Zero-sequence circulating current cooperative control method and related equipment

Abstract

The invention discloses a zero-sequence circulating current cooperative control method and related equipment, working conditions are judged by detecting a zero-sequence circulating current amplitude and a converter impedance parameter, and the zero-sequence circulating current is cooperatively inhibited by using harmonic injection control and dynamic virtual impedance control under steady-state, transient-state and parameter mismatch working conditions. Harmonic injection counteracts a high-frequency zero-sequence voltage component by superposing third harmonics, dynamic virtual impedance generates frequency-variable impedance to hinder low-frequency circulating current, and full-band suppression of high-frequency and low-frequency zero-sequence circulating current is achieved through cooperation of the frequency-variable impedance and the dynamic virtual impedance. And the harmonic injection amplitude and the virtual impedance parameter are optimized only through a software algorithm, so that the cost is greatly reduced. For interference such as DC side voltage fluctuation and AC side asymmetric faults, the strategy can be adjusted in real time. In a transient state, the harmonic amplitude is rapidly determined and the range is limited to suppress circulating current sudden change, and in parameter mismatch, the virtual impedance is corrected to realize impedance equalization, so that extremely strong anti-interference performance is shown, and the stability and reliability of grid-connected operation of an alternating current and direct current power grid are effectively improved.

Description

Technical Field

[0001] This application relates to the field of converter control technology, and more specifically, to a zero-sequence circulating current coordinated control method and related equipment. Background Technology

[0002] In modern power systems, the integrated operation of AC and DC power grids has become a development trend, greatly improving the overall performance of the power system with its efficient power transmission and flexible power supply methods. However, during the integration of AC and DC power grids, the zero-sequence circulating current problem between converters gradually becomes prominent, seriously affecting the stability, reliability, and power quality of the system, becoming a key bottleneck restricting the large-scale application of AC and DC power grids.

[0003] In existing technologies, the factors inducing zero-sequence circulating current are complex and diverse. From a hardware perspective, differences in line impedance of parallel converters lead to varying voltage drops in each branch, while differences in switching device characteristics result in inconsistent conduction losses and voltage drops. These two factors combined cause zero-sequence voltage imbalance, thus forming a circulating current path. At the control level, asynchronous control signals cause deviations in the switching timing of each converter, and differences in modulation strategies, such as inconsistent carrier phases, introduce high-frequency harmonic components into the system, thereby inducing high-frequency zero-sequence circulating current. Furthermore, DC-side bus voltage fluctuations disrupt the power balance on the DC side, and AC-side asymmetrical faults generate negative-sequence and zero-sequence components. These imbalances all contribute to low-frequency zero-sequence circulating current. Simultaneously, the common DC bus structure used in parallel converter topologies lacks electrical isolation, providing a natural closed loop for zero-sequence circulating current.

[0004] Existing control algorithms for the aforementioned zero-sequence circulating current problem have several shortcomings in practical applications. Firstly, they struggle to simultaneously cover both high-frequency and low-frequency zero-sequence circulating currents, failing to achieve effective suppression across the entire frequency band. Secondly, achieving a certain control effect often requires additional hardware, leading to high hardware costs. Furthermore, existing algorithms exhibit weak anti-interference capabilities in the face of complex and variable imbalance conditions on both the DC and AC sides, making it difficult to guarantee stable system operation under various conditions and failing to meet practical engineering requirements. Therefore, there is an urgent need to develop a more efficient, comprehensive, and low-cost zero-sequence circulating current control method. Summary of the Invention

[0005] This application provides a zero-sequence circulating current coordinated control method and related equipment, which determines the operating condition type by detecting the zero-sequence circulating current amplitude and converter impedance parameters. Under steady-state, transient, and parameter mismatch conditions, harmonic injection control and dynamic virtual impedance control are used in synergy to achieve precise suppression of the zero-sequence circulating current.

[0006] A zero-sequence circulating current cooperative control method, comprising:

[0007] Detect the amplitude of the zero-sequence circulating current and the impedance parameters of each converter to determine the current operating condition type;

[0008] When the current operating condition is steady-state, the zero-sequence circulating current amplitude is reduced through the coordinated processing of harmonic injection control and dynamic virtual impedance control. The harmonic injection control involves superimposing a third harmonic component into the converter modulation wave and coordinating the injection phase of the harmonics through the phase difference between adjacent converters, so that the zero-sequence voltage components output by each converter cancel each other out. The harmonic injection amplitude is adjusted in real time based on zero-sequence current error compensation. The dynamic virtual impedance control generates a frequency-varying impedance covering the fundamental frequency and multiple harmonic frequency bands, and increases the equivalent impedance of the circulating current path by connecting it in parallel to the zero-sequence circulating current path. The amplitude-frequency characteristics of the frequency-varying impedance are optimized in real time according to the circulating current harmonic content.

[0009] During transient operating conditions, the transient circulation suppression time is shortened by the harmonic injection control, wherein the harmonic injection amplitude is determined based on the zero-sequence circulation mutation rate and the amplitude range is limited by a first-order inertial element to suppress circulation mutations;

[0010] When the parameters are mismatched, impedance equalization compensation is performed through the dynamic virtual impedance control, wherein the frequency-varying impedance is corrected based on the online identified and updated impedance parameters of each converter, so that the equivalent impedance deviation of the parallel converters converges to the preset tolerance range.

[0011] Optionally, the calculation formula for the harmonic injection amplitude based on zero-sequence current error adjustment is as follows:

[0012]

[0013]

[0014] in, The compensation value for the harmonic injection amplitude at time t. Let be the zero-sequence current error at time t. , , These are the proportional coefficient, integral coefficient, and differential coefficient, respectively, where t is time . For the damping ratio, For natural frequency, This represents the system's open-loop gain.

