Unbalanced adaptive control method and system for three-phase four-wire grid-connected converter

Abstract

The embodiment of the application relates to the field of new energy power generation and power electronic control technology, in particular to an unbalanced adaptive control method of a three-phase four-wire grid-connected converter, which comprises the following steps: collecting AC side electrical quantities, extracting positive sequence, negative sequence and zero sequence components and positive sequence voltage phase frequency after sampling; generating a positive sequence voltage instruction according to the positive sequence component; generating a negative sequence compensation voltage instruction according to the negative sequence component through negative sequence current regulation and adaptive negative sequence virtual impedance; generating a zero sequence voltage instruction according to the neutral point potential and the zero sequence component; extracting double-frequency power fluctuation and separating to generate a feedforward compensation quantity for negative sequence current regulation; synthesizing three-phase modulation voltage instructions and generating a switching control signal. The application can suppress negative sequence current and zero sequence drift, reduce double-frequency fluctuation of a DC bus, and improve grid-connected operation stability.

Description

Technical Field

[0001] This application relates to the fields of new energy power generation and power electronic control technology, and in particular to an unbalanced adaptive control method and system for a three-phase four-wire grid converter. Background Technology

[0002] Grid-based converters can simulate the inertia and excitation characteristics of synchronous generators, providing voltage and frequency support for the power grid. However, in actual distribution networks and microgrids, single-phase loads, asymmetrical faults, and three-phase four-wire power supply scenarios are prevalent, resulting in severe voltage and current imbalances in the grid. Traditional grid-based control strategies are typically designed based on the assumption of three-phase balance and employ... The symmetric control framework under dq transform has difficulty effectively handling unbalanced components, leading to the following technical problems:

[0003] (1) Negative sequence current causes overcurrent in the converter and second-frequency fluctuations in DC bus voltage. Traditional grid control cannot suppress negative sequence current when the grid voltage is unbalanced. The negative sequence component is manifested as a second-frequency component in the synchronous rotating coordinate system, causing second-frequency oscillations of active and reactive power. In severe cases, it leads to violent fluctuations in DC bus voltage, threatening the safety of energy storage batteries.

[0004] (2) For a three-phase four-wire topology, the zero-sequence current can flow through the neutral line or the neutral point forming unit. When the zero-sequence voltage is not set with independent closed-loop control, the neutral point potential deviation may cause the three-phase phase voltage amplitudes to be inconsistent and increase the neutral line current.

[0005] (3) Existing control methods usually set up positive sequence, negative sequence and zero sequence control branches respectively, without fully considering the impact of negative sequence compensation action on the positive sequence power calculation results. The positive sequence power loop may repeatedly adjust the second harmonic power component.

[0006] (4) Existing fault ride-through control systems mostly set up a unified current limiting strategy for three-phase symmetrical voltage drops, which makes it difficult to simultaneously handle positive sequence voltage support, negative sequence current suppression and zero sequence current limitation under asymmetrical faults.

[0007] Therefore, developing a grid-type converter control method suitable for three-phase four-wire topology and with unbalanced adaptive capability has important practical engineering value. Summary of the Invention

[0008] To address this, the present invention provides an unbalanced adaptive control method and system for a three-phase four-wire grid converter, which overcomes the problems in the prior art where grid converters suffer from poor suppression of negative sequence current, large DC bus second harmonic fluctuations, and neutral point potential shifts due to positive, negative, and zero sequence coupling under grid imbalance conditions.

[0009] To achieve the above objectives, the present invention provides an unbalanced adaptive control method for a three-phase four-wire grid converter, comprising:

[0010] Step S1: Collect electrical quantity information from the AC side of the three-phase four-wire grid converter, and filter and synchronously sample the electrical quantity information to obtain digital electrical quantity signals;

[0011] Step S2: Sequence component separation is performed on the digital electrical quantity signal to extract the positive sequence component, negative sequence component and zero sequence component, and the phase and frequency information of the positive sequence voltage is obtained based on the positive sequence component;

[0012] Step S3: Calculate the positive sequence power based on the positive sequence component, and generate a positive sequence voltage command based on the phase and frequency information of the positive sequence voltage;

[0013] Step S4: Determine the negative sequence current based on the negative sequence component, and generate a negative sequence compensation voltage command by adjusting the negative sequence virtual impedance based on the negative sequence current and the voltage imbalance.

[0014] Step S5: Based on the neutral point potential information and the zero-sequence component, a zero-sequence voltage command is generated through zero-sequence voltage adjustment and neutral point potential balance control.

[0015] Step S6: Extract the second harmonic power fluctuation based on the positive sequence component and the negative sequence component, separate the second harmonic power fluctuation from the positive sequence power, generate a feedforward compensation amount based on the second harmonic power fluctuation, and use the feedforward compensation amount for the negative sequence current regulation.

[0016] Step S7: Combine the positive sequence voltage command, the negative sequence compensation voltage command, and the zero sequence voltage command into a three-phase modulation voltage command; perform modulation and drive processing on the three-phase modulation voltage command to generate a switch control signal; and control the three-phase four-wire grid converter based on the switch control signal.

[0017] Further, step S2 includes:

[0018] The three-phase voltage and three-phase current in the digital electrical quantity signal are transformed by coordinate transformation to obtain the voltage component and current component in a two-phase stationary coordinate system.

[0019] Using the voltage and current components in the two-phase stationary coordinate system, orthogonal signals are constructed respectively. Based on the combination relationship between the same-direction rotational components and the opposite-direction rotational components, positive-sequence components and negative-sequence components are separated from the results of the orthogonal signal construction.

[0020] The zero-sequence fundamental component is obtained by using the zero-sequence path component in the voltage and current components in the two-phase stationary coordinate system and extracting the fundamental component through a third-order generalized integrator.

[0021] The separated positive-sequence component, negative-sequence component, and zero-sequence fundamental component are filtered by an adaptive notch filter whose center frequency changes with the real-time angular frequency, and the filtered positive-sequence component, negative-sequence component, and zero-sequence fundamental component are output.

[0022] Furthermore, the orthogonal signal construction is implemented using a dual second-order generalized integrator, and the fundamental channel transfer function of the dual second-order generalized integrator is:

[0023] ;

[0024] in, For the fundamental channel transfer function of the dual second-order generalized integrator, For the real-time fundamental angular frequency, Here, s is the damping coefficient, and s is the Laplace operator;

[0025] The transfer function of the third-order generalized integrator is:

[0026] ;

[0027] in, Let be the transfer function of the third-order generalized integrator. This is the zero-sequence channel damping coefficient;

[0028] The center frequencies of the adaptive notch filter are respectively set to and .

[0029] Further, in step S3, a positive-sequence voltage command is generated, including:

[0030] Calculate the positive-sequence active power and positive-sequence reactive power based on the positive-sequence voltage and positive-sequence current in the positive-sequence components.

[0031] Using the phase and frequency information of the positive sequence voltage as a synchronization reference, the calculated positive sequence active power and positive sequence reactive power are compared with the given active power command and reactive power command respectively to obtain the active power deviation and reactive power deviation.

[0032] The active power deviation is adjusted by the mechanical equation controlled by the virtual synchronous machine to obtain the angular frequency of the positive sequence voltage, and the phase of the positive sequence voltage is obtained by integrating the angular frequency.

[0033] The reactive power deviation is adjusted by reactive voltage droop control to obtain the amplitude of the positive sequence voltage.

[0034] The positive sequence voltage command is synthesized based on the angular frequency, phase, and amplitude of the positive sequence voltage.

