-phase output, so it is extracted by solving the average value of the three-phase bridge arm voltage. The specific conversion equation is:

[0039] ;

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

[0041] As a preferred solution of the present invention, a separate-sequence imbalance 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 Park transformation. The specific transformation equation is:

[0042] ;

[0043] ;

[0044] in, 、 、 They 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;

[0045] A split-sequence unbalanced compensation control loop is designed in the three-sequence dq0 coordinate system. The split-sequence unbalanced compensation control loop adopts positive and negative sequence PI controllers and zero sequence PR controllers to suppress the unbalanced negative sequence and zero sequence components of the three-phase output voltage, while achieving zero static error tracking of the positive sequence fundamental component, thereby realizing high-precision three-phase unbalanced bridge arm voltage compensation control.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

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

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

[0049] The sequence unbalance compensation control loop designed in the dq0 coordinate system of the present invention realizes the decoupling control of the positive sequence component, negative sequence component and zero sequence component. By rationally designing the controller parameters, the positive sequence fundamental component is tracked without static error, the negative sequence and zero sequence unbalance components are strongly suppressed, and high-precision three-phase unbalanced bridge arm voltage compensation control is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0051] Figure 2 Schematic diagram of PD-SPWM switch modulation waveform.

[0052] Figure 3 Transfer function block diagram of SOGI-pro module.

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

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

[0055] Figure 6 This is the block diagram of the sequence imbalance compensation control strategy.

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

[0057] Figure 8 Simulate waveform for imbalance compensation control. DETAILED DESCRIPTION

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

[0059] The present invention provides a method for compensating unbalanced load of an improved three-phase four-wire converter. The three-phase four-wire converter off-grid with unbalanced load circuit of the present invention is as follows: Figure 1As shown, the positive and negative DC busbars are connected to the upper and lower bridge arms respectively, and the output of the three-phase bridge arm of the converter is connected to the three-phase load after passing through the LCL filter. The load neutral line 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 the DC bus capacitor, greatly improving the zero-sequence control performance of the inverter, and having significant advantages in the microgrid system energy storage converter with unbalanced load scenario. The DC bus capacitor is composed of electrolytic capacitors of appropriate capacitance in series and parallel. In order to improve the ability of the DC side to store zero-sequence energy, the bus capacitor capacitance is generally designed to be larger. In addition to being used for NPC three-level topology, the method of the present invention is also applicable to other traditional three-level topologies, such as T-type three-level topology, ANPC three-level topology, etc.

[0060] The present invention provides a method for compensating unbalanced load of a three-phase four-wire converter based on an improved SOGI filter. The specific process is as follows:

[0061] 1) Establish a 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;

[0062] 2) Designing the SOGI-pro module based on the SOGI filter. Specifically, adding an error integration circuit to the front end of the SOGI filter's D-axis input, transforming the SOGI's D-axis second-order bandpass filter into a third-order bandpass filter, and transforming the Q-axis second-order low-pass filter into a third-order bandpass filter.

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

[0064] 4) Design a sequence-wise 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, a three-phase four-wire three-level converter off-grid with unbalanced load is used for illustration. Figure 2 As shown in Figure 1, the three-level converter uses PD-SPWM modulation, with the upper and lower sinusoidal half cycles modulated by in-phase stacked triangular carriers. In-phase stacking concentrates the signal energy in the frequency / time domain, which helps improve power transmission efficiency. Multi-carrier stacking provides more flexible signal control capabilities, ensuring reliable transmission of other carriers by shutting down the disturbed subcarrier layer. Since the switching function has periodic characteristics, the switching function expression S for the upper and lower bridge arms can be further calculated based on double Fourier analysis. 1,x \S 2,x for:

[0066]

[0067] Among them A 0,0 、D 0,0Represents the DC component amplitude of the corresponding switching function, A 0,n 、D 0,n A represents the amplitude of the n-fold frequency component of the fundamental frequency of the corresponding switching function, where n is an integer ≥ 1. 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 switching frequency component frequency, n represents the fundamental frequency component frequency, 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 baseband multiplier component, and a high-frequency component of the carrier frequency band. The amplitude expressions of each frequency component are:

[0069]

[0070]

[0071]

[0072]

[0073] In the positive half modulation cycle, S 1,x 、S 2,x The difference in the switching function can represent the duty cycle of the NPC topology in the "0" level state, and the difference in the switching function S is obtained. 2,x -S 1,x It mainly includes DC component and baseband frequency multiplication component, and the baseband frequency multiplication component is mainly double frequency even harmonic. After filtering out the high frequency component of the carrier band, the sum of the switching function S can be obtained. 2,x +S 1,x , its main components are DC component and fundamental frequency component, and their expressions are as follows

[0074] ;

[0075] ;

[0076] in Indicates the duty cycle of the DC component, Indicates the duty cycle of the twice frequency component, Indicates the duty cycle of the fundamental frequency component.

