A Measurement and Correction Method for the Frequency Coupling Sequence Admittance of a Grid-Forming Inverter System

Abstract

The present invention discloses a method for measuring and correcting the frequency-coupled sequence admittance of a network-forming converter system. The method includes: injecting three-phase voltage disturbances with several different frequency values into the network-forming converter system to obtain the output voltage and current response data; obtaining the measurement result of the frequency-coupled sequence admittance according to the voltage and current response data; obtaining the compensation factor for phase correction; and using the compensation factor for phase correction to obtain the optimized sequence admittance result. The technical solution provided by the present invention realizes the impedance measurement of the grid-connected system by injecting disturbance signals at the point of common coupling, estimates the grid impedance through the collected impedance data, and corrects the phase of the measured frequency-coupled sequence admittance coupling component considering the influence of the grid impedance, thereby improving the measurement accuracy of the frequency-coupled sequence admittance of the network-forming converter grid-connected system, facilitating the accurate analysis of the stability of the network-forming converter grid-connected system, and thus optimizing the system scheduling and operation strategy.

Description

Technical Field

[0001] The present invention relates to the field of frequency-coupled sequence admittance measurement, and particularly to a method for measuring and correcting the frequency-coupled sequence admittance of a network-forming converter system. Background Art

[0002] In the context of the current global energy transition, new energy power generation equipment, such as wind and solar power generation, has increasingly become an important part of achieving sustainable development. Due to the use of asymmetric control structures in these power electronic devices, they often face the problem of frequency coupling during grid connection operation, which may lead to a decrease in system stability and power quality. Frequency-coupled sequence admittance refers to the admittance characteristics of a circuit or system to different frequency signals in the presence of frequency coupling effects. In new energy grid-connected power generation equipment, the modeling, measurement, and analysis of frequency-coupled sequence admittance are of great significance for ensuring the stable operation of the power grid.

[0003] In response to this problem, traditional measurement methods often ignore the influence of the large grid impedance on the phase of the coupled sequence admittance component current under a weak grid, thus affecting the measurement accuracy. Moreover, due to the unique structure and operating characteristics of network-forming new energy power generation equipment, it is more sensitive to frequency coupling, which further affects the safety and reliability of the overall power system. Since the influence of the grid impedance cannot be ignored in the power system connected to the network-forming grid-connected converter. Therefore, for the network-forming converter grid-connected system, a new measurement and correction method for frequency-coupled sequence admittance is needed to meet the requirements for measurement accuracy. Summary of the Invention

[0004] In view of the above deficiencies in the current technology, the present invention provides a method for measuring and correcting the frequency-coupled sequence admittance of a network-forming converter system. By using the method of correcting the phase of the frequency-coupled sequence admittance component, accurate measurement of the frequency-coupled sequence admittance of the network-forming grid-connected converter is achieved, which is beneficial to the accurate analysis of the stability of the network-forming converter grid-connected system.

[0005] To achieve the above object, the embodiments of the present invention adopt the following technical solutions:

[0006] Inject three-phase voltage disturbances with several different frequency values into the network-forming converter system to obtain the output voltage and current response data;

[0007] According to the voltage and current response data, obtain the measurement result of the frequency-coupled sequence admittance;

[0008] According to the characteristics of the network-forming converter system and the voltage and current response data, obtain the compensation factor for phase correction;

[0009] Use the compensation factor to perform phase correction on the sequence admittance measurement result to obtain an optimized sequence admittance result.

[0010] According to one aspect of the present invention, injecting three-phase voltage disturbances with several different frequency values into the grid-forming converter system and obtaining the output voltage and current response data includes:

[0011] Setting a fundamental frequency f1 and a frequency table including several frequency points;

[0012] Traversing the frequency points in the frequency table, and injecting three-phase voltage disturbances with a certain frequency value into the grid-forming converter system according to the frequency point value and the fundamental frequency, and obtaining the output voltage and current response data.

[0013] According to one aspect of the present invention, the setting of the fundamental frequency f1 and the frequency table including several frequency points further includes:

[0014] The frequency point values in the frequency table are set in ascending order.

[0015] According to one aspect of the present invention, injecting three-phase voltage disturbances with several different frequency values into the grid-forming converter system and obtaining the output voltage and current response data further includes:

[0016] Presetting a cut-off frequency;

[0017] When traversing the frequency points in the frequency table, when the frequency point is greater than or equal to the sweep cut-off frequency, end the measurement.