[0015] Optionally, the expression for the frequency-varying impedance in the dynamic virtual impedance control is:

[0016]

[0017] The objective function for frequency-varying impedance optimization is:

[0018]

[0019] in, For frequency-varying impedance, For the Laplace operator, , , These are the parameters of the frequency-varying resistor, inductor, and capacitor, respectively. The harmonic gain coefficient is dynamically adjusted according to the circulating harmonic content. The harmonic order is 3, 6, or 9. The fundamental angular frequency, The highest harmonic order, and The weighting coefficient is used to balance the harmonic suppression effect with the injection amplitude constraint.

[0020] Optionally, under the parameter mismatch condition, the update formula for updating the impedance parameters of each converter is identified online using the recursive least squares method:

[0021]

[0022] ,

[0023] in, This is the updated impedance parameter vector of the k-th converter at time t. Let be the zero-sequence impedance parameter vector of the k-th converter. Zero-sequence resistance, It is a zero-sequence inductance. For the observation vector, It is the zero-sequence current. Forgetting factor, Let covariance matrix be the variance matrix. The zero-sequence voltage measurement value at time t.

[0024] Optionally, the phase difference coordination formula for adjacent converters in the harmonic injection control is:

[0025]

[0026] in, The phase compensation angle for the k-th converter is... Let be the zero-sequence current phase angle of the i-th adjacent converter. Let n be the zero-sequence current phase angle of the k-th converter, and n be the number of adjacent converters.

[0027] Optionally, in the harmonic injection control, a third harmonic component is superimposed on the converter modulation wave, and the reconstructed modulation wave generated after coordinating the injection phase of the harmonics through the phase difference between adjacent converters is:

[0028]

[0029] in, This represents the reconstructed modulated wave signal of the k-th converter after superposition and correction in phase x. This represents the original converter modulation signal of the k-th converter before phase x-correction. To compensate for the amplitude of harmonics. The phase compensation angle for the k-th converter is... ω is the fundamental angular frequency.

[0030] A zero-sequence circulating current cooperative control device, comprising:

[0031] The operating condition detection unit is used to detect the amplitude of the zero-sequence circulating current and the impedance parameters of each converter to determine the current operating condition type.

[0032] The steady-state operating condition unit is used to reduce the zero-sequence circulating current amplitude by coordinating harmonic injection control and dynamic virtual impedance control when the current operating condition is steady-state. The harmonic injection control involves superimposing a third harmonic component into the converter modulation wave and coordinating the injection phase of the harmonics through the phase difference between adjacent converters, so that the zero-sequence voltage components output by each converter cancel each other out. The harmonic injection amplitude is adjusted in real time based on zero-sequence current error compensation. The dynamic virtual impedance control generates a frequency-varying impedance covering the fundamental frequency and multiple harmonic frequency bands, and increases the equivalent impedance of the circulating current path by connecting it in parallel to the zero-sequence circulating current path. The amplitude-frequency characteristics of the frequency-varying impedance are optimized in real time according to the circulating current harmonic content.

[0033] The transient operating condition unit is used to shorten the transient circulation suppression time during transient operating conditions by means of the harmonic injection control, wherein the harmonic injection amplitude is determined based on the zero-sequence circulation mutation rate and the amplitude range is limited by a first-order inertial element to suppress circulation mutation;

[0034] The mismatch condition unit is used to perform impedance equalization compensation through the dynamic virtual impedance control when the parameters are mismatched. The frequency-varying impedance is corrected based on the online identified and updated impedance parameters of each converter, so that the equivalent impedance deviation of the parallel converters converges to a preset tolerance range.

[0035] A zero-sequence circulating current cooperative control device includes a memory and a processor;

[0036] The memory is used to store programs;

[0037] The processor is used to execute the program to implement the various steps of the zero-sequence circulating current cooperative control method as described in any of the above claims.

[0038] A readable storage medium having a computer program stored thereon, characterized in that, when the computer program is executed by a processor, it implements the steps of the zero-sequence circulating current cooperative control method as described in any of the preceding claims.

[0039] A computer program product includes a computer program, characterized in that the computer program, when run by a processor, executes the various steps of the zero-sequence circulating current cooperative control method as described in any of the preceding claims.

[0040] As can be seen from the above technical solutions, the zero-sequence circulating current collaborative control method and related equipment provided in this application determine the operating condition type by detecting the zero-sequence circulating current amplitude and the converter impedance parameters. Under steady-state, transient, and parameter mismatch operating conditions, the method utilizes the synergistic effect of harmonic injection control and dynamic virtual impedance control to achieve precise suppression of the zero-sequence circulating current.

[0041] This application achieves full-band coverage through the synergy of harmonic injection and dynamic virtual impedance. Harmonic injection targets high-frequency zero-sequence circulating current by superimposing third harmonics and coordinating their phases to cancel the zero-sequence voltage component at the output of each converter. Dynamic virtual impedance generates frequency-varying impedance covering the fundamental and multiple harmonic bands, effectively hindering low-frequency zero-sequence circulating current. The two work together to achieve full-band suppression of high-frequency and low-frequency zero-sequence circulating current. In terms of low hardware consumption, compared to the traditional approach of relying on adding hardware to improve control performance, this application only optimizes the software algorithm to dynamically adjust the harmonic injection amplitude and virtual impedance parameters, eliminating the need for additional complex hardware and significantly reducing costs. Faced with interference factors such as DC-side voltage fluctuations and AC-side asymmetrical faults, this application can adjust the control strategy in real time according to the operating conditions. Under transient conditions, the amplitude of harmonic injection can be quickly determined based on the zero-sequence circulating current mutation rate, and the amplitude range can be limited to effectively suppress circulating current mutations. Under parameter mismatch conditions, the virtual impedance can be corrected based on the online identified converter impedance parameters to achieve impedance balance compensation, ensure stable system operation, and exhibit strong anti-interference characteristics, significantly improving the stability and reliability of AC / DC grid-connected operation. Attached Figure Description

