Method and system for suppressing negative-sequence current of network-forming inverter based on DPLSC

Abstract

The invention provides a DPLSC-based negative-sequence current suppression method and system for a network-building inverter. The method comprises the following steps: extracting positive and negative sequence components of voltage and current through a positive and negative sequence separation module, and calculating corresponding power; using a DPLSC control module to compare the positive and negative sequence actual power with the reference power, and generating a superposed voltage reference signal; through a dynamic virtual impedance module, virtual impedance is dynamically adjusted according to the current amplitude so as to suppress impact; through voltage and current inner loop control and PWM driving, inverter output control is realized, the problems of negative sequence current and transient impact are solved, the impedance coefficient is adaptively adjusted according to the unbalance degree of output current and voltage, transient overcurrent is effectively limited, and the system stability is improved.

Description

Technical Field

[0001] This application relates to the field of grid-connected inverter control technology, specifically a method and system for suppressing negative sequence current in grid-connected inverters based on DPLSC. Background Technology

[0002] With the increasing complexity of power grid operation conditions, unbalanced grid voltage scenarios are becoming more and more common. Traditional grid-connected inverter control strategies lack effective control mechanisms for negative sequence components. Under unbalanced grid conditions, the grid current will generate negative sequence components, which will lead to inverter output power fluctuations and current surges, seriously affecting the safe and stable operation of the system.

[0003] In existing technologies, the MHC strategy only optimizes for grid harmonic conditions and cannot solve the problem of negative sequence current. Although traditional grid-type inverters can simulate the characteristics of synchronous generators through active frequency and reactive voltage control, the negative sequence circuit is equivalent to the negative sequence grid voltage being directly grounded through a small line impedance, resulting in excessive negative sequence current. At the same time, the integral stage of reactive voltage control in traditional control makes the system's response speed to grid voltage imbalance slow, and the set virtual impedance needs to be repeatedly switched, which reduces system performance and makes it difficult to meet the dual requirements of negative sequence current suppression and current surge suppression. Summary of the Invention

[0004] The purpose of this application is to provide a method and system for suppressing negative sequence current in grid-connected inverters based on DPLSC, which effectively eliminates negative sequence current, significantly reduces power fluctuations, and accurately suppresses current surges, ultimately improving the safety, stability, and operational reliability of grid-connected inverter systems under unbalanced grid conditions.

[0005] To achieve the above objectives, this application provides the following technical solution:

[0006] In a first aspect, embodiments of this application provide a method for suppressing negative sequence current in a grid-connected inverter based on DPLSC, comprising the following steps:

[0007] The positive and negative sequence components of voltage and current are extracted by the positive and negative sequence separation module and the corresponding power is calculated.

[0008] Using the DPLSC control module, the actual power in positive and negative sequence is compared with the reference power to generate a superimposed voltage reference signal;

[0009] The virtual impedance is dynamically adjusted according to the current amplitude through the dynamic virtual impedance module to suppress the impact.

[0010] Inverter output control is achieved through voltage and current inner loop control and PWM drive, solving the problems of negative sequence current and transient impact.

[0011] The positive and negative sequence components of voltage and current are extracted by the positive and negative sequence separation module, and the corresponding power is calculated as follows:

[0012] Collect the three-phase signals at the output of the grid-type inverter;

[0013] The three-phase signal is input into the Clark transform module and converted into V in the αβ coordinate system. α V β Quantity;

[0014] Input Vα and Vβ into the SOGI-QSG module to generate the filtered signal and the corresponding orthogonal signals qVα and qVβ;

[0015] Inputting Vα, Vβ, qVα, and qVβ into the direct decoupling module yields the positive-sequence components ( , ) and negative order components ( , ), thus completing the separation of positive and negative orders.

[0016] Using the DPLSC control module, the actual power in positive and negative sequences is compared with the reference power to generate a superimposed voltage reference signal.