[0018] According to one aspect of the present invention, injecting three-phase voltage disturbances with a certain frequency value into the grid-forming converter system according to the frequency point value and the fundamental frequency and obtaining the output voltage and current response data includes:

[0019] When the frequency point value fp < 2f1, first inject a positive-sequence voltage disturbance with a frequency of fp, three-phase amplitudes of A1, B1, C1, and phases of 10, 240, 120 into the grid-forming converter system. After the system is stable, obtain the output voltage and current response data;

[0020] Then inject a positive-sequence voltage disturbance with a frequency of 2f1 - fp, three-phase amplitudes of A2, B2, C2, and phases of 10, 240, 120 into the grid-forming converter system. After the system is stable, obtain the output voltage and current response data.

[0021] According to one aspect of the present invention, injecting three-phase voltage disturbances with a certain frequency value into the grid-forming converter system according to the frequency point value and obtaining the output voltage and current response data includes:

[0022] When the frequency point value fp ≥ 2f1, first inject a positive-sequence voltage disturbance with a frequency of fp into the grid-forming converter system, where the three-phase amplitudes are A1, B1, and C1 respectively, and the phases are 0, 240, and 120 respectively. After the system stabilizes, obtain the output voltage and current response data;

[0023] Then inject a negative-sequence voltage disturbance with a frequency of fp - 2f1 into the grid-forming converter system, where the three-phase amplitudes are A2, B2, and C2 respectively, and the phases are 0, 240, and 120 respectively. After the system stabilizes, obtain the output voltage and current response data.

[0024] According to one aspect of the present invention, the fundamental frequency f1 is set to 50 Hz.

[0025] According to one aspect of the present invention, the obtaining of the phase correction compensation factor based on the characteristics of the grid-forming converter system and the voltage and current response data includes:

[0026] Estimate the grid impedance based on the voltage and current response data;

[0027] Introduce an adjustment coefficient according to the grid impedance and calculate the phase correction compensation factor.

[0028] According to one aspect of the present invention, the sequence admittance measurement results include: sequence admittance components Y 11 、Y 12 、Y 21 、Y 22 .

[0029] According to one aspect of the present invention, the measurement and correction method for the frequency-coupled sequence admittance of the grid-forming converter system further includes:

[0030] Plot the sequence admittance measurement results and the optimized sequence admittance results as curves to compare the effects before and after phase correction.

[0031] Advantages of the implementation of the present invention:

[0032] The present invention provides a measurement and correction method for the frequency-coupled sequence admittance of a grid-forming converter system, which realizes the impedance measurement of the grid-connected system by injecting a disturbance signal at the point of common coupling, estimates the grid impedance through the collected impedance data, corrects the phase of the measured frequency-coupled sequence admittance coupling component considering the influence of the grid impedance, thereby improving the measurement accuracy of the frequency-coupled sequence admittance of the grid-forming converter grid-connected system, facilitating the accurate analysis of the stability of the grid-forming converter grid-connected system, optimizing the system scheduling and operation strategies, and improving the access capacity of renewable energy. Brief Description of the Drawings

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the accompanying drawings required for use in the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0034] Figure 1 Flow chart of a method for measuring and correcting the frequency coupling sequence admittance of a network-forming converter system according to the present invention;

[0035] Figure 2 A new energy converter grid-connected system according to the present invention;

[0036] Figure 3 A network-forming converter control loop according to the present invention;

[0037] Figure 4 Equivalent schematic diagram of frequency coupling of a new energy grid-connected system according to the present invention;

[0038] Figure 5 Frequency coupling sequence admittance curve without phase correction according to the present invention;

[0039] Figure 6 Frequency coupling sequence admittance curve with phase correction according to the present invention. Specific embodiments

[0040] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0041] Embodiment 1

[0042] As Figure 1 shown, a method for measuring and correcting the frequency coupling sequence admittance of a network-forming converter system includes the following steps:

[0043] S1: Inject three-phase voltage disturbances with several different frequency values into the network-forming converter system to obtain the output voltage and current response data.

[0044] In practical applications, frequency sweep programs for frequency coupling sequence admittance can be developed using MATLAB, Simulink, or python to measure and analyze the frequency response characteristics of new energy grid-connected power generation equipment.

[0045] Specifically, the frequency sweep program mainly includes the following steps:

[0046] (1) According to the power grid system architecture, build a grid-forming converter system model, including a sequence impedance model considering frequency coupling.

