A general sequential impedance modeling method of grid-connected converter considering frequency coupling effect

Abstract

A general sequence impedance modeling method of grid-connected converter considering frequency coupling effect is disclosed. The method comprises the following steps: S1, obtaining time domain expression and frequency domain expression of three-phase voltage and current at a common connection point; S2, deducing a relationship equation of a modulation signal and a disturbance voltage and current; S3, deducing a frequency domain expression of a small signal phase angle; S4, deducing a frequency domain expression of a modulation signal of a current loop output of a control inner loop; S5, respectively obtaining a relationship matrix of a phase-locked loop and a control inner loop about an influence of a voltage disturbance on an output voltage; S6, obtaining a relationship matrix of a control outer loop about an influence of a voltage disturbance on an output voltage; S7, obtaining a multi-input multi-output frequency coupling admittance matrix of the grid-connected converter; and S8, obtaining positive and negative sequence impedances of the converter considering frequency coupling. The method provided by the application is consistent with impedance data obtained by simulation sweep through simulation sweep results.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, specifically to a general sequence impedance modeling method for grid-connected converters that considers frequency coupling effects. Background Technology

[0002] In recent years, the rapid development of renewable energy sources, represented by solar and wind power, has continuously driven profound changes in the power system structure. The main power generation source is accelerating its transformation from traditional coal-fired power plants to new energy sources, and the proportion of renewable energy in the power supply has significantly increased. In this process, power electronic devices are widely used in all aspects of power generation, transmission, and distribution, prompting the power grid to increasingly evolve into a "dual-high" system characterized by both a high proportion of renewable energy and a high proportion of power electronic equipment.

[0003] In this system, the converter, as a key component, plays a crucial role in supporting the stable operation of the power grid due to its dynamic response capability. With the increasing penetration rate of new energy sources, the overall damping characteristics of the power system tend to weaken, and even negative damping phenomena may occur. Especially in areas with relatively weak grid strength, the system stability margin decreases, easily leading to broadband oscillations, which in turn threaten the safety and stability of the power system.

[0004] To investigate broadband oscillation phenomena, existing research mainly employs state-space analysis, torque analysis, and impedance analysis. Among these, impedance analysis is widely used in practice due to its clear theory and ability to directly obtain the actual impedance characteristics of the converter using impedance measurement equipment. This method typically models sequence impedance in a stationary coordinate system. Specifically, it injects a voltage disturbance of a specific frequency into the PCC point through harmonic linearization and converts the periodic AC signal in the time domain to the frequency domain for analysis. The resulting impedance model is more comprehensive and has a wider range of applications.

[0005] The invention application with publication number "CN121615297A" discloses a dual-frequency positive sequence impedance modeling method for grid-type energy storage converters considering frequency coupling. For grid-type energy storage converters, a virtual synchronous machine control strategy is adopted. The grid voltage and frequency are autonomously constructed by simulating the inertia and damping characteristics of synchronous generators. At the same time, a dual-frequency positive sequence disturbance injection method is adopted to construct a 4×4 frequency coupling impedance matrix. However, the model has the problem of being detailed but highly complex.

[0006] The invention application with publication number "CN121036169A" discloses "A Modeling Method for Equivalent Sequence Impedance of Grid-type Converters Considering Frequency Coupling and Delay," which also targets grid-type converters, adopts VSG control, and incorporates the digital control delay element into the model, focusing on analyzing the impact of delay on stability at high frequencies. However, this patent is closely related to the specific topology of VSG, limiting the model's transferability.

[0007] The invention application with publication number "CN120824820A" discloses "A method and system for impedance modeling of grid-connected inverters under multivariable control mode". This patent adopts a harmonic linearization method and positive and negative sequence small signal disturbance injection method, and considers the influence of phase-locked loop phase angle disturbance on the impedance model. It has certain similarities in technical route, but uses an LCL type filter with a high filter order. It needs to deal with the complex frequency response caused by capacitor branch and damping resistor. In terms of control link processing, it introduces grid voltage feedforward, inverter side inductor current feedforward, and capacitor current feedforward multivariable feedback control to reshape the impedance model, focusing on the adjustment effect of different feedforward variables on impedance characteristics, but does not clearly establish a power outer loop model.

