An impedance measurement method for a new energy grid-connected system

Abstract

The present invention discloses an impedance measurement method for a new energy grid-connected system. The method corrects the impedance of the grid-connected system obtained based on the output phase of the phase-locked loop according to the relationship between the output phase of the phase-locked loop and the actual phase of the grid-connected point, which can eliminate the influence of the dynamic characteristics of the phase-locked loop on the impedance measurement. The corrected impedance is more accurate, and the selection requirements of the phase-locked loop parameters can also be reduced. The cut-off frequency can be higher, thereby improving the speed of impedance measurement. The impedance measurement method for the new energy grid-connected system of the present invention can obtain more accurate impedance frequency characteristics, which is conducive to further research such as wind turbine equivalent or stability analysis.

Description

Technical field:

[0001] The invention belongs to the field of power system stability analysis, and in particular relates to an impedance measurement method for a new energy grid-connected system. Background technology:

[0002] With the exhaustion of fossil energy, new energy has become the best choice for adjusting the energy structure and achieving sustainable development. When new energy stations are connected to the grid through the grid connection point, it often causes oscillation instability in the broadband of the entire grid-connected system, which seriously threatens the safe and stable operation of the grid. For the oscillation problem of the grid-connected system, the impedance analysis method is generally used to analyze the stability of the grid-connected system.

[0003] The impedance analysis method has a clear physical meaning and is the most widely used and effective stability analysis method. The impedance analysis method obtains the small signal impedance frequency characteristics of the system through the impedance measurement method, and then studies the small disturbance stability of the system through stability criteria such as Nyquist criterion, norm criterion, Bode diagram criterion and impedance determinant zero point criterion. Among them, the impedance measurement method is to inject a small disturbance into the grid-connected point and calculate its impedance characteristics based on the measured small disturbance response. In the process of injecting disturbances and measuring responses, there will be a dq-abc or abc-dq coordinate transformation process, and its phase angle is often obtained from the grid-connected point using a phase-locked loop. However, the phase-locked loop itself also has dynamic characteristics, which will cause errors in the impedance measurement results. Although the error can be reduced by limiting the parameters of the phase-locked loop, it will affect its dynamic performance, which is not conducive to the efficiency of impedance measurement. Summary of the invention:

[0004] The purpose of the present invention is to provide an impedance measurement method for a new energy grid-connected system. The impedance measurement method of the present invention takes into account the dynamic characteristics of a phase-locked loop during the impedance measurement process, eliminates the error in the impedance measurement result caused by the introduction of the phase-locked loop, thereby obtaining the true impedance of the grid-connected system, which is conducive to a more accurate application of the impedance characteristics of the grid-connected system.

[0005] In order to achieve the above object, the technical solution of the present invention is:

[0006] A method for measuring impedance of a new energy grid-connected system, wherein a new energy station in the new energy grid-connected system is connected to a power grid via a grid connection point, comprising:

[0007] A phase-locked loop is used to phase-lock the three-phase voltage at the grid connection point to obtain the phase of the phase-locked loop output; a small signal disturbance response is obtained according to the phase-locked loop output phase; the impedance of the new energy grid-connected system is calculated according to the small signal disturbance response; based on the relationship between the phase of the phase-locked loop output phase and the actual phase of the grid connection point, the impedance is corrected to obtain the actual impedance of the new energy grid-connected system.