[0042] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of this application. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0043] Figure 1 This is a flowchart of a zero-sequence circulating current cooperative control method disclosed in an embodiment of this application;

[0044] Figure 2This is a schematic diagram of a zero-sequence circulating current cooperative control device disclosed in an embodiment of this application;

[0045] Figure 3 This is a hardware structure block diagram of a zero-sequence circulating current cooperative control device disclosed in an embodiment of this application. Detailed Implementation

[0046] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0047] This application can be used in a wide variety of general-purpose or special-purpose computing device environments or configurations. For example: personal computers, server computers, handheld or portable devices, tablet devices, multiprocessor devices, distributed computing environments including any of the above devices, etc.

[0048] The following section introduces the solution proposed in this application. The technical solution is as follows, and details are provided below.

[0049] Figure 1 This is a flowchart of a zero-sequence circulating flow cooperative control method disclosed in an embodiment of this application.

[0050] like Figure 1 As shown, the method may include:

[0051] Step S1: Detect the amplitude of the zero-sequence circulating current and the impedance parameters of each converter to determine the current operating condition type.

[0052] Specifically, current sensors are placed along the zero-sequence circulating current path to collect the zero-sequence circulating current signal in real time. The circulating current amplitude is obtained through signal processing and calculation. Impedance parameters of each converter are acquired using impedance measurement circuits or model-based parameter identification methods. Based on the fluctuation characteristics and rate of change of the circulating current amplitude, as well as the consistency of the converter impedance parameters, the results are compared with pre-set operating condition determination rules to determine whether the current operating condition is steady-state, transient, or parameter mismatch.

[0053] One possible approach is to acquire the zero-sequence circulating current signal in real time using a high-precision current sensor. After signal processing steps such as filtering and amplification, the analog signal is converted into a digital signal using an analog-to-digital converter (ADC), and then the amplitude of the zero-sequence circulating current is calculated using a Fast Fourier Transform (FFT). For obtaining the impedance parameters of each converter, an impedance measurement method based on high-frequency injected signals can be used. A voltage or current signal of a specific frequency is injected into the converter, the corresponding response signal is measured, and the impedance parameters are calculated based on Ohm's law and related circuit principles. The measured zero-sequence circulating current amplitude is compared with the rated current. If the zero-sequence circulating current amplitude is less than 5% of the rated current, it is determined to be a steady-state condition; if the zero-sequence circulating current amplitude is greater than or equal to 20% of the rated current, it is determined to be a transient condition. Simultaneously, the deviation between the zero-sequence impedances of each converter is calculated. If the deviation exceeds 15%, it is determined to be a parameter mismatch condition. Through these judgment rules, the current operating condition type is accurately determined.

[0054] Step S2: When the current operating condition is a steady-state condition, the zero-sequence circulating current amplitude is reduced by the coordinated processing of harmonic injection control and dynamic virtual impedance control.

[0055] Specifically, in terms of harmonic injection control, the third harmonic component is first superimposed on the converter modulation wave. By accurately measuring and calculating the phase difference between adjacent converters, the injection phase of the third harmonic is coordinated, so that the zero-sequence voltage components output by each converter spatially cancel each other out. Simultaneously, a zero-sequence current error feedback mechanism is established to monitor the error between the actual and target values ​​of the zero-sequence current in real time. Based on this error, the harmonic injection amplitude is adjusted in real time to ensure that the harmonic injection amount accurately matches the current zero-sequence circulating current suppression requirements.

[0056] In terms of dynamic virtual impedance control, frequency-varying impedances covering the fundamental and multiple harmonic frequency bands are generated based on digital signal processing technology. These frequency-varying impedances are connected in parallel to the zero-sequence circulating current path, suppressing the zero-sequence circulating current by changing the equivalent impedance of the circulating current path. The circulating current harmonic content is analyzed in real time, and the amplitude-frequency characteristics of the frequency-varying impedance are optimized based on the analysis results. This allows the frequency-varying impedance to better adapt to the zero-sequence circulating current suppression requirements under different harmonic contents, thereby synergistically reducing the zero-sequence circulating current amplitude.

[0057] The following sections will provide a detailed introduction to harmonic injection control and dynamic virtual impedance control:

[0058] Harmonic injection control:

[0059] The harmonic injection control involves superimposing a third harmonic component into the converter modulation wave, coordinating the injection phase of the harmonics through the phase difference between adjacent converters, so that the zero-sequence voltage components output by each converter cancel each other out, and adjusting the harmonic injection amplitude in real time based on zero-sequence current error compensation.

[0060] Specifically, harmonic injection control aims to reduce zero-sequence circulating current between converters when connected to AC / DC power grids. Its core idea is to add a third harmonic component to the converter modulation wave. In practice, the phase relationship between adjacent converters must first be determined. By coordinating this phase difference, the zero-sequence voltage components output by each converter can cancel each other out. This is analogous to several people pulling a rope together; by adjusting the timing and direction of their respective efforts, the overall force is balanced, reducing unnecessary fluctuations. Furthermore, to improve the effectiveness of harmonic injection, the amplitude of the harmonic injection is adjusted in real time based on the error in the zero-sequence current.

