A method for early warning of converter AC filter element faults based on the variation of harmonic coefficients in the injection system.

Abstract

This invention discloses a method for early warning of AC filter element faults in a converter based on the variation of the injected system harmonic coefficients. The method includes: acquiring discrete waveform sequences of the AC grid-connected current of the converter; determining the harmonic filtering order and corresponding frequency group of the AC filter; dynamically detecting the instantaneous amplitude and instantaneous phase sequences of specific sub-current harmonics; calculating the theoretical injected system harmonic coefficient values ​​of the sub-harmonics; calculating the actual injected system harmonic coefficient values ​​of the sub-harmonics; calculating the equivalent capacitance deviation and equivalent inductance deviation of the AC filter elements; obtaining the relative change statistics of the filter elements; and realizing the element fault analysis and alarm of the corresponding tuned filter group. This invention utilizes only existing grid-side harmonic monitoring data of the converter to calculate the equivalent parameter deviations of the AC filter elements, achieving early warning and judgment of AC filter element faults in the converter. It features low cost, resistance to grid fundamental frequency disturbances, and ease of engineering implementation.

Description

Technical Field

[0001] This invention belongs to the field of power system and automation technology, and relates to AC filter element fault early warning technology, specifically to a converter AC filter element fault early warning method based on the change value of the injected system harmonic coefficient. Background Technology

[0002] As a crucial component of converters, AC filters play a vital role in harmonic suppression and reactive power compensation. AC filter banks are widely used in power grid construction, significantly maintaining the stable operation of the power system, improving the power factor and supply efficiency, reducing potential losses in transmission lines, and improving the overall operating environment of the power grid. However, the failure of a capacitor or inductor component within an AC filter bank can lead to overcurrent or overvoltage in other components, resulting in a more severe fault affecting the entire filter bank. This significantly hinders the normal operation of the converter, causing harmonic pollution of the power grid, overheating damage to components, and even threatening the safety of the entire substation. The causes of AC filter component failures are diverse, including internal short circuits in capacitors, capacitor component breakdown, capacitor component leakage, blown capacitor fuses, and inter-turn short circuits in the coil. This variety of causes increases the difficulty of monitoring and early warning of AC filter component failures.

[0003] Currently, fault early warning and diagnosis of AC filters and their components have played a certain role in detecting defects and preventing accidents, but some shortcomings still exist. One researcher proposed an early fault warning method for capacitor banks based on the change in LC oscillation frequency. This method involves collecting discharge PT voltage and bus voltage, calculating the oscillation frequency of the LC circuit, recording the initial value of this frequency, and then using the change in this frequency during capacitor operation for early warning judgment. This method involves complex data acquisition and requires various parameters of the capacitor bank for judgment. Another researcher proposed a fault early warning technology based on monitoring filter component parameter values. This involves obtaining voltage signals from the bus-side voltage transformer and current signals from the loop current transformer, and studying a power capacitor capacitance C value monitoring scheme. Although the principle of this method is simple, it requires the installation of new monitoring equipment, which is costly and difficult to install. It also requires eliminating the influence of the reactor inductance value L, making it relatively complex. Furthermore, some literature has proposed filter fault diagnosis based on hysteresis control and establishing a harmonic compensation current model for predictive fault-tolerant control. Both methods can effectively diagnose faults, but they are only applicable to active filters and cannot be adapted to AC filters lacking controllers. Since the aforementioned factors have not been considered in existing methods, new methods for island detection still need to be researched. Summary of the Invention

[0004] Purpose of the invention: In order to overcome the shortcomings of the existing technology, a fault early warning method for AC filter components of converters based on the change value of the harmonic coefficient of the injected system is provided. The method only uses the existing grid-side harmonic monitoring data of the converter to calculate the equivalent parameter deviation of the AC filter components, thereby realizing the fault early warning judgment of the AC filter components of the converter. It has the characteristics of low cost, resistance to grid fundamental frequency disturbance, and easy engineering implementation.

