Capacitor voltage fluctuation suppression method and system for modular multilevel converter

Abstract

The invention discloses a modular multilevel converter capacitor voltage fluctuation suppression method and system, and belongs to the technical field of power electronics. The system comprises an MMC main body circuit and a double-band-pass filtering module. The MMC main body circuit is composed of a three-phase bridge arm group, a bridge arm inductor and a plurality of sub-modules, the double-band-pass filtering module is composed of a first band-pass filter and a second band-pass filter, and the first band-pass filter and the second band-pass filter both adopt a topological structure in which an inductor and a capacitor are connected in series and are directly connected with each sub-module capacitor in parallel. The center frequency of the first band-pass filter is matched with a fluctuation component corresponding to a fundamental frequency in the capacitor voltage of the MMC sub-module, and the center frequency of the second band-pass filter is matched with a fluctuation component corresponding to a second harmonic frequency in the capacitor voltage of the sub-module. According to the invention, a complex control algorithm is not needed, the structure is simple, the capacitor voltage fluctuation amplitude can be obviously reduced, the submodule capacitor type selection capacity is reduced, and the method is suitable for various MMC application scenes.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronic conversion, in particular to a modular multilevel converter capacitor voltage fluctuation suppression method and system. BACKGROUND

[0002] As an advanced power electronic conversion technology, MMC (Modular Multilevel Converter) has outstanding advantages such as modular design, easy expansion, and low energy loss, and has become a research hotspot in recent years in the fields of flexible HVDC transmission, new energy grid connection, and traction power supply. As a core component of MMC, the voltage stability of the sub-module energy storage capacitor directly determines the operational reliability of the system, and the voltage fluctuation problem has always been a key bottleneck restricting the performance improvement of MMC.

[0003] The essence of MMC sub-module capacitor voltage fluctuation is the unbalanced charging and discharging of the capacitor caused by the alternating component in the instantaneous power of the bridge arm, among which the fundamental and second harmonic components are the dominant factors. This voltage fluctuation not only increases the design requirements of the capacitor value, improves the equipment cost and volume, but also may cause uneven electrical stress, leading to increased voltage and current harmonic content, and thus affecting the control accuracy and reliability of the entire system.

[0004] To solve this problem, the existing technology proposes various suppression strategies. CN117895762A discloses an M3C sub-module capacitor voltage fluctuation suppression method based on active power decoupling, which transfers the pulsating energy to the capacitor of the decoupling circuit by parallel connecting an active power decoupling circuit at the DC side of the sub-module. CN119420149B proposes a SiC MMC sub-module capacitor voltage fluctuation suppression method, which realizes fluctuation suppression by simultaneously injecting 2nd, 4th, and 8th harmonic circulating currents and 3rd harmonic voltage. CN111682575B describes a three-phase series CA-MMC system with a voltage-stabilizing capacitor bridge arm, which uses a PR controller and a special modulation method to reduce the bridge arm capacitor.

[0005] In addition, scholars have also proposed solutions based on high-frequency chain interconnection. CN112271746B discloses a high-frequency chain interconnected electrolytic capacitor-free MMC topology structure, which interconnects all isolated sub-modules through the high-frequency link of the full-bridge structure to eliminate capacitor voltage ripple and 2x frequency circulating current in the bridge arm. CN118316289A proposes a virtual capacitor-based AC-AC MMC sub-module ripple voltage suppression method, which uses a bandpass filter to extract the ripple component and realizes suppression through virtual capacitor design.

[0006] Although various suppression strategies have been proposed in the prior art, the prior art solutions generally have the following shortcomings: first, the method based on control strategy can suppress the capacitor voltage fluctuation to a certain extent, but it often needs complex algorithm to realize, which increases the complexity and calculation burden of the control system, and the robustness in a wide power range needs to be improved; second, the active power decoupling method has good suppression effect, but it needs additional power devices and control circuit, which significantly increases the system cost and power consumption loss; third, the existing passive filtering scheme mostly uses single frequency design, which is difficult to effectively suppress the fluctuation components corresponding to the fundamental wave and the second harmonic wave at the same time, and the adaptability is poor; finally, some schemes have limitations in dynamic response speed, and it is difficult to quickly respond to load changes or system disturbances.