[0008] Although some progress has been made in the study of sequence impedance modeling, existing techniques still have some limitations. They still treat the sequence impedance modeling of different control strategies as an isolated problem, usually relying on complicated formula derivations, and failing to fully consider the inherent connections and common structures that may exist in the modeling process. Summary of the Invention

[0009] The purpose of this invention is to propose a general sequence impedance modeling method for grid-connected converters that considers frequency coupling effects. This method fully considers frequency coupling effects in the modeling process, thereby reducing the complexity of the modeling process, revealing the potential correlation between impedance models under different control strategies, improving the accuracy of the impedance model of grid-connected converters, and enabling more accurate description and prediction of system dynamic characteristics through this modeling method, thus overcoming the limitations of existing technologies in this regard.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The present invention provides a general sequence impedance modeling method for grid-type converters that considers frequency coupling effects, comprising the following steps:

[0011] Step S1: Inject positive and negative sequence small signal voltage disturbances at the point of common coupling of the grid-connected system with the grid-connected converter to obtain the time domain expressions of the three-phase voltage and current at the point of common coupling, and obtain the frequency domain expressions of the three-phase voltage and current through Fourier transform.

[0012] Step S2: Based on the main circuit topology formed by the grid-connected converter and the grid connection and the average model of the converter, obtain the relationship equation between the converter internal potential, the voltage and current at the common coupling point, and the relationship equation between the converter internal potential and the modulation signal, and derive the relationship equation between the modulation signal and the disturbance voltage and current.

[0013] Step S3: Based on the small-signal phase angle generated by the phase-locked loop after the small-signal voltage disturbance, derive the frequency domain expression of the small-signal phase angle based on the mathematical model of the phase-locked loop;

[0014] Step S4: Based on the control structure of the grid-connected converter, ignoring the outer control loop, derive the frequency domain expression of the modulation signal output of the inner control loop current loop;

[0015] Step S5: Based on the relationship equation between the modulation signal and the disturbance voltage and current, derive the admittance matrix of the converter when the outer control loop is ignored, and obtain the relationship matrices of the phase-locked loop and the inner control loop regarding the influence of voltage disturbance on the output voltage.

[0016] Step S6: Considering the outer control loop, based on the instantaneous power theory, derive the frequency expression of the output power, obtain the frequency domain expression of the dq axis reference current value, substitute it into the inner control loop, and obtain the relationship matrix of the outer control loop regarding the influence of voltage disturbance on the output voltage;

[0017] Step S7: Based on the relationship matrix of the phase-locked loop, the inner control loop, and the outer control loop, obtain the multi-input multi-output frequency coupling admittance matrix of the grid converter;

[0018] Step S8: Establish the grid-side sequence impedance matrix. Based on the circuit coupling relationship between the converter side and the grid side, transform the multi-input multi-output frequency coupling admittance model into a single-input single-output impedance model, thereby obtaining the positive and negative sequence impedances of the converter considering frequency coupling.

[0019] As a further improvement, in step S1, after injecting positive and negative sequence small-signal voltage disturbances at the common connection point of the converter, the voltage obtained by sampling phase A is... and current The time-domain expressions are as follows:

[0020] ;

[0021] ;

[0022] in, , and These represent the voltage amplitudes under the fundamental frequency, positive-sequence disturbance signal, and coupled disturbance signal, respectively, with corresponding initial phase angles of 0°, 0°, and 0° under the disturbances, respectively. and , , and The current amplitudes under the fundamental frequency, positive-sequence disturbance signal, and coupled disturbance signal are respectively, and the initial phase angles under the corresponding disturbances are respectively. , and , , and These are the fundamental angular frequency, the disturbance angular frequency, and the coupling angular frequency, respectively, where t represents the time domain;

[0023] The corresponding frequency domain expression obtained from the Fourier transform is as follows:

[0024] ; ;

[0025] in, , , , , , The subscript p indicates the component at the disturbance frequency, the subscript n indicates the component at the coupling frequency, f represents the frequency domain, j represents the imaginary unit, f1 represents the fundamental frequency, and f p f represents the perturbation frequency. n This represents the coupling frequency.