[0005] Technical Solution: To achieve the above objectives, this invention provides a method for early warning of faults in AC filter components of a converter based on the variation value of the injected system harmonic coefficients, comprising the following steps:

[0006] S1: The discrete waveform sequence of the AC grid-connected current of the converter is acquired using the synchronous sampling method;

[0007] S2: Based on the AC filter configuration structure and component parameters of the converter, determine the harmonic filtering order and corresponding frequency group of the AC filter;

[0008] S3: The improved adaptive notch filter digital filtering algorithm is used to adaptively filter the discrete instantaneous waveform sequence of the three-phase current on the grid side of the converter, so as to realize the dynamic measurement of the amplitude of a given harmonic with transient response capability, and dynamically detect the instantaneous amplitude sequence and instantaneous phase sequence of a specific current harmonic.

[0009] S4: For a specific subharmonic, calculate the theoretical injection system harmonic coefficient value of the subharmonic based on the given AC filter structure and component group parameters;

[0010] S5: Calculate the actual subharmonic coefficient value of the injected subharmonic based on the specific subharmonic amplitude of the current dynamically detected by the improved adaptive notch filter digital filtering algorithm and the current harmonic amplitude before the converter filter.

[0011] S6: Compare the theoretical and actual harmonic coefficient values ​​of the injected system to obtain the change in the harmonic coefficient of the injected system. Calculate the change in the resonant frequency of the AC filter. Combine this with the harmonic frequency deviation estimated by the instantaneous phase output to deduce the equivalent capacitance deviation and equivalent inductance deviation of the AC filter components.

[0012] S7: Statistically analyze the equivalent component deviation change sequence for a certain period of time, and use the maximum value with a 95% probability to obtain the statistical value of the relative change of the filter component for that period of time;

[0013] S8: Compare the statistical value of the relative change of filter element parameters in the current period with the set threshold for the change of element parameters. If the former exceeds 50% of the latter, it will indicate that the filter element group may be abnormal or faulty. If the former exceeds 100% of the latter, it will issue a fault alarm for the filter element group and take countermeasures.

[0014] S9: Based on the other filtering harmonic frequencies of the AC filter, adjust the notch parameters of the improved adaptive notch digital filter algorithm in sequence, and repeat steps S3 to S8. By analyzing the statistical values ​​of the relative changes of the corresponding components, the component fault analysis and alarm of the corresponding tuned filter group can be realized.

[0015] Furthermore, in step S2, the AC filter configuration type structure mainly considers single-tuned filters and double-tuned filters. The parameters mainly consider the number of groups of tuned filters, center frequency, and quality parameters. The harmonic filtering number of the AC filter is equal to the number of groups of single-tuned filters plus twice the number of groups of double-tuned filters. The corresponding frequency group is constructed from the center frequency sequence of single-tuned filters and double-tuned filters.

[0016] Furthermore, the system equation for the improved adaptive notch digital filter in step S3 is as follows:

[0017]

[0018]

[0019] Where the input u(k) = A0sin(2πf) n0 t+δ0)+g(t), g(t) does not contain f n0 The frequency components, W1 and W2, are normal coefficients. The state mean of this notch filter system converges to the amplitude and initial phase (A0, δ0) of a specific frequency sinusoidal component of the external input. The system has three output quantities.

[0020] The most significant feature of the improved adaptive notch filter digital filter system described above is that Each can quickly track the f of the external input. n0 The harmonic components and their corresponding instantaneous amplitude and instantaneous phase are effectively converged even when there are small changes in the fundamental frequency and harmonic frequency of the external input signal. This is achieved through the f... n0 The instantaneous amplitude output A(k) of the harmonic component can detect the dynamic changes in the amplitude of specific harmonics of the AC grid side current of the converter, including the start and end times of the changes, amplitude, phase and other characteristic quantities.

[0021] Furthermore, in step S3, the improved adaptive notch filter digital filter system forms a closed-loop phase feedback control system that tracks the instantaneous phase of the input nth harmonic in real time; where x is the input signal, e is the error signal, y is the nth harmonic component signal of x, A is the instantaneous amplitude of y, and ω... n0 ω is the reference frequency of the nth harmonic (without considering frequency deviation), ω is the instantaneous angular frequency of the nth harmonic, and φ is the instantaneous phase of the nth harmonic.