[0017] Power definition and control objectives

[0018] Positive-sequence power and negative-sequence power are defined as follows:

[0019] in, , These represent the positive-sequence actual active power and the positive-sequence actual reactive power. , These represent negative-sequence actual active power and negative-sequence actual reactive power. , , , These are the positive-sequence αβ components of voltage and current, respectively; , , , These are the negative-order αβ components of voltage and current, respectively;

[0020] Control objective: The positive-sequence power module tracks the given power. , This enables the system to output normal power; the negative sequence power module controls the negative sequence reference power. , Eliminate negative sequence current;

[0021] The expression for the DPLSC control algorithm is:

[0022] Where ω0 is the system's rated angular frequency, θ +θ and θ represent the positive-sequence and negative-sequence reference voltage phases, respectively; , These are the positive-sequence and negative-sequence reference voltage amplitudes, respectively. , For frequency loop proportional and integral coefficients; , For voltage loop proportional and integral coefficients; This is the positive sequence rated voltage amplitude;

[0023] Reference voltage generation

[0024] Based on the above control algorithm, a positive-sequence reference voltage is generated. and negative sequence reference voltage The total reference voltage of the inner voltage and current loop is obtained after superposition. ,Right now:

[0025]

[0026] The expression for the positive-sequence reference voltage in the αβ coordinate system is as follows:

[0027]

[0028] The expression for the negative sequence reference voltage in the αβ coordinate system is:

[0029]

[0030] Under DPLSC control with grid-connected equivalent circuit and adaptive optimization, the grid-connected inverter is equivalent to a series connection of a positive-sequence voltage source and a negative-sequence voltage source. To adapt to the dynamic changes in the negative-sequence voltage of the grid, the amplitude of the negative-sequence component in the PCC voltage at the grid connection point is detected in real time. So that the magnitude of the negative sequence reference voltage satisfies:

[0031]

[0032] To ensure that the negative sequence voltage can accurately offset the influence of the grid's negative sequence voltage, eliminating the negative sequence current at its source, the output power fluctuation amplitude is only related to the degree of voltage imbalance, as expressed by:

[0033]

[0034] in, This is the positive sequence voltage amplitude. The magnitude of the negative sequence voltage. This represents the phase difference between positive and negative sequence voltages.

[0035] The dynamic virtual impedance module dynamically adjusts the virtual impedance based on the current amplitude to suppress surges.

[0036] The positive and negative sequence virtual impedances must meet the following constraints to ensure that the output current does not exceed the threshold:

[0037]

[0038] Will and Substituting the constraints and considering the change in PCC voltage at the grid connection point... , The negative sequence impedance coefficient can be obtained as follows:

[0039]

[0040] The final dynamic virtual impedance value is:

[0041]

[0042] When the PCC voltage at the grid connection point is balanced = =0, = =0, the virtual impedance is 0, which does not affect the steady-state operation of the system; when the power grid is unbalanced, the virtual impedance is dynamically engaged to suppress the inrush current to within 1.3 times the rated current; after the power grid is restored to balance, the virtual impedance automatically drops to 0, realizing dynamic adaptation.

[0043] Inverter output control is achieved through voltage and current inner-loop control and PWM drive, specifically addressing the issues of negative sequence current and transient impact.

[0044] The reference voltage U after superimposing the dynamic virtual impedance voltage drop ref The input voltage and current inner loop control module uses a proportional-resonant PR controller for the voltage inner loop and a proportional controller for the current inner loop to achieve accurate tracking of the output voltage and current. The transfer function of the voltage inner loop PR controller is:

[0045]

[0046] in, This is the voltage proportionality coefficient. The voltage resonance coefficient, For the resonant bandwidth, The resonant frequency,

[0047] The transfer function of the current inner loop proportional controller is:

[0048]

[0049] in, This is the current proportionality coefficient.

[0050] The output signal of the voltage and current inner loop is modulated by the PWM module to generate a PWM signal that drives the inverter switching devices, controls the inverter to output the required voltage and current, and ultimately achieves negative sequence current suppression and current surge suppression.