[0035] Furthermore, the positive-sequence active power is:

[0036] ;

[0037] The positive sequence reactive power is:

[0038] ;

[0039] The mechanical equations controlled by the virtual synchronous machine satisfy:

[0040] ;

[0041] as well as

[0042] ;

[0043] in, This represents positive-sequence active power. This is positive sequence reactive power. , where represents the d-axis component of the positive-sequence voltage in a synchronously rotating coordinate system. This represents the q-axis component of the positive-sequence voltage in a synchronously rotating coordinate system. Let d be the positive-sequence current in a synchronously rotating coordinate system. This represents the q-axis component of the positive-sequence current in a synchronously rotating coordinate system. For virtual inertia, The damping coefficient is... It is the positive sequence voltage angular frequency. The rated angular frequency, The angular frequency of the power grid. Active power command, This is the voltage regulation coefficient. This is the reactive voltage droop factor. This is the rated voltage amplitude. This is a reactive power command. This represents the positive sequence voltage amplitude.

[0044] Further, in step S4, a negative sequence compensation voltage command is generated, including:

[0045] The negative sequence current is determined based on the negative sequence component;

[0046] Using the negative sequence current as the negative sequence current feedback quantity, the negative sequence current regulator is used to adjust the negative sequence voltage adjustment quantity.

[0047] The value of the negative sequence virtual impedance is determined based on the current voltage imbalance. The negative sequence virtual impedance includes negative sequence virtual resistance and negative sequence virtual inductance.

[0048] The negative sequence current feedback quantity is used to calculate the voltage drop through the negative sequence virtual impedance to obtain the negative sequence virtual impedance voltage drop.

[0049] The negative sequence voltage adjustment amount is superimposed with the negative sequence virtual impedance voltage drop to generate the negative sequence compensation voltage command.

[0050] Further, in step S5, a zero-sequence voltage command is generated, including:

[0051] Based on the neutral point potential information and the zero-sequence component, determine the neutral point potential deviation and the zero-sequence current;

[0052] The neutral point potential deviation is adjusted by a zero-sequence voltage regulator to obtain the zero-sequence voltage adjustment amount;

[0053] The zero-sequence current is adjusted through the midpoint potential balance loop to obtain the midpoint potential balance adjustment amount;

[0054] The zero-sequence voltage adjustment amount is superimposed with the midpoint potential balance adjustment amount to generate the zero-sequence voltage command.

[0055] Further, in step S6, the feedforward compensation amount is generated, including:

[0056] Based on the positive sequence voltage and positive sequence current in the positive sequence component, and the negative sequence voltage and negative sequence current in the negative sequence component, calculate the second harmonic active power fluctuation and the second harmonic reactive power fluctuation.

[0057] The second harmonic active power fluctuation and the second harmonic reactive power fluctuation are filtered by a notch filter to extract the AC component from the second harmonic active power fluctuation and the second harmonic reactive power fluctuation.

[0058] The DC power component in the positive sequence power is input into the positive sequence grid control.

[0059] The extracted AC component is converted into a negative sequence voltage feedforward compensation amount, and the negative sequence voltage feedforward compensation amount is superimposed on the negative sequence compensation voltage command.

[0060] Further, in step S7, a switch control signal is generated, including:

[0061] The positive-sequence voltage command, the negative-sequence compensation voltage command, and the zero-sequence voltage command are respectively transformed to a three-phase stationary coordinate system to obtain a three-phase voltage command;

[0062] The three-phase voltage command is compared with the carrier signal to generate an initial pulse width modulation signal;

[0063] The initial pulse width modulation signal is subjected to dead-zone compensation processing to generate a dead-zone compensated pulse width modulation signal;

[0064] The pulse width modulation signal after dead zone compensation is subjected to minimum pulse width limiting processing to remove pulses with widths less than a preset minimum pulse width threshold, thereby generating the final switching control signal.

[0065] An unbalanced adaptive control system for a three-phase four-wire grid converter includes:

[0066] The signal acquisition and conditioning unit is used to acquire electrical quantity information on the AC side of the three-phase four-wire grid converter, filter and synchronously sample the electrical quantity information, and output digital electrical quantity signals.

[0067] The sequence component separation unit is connected to the signal acquisition and conditioning unit and is used to perform sequence component separation on the digital electrical quantity signal, extract the positive sequence component, negative sequence component and zero sequence component, and obtain the phase and frequency information of the positive sequence voltage based on the positive sequence component.

[0068] The positive sequence network control unit is connected to the positive sequence component separation unit and is used to calculate the positive sequence power based on the positive sequence component and generate a positive sequence voltage command based on the phase and frequency information of the positive sequence voltage.

[0069] A negative sequence suppression control unit, connected to the sequence component separation unit, is used to determine the negative sequence current based on the negative sequence component, and generate a negative sequence compensation voltage command by adjusting the negative sequence current and the negative sequence virtual impedance adaptively adjusted according to the voltage imbalance.

[0070] A zero-sequence voltage control unit, connected to the signal acquisition and conditioning unit and the sequence component separation unit, is used to generate a zero-sequence voltage command based on the neutral point potential information and the zero-sequence component through zero-sequence voltage regulation and neutral point potential balance control.

[0071] An unbalanced power decoupling unit, connected to the sequence component separation unit, the positive sequence network control unit, and the negative sequence suppression control unit, is used to extract second harmonic power fluctuations based on the positive sequence components and the negative sequence components, separate the second harmonic power fluctuations from the positive sequence power, generate a feedforward compensation amount based on the second harmonic power fluctuations, and output the feedforward compensation amount to the negative sequence suppression control unit.

[0072] A voltage command synthesis unit, connected to the positive sequence network control unit, the negative sequence suppression control unit, and the zero sequence voltage control unit, is used to synthesize the positive sequence voltage command, the negative sequence compensation voltage command, and the zero sequence voltage command into a three-phase modulation voltage command;

[0073] The modulation and drive unit, connected to the voltage command synthesis unit, is used to modulate and drive the three-phase modulation voltage command to generate a switching control signal to control the three-phase four-wire grid converter.

[0074] Compared with existing technologies, the beneficial effects of this invention are as follows: By employing an improved sequence component separation method combining a dual second-order generalized integrator and a third-order generalized integrator, this invention achieves rapid and accurate extraction of positive-sequence, negative-sequence, and zero-sequence components. Furthermore, by setting an adaptive notch filter whose center frequency changes with the real-time angular frequency, residual harmonic ripple during the separation process is effectively suppressed, improving the accuracy of sequence component extraction and dynamic response speed. Simultaneously, based on the accurately separated positive-sequence, negative-sequence, and zero-sequence components, a positive-sequence grid-building control branch, a negative-sequence suppression control branch, and a zero-sequence voltage control branch are constructed respectively, forming a three-sequence independent and coordinated control framework. By extracting and separating the second harmonic power fluctuation from the positive-sequence power, the influence of the negative-sequence component on the positive-sequence power calculation is eliminated, avoiding repeated adjustments of the second harmonic component by the positive-sequence power loop. Ultimately, under conditions of grid voltage imbalance, single-phase load, and asymmetrical faults, negative-sequence current and zero-sequence drift are effectively suppressed, reducing DC bus second harmonic fluctuations and improving the operational stability and fault ride-through capability of the grid-type converter under unbalanced conditions.

