A Modular Multilevel Converter Capacitor Voltage Fluctuation Suppression Method and System

Abstract

This invention discloses a method and system for suppressing capacitor voltage fluctuations in a modular multilevel converter (MMC), belonging to the field of power electronics technology. The system includes a main MMC 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 dual bandpass filter module consists of a first bandpass filter and a second bandpass filter, both employing a series inductor and capacitor topology, and are directly connected in parallel with the capacitors of each sub-module. The center frequency of the first bandpass filter matches the fluctuation component corresponding to the fundamental frequency of the MMC sub-module capacitor voltage, and the center frequency of the second bandpass filter matches the fluctuation component corresponding to the second harmonic frequency of the sub-module capacitor voltage. This invention requires no complex control algorithm, has a simple structure, can significantly reduce the capacitor voltage fluctuation amplitude, and reduces the capacity required for sub-module capacitor selection, making it suitable for various MMC application scenarios.

Description

Technical Field

[0001] This invention relates to the field of power electronic conversion technology, specifically to a method and system for suppressing capacitor voltage fluctuations in a modular multilevel converter. Background Technology

[0002] Modular Multilevel Converter (MC), as an advanced power electronic conversion technology, has emerged as a research hotspot in recent years due to its outstanding advantages such as modular design, easy expansion, and low energy loss, particularly in high-voltage, high-power fields like flexible DC transmission, renewable energy grid connection, and traction power supply. As a core component of the MMC, the voltage stability of the submodule energy storage capacitor directly determines the system's operational reliability, and voltage fluctuation has always been a key bottleneck restricting the improvement of MMC performance.

[0003] The essence of voltage fluctuations in the MMC submodule capacitors is the imbalance in capacitor charging and discharging caused by the alternating component in the instantaneous power of the bridge arm, with the fundamental and second harmonic components being the dominant factors. This voltage fluctuation not only increases the design requirements for capacitor capacitance, raising equipment costs and size, but may also cause imbalances in electrical stress, leading to increased voltage and current harmonic content, thereby affecting the control accuracy and reliability of the entire system.

[0004] To address this issue, existing technologies have proposed various suppression strategies. CN117895762A discloses a method for suppressing capacitor voltage fluctuations in M3C submodules based on active power decoupling. This method transfers pulsating energy to the capacitor of the decoupling circuit by connecting an active power decoupling circuit in parallel on the DC side of the submodule. CN119420149B proposes a method for suppressing capacitor voltage fluctuations in SiC MMC submodules by simultaneously injecting 2nd, 4th, and 8th harmonic circulating currents and a 3rd harmonic voltage to achieve fluctuation suppression. CN111682575B describes a three-phase series CA-MMC system with a voltage-regulating capacitor bridge arm, utilizing a PR controller and a special modulation method to reduce the bridge arm capacitance.

[0005] In addition, some scholars have proposed solutions based on high-frequency chain interconnects. CN112271746B discloses a capacitorless MMC topology with high-frequency chain interconnects, which interconnects all isolated submodules through a full-bridge high-frequency link, eliminating capacitor voltage ripple and second-harmonic circulating current in the bridge arms. CN118316289A proposes a method for suppressing ripple voltage in AC-AC MMC submodules based on virtual capacitors, using a bandpass filter to extract ripple components and suppressing them through virtual capacitor design.

[0006] Although various suppression strategies have been proposed in the existing technology, the existing technical solutions generally have the following shortcomings: First, although control-based methods can suppress capacitor voltage fluctuations to a certain extent, they often require complex algorithms, increasing the complexity and computational burden of the control system, and their robustness over a wide power range needs to be improved; Second, although methods such as active power decoupling have good suppression effects, they require additional power devices and control circuits, significantly increasing system cost and power consumption; Third, existing passive filtering schemes mostly adopt a single-frequency design, making it difficult to simultaneously and effectively suppress the fluctuation components corresponding to the fundamental and second harmonics, resulting in poor adaptability; Finally, some schemes have limitations in dynamic response speed, making it difficult to quickly respond to load changes or system disturbances.