[0008] Furthermore, the method for obtaining the small signal disturbance response comprises the following steps:

[0009] Generate a disturbance signal: generate a first disturbance signal and a second disturbance signal in a dq coordinate system, wherein the first disturbance signal keeps the disturbance at 0 on the q axis and keeps a sinusoidal voltage disturbance of a set frequency on the d axis; the second disturbance signal keeps the disturbance at 0 on the d axis and keeps a sinusoidal voltage disturbance of a set frequency on the q axis;

[0010] Performing coordinate transformation on the disturbance signal: performing coordinate transformation on the first and second disturbance signals respectively according to the phase-locked output phase to obtain the first and second disturbance signals in the abc coordinate system;

[0011] Perform disturbance injection: for the first disturbance injection, inject the first disturbance signal in series into the grid connection point through the controlled voltage source, and measure the voltage and current responses of the grid connection point during the first disturbance injection; for the second disturbance injection, inject the second disturbance signal in series into the grid connection point through the controlled voltage source, and measure the voltage and current responses of the grid connection point during the second disturbance injection;

[0012] Perform inverse coordinate transformation on the disturbance signal: According to the phase-locked output phase, perform inverse coordinate transformation on the voltage and current responses of the grid connection point when the first and second disturbances are injected, respectively, and obtain the voltage and current responses of the grid connection point when the first and second disturbances are injected in the dq coordinate system.

[0013] Calculate the small signal disturbance response: subtract the steady-state voltage component and the steady-state current component from the voltage and current responses of the grid-connected point during the first and second disturbance injections in the dq coordinate system, respectively, to obtain the small signal disturbance response of the grid-connected point during the first and second disturbance injections.

[0014] Furthermore, the expression of the first disturbance signal is:

[0015]

[0016] The expression of the second disturbance signal is:

[0017]

[0018] In the formula, is the d-axis component of the first disturbance signal; is the q-axis component of the first disturbance signal; ampd is the amplitude of the d-axis component of the first disturbance signal, and ampd is 1-5% of the rated voltage amplitude of the grid connection point; f is the set frequency, which depends on the specific system; t is the time; is the d-axis component of the second disturbance signal; is the q-axis component of the second disturbance signal; ampq is the amplitude of the q-axis component of the second disturbance signal, and ampq is 1-5% of the rated voltage amplitude of the grid connection point.

[0019] Furthermore, the specific method of performing coordinate transformation on the disturbance signal is:

[0020]

[0021]

[0022] In the formula, θ pll The output phase is phase-locked; and are the a-phase, b-phase and c-phase components of the first disturbance signal respectively; and They are respectively the a-phase, b-phase and c-phase components of the second disturbance signal.

[0023] Furthermore, the specific method of performing inverse coordinate transformation on the disturbance signal is:

[0024]

[0025]

[0026]

[0027]

[0028] In the formula, They are the d, q, and 0-axis voltage responses of the grid connection point when the first disturbance is injected; They are the d, q, and 0-axis current responses of the grid connection point when the first disturbance is injected; They are the voltage responses of phase a, phase b, and phase c at the grid connection point when the first disturbance is injected, They are the current responses of phase a, phase b and phase c of the grid connection point when the first disturbance is injected;

[0029] They are the d, q, and 0-axis voltage responses of the grid connection point when the second disturbance is injected; They are the d, q and 0 axis current responses of the grid connection point when the second disturbance is injected; They are the voltage responses of phase a, phase b, and phase c at the grid connection point when the second disturbance is injected, They are the current responses of phase a, phase b and phase c of the grid-connected point during the second disturbance injection.

[0030] Furthermore, the specific method for calculating the small signal disturbance response is:

[0031]

[0032]

[0033]

[0034]

[0035] In the formula, They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the first disturbance is injected;

[0036] They are the d-axis and q-axis components of the small signal current response of the grid connection point when the first disturbance is injected;

[0037] They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the second disturbance is injected;

[0038] They are the d-axis and q-axis components of the small signal current response of the grid connection point when the second disturbance is injected;

[0039] U d0 , U q0 They are the d-axis and q-axis components of the steady-state voltage at the grid connection point when no disturbance is injected;

[0040] I d0 ,I q0 They are the d-axis and q-axis components of the steady-state current at the grid-connected point when no disturbance is injected.

[0041] Furthermore, the impedance of the new energy grid-connected system is calculated as follows:

[0042]

[0043] In the formula, is the impedance of the new energy grid-connected system.