[0075] Furthermore, this invention simulates the inertia and damping characteristics of a synchronous generator through virtual synchronous machine control, enabling the grid-connected converter to possess autonomous voltage and frequency support capabilities. Simultaneously, reactive power is rationally allocated through reactive voltage droop control. Based on this, positive-sequence active and reactive power are calculated using accurately extracted positive-sequence voltage and current. Using the phase and frequency of the positive-sequence voltage as a synchronization reference, the angular frequency, phase, and amplitude of the positive-sequence voltage are obtained through the virtual synchronous machine's mechanical equations and reactive voltage droop control, respectively. Finally, a three-phase positive-sequence voltage command is synthesized, providing a stable voltage reference for positive-sequence grid control and ensuring the basic symmetry of the positive-sequence voltage and effective support for the grid under grid imbalance conditions.

[0076] Furthermore, this invention constructs a negative-sequence suppression control module, decomposing the negative-sequence current into two components, direct and quadrature axes, in a reverse synchronous rotating coordinate system. The deviation between the negative-sequence current command and the negative-sequence current feedback is used as input, and a proportional-integral regulator generates a negative-sequence voltage regulation, achieving closed-loop control of the negative-sequence current. Simultaneously, a negative-sequence virtual impedance, composed of virtual resistance and virtual inductance, is introduced into the negative-sequence branch. This impedance is adaptively adjusted in segments according to the current voltage imbalance. When the voltage imbalance exceeds a first threshold, the negative-sequence virtual resistance is increased to enhance negative-sequence damping and limit the negative-sequence current. When the voltage imbalance falls below a certain threshold, the negative-sequence virtual resistance is increased. At the second threshold, the negative sequence virtual resistance is reduced to lower normal operation losses, and the first threshold is greater than the second threshold to form a hysteresis region to prevent frequent switching. The negative sequence voltage regulation amount is superimposed with the negative sequence virtual impedance voltage drop to generate a negative sequence compensation voltage command. This can suppress the negative sequence current to zero under normal grid operating conditions to avoid injecting negative sequence components into the grid, and can also provide limited negative sequence support during asymmetrical faults according to the negative sequence current command given by the fault ride-through coordinator. At the same time, it avoids the negative sequence current regulator from entering saturation when the external grid negative sequence voltage is large, effectively balancing fault ride-through capability and normal operation losses.

[0077] Furthermore, this invention constructs a zero-sequence voltage control module. After comparing the neutral point-to-ground potential with a zero-potential reference value, a quasi-proportional resonant controller generates a zero-sequence voltage regulation quantity. Simultaneously, the zero-sequence current is regulated by a neutral point potential balancing loop to generate a neutral point potential balancing regulation quantity. These two are then superimposed to generate a zero-sequence voltage command, achieving independent closed-loop control of the zero-sequence voltage and precise regulation of the neutral point potential. This effectively suppresses the inconsistency in the three-phase phase voltage amplitude caused by neutral point potential deviation. Simultaneously, the zero-sequence current is detected, and when it exceeds a safety threshold, amplitude and rate of change of the zero-sequence voltage command are limited to prevent zero-sequence overcurrent. Furthermore, for the fourth-arm topology and the split capacitor midpoint lead-out topology, the zero-sequence voltage command is used to determine the fourth-arm modulation voltage to enable controllable return of the neutral line current. A compensation quantity is also generated based on the voltage difference between the upper and lower split capacitors and superimposed on the zero-sequence duty cycle. This effectively limits the zero-sequence current and neutral line current under different three-phase four-wire topologies, preventing long-term bias of the split capacitor voltage and ensuring stable operation of the grid-type converter under single-phase load and asymmetrical operating conditions.

[0078] Furthermore, this invention constructs an unbalanced power decoupling and coordination module. Based on the positive-sequence voltage, negative-sequence voltage, positive-sequence current, and negative-sequence current, it calculates the second-harmonic active power fluctuation and second-harmonic reactive power fluctuation formed by the cross-coupling of positive and negative sequences. Then, it removes these fluctuations from the positive-sequence instantaneous power using a second-harmonic notch filter network, inputting only the DC component into the virtual synchronous machine power loop. This eliminates the influence of the negative-sequence component on the positive-sequence power calculation and avoids repeated adjustments of the second-harmonic component by the positive-sequence power loop. Simultaneously, the second-harmonic fluctuation, after phase compensation and gain tuning, is fed forward to the output of the negative-sequence voltage regulator, preventing the negative-sequence compensation action from repeatedly triggering positive-sequence power regulation. This achieves decoupling and coordination between positive-sequence grid control and negative-sequence suppression control, effectively reducing DC bus second-harmonic fluctuations under grid voltage imbalance conditions and improving the operational stability and dynamic response performance of the grid-type converter under asymmetrical faults.

[0079] Furthermore, this invention synthesizes the positive-sequence voltage command, negative-sequence compensation voltage command, and zero-sequence voltage command into a three-phase modulated voltage command, and generates corresponding switching control signals according to different topologies: For the fourth-arm topology, the duty cycles of the four arms are generated based on the three-phase modulated voltage command and the zero-sequence voltage command; for the split capacitor midpoint lead-out topology, carrier phase-shift pulse width modulation, space vector pulse width modulation, or three-level space vector pulse width modulation is used, and the zero-sequence vector action time is corrected according to the midpoint potential balance compensation; the modulated signal is output to the power switch after voltage limiting, vector scaling, dead-time compensation, and minimum pulse width limitation, thereby realizing the effective synthesis and precise driving of the three-sequence voltage command, ensuring the uniform applicability of the control strategy under different three-phase four-wire topologies and the reliability of the switching signal, while avoiding the shoot-through risk and narrow pulse problem of the power switch through dead-time compensation and minimum pulse width limitation, thus improving the safety of system operation. Attached Figure Description

[0080] Figure 1 This is a flowchart of the unbalanced adaptive control method for a three-phase four-wire grid converter according to an embodiment of this application;

[0081] Figure 2 This is a block diagram illustrating the unbalanced adaptive control principle of a three-phase four-wire grid converter according to an embodiment of this application.

[0082] Figure 3 This is a schematic diagram of the unbalanced adaptive control system of a three-phase four-wire grid converter according to an embodiment of this application. Detailed Implementation

[0083] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0084] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0085] Please see Figures 1-3 As shown, Figure 1 This is a flowchart of the unbalanced adaptive control method for a three-phase four-wire grid converter according to an embodiment of this application; Figure 2 This is a block diagram illustrating the unbalanced adaptive control principle of a three-phase four-wire grid converter according to an embodiment of this application. Figure 3 This is a schematic diagram of the unbalanced adaptive control system of a three-phase four-wire grid converter according to an embodiment of this application.

[0086] The unbalance adaptive control method for a three-phase four-wire grid converter according to embodiments of this application includes the following steps:

[0087] Step S1: Collect electrical quantity information from the AC side of the three-phase four-wire grid converter, and filter and synchronously sample the electrical quantity information to obtain digital electrical quantity signals;

[0088] Step S2: Sequence component separation is performed on the digital electrical quantity signal to extract the positive sequence component, negative sequence component and zero sequence component, and the phase and frequency information of the positive sequence voltage is obtained based on the positive sequence component;

[0089] Step S3: Calculate the positive sequence power based on the positive sequence component, and generate a positive sequence voltage command based on the phase and frequency information of the positive sequence voltage;

[0090] Step S4: Determine the negative sequence current based on the negative sequence component, and generate a negative sequence compensation voltage command by adjusting the negative sequence virtual impedance based on the negative sequence current and the voltage imbalance.