[0007] Therefore, in the face of the need to suppress voltage fluctuations in MMC submodule capacitors, how to design a technical solution that is highly adaptable, responsive, and has a stable suppression effect has become a key issue that urgently needs to be addressed. Summary of the Invention

[0008] The technical problem this invention aims to solve is as follows: In existing technologies, the capacitor voltage of MMC submodules fluctuates excessively due to fundamental and second harmonic waves. Furthermore, existing suppression solutions suffer from problems such as complex structure, high cost, high losses, or poor adaptability. This invention provides a method and system for suppressing capacitor voltage fluctuations in modular multilevel converters. By directly connecting a specially designed dual bandpass filter module in parallel with the submodule capacitor, fundamental and second harmonic components are accurately filtered out. Without altering the MMC topology, this reduces the amplitude of capacitor voltage fluctuations, lowers the capacity of power devices and the total amount of capacitors, and improves the system's economy and stability.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0010] First, this invention provides a modular multilevel converter capacitor voltage fluctuation suppression system, whose core topology is an integrated structure of the 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 upper bridge arm group and 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 as a whole to a common DC bus. The upper and lower bridge arms are connected to each other through the bridge arm inductor connection point as AC connection terminals. Each sub-module adopts a half-bridge or full-bridge structure and has a built-in energy storage capacitor. Multiple sub-modules are connected in series to form the main part of each bridge arm, realizing the conversion and transmission of electrical energy.

[0011] The dual bandpass filter module is the key innovation of this invention. It consists of a first bandpass filter and a second bandpass filter. Both bandpass filters adopt a series inductor and capacitor topology, and their ends are directly connected in parallel with the positive and negative terminals of the energy storage capacitor of each submodule. This allows for direct integration into existing MMC submodules without altering the structure of the main MMC circuit. The center frequency of the first bandpass filter matches the fluctuation component corresponding to the fundamental frequency of the submodule capacitor voltage, and the center frequency of the second bandpass filter matches the fluctuation component corresponding to the second harmonic frequency of the submodule capacitor voltage.

[0012] Furthermore, in the solution proposed in this invention, the parameters of the inductor and capacitor in the first and second bandpass filters satisfy the following resonance condition:

[0013] , ,

[0014] 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.

[0015] On the other hand, this invention proposes a modular multilevel converter capacitor voltage fluctuation suppression method, which effectively suppresses voltage fluctuation components corresponding to the fundamental frequency and the second harmonic frequency through a dual bandpass filter module; wherein

[0016] The dual bandpass filter module consists of a first bandpass filter and a second bandpass filter. Both bandpass filters adopt a series topology of inductor and capacitor, and their two ends are connected in parallel with the positive and negative terminals of the submodule capacitor, respectively. The center frequency of the first bandpass filter matches the fluctuation component corresponding to the fundamental frequency in the voltage of the submodule capacitor, and the center frequency of the second bandpass filter matches the fluctuation component corresponding to the second harmonic frequency in the voltage of the submodule capacitor.

[0017] Furthermore, to achieve this function, the center frequency of the first bandpass filter needs to be precisely matched with the fundamental frequency f1, and the parameters of its inductor L1 and capacitor C1 need to satisfy the resonance condition:

[0018] ,

[0019] 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.

[0020] The center frequency of the second bandpass filter needs to be precisely matched with the second harmonic frequency f2, and the parameters of its inductor L2 and capacitor C2 need to meet the resonance condition.

[0021] ,

[0022] Where 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.

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

[0024] With the above parameter design, when the MMC is running, the first bandpass filter resonates at the fundamental frequency and presents low impedance, 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 and presents low impedance, bypassing and filtering out the fluctuations caused by the second harmonic component of the circulating current. The two work together to suppress capacitor voltage fluctuations from the source.

[0025] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0026] The technical solution proposed in this invention, through the integrated structure design of a dual bandpass filter module and the main circuit of the MMC, can effectively filter out the fundamental and second harmonic components in the capacitor current, significantly reducing the fluctuation of the capacitor voltage in the submodule. Compared with the prior art, PSCAD simulation test results show that the capacitor voltage fluctuation range of traditional MMC is 0kV-5kV, while after adopting the system of this invention, the capacitor voltage fluctuation range is 1.5kV-2kV, a reduction of 90% in fluctuation amplitude. This invention makes the capacity requirements of power devices more reasonable, without the need to further increase the withstand voltage level of devices due to capacitor voltage fluctuations. Without changing the main topology of the MMC, it reduces the amplitude of capacitor voltage fluctuations, reduces the capacity of power devices and the total capacitance, and improves the economy and stability of the system. It is suitable for capacitor voltage stabilization control of MMC submodules in scenarios such as flexible DC transmission and new energy grid connection. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the overall structure of the suppression system in an embodiment of the present invention.

[0028] Figure 2 This is a schematic diagram of the parallel topology of the dual bandpass filter module and the sub-module in an embodiment of the present invention, wherein (a) is a schematic diagram of the sub-module connected in parallel with the dual bandpass filter, and (b) is a schematic diagram of the dual bandpass filter structure.