[0044] Furthermore, the specific method of correcting the impedance to obtain the actual impedance of the new energy grid-connected system includes:

[0045] Assume that the phase difference between the actual phase of the grid-connected point and the phase of the phase-locked loop output during the first and second disturbance injections are Δθ1 and Δθ2 respectively. Then, in the dq coordinate system, the relationship between the actual voltage of the grid-connected point and the first and second disturbance signals is:

[0046]

[0047]

[0048] In the formula, u d1 is the d-axis component of the actual voltage at the grid connection point when the first disturbance is injected; u q1 is the q-axis component of the actual voltage at the grid connection point when the first disturbance is injected; u d2 is the d-axis component of the actual voltage at the grid connection point when the second disturbance is injected; u q2 is the q-axis component of the actual voltage at the grid connection point when the second disturbance is injected;

[0049] is the d-axis component of the first disturbance signal; is the q-axis component of the first disturbance signal; is the d-axis component of the second disturbance signal; is the q-axis component of the second disturbance signal;

[0050] By performing small signal linearization on the relationship between the actual voltage at the grid connection point and the first and second disturbance signals, it can be obtained that:

[0051]

[0052]

[0053] In the formula, They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the first disturbance is injected;

[0054] They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the second disturbance is injected;

[0055] Δu d1 , Δu q1 They are the d-axis and q-axis components of the actual small signal voltage response of the grid connection point when the first disturbance is injected;

[0056] Δu d2 , Δu q2 They are the d-axis and q-axis components of the actual small signal voltage response of the grid connection point when the second disturbance is injected;

[0057] In the dq coordinate system, the small signal voltage and current response of the grid connection point during disturbance injection and the actual small signal voltage and current response of the grid connection point satisfy the following relationship:

[0058]

[0059]

[0060] in,

[0061] In the formula, K p , K iare the proportional and integral coefficients in the phase-locked loop PI controller, ω0 is the rated angular frequency of the power grid; Δθ pll is the phase disturbance of the phase-locked loop output;

[0062] According to the above formula, the phase-locked loop output phase disturbance Δθ pll About the q-axis component Δu of the actual small signal voltage at the grid connection point q The transfer function is:

[0063]

[0064] In the formula, G pll (s) is the transfer function of the phase-locked loop;

[0065] Then the relationship between the actual small signal voltage at the grid connection point and the small signal voltage at the grid connection point during disturbance injection can be transformed into:

[0066]

[0067] Similarly, the relationship between the actual small signal current at the grid-connected point and the small signal current at the grid-connected point during disturbance injection can be transformed into:

[0068]

[0069] It can be obtained that in the dq coordinate system, the relationship between the actual impedance of the new energy grid-connected system and the grid-connected point impedance obtained according to the phase of the phase-locked loop output is:

[0070]

[0071] By transforming the above formula, the actual impedance of the renewable energy grid-connected system can be obtained as:

[0072]

[0073] Compared with the prior art, the present invention has the following beneficial effects

[0074] The impedance measurement method of the new energy grid-connected system of the present invention corrects the grid-connected system impedance obtained based on the output phase of the phase-locked loop according to the relationship between the output phase of the phase-locked loop and the actual phase of the grid-connected point, which can eliminate the influence of the dynamic characteristics of the phase-locked loop on the impedance measurement. The corrected impedance is more accurate, and the selection requirements of the phase-locked loop parameters can also be reduced. The cut-off frequency can be higher, thereby improving the speed of impedance measurement;

[0075] The impedance measurement method of the new energy grid-connected system of the present invention can obtain more accurate impedance frequency characteristics, which is conducive to further research such as wind turbine equivalence or stability analysis. Description of the drawings:

[0076] The above and other features of the present invention will become more apparent from the following detailed description of preferred non-limiting embodiments of the present invention with reference to the accompanying drawings, in which:

[0077] Figure 1 is a flow chart of the impedance measurement method of the present invention;

[0078] Figure 2 Schematic diagram of voltage source disturbance injection;

[0079] Figure 3 It is a schematic block diagram of a phase-locked loop control system;

[0080] Figure 4 This is a schematic diagram of the equivalent model of a single-unit infinite system of a direct-drive wind turbine;

[0081] Figure 5(a) shows the Y before and after correction dd Comparison of the real and imaginary characteristic curves of (s) with the exact value.