[0007] Therefore, in the face of the demand for suppressing the capacitor voltage fluctuation of the MMC sub-module, how to design a technical solution with strong adaptability, rapid response and stable suppression effect has become a key problem to be solved at present. SUMMARY

[0008] The technical problem to be solved by the present application: In view of the problem that the capacitor voltage of the MMC sub-module fluctuates too much due to the fundamental wave and the second harmonic wave in the prior art, and the existing suppression schemes have the problems of complex structure, high cost, large loss or poor adaptability, the present application provides a modular multilevel converter capacitor voltage fluctuation suppression method and system, which directly connects the double-bandpass filtering module designed for the capacitor of the sub-module in parallel, accurately filters out the fundamental wave and the second harmonic wave component, reduces the capacitor voltage fluctuation amplitude without changing the MMC main topology, reduces the power device capacity and the total amount of capacitor, and improves the economy and stability of the system.

[0009] The present application adopts the following technical solutions to solve the above technical problems:

[0010] Firstly, the present application provides a modular multilevel converter capacitor voltage fluctuation suppression system, the core topology design of which is the integrated structure of MMC main circuit and double-bandpass filtering module. The MMC main circuit is composed of three-phase bridge arm groups, bridge arm inductors and a plurality of sub-modules. The three-phase bridge arm groups are divided into upper bridge arm groups and lower bridge arm groups. Each upper bridge arm group and the corresponding lower bridge arm group are connected in series through a bridge arm inductor, and the series connection is connected to a common DC bus. The connection points of the upper bridge arm and the lower bridge arm through the bridge arm inductor are taken out as AC connection ends. Each sub-module adopts a half-bridge or full-bridge structure and has an internal energy storage capacitor. A plurality of sub-modules are connected in series to form the main part of each bridge arm, realizing the conversion and transmission of electric energy.

[0011] The double-band-pass filtering module is a key innovative part of the application, which is composed of a first band-pass filter and a second band-pass filter, both of which adopt a topology structure of inductance and capacitance in series, and are directly connected in parallel with the positive and negative poles of the energy storage capacitor of each sub-module at both ends, without changing any structure of the MMC main circuit, and can be directly integrated in the existing MMC sub-module. The center frequency of the first band-pass filter matches the fluctuation component corresponding to the fundamental frequency in the sub-module capacitor voltage, and the center frequency of the second band-pass filter matches the fluctuation component corresponding to the second harmonic frequency in the sub-module capacitor voltage.

[0012] Further, in the scheme proposed in the application, the parameters of the inductance and capacitance in the first and second band-pass filters satisfy the following resonance condition:

[0013]

[0014] Where f1 is the fundamental frequency, L1 is the inductance value of the first band-pass filter, and C1 is the capacitance value of the first band-pass filter; f2=2f1 is the second harmonic frequency, L2 is the inductance value of the second band-pass filter, and C2 is the capacitance value of the second band-pass filter.

[0015] On the other hand, the application proposes a method for suppressing the capacitor voltage fluctuation of a modular multilevel converter, which realizes effective suppression of the voltage fluctuation components corresponding to the fundamental frequency and the second harmonic frequency through a double-band-pass filtering module; wherein

[0016] The double-band-pass filtering module is composed of a first and a second band-pass filter, both of which adopt a topology structure of inductance and capacitance in series, and are connected in parallel with the positive and negative poles of the sub-module capacitor at both ends; the center frequency of the first band-pass filter matches the fluctuation component corresponding to the fundamental frequency in the sub-module capacitor voltage, and the center frequency of the second band-pass filter matches the fluctuation component corresponding to the second harmonic frequency in the sub-module capacitor voltage.

[0017] Further, to realize this function, the center frequency of the first band-pass filter needs to accurately match the fundamental frequency f1, and the inductance L1 and capacitance C1 parameters need to satisfy the resonance condition:

[0018]

[0019] Where f1 is the fundamental frequency, L1 is the inductance value of the first band-pass filter, and C1 is the capacitance value of the first band-pass filter.

[0020] The center frequency of the second band-pass filter needs to accurately match the second harmonic frequency f2, and the inductance L2 and capacitance C2 parameters need to satisfy the resonance condition:

[0021] ​​​​

[0022] wherein f2=2f1 is the second harmonic frequency, L2 is the inductance value of the second band-pass filter, and C2 is the capacitance value of the second band-pass filter.