[0029] Figure 3 This is a simulation diagram comparing the capacitor voltage fluctuation of the embodiment of the present invention with that of a traditional MMC.

[0030] Figure reference numerals: 1-MMC main circuit; 2-upper bridge arm group; 3-lower bridge arm group; 4-bridge arm inductor; 5-sub-module; 6-capacitor; 7-dual bandpass filter module; 8-first bandpass filter; 9-second bandpass filter.

[0031] Among them, u a u b u c These are the three-phase voltages on the AC side of the MMC, u au u bu u cu These are the upper arm voltages of the MMC, u al u bl u cl These are the lower arm voltages of the MMC, U dc For DC side voltage, i au i bu i cu These are the upper arm currents of the MMC, i al i bl i cl These are the lower arm currents of the MMC, u sm SM represents the voltage across the MMC submodule. Detailed Implementation

[0032] The present invention will now be described in detail with reference to specific embodiments and accompanying drawings. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way.

[0033] Example 1: This invention provides a modular multilevel converter capacitor voltage fluctuation suppression system, as shown in the attached figure. Figure 1 As shown, the overall structure of the suppression system in this embodiment includes an MMC main circuit 1 and a dual bandpass filter module 7. The MMC main circuit 1 consists of a three-phase bridge arm group (upper bridge arm group 2, lower bridge arm group 3), a bridge arm inductor 4, and several sub-modules 5. The upper bridge arm group 2 and the corresponding lower bridge arm group 3 of the three-phase bridge arm group are connected in series through the bridge arm inductor 4 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 5 adopts a half-bridge structure and has a built-in energy storage capacitor 6. The voltage output is regulated by controlling the on / off state of the switching device. Multiple sub-modules 5 are connected in series to form the main body 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 attached Figure 2 As shown, the dual bandpass filter module 7 consists of a first bandpass filter 8 and a second bandpass filter 9. The first bandpass filter is composed of an inductor L1 and a capacitor C1 connected in series, and the second bandpass filter is composed of an inductor L2 and a capacitor C2 connected in series. The two bandpass filters are connected in parallel with the capacitor of the submodule.

[0035] The center frequency of the first bandpass filter matches the fluctuation component corresponding to the fundamental frequency in the capacitor voltage of the MMC submodule, and the center frequency of the second bandpass filter matches the fluctuation component corresponding to the second harmonic frequency in the capacitor voltage of the submodule.

[0036] Based on the MMC mathematical model, this invention further derives the correlation between bridge arm current, circulating current, and submodule capacitor voltage fluctuation. Taking phase A as an example, the bridge arm current mainly contains a DC component and a fundamental frequency AC component. The upper and lower bridge arm currents of phase A can be expressed as:

[0037] ,

[0038] in, and These are the upper and lower bridge arm currents of phase A, respectively. This is the DC side current. ϕ is the amplitude of the AC side current, and ϕ is the power factor angle. The A-phase circulating current mainly includes the 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] During normal operation, all capacitor voltages remain dynamically balanced, so the average switching functions Spa and Sna of the upper and lower arms of phase A can be expressed as follows:

[0042] ,

[0043] The activation and deactivation states of submodules are controlled by a switching function, so the current flowing through the submodule capacitor can be expressed as:

[0044] ,

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

[0046] .

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

[0048] ,

[0049] By substituting the higher-order current harmonics, the second harmonic fluctuation of the submodule capacitor voltage can be derived as follows:

[0050] .

[0051] As can be seen from the above analysis, the fundamental component and the second harmonic component are the dominant factors of capacitor voltage fluctuation. Their corresponding fluctuation frequencies are the fundamental frequency f1 and the second harmonic frequency f2, respectively. Therefore, by selectively filtering out the fluctuation components corresponding to these two frequencies, the capacitor voltage fluctuation can be effectively suppressed.

[0052] Based on the above analysis, the core function of the dual bandpass filter module of this invention is to bypass and filter out the wave components corresponding to the fundamental and second harmonics through resonant characteristics. The core characteristic of a bandpass filter is that it exhibits low impedance at the center frequency, at which point the attenuation of the component at that frequency is minimal and the conductivity is strongest, allowing the corresponding wave components to be bypassed and extracted.

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

[0054] The parameters of the dual bandpass filter module are calculated and determined based on the resonance condition as follows:

[0055] The inductor L1 and capacitor C1 of the first bandpass filter must satisfy the following conditions: ,

[0056] Choose L1 = 2mH and C1 = 5.132mF;

[0057] The inductor L2 and capacitor C2 of the second bandpass filter must satisfy the following conditions: ,

[0058] We selected L2=1mH and C2=2.533mF.

