Improved vsg inverter control method and system for suppressing grid current imbalance

Abstract

The application belongs to the technical field of power electronic converter control, and is especially an improved VSG converter control method and system for suppressing grid current imbalance. The method comprises the following steps: S1: constructing a converter basic control architecture based on a virtual synchronous generator; S2: performing real-time extraction of positive sequence, negative sequence and zero sequence components of three-phase currents on the load side; S3: generating an outer loop reference given value of positive sequence voltage control; S4: constructing a control loop containing positive sequence, negative sequence and zero sequence; and S5: according to the control requirements of the sequence control loop, adopting a sliding mode controller to replace a traditional PI controller and applying the sliding mode controller to current loop control of the converter, and designing a differential sliding mode surface and combining an exponential reaching law to optimize the sliding mode controller. The application can ensure high symmetry of grid-side currents under various asymmetric conditions, expand an equal-area deceleration region of transient stability of the system, and has strong robustness and fast response speed.

Description

Technical Field

[0001] This invention relates to the field of power electronic converter control technology, specifically to an improved VSG converter control method and system for suppressing grid current imbalance. Background Technology

[0002] With the large-scale integration of renewable energy sources such as photovoltaics and wind power into the power system, the power system with electronic power exhibits characteristics of low inertia and low damping. Virtual synchronous generator (VSG) control strategies, due to their ability to provide frequency and voltage support to the grid, demonstrate excellent characteristics in weak or even extremely weak grids, and have become the mainstream control method for grid-connected converters. The VSG control strategy simulates the rotational inertia and damping characteristics of a synchronous generator, achieving frequency and voltage regulation through active power loops and reactive voltage loops. Compared to grid-connected control strategies, it possesses inertia support capabilities.

[0003] However, traditional VSG control strategies do not include control logic for negative-sequence and zero-sequence components. When asymmetrical faults such as single-phase voltage drop, load phase loss, or unbalanced load occur in the AC system, the grid-side voltage and current will generate a large number of negative-sequence and zero-sequence components, resulting in severe imbalance of grid current and voltage, and even generating large inrush currents, which can damage converter switching transistors and reduce the reliability and safety of the power system.

[0004] Existing research largely focuses on the transient synchronization stability of VSG converter systems under symmetrical faults, with limited research on capability analysis and enhanced control techniques for asymmetrical faults. Some studies analyze the stability of VSG systems using the equal area criterion, but do not consider the negative sequence current injection requirement under asymmetrical faults. Some studies analyze the principle of three-phase voltage and current asymmetry when the load is not in full phase, but do not propose effective improvement methods. Other studies analyze the operating characteristics of VSG under asymmetrical faults, but do not cover the single-phase voltage drop condition, and still use traditional PI controllers, which have problems such as slow control error response speed, long fault recovery time, and poor system robustness.

[0005] Therefore, there is an urgent need for an improved control strategy that can effectively suppress grid current imbalance and improve the transient stability and fault recovery rate of VSG converters under asymmetrical faults, so as to meet the needs of new power system operation. Summary of the Invention

[0006] The purpose of this section is to outline some aspects of the embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0007] To address the aforementioned technical problems, according to one aspect of the present invention, the present invention provides the following technical solution:

[0008] An improved VSG converter control method for suppressing grid current imbalance includes the following steps:

[0009] S1: Construct a converter basic control architecture based on a virtual synchronous generator to simulate the mechanical and electromagnetic characteristics of a synchronous generator;

[0010] S2: Based on the basic control architecture, a sequence component extraction module is built using a second-order generalized integrator to extract the positive sequence, negative sequence, and zero sequence components of the three-phase current on the load side in real time, and to construct a sequence control loop.

[0011] S3: Calculate the instantaneous active and reactive power output of the converter based on the collected data, input the active power loop and reactive voltage loop of the virtual synchronous generator, and generate the outer loop reference setpoint for positive sequence voltage control.

[0012] S4: Construct a control loop that includes positive sequence, negative sequence and zero sequence. Use the sampled values ​​of the negative sequence and zero sequence current on the load side as the reference values ​​of the negative sequence and zero sequence current loops, respectively. By controlling the inverter to output the corresponding negative sequence and zero sequence components, the negative sequence and zero sequence current generated by the grid asymmetric fault can be offset, thereby achieving symmetrical compensation of the grid-side current.

[0013] S5: Based on the control requirements of the sequence control loop, a sliding mode controller is used to replace the traditional PI controller for converter current loop control. A differential sliding surface is designed and the sliding mode controller is optimized by combining the exponential reaching law.

[0014] As a preferred embodiment of the improved VSG converter control method for suppressing grid current imbalance described in this invention, the converter basic control architecture in S1 adopts an NPC-type three-level converter controlled by a virtual synchronous generator. The specific construction method is as follows: An NPC-type three-level converter simulation model is built, which includes the inverter main circuit, filter circuit, and VSG grid-connected topology with equivalent impedance on the grid side; active power loop and reactive power loop are built; and the VSG mechanical motion equations are established.

[0015]

[0016] And the voltage regulation equation:

[0017]

[0018] In the formula, t is the time variable; J and D are the inertia coefficient and damping coefficient of the VSG, respectively; E is the virtual electromotive force inside the inverter; P e Q o ω oThese represent the active power, reactive power, and angular velocity output by the VSG, respectively; P ref Q ref , ω, U ref These are the active power, reactive power, angular velocity, and rated voltage reference values ​​of the VSG; K ω K q These are the frequency droop coefficient and the reactive power droop coefficient, respectively.