[0026] As a further improvement, in step S2, taking phase A as an example, the relationship equation between the internal potential of the converter, the voltage at the common coupling point, and the current in the main circuit is as follows:

[0027] ,in, Represents the internal potential of the converter. represents the voltage at point PCC, and s represents the frequency domain;

[0028] Based on the converter's average model, the equation relating the converter's internal potential to the modulation signal is as follows:

[0029] ;

[0030] in, , The modulation coefficient, DC side voltage For delay function, Indicates the modulated signal;

[0031] Combining the above equations, the relationship between the modulated signal and the voltage and current at the point of common connection is as follows:

[0032] ;

[0033] The equations relating the modulated signal to the disturbance voltage and current are as follows:

[0034] ;

[0035] Where F is a diagonal matrix with diagonal elements of sL, and E is the identity matrix. , These represent the modulation signals at the perturbation frequency and the coupling frequency, respectively.

[0036] As a further improvement, in step S3, without considering small disturbance phase angles, the frequency domain expression of the dq-axis voltage is:

[0037] Where dc represents DC frequency, This represents the frequency at fp-f1;

[0038] ;

[0039] Assume the frequency domain expression for the phase angle of the small perturbation is:

[0040] ,in, , These represent the phase angles at the disturbance frequency and the coupling frequency, respectively.

[0041] Considering small perturbation phase angles, the frequency domain expression for the dq-axis voltage is:

[0042] ,in, represent ;

[0043] Closed-loop transfer function of phase-locked loop for:

[0044] ;

[0045] in, , and These are the parameters of the phase-locked loop PI controller;

[0046] The frequency domain expression for the small-signal phase angle is:

[0047] .

[0048] As a further improvement, in step S4, based on the control structure of the grid-connected converter, without considering the outer control loop, the frequency domain expression of the modulation signal output by the inner control loop current loop is derived as follows:

[0049] ;

[0050] in, and For an ideal modulated signal, and This is the reference current value for the dq axis. and This is the measured value of the dq-axis current. For current loop controller, These are the decoupling coefficients;

[0051] Therefore, the frequency domain expression of the modulated signal considering the small signal phase angle is obtained as follows:

[0052] .

[0053] in, and This is the complete modulated signal along the dq axis.

[0054] As a further improvement, the frequency domain expression of the modulation signal after the inverse Parker transform in step S5 is as follows:

[0055] ;

[0056] in, This is the A-phase modulated signal. and Here are the frequency domain expressions for the sine and cosine functions.

[0057] The equations relating the modulated signal to the disturbance voltage and disturbance current are as follows:

[0058] ;

[0059] The admittance matrix of the converter is not considered when the outer control loop is not considered. for:

[0060] ;

[0061] Where S is the phase-locked loop's relationship matrix regarding the effect of voltage disturbance on the output voltage, and the specific elements in the matrix are shown below:

[0062] ;

[0063] Where C1 represents the modulation signal at the fundamental frequency, C1 * Let I1 be the conjugate of C1. * It is the conjugate complex number of I1;

[0064] To control the inner loop's relationship matrix regarding the impact of voltage disturbances on the converter output voltage, the specific elements of the matrix are shown below:

[0065] ;

[0066] .

[0067] As a further improvement, in step S6, when considering the outer control loop, the active power is established. and reactive power The frequency domain expression is:

[0068] V1 * Let V1 be the conjugate complex number;

[0069] The frequency domain expression for the dq-axis reference current value is:

[0070] ;

[0071] in, and It is the power command value. For the outer loop controller, , All of these represent the frequency domain expression of the output power.

[0072] The dq-axis reference current value is substituted into the inner control loop. After propagation through subsequent stages, the relationship equation between the converter's internal potential and the disturbance voltage and current under the action of the outer control loop is obtained as follows:

[0073] ;

[0074] in, and To control the relationship matrix of the outer loop regarding the effect of voltage disturbance on the output voltage, and The modulation signals added to the outer loop are controlled at the disturbance frequency and the coupling frequency, respectively. The specific elements of the matrix are shown below:

[0075] ; ;

[0076] .

[0077] As a further improvement, in step S7, based on the relationship matrix of the phase-locked loop, the inner control loop, and the outer control loop, the relationship equation between the modulation signal and the disturbance voltage and current is obtained as follows:

[0078] ;

[0079] The equation relating the internal potential of the converter to the disturbance voltage and current is as follows:

[0080] ;

[0081] Frequency coupling admittance matrix of mesh converter for:

[0082] .