[0047] (2) Inject a small disturbance signal: Inject a small disturbance signal into the model. Generally, a sine signal is used as the disturbance signal, and its frequency varies within a certain range to ensure that the required frequency response range can be covered.

[0048] (3) Sweep frequency simulation: Sweep the frequency of the disturbance signal and record the system current and voltage response data at each frequency.

[0049] (4) Data processing and plotting: Use the sweep frequency results to plot the Bode diagram of the sequence impedance considering frequency coupling, where the abscissa is the frequency and the ordinate is the magnitude and phase of the sequence impedance.

[0050] Figure 2 A grid-forming new energy converter grid-connected system provided by this method, where PCC is the point of common coupling between the new energy converter and the power grid, V dc is the DC side voltage, L f is the filter inductor, R d is the filter resistor, i abc are the A, B, and C phase currents sampled at PCC, v abc are the A, B, and C phase voltages sampled at PCC, C f is the filter capacitor, i gabc is the grid side current, L g is the grid side inductor, R g is the grid side resistor. Specifically, L g can be set to 20 mH, R g can be set to 0.2 Ω.

[0051] Figure 3 A grid-forming converter control loop adopted by this method, where P ref is the active power reference value, Q ref is the reactive power loop reference value, P e is the active power instantaneous value, Q e is the reactive power instantaneous value, , Dp is the active damping coefficient, J is the inertia coefficient, and K is the reactive loop droop coefficient. V ref is the given value of the reactive loop voltage amplitude. V m is the given value of the single-phase grid voltage amplitude. D q is the virtual coefficient of the reactive loop. θ GFM is the output phase angle of the active loop. E m is the output voltage amplitude of the reactive loop. The synchronous modulation waves ma, mb, and mc can be expressed as:

[0052]

[0053] In the formula, K f = 2 / V dc .

[0054] Specifically, step S1 includes:

[0055] S11: Set the fundamental frequency f1 and a frequency table containing several frequency points.

[0056] In practical applications, the frequency sweep program can be initialized. Set the fundamental frequency f1 to 50 Hz. The frequency table contains 35 frequency points, which are set to [10, 12, 15, 18, 22, 27, 33, 40, 60, 72, 88, 107, 130, 158, 193, 235, 287, 349, 425, 518, 631, 769, 936, 1141, 1389, 1693, 2062, 2512, 3060, 3728, 4541, 5532, 6739, 8209, 10000].

[0057] In addition, the frequency sweep program also needs to set parameters such as the communication port and the measurement storage address.

[0058] S12: Traverse the frequency points in the frequency table. According to the frequency point value and the fundamental frequency, inject a three-phase voltage disturbance with a certain frequency value into the grid-forming converter system, and obtain the output voltage and current response data.

[0059] Specifically, there are two cases when the frequency sweep program measures:

[0060] 1. When the frequency point value fp < 2f1, that is, when fp < 100 Hz:

[0061] (1) First, inject a positive-sequence voltage disturbance with a frequency of fp, three-phase amplitudes of A1, B1, and C1, and phases of 10, 240, and 120 respectively into the grid-forming converter system. After the system stabilizes, obtain the output voltage and current response data:

[0062]

[0063]

[0064]

[0065]

[0066] (2) Then, inject a positive-sequence voltage disturbance with a frequency of 2f1 - fp, three-phase amplitudes of A2, B2, and C2, and phases of 10, 240, and 120 respectively into the grid-forming converter system. After the system stabilizes, obtain the output voltage and current response data:

[0067]

[0068]

[0069]

[0070]

[0071] Among them, t 0' and t 0" represent the first sampling time and the second sampling time respectively; V is voltage, I is current; φ is phase angle.

[0072] 2. When the frequency point value fp ≥ 2f1, that is, when fp ≥ 100 Hz:

[0073] (1) First, inject a positive-sequence voltage disturbance with a frequency of fp, three-phase amplitudes of A1, B1, and C1, and phases of 0, 240, and 120 respectively into the grid-forming converter system. After the system stabilizes, obtain the output voltage and current response data:

[0074]

[0075]

[0076]

[0077]

[0078]

[0079] (2) Then, inject a negative-sequence voltage disturbance with a frequency of fp - 2f1 into the grid-forming converter system, where the three-phase amplitudes are A2, B2, and C2 respectively, and the phases are 0, 240, and 120 respectively. After the system stabilizes, obtain the output voltage and current response data:

[0080]

[0081]

[0082]

[0083]

[0084] S2: According to the voltage and current response data, obtain the measurement results of the sequence admittance with frequency coupling.