[0019] As a preferred embodiment of the improved VSG converter control method for suppressing grid current imbalance described in this invention, the basic control architecture employs a second-order generalized integrator to construct a sequence component extraction module. Specifically, the acquired three-phase current signal from the load side is converted into components in the αβ coordinate system using Clark transformation. The Clark transformation matrix is ​​as follows:

[0020]

[0021] The αβ components are input to a second-order generalized integrator, and the SOGI damping coefficient of the second-order generalized integrator is set to k=2, and the center frequency ω is set to... c =100π, s is the Laplace operator, and orthogonal signals are generated through the transfer function, which is:

[0022]

[0023]

[0024] Where D(s) is the bandpass transfer function of the second-order generalized integrator; Q(s) is the lowpass transfer function of the second-order generalized integrator; f(s) is the original input signal; d(s) is the in-phase fundamental component of f(s) after bandpass filtering; q(s) is the 90° hysteresis quadrature component of f(s) after lowpass filtering; combining the instantaneous symmetric component method and the phase shift matrix, the positive-sequence, negative-sequence, and zero-sequence components are extracted from the αβ components.

[0025] As a preferred embodiment of the improved VSG converter control method for suppressing grid current imbalance described in this invention, the specific method for constructing control loops containing positive sequence, negative sequence, and zero sequence in step S4 is as follows: based on the traditional VSG positive sequence voltage and current loop control loop, a negative sequence control loop and a zero sequence control loop are added, so that the three sequence component control loops operate independently and decoupled; the sampled values ​​of the negative sequence current and the zero sequence current on the load side are used as reference values ​​for the negative sequence and zero sequence current loops, respectively, and the inverter outputs negative sequence and zero sequence currents that are equal in magnitude and opposite in direction to those on the load side to compensate for the unbalanced current on the grid side, so that the three-phase currents on the grid side are restored to symmetry.

[0026] As a preferred embodiment of the improved VSG converter control method for suppressing grid current imbalance as described in this invention, the specific method of step S5 is to calculate the current loop control error:

[0027]

[0028] In the formula: i ref This is the current reference value; i act x is the actual sampled value of the current. e To control the current loop error; define the differential sliding surface:

[0029]

[0030] In the formula k p1 k p2 These are the proportional coefficients for the dq axes; k d1 k d2 These are the coefficients of the differential terms along the dq axis; σ1 and σ2 are the differential sliding surface functions along the dq axis; x e1 ,x e2 These represent the dq-axis current errors; t is the time variable; the dq-axis components are obtained from the αβ-axis components extracted by the positive / negative sequence separation method through Park transform; the exponential reaching law is adopted, and its expression is:

[0031]

[0032] in, and ε1 and ε2 are the first derivatives of the corresponding dq-axis differential sliding surface functions with respect to time t; ε1 and ε2 are the dq-axis approach velocity coefficients, k1 and k2 are the dq-axis exponential convergence coefficients, and sat(σ) is the saturation function used to suppress chattering in sliding mode control. The control input of the sliding mode controller is designed based on the sliding surface and the exponential approach law, so that the system state trajectory reaches the sliding surface in a finite time and maintains stable operation.

[0033] An improved VSG converter control system for suppressing grid current imbalance includes:

[0034] Data acquisition module: used to acquire three-phase voltage and current signals from the converter output side, grid-side PCC point, and unbalanced load side in real time;

[0035] Sequence component extraction module: used to build a sequence component extraction unit based on a second-order generalized integrator, and to extract the positive sequence, negative sequence and zero sequence components of the three-phase current on the load side in real time;

[0036] Power control module: Simulates the mechanical and electromagnetic characteristics of a synchronous generator, realizes frequency and voltage regulation functions through active power loop and reactive voltage loop, establishes VSG mechanical motion equation and voltage regulation equation, and provides frequency and voltage support for the system;

[0037] Voltage and current control module: used to construct independent decoupled voltage and current control loops for positive sequence, negative sequence, and zero sequence, and to use the negative sequence and zero sequence currents output by the sequence component extraction module as the non-zero reference setpoints of the corresponding inner loops;

[0038] Sliding mode control and PWM module: Used to replace the PI controller with a sliding mode controller with an exponential reaching law to calculate voltage commands in the inner current loop, and generate PWM signals to drive the converter through space vector or sinusoidal pulse width modulation.

[0039] Compared with existing technologies, the advantages of this invention are as follows: First, this invention accurately extracts the sequence components of the load-side current using a second-order generalized integrator (SOGI); it breaks the limitation of traditional zero-sequence control by setting the setpoint to 0, and creatively uses the sampled values ​​of the negative-sequence and zero-sequence currents on the load side as the reference setpoints for the inner current loop, actively outputting a reverse compensation current to offset the unbalanced components; furthermore, it introduces a differential sliding mode controller (SMC) with an exponential reaching law into the inner current loop, significantly improving the current's steady-state error-free tracking speed and suppressing chattering. This invention can ensure the high symmetry of the grid-side current under various asymmetrical operating conditions, expands the equal-area deceleration region of the system's transient stability, and exhibits strong robustness and fast response speed. Attached Figure Description

[0040] To more clearly illustrate the technical solutions of the embodiments of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and detailed embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0041] Figure 1 This is a framework diagram of the control method of the present invention, wherein (a) is the main circuit of the NPC type three-level converter; (b) is the coordinate transformation module; and (c) is the improved VSG control module.

[0042] Figure 2 This is a schematic diagram of the second-order generalized integrator structure of the present invention;

[0043] Figure 3 This is a flowchart of the positive and negative order component extraction process of the present invention;

[0044] Figure 4 This is an equivalent model diagram of the converter grid-connected system of the present invention;

[0045] Figure 5This is a diagram showing the power angle curve of the converter grid-connected system of the present invention;

[0046] Figure 6 The following are simulation results of unbalanced loads according to the present invention, wherein (a) is the grid-side current waveform under the traditional strategy; (b) is the grid-side current waveform under the improved strategy; (c) is the output power waveform under the improved strategy; and (d) is the power angle change waveform.

[0047] Figure 7 The following are simulation results of the non-full-phase operation of the present invention, where (a) is the grid-side current waveform under the traditional strategy; (b) is the grid-side current waveform under the improved strategy; (c) is the output power waveform under the improved strategy; and (d) is the power angle change waveform.