[0051] Secondly, this application provides a negative sequence current suppression system for grid-connected inverters based on DPLSC. The system consists of a positive and negative sequence separation module, a DPLSC control module, a positive and negative sequence dynamic virtual impedance module, and a voltage and current inner loop control and PWM module. The positive and negative sequence separation module uses SOGI-QSG combined with direct decoupling synchronization method to extract the positive and negative sequence fundamental frequency components of the inverter output voltage and current, achieving steady-state error-free operation with a settling time of only about 0.3s. The DPLSC control module includes two power synchronization loops: a positive sequence loop and a negative sequence loop. The positive sequence loop controls the normal power output of the inverter, while the negative sequence loop generates a negative sequence reference voltage by setting the negative sequence reference power to 0 to offset the influence of the grid's negative sequence voltage. The dynamic virtual impedance module detects the positive and negative sequence current amplitudes in real time and compares them with a 1.3 times rated current threshold to dynamically adjust the virtual impedance, avoiding repeated switching. The voltage and current inner loop control and PWM module generate drive signals to control the inverter based on the superimposed reference voltage and the virtual impedance voltage drop.

[0052] Compared with the prior art, the beneficial effects of the present invention are:

[0053] (1) A negative sequence current suppression method based on dual power loop synchronous control (DPLSC) is proposed, which can actively suppress the negative sequence current of the inverter under unbalanced grid conditions. Compared with the traditional grid-connected inverter, which is based solely on positive sequence power control and cannot effectively handle negative sequence voltage disturbances, this invention constructs a dual synchronous control structure of positive sequence power loop and negative sequence power loop. It can generate a negative sequence voltage reference in real time according to the negative sequence power deviation, thereby offsetting the negative sequence current caused by grid imbalance at the source. This control method does not require additional switching of operating modes, keeping the inverter always in grid-connected voltage source mode, and the control is continuous and highly stable.

[0054] (2) Furthermore, this invention introduces a dynamic virtual impedance in the positive and negative sequence control channel and adaptively adjusts the impedance value according to the output current amplitude. This can increase the inverter's grid-connected equivalent impedance at the moment of grid imbalance or recovery, effectively suppressing current surges and avoiding overcurrent risks. The activation and deactivation of the dynamic virtual impedance are based entirely on the real-time status of the grid and current, without the need for external commands or mode switching, which significantly improves the system's robustness to voltage imbalance disturbances.

[0055] (3) The overall structure of the present invention does not require modification of the inverter main circuit and does not add hardware components. It can achieve multiple effects such as negative sequence current suppression, power fluctuation reduction and transient impact suppression by simply improving the control strategy. It has the advantages of simple implementation, strong adaptability and high engineering application value. It is particularly suitable for grid-type inverter systems in weak grids, scenarios with a large number of unbalanced loads or fault disturbances. Attached Figure Description

[0057] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0058] Figure 1 This is a schematic diagram of the overall structure of the negative sequence current suppression strategy based on DPLSC;

[0059] Figure 2 This is a control block diagram of the positive and negative sequence separation module based on SOGI-QSG;

[0060] Figure 3 This is a schematic diagram of the control structure of DPLSC;

[0061] Figure 4 This is the control block diagram of DPLSC;

[0062] Figure 5 It is the grid-connected equivalent circuit of DPLSC;

[0063] Figure 6 It is a control block diagram of positive and negative sequence virtual impedance. Detailed Implementation

[0066] The technical solutions of the embodiments of this application will now be described with reference to the accompanying drawings. It should be noted that similar reference numerals and letters in the following drawings indicate similar items; therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0067] The terms “comprising,” “including,” or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase “comprising one…” does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0068] The terms “first,” “second,” etc., are used only to distinguish one entity or operation from another, and should not be construed as indicating or implying relative importance, nor as requiring or implying any such actual relationship or order between these entities or operations.