[0082] Figure 5(b) shows the Y before and after correction qq Comparison of the real and imaginary characteristic curves of (s) with the exact value. Specific implementation method:

[0083] The present invention will be further described below in conjunction with specific embodiments and corresponding drawings.

[0084] Embodiment 1:

[0085] An impedance measurement method for a new energy grid-connected system, in which a new energy station is connected to a power grid via a grid connection point, comprises:

[0086] The three-phase voltage at the grid connection point is phase-locked using a phase-locked loop to obtain the phase of the phase-locked loop output; the small signal disturbance response is obtained based on the phase-locked loop output phase; the impedance of the new energy grid-connected system is calculated based on the small signal disturbance response; based on the relationship between the phase of the phase-locked loop output and the actual phase of the grid connection point, the impedance is corrected to obtain the actual impedance of the new energy grid-connected system.

[0087] In the above steps, the method for obtaining the small signal disturbance response includes the following steps:

[0088] Generate a disturbance signal: Generate the first and second disturbance signals in the dq coordinate system, wherein the first disturbance signal keeps the disturbance at 0 on the q axis and keeps the sinusoidal voltage disturbance of the set frequency on the d axis; the second disturbance signal keeps the disturbance at 0 on the d axis and keeps the sinusoidal voltage disturbance of the set frequency on the q axis; wherein the expression of the first disturbance signal is:

[0089]

[0090] The expression of the second disturbance signal is:

[0091]

[0092] In the formula, is the d-axis component of the first disturbance signal; is the q-axis component of the first disturbance signal; ampd is the amplitude of the d-axis component of the first disturbance signal, and ampd is 1-5% of the rated voltage amplitude of the grid connection point; f is the set frequency, which depends on the specific system; t is the time; is the d-axis component of the second disturbance signal; is the q-axis component of the second disturbance signal; ampq is the amplitude of the q-axis component of the sinusoidal voltage disturbance, and ampq is 1-5% of the rated voltage amplitude of the grid-connected point.

[0093] Coordinate transformation of the disturbance signal: coordinate transformation of the first and second disturbance signals is performed respectively according to the phase of the phase-locked output to obtain the first and second disturbance signals in the abc coordinate system; the specific method of coordinate transformation of the disturbance signal is:

[0094]

[0095]

[0096] In the formula, θ pll The output phase is phase-locked; and are the a-phase, b-phase and c-phase components of the first disturbance signal respectively; and They are respectively the a-phase, b-phase and c-phase components of the second disturbance signal.

[0097] like Figure 2 As shown, disturbance injection is performed, where Δi a , Δi b and Δi c is the small signal three-phase current response, Δu a , Δu b and Δu c It is a small signal three-phase voltage response: for the first disturbance injection, the first disturbance signal is injected into the grid connection point in series through the controlled voltage source, and the voltage and current responses of the grid connection point during the first disturbance injection are measured; for the second disturbance injection, the second disturbance signal is injected into the grid connection point in series through the controlled voltage source, and the voltage and current responses of the grid connection point during the second disturbance injection are measured;

[0098] Perform inverse coordinate transformation on the disturbance signal: According to the phase-locked output phase, perform inverse coordinate transformation on the voltage and current responses of the grid connection point during the first and second disturbance injections, respectively, and obtain the voltage and current responses of the grid connection point during the first and second disturbance injections in the dq coordinate system; the specific method for performing inverse coordinate transformation on the disturbance signal is:

[0099]

[0100]

[0101]

[0102]