[0048] Figure 8 The following are simulation results of the C-phase voltage drop of the present invention, where (a) is the grid-side current waveform under the traditional strategy; (b) is the grid-side current waveform under the improved strategy; (c) is the output power waveform under the improved strategy; and (d) is the power angle change waveform. Detailed Implementation

[0049] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be further described below with reference to the accompanying drawings. The described embodiments are only some embodiments of the present invention, and not all embodiments; other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort should all fall within the protection scope of the present invention.

[0050] An improved VSG converter control method for suppressing grid current imbalance includes the following steps:

[0051] S1: Construct a basic control architecture for a converter based on a virtual synchronous generator, simulating the mechanical and electromagnetic characteristics of a synchronous generator. This basic control architecture employs an NPC-type three-level converter controlled by a virtual synchronous generator. The specific construction method is as follows: Build an NPC-type three-level converter simulation model, including the inverter main circuit, filter circuit, and a VSG grid-connected topology with equivalent impedance on the grid side. Establish active power loops and reactive power loops, and implement frequency and voltage regulation functions through these loops to provide frequency and voltage support to the grid. Establish the VSG mechanical motion equations.

[0052]

[0053] And the voltage regulation equation:

[0054]

[0055] In the formula, t is the time variable; J and D are the inertia coefficient and damping coefficient of the VSG, respectively; E is the virtual electromotive force inside the inverter; P e Q o ω o These represent the active power, reactive power, and angular velocity output by the VSG, respectively; P ref Q ref , ω, U ref These are the active power, reactive power, angular velocity, and rated voltage reference values ​​of the VSG; K ω K q These are the frequency droop coefficient and the reactive power droop coefficient, respectively.

[0056] The active power loop achieves active power regulation and frequency modulation functions based on the frequency deviation of the PCC point, and the reactive power loop adjusts the virtual electromotive force to achieve reactive power regulation and voltage regulation functions, providing frequency and voltage support for the power grid, and providing a basic operating architecture for subsequent sequence component extraction and sequence control.

[0057] The basic control architecture employs a second-order generalized integrator (SOGI) to build a sequence component extraction module, which performs real-time extraction of positive-sequence, negative-sequence, and zero-sequence components of the three-phase current on the load side. Specifically, the acquired three-phase current signal on the load side is converted into components in the αβ coordinate system through Clark transformation, and the Clark transformation matrix... for:

[0058]

[0059] Input the αβ components into SOGI, and set the SOGI damping coefficient k=2 and the center frequency ω. c =100π, s is the Laplace operator, and orthogonal signals are generated through the transfer function, which is:

[0060]

[0061]

[0062] Where D(s) is the bandpass transfer function of the second-order generalized integrator; Q(s) is the lowpass transfer function of the second-order generalized integrator; f(s) is the original input signal; d(s) is the in-phase fundamental component of f(s) after bandpass filtering; and q(s) is the 90° hysteresis quadrature component of f(s) after lowpass filtering. By combining the instantaneous symmetrical component method and the phase shift matrix, the positive-sequence, negative-sequence, and zero-sequence components can be extracted from the αβ components, completing the sequence component decoupling of the three-phase unbalanced voltage. This decoupling method can also be used for the sequence component decoupling of the current.

[0063]

[0064]

[0065] In the formula: The three-phase positive sequence voltage component matrix is ​​extracted for decoupling; The three-phase negative sequence voltage component matrix extracted for decoupling; U a U b U c These are the voltage sample values ​​for phases A, B, and C, respectively. For U a U b U c The three-phase voltage column vector is formed; h is the phase shift factor, whose value is e. j120° j is the imaginary unit; T + T is the forward transformation matrix; - It is a negative order transformation matrix;

[0066] S2: Based on the aforementioned basic control architecture, a sequence component extraction module is built using a second-order generalized integrator to extract the positive-sequence, negative-sequence, and zero-sequence components of the three-phase current on the load side in real time. Based on the extraction results of the sequence component extraction module, a separate sequence control loop with independent decoupling of positive-sequence, negative-sequence, and zero-sequence components is constructed to achieve individual control of different sequence components and avoid cross-coupling interference between sequence components. Unlike traditional zero-sequence control which sets the reference value to 0, this invention uses the sampled values ​​of the negative-sequence current and zero-sequence current on the load side as the reference setpoints for the negative-sequence current loop and the zero-sequence current loop, respectively. The reference setpoint of the positive-sequence current loop remains unchanged, and the inverter outputs the corresponding compensation component to offset the unbalanced component on the grid side.

[0067] S3: Calculate the instantaneous active and reactive power output of the converter based on the collected data, input the active power loop and reactive voltage loop of the virtual synchronous generator, and generate the outer loop reference setpoint for positive sequence voltage control.

[0068] S4: Construct a control loop containing positive sequence, negative sequence, and zero sequence currents. Use the sampled values ​​of the negative sequence and zero sequence currents on the load side as reference values ​​for the negative sequence and zero sequence current loops, respectively. By controlling the inverter to output the corresponding negative sequence and zero sequence components, the negative sequence and zero sequence currents generated by grid asymmetric faults are offset, thereby achieving symmetrical compensation of grid-side currents. Asymmetric faults include one or more combinations of unbalanced loads, non-full-phase operation, and single-phase voltage dips. Under the asymmetric fault conditions, after adopting the control method of this invention, the grid-side current can achieve rapid symmetry, and the output power and system power angle can recover to a stable state within milliseconds.

[0069] Based on the traditional VSG positive sequence voltage and current loop control loop, a negative sequence control loop and a zero sequence control loop are added to enable the three sequence component control loops to operate independently and decoupled. The negative sequence current sampling value and the zero sequence current sampling value on the load side are used as the reference setpoint values ​​for the negative sequence and zero sequence current loops, respectively. The inverter output is controlled to output negative sequence and zero sequence currents that are equal in magnitude and opposite in direction to the load side, to compensate for the unbalanced current on the grid side and restore the symmetry of the three-phase current on the grid side.