[0069] This embodiment discloses a negative sequence current suppression method based on Dual Power-Loop Synchronous Control (DPLSC), which aims to solve the problems of negative sequence current and current surge in grid-connected inverters under unbalanced power grids. The method includes the following steps:

[0070] S1, Overall System Structure

[0071] The overall structure of this strategy is as follows: Figure 1 As shown, the system includes a DC-side power supply U dc Filter inductor L, filter capacitor C, line impedance Z g The main body of the grid-type inverter includes a positive and negative sequence separation module, a DPLSC control module, a dynamic virtual impedance module, a voltage and current inner loop control module, and a PWM module.

[0072] Among them, I L I is the inverter filter inductor current. o U is the inverter output current. o This refers to the inverter output voltage (i.e., the filter capacitor voltage); I Lαβ U oαβ I L U o Components after transformation from the abc coordinate system to the αβ coordinate system; P ref Q ref Given the system power, P e Q e The actual output power of the system is E; the reference voltage amplitude is E; the reference voltage phase is θ; the superscripts “+” and “-” represent the positive and negative sequence components of each physical quantity, respectively.

[0073] The system workflow is as follows: First, the output voltage U is extracted through the positive and negative sequence separation module. oand output current I o The positive and negative sequence components in the power calculation module are used to obtain the actual output power in the positive and negative sequence. Then, the positive and negative sequence reference voltages are generated by the DPLSC control module, and dynamic virtual impedance is incorporated to suppress current surges. Finally, the inverter drive signal is generated by the voltage and current inner loop control and PWM module to achieve negative sequence current suppression and stable system operation.

[0074] S2, Implementation of the positive / negative sequence separation module

[0075] S21. The positive and negative sequence components are separated by using SOGI-QSG combined with the direct decoupling synchronization method. SOGI-QSG can filter out harmonics in the input signal and generate orthogonal signals, while the direct decoupling synchronization method achieves decoupling of the positive and negative sequence components.

[0076] 1. First, perform Clark transform on the three-phase signals. The transform matrix Tαβ is:

[0077]

[0078] The three-phase signal is converted into V components in the αβ coordinate system using Clark transform. α V β .

[0079] 2. For example Figure 2 After processing by the SOGI-QSG module, the filtered components and their orthogonal signals qV with a 90° lag are obtained. α ,qV β (q is the phase shift operator with a 90° lag).

[0080] 3. Using the positive-order transformation matrix [T+] and negative-order transformation matrix [T-] in the symmetric component method, combined with the direct decoupling algorithm, the decoupling formulas for the positive and negative order components are obtained:

[0081]

[0082] in, , For the positive-order components in the αβ coordinate system , It represents the negative order component in the αβ coordinate system.

[0083] S22. Module Implementation Flow: The control block diagram of the positive / negative sequence separation module is as follows: Figure 2 As shown, the specific implementation steps are as follows:

[0084] 1. Acquire the three-phase signals (voltage U) at the output of the grid-connected inverter. o or current I o );

[0085] 2. Input the three-phase signal into the Clark transform module to convert it into V in the αβ coordinate system. α V β Quantity;

[0086] 3. V α V β Input the SOGI-QSG module to generate the filtered signal and the corresponding quadrature signal qV. α ,qV β ;

[0087] 4. V α V β ,qV α ,qV β The input is directly decoupled, and the positive-sequence component is calculated according to the above decoupling formula. , ) and negative order components ( , ), thus completing the separation of positive and negative orders.

[0088] Implementation of S3 and DPLSC control modules

[0089] S31. Power Definition and Control Objectives

[0090] 1. For example Figure 3 Positive-sequence power and negative-sequence power are defined as follows:

[0091]

[0092] in, , These represent the positive-sequence actual active power and the positive-sequence actual reactive power. , These represent negative-sequence actual active power and negative-sequence actual reactive power. , , , These are the positive-sequence αβ components of voltage and current, respectively; , , , These are the negative-sequence αβ components of voltage and current, respectively.