[0103] In the formula, They are the d, q, and 0-axis voltage responses of the grid connection point when the first disturbance is injected; They are the d, q, and 0-axis current responses of the grid connection point when the first disturbance is injected; They are the voltage responses of phase a, phase b, and phase c at the grid connection point when the first disturbance is injected, They are the current responses of phase a, phase b and phase c of the grid connection point when the first disturbance is injected;

[0104] They are the d, q, and 0-axis voltage responses of the grid connection point when the second disturbance is injected; They are the d, q and 0 axis current responses of the grid connection point when the second disturbance is injected; They are the voltage responses of phase a, phase b, and phase c at the grid connection point when the second disturbance is injected, They are the current responses of phase a, phase b and phase c of the grid-connected point during the second disturbance injection.

[0105] Calculate the small signal disturbance response: subtract the steady-state voltage component and the steady-state current component from the voltage and current responses of the grid connection point when the first and second disturbances are injected in the dq coordinate system, and obtain the small signal disturbance response of the grid connection point when the first and second disturbances are injected. The specific method is:

[0106]

[0107]

[0108]

[0109]

[0110] In the formula, They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the first disturbance is injected;

[0111] They are the d-axis and q-axis components of the small signal current response of the grid connection point when the first disturbance is injected;

[0112] They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the second disturbance is injected;

[0113] They are the d-axis and q-axis components of the small signal current response of the grid connection point when the second disturbance is injected;

[0114] U d0 , U q0 They are the d-axis and q-axis components of the steady-state voltage at the grid connection point when no disturbance is injected;

[0115] I d0 ,I q0 They are the d-axis and q-axis components of the steady-state current at the grid-connected point when no disturbance is injected.

[0116] The calculation method of the impedance of the new energy grid-connected system in the above steps is as follows:

[0117]

[0118] In the formula, is the impedance of the new energy grid-connected system; is the dd-axis component of impedance; is the dq axis component of impedance; is the qd-axis component of impedance; is the qq-axis component of impedance.

[0119] The specific method for correcting the impedance in the above steps to obtain the actual impedance of the new energy grid-connected system includes:

[0120] Assume that the phase difference between the actual phase of the grid-connected point and the phase of the phase-locked loop output during the first and second disturbance injections are Δθ1 and Δθ2 respectively. Then, in the dq coordinate system, the relationship between the actual voltage of the grid-connected point and the first and second disturbance signals is:

[0121]

[0122]

[0123] In the formula, u d1 is the d-axis component of the actual voltage at the grid connection point when the first disturbance is injected; u q1 is the q-axis component of the actual voltage at the grid connection point when the first disturbance is injected; u d2is the d-axis component of the actual voltage at the grid connection point when the second disturbance is injected; u q2 is the q-axis component of the actual voltage at the grid connection point when the second disturbance is injected;

[0124] is the d-axis component of the first disturbance signal; is the q-axis component of the first disturbance signal; is the d-axis component of the second disturbance signal; is the q-axis component of the second disturbance signal;

[0125] The relationship between the actual voltage at the grid connection point and the first and second disturbance signals is linearized by small signals to obtain:

[0126]

[0127]

[0128] In the formula, They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the first disturbance is injected;

[0129] They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the second disturbance is injected;

[0130] Δu d1 , Δu q1 They are the d-axis and q-axis components of the actual small signal voltage response of the grid connection point when the first disturbance is injected;

[0131] Δu d2 , Δu q2 They are the d-axis and q-axis components of the actual small signal voltage response of the grid connection point when the second disturbance is injected;

[0132] In the dq coordinate system, the small signal voltage and current response of the grid connection point during disturbance injection and the actual small signal voltage and current response of the grid connection point satisfy the following relationship:

[0133]

[0134]

[0135] Among them, Δθ pll is the phase-locked loop output phase disturbance.