[0022] Furthermore, the specific calculation process for the theoretically injected system harmonic coefficient values ​​in step S4 is as follows:

[0023] A1: Based on the common double-tuned filter structure used in AC filters, its impedance is:

[0024]

[0025] A2: Obtain the impedance-frequency response curve of the double-tuned AC filter based on the impedance formula;

[0026] A3: To minimize the impedance of the dual-tuned AC filter, take the two lowest points of the impedance-frequency response curve to obtain the two resonant frequencies of the filter.

[0027] A4: Calculate the filter harmonic impedance and system harmonic impedance at two resonant frequencies using the given component parameters of the AC filter.

[0028] A5: Calculate the theoretical value of the injected system harmonic coefficient from the filter harmonic impedance and the system harmonic impedance, using the following formula;

[0029]

[0030] Among them, Z fn Z sn These are the filter harmonic impedance and the system harmonic impedance, respectively.

[0031] Furthermore, the calculation process for the actual injected harmonic coefficient values ​​of the subharmonics in step S5 is as follows:

[0032] B1: Setting the notch parameter f of the improved adaptive notch digital filter system n0 The frequency estimate of the harmonic component to be detected is the harmonic order multiplied by 50Hz.

[0033] B2: After the output of the digital filtering system stabilizes, the output signal sequence A(k) is extracted to obtain the actual amplitude of the harmonic current after AC filtering of the converter;

[0034] B3: Read the current waveform at the front end of the converter filter and use the fast Fourier transform to obtain the amplitude of the harmonic of the current before the converter filter.

[0035] B4: Divide the actual amplitude of the filtered secondary current harmonic by the amplitude of the secondary harmonic of the current before filtering in the converter to calculate the actual injected system harmonic coefficient value of the secondary harmonic.

[0036] Furthermore, the calculation process for the equivalent capacitance deviation and equivalent inductance deviation of the AC filter element in step S6 is as follows:

[0037] C1: Based on the actual harmonic coefficient values ​​of the subharmonic injected into the system, and considering the type and quality factor of the corresponding filter bank, calculate the equivalent frequency deviation δ of the filter bank. f ;

[0038] C2: After the output of the digital filtering system stabilizes, extract the instantaneous phase output signal sequence.

[0039] C3: The actual frequency value of the subharmonic is calculated from the instantaneous phase output signal sequence, using the following formula:

[0040]

[0041] Where Ts is the sampling step size, f n This is the actual frequency value of the harmonic.

[0042] C4: Calculate the actual frequency deviation value δ f0 The formula is as follows:

[0043]

[0044] Where n is the harmonic order, and f0 is the theoretical fundamental frequency of the power grid, which is 50Hz;

[0045] C5: Considering that the probability of simultaneous failure of the filter components capacitors and inductors is very small, and ignoring the inductor parameter deviation, the equivalent capacitance deviation of the filter bank can be calculated using the following formula:

[0046] ΔC=2(δ f -δ f0 ).

[0047] Furthermore, the calculation process for the equivalent component deviation in step S7, using the maximum value with a 95% probability, is as follows:

[0048] The equivalent capacitance deviation value or equivalent capacitance deviation value of the AC filter is counted at each moment in the first 5 minutes to obtain an array of equivalent component deviation values. The array is sorted from largest to smallest according to absolute size. The first 5% of the maximum values ​​in the sorted array are removed. Then, the maximum value of the remaining 95% of the values ​​in the array is used as the final equivalent component deviation value ΔC for the first 5 minutes.

[0049] Furthermore, in step S8, if the equivalent element deviation value is small, the analysis of the next harmonic frequency is performed, i.e., step S9 is entered; if the equivalent element deviation value exceeds the threshold, it is determined that the filter element may be abnormal or faulty and countermeasures are taken.

[0050] The judgment rules are as follows:

[0051] If ΔC ≥ 0.1C0 and A(k) > 0.01I, where C0 is the capacitor's rated value and I is the effective value of the current, it indicates that the filter element may be abnormal or faulty; if ΔC ≥ 0.2C0 and A(k) > 0.02I, a filter element fault alarm is triggered, and countermeasures are taken.