[0093] 2. Control objective: The positive-sequence power module tracks the given power. , This enables the system to output normal power; the negative sequence power module controls the negative sequence reference power. , Eliminate negative sequence current.

[0094] S32, DPLSC control algorithm. The expression for the DPLSC control algorithm is:

[0095]

[0096] Where ω0 is the system's rated angular frequency, θ + θ and θ represent the positive-sequence and negative-sequence reference voltage phases, respectively; , These are the positive-sequence and negative-sequence reference voltage amplitudes, respectively. , For frequency loop proportional and integral coefficients; , For voltage loop proportional and integral coefficients; This is the positive sequence rated voltage amplitude.

[0097] S33, Reference Voltage Generation

[0098] Based on the above control algorithm, a positive-sequence reference voltage is generated. and negative sequence reference voltage The total reference voltage of the inner voltage and current loop is obtained after superposition. ,Right now:

[0099]

[0100] The expression for the positive-sequence reference voltage in the αβ coordinate system is as follows:

[0101]

[0102] The expression for the negative sequence reference voltage in the αβ coordinate system is:

[0103]

[0104] S34. Grid-connected equivalent circuit and adaptation optimization: Under DPLSC control, the grid-connected inverter is equivalent to a series connection of a positive-sequence voltage source and a negative-sequence voltage source. The grid-connected equivalent circuit is as follows: Figure 4 As shown. To adapt to the dynamic changes in the negative sequence voltage of the power grid, the amplitude of the negative sequence component in the voltage at the point of connection (PCC) is detected in real time. So that the magnitude of the negative sequence reference voltage satisfies:

[0105]

[0106] To ensure that the negative sequence voltage can accurately offset the influence of the grid's negative sequence voltage, eliminating the negative sequence current at its source, the output power fluctuation amplitude is only related to the degree of voltage imbalance, as expressed by:

[0107]

[0108] in, This is the positive sequence voltage amplitude. The magnitude of the negative sequence voltage. This represents the phase difference between positive and negative sequence voltages.

[0109] S4. Implementation of Positive and Negative Sequence Dynamic Virtual Impedance

[0110] S41. During the occurrence and recovery of an unbalanced power grid, the inverter output current contains both positive-sequence and negative-sequence surge components, which, when superimposed, result in an excessively large total surge current. By adding a dynamic virtual impedance to the positive and negative sequence control loop, the equivalent impedance from the output terminal to the PCC point is increased, reducing the short-circuit current and suppressing the surge. After adding the virtual impedance, as... Figure 5 The positive and negative sequence voltage drops are respectively:

[0111]

[0112] in, , These are the positive-sequence and negative-sequence dynamic virtual impedances, respectively. , This is a virtual resistance component; , This is a virtual reactance component; , The positive and negative sequence αβ components of the current.

[0113] S42. Dynamic Virtual Impedance Coefficient Design: The control block diagram for dynamic virtual impedance is shown below. Figure 6 As shown, the impedance coefficient and impedance value are calculated as follows:

[0114] 1. Define the current threshold ,in This refers to the rated output current of the inverter.

[0115] 2. Calculate the d-axis and q-axis components of the output current. , And determine the maximum amplitude of the positive and negative sequence currents. , ;

[0116] 3. Impedance coefficient calculation:

[0117]

[0118] in, It is the basic impedance coefficient, which is tuned according to the system parameters;

[0119] 4. Calculation of virtual impedance components:

[0120]

[0121] in, is the reference resistor, and n is the virtual impedance ratio (the ratio of reactance to resistance), which is set according to the system dynamic response requirements.