[0083] As a further improvement, in step S8, a grid-side positive and negative sequence impedance matrix is ​​established. for:

[0084] ;

[0085] ;

[0086] in, , For the grid resistance and grid inductance, , For filter resistors and filter capacitors, Parallel operation representing impedance;

[0087] Since a normal power grid has a three-phase symmetrical structure, ;

[0088] Based on the circuit coupling relationship between the converter side and the grid side, the positive and negative sequence impedances of the grid-connected converter considering frequency coupling are obtained. and for:

[0089] .

[0090] The present invention also provides an electronic device, including a memory and a processor, wherein the memory is used to store a program that supports the processor in executing the general sequence impedance modeling method for grid converters that takes into account frequency coupling effects, and the processor is configured to execute the program stored in the memory.

[0091] The present invention also provides a computer-readable storage medium storing a computer program, wherein the computer program, when executed by a processor, performs the steps of the general sequence impedance modeling method for grid-type converters considering frequency coupling effects.

[0092] Compared with the prior art, the beneficial effects of the present invention are:

[0093] (1) Modeling accuracy: The method fully considers the influence of frequency coupling effect in the derivation process. By establishing the relationship matrix of phase-locked loop, inner control loop and outer control loop with respect to voltage disturbance, the coupling mechanism of each control link is systematically incorporated. The simulation frequency sweep results show that the theoretically calculated positive and negative sequence impedance curves have good consistency with the impedance data obtained by simulation frequency sweep, which verifies the accuracy of the model.

[0094] (2) In terms of computational efficiency: This method reveals the inherent connection and common structure between different control links, unifies each link into a relation matrix form, avoids the complicated derivation caused by isolated modeling in traditional methods, and effectively reduces computational complexity.

[0095] (3) In terms of scalability: This method uses the relation matrix as the basic unit to construct a multi-input multi-output frequency coupling admittance model. Each control link can be flexibly added, deleted or replaced according to the actual control strategy. It has strong versatility and scalability and is suitable for impedance modeling of grid-type converters with different control structures.

[0096] (4) Engineering applications: This method ultimately transforms the multi-input multi-output model into a single-input single-output positive and negative sequence impedance model, which can be directly used for system stability analysis based on classic methods such as the Nyquist criterion, providing a reliable theoretical basis for parameter design and stability margin assessment of grid connection schemes for new energy power plants. Attached Figure Description

[0097] Figure 1 This is a flowchart illustrating a general sequence impedance modeling method for grid-type converters that considers frequency coupling effects, according to the present invention.

[0098] Figure 2 This is a schematic diagram of the topology of the grid converter of the present invention;

[0099] Figure 3 This is a block diagram of the control structure of the grid converter of the present invention;

[0100] Figure 4 This is a schematic diagram of the converter and the equivalent network of the power grid of the present invention;

[0101] Figure 5 Bode plot for verification of the positive sequence impedance and frequency sweep of the grid converter in this invention;

[0102] Figure 5 (a) is the Bode plot for verifying the positive sequence impedance of the grid converter of the present invention;

[0103] Figure 5 (b) is the Bode plot for the positive-sequence frequency sweep verification of the grid converter of the present invention;

[0104] Figure 6 This invention is based on the Bode plot for verifying the negative sequence impedance and frequency sweep of the grid converter.

[0105] Figure 6 (a) is the Bode plot for verifying the negative sequence impedance of the grid-type converter of this invention;

[0106] Figure 6 (b) is the Bode plot for the negative sequence frequency sweep verification of the grid converter of the present invention. Detailed Implementation

[0107] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0108] In this embodiment, addressing the limitations of traditional impedance modeling methods—such as high complexity, difficulty in accurately describing frequency coupling effects, and inability to precisely reflect the wideband dynamic characteristics of the system—a general sequence impedance modeling method for grid-connected converters considering frequency coupling is proposed. This method establishes a frequency domain expression for the small-signal phase angle by superimposing the disturbance voltage signal with the voltage signal at the PCC. It then constructs the relationship matrices of the inner and outer control loops with respect to the small disturbance signal, transforming them into a single-input single-output equivalent model. This yields the positive and negative sequence impedances considering frequency coupling, achieving accurate modeling of the frequency coupling characteristics of the grid-connected converter.