[0052] This invention utilizes existing harmonic detection data and employs an improved adaptive notch filter digital filtering algorithm to detect and analyze the changes in harmonic coefficients of the injected system. Furthermore, it determines whether the AC filter components of the converter are faulty by analyzing the equivalent parameter deviation of the AC filter components. This invention features low cost, resistance to power grid fundamental frequency disturbances, and ease of engineering implementation.

[0053] The method of the present invention does not require the installation of a new filter element fault monitoring device, nor does it affect the normal operation of the filter. It only uses the existing harmonic monitoring data of the equipment to realize the component fault diagnosis. It has the characteristics of low cost, strong resistance to power grid fundamental frequency disturbance, and easy engineering implementation.

[0054] Beneficial effects: Compared with existing technologies, this invention does not affect the quality of the converter's output power, nor does it interfere with the normal filtering function of the AC filter. Furthermore, it can detect the deviation of the equivalent component parameters of the AC filter in real time and dynamically and quickly under the condition of grid frequency deviation, realizing robust early warning of AC filter component faults. It is applicable to the component fault conditions of various AC filters, tuned filters, and high-pass filters. It has the characteristics of high detection accuracy and good dynamic performance. It does not require the installation of new equipment, has low cost, and has good practicality. It is of great significance for improving the safe and stable operation of converters and AC filters and improving the power quality of the system. Attached Figure Description

[0055] Figure 1 This is a basic flowchart of the method of the present invention;

[0056] Figure 2 The principle block diagram of the improved adaptive notch filter digital filter system provided by the present invention. Detailed Implementation

[0057] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. After reading this invention, any modifications of the invention in various equivalent forms by those skilled in the art will fall within the scope defined by the appended claims.

[0058] This invention provides a method for early warning of converter AC filter element faults based on the variation value of the injected system harmonic coefficient, such as... Figure 1 As shown, it includes the following steps:

[0059] S1: The discrete waveform sequence of the AC grid-connected current of the converter is acquired using the synchronous sampling method;

[0060] S2: Based on the AC filter configuration structure and component parameters of the converter, determine the harmonic filtering order and corresponding frequency group of the AC filter;

[0061] S3: The improved adaptive notch filter digital filtering algorithm is used to adaptively filter the discrete instantaneous waveform sequence of the three-phase current on the grid side of the converter, so as to realize the dynamic measurement of the amplitude of a given harmonic with transient response capability, and dynamically detect the instantaneous amplitude sequence and instantaneous phase sequence of a specific current harmonic.

[0062] S4: For a specific subharmonic, calculate the theoretical injection system harmonic coefficient value of the subharmonic based on the given AC filter structure and component group parameters;

[0063] S5: Calculate the actual subharmonic coefficient value of the injected subharmonic based on the specific subharmonic amplitude of the current dynamically detected by the improved adaptive notch filter digital filtering algorithm and the current harmonic amplitude before the converter filter.

[0064] S6: Compare the theoretical and actual harmonic coefficient values ​​of the injected system to obtain the change in the harmonic coefficient of the injected system. Calculate the change in the resonant frequency of the AC filter. Combine this with the harmonic frequency deviation estimated by the instantaneous phase output to deduce the equivalent capacitance deviation and equivalent inductance deviation of the AC filter components.

[0065] S7: Statistically analyze the equivalent component deviation change sequence for a certain period of time, and use the maximum value with a 95% probability to obtain the statistical value of the relative change of the filter component for that period of time;

[0066] S8: Compare the statistical value of the relative change of filter element parameters in the current period with the set threshold for the change of element parameters. If the former exceeds 50% of the latter, it will indicate that the filter element group may be abnormal or faulty. If the former exceeds 100% of the latter, it will issue a fault alarm for the filter element group and take countermeasures.

[0067] S9: Based on the other filtering harmonic frequencies of the AC filter, adjust the notch parameters of the improved adaptive notch digital filter algorithm in sequence, and repeat steps S3 to S8. By analyzing the statistical values ​​of the relative changes of the corresponding components, the component fault analysis and alarm of the corresponding tuned filter group can be realized.