[0023] Preferably, when the fundamental frequency f1=50Hz and the second harmonic frequency f2=100Hz, the inductance L1 and the capacitance C1 of the first band-pass filter can be selected as L1=2mH and C1=5.132mF, and the inductance L2 and the capacitance C2 of the second band-pass filter can be selected as L2=1mH and C2=2.533mF.

[0024] Through the above parameter design, when the MMC is running, the first band-pass filter resonates at the fundamental frequency and presents low impedance to filter out the fluctuation caused by the fundamental component of the bridge arm current, and the second band-pass filter resonates at the second harmonic frequency and presents low impedance to filter out the fluctuation caused by the second harmonic component of the circulating current, and the two work together to suppress the capacitor voltage fluctuation from the source.

[0025] Compared with the prior art, the above technical scheme of the present application has the following technical effects:

[0026] The technical scheme of the present application can effectively filter out the fundamental and second harmonic components in the capacitor current and significantly reduce the fluctuation of the sub-module capacitor voltage through the integrated structure design of the double band-pass filter module and the MMC main circuit. Compared with the prior art, the PSCAD simulation test results show that the capacitor voltage fluctuation range of the traditional MMC is 0kV-5kV, and the capacitor voltage fluctuation range after using the system of the present application is 1.5kV-2kV, and the fluctuation amplitude is reduced by 90%. The present application makes the power device capacity demand more reasonable, and there is no need to further improve the device voltage withstand level due to the capacitor voltage fluctuation, reduces the capacitor voltage fluctuation amplitude and the power device capacity and the total amount of capacitors without changing the MMC main topology, improves the economic efficiency and stability of the system, and is suitable for the capacitor voltage stability control of MMC sub-modules in the scenes of flexible DC power transmission and new energy grid connection. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The figure is a schematic diagram of the overall structure of the system for suppressing in the embodiment of the present application.

[0028] Figure 2 The figure is a schematic diagram of the parallel topology structure of the double band-pass filter module and the sub-module in the embodiment of the present application, wherein (a) is a schematic diagram of the sub-module parallel double band-pass filter structure, and (b) is a schematic diagram of the double band-pass filter structure.

[0029] Figure 3 The figure is a simulation diagram of the capacitor voltage fluctuation of the embodiment of the present application and the traditional MMC.

[0030] Reference signs: 1-MMC main circuit; 2-upper bridge arm group; 3-lower bridge arm group; 4-bridge arm inductor; 5-submodule; 6-capacitor; 7-double band-pass filter module; 8-first band-pass filter; 9-second band-pass filter.

[0031] Wherein, u a , u b , u c are MMC AC side three-phase voltages, u au , u bu , u cu are MMC upper bridge arm voltages, u al , u bl , u cl are MMC lower bridge arm voltages, U dc is DC side voltage, i au , i bu , i cu are MMC upper bridge arm currents, i al , i bl , i cl are MMC lower bridge arm currents, u sm is MMC submodule voltage, and SM is MMC submodule. DETAILED DESCRIPTION

[0032] The present application will be described in detail below with specific embodiments and drawings, and the following embodiments will help those skilled in the art to further understand the present application, but do not limit the present application in any form.

[0033] Embodiment 1: The present application provides a modular multilevel converter capacitor voltage fluctuation suppression system, as shown in the accompanying Figure 1 The overall structure of the suppression system in this embodiment includes MMC main circuit 1 and double band-pass filter module 7. The MMC main circuit 1 is composed of three-phase bridge arm groups (upper bridge arm group 2, lower bridge arm group 3), bridge arm inductors 4 and a plurality of submodules 5. The upper bridge arm group 2 of the three-phase bridge arm group and the corresponding lower bridge arm group 3 are connected to the common DC bus through the bridge arm inductor 4 in series, and the connection point of the upper bridge arm and the lower bridge arm is led out as an AC connection end. Each submodule 5 adopts a half-bridge structure, and an energy storage capacitor 6 is built-in. The voltage output is adjusted by controlling the on-off of the switching device, and a plurality of submodules 5 are connected in series to form the main part of each bridge arm. The function of the bridge arm inductor is to limit the circulating current between the bridge arms and ensure the stable operation of the system.