[0061] The phase difference coordination principle utilizes the phase difference between adjacent converters to coordinate the harmonic injection phase. The phase compensation angle of the converter is calculated using the phase difference coordination formula. By calculating the relevant values ​​of the zero-sequence current phase angle of adjacent converters, a suitable phase compensation angle is obtained, thereby achieving the purpose of mutual cancellation of the zero-sequence voltage components output by each converter.

[0062] The formula for coordinating the phase difference between adjacent converters in the harmonic injection control is as follows:

[0063]

[0064] in, The phase compensation angle for the k-th converter is... Let be the zero-sequence current phase angle of the i-th adjacent converter. Let n be the zero-sequence current phase angle of the k-th converter, and n be the number of adjacent converters.

[0065] Reconstructed modulation wave generation: In the harmonic injection control, a third harmonic component is superimposed on the converter modulation wave. The reconstructed modulation wave generated after coordinating the injection phase of the harmonics through the phase difference between adjacent converters is as follows:

[0066]

[0067] in, This represents the reconstructed modulated wave signal of the k-th converter after superposition and correction in phase x. This represents the original converter modulation signal of the k-th converter before phase x-correction. To compensate for the amplitude of harmonics. The phase compensation angle for the k-th converter is... ω is the fundamental angular frequency.

[0068] With the initial phase of each phase as For example, by injecting third harmonic components The modulated wave signal is reconstructed so that the zero-sequence voltage components at the converter output cancel each other out.

[0069]

[0070] Because the third harmonic of the three phases is in phase The zero-sequence voltage after superposition is:

[0071]

[0072] Amplitude Adjustment Mechanism: The harmonic injection amplitude is adjusted in real time based on the zero-sequence current error. The harmonic injection amplitude compensation value is dynamically adjusted according to the current value, past integral value, and rate of change of the zero-sequence current error, achieving precise suppression of the zero-sequence circulating current. The calculation formula for the harmonic injection amplitude adjustment based on the zero-sequence current error is as follows:

[0073]

[0074]

[0075] in, The compensation value for the harmonic injection amplitude at time t. Let be the zero-sequence current error at time t. , , These are the proportional coefficient, integral coefficient, and differential coefficient, respectively, where t is time . For the damping ratio, For natural frequency, This represents the system's open-loop gain.

[0076] Dynamic virtual impedance control:

[0077] The dynamic virtual impedance control generates a frequency-varying impedance covering the fundamental and multiple harmonic frequency bands. By connecting it in parallel to the zero-sequence circulating current path, the equivalent impedance of the circulating current path is increased. The amplitude-frequency characteristics of the frequency-varying impedance are optimized in real time according to the circulating current harmonic content.

[0078] Specifically, dynamic virtual impedance control is used to suppress zero-sequence circulating current between converters when connected to AC / DC power grids. Its principle is based on circuit characteristics to generate a frequency-varying impedance that covers the fundamental frequency and specific harmonic frequency bands (such as the 3rd, 6th, and 9th harmonics). This frequency-varying impedance is connected in parallel to the zero-sequence circulating current path. According to Ohm's law, when the voltage remains constant, increasing the equivalent impedance can reduce the zero-sequence circulating current. Simultaneously, the circulating current harmonic content is monitored in real time, and the amplitude-frequency characteristics of the frequency-varying impedance are adjusted according to its changes, ensuring that the impedance presents appropriate impedance values ​​in different harmonic frequency bands, thereby continuously and effectively suppressing the zero-sequence circulating current.

[0079] Frequency-variable impedance generation: Frequency-variable impedance is generated by determining the parameters of the frequency-variable resistor, inductor, and capacitor, as well as the harmonic gain coefficient, which is dynamically adjusted according to the harmonic content, based on the frequency-variable impedance expression. The frequency-variable impedance expression in the dynamic virtual impedance control is:

[0080]

[0081] Parallel connection: The generated frequency-varying impedance is connected in parallel to the zero-sequence circulating current path to change the equivalent impedance of the zero-sequence circulating current path and achieve initial suppression of the zero-sequence circulating current.

[0082] Real-time optimization: Guided by the objective function of frequency-varying impedance optimization, the amplitude-frequency characteristics of the frequency-varying impedance are adjusted according to the objective function by monitoring the circulating current harmonic content, thereby dynamically optimizing the frequency-varying impedance and continuously improving the suppression effect on zero-sequence circulating current. The objective function for frequency-varying impedance optimization is:

[0083]

[0084] in, For frequency-varying impedance, For the Laplace operator, , , These are the parameters of the frequency-varying resistor, inductor, and capacitor, respectively. The harmonic gain coefficient is dynamically adjusted according to the circulating harmonic content. The harmonic order is 3, 6, or 9. The fundamental angular frequency, The highest harmonic order, and The weighting coefficient is used to balance the harmonic suppression effect with the injection amplitude constraint.

[0085] Step S3: During transient operating conditions, the transient circulation suppression time is shortened by the harmonic injection control, wherein the harmonic injection amplitude is determined based on the zero-sequence circulation mutation rate, and the amplitude range is limited by a first-order inertial element to suppress circulation mutation.

[0086] Specifically, under transient operating conditions, the system faces a significant risk of sudden changes in zero-sequence circulating current, which may damage the equipment. Harmonic injection control is employed to shorten the transient circulating current suppression time. The principle is to quickly determine an appropriate harmonic injection amplitude based on the rate of change of the zero-sequence circulating current. Because the faster the zero-sequence circulating current changes, a larger harmonic amplitude needs to be injected to more quickly cancel the zero-sequence voltage and suppress the circulating current. However, to prevent excessive injection amplitude from causing new problems, a first-order inertial element is used to limit the range of the harmonic injection amplitude, avoiding over-adjustment, thereby ensuring system stability while rapidly suppressing sudden changes in circulating current.