[0109] Example 1:

[0110] like Figure 1 As shown, this embodiment provides a general sequence impedance modeling method for grid-type converters that considers frequency coupling, specifically including the following steps:

[0111] Injecting positive and negative sequence small-signal voltage disturbances at the PCC of the grid-connected system with the grid converter, and sampling the voltage of phase A... and current The time-domain expressions are as follows:

[0112] ;

[0113] ;

[0114] in, , and These represent the voltage amplitudes under the fundamental frequency, positive-sequence disturbance signal, and coupled disturbance signal, respectively, with corresponding initial phase angles of 0°, 0°, and 0° under the disturbances, respectively. and , , and The current amplitudes under the fundamental frequency, positive-sequence disturbance signal, and coupled disturbance signal are respectively, and the initial phase angles under the corresponding disturbances are respectively. , and , , and These are the fundamental angular frequency, the disturbance angular frequency, and the coupling angular frequency, respectively, where t represents the time domain. The corresponding frequency domain expression obtained from the Fourier transform is:

[0115] ; ;

[0116] in ; ; ; ; ; The subscript p indicates the component at the disturbance frequency, the subscript n indicates the component at the coupling frequency, f represents the frequency domain, j represents the imaginary unit, f1 represents the fundamental frequency, and f p f represents the perturbation frequency. n This represents the coupling frequency.

[0117] Based on the main circuit topology formed by the grid-connected converter and the power grid, as follows: Figure 2 As shown, taking phase A as an example, the relationship equations between the converter internal potential, PCC voltage, and current in the main circuit are as follows:

[0118] ,in, Represents the internal potential of the converter. represents the voltage at point PCC, and s represents the frequency domain;

[0119] Based on the converter's average model, the equation relating the converter's internal potential to the modulation signal is as follows:

[0120] ;

[0121] in, , The modulation coefficient, DC side voltage For delay function, This indicates the modulated signal.

[0122] Combining the above equations, the relationship between the modulation signal and the voltage and current of the PCC is obtained as follows:

[0123] ;

[0124] The equations relating the modulated signal to the disturbance voltage and current are as follows:

[0125] ;

[0126] Where F is a diagonal matrix with diagonal elements of sL, and E is the identity matrix. , These represent the modulation signals at the perturbation frequency and the coupling frequency, respectively.

[0127] Overall control block diagram as follows Figure 3 As shown, grid phase tracking is achieved through a phase-locked loop (PLL). The outer control loop obtains the dq-axis current reference value by passing the active and reactive power values ​​based on instantaneous power theory through a PI controller. The inner control loop calculates the difference between the dq-axis current reference value and the actual value, then passes the result through a PI controller and decouples to obtain the modulation signal.

[0128] A small-signal voltage disturbance applied at the PCC point will affect the output phase angle through the phase-locked loop. Ignoring the small disturbance phase angle, the frequency domain expression for the dq-axis voltage is:

[0129] Where dc represents DC frequency, This represents the frequency at fp-f1;

[0130] ;

[0131] Assume the frequency domain expression for the phase angle of the small perturbation is:

[0132] ,in, , These represent the phase angles at the disturbance frequency and the coupling frequency, respectively.

[0133] Therefore, considering the small perturbation phase angle, the frequency domain expression of the q-axis voltage is:

[0134] ,in, represent ;

[0135] The closed-loop transfer function of the phase-locked loop for:

[0136] ;

[0137] in, , and These are the parameters of the phase-locked loop PI controller.

[0138] The frequency domain expression for the small-signal phase angle is:

[0139] ;

[0140] The control structure block diagram of the grid converter is as follows: Figure 3 The outer control loop calculates the reference current value required by the inner control loop based on the power command and output power, and the inner control loop calculates the modulation signal based on the reference current value and the actual current.

[0141] Ignoring the outer control loop, the frequency domain expression of the modulation signal output by the inner control loop is:

[0142] ;

[0143] in, and For an ideal modulated signal, and This is the reference current value for the dq axis. and This is the measured value of the dq-axis current. For current loop controller, is the decoupling coefficient.