[0070] S5: Based on the control requirements of the sequence control loop, a sliding mode controller is used to replace the traditional PI controller for the converter current loop control. A differential sliding surface is designed and the sliding mode controller is optimized by combining the exponential reaching law to meet the current loop control requirements of the VSG converter sequence control loop and improve the control error response speed and system robustness.

[0071] The specific method is to calculate the current loop control error:

[0072]

[0073] In the formula: i ref This is the current reference value; i act x is the actual sampled value of the current. e To control the current loop error; define the differential sliding surface:

[0074]

[0075] In the formula, σ1 and σ2 are the differential sliding mode surface functions of the dq axis, respectively; t is the time variable; k p1 k p2 These are the proportional coefficients for the dq axes; k d1 k d2 These are the coefficients of the differential terms along the dq axes; x e1 ,x e2 These represent the dq-axis current errors, respectively; the dq-axis components are obtained by extracting the αβ-axis components using the positive and negative sequence separation method and then applying the Park transform; the exponential reaching law is used, and its expression is:

[0076]

[0077] in, and ε1 and ε2 are the first derivatives of the corresponding dq-axis differential sliding surface functions with respect to time t; ε1 and ε2 are the dq-axis approach velocity coefficients, k1 and k2 are the dq-axis exponential convergence coefficients, and sat(σ) is the saturation function used to suppress chattering in sliding mode control. The control input of the sliding mode controller is designed based on the sliding surface and the exponential approach law, so that the system state trajectory reaches the sliding surface in a finite time and maintains stable operation.

[0078] The expression for the saturation function sat(σ) is:

[0079]

[0080] Where Δ is the boundary layer thickness of the saturation function, and its value is a positive real number greater than zero. Based on the designed differential sliding surface and exponential reaching law, and combined with the Lyapunov stability criterion, the control input of the sliding controller is designed so that the system state trajectory meets the arrival conditions, reaches the sliding surface in a finite time and maintains stable sliding motion, thereby achieving fast and accurate tracking of current, improving the control error response speed and the system's anti-disturbance capability, and shortening the fault recovery time of grid-side voltage and current;

[0081] Based on the optimized current loop control method of sliding mode control, a transient stability analysis model is constructed based on the equal area criterion, as follows:

[0082] Treating the VSG-controlled converter as a voltage source, we establish the equivalent circuit of the VSG converter grid-connected system and derive the system power transfer equation:

[0083]

[0084] In the formula, E is the virtual electromotive force inside the inverter, and U g X is the grid line voltage. g P is the system equivalent reactance, δ is the power angle; e Q o The active and reactive power outputs of the VSG-controlled converter are respectively identified. Based on the power transfer equation, the power angle characteristic curve of the VSG converter system is derived. The trajectory of the system power angle change under different control strategies when an asymmetrical fault occurs is analyzed. The acceleration and deceleration areas under different control methods are compared to verify their impact on system stability.

[0085] An improved VSG converter control system for suppressing grid current imbalance includes:

[0086] Data acquisition module: used to acquire three-phase voltage and current signals from the converter output side, grid-side PCC point, and unbalanced load side in real time;

[0087] Sequence component extraction module: used to build a sequence component extraction unit based on a second-order generalized integrator, and to extract the positive sequence, negative sequence and zero sequence components of the three-phase current on the load side in real time;

[0088] Power control module: Simulates the mechanical and electromagnetic characteristics of a synchronous generator, realizes frequency and voltage regulation functions through active power loop and reactive voltage loop, establishes VSG mechanical motion equation and voltage regulation equation, and provides frequency and voltage support for the system;

[0089] Voltage and current control module: used to construct independent decoupled voltage and current control loops for positive sequence, negative sequence, and zero sequence, and to use the negative sequence and zero sequence currents output by the sequence component extraction module as the non-zero reference setpoints of the corresponding inner loops;

[0090] Sliding mode control and PWM module: Used to replace the PI controller with a sliding mode controller with an exponential reaching law to calculate voltage commands in the inner current loop, and generate PWM signals to drive the converter through space vector or sinusoidal pulse width modulation.

[0091] Example:

[0092] Before detailing the method steps of this invention, the parameters in the hardware topology and control architecture of the system will be explained first.

[0093] Reference Figure 1 The following is a specific framework diagram of the present invention, wherein (a) is the main circuit of the NPC type three-level converter, including a DC side capacitor, a three-phase switch bridge arm, an LCL filter, an unbalanced load, and a three-phase power grid; (b) is a coordinate transformation module, which uses a second-order generalized integrator (SOGI) to separate the positive, negative, and zero-sequence components of the sampled three-phase voltage and current signals; (c) is an improved VSG control module, which consists of a VSG power outer loop, a positive and negative sequence dual-loop control based on SMC, and a QPIR zero-sequence control, used to generate an SPWM signal; the present invention provides an improved VSG converter control method for suppressing grid current imbalance, taking the NPC type three-level VSG converter grid-connected system as the application object, where the superscript "+" represents the positive sequence component, the superscript "-" represents the negative sequence component, the superscript "*" represents the reference value, the subscript containing "z" represents the zero-sequence component, and the subscript containing "dq" represents the direct axis and quadrature axis components of the parameter in the dq coordinate system;