[0136] The control block diagram of the phase-locked loop is as follows: Figure 2 As shown, where u a 、u b and u c is the three-phase voltage at the grid connection point, u a 、u a is the d-axis and q-axis components of the grid-connected point voltage, θpll is the phase-locked loop output phase, and its mathematical model is:

[0137]

[0138] In the formula, K p , K i are the proportional and integral coefficients in the phase-locked loop PI controller, respectively, and ω0 is the rated angular frequency of the power grid;

[0139] According to the above formula, the phase-locked loop output phase disturbance Δθ pll About the q-axis component Δu of the actual small signal voltage at the grid connection point q The transfer function is:

[0140]

[0141] In the formula, G pll (s) is the transfer function of the phase-locked loop;

[0142] Then the relationship between the actual small signal voltage at the grid connection point and the small signal voltage at the grid connection point during disturbance injection can be transformed into:

[0143]

[0144] Similarly, the relationship between the actual small signal current at the grid-connected point and the small signal current at the grid-connected point during disturbance injection can be transformed into:

[0145]

[0146] It can be obtained that in the dq coordinate system, the relationship between the actual impedance of the new energy grid-connected system and the grid-connected point impedance obtained according to the phase of the phase-locked loop output is:

[0147]

[0148] By transforming the above formula, the actual impedance of the renewable energy grid-connected system can be obtained as:

[0149]

[0150] Embodiment 2:

[0151] This embodiment uses the impedance measurement method of the present invention to simulate a direct-drive wind turbine single-machine grid-connected system and measure its admittance. Since impedance and admittance are inversely proportional to each other, admittance measurement is equivalent to impedance measurement. The infinite system equivalent model diagram of the direct-drive wind turbine single-machine grid-connected system is shown in Figure 2. Figure 4As shown in the figure, the specific structure of the grid-connected system includes a wind turbine (WindTurbine), a permanent magnet synchronous generator (PMSG), a machine-side converter (MSC), a DC link (DC Link), a grid-side converter (GSC) and an equivalent power grid. Among them, PCC is the common coupling point, i.e., the grid connection point, C dc is the DC capacitor, set to 0.04, R f is the filter resistor, set to 0.00035, L f is the filter inductor, set to 0.00035, R g is the grid-side equivalent resistance, set to 0, L g is the grid-side equivalent inductance, set to 0.00030.

[0152] In order to ensure the accuracy of the impedance measurement method of the present invention, this example first measures the admittance of the grid-connected system using a given phase of a triangular wave as the accurate value of the admittance of the grid-connected system; then measures the admittance using the output phase of a phase-locked loop, and obtains Among them, the amplitude ampd of the d-axis component of the first disturbance signal and the amplitude ampq of the q-axis component of the second disturbance signal are both set to 30V, and the frequency f is set to 1-200Hz; the cutoff frequency of the phase-locked loop is set to 10Hz, and the damping ratio is set to 1. The ratio K in the phase-locked loop PI controller can be calculated. p Set to 62, the integral coefficient K in the phase-locked loop PI controller i Set to 3820. After correction, we get Y dq (s).

[0153] This embodiment mainly considers the commonly used unity power factor control situation, and ignores the influence of the fan admittance coupling term at this time, so only focus on Y in the admittance matrix dd (s) and Y qq (s) Two items. Y before and after correction dd (s) and Y qq The comparison of the real and imaginary characteristic curves of (s) with the accurate value is shown in Figure 5(a) and Figure 5(b), respectively, where the horizontal axis is the frequency f and the vertical axis is Y dd (s) and Y qq The real and imaginary parts of (s), the "O" line type is the accurate value of the admittance, the "▽" line type is the admittance value obtained according to the phase of the phase-locked loop output, that is, the admittance value before PLL correction, and the "×" line type is the admittance value corrected by the method of the present invention, that is, the admittance value after PLL correction. According to Figure 5(b), it can be directly seen that the accurate value of the admittance represented by the "O" line type is basically the same as the admittance value corrected by the method of the present invention represented by the "×" line type. By analyzing Figure 5(a) and Figure (b), it can be seen that due to the error introduced by the phase-locked loop, the small signal output admittance measurement value Y of the systemp qq (s) and the exact value Y qq (s) is obviously different, and the admittance measurement value Y p dd (s) and the exact value Y dd (s) is almost the same, indicating that the phase-locked loop mainly affects the q-axis component of the output admittance. The frequency response characteristics of the corrected small-signal output admittance are almost consistent with the accurate value, eliminating the measurement error introduced by the phase-locked loop, verifying the correctness and effectiveness of the sufficient measurement method of the present invention.