[0059] This parameter configuration ensures that the filter can achieve accurate resonance characteristics at the corresponding frequency.

[0060] The system operates as follows: During MMC operation, the fundamental component in the bridge arm current and the second harmonic component in the circulating current act on the submodule capacitor 6 through a switching function, causing voltage fluctuations. At this time, the first bandpass filter 8 resonates at a frequency of 50Hz, exhibiting low impedance characteristics, and bypasses the fluctuations corresponding to the fundamental component, preventing this frequency component from affecting the capacitor voltage. Simultaneously, the second bandpass filter 9 resonates at a frequency of 100Hz, exhibiting low impedance characteristics, and bypasses the fluctuations corresponding to the second harmonic component.

[0061] By employing this dual-frequency selective filtering method, the system can effectively suppress the main fluctuation components in the capacitor voltage of the MMC submodule. The first bandpass filter is specifically designed to suppress fluctuations in the fundamental frequency, while the second bandpass filter is specifically designed to suppress fluctuations in the second harmonic frequency. Working together, the two filters significantly reduce the amplitude of capacitor voltage fluctuations, thereby improving the operational stability and power quality of the MMC system.

[0062] As attached Figure 3 As shown, real-time simulation tests were performed using PSCAD to compare the capacitor voltage fluctuations of the traditional MMC system and the system of this invention. (See attached image.) Figure 3 As shown, the voltage fluctuation of a conventional MMC is displayed 5 seconds before the image, while the voltage fluctuation after the image shows the voltage fluctuation after the system of this invention is displayed 5 seconds later. The results show that the voltage fluctuation range of a conventional MMC is 0kV-5kV, while the voltage fluctuation range of the system of this invention is 1.5kV-2kV, a 90% reduction in fluctuation amplitude, verifying the effectiveness and superiority of the system of this invention.

[0063] Example 2: This example provides a method for suppressing voltage fluctuations in a modular multilevel converter capacitor. This method uses a dual bandpass filter module to effectively suppress voltage fluctuation components corresponding to the fundamental frequency and the second harmonic frequency.

[0064] The dual bandpass filter module consists of a first bandpass filter and a second bandpass filter. Both bandpass filters employ a series inductor and capacitor topology, with their ends connected in parallel to 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 of the MMC submodule's capacitor voltage, while the center frequency of the second bandpass filter matches the fluctuation component corresponding to the second harmonic frequency of the submodule's capacitor voltage.

[0065] In the design of the parameters for the first bandpass filter, the inductor L1 and capacitor C1 of the first bandpass filter must satisfy the resonance condition. This results in low impedance characteristics at the fundamental frequency, thereby bypassing and filtering out voltage fluctuations corresponding to the fundamental component.

[0066] In the design of the parameters for the second bandpass filter, the inductor L2 and capacitor C2 of the second bandpass filter must satisfy the resonance condition. This results in low impedance characteristics at the second harmonic frequency, thereby bypassing and filtering out voltage fluctuations corresponding to the second harmonic component.

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

[0068] The working principle of this method is as follows: the fundamental component in the bridge arm current and the second harmonic component in the circulating current are controlled by a switching function to act on the submodule capacitor, causing voltage fluctuations. The first bandpass filter resonates at a frequency of 50Hz, exhibiting low impedance characteristics, and bypasses the fluctuations corresponding to the fundamental component; the second bandpass filter resonates at a frequency of 100Hz, exhibiting low impedance characteristics, and bypasses the fluctuations corresponding to the second harmonic component.

[0069] Real-time simulation tests verified that the capacitor voltage fluctuation range of traditional MMC is 0kV-5kV. After adopting this 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. This method can significantly suppress the fluctuation of submodule capacitor voltage in MMC systems and improve the system's operational stability.

[0070] 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 process, method, article, or apparatus.

[0071] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A system for suppressing voltage fluctuations in MMC capacitors based on a parallel bandpass filter, 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 a bridge arm inductor 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. 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.

2. 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.

3. The MMC capacitor voltage fluctuation suppression system based on a parallel bandpass filter according to claim 2, 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.

4. The MMC capacitor voltage fluctuation suppression system based on a parallel bandpass filter according to any one of claims 1-3, 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.

5. A method for suppressing capacitor voltage fluctuations in a modular multilevel converter applied to the system described in 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.

6. The method according to claim 5, 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.

7. The method according to claim 5, 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.

8. The method according to claim 7, 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.

9. The method according to claim 5, characterized in that, Both bandpass filters employ an inductor and capacitor series topology.

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