[0085] Admittance is a general term for conductance and susceptance, used to describe the ease of alternating current passing through a circuit or system. Admittance is the reciprocal of impedance, denoted by Y, and the unit is Siemens (S). Admittance is a complex number, consisting of a real part (conductance) and an imaginary part (susceptance).

[0086] When calculating the sequence admittance results with frequency coupling of the grid-forming converter system, the sequence admittance measurement results include: sequence admittance components Y 11 、Y 12 、Y 21 、Y 22 , and the calculation formula is as follows:

[0087]

[0088] S3: According to the characteristics of the grid-forming converter system and the voltage and current response data, obtain the compensation factor for phase correction.

[0089] Step S3 includes:

[0090] (1) Estimate the grid impedance according to the voltage and current response data.

[0091] Assume that a total of n frequency disturbances are injected, that is, n sequence admittance measurements are performed. Then, the estimated value of the grid impedance Zg can be expressed as:

[0092]

[0093] Where, V pi ' and I gpi ' are the positive-sequence voltage and positive-sequence current measured after the i-th injection of disturbance in step S2 respectively, fpi is the disturbance source frequency for the i-th injection.

[0094] (2) According to the grid impedance, introduce an adjustment coefficient to calculate the compensation factor for phase correction.

[0095] Figure 4 is the equivalent schematic diagram of frequency coupling in the new energy grid-connected system. After the system injects a disturbance source V p ( f p ), two response excitations will be generated through the converter control loop e '( f p ) and e '( f p - 2 f 1). Under the action of the two response excitations, coupled excitations e "( f p - 2 f 1) and e "( f p ) are respectively generated. Therefore, the disturbance frequencies collected in the actual process are the current components of fp and the current components of fp - 2f1, which include both the current components generated by the response excitation acting on the converter impedance and the grid impedance, and the current components under the action of the coupled response excitation. In this process, the influence of the grid impedance on the phase of the coupled component, especially the current component, cannot be ignored. The coupled current component contains the influence of the corresponding grid impedance. From engineering experience, it can be obtained that the impedance of the converter in the high-frequency band is much larger than the grid impedance. Therefore, the influence of the grid on the converter is mainly reflected in the low-frequency band. Taking the voltage component as the reference quantity, this method reduces the influence in the coupled component to the phase of the coupled current component and introduces a compensation factor λ to achieve phase correction of the frequency coupling measurement sequence admittance.

[0096] The calculation formula for the compensation factor λ is:

[0097]

[0098] In the formula, Z GFM ( f p ) = ; α is the adjustment coefficient, and its value range is 0.6 to 1; is the grid impedance angle; angle( ) represents the phase of the vector in the parentheses; | | represents the magnitude of the vector.

[0099] S4: Use the compensation factor to perform phase correction on the sequence admittance measurement results to obtain optimized sequence admittance results.

[0100] Specifically, the result after sequence admittance phase correction is expressed as follows:

[0101]

[0102] Preferably, this method further includes:

[0103] S5: Plot the sequence admittance measurement results and the optimized sequence admittance results as curves to compare the effects before and after phase correction.

[0104] Use this method to measure the grid-forming new energy converter grid-connected system described above, plot the obtained frequency coupling sequence admittance results as curves. The curve without phase correction is as Figure 5 shown, and the curve after adding phase correction is as Figure 6 shown.

[0105] Figure 5 In Figure 6 , the solid line part represents the theoretically derived frequency coupling sequence admittance curve of the grid-forming new energy converter. The formula in the complex frequency domain of the curve is as follows:

[0106]

[0107] In the formula, N, M, T, W, N l , M l , T l , W l are expressed as follows:

[0108]

[0109] In the formula, V1 = V1 * = V 1 / 2; I1 = ( I 1 / 2)e ±jφi1 ; I1 * = ( I 1 / 2)e ∓jφi1 ; V 1 = V ref ; I 1 = P ref / (1.5 * V ref ); φ i1 is the phase of the fundamental current, and its value is obtained by simulation measurement; s 1 = 2π f p , s2 = 2π f p -4π f 1, s 0 = 2π f p -2π f 1; PWM modulation coefficient K pwm = 0.5; T ( s ) = 1 / s * ( J * s + D p );Delay function G del ( s ) = e -1.5Tss , T s is the sampling time step; s is the Laplace operator; j is the imaginary unit of a complex number; D q is the reactive power loop damping coefficient.