[0154] The above embodiments are only for illustrating the technical idea of ​​the present invention, and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. An impedance measurement method for a new energy grid-connected system, wherein a new energy station in the new energy grid-connected system is connected to the power grid via a grid connection point, characterized in that: include: The three-phase voltage at the grid connection point is phase-locked using a phase-locked loop to obtain the phase of the phase-locked loop output; Acquiring a small signal disturbance response according to the phase-locked loop output phase; Calculating the impedance of the new energy grid-connected system according to the small signal disturbance response; Based on the relationship between the phase of the phase-locked loop output and the actual phase of the grid-connected point, the impedance is corrected to obtain the actual impedance of the new energy grid-connected system; The method for obtaining the small signal disturbance response comprises the following steps: Generate disturbance signals: generate first and second disturbance signals in a dq coordinate system; The first disturbance signal keeps the disturbance at 0 on the q-axis and keeps the sinusoidal voltage disturbance at the set frequency on the d-axis; the second disturbance signal keeps the disturbance at 0 on the d-axis and keeps the sinusoidal voltage disturbance at the set frequency on the q-axis; Performing coordinate transformation on the disturbance signal: performing coordinate transformation on the first and second disturbance signals respectively according to the phase output of the phase-locked loop to obtain the first and second disturbance signals in the abc coordinate system; Perform disturbance injection: for the first disturbance injection, inject the first disturbance signal in series into the grid connection point through the controlled voltage source, and measure the voltage and current responses of the grid connection point during the first disturbance injection; for the second disturbance injection, inject the second disturbance signal in series into the grid connection point through the controlled voltage source, and measure the voltage and current responses of the grid connection point during the second disturbance injection; Performing inverse coordinate transformation on the disturbance signal: performing inverse coordinate transformation on the voltage and current responses of the grid connection point when the first and second disturbances are injected, respectively, according to the phase-locked output phase, and obtaining the voltage and current responses of the grid connection point when the first and second disturbances are injected in the dq coordinate system; Calculating the small signal disturbance response: subtracting the steady-state voltage component and the steady-state current component from the voltage and current responses of the grid-connected point during the first and second disturbance injections in the dq coordinate system, respectively, to obtain the small signal disturbance response of the grid-connected point during the first and second disturbance injections; The expression of the first disturbance signal is: The expression of the second disturbance signal is: In the formula, is the d-axis component of the first disturbance signal; is the q-axis component of the first disturbance signal; ampd is the amplitude of the d-axis component of the first disturbance signal, and ampd is 1-5% of the rated voltage amplitude of the grid connection point; f is the set frequency, which depends on the specific system; t is the time; is the d-axis component of the second disturbance signal; is the q-axis component of the second disturbance signal; ampq is the amplitude of the q-axis component of the second disturbance signal, and ampq is 1 to 5% of the rated voltage amplitude of the grid connection point; The specific method for coordinate transformation of the disturbance signal is: In the formula, θ pll The output phase is phase-locked; and are the a-phase, b-phase and c-phase components of the first disturbance signal respectively; and are the a-phase, b-phase and c-phase components of the second disturbance signal respectively; The specific method of performing inverse coordinate transformation on the disturbance signal is: In the formula, They are the d, q, and 0-axis voltage responses of the grid connection point when the first disturbance is injected; They are the d, q, and 0-axis current responses of the grid connection point when the first disturbance is injected; They are the voltage responses of phase a, phase b, and phase c at the grid connection point when the first disturbance is injected, They are the current responses of phase a, phase b and phase c of the grid connection point when the first disturbance is injected; They are the d, q, and 0-axis voltage responses of the grid connection point when the second disturbance is