[0068] In step S2, the AC filter configuration type structure mainly considers single-tuned filters and double-tuned filters. The parameters mainly consider the number of tuning filter groups, center frequency, and quality parameters. The harmonic filtering number of the AC filter is equal to the number of single-tuned filter groups plus twice the number of double-tuned filter groups. The corresponding frequency group is constructed from the center frequency sequence of the single-tuned filter and the double-tuned filter.

[0069] The system equation for the improved adaptive notch digital filter in step S3 is as follows:

[0070]

[0071]

[0072] Where the input u(k) = A0sin(2πf) n0 t+δ0)+g(t), g(t) does not contain f n0 The frequency components, W1 and W2, are normal coefficients. The state mean of this notch filter system converges to the amplitude and initial phase (A0, δ0) of a specific frequency sinusoidal component of the external input. The system has three output quantities.

[0073] The most significant feature of the improved adaptive notch filter digital filter system described above is that Each can quickly track the f of the external input. n0 The harmonic components and their corresponding instantaneous amplitude and instantaneous phase are effectively converged even when there are small changes in the fundamental frequency and harmonic frequency of the external input signal. This is achieved through the f... n0 The instantaneous amplitude output A(k) of the harmonic component can detect the dynamic changes in the amplitude of specific harmonics of the AC grid side current of the converter, including the start and end times of the changes, amplitude, phase and other characteristic quantities.

[0074] The specific structure of the improved adaptive notch digital filter system in step S3 is as follows: Figure 2 As shown, this system forms a closed-loop phase feedback control system that tracks the instantaneous phase of the input nth harmonic in real time; where x is the input signal, e is the error signal, y is the nth harmonic component of x, A is the instantaneous amplitude of y, and ω... n0 ω is the reference frequency of the nth harmonic (without considering frequency deviation), ω is the instantaneous angular frequency of the nth harmonic, and φ is the instantaneous phase of the nth harmonic.

[0075] The specific calculation process for the theoretically injected system harmonic coefficient values ​​in step S4 is as follows:

[0076] A1: Based on the common double-tuned filter structure used in AC filters, its impedance is:

[0077]

[0078] A2: Obtain the impedance-frequency response curve of the double-tuned AC filter based on the impedance formula;

[0079] A3: To minimize the impedance of the dual-tuned AC filter, take the two lowest points of the impedance-frequency response curve to obtain the two resonant frequencies of the filter.

[0080] A4: Calculate the filter harmonic impedance and system harmonic impedance at two resonant frequencies using the given component parameters of the AC filter.

[0081] A5: Calculate the theoretical value of the injected system harmonic coefficient from the filter harmonic impedance and the system harmonic impedance, using the following formula;

[0082]

[0083] Among them, Z fn Z sn These are the filter harmonic impedance and the system harmonic impedance, respectively.

[0084] The calculation process for the actual injected harmonic coefficient values ​​of the subharmonic in step S5 is as follows:

[0085] B1: Setting the notch parameter f of the improved adaptive notch digital filter system n0 The frequency estimate of the harmonic component to be detected is the harmonic order multiplied by 50Hz.

[0086] B2: After the output of the digital filtering system stabilizes, the output signal sequence A(k) is extracted to obtain the actual amplitude of the harmonic current after AC filtering of the converter;

[0087] B3: Read the current waveform at the front end of the converter filter and use the fast Fourier transform to obtain the amplitude of the harmonic of the current before the converter filter.

[0088] B4: Divide the actual amplitude of the filtered secondary current harmonic by the amplitude of the secondary harmonic of the current before filtering in the converter to calculate the actual injected system harmonic coefficient value of the secondary harmonic.

[0089] The calculation process for the equivalent capacitance deviation and equivalent inductance deviation of the AC filter components in step S6 is as follows:

[0090] C1: Based on the actual harmonic coefficient values ​​of the subharmonic injected into the system, and considering the type and quality factor of the corresponding filter bank, calculate the equivalent frequency deviation δ of the filter bank. f ;

[0091] C2: After the output of the digital filtering system stabilizes, extract the instantaneous phase output signal sequence.