[0144] Therefore, considering the small signal phase angle, the frequency domain expression of the modulated signal is:

[0145] ,in, and This is the complete modulated signal along the dq axis;

[0146] Therefore, the frequency domain expression of the modulated signal after the inverse Parker transform is:

[0147] , This is the A-phase modulated signal. and Here are the frequency domain expressions for the sine and cosine functions;

[0148] The equations relating the modulation signal to the disturbance voltage and current are as follows:

[0149] ;

[0150] Thus, the admittance matrix of the converter is obtained when the outer control loop is ignored. for:

[0151] ;

[0152] Where S is the phase-locked loop's relationship matrix regarding the effect of voltage disturbance on the output voltage, and the specific elements in the matrix are shown below:

[0153] ;

[0154] Where C1 represents the modulation signal at the fundamental frequency, C1 * Let I1 be the conjugate of C1. * It is the conjugate complex number of I1;

[0155] To control the inner loop's relationship matrix regarding the impact of voltage disturbances on the converter output voltage, the specific elements of the matrix are shown below:

[0156] ;

[0157] ;

[0158] In step S6, considering the outer control loop, based on instantaneous power theory, the time-domain expression for the converter output power is established as follows:

[0159] ;

[0160] The frequency domain expressions for active and reactive power under voltage disturbances are obtained as follows:

[0161] V1 * Let V1 be the conjugate complex number;

[0162] The frequency domain expression for the dq-axis reference current value obtained by controlling the outer loop output is derived as follows:

[0163] ,in, and It is the power command value. For the outer loop controller, , Both represent the frequency domain expression of the output power;

[0164] Then, the dq-axis reference current value is input into the inner control loop. After propagation through subsequent stages, the relationship matrix of the outer control loop regarding the influence of voltage disturbance and response current disturbance on the output voltage is obtained as follows:

[0165] ; ;

[0166] ;

[0167] Based on the relationship matrix of the phase-locked loop, the inner control loop, and the outer control loop, the relationship equations between the modulation signal and the disturbance voltage and current are obtained as follows:

[0168] , and These are the modulation signals added to the outer loop at the disturbance frequency and the coupling frequency, respectively.

[0169] The equation relating the internal potential of the converter to the disturbance voltage and current is as follows:

[0170] ;

[0171] The frequency coupling admittance matrix of the grid converter is obtained as follows:

[0172] ;

[0173] To simplify stability analysis, the multi-input multi-output model is transformed into a single-input single-output model. The positive and negative sequence impedance matrices on the network side are established as follows:

[0174] ;

[0175] ;

[0176] in, , For the grid resistance and grid inductance, , For filter resistors and capacitors, This represents the parallel operation of impedances. Since the power grid has a three-phase symmetrical structure, then... .

[0177] Based on the converter and the equivalent network of the power grid, such as Figure 4 As shown, the circuit coupling relationship between the converter side and the grid side is established, and the positive and negative sequence impedances of the grid-connected converter considering frequency coupling are obtained as follows:

[0178] ;

[0179] The following are the main parameters of the circuit in this embodiment:

[0180] Table 1

[0181]

[0182] To evaluate the reliability of the proposed impedance modeling method, a grid-type converter simulation model was constructed in the MATLAB / Simulink environment. By performing a frequency scan of the system, impedance data at different frequency points were obtained, and these data were fitted and compared with the theoretically derived analytical expressions to verify the correctness of the model.

[0183] Based on the established grid-connected converter simulation model, frequency scanning is performed, and the actual system parameters are substituted into the theoretical model. This yields analytical results for the positive and negative sequence impedances of the converter, along with corresponding Bode plots for frequency sweep verification. Figure 5 and Figure 6 As shown, where Figure 5 (a) is the Bode plot for verifying the positive sequence impedance of the grid converter of the present invention. Figure 5 (b) is the Bode plot for the positive-sequence frequency sweep verification of the grid converter of the present invention; Figure 6 (a) is the Bode plot for verifying the negative sequence impedance of the grid-type converter of this invention; Figure 6 (b) shows the Bode plot for the negative-sequence sweep frequency verification of the grid converter in this invention. Comparative analysis shows that the impedance curve calculated theoretically has good consistency with the impedance data obtained from the simulated sweep frequency, indicating that the constructed sequence impedance model can effectively characterize the impedance characteristics of the grid converter, thus verifying the correctness and reliability of the modeling method in this paper.