[0034] As shown in the accompanying Figure 2 , the double band-pass filter module 7 is composed of a first band-pass filter 8 and a second band-pass filter 9. The first band-pass filter is composed of an inductor L1 and a capacitor C1 in series, and the second band-pass filter is composed of an inductor L2 and a capacitor C2 in series. The two band-pass filters are connected in parallel with the submodule capacitor.

[0035] The center frequency of the first band-pass filter matches the fluctuation component corresponding to the fundamental frequency in the MMC sub-module capacitor voltage, and the center frequency of the second band-pass filter matches the fluctuation component corresponding to the second harmonic frequency in the sub-module capacitor voltage.

[0036] Based on the MMC mathematical model, the application further deduces the correlation between the bridge arm current, the circulating current and the sub-module capacitor voltage fluctuation. Taking the A phase as an example, the bridge arm current mainly contains a direct current component and an alternating current component of the fundamental frequency, and the upper and lower bridge arm currents of the A phase can be expressed as:

[0037] ,

[0038] Among them, and are the upper and lower bridge arm currents of the A phase, is the direct current, is the alternating current amplitude, and φ is the power factor angle, is the A phase circulating current mainly including a second harmonic component:

[0039] =I2sin(2ωt+θ),

[0040] where I2 is the amplitude of the second harmonic component in the A phase circulating current.

[0041] In normal operation, all capacitor voltages are dynamically balanced, so the average switching functions Spa and Sna of the upper and lower bridge arms of the A phase can be expressed as

[0042] ,

[0043] The sub-module is controlled by the switching function, so the current flowing through the sub-module capacitor can be expressed as:

[0044] ,

[0045] Therefore, the fundamental frequency fluctuation of the capacitor voltage of the upper bridge arm can be calculated as:

[0046] .

[0047] Similarly, the fundamental frequency fluctuation of the capacitor voltage of the lower bridge arm can be calculated as:

[0048] ,

[0049] By substituting the high-order current harmonics, the second harmonic fluctuation of the sub-module capacitor voltage can be obtained as follows:

[0050] .

[0051] From the above analysis, the fundamental component and the second harmonic component are the dominant factors of the capacitor voltage fluctuation, and the corresponding fluctuation frequencies are the fundamental frequency f1 and the second harmonic frequency f2, respectively, so that the fluctuation components corresponding to the two frequencies are filtered out, thereby realizing efficient suppression of the capacitor voltage fluctuation.

[0052] Based on the above analysis, the core function of the double-bandpass filtering module of the application is to bypass and filter out the fluctuation components corresponding to the fundamental and second harmonic through the resonance characteristic. The core characteristic of the bandpass filter is to present a low impedance characteristic at the center frequency, at which the attenuation of the frequency component is minimal and the conduction is strongest, and the corresponding fluctuation component can be bypassed and conducted out.

[0053] In this embodiment, the MMC main circuit parameters can adapt to the general MMC topology under the power frequency of 50Hz: each phase bridge arm contains a plurality of sub-modules (the sub-modules adopt half-bridge or full-bridge structure); the bridge arm inductance, DC side voltage, AC side voltage and other parameters can be configured according to actual engineering requirements, the fundamental frequency f1 corresponds to the power frequency of 50Hz, and the second harmonic frequency f2 is 100Hz, and the filtering topology and method of the application can adapt to the capacitor voltage fluctuation suppression requirement of MMC.

[0054] The parameters of the double-bandpass filtering module are determined according to the resonance condition as follows:

[0055] The inductance L1 and the capacitance C1 of the first bandpass filter need to satisfy

[0056] L1=2mH, C1=5.132mF are selected;

[0057] The inductance L2 and the capacitance C2 of the second bandpass filter need to satisfy

[0058] L2=1mH, C2=2.533mF are selected.

[0059] Such parameter configuration ensures that the filter can realize accurate resonance characteristic at the corresponding frequency.

[0060] The working principle of the system is as follows: during the operation of the MMC, the fundamental component contained in the bridge arm current and the second harmonic component contained in the circulating current act on the sub-module capacitor 6 through the switching function control, and induce voltage fluctuation. At this time, the first bandpass filter 8 resonates at 50Hz frequency and presents a low impedance characteristic, bypasses and filters out the fluctuation corresponding to the fundamental component, and avoids the influence of the frequency component on the capacitor voltage fluctuation; at the same time, the second bandpass filter 9 resonates at 100Hz frequency and presents a low impedance characteristic, bypasses and filters out the fluctuation corresponding to the second harmonic component.