[0077] Based on the periodic switching function expression, the mathematical model of the bridge arm voltage of the three-phase four-wire three-level converter can be derived. The output voltage of the converter is the convolution of the switching function and the voltage components on the DC side. When the upper bridge arm is turned on or the lower bridge arm is turned on, the switching function S 2,x +S1,x – 1 are 1 and -1 respectively, and the output voltage amplitude after multiplying by half of the DC bus voltage is expressed as U dc / 2, -U dc / 2, considering the duty cycle of the switching function within the modulation period, the three-phase symmetrical fundamental frequency output voltage u can be obtained. bal , and when the bridge arm is in the freewheeling state and connected to the midpoint of the DC bus capacitor, the switching function S 2,x -S 1,x The value is 1, multiplied by the busbar capacitor midpoint triple frequency voltage fluctuation, and then superimposed on the output side midpoint unbalanced component u ubl , the specific expression is as follows:

[0078] ;

[0079] 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:

[0080]

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

[0082]

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

[0084] According to the difference of the switching function S 2,x -S 1,x With midpoint ripple voltage u np (t) The unbalanced component u of the bridge arm voltage midpoint can be obtained ubl , its main components are the fundamental frequency negative sequence component, the triple frequency zero sequence component, and the quintuple frequency positive sequence component. The specific expression is:

[0085] .

[0086] The establishment of a three-phase four-wire three-level converter bridge arm voltage model accurately describes the harmonic components of the output side voltage, which helps to better design filters to suppress the interference of various harmonic components under unbalanced load conditions and achieve high-precision positive and negative sequence separation and unbalanced component compensation.

[0087] The SOGI-pro module of the present invention adds an error integration link to the front end of the SOGI filter input, and its transfer function block diagram is as follows: Figure 3 Specifically, the error integral positive feedback is added to the front end of the SOGI filter input, the traditional SOGI D-axis second-order bandpass filter is transformed into a third-order bandpass filter, and the Q-axis second-order low-pass filter is transformed into a third-order bandpass filter. The transfer functions of the modified D-axis and Q-axis filters are expressed as follows:

[0088]

[0089]

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

[0091] The Bode diagrams of the modified D and Q axis filters are as follows: Figure 4 、 Figure 5 As shown in the figure, the two maintain the gain of the input reference frequency unchanged while 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. Based on the above improved SOGI-pro filter, high-precision positive and negative sequence separation of unbalanced voltage is achieved in the αβ coordinate system, and the positive and negative sequence components are extracted through the linear combination of orthogonal components. The specific conversion equation can be expressed as

[0092]

[0093]

[0094] in, 、 They are the bridge arm voltages in the αβ coordinate system The positive and negative sequence components of the signal are represented by j, which is the sign of the orthogonal transformation, equivalent to a 90° leading phase shift.

[0095] The SOGI-pro module improves the harmonic suppression performance of the orthogonal D and Q axis filters across the entire frequency range, generating an orthogonal signal for the fundamental frequency component while filtering out harmonic interference at other frequencies of the three-phase unbalanced bridge arm voltage, thereby achieving highly accurate positive and negative sequence separation. The zero-sequence component is equivalent to the common-mode component of the three-phase output, so it is extracted by solving the average value of the three-phase bridge arm voltage. The specific conversion equation is:

[0096] ;

[0097] in, is the zero-sequence component of the bridge arm voltage, The control block diagram of the sequence imbalance compensation strategy in the dq0 coordinate system of the present invention is as follows: Figure 6 As shown. Among them, U pdref 、U pqref Respectively represent the d-axis and q-axis reference values 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 wave 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 into the dq0 coordinate system through Park transformation. The specific transformation equation is:

[0098]

[0099]

[0100] in, 、 、 They 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 split-sequence unbalanced compensation control loop is designed in the three-sequence dq0 coordinate system. The split-sequence unbalanced compensation control loop adopts a positive- and negative-sequence PI controller and a zero-sequence PR controller to suppress the unbalanced negative-sequence and zero-sequence components of the three-phase output voltage, while achieving zero-static error tracking of the positive-sequence fundamental component, thereby realizing high-precision three-phase unbalanced bridge arm voltage compensation control.