[0184] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any modifications or equivalent changes made based on the technical essence of the present invention shall still fall within the scope of protection claimed by the present invention.

Claims

1. A general sequence impedance modeling method for grid-type converters considering frequency coupling effects, characterized in that, Includes the following steps: Step S1: Inject positive and negative sequence small signal voltage disturbances at the point of common coupling of the grid-connected system with the grid-connected converter to obtain the time domain expressions of the three-phase voltage and current at the point of common coupling, and obtain the frequency domain expressions of the three-phase voltage and current through Fourier transform. Step S2: Based on the main circuit topology formed by the grid-connected converter and the grid connection and the average model of the converter, obtain the relationship equation between the converter internal potential, the voltage and current at the common coupling point, and the relationship equation between the converter internal potential and the modulation signal, and derive the relationship equation between the modulation signal and the disturbance voltage and current. Step S3: Based on the small-signal phase angle generated by the phase-locked loop after the small-signal voltage disturbance, derive the frequency domain expression of the small-signal phase angle based on the mathematical model of the phase-locked loop; Step S4: Based on the control structure of the grid-connected converter, ignoring the outer control loop, derive the frequency domain expression of the modulation signal output of the inner control loop current loop; Step S5: Based on the relationship equation between the modulation signal and the disturbance voltage and current, derive the admittance matrix of the converter when the outer control loop is ignored, and obtain the relationship matrices of the phase-locked loop and the inner control loop regarding the influence of voltage disturbance on the output voltage. Step S6: Considering the outer control loop, based on the instantaneous power theory, derive the frequency expression of the output power, obtain the frequency domain expression of the dq axis reference current value, substitute it into the inner control loop, and obtain the relationship matrix of the outer control loop regarding the influence of voltage disturbance on the output voltage; Step S7: Based on the relationship matrix of the phase-locked loop, the inner control loop, and the outer control loop, obtain the multi-input multi-output frequency coupling admittance matrix of the grid converter; Step S8: Establish the grid-side sequence impedance matrix. Based on the circuit coupling relationship between the converter side and the grid side, transform the multi-input multi-output frequency coupling admittance model into a single-input single-output impedance model, thereby obtaining the positive and negative sequence impedances of the converter considering frequency coupling.

2. The general sequence impedance modeling method for grid-type converters considering frequency coupling effects according to claim 1, characterized in that, In step S1, after injecting positive and negative sequence small-signal voltage disturbances at the common connection point of the converter, the voltage sampled from phase A is... and current The time-domain expressions are as follows: ； ； in, , and These represent the voltage amplitudes under the fundamental frequency, positive-sequence disturbance signal, and coupled disturbance signal, respectively, with corresponding initial phase angles of 0°, 0°, and 0° under the disturbances, respectively. and , , and The current amplitudes under the fundamental frequency, positive-sequence disturbance signal, and coupled disturbance signal are respectively, and the initial phase angles under the corresponding disturbances are respectively. , and , , and These are the fundamental angular frequency, the disturbance angular frequency, and the coupling angular frequency, respectively, where t represents the time domain; The corresponding frequency domain expression obtained from the Fourier transform is as follows: ； ； in, , , , , , The subscript p indicates the component at the disturbance frequency, the subscript n indicates the component at the coupling frequency, f represents the frequency domain, j represents the imaginary unit, f1 represents the fundamental frequency, and f p f represents the perturbation frequency. n This represents the coupling frequency.

3. The general sequence impedance modeling method for grid-type converters considering frequency coupling effects according to claim 2, characterized in that, In step S2, for phase A, the relationship equation between the internal potential of the converter, the voltage at the common coupling point, and the current in the main circuit is as follows: ,in, Represents the internal potential of the converter. represents the voltage at point PCC, and s represents the frequency domain; Based on the converter's average model, the equation relating the converter's internal potential to the modulation signal is as follows: ； in, , The modulation coefficient, DC side voltage For delay function, Indicates the modulated signal; Combining the above equations, the relationship between the modulated signal and the voltage and current at the point of common connection is as follows: ； The equations relating the modulated signal to the disturbance voltage and current are as follows: ； Where F is a diagonal matrix with diagonal elements of sL, and E is the identity matrix. , These represent the modulation signals at the perturbation frequency and the coupling frequency, respectively.

