S1, superimposing a positive sequence current disturbance in a current sampling value, and recording a first output voltage and current waveform of the converter;

S2, superimposing a positive sequence voltage disturbance in a voltage sampling value, and recording a second output voltage and current waveform of the converter;

S3, determining whether measurement data of the converter at two or more current operating points are obtained, when a result is no, waiting for a change of the two or more current operating points of the converter, repeating above measurement process, measuring impedance of the converter at new current operating points and recording waveform data, when the result is yes, proceeding S4;

S4, based on the measurement data of the converter at two current operating points, calculating values of expressions A_s(s) and B_s(s) at different frequency points;

S5, further calculating values of coefficients a₀, a₁, b₀, b₁ at each frequency point, and finally obtaining an impedance value of the converter at a plurality of current operating points by a calculation;

before the S1, the measurement method further comprises the following: a converter with an inductance capacitance inductance (LCL) filter is adopted, a controller is controlled by a single current loop, and a phase angle is generated by a phase-locked loop, a theoretical model of the positive sequence impedance on an alternating current (AC) side of the converter is as follows:

wherein s is a differential operator, L₁ and L₂ are filter inductance values, C_f is a filter capacitance value, R_d is a resistance value in series with a filter capacitance, G_i(s−j2πf₁) is a transfer function of a current loop controller of the converter, f₁ is a frequency of 50 Hz, I₁e^jφ_i1 is a current operating point of the converter, I₁ is a current amplitude, V₁ is a voltage operating point of the converter, H_pll(s−j2πf₁) is a transfer function of a phase-locked loop controller, K_f is a voltage feedforward coefficient, G_id is an output current sampling delay, G_vd is a sampling delay of a voltage at point of common coupling (PCC), and K_PWM is a voltage gain of the converter;

a formula (1) is simplified as:

in the S1, the positive sequence current disturbance Δi_t is superimposed on the current sampling value i_g1, and a calculation expression of the converter under a secondary-side current disturbance is:

wherein v_p1 is a voltage of the converter at a point of common coupling when the positive sequence current disturbance is superimposed on the current sampling value;

in the S2, the positive sequence voltage disturbance Δv_t is injected into the voltage sampling value v_p2, and a calculation expression of the converter under a secondary-side voltage disturbance is:

wherein i_g2 is an output current of the converter when the positive sequence voltage disturbance is superimposed on the voltage sampling value, and a value of G_c is obtained by solving simultaneous equations (5) and (7):

the S4 is as follows: according to a formula (6) and a formula (8), a value of A_s(s) is obtained:

a formula (9) is substituted into a formula (7), and a value of B_s(s) is obtained:

in the S5, since the voltage operating point of the converter generally remains unchanged or changes in a small range without major changes in a system operation, V₁ is considered to be basically unchanged under a predetermined stable operating condition, according to the formula (1) and a formula (2), A_s(s) and B_s(s) are expressed as a linear function of a current operating point I₁ of the converter, wherein

a relational expression between the impedance value of the converter and the plurality of current operating points is obtained by solving values of the coefficients a₀, a₁, b₀, b₁ under a disturbance frequency f_p, and then the impedance value of the converter at other current operating points is calculated by the relational expression without a repeated measurement at each operating point.