[0091] Step S5: Based on the neutral point potential information and the zero-sequence component, a zero-sequence voltage command is generated through zero-sequence voltage adjustment and neutral point potential balance control.

[0092] Step S6: Extract the second harmonic power fluctuation based on the positive sequence component and the negative sequence component, separate the second harmonic power fluctuation from the positive sequence power, generate a feedforward compensation amount based on the second harmonic power fluctuation, and use the feedforward compensation amount for the negative sequence current regulation.

[0093] Step S7: Combine the positive sequence voltage command, the negative sequence compensation voltage command, and the zero sequence voltage command into a three-phase modulation voltage command; perform modulation and drive processing on the three-phase modulation voltage command to generate a switch control signal; and control the three-phase four-wire grid converter based on the switch control signal.

[0094] In this embodiment of the invention, voltage and current sampling is performed as follows: Voltage Hall sensors and current Hall sensors are used to collect the three-phase phase voltages, three-phase line currents, and neutral line current on the AC side of the three-phase four-wire grid converter. Simultaneously, the positive and negative DC bus voltages are collected through a DC bus voltage sampling circuit, and the neutral point potential is calculated based on these voltages. For the fourth arm topology, the neutral point potential can also be obtained directly by collecting the voltage between the midpoint of the fourth arm and ground using a voltage sensor. Filtering is then performed: the analog voltage and current signals collected in the first step are input into a low-pass active filter. A second-order Butterworth low-pass filter structure is used, with its cutoff frequency set to one-tenth to one-fifth of the converter switching frequency to filter out subharmonics and high-frequency noise interference at the switching frequency, while maintaining the amplitude and phase distortion of the fundamental frequency component within acceptable limits.

[0095] Synchronous Sampling and Analog-to-Digital Conversion: The filtered analog signal is input to the analog-to-digital conversion module of the digital signal processor. The sampling trigger pulse is controlled by a synchronization signal generated by a phase-locked loop or system clock, ensuring that the three-phase voltage, three-phase current, and neutral current are sampled at the same time to maintain the synchronicity of the sampled data. The analog-to-digital converter adopts a successive approximation or differential summation-incremental structure, with a sampling frequency not less than 256 times the fundamental frequency and a quantization bit depth not less than 12 bits. Sampled Data Verification and Calibration: The digitized sampled data after analog-to-digital conversion is verified, including checking whether the sampled values ​​exceed the preset range and whether data overflow occurs. Simultaneously, the sampled data is calibrated according to the sensor's proportional gain and zero-point offset, converting the digital quantities into actual physical quantities, resulting in digitized three-phase voltage sampled values, three-phase current sampled values, neutral current sampled values, and neutral point potential sampled values—a total of eight digitized electrical quantity signals.

[0096] Specifically, step S2 includes:

[0097] The three-phase voltage and three-phase current in the digital electrical quantity signal are transformed by coordinate transformation to obtain the voltage component and current component in a two-phase stationary coordinate system.

[0098] Using the voltage and current components in the two-phase stationary coordinate system, orthogonal signals are constructed respectively. Based on the combination relationship between the same-direction rotational components and the opposite-direction rotational components, positive-sequence components and negative-sequence components are separated from the results of the orthogonal signal construction.

[0099] The zero-sequence fundamental component is obtained by using the zero-sequence path component in the voltage and current components in the two-phase stationary coordinate system and extracting the fundamental component through a third-order generalized integrator.

[0100] The separated positive-sequence component, negative-sequence component, and zero-sequence fundamental component are filtered by an adaptive notch filter whose center frequency changes with the real-time angular frequency, and the filtered positive-sequence component, negative-sequence component, and zero-sequence fundamental component are output.

[0101] Specifically, the orthogonal signal construction is implemented using a dual second-order generalized integrator, and the fundamental channel transfer function of the dual second-order generalized integrator is:

[0102] ;

[0103] in, For the fundamental channel transfer function of the dual second-order generalized integrator, For the real-time fundamental angular frequency, Here, s is the damping coefficient, and s is the Laplace operator;

[0104] The transfer function of the third-order generalized integrator is:

[0105] ;

[0106] in, Let be the transfer function of the third-order generalized integrator. This is the zero-sequence channel damping coefficient;

[0107] The center frequencies of the adaptive notch filter are respectively set to and .

[0108] In this embodiment of the invention, the first step is coordinate transformation: the three-phase voltage and three-phase current in the digital electrical quantity signal output in step S1 are transformed from a three-phase natural coordinate system to a two-phase stationary coordinate system to obtain voltage and current components in the two-phase stationary coordinate system. This transformation maintains the amplitude unchanged, and the transformed two-phase components are orthogonal to each other. The second step is positive-sequence and negative-sequence component separation: orthogonal signals are constructed using the voltage and current components in the two-phase stationary coordinate system to obtain orthogonal signals of each component in the stationary coordinate system; based on the combination relationship between the same-direction rotating components and the opposite-direction rotating components, the positive-sequence and negative-sequence components are separated from the results of the orthogonal signal construction through positive-sequence and negative-sequence calculations. The orthogonal signal construction is implemented using a dual second-order generalized integrator. The fundamental channel transfer function of the dual second-order generalized integrator is determined by the real-time fundamental angular frequency and the damping coefficient. The transfer function expression is a second-order rational fraction in the complex frequency domain. The numerator is the product of the real-time fundamental angular frequency and the Laplace operator multiplied by the damping coefficient. The denominator is the sum of the square term of the Laplace operator, the product term of the damping coefficient and the real-time fundamental angular frequency and the Laplace operator, and the square term of the real-time fundamental angular frequency.

[0109] The third step is the extraction of the zero-sequence fundamental component: using the zero-sequence path component in the voltage and current components under the two-phase stationary coordinate system, the fundamental component is extracted using a third-order generalized integrator to obtain the zero-sequence fundamental component. The transfer function of the third-order generalized integrator is determined by the real-time fundamental angular frequency and the zero-sequence path damping coefficient. The transfer function expression is a third-order rational fraction in the complex frequency domain. The numerator is the product of the real-time fundamental angular frequency and the Laplace operator multiplied by the zero-sequence path damping coefficient. The denominator is the sum of the cubic term of the Laplace operator, the product of the zero-sequence path damping coefficient, the real-time fundamental angular frequency, and the square term of the Laplace operator, the product of the square of the real-time fundamental angular frequency and the Laplace operator, and the cubic term of the zero-sequence path damping coefficient and the real-time fundamental angular frequency. The fourth step is adaptive notch filtering: the separated positive-sequence component, negative-sequence component, and zero-sequence fundamental component are filtered by an adaptive notch filter whose center frequency varies with the real-time angular frequency to remove high-frequency ripples remaining during the separation process. The center frequency of the adaptive notch filter is set to a multiple of the real-time fundamental angular frequency.

[0110] This invention employs an improved sequence component separation method combining a dual second-order generalized integrator and a third-order generalized integrator. This method achieves rapid and accurate extraction of positive-sequence, negative-sequence, and zero-sequence components. Furthermore, by setting an adaptive notch filter whose center frequency changes with the real-time angular frequency, residual harmonic ripple during the separation process is effectively suppressed, improving the accuracy and dynamic response speed of sequence component extraction. Simultaneously, based on the accurately separated positive-sequence, negative-sequence, and zero-sequence components, a positive-sequence grid-building control branch, a negative-sequence suppression control branch, and a zero-sequence voltage control branch are constructed, forming a three-sequence independent and coordinated control framework. By extracting and separating the second harmonic power fluctuation from the positive-sequence power, the influence of the negative-sequence component on the positive-sequence power calculation is eliminated, avoiding repeated adjustments of the second harmonic component by the positive-sequence power loop. Ultimately, under conditions of grid voltage imbalance, single-phase load, and asymmetrical faults, negative-sequence current and zero-sequence drift are effectively suppressed, reducing DC bus second harmonic fluctuations and improving the operational stability and fault ride-through capability of the grid-type converter under unbalanced conditions.