[0092] C3: The actual frequency value of the subharmonic is calculated from the instantaneous phase output signal sequence, using the following formula:

[0093]

[0094] Where Ts is the sampling step size, f n This is the actual frequency value of the harmonic.

[0095] C4: Calculate the actual frequency deviation value δ f0 The formula is as follows:

[0096]

[0097] Where n is the harmonic order, and f0 is the theoretical fundamental frequency of the power grid, which is 50Hz;

[0098] C5: Considering that the probability of simultaneous failure of the filter components capacitors and inductors is very small, and ignoring the inductor parameter deviation, the equivalent capacitance deviation of the filter bank can be calculated using the following formula:

[0099] ΔC=2(δ f -δ f0 ).

[0100] The calculation process for the equivalent component deviation in step S7, using the maximum value with a 95% probability, is as follows:

[0101] The equivalent capacitance deviation value or equivalent capacitance deviation value of the AC filter is counted at each moment in the first 5 minutes to obtain an array of equivalent component deviation values. The array is sorted from largest to smallest according to absolute size. The first 5% of the maximum values ​​in the sorted array are removed. Then, the maximum value of the remaining 95% of the values ​​in the array is used as the final equivalent component deviation value ΔC for the first 5 minutes.

[0102] If the equivalent element deviation value is small in step S8, the analysis of the next harmonic frequency is performed, i.e., step S9 is entered; if the equivalent element deviation value exceeds the threshold, it is determined that the filter element may be abnormal or faulty and countermeasures are taken.

[0103] The judgment rules are as follows:

[0104] If ΔC ≥ 0.1C0 and A(k) > 0.01I, where C0 is the capacitor's rated value and I is the effective value of the current, it indicates that the filter element may be abnormal or faulty; if ΔC ≥ 0.2C0 and A(k) > 0.02I, a filter element fault alarm is triggered, and countermeasures are taken.

[0105] To verify the effectiveness of the above scheme, this embodiment uses a typical DC-DC converter scheme and a simulation is built in Matlab / Simulnk. The converter is a 12-pulse rectifier, and the AC filter includes one 11th / 13th order double-tuned filter and one 24th harmonic high-pass filter. The simulation results of component fault detection are shown in the table below:

[0106] Table 1 Harmonic coefficients of the converter grid-side 11th / 13th harmonic injection system under different component failure magnitudes.

[0107]

[0108] Matlab simulation results show that changes in the component parameter values ​​of the filter cause significant changes in the harmonic coefficients of the AC injection system in the converter, thereby revealing the severity of faults in the AC filter components. This demonstrates the correctness and reliability of the early warning and troubleshooting method proposed in this invention. The simulation results prove the correctness of the novel AC filter component fault detection method proposed in this embodiment for converters, which can reduce or even eliminate the detection blind zone while ensuring high detection efficiency and high power quality.

[0109] Based on the technical solution of the method of the present invention and the simulation results of the embodiments, the following conclusions and principles can be obtained:

[0110] The method of this invention can comprehensively consider the accidental changes in harmonic voltage caused by distributed power sources and power grid systems under normal grid-connected operation and islanded conditions. It has the characteristics of high detection accuracy, small detection blind zone, and easy engineering implementation.

[0111] The technical principle of the method of the present invention is as follows:

[0112] First, based on the AC filter configuration and component parameters of the converter, the harmonic filtering order and corresponding frequency group of the AC filter are determined. Then, a customized improved adaptive notch filter digital filtering algorithm is used to dynamically detect harmonics in the acquired AC side current of the converter, obtaining the instantaneous amplitude and frequency of the current harmonics. Next, the actual injected system harmonic coefficient of this harmonic is calculated and compared with the theoretical injected system harmonic coefficient calculated based on the given AC filter structure type and component parameters to obtain the change value of the injected system harmonic coefficient. This allows for the estimation of the equivalent capacitance deviation and equivalent inductance deviation of the AC filter components. Finally, based on the 95% probability maximum value of the equivalent component deviation during a statistical period, the relative change threshold of the components is compared to determine the fault status of the AC filter components. If the threshold is exceeded, an AC filter component fault alarm is issued, and corresponding measures are taken. This embodiment of the invention does not require the installation of a new filter component fault monitoring device and will not affect the normal operation of the filter. It only utilizes the existing harmonic monitoring data of the equipment to achieve component fault diagnosis, featuring low cost, strong resistance to power grid fundamental frequency disturbances, and ease of engineering implementation.