[0087] Formula for determining harmonic injection amplitude:

[0088]

[0089] in, It is the compensation value for the harmonic injection amplitude. It is the correlation coefficient with the zero-order circulation abrupt change rate. The abrupt change rate of the zero-order circulation is represented. It is a limiting function, with the amplitude limited to ±0.15.

[0090] This formula obtains a suitable harmonic injection amplitude by multiplying the mutation rate by the correlation coefficient and then applying a limiting function. The correlation coefficient of the zero-sequence circulating mutation rate comprehensively considers the dynamic characteristics of the system, allowing the coefficient to be reasonably adjusted according to actual operating conditions, thereby accurately determining the harmonic injection amplitude.

[0091] Step S4: In the case of parameter mismatch, impedance equalization compensation is performed through the dynamic virtual impedance control, wherein the frequency-varying impedance is corrected based on the online identified and updated impedance parameters of each converter, so that the equivalent impedance deviation of the parallel converters converges to the preset tolerance range.

[0092] Specifically, under parameter mismatch conditions, the significant impedance differences between converters lead to increased zero-sequence circulating current, affecting system stability. Dynamic virtual impedance control is used for impedance balancing compensation in this situation. First, an algorithm based on recursive least squares is used to continuously identify and update the impedance parameters (including zero-sequence resistance and inductance) of each converter online, obtaining more accurate real-time parameters. Then, based on the updated parameters, the frequency-varying impedance is corrected, gradually reducing the equivalent impedance deviation of the parallel converters until it converges to a preset tolerance range. This balances the impedance between the converters, effectively suppressing the zero-sequence circulating current problem caused by parameter mismatch and ensuring stable system operation.

[0093] Converter impedance parameter update: The impedance parameter vector of the k-th converter at time t is updated using the recursive least squares update formula, which includes zero-sequence resistance and zero-sequence inductance.

[0094] Under the parameter mismatch condition, the update formula for updating the impedance parameters of each converter is identified online using the recursive least squares method:

[0095]

[0096] ,

[0097] in, This is the updated impedance parameter vector of the k-th converter at time t. Let be the zero-sequence impedance parameter vector of the k-th converter. Zero-sequence resistance, It is a zero-sequence inductance. For the observation vector, It is the zero-sequence current. Forgetting factor, Let covariance matrix be the variance matrix. The zero-sequence voltage measurement value at time t.

[0098] Frequency-dependent impedance correction: Based on the impedance balance compensation formula and the updated converter impedance parameters, the frequency-dependent impedance is corrected so that the equivalent impedance deviation of the parallel converters converges to the preset tolerance range.

[0099] The impedance equalization compensation formula is:

[0100]

[0101]

[0102] in, It is the corrected frequency-varying impedance of the k-th converter. This is the frequency-varying impedance before correction; It is the average value of the zero-sequence impedance of all converters. It is the zero-sequence impedance of the k-th converter; μ is set to 0.05 as the convergence rate, which determines how fast the impedance is adjusted; ϵ=1e−6 is the zero-division parameter to avoid the case where the denominator is zero.

[0103] As can be seen from the above technical solutions, the zero-sequence circulating current collaborative control method and related equipment provided in this application determine the operating condition type by detecting the zero-sequence circulating current amplitude and the converter impedance parameters. Under steady-state, transient, and parameter mismatch operating conditions, the method utilizes the synergistic effect of harmonic injection control and dynamic virtual impedance control to achieve precise suppression of the zero-sequence circulating current.

[0104] This application achieves full-band coverage through the synergy of harmonic injection and dynamic virtual impedance. Harmonic injection targets high-frequency zero-sequence circulating current by superimposing third harmonics and coordinating their phases to cancel the zero-sequence voltage component at the output of each converter. Dynamic virtual impedance generates frequency-varying impedance covering the fundamental and multiple harmonic bands, effectively hindering low-frequency zero-sequence circulating current. The two work together to achieve full-band suppression of high-frequency and low-frequency zero-sequence circulating current. In terms of low hardware consumption, compared to the traditional approach of relying on adding hardware to improve control performance, this application only optimizes the software algorithm to dynamically adjust the harmonic injection amplitude and virtual impedance parameters, eliminating the need for additional complex hardware and significantly reducing costs. Faced with interference factors such as DC-side voltage fluctuations and AC-side asymmetrical faults, this application can adjust the control strategy in real time according to the operating conditions. Under transient conditions, the amplitude of harmonic injection can be quickly determined based on the zero-sequence circulating current mutation rate, and the amplitude range can be limited to effectively suppress circulating current mutations. Under parameter mismatch conditions, the virtual impedance can be corrected based on the online identified converter impedance parameters to achieve impedance balance compensation, ensure stable system operation, and exhibit strong anti-interference characteristics, significantly improving the stability and reliability of AC / DC grid-connected operation.

[0105] The following describes a zero-sequence circulating current cooperative control device provided in the embodiments of this application. The zero-sequence circulating current cooperative control device described below and the zero-sequence circulating current cooperative control method described above can be referred to and correspond to each other.

[0106] See Figure 2 , Figure 2 This is a schematic diagram of a zero-sequence circulating flow cooperative control device disclosed in an embodiment of this application.

[0107] like Figure 2 As shown, the zero-sequence circulating current cooperative control device may include:

[0108] The operating condition detection unit 110 is used to detect the circulating current amplitude of the zero-sequence circulating current and the impedance parameters of each converter to determine the current operating condition type.