[0094] Figure 1 (a): U dc This refers to the DC-side bus voltage; u dc +,u dc - These represent the capacitor voltages of the upper and lower half-bridge on the DC side, respectively; T a1 ,T a2 ,T a3 ,T a4 For phase A NPC inverter bridge arm switching transistors; D a1 D a2 For the A-phase NPC inverter bridge arm clamping diode; u xa ,u xb ,u xc These represent the output voltages of each phase arm of the inverter; N is the neutral point of the filter capacitor; R a ,R b ,R c These are the three-phase equivalent resistances of the unbalanced load; C a Cb C c For three-phase filter capacitors; L a1 ,L b1 ,L c1 The L is the three-phase inductor closest to the inverter side in the LCL filter; a2 ,L b2 ,L c2 For three-phase inductors close to the grid side; L ga ,L gb ,L gc For the grid-side equivalent inductance; i Labc (i La i Lb i Lc ) represents the three-phase current flowing through the inductor of the LCL filter; i oabc (i oa i ob i oc ) represents the three-phase current on the inverter output side; i loadabc For unbalanced loads, the three-phase current is i. gabc (i ga i gb i gc ) represents the actual three-phase current injected into the power grid; u cabc This refers to the voltage across the three-phase filter capacitor; u ga ,u gb ,u gc These are the phase voltages of the three-phase power grid; PCC is the three-phase common coupling point; i z QF represents the current flowing from the unbalanced load side to the neutral line; QF is the grid-connected circuit breaker.

[0095] Figure 1 (b) θ + θ - These are the positive-sequence synchronous rotation electrical angles and the negative-sequence synchronous rotation electrical angles, respectively; i Ld + i Lq + These are the positive-sequence d-axis and q-axis components of the inverter-side current; i Ld - i Lq - These are the negative-sequence d-axis and q-axis components of the inverter-side current; i Lz i represents the zero-sequence component of the inverter-side current. od + i oq + These are the positive-sequence d-axis and q-axis components of the grid-side output current; i od - i oq - These are the negative-sequence d-axis and q-axis components of the grid-side output current; ioz The zero-sequence component of the grid-side output current; u cd + ,u cq + The positive-sequence d-axis and q-axis components of the filter capacitor voltage; u cd - ,u cq - The negative-sequence d-axis and q-axis components of the filter capacitor voltage; i od * i oq * The negative sequence d-axis and q-axis current reference values ​​are extracted from the load current; i oz * The zero-sequence current reference value is extracted from the load current;

[0096] Figure 1 (c) P ref Q ref These are the active and reactive power reference values ​​set for the converter, respectively; P e Q o These represent the actual active and reactive power outputs of the converter, respectively; ΔP and ΔQ represent the active and reactive power deviations, respectively; ω is the set reference angular velocity; ω o Δω is the actual angular velocity output by the VSG; K is the angular velocity deviation value. ω denoted by , where is the frequency droop factor; D is the damping factor of the VSG active loop; J is the inertia factor of the VSG active loop; K is the frequency droop factor. q K is the reactive power droop factor. dv K is the proportional adjustment coefficient of the reactive power control loop; iv U is the integral regulation coefficient of the reactive voltage control loop; ref The rated voltage reference value; E is the virtual electromotive force inside the inverter; ud+* and uq+* are the positive sequence dq axis modulation voltage commands, respectively; SMC is the sliding mode controller; QPIR is the quasi-proportional integral resonant controller; G i This is an inner-loop current proportional controller; dq0 / abc and abc / dq0 are Parker's inverse and forward transform modules, respectively; u abc *+ The positive-sequence three-phase modulated voltage command output by the positive-sequence control loop; u abc *- The negative-sequence three-phase modulated voltage command output by the negative-sequence control loop; u abc *0 This is a zero-sequence three-phase modulation voltage command; u abc* This is the final three-phase AC modulation voltage integrated command obtained by superimposing the various sequence commands;

[0097] like Figure 2 The diagram shown illustrates the structure of the second-order generalized integrator used in this invention. To adapt to operation under asymmetrical power grid conditions, the most widely used pre-filter in PLL phase-locked loop (PLL) improvements is the pre-filter, specifically the second-order generalized integrator (SOGI) integrator. The specific parameters involved in the control architecture diagram include: f is the input signal, k is the damping coefficient, and ω... c With the center frequency as the center frequency, the output d and q components are a set of orthogonal signals.

[0098] like Figure 3 The diagram shown is a flowchart of the positive and negative order component extraction process of this invention. Specific parameters involved in the control architecture diagram include: U α U β These are the two-phase stationary coordinate system components of the voltage obtained after Clark transformation; d α q α and d β q β U represents the phase-shifted orthogonal components of the α-axis and β-axis; α + U α - These are the positive-sequence voltage components along the α-axis and the negative-sequence voltage components along the α-axis, respectively; U β + U β - These are the positive-sequence voltage components along the β-axis and the negative-sequence voltage components along the β-axis, respectively; U d + U d - These represent the positive-sequence voltage component and the negative-sequence voltage component along the d-axis, respectively; U q + U q - These are the positive-sequence voltage component and the negative-sequence voltage component along the q-axis, respectively.

[0099] like Figure 4 The figure shows the equivalent model of the VSG-controlled converter grid-connected system of the present invention. Wherein, E is the virtual electromotive force inside the inverter, and U... g X is the grid line voltage. g P is the system equivalent reactance, δ is the power angle; e Q o These represent the active and reactive power outputs of the VSG-controlled converter, respectively; θ g The phase angle of the grid voltage;

[0100] like Figure 5The figure shows the power angle curve of the VSG-controlled converter grid-connected system of this invention. This invention analyzes fault types such as incomplete phase operation and load imbalance. It is assumed that stable equilibrium points b and c exist in the system after a fault. Figure 5 In the diagram, a, b, and c represent the stable equilibrium points of the system before the fault, the stable equilibrium points of the improved control system after the fault, and the stable equilibrium points of the traditional virtual synchronous generator, respectively. Other letters represent the intersection points between the curves. The bus characteristic curve before the fault is P. S1 When a fault occurs, the power angle characteristic curve will instantaneously change to P. S3 In the power angle equation, the introduction of a virtual inertia coefficient helps suppress abrupt changes in rotor angle. Therefore, the power angle trajectory of a traditional VSG is a→e→c. Improved control technology effectively reduces negative-sequence and zero-sequence currents on the grid side. In the event of a fault, the improved control strategy will rapidly shift the power angle characteristic curve to P. S2 In the power angle equation of the improved control strategy, the trajectory of the power angle is a→d→b. The equation reveals the relationship between the acceleration region and power angle characteristics under both the traditional VSG and the improved control strategy:

[0101]

[0102] Where: δ cm1 To improve the maximum value of the power angle oscillation under the control strategy; δ cm2 S represents the maximum value of the power angle oscillation under the traditional control strategy. adb To improve the acceleration area under the control strategy; S aec The acceleration area under traditional control strategies;

[0103] Its specific implementation method includes the following steps:

[0104] Step 1: System operation status monitoring and core electrical quantity acquisition:

[0105] The system collects three-phase voltage and current signals from the inverter output side, the grid-side three-phase voltage and current signals from the point of common coupling (PCC), and the three-phase voltage and current signals from the unbalanced load side in real time. The collected analog signals are then converted from analog to digital signals and sent to the digital control system as the basic physical quantities for subsequent control loops.

[0106] Step 2: Separation of positive, negative, and zero sequences of voltage and current signals based on SOGI:

[0107] For the acquired asymmetrical three-phase voltage / current signals, decoupling and separation are performed using a second-order generalized integrator (SOGI) extraction module combined with the instantaneous symmetrical component method. The specific process is as follows: First, the SOGI module is introduced to extract the fundamental component to be measured in the system. Utilizing the bandpass filtering characteristics of SOGI at the center frequency, a corresponding orthogonal signal is generated. Then, the positive-sequence, negative-sequence, and zero-sequence components in the asymmetrical signal are extracted. The acquired three-phase voltage / current signals are converted into α and β components using Clark transform, and the Clark transform matrix is ​​expressed as follows:

[0108]

[0109] Based on the instantaneous symmetrical component method, the positive-sequence, negative-sequence, and zero-sequence components of the voltage / current are calculated, and their calculation formulas are expressed as follows:

[0110]

[0111]

[0112]

[0113] In the formula, , , These represent the positive-sequence, negative-sequence, and zero-sequence components of the inverter's output three-phase voltage, respectively; h is the phase-shift factor and h = e j120° ;T + T is the forward transformation matrix; - T is the negative order transformation matrix; 0 This is the zero-sequence transformation matrix; this method can be applied simultaneously to the decoupling of current sequence components; combined with the Clark transformation matrix, the positive and negative sequence components in the αβ coordinate system can be obtained:

[0114]

[0115]

[0116] In the formula: U + αβ U - αβ T represents the positive-sequence voltage component and the negative-sequence voltage component in the αβ coordinate system. abc / αβ U is the Clark transformation matrix; abc The inverter outputs three-phase voltage; U αβ The voltage vector in the αβ coordinate system after Clark transformation;

[0117] Step 3: Instantaneous power calculation and VSG active-reactive loop control:

[0118] Based on the voltage and current signals acquired in step 1 and transformed by Clark and Park, the instantaneous active power P output by the converter is calculated. e With reactive power Q o The active power loop and reactive voltage loop of the VSG are fed into the VSG to simulate the inertia and damping characteristics of a traditional synchronous generator, thereby realizing the frequency and voltage regulation functions of the system. The mechanical motion equations of the VSG (active power loop) are expressed as follows:

[0119]

[0120] The voltage regulation control equation (reactive power loop) is expressed as:

[0121]

[0122] In the formula, t is the time variable; J and D are the inertia coefficient and damping coefficient of the VSG, respectively; P e Q o ω o These represent the active power, reactive power, and angular velocity output by the VSG, respectively; P ref Q ref , ω, U ref These are the active power, reactive power, angular velocity, and rated voltage reference values ​​of the VSG; K ω K q These are the frequency droop coefficient and the reactive power droop coefficient, respectively.

[0123] Step 4: Construct independent and decoupled sequence control loops for positive sequence, negative sequence, and zero sequence:

[0124] To achieve synchronized and coordinated control under asymmetric faults, independent decoupled control loops for positive sequence, negative sequence, and zero sequence are established respectively:

[0125] (1) Positive sequence control loop: The virtual electromotive force E output by VSG in step 3 is used as the reference setpoint for the outer loop of positive sequence voltage control. A dual-loop control structure of voltage outer loop and current inner loop is adopted to maintain the stability of the system's base voltage and frequency.

[0126] (2) Negative-sequence and zero-sequence control loop: The negative-sequence current sampling value and zero-sequence current sampling value of the unbalanced load side collected in step 1 and extracted in step 2 are directly used as the reference setpoint values ​​for the inner loop of the negative-sequence and zero-sequence current of the converter. Its control objective is to control the inverter to actively output negative-sequence and zero-sequence compensation currents that are equal in magnitude and opposite in direction to those generated on the load side, so as to counteract the adverse effects of the negative-sequence and zero-sequence components.

[0127] Step 5: Current inner loop optimization and fast tracking based on sliding mode controller:

[0128] In the sequence control loops constructed in step 4, a sliding mode controller (SMC) is used instead of a traditional PI controller as the inner current loop regulator to improve the tracking speed of the reference current and the dynamic response capability to asymmetrical faults. The differential sliding surface function of the current error is defined as:

[0129]

[0130] In the formula, σ1 and σ2 are the differential sliding mode surface functions of the dq axis, respectively; k p1 k p2 These are the proportional coefficients for the dq axes; k d1 k d2 These are the coefficients of the differential terms along the dq axes; x e1 ,x e2 denoted as dq-axis current error; t is the time variable; to reduce the inherent chattering phenomenon of sliding mode control and improve the response speed, an exponential reaching law combined with the SAT function is used for control law design:

[0131]

[0132] In the formula, and ε1 and ε2 are the first derivatives of the corresponding dq-axis differential sliding surface functions with respect to time t; ε1 and ε2 are the dq-axis approach velocity coefficients, k1 and k2 are the dq-axis exponential convergence coefficients, and sat(σ) is the saturation function used to suppress chattering in sliding mode control. The control input of the sliding mode controller is designed based on the sliding surface and the exponential reaching law, ensuring that the system state trajectory reaches the sliding surface within a finite time and maintains stable operation. The expression for the saturation function sat(σ) is:

[0133]

[0134] Where Δ is the boundary layer thickness of the saturation function, and its value is a positive real number greater than zero.