[0102] To demonstrate the effectiveness of the unbalanced load compensation method for a three-phase, four-wire, three-level converter described in the present invention, a three-phase, four-wire, three-level converter off-grid with unbalanced load simulation model was built based on the MATLAB / Simulink simulation tool for verification. The main parameters of the simulation model are as follows: 850V DC input, 220V AC phase effective value output voltage, 30kHz switching frequency, 2800μF DC side capacitance, 130μH LC filter inductance, 40μF filter capacitance, 10kW resistive load for both A / B phases, and no load for phase C. Tests were conducted under 100% unbalanced operating conditions with no load on phase C and 10kW resistive load on phases A / B to verify the unbalanced compensation control effects based on positive and negative sequence separation of the SOGI module and the SOGI-pro module. Figure 7 (a) and (b) in the figure respectively give the positive and negative sequence components of the bridge arm voltage after the positive and negative sequence separation of SOG module and SOGII-pro module and conversion to dq0 coordinate system. Figure 7 (a) in Figure 7 The d-axis positive sequence component U in (b) pd , q-axis positive sequence component U pq , d-axis negative sequence component U nd , q-axis negative sequence component U nq The second harmonic components in the signal are significantly suppressed, which verifies the effectiveness of the SOGI-pro module in improving the accuracy of positive and negative sequence separation. Figure 8 (a) and (b) in the figure 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 (a) shows that after starting the unbalance control based on the SOGI module, the maximum difference in the effective value of the three-phase bridge arm voltage is reduced from 11.4V to 1.7V, and the degree of unbalance compensation reaches 85.1%. Figure 8 Figure (b) shows that after enabling unbalanced control using the SOGI-pro module, the maximum difference in the effective values of the three-phase bridge arm voltages decreases from 11.4V to 0.7V, and the degree of unbalance compensation reaches 93.9%. This demonstrates that the accuracy of positive and negative sequence separation achieved with the SOGI-pro module improves the effectiveness of unbalanced compensation control. These simulation results validate 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 methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for compensating unbalanced load of a three-phase four-wire three-level converter, characterized in that: The following steps are involved: 1) Establish a 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. The three-phase four-wire three-level converter adopts a sinusoidal carrier co-phase stacking modulation method. By stacking and multiplexing the upper and lower sinusoidal half-cycles of the carrier, the signal energy in the time or frequency domain is concentrated to achieve improved power transmission efficiency and enhanced anti-interference capability of the output sinusoidal component. The switching function of the four fully controlled switches in each phase bridge arm is defined as , where y = 1, 2, 3, 4 represents the serial number of the switch device, and x = a, b, c represents the serial number of the three phases; =0 means off, =1 means conduction; The switching function S is further calculated based on the double Fourier analysis. 1,x 、S 2,x The expression is: ； ； Among them A 0,0 、D 0,0 Represents the DC component amplitude of the corresponding switching function, A 0,n 、D 0,n A represents the amplitude of the n-fold frequency component of the fundamental frequency of the corresponding switching function, where n is an integer ≥ 1. m,n 、D m,n Represents the high-frequency component of the carrier frequency band of the corresponding switching function, m represents the switching frequency component frequency, n represents the fundamental frequency component frequency, ω0 represents the fundamental frequency angular frequency, ω c represents the high-frequency carrier angular frequency, represents the phase angle of the three-phase modulation wave, where phase A = 0°, phase B = 120°, phase C = 240°, and t represents time; In the positive half modulation cycle, the difference between the switching functions S 1,x 、S 2,x The duty cycle of the three-level converter topology in the "0" state is expressed as follows: 2,x -S 1,x It contains DC component and baseband frequency multiplication component, and the baseband frequency multiplication component is mainly composed of double frequency even harmonics; after filtering out the high frequency component of the carrier band, the sum of the switching functions S is obtained 2,x +S 1,x , its main components are DC component and fundamental frequency component; ； ； in Indicates the duty cycle of the DC component, Indicates the duty cycle of the twice frequency component, Indicates the duty cycle of the fundamental frequency component; 2) Designing the SOGI-pro module based on the SOGI filter. Specifically, adding an error integration circuit to the front end of the SOGI filter's D-axis input, transforming the SOGI's D-axis second-order bandpass filter into a third-order bandpass filter, and transforming the Q-axis second-order low-pass filter into a third-order bandpass filter. 3) Based on the SOGI-pro module, high-precision positive and negative sequence separation of three-phase unbalanced load voltage is performed in the αβ coordinate system; 4) Design a sequence-wise 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 tie-back topology, which is connected to the DC bus midpoint through the output load neutral line, providing a loop for the zero-sequence current component in the three-phase four-wire three-level converter off-grid loaded system, thereby improving the converter's ability to manage 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: Bridge arm voltage of three-phase four-wire three-level converter The main components include the midpoint balance component u bal and midpoint unbalanced component u ubl , the midpoint balance component u bal is the three-phase fundamental frequency component after the busbar DC voltage is inverted, and the midpoint unbalanced component u ubl It is the multiple frequency components generated by multiplying the triple frequency midpoint ripple 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 midpoint voltage fluctuation amplitude and phase; According to the above analysis, the midpoint balance component u of the bridge arm voltage is obtained bal , its main component is the fundamental frequency component, and the specific expression is: ； is the switching function S 1,x The duty cycle of the fundamental frequency component, is the switching function S 2,x The duty cycle of the fundamental frequency component; According to the difference between the switching function and the midpoint ripple voltage u np (t) Obtain the unbalanced component u of the bridge arm voltage midpoint ubl , its main components are the fundamental frequency negative sequence component, the triple frequency zero sequence component, and the quintuple frequency positive sequence component. The specific expression is 。 4. The method for compensating unbalanced load of a three-phase four-wire three-level converter according to claim 1, wherein: 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 capability of full-band harmonics, and realizes adaptive tracking of the reference frequency. The modified D-axis filter transfer function , Q-axis filter transfer function Respectively expressed as: ； ； in, Indicates the D-axis output of the SOGI-pro module; Indicates the Q-axis output of the SOGI-pro module; Represents the SOGI-pro module input; represents the reference frequency; k represents the gain coefficient; s represents the Laplace operator.