[0122] S43. Constraints and Dynamic Adaptation

[0123] The positive and negative sequence virtual impedances must meet the following constraints to ensure that the output current does not exceed the threshold:

[0124]

[0125] Will and Substituting the constraints and considering the voltage change at point PCC , The negative sequence impedance coefficient can be obtained as follows:

[0126]

[0127] The final dynamic virtual impedance value is:

[0128]

[0129] When the voltage at point PCC is balanced = =0, = =0, the virtual impedance is 0, which does not affect the steady-state operation of the system; when the power grid is unbalanced, the virtual impedance is dynamically engaged to suppress the inrush current to within 1.3 times the rated current; after the power grid is restored to balance, the virtual impedance automatically drops to 0, realizing dynamic adaptation.

[0130] S5, the voltage and current inner loop control and PWM module will superimpose the reference voltage U after the dynamic virtual impedance voltage drop. ref The input voltage and current inner loop control module employs a proportional-resonant (PR) controller for the voltage inner loop and a proportional (P) controller for the current inner loop, achieving precise tracking of the output voltage and current. The transfer function of the voltage inner loop PR controller is as follows:

[0131]

[0132] in, This is the voltage proportionality coefficient. The voltage resonance coefficient, For the resonant bandwidth, It is the resonant frequency.

[0133] The transfer function of the current inner loop P controller is:

[0134]

[0135] in, This is the current proportionality coefficient.

[0136] The output signal of the voltage and current inner loop is modulated by the PWM module to generate a PWM signal that drives the inverter switching devices, controls the inverter to output the required voltage and current, and ultimately achieves negative sequence current suppression and current surge suppression.

[0137] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for suppressing negative sequence current in a grid-connected inverter based on DPLSC, characterized in that, Includes the following steps: The positive and negative sequence components of voltage and current are extracted by the positive and negative sequence separation module and the corresponding power is calculated. Using the DPLSC control module, the actual power in positive and negative sequence is compared with the reference power to generate a superimposed voltage reference signal; The virtual impedance is dynamically adjusted according to the current amplitude through the dynamic virtual impedance module to suppress the impact. Inverter output control is achieved through voltage and current inner loop control and PWM drive, solving the problems of negative sequence current and transient impact.

2. The method for suppressing negative sequence current in a grid-type inverter based on DPLSC according to claim 1, characterized in that, The positive and negative sequence components of voltage and current are extracted by the positive and negative sequence separation module, and the corresponding power is calculated as follows: Collect the three-phase signals at the output of the grid-type inverter; The three-phase signal is input into the Clark transform module and converted into V in the αβ coordinate system. α V β Quantity; Input Vα and Vβ into the SOGI-QSG module to generate the filtered signal and the corresponding orthogonal signals qVα and qVβ; Inputting Vα, Vβ, qVα, and qVβ into the direct decoupling module yields the positive-sequence components ( , ) and negative order components ( , ), thus completing the separation of positive and negative orders.

3. The method for suppressing negative sequence current in a grid-type inverter based on DPLSC according to claim 1, characterized in that, Using the DPLSC control module, the actual power in positive and negative sequences is compared with the reference power to generate a superimposed voltage reference signal. Power definition and control objectives Positive-sequence power and negative-sequence power are defined as follows: in, , These represent the positive-sequence actual active power and the positive-sequence actual reactive power. , These represent negative-sequence actual active power and negative-sequence actual reactive power. , , , These are the positive-sequence αβ components of voltage and current, respectively; , , , These are the negative-order αβ components of voltage and current, respectively; Control objective: The positive-sequence power module tracks the given power. , This enables the system to output normal power; the negative sequence power module controls the negative sequence reference power. , Eliminate negative sequence current; The expression for the DPLSC control algorithm is: Where ω0 is the system's rated angular frequency, θ + θ and θ represent the positive-sequence and negative-sequence reference voltage phases, respectively; , These are the positive-sequence and negative-sequence reference voltage amplitudes, respectively. , For frequency loop proportional and integral coefficients; , For voltage loop proportional and integral coefficients; This is the positive sequence rated voltage amplitude; Reference voltage generation Based on the above control algorithm, a positive-sequence reference voltage is generated. and negative sequence reference voltage The total reference voltage of the inner voltage and current loop is obtained after superposition. ,Right now: The expression for the positive-sequence reference voltage in the αβ coordinate system is as follows: The expression for the negative sequence reference voltage in the αβ coordinate system is: Under DPLSC control with grid-connected equivalent circuit and adaptive optimization, the grid-connected inverter is equivalent to a series connection of a positive-sequence voltage source and a negative-sequence voltage source. To adapt to the dynamic changes in the negative-sequence voltage of the grid, the amplitude of the negative-sequence component in the PCC voltage at the grid connection point is detected in real time. So that the magnitude of the negative sequence reference voltage satisfies: To ensure that the negative sequence voltage can accurately offset the influence of the grid's negative sequence voltage, eliminating the negative sequence current at its source, the output power fluctuation amplitude is only related to the degree of voltage imbalance, as expressed by: in, This is the positive sequence voltage amplitude. The magnitude of the negative sequence voltage. This represents the phase difference between positive and negative sequence voltages.