[0135] Step 6: Inverse transformation of control signal and generation of inverter PWM drive signal:

[0136] The positive-sequence, negative-sequence, and zero-sequence voltage control signals output by the sliding mode controller in step 5 are aggregated and synthesized into a three-phase AC modulation voltage command through corresponding Park inverse transformation (from the dq rotating coordinate system to the αβ stationary coordinate system) and Clark inverse transformation (from the αβ stationary coordinate system to the abc three-phase natural coordinate system). Finally, the three-phase AC modulation voltage command is sent to the SPWM modulation module, compared with the high-frequency carrier, and the required PWM drive pulse signal is generated. This PWM drive signal is applied to the switching transistors of the NPC three-level inverter to drive the converter to output the required three-phase voltage and current, ultimately achieving effective suppression of grid-side current imbalance and improvement of system transient stability.

[0137] The following scheme is used to apply, verify, and compare the above methods.

[0138] 1. Parameter settings:

[0139] The performance of a single VSG grid-connected system under different imbalance conditions was analyzed using Matlab / Simulink simulation software. System parameters are shown in Table 1. The grid-side line voltage is 380V, frequency is 50Hz; external filter inductance is 0.1mH; DC voltage is 800V; switching frequency is 10kHz. Specific parameter settings are shown in Table 1 below.

[0140] Table 1: Case Study Parameters

[0141]

[0142] 2. Case Study 1: Performance Verification under Unbalanced Load Conditions

[0143] The specific operating conditions are set as follows: phases A and B each carry a 2kW load, while phase C carries a 1kW load. For example... Figure 6 The figure shows a comparison waveform of the traditional control strategy and the strategy of the present invention under this operating condition.

[0144] (1) When using the traditional VSG control strategy ( Figure 6 (a) Without controlling the negative sequence and zero sequence, the three-phase current on the grid side will show obvious asymmetrical distortion.

[0145] (2) When using the improved control strategy of the embodiments of the present invention ( Figure 6 (b) Figure 6 (c) Figure 6 (d) Due to the introduction of zero-sequence and negative-sequence current feedforward compensation on the load side, the symmetry of the three-phase current waveform on the grid side is significantly improved. At the same time, compared with the traditional control strategy, after the system reaches a steady state, the present invention effectively reduces the operating power angle of the system and further improves the transient stability margin of the system, which is consistent with the theoretical analysis of the aforementioned equal area criterion.

[0146] 3. Case Study 2: Performance Verification under Non-Full-Phase Operation Conditions:

[0147] The system is configured to encounter a more extreme non-full-phase operation fault. Specifically, phases A and B each carry a 2kW load, while phase C is disconnected and operating without load. Figure 7 The figure shows a comparison waveform of the traditional control strategy and the strategy of the present invention under this operating condition.

[0148] (1) When using the traditional VSG control strategy ( Figure 7 (a) Non-full-phase operation leads to severe imbalance of grid-side current, which can easily trigger overcurrent protection.

[0149] (2) When using the improved control strategy proposed in this invention ( Figure 7 (b) Figure 7 (c) Figure 7 (d) The control system responds rapidly. During non-full-phase operation, the peak current on the grid side of phase C exceeds the peak current on the grid side only by 1.1A compared to the peak current during normal operation, and the fluctuation is strictly limited within a safe range. At the same time, the power grid smoothly absorbs the extra 4kW of active power output from the converter. This invention effectively ensures that the converter does not disconnect from the grid under extreme conditions of phase loss, maintaining the safe operation of the equipment.

[0150] 4. Case Study 3: Single-phase (C-phase) voltage dip condition:

[0151] The scenario involves an asymmetrical voltage dip fault in the power grid: After 0.4 seconds of operation, the C-phase voltage drops by 30% instantaneously. For example... Figure 8 The figure shows a comparison of the system response waveforms under voltage drop conditions.

[0152] (1) When using the traditional VSG control strategy ( Figure 8 (a) After the C-phase voltage drops, the grid-side current immediately exhibits severe asymmetry and high-order harmonics.

[0153] When using the improved control strategy of the present invention () Figure 8 (b) Figure 8 (c) Figure 8 (d) The sequence control loop effectively counteracts the negative sequence current injection caused by unbalanced voltage. The harmonic distortion (THD) rate of the grid-side current is successfully suppressed to below 5%. Notably, thanks to the sliding mode controller (SMC) with exponential reaching law introduced in the inner current loop of this invention, the output power and system power angle recover rapidly and remain stable within 0.1 seconds after the fault occurs.

[0154] Although the present invention has been described above with reference to embodiments, various modifications can be made and components can be replaced with equivalents without departing from the scope of the invention. In particular, as long as there is no structural conflict, the features in the disclosed embodiments can be combined with each other in any manner. The lack of an exhaustive description of these combinations in this specification is merely for the sake of brevity and resource conservation. Therefore, the present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. An improved VSG converter control method to suppress grid current imbalance, characterized in that, Includes the following steps: S1: Construct a converter basic control architecture based on a virtual synchronous generator to simulate the mechanical and electromagnetic characteristics of a synchronous generator; S2: Based on the basic control architecture, a sequence component extraction module is built using a second-order generalized integrator to extract the positive sequence, negative sequence, and zero sequence components of the three-phase current on the load side in real time, and to construct a sequence control loop. S3: Calculate the instantaneous active and reactive power output of the converter based on the collected data, input the active power loop and reactive voltage loop of the virtual synchronous generator, and generate the outer loop reference setpoint for positive sequence voltage control. S4: Construct a control loop that includes positive sequence, negative sequence and zero sequence. Use the sampled values ​​of the negative sequence and zero sequence current on the load side as the reference values ​​of the negative sequence and zero sequence current loops, respectively. By controlling the inverter to output the corresponding negative sequence and zero sequence components, the negative sequence and zero sequence current generated by the grid asymmetric fault can be offset, thereby achieving symmetrical compensation of the grid-side current. S5: Based on the control requirements of the sequence control loop, a sliding mode controller is used to replace the traditional PI controller for converter current loop control. A differential sliding surface is designed and the sliding mode controller is optimized by combining the exponential reaching law.