[0111] Specifically, in step S3, generating a positive-sequence voltage command includes:

[0112] Calculate the positive-sequence active power and positive-sequence reactive power based on the positive-sequence voltage and positive-sequence current in the positive-sequence components.

[0113] Using the phase and frequency information of the positive sequence voltage as a synchronization reference, the calculated positive sequence active power and positive sequence reactive power are compared with the given active power command and reactive power command respectively to obtain the active power deviation and reactive power deviation.

[0114] The active power deviation is adjusted by the mechanical equation controlled by the virtual synchronous machine to obtain the angular frequency of the positive sequence voltage, and the phase of the positive sequence voltage is obtained by integrating the angular frequency.

[0115] The reactive power deviation is adjusted by reactive voltage droop control to obtain the amplitude of the positive sequence voltage.

[0116] The positive sequence voltage command is synthesized based on the angular frequency, phase, and amplitude of the positive sequence voltage.

[0117] Specifically, the positive-sequence active power is:

[0118] ;

[0119] The positive sequence reactive power is:

[0120] ;

[0121] The mechanical equations controlled by the virtual synchronous machine satisfy:

[0122] ;

[0123] as well as

[0124] ;

[0125] in, This represents positive-sequence active power. This is positive sequence reactive power. , where represents the d-axis component of the positive-sequence voltage in a synchronously rotating coordinate system. This represents the q-axis component of the positive-sequence voltage in a synchronously rotating coordinate system. Let d be the positive-sequence current in a synchronously rotating coordinate system. This represents the q-axis component of the positive-sequence current in a synchronously rotating coordinate system. For virtual inertia, The damping coefficient is... It is the positive sequence voltage angular frequency. The rated angular frequency, The angular frequency of the power grid. Active power command, This is the voltage regulation coefficient. This is the reactive voltage droop factor. This is the rated voltage amplitude. This is a reactive power command. This represents the positive sequence voltage amplitude.

[0126] In this embodiment of the invention, the first step is positive-sequence power calculation: the positive-sequence voltage and positive-sequence current in the filtered positive-sequence components output in step S2 are transformed by synchronous rotating coordinates to obtain the direct-axis and quadrature-axis components of the positive-sequence voltage, and the direct-axis and quadrature-axis components of the positive-sequence current in the synchronous rotating coordinate system. Based on the direct-axis and quadrature-axis components of the positive-sequence voltage and the direct-axis and quadrature-axis components of the positive-sequence current, the positive-sequence active power and positive-sequence reactive power are calculated according to the active power calculation formula and the reactive power calculation formula, respectively. Specifically, the positive-sequence active power is equal to the sum of the products of the direct-axis components of the positive-sequence voltage and the positive-sequence current, and the products of the quadrature-axis components of the positive-sequence voltage and the positive-sequence current, multiplied by three-half; the positive-sequence reactive power is equal to the difference between the products of the quadrature-axis components of the positive-sequence voltage and the positive-sequence current, and the products of the direct-axis components of the positive-sequence voltage and the positive-sequence current, multiplied by three-half.

[0127] The second step is to obtain the power deviation: using the phase and frequency information of the positive sequence voltage obtained in step S2 as a synchronization reference, the calculated positive sequence active power and positive sequence reactive power are compared with the given active power command and reactive power command, respectively. The active power command is the active power target value issued by the scheduling command or the upper-level energy management system, and the reactive power command is the reactive power target value issued by the scheduling command or the upper-level energy management system. The active power deviation is obtained by subtracting the active power command from the positive sequence active power; the reactive power deviation is obtained by subtracting the reactive power command from the positive sequence reactive power.

[0128] The third step is to obtain the angular frequency and phase: The active power deviation is adjusted using the mechanical equations controlled by the virtual synchronous machine to obtain the angular frequency of the positive sequence voltage, and the phase of the positive sequence voltage is obtained by integrating the angular frequency. The mechanical equations controlled by the virtual synchronous machine satisfy the following: the virtual inertia multiplied by the differential of the positive sequence voltage angular frequency with respect to time is equal to the difference between the active power command and the positive sequence active power divided by the rated angular frequency, minus the product of the damping coefficient and the difference between the positive sequence voltage angular frequency and the grid angular frequency.

[0129] Step 4, Amplitude Acquisition: The reactive power deviation is adjusted by reactive voltage droop control to obtain the amplitude of the positive sequence voltage. The reactive voltage droop control satisfies the following condition: the voltage regulation coefficient multiplied by the derivative of the positive sequence voltage amplitude with respect to time equals the difference between the reactive power command and the positive sequence reactive power, plus the product of the reactive voltage droop coefficient and the difference between the rated voltage amplitude and the positive sequence voltage amplitude. Step 5, Positive Sequence Voltage Command Synthesis: Based on the angular frequency, phase, and amplitude of the positive sequence voltage, a three-phase positive sequence voltage command is generated according to a sine wave synthesis method.

[0130] This invention simulates the inertia and damping characteristics of a synchronous generator through virtual synchronous machine control, enabling grid-connected converters to have autonomous voltage and frequency support capabilities. Simultaneously, it achieves rational distribution of reactive power through reactive voltage droop control. Based on this, positive-sequence active and reactive power are calculated using accurately extracted positive-sequence voltage and current. Using the phase and frequency of the positive-sequence voltage as a synchronization reference, the angular frequency, phase, and amplitude of the positive-sequence voltage are obtained through the virtual synchronous machine's mechanical equations and reactive voltage droop control, respectively. Finally, a three-phase positive-sequence voltage command is synthesized, providing a stable voltage reference for positive-sequence grid control and ensuring the basic symmetry of the positive-sequence voltage and effective support for the grid under unbalanced grid conditions.

[0131] Specifically, in step S4, a negative sequence compensation voltage command is generated, including:

[0132] The negative sequence current is determined based on the negative sequence component;

[0133] Using the negative sequence current as the negative sequence current feedback quantity, the negative sequence current regulator is used to adjust the negative sequence voltage adjustment quantity.

[0134] The value of the negative sequence virtual impedance is determined based on the current voltage imbalance. The negative sequence virtual impedance includes negative sequence virtual resistance and negative sequence virtual inductance.

[0135] The negative sequence current feedback quantity is used to calculate the voltage drop through the negative sequence virtual impedance to obtain the negative sequence virtual impedance voltage drop.

[0136] The negative sequence voltage adjustment amount is superimposed with the negative sequence virtual impedance voltage drop to generate the negative sequence compensation voltage command.

[0137] In this embodiment of the invention, a negative sequence suppression control module is constructed. The negative sequence current is converted into a reverse synchronous rotating coordinate system. and negative sequence current command , The deviation between the negative sequence current feedback and the input is used to generate the negative sequence voltage regulation quantity via a PI regulator. When the control objective is to prevent the injection of negative sequence current into the grid, the negative sequence current command is set to zero; when limited negative sequence support is required during asymmetrical faults, the negative sequence current command is given by the fault ride-through coordinator.