[0109] The steady-state operating condition unit 120 is used to reduce the zero-sequence circulating current amplitude by coordinating harmonic injection control and dynamic virtual impedance control when the current operating condition is steady-state. The harmonic injection control involves superimposing a third harmonic component into the converter modulation wave and coordinating the injection phase of the harmonics through the phase difference between adjacent converters, so that the zero-sequence voltage components output by each converter cancel each other out. The harmonic injection amplitude is adjusted in real time based on zero-sequence current error compensation. The dynamic virtual impedance control generates a frequency-varying impedance covering the fundamental frequency and multiple harmonic frequency bands, and increases the equivalent impedance of the circulating current path by connecting it in parallel to the zero-sequence circulating current path. The amplitude-frequency characteristics of the frequency-varying impedance are optimized in real time according to the circulating current harmonic content.

[0110] The transient operating condition unit 130 is used to shorten the transient circulation suppression time during transient operating conditions by means of the harmonic injection control, wherein the harmonic injection amplitude is determined based on the zero-sequence circulation mutation rate and the amplitude range is limited by a first-order inertial element to suppress circulation mutation.

[0111] The mismatch condition unit 140 is used to perform impedance equalization compensation through the dynamic virtual impedance control when the parameters are mismatched. The frequency-varying impedance is corrected based on the online identified and updated impedance parameters of each converter, so that the equivalent impedance deviation of the parallel converters converges to a preset tolerance range.

[0112] As can be seen from the above technical solutions, the zero-sequence circulating current collaborative control method and related equipment provided in this application determine the operating condition type by detecting the zero-sequence circulating current amplitude and the converter impedance parameters. Under steady-state, transient, and parameter mismatch operating conditions, the method utilizes the synergistic effect of harmonic injection control and dynamic virtual impedance control to achieve precise suppression of the zero-sequence circulating current.

[0113] This application achieves full-band coverage through the synergy of harmonic injection and dynamic virtual impedance. Harmonic injection targets high-frequency zero-sequence circulating current by superimposing third harmonics and coordinating their phases to cancel the zero-sequence voltage component at the output of each converter. Dynamic virtual impedance generates frequency-varying impedance covering the fundamental and multiple harmonic bands, effectively hindering low-frequency zero-sequence circulating current. The two work together to achieve full-band suppression of high-frequency and low-frequency zero-sequence circulating current. In terms of low hardware consumption, compared to the traditional approach of relying on adding hardware to improve control performance, this application only optimizes the software algorithm to dynamically adjust the harmonic injection amplitude and virtual impedance parameters, eliminating the need for additional complex hardware and significantly reducing costs. Faced with interference factors such as DC-side voltage fluctuations and AC-side asymmetrical faults, this application can adjust the control strategy in real time according to the operating conditions. Under transient conditions, the amplitude of harmonic injection can be quickly determined based on the zero-sequence circulating current mutation rate, and the amplitude range can be limited to effectively suppress circulating current mutations. Under parameter mismatch conditions, the virtual impedance can be corrected based on the online identified converter impedance parameters to achieve impedance balance compensation, ensure stable system operation, and exhibit strong anti-interference characteristics, significantly improving the stability and reliability of AC / DC grid-connected operation.

[0114] Optionally, the calculation formula for the harmonic injection amplitude based on zero-sequence current error adjustment is as follows:

[0115]

[0116]

[0117] in, The compensation value for the harmonic injection amplitude at time t. Let be the zero-sequence current error at time t. , , These are the proportional coefficient, integral coefficient, and differential coefficient, respectively, where t is time . For the damping ratio, For natural frequency, This represents the system's open-loop gain.

[0118] Optionally, the expression for the frequency-varying impedance in the dynamic virtual impedance control is:

[0119]

[0120] The objective function for frequency-varying impedance optimization is:

[0121]

[0122] in, For frequency-varying impedance, For the Laplace operator, , , These are the parameters of the frequency-varying resistor, inductor, and capacitor, respectively. The harmonic gain coefficient is dynamically adjusted according to the circulating harmonic content. The harmonic order is 3, 6, or 9. The fundamental angular frequency, The highest harmonic order, and The weighting coefficient is used to balance the harmonic suppression effect with the injection amplitude constraint.

[0123] Optionally, under the parameter mismatch condition, the update formula for updating the impedance parameters of each converter is identified online using the recursive least squares method:

[0124]

[0125] ,

[0126] in, This is the updated impedance parameter vector of the k-th converter at time t. Let be the zero-sequence impedance parameter vector of the k-th converter. Zero-sequence resistance, It is a zero-sequence inductance. For the observation vector, It is the zero-sequence current. Forgetting factor, Let covariance matrix be the variance matrix. The zero-sequence voltage measurement value at time t.

[0127] Optionally, the phase difference coordination formula for adjacent converters in the harmonic injection control is:

[0128]

[0129] in, The phase compensation angle for the k-th converter is... Let be the zero-sequence current phase angle of the i-th adjacent converter. Let n be the zero-sequence current phase angle of the k-th converter, and n be the number of adjacent converters.

[0130] Optionally, in the harmonic injection control, a third harmonic component is superimposed on the converter modulation wave, and the reconstructed modulation wave generated after coordinating the injection phase of the harmonics through the phase difference between adjacent converters is:

[0131]

[0132] in, This represents the reconstructed modulated wave signal of the k-th converter after superposition and correction in phase x. This represents the original converter modulation signal of the k-th converter before phase x-correction. To compensate for the amplitude of harmonics. The phase compensation angle for the k-th converter is... ω is the fundamental angular frequency.

[0133] The zero-sequence circulating current cooperative control device provided in this application embodiment can be applied to zero-sequence circulating current cooperative control equipment. Figure 3 The hardware structure block diagram of the zero-sequence circulating current cooperative control device is shown, with reference to... Figure 3 The hardware structure of the zero-sequence circulating current cooperative control device may include: at least one processor 1, at least one communication interface 2, at least one memory 3, and at least one communication bus 4;

[0134] In this embodiment, the number of processor 1, communication interface 2, memory 3, and communication bus 4 is at least one, and processor 1, communication interface 2, and memory 3 communicate with each other through communication bus 4.