[0110] Figure 5 and Figure 6 In, the "*" part represents the numerical values of the sequence admittance at the disturbance frequencies fp and fp - 2f1 measured by the frequency sweep program. It can be seen from Figure 5 that for the coupling components Y 12 and Y 21 of the grid-forming frequency coupling sequence admittance without phase correction, there are large errors between the measured phase values and the theoretical curve at frequencies f < 100 Hz. Under the same simulation conditions, the measured values of the grid-forming frequency coupling sequence admittance after phase correction are as shown in Figure 6 , and the measured values are in good agreement with the theoretical values, verifying the effectiveness of this method.

[0111] The beneficial effects of this embodiment are as follows:

[0112] This method considers that the grid-forming new energy converter needs to be connected to a weak grid to achieve stable operation, and the grid impedance cannot be ignored, so its influence on the frequency coupling component cannot be ignored.

[0113] This method uses the method of injecting disturbance signals at the point of common coupling to measure the impedance of the grid-connected system, estimates the grid impedance through the collected impedance data; considers the influence of the grid impedance to correct the phase of the measured frequency coupling sequence admittance coupling component, thereby improving the measurement accuracy of the frequency coupling sequence admittance of the grid-forming converter grid-connected system, facilitating the accurate analysis of the stability of the grid-forming converter grid-connected system, optimizing the system scheduling and operation strategies, improving the access capacity of renewable energy, and promoting the further development of new energy power generation technology.

[0114] Example 2

[0115] As Figure 1 shown, a method for measuring and correcting the frequency-coupled sequence admittance of a network-forming converter system includes the following steps:

[0116] S1: Inject three-phase voltage disturbances with several different frequency values into the network-forming converter system to obtain the output voltage and current response data.

[0117] In practical applications, frequency-sweeping programs for frequency-coupled sequence admittance can be developed using MATLAB, Simulink, or python to measure and analyze the frequency response characteristics of new energy grid-connected power generation equipment.

[0118] Specifically, the frequency-sweeping program mainly includes the following steps:

[0119] (1) According to the power grid system architecture, build a network-forming converter system model, including a sequence impedance model considering frequency coupling.

[0120] (2) Inject a small disturbance signal: Inject a small disturbance signal into the model. Generally, a sine signal is used as the disturbance signal, and the frequency varies within a certain range to ensure that the required frequency response range can be covered.

[0121] (3) Frequency-sweeping simulation: Sweep the frequency of the disturbance signal and record the system current and voltage response data at each frequency.

[0122] (4) Data processing and plotting: Use the frequency-sweeping results to plot the Bode diagram of the sequence impedance considering frequency coupling, where the abscissa is the frequency and the ordinate is the amplitude and phase of the sequence impedance.

[0123] Figure 2 A network-forming new energy converter grid-connected system provided by this method, where PCC is the point of common coupling between the new energy converter and the power grid, V dc is the DC-side voltage, L f is the filter inductor, R d is the filter resistor, i abc are the three-phase currents of A, B, and C sampled at PCC, v abc are the three-phase voltages of A, B, and C sampled at PCC, C f is the filter capacitor, i gabc is the grid-side current, L g is the grid-side inductor,R g is the resistance on the grid side. Specifically, L g can be set to 20 mH, R g and can be set to 0.2 Ω.

[0124] Figure 3 is the control loop of the grid-forming converter adopted by this method, where P ref is the active power reference value, Q ref is the reactive power loop reference value, P e is the active power instantaneous value, Q e is the reactive power instantaneous value, , D p is the active damping coefficient, J is the inertia coefficient, K is the reactive power loop droop coefficient, V ref is the reactive power loop voltage amplitude reference value, V m is the grid single-phase phase voltage amplitude reference value, D q is the reactive power loop virtual coefficient, θ GFM is the active power loop output phase angle, E m is the reactive power loop output voltage amplitude. The synchronous modulation waves ma, mb, and mc can be expressed as:

[0125]

[0126] In the formula, K f = 2 / V dc .

[0127] Specifically, step S1 includes:

[0128] S11: Set the fundamental frequency f1 and a frequency table including several frequency points.

[0129] In practical applications, the frequency sweep program can be initialized, and the fundamental frequency f1 is set to 50 Hz. The frequency table contains 35 frequency points, which are set to [10, 12, 15, 18, 22, 27, 33, 40, 60, 72, 88, 107, 130, 158, 193, 235, 287, 349, 425, 518, 631, 769, 936, 1141, 1389, 1693, 2062, 2512, 3060, 3728, 4541, 5532, 6739, 8209, 10000] respectively.