2. The improved VSG converter control method for suppressing grid current imbalance according to claim 1, wherein, The converter basic control architecture described in S1 adopts an NPC-type three-level converter controlled by a virtual synchronous generator. The specific construction method is as follows: An NPC-type three-level converter simulation model is built, which includes the inverter main circuit, filter circuit, and VSG grid-connected topology with equivalent impedance on the grid side; active power loop and reactive power loop are built; and the VSG mechanical motion equations are established. And the voltage regulation equation: In the formula, t is the time variable; J and D are the inertia coefficient and damping coefficient of the VSG, respectively; E is the virtual electromotive force inside the inverter; P e Q o ω o These represent the active power, reactive power, and angular velocity output by the VSG, respectively; P ref Q ref , ω, U ref These are the active power, reactive power, angular velocity, and rated voltage reference values ​​of the VSG; K ω K q These are the frequency droop coefficient and the reactive power droop coefficient, respectively.

3. The improved VSG converter control method for suppressing grid current imbalance according to claim 2, wherein, The basic control architecture employs a second-order generalized integrator to construct a sequence component extraction module. Specifically, it converts the acquired three-phase current signal from the load side into components in the αβ coordinate system using Clark transformation. The Clark transformation matrix is ​​as follows: The αβ component is input to a second-order generalized integrator, and a second-order generalized integrator SOGI damping coefficient k=2, center frequency ω c =100π, s is a Laplace operator, and an orthogonal signal is generated through a transfer function, and the transfer function is: Where D(s) is the bandpass transfer function of the second-order generalized integrator; Q(s) is the lowpass transfer function of the second-order generalized integrator; f(s) is the original input signal; d(s) is the in-phase fundamental component of f(s) after bandpass filtering; q(s) is the 90° hysteresis quadrature component of f(s) after lowpass filtering; combining the instantaneous symmetric component method and the phase shift matrix, the positive-sequence, negative-sequence, and zero-sequence components are extracted from the αβ components.

4. The improved VSG converter control method for suppressing grid current imbalance according to claim 1, wherein, The specific method for constructing a control loop containing positive sequence, negative sequence, and zero sequence in S4 is as follows: On the basis of the traditional VSG positive sequence voltage and current loop control loop, a negative sequence control loop and a zero sequence control loop are added to enable the three sequence component control loops to operate independently and decoupled; the sampled values ​​of the negative sequence current and the zero sequence current on the load side are used as the reference values ​​of the negative sequence and zero sequence current loops, respectively, to control the inverter to output negative sequence and zero sequence currents that are equal in magnitude and opposite in direction to the load side, to compensate for the unbalanced current on the grid side, and to restore the symmetry of the three-phase current on the grid side.

5. An improved VSG converter control method for suppressing grid current imbalance according to claim 1, characterized in that, The specific method of S5 is to calculate the current loop control error: In the formula: i ref This is the current reference value; i act x is the actual sampled value of the current. e To control the current loop error; define the differential sliding surface: In the formula k p1 k p2 These are the proportional coefficients for the dq axes; k d1 k d2 These are the coefficients of the differential terms along the dq axis; σ1 and σ2 are the differential sliding surface functions along the dq axis; x e1 ,x e2 These represent the dq-axis current errors; t is the time variable; the dq-axis components are obtained from the αβ-axis components extracted by the positive / negative sequence separation method through Park transform; the exponential reaching law is adopted, and its expression is: in, and ε1 and ε2 are the first derivatives of the corresponding dq-axis differential sliding surface functions with respect to time t; ε1 and ε2 are the dq-axis approach velocity coefficients, k1 and k2 are the dq-axis exponential convergence coefficients, and sat(σ) is the saturation function used to suppress chattering in sliding mode control. The control input of the sliding mode controller is designed based on the sliding surface and the exponential approach law, so that the system state trajectory reaches the sliding surface in a finite time and maintains stable operation.

6. An improved VSG converter control system for suppressing grid current imbalance, for implementing the improved VSG converter control method for suppressing grid current imbalance according to any one of claims 1-5, characterized in that, include: Data acquisition module: used to acquire three-phase voltage and current signals from the converter output side, grid-side PCC point, and unbalanced load side in real time; Sequence component extraction module: used to build a sequence component extraction unit based on a second-order generalized integrator, and to extract the positive sequence, negative sequence and zero sequence components of the three-phase current on the load side in real time; Power control module: Simulates the mechanical and electromagnetic characteristics of a synchronous generator, realizes frequency and voltage regulation functions through active power loop and reactive voltage loop, establishes VSG mechanical motion equation and voltage regulation equation, and provides frequency and voltage support for the system; Voltage and current control module: used to construct independent decoupled voltage and current control loops for positive sequence, negative sequence, and zero sequence, and to use the negative sequence and zero sequence currents output by the sequence component extraction module as the non-zero reference setpoints of the corresponding inner loops; Sliding mode control and PWM module: Used to replace the PI controller with a sliding mode controller with an exponential reaching law to calculate voltage commands in the inner current loop, and generate PWM signals to drive the converter through space vector or sinusoidal pulse width modulation.

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