[0113] This invention does not affect the quality of the converter's output power, nor does it interfere with the normal filtering function of the AC filter. Furthermore, it can dynamically and rapidly detect deviations in the equivalent component parameters of the AC filter in real time under grid frequency deviations, achieving robust early warning of AC filter component faults. It is applicable to various AC filters, including various tuned filters and high-pass filters, and features high detection accuracy and good dynamic performance. It requires no new equipment installation, is low-cost, and has good practicality. It is of great significance for improving the safe and stable operation of converters and AC filters and for improving system power quality.

Claims

1. A method for early warning of faults in AC filter components of a converter based on the variation value of the harmonic coefficient of the injected system, characterized in that, Includes the following steps: S1: The discrete waveform sequence of the AC grid-connected current of the converter is acquired using the synchronous sampling method; S2: Based on the AC filter configuration structure and component parameters of the converter, determine the harmonic filtering order and corresponding frequency group of the AC filter; S3: The improved adaptive notch filter digital filtering algorithm is used to adaptively filter the discrete instantaneous waveform sequence of the three-phase current on the grid side of the converter, so as to realize the dynamic measurement of the amplitude of a given harmonic with transient response capability, and dynamically detect the instantaneous amplitude sequence and instantaneous phase sequence of a specific current harmonic. S4: For a specific subharmonic, calculate the theoretical injection system harmonic coefficient value of the subharmonic based on the given AC filter structure and component group parameters; S5: Calculate the actual subharmonic coefficient value of the injected subharmonic based on the specific subharmonic amplitude of the current dynamically detected by the improved adaptive notch filter digital filtering algorithm and the current harmonic amplitude before the converter filter. S6: Compare the theoretical and actual harmonic coefficient values ​​of the injected system to obtain the change in the harmonic coefficient of the injected system, calculate the change in the resonant frequency of the AC filter, and then combine the harmonic frequency deviation estimated by the instantaneous phase output to deduce the equivalent capacitance deviation and equivalent inductance deviation of the AC filter components. S7: Statistically analyze the equivalent component deviation change sequence for a certain period of time, and use the value with the highest probability of 95% to obtain the statistical value of the relative change of the filter component for that period of time; S8: Compare the statistical value of the relative change of filter element parameters in the current period with the set threshold for the change of element parameters. If the former exceeds 50% of the latter, it will indicate that the filter element group is abnormal or faulty. If the former exceeds 100% of the latter, it will issue a fault alarm for the filter element group and take countermeasures. S9: Based on the other filtering harmonic frequencies of the AC filter, adjust the notch parameters of the improved adaptive notch digital filter algorithm in sequence, and repeat steps S3 to S8. By analyzing the statistical values ​​of the relative changes of the corresponding components, the component fault analysis and alarm of the corresponding tuned filter group can be realized. The system equation for the improved adaptive notch digital filter in step S3 is as follows: ； Among them, input , Not included Frequency components, W1 and W2 are normal coefficients. The state mean of the notch filter system converges to the amplitude and initial phase of the specific frequency sinusoidal component of the external input. The notch filter system has three outputs. ; The specific calculation process for the theoretically injected system harmonic coefficient values ​​in step S4 is as follows: A1: Based on the fact that the AC filter adopts a double-tuned filter structure, its impedance is: ； A2: Obtain the impedance-frequency response curve of the double-tuned AC filter based on the impedance formula; A3: To minimize the impedance of the dual-tuned AC filter, take the two lowest points of the impedance-frequency response curve to obtain the two resonant frequencies of the filter. A4: Calculate the filter harmonic impedance and system harmonic impedance at two resonant frequencies using the given component parameters of the AC filter. A5: Calculate the theoretical value of the injected system harmonic coefficient from the filter harmonic impedance and the system harmonic impedance, using the following formula; ； in, These are the filter harmonic impedance and the system harmonic impedance, respectively.