[0135] Processor 1 may be a central processing unit (CPU), an application-specific integrated circuit (ASIC), or one or more integrated circuits configured to implement embodiments of the present invention.

[0136] Memory 3 may include high-speed RAM, and may also include non-volatile memory, such as at least one disk storage device;

[0137] The memory stores a program, which the processor can call. The program is used for:

[0138] Detect the amplitude of the zero-sequence circulating current and the impedance parameters of each converter to determine the current operating condition type;

[0139] When the current operating condition is steady-state, the zero-sequence circulating current amplitude is reduced through the coordinated processing of harmonic injection control and dynamic virtual impedance control. The harmonic injection control involves superimposing a third harmonic component into the converter modulation wave and coordinating the injection phase of the harmonics through the phase difference between adjacent converters, so that the zero-sequence voltage components output by each converter cancel each other out. The harmonic injection amplitude is adjusted in real time based on zero-sequence current error compensation. The dynamic virtual impedance control generates a frequency-varying impedance covering the fundamental frequency and multiple harmonic frequency bands, and increases the equivalent impedance of the circulating current path by connecting it in parallel to the zero-sequence circulating current path. The amplitude-frequency characteristics of the frequency-varying impedance are optimized in real time according to the circulating current harmonic content.

[0140] During transient operating conditions, the transient circulation suppression time is shortened by the harmonic injection control, wherein the harmonic injection amplitude is determined based on the zero-sequence circulation mutation rate and the amplitude range is limited by a first-order inertial element to suppress circulation mutations;

[0141] When the parameters are mismatched, impedance equalization compensation is performed through the dynamic virtual impedance control, wherein the frequency-varying impedance is corrected based on the online identified and updated impedance parameters of each converter, so that the equivalent impedance deviation of the parallel converters converges to the preset tolerance range.

[0142] Optionally, the refined and extended functions of the program can be referred to the above description.

[0143] This application embodiment also provides a readable storage medium that can store a program suitable for execution by a processor, the program being used for:

[0144] Detect the amplitude of the zero-sequence circulating current and the impedance parameters of each converter to determine the current operating condition type;

[0145] When the current operating condition is steady-state, the zero-sequence circulating current amplitude is reduced through the coordinated processing of harmonic injection control and dynamic virtual impedance control. The harmonic injection control involves superimposing a third harmonic component into the converter modulation wave and coordinating the injection phase of the harmonics through the phase difference between adjacent converters, so that the zero-sequence voltage components output by each converter cancel each other out. The harmonic injection amplitude is adjusted in real time based on zero-sequence current error compensation. The dynamic virtual impedance control generates a frequency-varying impedance covering the fundamental frequency and multiple harmonic frequency bands, and increases the equivalent impedance of the circulating current path by connecting it in parallel to the zero-sequence circulating current path. The amplitude-frequency characteristics of the frequency-varying impedance are optimized in real time according to the circulating current harmonic content.

[0146] During transient operating conditions, the transient circulation suppression time is shortened by the harmonic injection control, wherein the harmonic injection amplitude is determined based on the zero-sequence circulation mutation rate and the amplitude range is limited by a first-order inertial element to suppress circulation mutations;

[0147] When the parameters are mismatched, impedance equalization compensation is performed through the dynamic virtual impedance control, wherein the frequency-varying impedance is corrected based on the online identified and updated impedance parameters of each converter, so that the equivalent impedance deviation of the parallel converters converges to the preset tolerance range.

[0148] Optionally, the refined and extended functions of the program can be referred to the above description.

[0149] This application also provides a computer program product, including a computer program, wherein the computer program is executed by a processor using the following method:

[0150] Detect the amplitude of the zero-sequence circulating current and the impedance parameters of each converter to determine the current operating condition type;

[0151] When the current operating condition is steady-state, the zero-sequence circulating current amplitude is reduced through the coordinated processing of harmonic injection control and dynamic virtual impedance control. The harmonic injection control involves superimposing a third harmonic component into the converter modulation wave and coordinating the injection phase of the harmonics through the phase difference between adjacent converters, so that the zero-sequence voltage components output by each converter cancel each other out. The harmonic injection amplitude is adjusted in real time based on zero-sequence current error compensation. The dynamic virtual impedance control generates a frequency-varying impedance covering the fundamental frequency and multiple harmonic frequency bands, and increases the equivalent impedance of the circulating current path by connecting it in parallel to the zero-sequence circulating current path. The amplitude-frequency characteristics of the frequency-varying impedance are optimized in real time according to the circulating current harmonic content.

[0152] During transient operating conditions, the transient circulation suppression time is shortened by the harmonic injection control, wherein the harmonic injection amplitude is determined based on the zero-sequence circulation mutation rate and the amplitude range is limited by a first-order inertial element to suppress circulation mutations;

[0153] When the parameters are mismatched, impedance equalization compensation is performed through the dynamic virtual impedance control, wherein the frequency-varying impedance is corrected based on the online identified and updated impedance parameters of each converter, so that the equivalent impedance deviation of the parallel converters converges to the preset tolerance range.

[0154] Optionally, the refined and extended functions of the program can be referred to the above description.

