EMD-based photovoltaic inverter capacitor ESR calculation method

Abstract

This invention belongs to the field of photovoltaic grid-connected technology and discloses a method for calculating the ESR of photovoltaic inverter capacitors based on EMD. The method includes: performing Empirical Mode Decomposition (EMD) on the sampled DC signals of the photovoltaic array output current, boost converter inductor current, and grid-connected inverter inductor current to select n high-content IMF AC components; selecting the IMF AC component with the least correlation to the IMF AC component of the photovoltaic array output current from the n high-content IMF AC components of the boost converter inductor current as the input capacitor feature IMF1; selecting the IMF AC component with the least correlation to the IMF AC component of the boost converter inductor current from the n high-content IMF AC components of the grid-connected inverter inductor current as the DC bus capacitor feature IMF2; and calculating the capacitor ESR based on the input capacitor feature IMF1 and the DC bus capacitor feature IMF2. This method does not require interrupting the normal operation of the photovoltaic inverter and has advantages such as low cost, unified monitoring, and strong anti-interference.

Description

Technical Field

[0001] This invention belongs to the field of photovoltaic grid connection technology, specifically relating to a photovoltaic inverter capacitor ESR calculation method based on EMD. Background Technology

[0002] The number and capacity of distributed photovoltaic (PV) power generation systems connected to medium- and low-voltage distribution networks are also rapidly increasing. With the increase in installed PV capacity and the lengthening of operating years, reports of PV system accidents are becoming increasingly common, highlighting the growing importance of safety and reliability. As the core power conversion unit of a PV power generation system, the safe and reliable operation of the PV inverter is crucial for improving grid security and reducing the cost of PV power generation. Affected by harsh natural environments such as high temperature, high humidity, and sandstorms, as well as harsh electrical environments such as power fluctuations and grid impacts, the reliability of PV inverters faces severe challenges, making them the component with the highest failure rate in PV power generation systems. Statistics show that inverter failure rates account for 43% to 70% in PV power generation systems, and power losses due to failure account for as much as 36%. Currently, to improve the reliability of PV inverters, manufacturers mainly focus on increasing design margins to ensure safe and reliable operation, while maintenance mainly adopts reactive repair and complete unit replacement methods. Both of these approaches significantly increase the cost of PV power generation. Therefore, it is urgent to research methods that can improve the safety and reliability of PV inverters while also considering cost.

[0003] The reliability of a photovoltaic (PV) inverter depends on its least reliable component, the capacitor, which accounts for 30% of failures in currently installed PV inverters. In distributed PV systems, the input-side capacitor and DC bus capacitor play crucial roles in absorbing high-frequency ripple current, supporting voltage, and suppressing low-frequency ripple. As degradation occurs, the dielectric loss of the capacitor increases, leading to a rise in operating temperature and further accelerating aging, creating a vicious cycle that ultimately results in device failure. Although advancements in capacitor technology have improved the lifespan of electrolytic capacitors, and longer-life metal film capacitors have partially or completely replaced them in some applications, documents indicate that capacitors remain the most vulnerable components in current power electronic systems. Furthermore, compared to indoor power electronic circuits, PV inverters operate under more complex and variable conditions, resulting in shorter capacitor lifespans. Therefore, research into DC-side capacitor condition monitoring technology in PV inverters is urgently needed to detect deteriorating capacitors and replace them promptly to prevent catastrophic failures.

[0004] Currently, numerous studies have focused on online monitoring of capacitor status, but most of these methods have limitations: on the one hand, some schemes only target the DC bus capacitor, ignoring the impact of input-side capacitor parameter changes on the overall system characteristics; on the other hand, while some methods can obtain the parameters of both capacitors separately, they require discrete and complex detection circuits and algorithms, increasing system cost and complexity, making them difficult to implement in engineering applications. This "separate" monitoring approach cannot uniformly and efficiently assess the overall health status of the entire system's capacitor bank at the system level. Especially in photovoltaic power generation systems, the output characteristics of the photovoltaic array change in real time with ambient illuminance and temperature. Coupled with the multi-stage switching modulation of the boost converter and grid-connected inverter, as well as the coupling interference of grid background harmonics, the ripple current on the input capacitor and DC bus capacitor exhibits significant wide-frequency, nonlinear, and non-stationary characteristics. This brings complexity to the online calculation of the equivalent series resistance (ESR), with multiple signal aliasing and difficulty in accurately decoupling characteristic parameters.

[0005] Therefore, developing a method that can adapt to complex photovoltaic scenarios and simultaneously perform online real-time calculation of the ESR of the input and DC bus side capacitors is of vital engineering significance for achieving comprehensive perception of system-level health status, further reducing the cost per kilowatt-hour, and improving the long-term operational reliability of photovoltaic power generation systems. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a photovoltaic inverter capacitor ESR calculation system and method based on EMD, which can calculate the ESR of the input and DC bus capacitors in real time.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] In a first aspect, this invention provides a method for calculating the ESR of a photovoltaic inverter capacitor based on EMD (Electronic Performance Measurement). The photovoltaic inverter includes a boost converter and a grid-connected inverter. The input terminal of the boost converter is connected to a photovoltaic array, and the output terminal is connected to the grid-connected inverter. The input capacitor is the capacitor at the input terminal of the boost converter. The ESR calculation method is as follows:

[0009] Empirical mode decomposition was performed on the DC signals of the photovoltaic array output current, the boost converter inductor current and the grid-connected inverter inductor current obtained from sampling, and n high-content IMF AC components were selected according to the effective value of each component, where n is a natural number greater than 1.