2. The converter AC filter element fault early warning method based on the change value of the injected system harmonic coefficient as described in claim 1, characterized in that, In step S2, the harmonic filtering number of the AC filter is equal to the number of single-tuned filter groups plus twice the number of double-tuned filter groups, and the corresponding frequency group is constructed from the center frequency sequence of the single-tuned filter and the double-tuned filter.

3. The converter AC filter element fault early warning method based on the change value of the injected system harmonic coefficient as described in claim 1, characterized in that, In step S3, the improved adaptive notch filter digital filter system forms a closed-loop phase feedback control system that tracks the instantaneous phase of the input nth harmonic in real time; where x is the input signal, e is the error signal, y is the nth harmonic component signal of x, A is the instantaneous amplitude of y, and ω n0 ω is the reference frequency of the nth harmonic, ω is the instantaneous angular frequency of the nth harmonic, and φ is the instantaneous phase of the nth harmonic.

4. The converter AC filter element fault early warning method based on the change value of the injected system harmonic coefficient as described in claim 1, characterized in that, The calculation process for the actual injected harmonic coefficient values ​​of the subharmonics in step S5 is as follows: B1: Setting the notch parameters for the improved adaptive notch digital filter system The frequency estimate of the harmonic component to be detected is the harmonic order multiplied by 50Hz. B2: After the output of the digital filtering system stabilizes, extract the output signal sequence. The actual amplitude of the subharmonic current after AC filtering of the converter is obtained. B3: Read the current waveform at the front end of the converter filter and use the fast Fourier transform to obtain the amplitude of the harmonic of the current before the converter filter. B4: Divide the actual amplitude of the filtered secondary current harmonic by the amplitude of the secondary harmonic of the current before filtering in the converter to calculate the actual injected system harmonic coefficient value of the secondary harmonic.

5. The converter AC filter element fault early warning method based on the change value of the injected system harmonic coefficient as described in claim 4, characterized in that, The calculation process for the equivalent capacitance deviation and equivalent inductance deviation of the AC filter element in step S6 is as follows: C1: Calculate the equivalent frequency deviation of the filter bank based on the actual harmonic coefficient values ​​injected into the system, combined with the type and quality factor of the corresponding filter bank. ; C2: After the output of the digital filtering system stabilizes, extract the instantaneous phase output signal sequence. ; C3: The actual frequency value of the subharmonic is calculated from the instantaneous phase output signal sequence, using the following formula: ； in, The sampling step size, This is the actual frequency value of the harmonic. C4: Calculate the actual frequency deviation value The formula is as follows: ； Where n is the harmonic order. This is the theoretical fundamental frequency of the power grid; C5: Considering that the probability of simultaneous failure of both the capacitor and inductor components of the filter is less than a set threshold, and ignoring the inductor parameter deviation, the equivalent capacitance deviation of the filter bank is calculated using the following formula: 。 6. The converter AC filter element fault early warning method based on the change value of the injected system harmonic coefficient as described in claim 1, characterized in that, The calculation process for the equivalent component deviation in step S7, using the maximum value with a 95% probability, is as follows: The equivalent capacitance deviation values ​​of the AC filter are recorded at various times during the first 5 minutes, resulting in an array of equivalent component deviation values. This array is then sorted from largest to smallest based on absolute value. The top 5% of the largest values ​​are removed, and the remaining 95% of the largest values ​​are used as the final equivalent component deviation values ​​for the first 5 minutes. .

7. A method for early warning of converter AC filter element faults based on the change value of the injected system harmonic coefficients according to claim 6, characterized in that, If the equivalent element deviation value is less than the threshold in step S8, the analysis of the next harmonic frequency is performed, i.e., step S9 is entered; if the equivalent element deviation value exceeds the threshold, the filter element is determined to be abnormal or faulty and countermeasures are taken. The judgment rules are as follows: like and Where C0 is the rated value of the capacitor. If the value is the effective current value, it indicates an abnormality or malfunction of the filter component; if... and If the filter element fails, a fault alarm will be triggered, and corresponding measures will be taken.

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