[0138] To prevent the negative-sequence current regulator from saturating when the external grid negative-sequence voltage is high, a virtual resistor is introduced into the negative-sequence branch. and virtual inductance The negative sequence virtual impedance, and its negative sequence compensation voltage command is:

[0139] ;

[0140] ;

[0141] Define voltage imbalance .when Above the first threshold At that time, the virtual impedance adaptive module increases according to the preset slope. To enhance negative sequence damping and limit negative sequence current; when Below the second threshold Gradually reduce To reduce normal operating losses, among which Greater than This creates a hysteresis region to prevent frequent switching. Preferably, 5%, It is 2%, but not limited to this.

[0142] This invention establishes a negative-sequence suppression control module, which decomposes the negative-sequence current into two components, direct and quadrature, in a reverse synchronous rotating coordinate system. The deviation between the negative-sequence current command and the negative-sequence current feedback is used as input, and a proportional-integral regulator generates a negative-sequence voltage regulation, thus achieving closed-loop control of the negative-sequence current. Simultaneously, a negative-sequence virtual impedance, composed of virtual resistance and virtual inductance, is introduced into the negative-sequence branch. This impedance is adaptively adjusted in segments according to the current voltage imbalance. When the voltage imbalance exceeds a first threshold, the negative-sequence virtual resistance is increased to enhance negative-sequence damping and limit the negative-sequence current; when the voltage imbalance falls below a second threshold... The negative sequence virtual resistance is reduced to lower normal operation losses, and the first threshold is greater than the second threshold to form a hysteresis zone to prevent frequent switching. The negative sequence voltage regulation is superimposed with the negative sequence virtual impedance voltage drop to generate a negative sequence compensation voltage command. This can suppress the negative sequence current to zero under normal grid operating conditions to avoid injecting negative sequence components into the grid, and provide limited negative sequence support during asymmetrical faults according to the negative sequence current command given by the fault ride-through coordinator. At the same time, it avoids the negative sequence current regulator from entering saturation when the external grid negative sequence voltage is large, effectively balancing fault ride-through capability and normal operation losses.

[0143] Specifically, in step S5, generating a zero-sequence voltage command includes:

[0144] Based on the neutral point potential information and the zero-sequence component, determine the neutral point potential deviation and the zero-sequence current;

[0145] The neutral point potential deviation is adjusted by a zero-sequence voltage regulator to obtain the zero-sequence voltage adjustment amount;

[0146] The zero-sequence current is adjusted through the midpoint potential balance loop to obtain the midpoint potential balance adjustment amount;

[0147] The zero-sequence voltage adjustment amount is superimposed with the midpoint potential balance adjustment amount to generate the zero-sequence voltage command.

[0148] In this embodiment of the invention, a zero-sequence voltage control module is constructed. The neutral point-to-ground potential is detected. This value is compared to a zero-potential reference, and the deviation is input to a quasi-proportional resonant controller. The quasi-proportional resonant controller provides high gain near the fundamental angular frequency to generate a zero-sequence voltage command. Synchronous detection of zero-sequence current ,when When the zero-sequence current safety threshold is exceeded, for Amplitude limiting, rate-of-change limitation, or zero-sequence current priority suppression are implemented. For a fourth-arm topology, the zero-sequence voltage command is used to determine the modulation voltage of the fourth arm, allowing the neutral current to flow back controllably through the fourth arm. For a split capacitor midpoint lead-out topology, the voltages of the upper and lower split capacitors are detected. and According to the voltage difference A midpoint potential balance compensation is formed, and this compensation is superimposed on the zero-sequence duty cycle to avoid long-term bias of the split capacitor voltage.

[0149] This invention constructs a zero-sequence voltage control module. After comparing the neutral point-to-ground potential with a zero-potential reference value, a quasi-proportional resonant controller generates a zero-sequence voltage regulation quantity. Simultaneously, the zero-sequence current is regulated by a neutral point potential balancing loop to generate a neutral point potential balancing regulation quantity. These two are then superimposed to generate a zero-sequence voltage command, achieving independent closed-loop control of the zero-sequence voltage and precise regulation of the neutral point potential. This effectively suppresses the inconsistency in the three-phase phase voltage amplitude caused by neutral point potential deviation. Simultaneously, the zero-sequence current is detected, and when it exceeds a safety threshold, amplitude and rate of change of the zero-sequence voltage command are limited to prevent zero-sequence overcurrent. Furthermore, for the fourth-arm topology and the split capacitor midpoint lead-out topology, the zero-sequence voltage command is used to determine the fourth-arm modulation voltage, enabling controllable return of the neutral line current. A compensation quantity based on the voltage difference between the upper and lower split capacitors is superimposed on the zero-sequence duty cycle. Therefore, under different three-phase four-wire topologies, the zero-sequence current and neutral line current can be effectively limited, avoiding long-term bias of the split capacitor voltage and ensuring stable operation of the grid-type converter under single-phase load and asymmetrical operating conditions.

[0150] Specifically, in step S6, the feedforward compensation amount is generated, including:

[0151] Based on the positive sequence voltage and positive sequence current in the positive sequence component, and the negative sequence voltage and negative sequence current in the negative sequence component, calculate the second harmonic active power fluctuation and the second harmonic reactive power fluctuation.

[0152] The second harmonic active power fluctuation and the second harmonic reactive power fluctuation are filtered by a notch filter to extract the AC component from the second harmonic active power fluctuation and the second harmonic reactive power fluctuation.

[0153] The DC power component in the positive sequence power is input into the positive sequence grid control.

[0154] The extracted AC component is converted into a negative sequence voltage feedforward compensation amount, and the negative sequence voltage feedforward compensation amount is superimposed on the negative sequence compensation voltage command.

[0155] In this embodiment of the invention, an unbalanced power decoupling and coordination module is constructed. The instantaneous power fluctuation caused by the cross-coupling of positive and negative sequences is calculated based on the positive-sequence voltage, negative-sequence voltage, positive-sequence current, and negative-sequence current. Its active power fluctuation can be expressed as:

[0156] ;

[0157] Its reactive power fluctuation can be expressed as:

[0158] ;

[0159] in, , These are the direct-axis and quadrature-axis components of the positive-sequence voltage, respectively. , These are the direct-axis and quadrature-axis components of the negative-sequence voltage, respectively. , These are the direct-axis and quadrature-axis components of the positive-sequence current, respectively. , These are the direct-axis and quadrature-axis components of the negative-sequence current, respectively. and These represent the cross terms formed by the phase relationships of the coordinate axes. They are removed from the positive-sequence instantaneous power using a second-harmonic notch filter network. and Only the DC component is input to the virtual synchronous machine power loop; at the same time, the second-harmonic fluctuation after phase compensation and gain tuning is fed forward to the output of the negative-sequence voltage regulator so that the negative-sequence compensation action will not repeatedly trigger the positive-sequence power regulation.

[0160] This invention constructs an unbalanced power decoupling and coordination module. Based on positive-sequence voltage, negative-sequence voltage, positive-sequence current, and negative-sequence current, it calculates the second-harmonic active power fluctuation and second-harmonic reactive power fluctuation formed by positive-negative sequence cross-coupling. Then, it removes these fluctuations from the positive-sequence instantaneous power using a second-harmonic notch filter network, inputting only the DC component into the virtual synchronous machine power loop. This eliminates the influence of the negative-sequence component on the positive-sequence power calculation and avoids repeated adjustments of the second-harmonic component by the positive-sequence power loop. Simultaneously, the second-harmonic fluctuation, after phase compensation and gain tuning, is fed forward to the output of the negative-sequence voltage regulator, preventing the negative-sequence compensation action from repeatedly triggering positive-sequence power regulation. This achieves decoupling and coordination between positive-sequence grid control and negative-sequence suppression control, effectively reducing DC bus second-harmonic fluctuations under grid voltage imbalance conditions and improving the operational stability and dynamic response performance of the grid-type converter under asymmetrical faults.