[0061] ​​By this way of dual-frequency selective filtering, the system can effectively suppress the main fluctuation component in the capacitor voltage of the MMC sub-module. The first band-pass filter is specially designed to suppress the fluctuation of the fundamental frequency, and the second band-pass filter is specially designed to suppress the fluctuation of the second harmonic frequency. The two work together to significantly reduce the fluctuation amplitude of the capacitor voltage and improve the operation stability and power quality of the MMC system.

[0062] As shown in the accompanying Figure 3 , real-time simulation test is conducted by PSCAD, and the capacitor voltage fluctuation of the traditional MMC and the system of the application is compared. As shown in the accompanying Figure 3 , the capacitor voltage fluctuation of the traditional MMC is shown in the first 5s, and the capacitor voltage fluctuation after the system of the application is used is shown in the last 5s. The results show that the fluctuation range of the capacitor voltage of the traditional MMC is 0kV-5kV, and the fluctuation range of the capacitor voltage after the system of the application is used is 1.5kV-2kV, and the fluctuation amplitude is reduced by 90%, which verifies the effectiveness and superiority of the system of the application.

[0063] Embodiment 2: The embodiment provides a capacitor voltage fluctuation suppression method of a modular multilevel converter. The method realizes effective suppression of voltage fluctuation components corresponding to the fundamental frequency and the second harmonic frequency through a dual-band-pass filtering module.

[0064] The dual-band-pass filtering module is composed of a first band-pass filter and a second band-pass filter. Both of the two band-pass filters adopt a topology structure of inductance and capacitance in series, and the two ends are respectively connected in parallel with the positive and negative poles of the energy storage capacitor of the sub-module. The center frequency of the first band-pass filter matches the fluctuation component corresponding to the fundamental frequency in the capacitor voltage of the MMC sub-module, and the center frequency of the second band-pass filter matches the fluctuation component corresponding to the second harmonic frequency in the capacitor voltage of the sub-module.

[0065] In the parameter design of the first band-pass filter, the inductance L1 and the capacitance C1 of the first band-pass filter need to satisfy the resonance condition , so as to present low impedance characteristics at the fundamental frequency, thereby bypassing and filtering out the voltage fluctuation corresponding to the fundamental component.

[0066] In the parameter design of the second band-pass filter, the inductance L2 and the capacitance C2 of the second band-pass filter need to satisfy the resonance condition , so as to present low impedance characteristics at the second harmonic frequency, thereby bypassing and filtering out the voltage fluctuation corresponding to the second harmonic component.

[0067] In specific engineering applications, the fundamental frequency f1=50Hz and the second harmonic frequency f2=100Hz. The inductance L1 and the capacitance C1 of the first band-pass filter can be selected as L1=2mH and C1=5.132mF, and the inductance L2 and the capacitance C2 of the second band-pass filter can be selected as L2=1mH and C2=2.533mF.

[0068] The working principle of the method is that the fundamental component contained in the bridge arm current and the second harmonic component contained in the circulating current act on the sub-module capacitor through the switching function control to cause voltage fluctuation. The first band-pass filter resonates at 50Hz frequency and presents low impedance characteristics to bypass and filter out the fluctuation corresponding to the fundamental component; the second band-pass filter resonates at 100Hz frequency and presents low impedance characteristics to bypass and filter out the fluctuation corresponding to the second harmonic component.

[0069] Through real-time simulation test verification, the capacitor voltage fluctuation range of the traditional MMC is 0kV-5kV, and after adopting the method, the capacitor voltage fluctuation range is reduced to 1.5kV-2kV, and the fluctuation amplitude is reduced by 90%, effectively verifying the effectiveness and superiority of the method. The method can significantly suppress the fluctuation of the sub-module capacitor voltage in the MMC system and improve the system operation stability.

[0070] It should be noted that in this text, 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. Moreover, the terms "include", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or equipment.