5. The unbalanced load compensation method for a three-phase four-wire three-level converter according to claim 1, characterized in that: In step 3), high-precision positive and negative sequence separation is performed in the αβ coordinate system, specifically: Construct a pair of orthogonal components and extract the positive and negative sequence components based on their linear combination. The specific mathematical implementation of the linear combination is as follows: ； ； in, 、 They are the bridge arm voltages in the αβ coordinate system The positive and negative sequence components of , j is the sign of the orthogonal transformation, which is equivalent to a phase shift of 90° ahead; The SOGI-pro module improves the harmonic suppression performance of the orthogonal D and Q axis filters across the entire frequency range, generating an orthogonal signal for the fundamental frequency component while filtering out harmonic interference at other frequencies of the three-phase unbalanced bridge arm voltage, thereby achieving highly accurate positive and negative sequence separation. The zero-sequence component is equivalent to the common-mode component of the three-phase output, so it is extracted by solving the average value of the three-phase bridge arm voltage. The specific conversion equation is: ； in, is the zero-sequence component of the bridge arm voltage, is the three-phase bridge arm voltage.

6. The method for compensating unbalanced load of a three-phase four-wire three-level converter according to claim 5, characterized in that: The step 4) includes: transforming the positive and negative sequence components in the αβ coordinate system extracted in step 3) into the dq0 coordinate system through Park transformation. The specific transformation equation is: ； ； in, 、 、 They 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; A split-sequence unbalanced compensation control loop is designed in the three-sequence dq0 coordinate system. The split-sequence unbalanced compensation control loop adopts positive and negative sequence PI controllers and zero sequence PR controllers to suppress the unbalanced negative sequence and zero sequence components of the three-phase output voltage, while achieving zero static error tracking of the positive sequence fundamental component, thereby realizing high-precision three-phase unbalanced bridge arm voltage compensation control.

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