[0161] Specifically, in step S7, generating a switch control signal includes:

[0162] The positive-sequence voltage command, the negative-sequence compensation voltage command, and the zero-sequence voltage command are respectively transformed to a three-phase stationary coordinate system to obtain a three-phase voltage command;

[0163] The three-phase voltage command is compared with the carrier signal to generate an initial pulse width modulation signal;

[0164] The initial pulse width modulation signal is subjected to dead-zone compensation processing to generate a dead-zone compensated pulse width modulation signal;

[0165] The pulse width modulation signal after dead zone compensation is subjected to minimum pulse width limiting processing to remove pulses with widths less than a preset minimum pulse width threshold, thereby generating the final switching control signal.

[0166] In this embodiment of the invention, the positive sequence voltage command is... Negative sequence compensation voltage command and zero-sequence voltage command Synthesize three-phase modulated voltage command:

[0167] ;

[0168] For the fourth-arm topology, the duty cycles of the four arms (a, b, c, and n) are generated based on the three-phase modulation voltage command and the zero-sequence voltage command. For the split capacitor midpoint lead-out topology, carrier phase-shift PWM, space vector PWM, or three-level space vector PWM are used, and the zero-sequence vector duration is corrected based on the midpoint potential balance compensation. The modulation signal is output to the power switching transistor after voltage limiting, vector scaling, dead-time compensation, and minimum pulse width limiting.

[0169] This invention synthesizes the positive-sequence voltage command, negative-sequence compensation voltage command, and zero-sequence voltage command into a three-phase modulated voltage command, and generates corresponding switching control signals according to different topologies: For a fourth-arm topology, the duty cycles of the four arms are generated based on the three-phase modulated voltage command and the zero-sequence voltage command; for a split capacitor midpoint lead-out topology, carrier phase-shift pulse width modulation, space vector pulse width modulation, or three-level space vector pulse width modulation is used, and the zero-sequence vector action time is corrected according to the midpoint potential balance compensation; the modulated signal is output to the power switch after voltage limiting, vector scaling, dead-time compensation, and minimum pulse width limitation, thereby realizing the effective synthesis and precise driving of the three-sequence voltage command, ensuring the uniform applicability of the control strategy under different three-phase four-wire topologies and the reliability of the switching signal, while avoiding the shoot-through risk and narrow pulse problem of the power switch through dead-time compensation and minimum pulse width limitation, thus improving the safety of system operation.

[0170] Specifically, an unbalanced adaptive control system for a three-phase four-wire grid converter includes:

[0171] The signal acquisition and conditioning unit is used to acquire electrical quantity information on the AC side of the three-phase four-wire grid converter, filter and synchronously sample the electrical quantity information, and output digital electrical quantity signals.

[0172] The sequence component separation unit is connected to the signal acquisition and conditioning unit and is used to perform sequence component separation on the digital electrical quantity signal, extract the positive sequence component, negative sequence component and zero sequence component, and obtain the phase and frequency information of the positive sequence voltage based on the positive sequence component.

[0173] The positive sequence network control unit is connected to the positive sequence component separation unit and is used to calculate the positive sequence power based on the positive sequence component and generate a positive sequence voltage command based on the phase and frequency information of the positive sequence voltage.

[0174] A negative sequence suppression control unit, connected to the sequence component separation unit, is used to determine the negative sequence current based on the negative sequence component, and generate a negative sequence compensation voltage command by adjusting the negative sequence current and the negative sequence virtual impedance adaptively adjusted according to the voltage imbalance.

[0175] A zero-sequence voltage control unit, connected to the signal acquisition and conditioning unit and the sequence component separation unit, is used to generate a zero-sequence voltage command based on the neutral point potential information and the zero-sequence component through zero-sequence voltage regulation and neutral point potential balance control.

[0176] An unbalanced power decoupling unit, connected to the sequence component separation unit, the positive sequence network control unit, and the negative sequence suppression control unit, is used to extract second harmonic power fluctuations based on the positive sequence components and the negative sequence components, separate the second harmonic power fluctuations from the positive sequence power, generate a feedforward compensation amount based on the second harmonic power fluctuations, and output the feedforward compensation amount to the negative sequence suppression control unit.

[0177] A voltage command synthesis unit, connected to the positive sequence network control unit, the negative sequence suppression control unit, and the zero sequence voltage control unit, is used to synthesize the positive sequence voltage command, the negative sequence compensation voltage command, and the zero sequence voltage command into a three-phase modulation voltage command;

[0178] The modulation and drive unit, connected to the voltage command synthesis unit, is used to modulate and drive the three-phase modulation voltage command to generate a switching control signal to control the three-phase four-wire grid converter.

[0179] The above embodiments are merely illustrative examples and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. An unbalanced adaptive control method for a three-phase four-wire grid converter, wherein the three-phase four-wire grid converter includes a three-phase full-bridge power module and a neutral point forming unit, wherein the neutral point forming unit is a fourth-arm neutral point forming unit or a split capacitor midpoint lead-out neutral point forming unit, characterized in that, include: Step S1: Collect electrical quantity information from the AC side of the three-phase four-wire grid converter, and filter and synchronously sample the electrical quantity information to obtain digital electrical quantity signals; Step S2: Sequence component separation is performed on the digital electrical quantity signal to extract the positive sequence component, negative sequence component and zero sequence component, and the phase and frequency information of the positive sequence voltage is obtained based on the positive sequence component; Step S3: Calculate the positive sequence power based on the positive sequence component, and generate a positive sequence voltage command based on the phase and frequency information of the positive sequence voltage; Step S4: Determine the negative sequence current based on the negative sequence component, and generate a negative sequence compensation voltage command by adjusting the negative sequence virtual impedance based on the negative sequence current and the voltage imbalance. Step S5: Based on the neutral point potential information and the zero-sequence component, a zero-sequence voltage command is generated through zero-sequence voltage adjustment and neutral point potential balance control. Step S6: Extract the second harmonic power fluctuation based on the positive sequence component and the negative sequence component, separate the second harmonic power fluctuation from the positive sequence power, generate a feedforward compensation amount based on the second harmonic power fluctuation, and use the feedforward compensation amount for the negative sequence current regulation. Step S7: Combine the positive sequence voltage command, the negative sequence compensation voltage command, and the zero sequence voltage command into a three-phase modulation voltage command; perform modulation and drive processing on the three-phase modulation voltage command to generate a switch control signal; and control the three-phase four-wire grid converter based on the switch control signal.

2. The method according to claim 1, characterized in that, Step S2 includes: The three-phase voltage and three-phase current in the digital electrical quantity signal are transformed by coordinate transformation to obtain the voltage component and current component in a two-phase stationary coordinate system. Using the voltage and current components in the two-phase stationary coordinate system, orthogonal signals are constructed respectively. Based on the combination relationship between the same-direction rotational components and the opposite-direction rotational components, positive-sequence components and negative-sequence components are separated from the results of the orthogonal signal construction. The zero-sequence fundamental component is obtained by using the zero-sequence path component in the voltage and current components in the two-phase stationary coordinate system and extracting the fundamental component through a third-order generalized integrator. The separated positive-sequence component, negative-sequence component, and zero-sequence fundamental component are filtered by an adaptive notch filter whose center frequency changes with the real-time angular frequency, and the filtered positive-sequence component, negative-sequence component, and zero-sequence fundamental component are output.