[0130] In addition, the frequency sweep program also needs to set parameters such as the communication port and the measurement storage address.

[0131] When constructing the frequency sweep program, the frequency sweep cut-off frequency, that is, the upper limit frequency of the frequency sweep, can also be set according to the actual power grid characteristics. When the frequency sweep program is executed and a frequency point greater than or equal to the frequency sweep cut-off frequency is encountered, there is no need to execute the frequency sweep program.

[0132] When initializing the frequency sweep program, preset the frequency sweep cut-off frequency.

[0133] When setting the frequency table, the numerical values of the frequency points in the frequency table are set in ascending order one by one.

[0134] When traversing the frequency points in the frequency table, the frequency points are processed in ascending order. When a frequency point is greater than or equal to the frequency sweep cut-off frequency, the frequency sweep measurement can be directly ended to reduce the calculation amount.

[0135] S12: Traverse the frequency points in the frequency table, and inject three-phase voltage disturbances with a certain frequency value into the grid-forming converter system according to the frequency point value and the fundamental frequency, and obtain the output voltage and current response data.

[0136] Specifically, there are two cases in the measurement of the frequency sweep program:

[0137] 1. When the frequency point value fp < 2f1, that is, when fp < 100 Hz:

[0138] (1) First, inject a positive-sequence voltage disturbance with a frequency of fp, three-phase amplitudes of A1, B1, and C1, and phases of 10, 240, and 120 respectively into the grid-forming converter system. After the system stabilizes, obtain the output voltage and current response data:

[0139]

[0140]

[0141]

[0142]

[0143] (2) Then, inject a positive-sequence voltage disturbance with a frequency of 2f1 - fp, three-phase amplitudes of A2, B2, and C2, and phases of 10, 240, and 120 into the grid-forming converter system. After the system stabilizes, obtain the output voltage and current response data:

[0144]

[0145]

[0146]

[0147]

[0148] Among them, t 0' and t 0" represent the time of the first sampling and the time of the second sampling respectively; V is the voltage, I is the current; φ is the phase angle.

[0149] 2. When the frequency point value fp ≥ 2f1, that is, when fp ≥ 100 Hz:

[0150] (1) First, inject a positive-sequence voltage disturbance with a frequency of fp, three-phase amplitudes of A1, B1, and C1, and phases of 0, 240, and 120 into the grid-forming converter system. After the system stabilizes, obtain the output voltage and current response data:

[0151]

[0152]

[0153]

[0154]

[0155]

[0156] (2) Then, inject a negative-sequence voltage disturbance with a frequency of fp - 2f1, three-phase amplitudes of A2, B2, and C2, and phases of 0, 240, and 120 into the grid-forming converter system. After the system stabilizes, obtain the output voltage and current response data:

[0157]

[0158]

[0159]

[0160]

[0161] S2: Obtain the sequence admittance measurement results of frequency coupling according to the voltage-current response data.

[0162] Admittance is a general term for conductance and susceptance, which is used to describe the ease of alternating current passing through a circuit or system. Admittance is the reciprocal of impedance, denoted by Y, and the unit is Siemens (S). Admittance is a complex number, consisting of a real part (conductance) and an imaginary part (susceptance).

[0163] When calculating the sequence admittance results of frequency coupling in a grid-forming converter system, the sequence admittance measurement results include: sequence admittance components Y 11 、Y 12 、Y 21 、Y 22 , and the calculation formula is as follows:

[0164]

[0165] S3: Obtain the compensation factor for phase correction according to the characteristics of the grid-forming converter system and the voltage-current response data.

[0166] Step S3 includes:

[0167] (1) Estimate the grid impedance according to the voltage-current response data.

[0168] Assume that n frequency disturbances are co-injected, that is, n sequence admittance measurements are performed. Then, the estimated value of the grid impedance Zg can be expressed as:

[0169]

[0170] In the formula, V pi ' and I gpi ' are the positive-sequence voltage and positive-sequence current measured after the i-th injection of disturbance in step S2 respectively, f pi is the disturbance source frequency injected for the i-th time.

[0171] (2) Introduce an adjustment coefficient according to the grid impedance and calculate the compensation factor for phase correction.