4. The method for suppressing negative sequence current in a grid-type inverter based on DPLSC according to claim 1, characterized in that, The dynamic virtual impedance module dynamically adjusts the virtual impedance based on the current amplitude to suppress surges. The positive and negative sequence virtual impedances must meet the following constraints to ensure that the output current does not exceed the threshold: Will and Substituting the constraints and considering the change in PCC voltage at the grid connection point... , The negative sequence impedance coefficient can be obtained as follows: The final dynamic virtual impedance value is: When the PCC voltage at the grid connection point is balanced = =0, = =0, the virtual impedance is 0, which does not affect the steady-state operation of the system; when the power grid is unbalanced, the virtual impedance is dynamically engaged to suppress the inrush current to within 1.3 times the rated current; after the power grid is restored to balance, the virtual impedance automatically drops to 0, realizing dynamic adaptation.

5. The method for suppressing negative sequence current in a grid-type inverter based on DPLSC according to claim 1, characterized in that, Inverter output control is achieved through voltage and current inner-loop control and PWM drive, specifically addressing the issues of negative sequence current and transient impact. The reference voltage U after superimposing the dynamic virtual impedance voltage drop ref The input voltage and current inner loop control module uses a proportional-resonant PR controller for the voltage inner loop and a proportional controller for the current inner loop to achieve accurate tracking of the output voltage and current. The transfer function of the voltage inner loop PR controller is: in, This is the voltage proportionality coefficient. The voltage resonance coefficient, For the resonant bandwidth, The resonant frequency, The transfer function of the current inner loop proportional controller is: in, This is the current proportionality coefficient. The output signal of the voltage and current inner loop is modulated by the PWM module to generate a PWM signal that drives the inverter switching devices, controls the inverter to output the required voltage and current, and ultimately achieves negative sequence current suppression and current surge suppression.

6. A negative sequence current suppression system for a grid-connected inverter based on DPLSC, characterized in that, The system consists of a positive-sequence separation module, a DPLSC control module, a positive-sequence dynamic virtual impedance module, and a voltage and current inner-loop control and PWM module. The positive-sequence separation module uses SOGI-QSG combined with a direct decoupling synchronization method to extract the positive and negative sequence fundamental frequency components of the inverter's output voltage and current, achieving steady-state error-free operation with a settling time of only about 0.3 seconds. The DPLSC control module includes two power synchronization loops: a positive-sequence loop controls the inverter's normal power output, while the negative-sequence loop generates a negative-sequence reference voltage by setting the negative-sequence reference power to 0 to offset the influence of the grid's negative-sequence voltage. The dynamic virtual impedance module monitors the positive and negative sequence current amplitudes in real time and compares them with a 1.3 times rated current threshold, dynamically adjusting the virtual impedance to avoid repeated switching. The voltage and current inner-loop control and PWM module generates drive signals to control the inverter based on the superimposed reference voltage and the virtual impedance voltage drop.

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