[0071] Although the embodiments of the present application have been shown and described, it can be understood by those of ordinary skill in the art that various changes, modifications, replacements and variations can be made to the embodiments without departing from the principles and spirits of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A modular multilevel converter capacitor voltage fluctuation suppression system, characterized in that, The system includes an MMC main circuit and a dual bandpass filter module. The MMC main circuit consists of a three-phase bridge arm group, bridge arm inductors, and several sub-modules. The three-phase bridge arm group is divided into an upper bridge arm group and a lower bridge arm group. Each upper bridge arm group and its corresponding lower bridge arm group are connected in series through bridge arm inductors and then connected to a common DC bus. The connection point between the upper and lower bridge arms is led out as an AC connection terminal. Each sub-module is equipped with a capacitor, and the dual bandpass filter module is directly connected in parallel with the energy storage capacitor of each sub-module.

2. The system according to claim 1, characterized in that, The dual bandpass filter module consists of a first bandpass filter and a second bandpass filter. Both bandpass filters adopt a series inductor and capacitor topology, and their two ends are connected in parallel with the positive and negative terminals of the submodule's energy storage capacitor, respectively. The center frequency of the first bandpass filter matches the fluctuation component corresponding to the fundamental frequency in the MMC submodule's capacitor voltage, and the center frequency of the second bandpass filter matches the fluctuation component corresponding to the second harmonic frequency in the submodule's capacitor voltage.

3. The system according to claim 1, characterized in that, The parameters of the inductor and capacitor in the first and second bandpass filters satisfy the following resonance condition: ， ， Where f1 is the fundamental frequency, L1 is the inductance value of the first bandpass filter, and C1 is the capacitance value of the first bandpass filter; f2=2f1 is the second harmonic frequency, L2 is the inductance value of the second bandpass filter, and C2 is the capacitance value of the second bandpass filter.

4. The system according to claim 3, characterized in that, When the fundamental frequency f1 = 50Hz and the second harmonic frequency f2 = 100Hz, the inductor L1 and capacitor C1 of the first bandpass filter are selected as follows: L1 = 2mH and C1 = 5.132mF; the inductor L2 and capacitor C2 of the second bandpass filter are selected as follows: L2 = 1mH and C2 = 2.533mF.

5. The modular multilevel converter capacitor voltage fluctuation suppression system according to any one of claims 1-4, characterized in that, The submodule is a half-bridge structure submodule or a full-bridge structure submodule, and the submodule capacitor is connected in parallel with two bandpass filters.

6. A method for suppressing capacitor voltage fluctuations in a modular multilevel converter applied to the system of claim 1, characterized in that, In the main circuit of the modular multilevel converter (MMC), the voltage fluctuation components corresponding to the fundamental frequency and the second harmonic frequency are effectively suppressed by a dual bandpass filter module. in The dual bandpass filter module consists of a first bandpass filter and a second bandpass filter, with its two ends connected in parallel to the positive and negative terminals of the MMC submodule capacitor, respectively. When the MMC is running, the first bandpass filter resonates at the fundamental frequency and exhibits low impedance, thus bypassing and filtering out the fluctuations caused by the fundamental component of the bridge arm current. The second bandpass filter resonates at the second harmonic frequency, exhibiting low impedance, and bypasses and filters out the fluctuations caused by the second harmonic component of the circulating current.

7. The method according to claim 6, characterized in that, The fluctuation component corresponding to the fundamental frequency in the center frequency matching submodule capacitor voltage of the first bandpass filter, and the fluctuation component corresponding to the second harmonic frequency in the center frequency matching submodule capacitor voltage of the second bandpass filter.

8. The method according to claim 6, characterized in that, The parameters of the inductor and capacitor in the first and second bandpass filters satisfy the following resonance condition: ， ， Where f1 is the fundamental frequency, L1 is the inductance value of the first bandpass filter, and C1 is the capacitance value of the first bandpass filter; f2=2f1 is the second harmonic frequency, L2 is the inductance value of the second bandpass filter, and C2 is the capacitance value of the second bandpass filter.

9. The method according to claim 8, characterized in that, The fundamental frequency f1 = 50Hz, the second harmonic frequency f2 = 100Hz, the inductance L1 of the first bandpass filter is 2mH, the capacitance C1 is 5.132mF, the inductance L2 of the second bandpass filter is 1mH, and the capacitance C2 is 2.533mF.

10. The method according to claim 6, characterized in that, Both bandpass filters employ an inductor and capacitor series topology.

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