3. The method according to claim 2, characterized in that, The orthogonal signal construction is implemented using a dual second-order generalized integrator, and the fundamental channel transfer function of the dual second-order generalized integrator is: ； in, For the fundamental channel transfer function of the dual second-order generalized integrator, For the real-time fundamental angular frequency, Here, s is the damping coefficient, and s is the Laplace operator; The transfer function of the third-order generalized integrator is: ； in, Let be the transfer function of the third-order generalized integrator. This is the zero-sequence channel damping coefficient; The center frequencies of the adaptive notch filters are respectively set to and .

4. The method according to claim 1, characterized in that, In step S3, a positive sequence voltage command is generated, including: Calculate the positive-sequence active power and positive-sequence reactive power based on the positive-sequence voltage and positive-sequence current in the positive-sequence components. Using the phase and frequency information of the positive sequence voltage as a synchronization reference, the calculated positive sequence active power and positive sequence reactive power are compared with the given active power command and reactive power command respectively to obtain the active power deviation and reactive power deviation. The active power deviation is adjusted by the mechanical equation controlled by the virtual synchronous machine to obtain the angular frequency of the positive sequence voltage, and the phase of the positive sequence voltage is obtained by integrating the angular frequency. The reactive power deviation is adjusted by reactive voltage droop control to obtain the amplitude of the positive sequence voltage. The positive sequence voltage command is synthesized based on the angular frequency, phase, and amplitude of the positive sequence voltage.

5. The method according to claim 4, characterized in that, The positive-sequence active power is: ； The positive sequence reactive power is: ； The mechanical equations controlled by the virtual synchronous machine satisfy: ； as well as ； in, This represents positive-sequence active power. This is positive sequence reactive power. , where represents the d-axis component of the positive-sequence voltage in a synchronously rotating coordinate system. This represents the q-axis component of the positive-sequence voltage in a synchronously rotating coordinate system. Let d be the positive-sequence current in a synchronously rotating coordinate system. This represents the q-axis component of the positive-sequence current in a synchronously rotating coordinate system. For virtual inertia, The damping coefficient is... It is the positive sequence voltage angular frequency. The rated angular frequency, The angular frequency of the power grid. This is an active power command. This is the voltage regulation coefficient. This is the reactive voltage droop factor. This is the rated voltage amplitude. This is a reactive power command. This represents the positive sequence voltage amplitude.

6. The method according to claim 1, characterized in that, In step S4, a negative sequence compensation voltage command is generated, including: The negative sequence current is determined based on the negative sequence component; Using the negative sequence current as the negative sequence current feedback quantity, the negative sequence current regulator is used to adjust the negative sequence voltage adjustment quantity. The value of the negative sequence virtual impedance is determined based on the current voltage imbalance. The negative sequence virtual impedance includes negative sequence virtual resistance and negative sequence virtual inductance. The negative sequence current feedback quantity is used to calculate the voltage drop through the negative sequence virtual impedance to obtain the negative sequence virtual impedance voltage drop. The negative sequence voltage adjustment amount is superimposed with the negative sequence virtual impedance voltage drop to generate the negative sequence compensation voltage command.

7. The method according to claim 1, characterized in that, In step S5, a zero-sequence voltage command is generated, including: Based on the neutral point potential information and the zero-sequence component, determine the neutral point potential deviation and the zero-sequence current; The neutral point potential deviation is adjusted by a zero-sequence voltage regulator to obtain the zero-sequence voltage adjustment amount; The zero-sequence current is adjusted through the midpoint potential balance loop to obtain the midpoint potential balance adjustment amount; The zero-sequence voltage adjustment amount is superimposed with the midpoint potential balance adjustment amount to generate the zero-sequence voltage command.

8. The method according to claim 1, characterized in that, In step S6, the feedforward compensation amount is generated, including: Based on the positive sequence voltage and positive sequence current in the positive sequence component, and the negative sequence voltage and negative sequence current in the negative sequence component, calculate the second harmonic active power fluctuation and the second harmonic reactive power fluctuation. The second harmonic active power fluctuation and the second harmonic reactive power fluctuation are filtered by a notch filter to extract the AC component from the second harmonic active power fluctuation and the second harmonic reactive power fluctuation. The DC power component in the positive sequence power is input into the positive sequence grid control. The extracted AC component is converted into a negative sequence voltage feedforward compensation amount, and the negative sequence voltage feedforward compensation amount is superimposed on the negative sequence compensation voltage command.

9. The method according to claim 1, characterized in that, In step S7, a switch control signal is generated, including: The positive-sequence voltage command, the negative-sequence compensation voltage command, and the zero-sequence voltage command are respectively transformed to a three-phase stationary coordinate system to obtain a three-phase voltage command; The three-phase voltage command is compared with the carrier signal to generate an initial pulse width modulation signal; The initial pulse width modulation signal is subjected to dead-zone compensation processing to generate a dead-zone compensated pulse width modulation signal; The pulse width modulation signal after dead zone compensation is subjected to minimum pulse width limiting processing to remove pulses with widths less than a preset minimum pulse width threshold, thereby generating the final switching control signal.

10. An unbalanced adaptive control system for a three-phase four-wire grid converter, characterized in that, include: The signal acquisition and conditioning unit is used to acquire electrical quantity information on the AC side of the three-phase four-wire grid converter, filter and synchronously sample the electrical quantity information, and output digital electrical quantity signals. The sequence component separation unit is connected to the signal acquisition and conditioning unit and is used to perform sequence component separation on the digital electrical quantity signal, extract the positive sequence component, negative sequence component and zero sequence component, and obtain the phase and frequency information of the positive sequence voltage based on the positive sequence component. The positive sequence network control unit is connected to the positive sequence component separation unit and is used to calculate the positive sequence power based on the positive sequence component and generate a positive sequence voltage command based on the phase and frequency information of the positive sequence voltage. A negative sequence suppression control unit, connected to the sequence component separation unit, is used to determine the negative sequence current based on the negative sequence component, and generate a negative sequence compensation voltage command by adjusting the negative sequence current and the negative sequence virtual impedance adaptively adjusted according to the voltage imbalance. A zero-sequence voltage control unit, connected to the signal acquisition and conditioning unit and the sequence component separation unit, is used to generate a zero-sequence voltage command based on the neutral point potential information and the zero-sequence component through zero-sequence voltage regulation and neutral point potential balance control. An unbalanced power decoupling unit, connected to the sequence component separation unit, the positive sequence network control unit, and the negative sequence suppression control unit, is used to extract second harmonic power fluctuations based on the positive sequence components and the negative sequence components, separate the second harmonic power fluctuations from the positive sequence power, generate a feedforward compensation amount based on the second harmonic power fluctuations, and output the feedforward compensation amount to the negative sequence suppression control unit. A voltage command synthesis unit, connected to the positive sequence network control unit, the negative sequence suppression control unit, and the zero sequence voltage control unit, is used to synthesize the positive sequence voltage command, the negative sequence compensation voltage command, and the zero sequence voltage command into a three-phase modulation voltage command; The modulation and drive unit, connected to the voltage command synthesis unit, is used to modulate and drive the three-phase modulation voltage command to generate a switching control signal to control the three-phase four-wire grid converter.

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