4. The general sequence impedance modeling method for grid-type converters considering frequency coupling effects according to claim 3, characterized in that, In step S3, without considering small disturbance phase angles, the frequency domain expression of the dq-axis voltage is: Where dc represents DC frequency, This represents the frequency at fp-f1; ； in, and This represents the dq-axis voltage without considering small perturbation phase angles; Assume the frequency domain expression for the phase angle of the small perturbation is: ,in, , These represent the phase angles at the disturbance frequency and the coupling frequency, respectively. Considering small perturbation phase angles, the frequency domain expression for the dq-axis voltage is: ,in, represent ; Closed-loop transfer function of phase-locked loop for: ； in, , and These are the parameters of the phase-locked loop PI controller; The frequency domain expression for the small-signal phase angle is: ； in, , These represent the phase angles at the disturbance frequency and the coupling frequency, respectively.

5. The general sequence impedance modeling method for grid-type converters considering frequency coupling effects according to claim 4, characterized in that, In step S4, based on the control structure of the grid-connected converter, without considering the outer control loop, the frequency domain expression of the modulation signal output by the inner control loop current loop is derived as follows: ； in, and For an ideal modulated signal, and This is the reference current value for the dq axis. and This is the measured value of the dq-axis current. For current loop controller, These are the decoupling coefficients; Therefore, the frequency domain expression of the modulated signal considering the small signal phase angle is obtained as follows: ； in, and This is the complete modulated signal along the dq axis.

6. The general sequence impedance modeling method for grid-type converters considering frequency coupling effects according to claim 5, characterized in that, The frequency domain expression of the modulation signal after the inverse Parker transform in step S5 is: ； in, This is the A-phase modulated signal. and Here are the frequency domain expressions for the sine and cosine functions; The equations relating the modulated signal to the disturbance voltage and disturbance current are as follows: ； The admittance matrix of the converter is not considered when the outer control loop is not considered. for: ； Where S is the phase-locked loop's relationship matrix regarding the effect of voltage disturbance on the output voltage, and the specific elements in the matrix are shown below: ； Where C1 represents the modulation signal at the fundamental frequency, C1 * Let I1 be the conjugate of C1. * It is the conjugate complex number of I1; To control the inner loop's relationship matrix regarding the impact of voltage disturbances on the converter output voltage, the specific elements of the matrix are shown below: ； 。 7. A general sequence impedance modeling method for grid-type converters considering frequency coupling effects according to claim 6, characterized in that, In step S6, when considering the control outer loop, the active power is established. and reactive power The frequency domain expression is: V1 * Let V1 be the conjugate complex number; The frequency domain expression for the dq-axis reference current value is: ； in, and It is the power command value. For the outer loop controller, , Both represent the frequency domain expression of the output power; The dq-axis reference current value is substituted into the inner control loop. After propagation through subsequent stages, the relationship equation between the converter's internal potential and the disturbance voltage and current under the action of the outer control loop is obtained as follows: ； in, and To control the relationship matrix of the outer loop regarding the effect of voltage disturbance on the output voltage, and The modulation signals added to the outer loop are controlled at the disturbance frequency and the coupling frequency, respectively. The specific elements of the matrix are shown below: ； ； 。 8. A general sequence impedance modeling method for grid-type converters considering frequency coupling effects according to claim 7, characterized in that, In step S7, based on the relationship matrix of the phase-locked loop, the inner control loop, and the outer control loop, the relationship equation between the modulation signal and the disturbance voltage and current is obtained as follows: ； The equation relating the internal potential of the converter to the disturbance voltage and current is as follows: ； Frequency coupling admittance matrix of mesh converter for: ； and These are the positive-order and negative-order self-admittances, respectively. and These are the positive-order and negative-order coupled mutual admittances, respectively.

9. A general sequence impedance modeling method for grid-type converters considering frequency coupling effects according to claim 8, characterized in that, In step S8, the positive and negative sequence impedance matrix of the network side is established. for: ； ； in, , For the grid resistance and grid inductance, , For filter resistors and filter capacitors, Parallel operation representing impedance; Since a normal power grid has a three-phase symmetrical structure, ; Based on the circuit coupling relationship between the converter side and the grid side, the positive and negative sequence impedances of the grid-connected converter considering frequency coupling are obtained. and for: 。

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