[0172] Figure 4 is the equivalent schematic diagram of frequency coupling in a new energy grid-connected system. After the system injects the disturbance source V p ( f p ), two response excitations will be generated through the converter control loop e '( f p) and e '( f p -2 f 1), under the action of two response excitations, coupled excitations are generated respectively e "( f p -2 f 1) and e "( f p ). Therefore, the current components of the disturbance frequency fp and the current component of fp - 2f1 collected in the actual process include both the current components generated by the response excitation acting on the converter impedance and the grid impedance, and the current components under the action of the coupled response excitation. In this process, the influence of the grid impedance on the phase of the coupled component, especially the current component, cannot be ignored. The coupled current component contains the influence of the corresponding grid impedance. From engineering experience, it can be obtained that the impedance of the converter in the high-frequency band is much larger than the grid impedance. Therefore, the influence of the grid on the converter is mainly reflected in the low-frequency band. Taking the voltage component as the reference quantity, this method reduces the influence in the coupled component to the phase of the coupled current component and introduces a compensation factor λ to realize the phase correction of the frequency-coupled measurement sequence admittance.

[0173] The calculation formula of the compensation factor λ is:

[0174]

[0175] Wherein, Z GFM ( f p ) = ; α is an adjustment coefficient, and its value range is 0.6 - 1; is the grid impedance angle; angle() represents the phase of the vector in the parentheses; || represents the magnitude of the vector.

[0176] S4: Use the compensation factor to perform phase correction on the sequence admittance measurement result to obtain an optimized sequence admittance result.

[0177] Specifically, the result after the sequence admittance phase correction is expressed as follows:

[0178]

[0179] Preferably, this method further includes:

[0180] S5: Plot the sequence admittance measurement result and the optimized sequence admittance result as curves to compare the effects before and after the phase correction.

[0181] Measure the grid-forming new energy converter grid-connected system described above using this method, and plot the obtained frequency coupling sequence admittance results as curves. The curve without phase correction is as shown in Figure 5 , and the curve with phase correction is as shown in Figure 6 .

[0182] Figure 5 In Figure 6 , the solid line part represents the frequency coupling sequence admittance curve of the grid-forming new energy converter obtained by theoretical derivation. The formula in the complex frequency domain of the curve is as follows:

[0183]

[0184] In the formula, N, M, T, W, N l , M l , T l , W l are expressed as follows:

[0185]

[0186] In the formula, V1 = V1 * = V 1 / 2; I1 = ( I 1 / 2)e ±jφi1 ; I1 * = ( I 1 / 2)e ∓jφi1 ; V 1 = V ref ; I 1 = P ref / (1.5 * V ref ); φ i1 is the phase of the fundamental current, and its value is obtained by simulation measurement; s 1 = 2π f p , s 2 = 2π f p -4π f 1, s 0 = 2π f p -2π f 1; The PWM modulation coefficient K pwm = 0.5; T ( s ) = 1 / s * ( J * s + D p ); Delay functionG del ( s ) = e -1.5Tss , T s is the sampling time step; s is the Laplace operator; j is the imaginary unit of a complex number; D q is the reactive loop damping coefficient.

[0187] Figure 5 and Figure 6 In, the "*" part represents the numerical values of the sequence admittance at the perturbation frequencies fp and fp - 2f1 measured by the frequency sweep program. From Figure 5 it can be seen that the coupling components Y 12 and Y 21 of the network-forming frequency coupling sequence admittance without phase correction have large errors in the phase measurement values compared with the theoretical curve at frequencies f < 100 Hz. Under the same simulation conditions, the measured values of the network-forming frequency coupling sequence admittance after phase correction are as Figure 6 shown, and the measured values are in good agreement with the theoretical values, verifying the effectiveness of this method.

[0188] The beneficial effect of this embodiment is that this method also presets a frequency sweep cut-off frequency, sets the numerical values of the frequency points in the frequency table in ascending order one by one, and when traversing the frequency points in the frequency table, processes the frequency points in ascending order one by one. When the frequency point is greater than or equal to the frequency sweep cut-off frequency, the frequency sweep program can be directly ended, thereby reducing the calculation amount.