[0155] Finally, it should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0156] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0157] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A zero-sequence circulating current cooperative control method, characterized in that, include: Detect the amplitude of the zero-sequence circulating current and the impedance parameters of each converter to determine the current operating condition type; When the current operating condition is steady-state, the zero-sequence circulating current amplitude is reduced through the coordinated processing of harmonic injection control and dynamic virtual impedance control. The harmonic injection control involves superimposing a third harmonic component into the converter modulation wave and coordinating the injection phase of the harmonics through the phase difference between adjacent converters, so that the zero-sequence voltage components output by each converter cancel each other out. The harmonic injection amplitude is adjusted in real time based on zero-sequence current error compensation. The dynamic virtual impedance control generates a frequency-varying impedance covering the fundamental frequency and multiple harmonic frequency bands, and increases the equivalent impedance of the circulating current path by connecting it in parallel to the zero-sequence circulating current path. The amplitude-frequency characteristics of the frequency-varying impedance are optimized in real time according to the circulating current harmonic content. During transient operating conditions, the transient circulation suppression time is shortened by the harmonic injection control, wherein the harmonic injection amplitude is determined based on the zero-sequence circulation mutation rate and the amplitude range is limited by a first-order inertial element to suppress circulation mutations; When the parameters are mismatched, impedance equalization compensation is performed through the dynamic virtual impedance control, wherein the frequency-varying impedance is corrected based on the online identified and updated impedance parameters of each converter, so that the equivalent impedance deviation of the parallel converters converges to the preset tolerance range.

2. The method according to claim 1, characterized in that, The formula for calculating the harmonic injection amplitude based on zero-sequence current error adjustment is as follows: in, The compensation value for the harmonic injection amplitude at time t. Let be the zero-sequence current error at time t. , , These are the proportional coefficient, integral coefficient, and differential coefficient, respectively, where t is time . For the damping ratio, For natural frequency, This represents the system's open-loop gain.

3. The method according to claim 1, characterized in that, The expression for the frequency-varying impedance in the dynamic virtual impedance control is: The objective function for frequency-varying impedance optimization is: in, For frequency-varying impedance, For the Laplace operator, , , These are the parameters of the frequency-varying resistor, inductor, and capacitor, respectively. The harmonic gain coefficient is dynamically adjusted according to the circulating harmonic content. The harmonic order is 3, 6, or 9. The fundamental angular frequency, The highest harmonic order, and The weighting coefficient is used to balance the harmonic suppression effect with the injection amplitude constraint.

4. The method according to claim 1, characterized in that, Under the parameter mismatch condition, the update formula for updating the impedance parameters of each converter is identified online using the recursive least squares method: ， in, This is the updated impedance parameter vector of the k-th converter at time t. Let be the zero-sequence impedance parameter vector of the k-th converter. Zero-sequence resistance, It is a zero-sequence inductance. For the observation vector, It is the zero-sequence current. Forgetting factor, Let covariance matrix be the variance matrix. The zero-sequence voltage measurement value at time t.

5. The method according to claim 1, characterized in that, The formula for coordinating the phase difference between adjacent converters in the harmonic injection control is as follows: in, The phase compensation angle for the k-th converter is... Let be the zero-sequence current phase angle of the i-th adjacent converter. Let n be the zero-sequence current phase angle of the k-th converter, and n be the number of adjacent converters.

6. The method according to claim 5, characterized in that, In the harmonic injection control, a third harmonic component is superimposed on the converter modulation wave. The reconstructed modulation wave generated after coordinating the injection phase of the harmonics through the phase difference between adjacent converters is as follows: in, This represents the reconstructed modulated wave signal of the k-th converter after superposition and correction in phase x. This represents the original converter modulation signal of the k-th converter before phase x-correction. To compensate for the amplitude of harmonics. The phase compensation angle for the k-th converter is... ω is the fundamental angular frequency.

7. A zero-sequence circulating current cooperative control device, characterized in that, include: The operating condition detection unit is used to detect the amplitude of the zero-sequence circulating current and the impedance parameters of each converter to determine the current operating condition type. The steady-state operating condition unit is used to reduce the zero-sequence circulating current amplitude by coordinating harmonic injection control and dynamic virtual impedance control when the current operating condition is steady-state. The harmonic injection control involves superimposing a third harmonic component into the converter modulation wave and coordinating the injection phase of the harmonics through the phase difference between adjacent converters, so that the zero-sequence voltage components output by each converter cancel each other out. The harmonic injection amplitude is adjusted in real time based on zero-sequence current error compensation. The dynamic virtual impedance control generates a frequency-varying impedance covering the fundamental frequency and multiple harmonic frequency bands, and increases the equivalent impedance of the circulating current path by connecting it in parallel to the zero-sequence circulating current path. The amplitude-frequency characteristics of the frequency-varying impedance are optimized in real time according to the circulating current harmonic content. The transient operating condition unit is used to shorten the transient circulation suppression time during transient operating conditions by means of the harmonic injection control, wherein the harmonic injection amplitude is determined based on the zero-sequence circulation mutation rate and the amplitude range is limited by a first-order inertial element to suppress circulation mutation; The mismatch condition unit is used to perform impedance equalization compensation through the dynamic virtual impedance control when the parameters are mismatched. The frequency-varying impedance is corrected based on the online identified and updated impedance parameters of each converter, so that the equivalent impedance deviation of the parallel converters converges to a preset tolerance range.

8. A zero-sequence circulating current cooperative control device, characterized in that, Including memory and processor; The memory is used to store programs; The processor is used to execute the program to implement each step of the zero-sequence circulating current cooperative control method as described in any one of claims 1-6.

9. A readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements each step of the zero-sequence circulating flow cooperative control method as described in any one of claims 1-6.

10. A computer program product, comprising a computer program, characterized in that, The computer program is executed by the processor to perform the various steps of the zero-sequence circulating flow cooperative control method as described in any one of claims 1-6.

Images