injected; They are the d, q and 0 axis current responses of the grid connection point when the second disturbance is injected; They are the voltage responses of phase a, phase b, and phase c at the grid connection point when the second disturbance is injected, They are the current responses of phase a, phase b and phase c of the grid connection point when the second disturbance is injected; The specific method for calculating the small signal disturbance response is: In the formula, They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the first disturbance is injected; They are the d-axis and q-axis components of the small signal current response of the grid connection point when the first disturbance is injected; They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the second disturbance is injected; They are the d-axis and q-axis components of the small signal current response of the grid connection point when the second disturbance is injected; U d0 , U q0 They are the d-axis and q-axis components of the steady-state voltage at the grid connection point when no disturbance is injected; I d0 ,I q0 They are the d-axis and q-axis components of the steady-state current at the grid connection point when no disturbance is injected; The calculation method of the impedance of the new energy grid-connected system is as follows: In the formula, is the impedance of the new energy grid-connected system; The specific method of correcting the impedance to obtain the actual impedance of the new energy grid-connected system includes: Assume that the phase difference between the actual phase of the grid-connected point and the phase of the phase-locked loop output during the first and second disturbance injections are Δθ1 and Δθ2 respectively. Then, in the dq coordinate system, the relationship between the actual voltage of the grid-connected point and the first and second disturbance signals is: Where ud1 is the d-axis component of the actual voltage at the grid connection point when the first disturbance is injected; q1 is the q-axis component of the actual voltage at the grid connection point when the first disturbance is injected; u d2 is the d-axis component of the actual voltage at the grid connection point when the second disturbance is injected; u q2 is the q-axis component of the actual voltage at the grid connection point when the second disturbance is injected; is the d-axis component of the first disturbance signal; is the q-axis component of the first disturbance signal; is the d-axis component of the second disturbance signal; is the q-axis component of the second disturbance signal; By performing small signal linearization on the relationship between the actual voltage at the grid connection point and the first and second disturbance signals, it can be obtained that: In the formula, They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the first disturbance is injected; They are the d-axis and q-axis components of the small signal voltage response of the grid connection point when the second disturbance is injected; Δu d1 , Δu q1 They are the d-axis and q-axis components of the actual small signal voltage response of the grid connection point when the first disturbance is injected; Δu d2 , Δu q2 They are the d-axis and q-axis components of the actual small signal voltage response of the grid connection point when the second disturbance is injected; In the dq coordinate system, the small signal voltage and current response of the grid connection point during disturbance injection and the actual small signal voltage and current response of the grid connection point satisfy the following relationship: in, In the formula, K p , K i are the proportional and integral coefficients in the phase-locked loop PI controller, ω0 is the rated angular frequency of the power grid; Δθ pll is the phase disturbance of the phase-locked loop output; According to the above formula, the phase-locked loop output phase disturbance Δθ pll About the q-axis component Δu of the actual small signal voltage at the grid connection point q The transfer function is: In the formula, G pll (s) is the transfer function of the phase-locked loop; Then the relationship between the actual small signal voltage at the grid connection point and the small signal voltage at the grid connection point during disturbance injection can be transformed into: Similarly, the relationship between the actual small signal current at the grid-connected point and the small signal current at the grid-connected point during disturbance injection can be transformed into: It can be obtained that in the dq coordinate system, the relationship between the actual impedance of the new energy grid-connected system and the grid-connected point impedance obtained according to the phase of the phase-locked loop output is: By transforming the above formula, the actual impedance of the renewable energy grid-connected system can be obtained as:

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