[0189] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A method for measuring and correcting frequency-coupled sequence admittance of a grid-type converter system, characterized in that: The following steps are involved: Injecting three-phase voltage disturbances of different frequency values ​​into the grid-connected converter system several times to obtain output voltage and current response data; Obtain frequency-coupled sequence admittance measurement results based on voltage and current response data; Obtain the compensation factor of phase correction according to the system characteristics of the grid-type converter and the voltage and current response data; The compensation factor is used to perform phase correction on the sequence admittance measurement result to obtain the optimized sequence admittance result; Wherein, the phase correction compensation factor is obtained according to the grid-type converter system characteristics and voltage and current response data, including: Estimate grid impedance based on voltage and current response data; According to the grid impedance, the adjustment coefficient is introduced to calculate the compensation factor of phase correction; Assuming that n frequency disturbances are injected, that is, n-order admittance measurements are performed, the estimated grid impedance Zg can be expressed as: In the formula, V pi ' and I gpi ' are the positive sequence voltage and positive sequence current measured after the ith disturbance injection, f pi is the frequency of the disturbance source injected for the ith time; fp is the frequency value; The calculation formula of the compensation factor λ is: In the formula, Z GFM ( f p ) = / , / is the output voltage and current response data, t 0' represents the moment of first sampling, φ is the phase angle, and f1 is the fundamental frequency; α is the adjustment coefficient, and its value range is 0.6~1; is the grid impedance angle; angle ( ) represents the phase of the vector in brackets; | | represents the magnitude of the vector modulus.

2. The method for measuring and correcting the frequency-coupled sequence admittance of a grid-connected converter system according to claim 1, characterized in that: The step of injecting a plurality of three-phase voltage disturbances of different frequency values ​​into the grid-connected converter system to obtain output voltage and current response data includes: Set the fundamental frequency f1 and a frequency table containing several frequency points; The frequency points in the frequency table are traversed, and according to the frequency point values ​​and the fundamental frequency, a three-phase voltage disturbance with a certain frequency value is injected into the grid-type converter system to obtain the output voltage and current response data.

3. The method for measuring and correcting the frequency-coupled sequence admittance of a grid-connected converter system according to claim 2, characterized in that: The setting of the fundamental frequency f1 and the frequency table containing a plurality of frequency points also includes: The frequency point values ​​in the frequency table are set from small to large.

4. The method for measuring and correcting the frequency-coupled sequence admittance of a grid-connected converter system according to claim 3, characterized in that: The step of injecting a plurality of three-phase voltage disturbances of different frequency values ​​into the grid-connected converter system to obtain output voltage and current response data further includes: Preset cutoff frequency; When traversing the frequency points in the frequency table, when the frequency point is greater than or equal to the sweep cutoff frequency, the measurement ends.

5. The method for measuring and correcting the frequency-coupled sequence admittance of a grid-type converter system according to claim 2, characterized in that: The step of injecting a three-phase voltage disturbance of a certain frequency value into the grid-connected converter system according to the frequency point value and the fundamental frequency to obtain the output voltage and current response data includes: When the frequency point value fp<2f1, first inject a positive sequence voltage disturbance with a frequency of fp, three-phase amplitudes of A1, B1, C1, and phases of 10, 240, and 120 into the grid-type converter system. After the system is stable, obtain the output voltage and current response data. Then, a positive-sequence voltage disturbance with a frequency of 2f1-fp, three-phase amplitudes of A2, B2, and C2, and phases of 10, 240, and 120 respectively, is injected into the grid-connected converter system. After the system is stable, the output voltage and current response data are obtained.

6. The method for measuring and correcting the frequency-coupled sequence admittance of a grid-connected converter system according to claim 2, characterized in that: The step of injecting a three-phase voltage disturbance of a certain frequency value into the grid-type converter system according to the frequency point value and obtaining the output voltage and current response data comprises: When the frequency point value fp≥2f1, first inject a positive sequence voltage disturbance with a frequency of fp, three-phase amplitudes of A1, B1, C1, and phases of 0, 240, and 120 into the grid-connected converter system. After the system is stable, obtain the output voltage and current response data. Then, a negative-sequence voltage disturbance with a frequency of fp-2f1, three-phase amplitudes of A2, B2, and C2, and phases of 0, 240, and 120 are injected into the grid-connected converter system. After the system is stable, the output voltage and current response data are obtained.

7. The method for measuring and correcting frequency-coupled sequence admittance of a grid-connected converter system according to claim 2, characterized in that: The fundamental frequency f1 is set to 50 Hz.

8. The method for measuring and correcting frequency-coupled sequence admittance of a grid-connected converter system according to claim 1, characterized in that: The sequence admittance measurement result includes: sequence admittance component Y 11 , Y 12 , Y 21 , Y 22 .

9. The method for measuring and correcting frequency-coupled sequence admittance of a grid-connected converter system according to claim 1, characterized in that: The method for measuring and correcting the frequency-coupled sequence admittance of the grid-type converter system also includes: The sequence admittance measurement results and the optimized sequence admittance results are plotted into curves to compare the effects before and after phase correction.

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