[0010] From the n high-content IMF AC components of the boost converter inductor current, the IMF AC component with the least correlation to the IMF AC component of the photovoltaic array output current is selected as the input capacitor feature IMF1.

[0011] From the n high-content IMF AC components of the grid-connected inverter inductor current, the IMF AC component with the least correlation to the IMF AC component of the boost converter inductor current is selected as the DC bus capacitor characteristic IMF2.

[0012] The equivalent series resistance (ESR1) of the input capacitor is calculated based on the input capacitor characteristic IMF. The calculation formula is as follows:

[0013] ;

[0014] Where T1 is the ESR1 calculation period, IMF1(t) is the instantaneous value of the input capacitance characteristic IMF, and v in (t) represents the instantaneous value of the input voltage;

[0015] The equivalent series resistance (ESR2) of the DC bus capacitor is calculated based on the IMF characteristic of the DC bus capacitor. The calculation formula is as follows:

[0016] ;

[0017] Where T2 is the calculation period of ESR1, IMF2(t) is the instantaneous value of the DC bus capacitance characteristic IMF, and v Cf（ t) is the instantaneous value of the DC bus voltage.

[0018] In some embodiments of the present invention, T1 = (5~10) / f1, where f1 is the main frequency of the input capacitor characteristic IMF1; T2 = (5~10) / f2, where f2 is the main frequency of the DC bus capacitor characteristic IMF2.

[0019] In some embodiments of the present invention, the step of selecting n high-content IMF AC components based on their effective values ​​specifically involves:

[0020] The effective values ​​of each IMF AC component obtained through empirical mode decomposition are calculated using the following formula. And select the n IMF AC components with the largest effective values ​​based on the calculation results;

[0021]

[0022] Among them, IMF denoted as the instantaneous value of the IMF AC component, and T is the sampling period.

[0023] In some embodiments of the present invention, the method for screening input capacitance feature IMF is as follows: for each AC component of the boost converter inductor current IMF, calculate its correlation coefficient with n high-content AC components of the photovoltaic array output current and sum them to obtain the cross-correlation coefficient, and select the AC component of the boost converter inductor current IMF with the smallest cross-correlation coefficient as input capacitance feature IMF1.

[0024] In some embodiments of the present invention, the method for screening DC bus capacitor characteristic IMF is as follows: for each grid-connected inverter inductor current IMF AC component, calculate its correlation coefficient with n high-content IMF AC components of the boost converter inductor current and sum them to obtain the cross-correlation coefficient, and select the grid-connected inverter inductor current IMF AC component with the smallest cross-correlation coefficient as DC bus capacitor characteristic IMF2.

[0025] In some embodiments of the present invention, n=3.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) To address the technical challenge of overlapping ripple current components and difficulty in directly extracting effective characteristic quantities under complex operating conditions of photovoltaic inverters, this invention introduces the Empirical Mode Decomposition (EMD) method. It adaptively decomposes various frequency components caused by MPPT control, switching action and grid disturbance in the output current of photovoltaic array, inductor current of boost converter and inductor current of grid-connected inverter. By screening several IMF AC components with the largest effective value, noise and secondary components are effectively removed, and the dominant ripple component with the highest correlation with capacitor aging characteristics is accurately extracted, providing effective characteristic information for subsequent ESR calculation.

[0028] (2) Based on the feature component screening after EMD decomposition, this invention simultaneously realizes the decoupled calculation of ESR of capacitors on both sides under the same algorithm framework, effectively improving the integration of photovoltaic inverter state perception and system-level monitoring capability.

[0029] (3) It is simple to implement and does not require changing the normal working state of the photovoltaic inverter. Real-time calculation can be completed.

[0030] (4) Online monitoring can be used to monitor the health status of the input and DC side capacitors in real time, and can prompt for replacement when the capacitor reaches the end of its service life. Attached Figure Description

[0031] These and / or other aspects and advantages of the present invention will become apparent and readily understood from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which:

[0032] Figure 1 A schematic diagram of the structure of a photovoltaic inverter capacitor ESR calculation system provided in one embodiment of the present invention;

[0033] Figure 2 A partial simulation waveform diagram of a photovoltaic inverter capacitor ESR calculation method provided for an embodiment of the present invention. Detailed Implementation

[0034] The technical solution of the present invention will be further described in detail below through embodiments and in conjunction with the accompanying drawings. The following description of the embodiments of the present invention with reference to the accompanying drawings is intended to explain the overall inventive concept of the present invention and should not be construed as a limitation of the present invention.

[0035] Empirical Mode Decomposition (EMD) is an adaptive time-frequency analysis method for signals, primarily applicable to the decomposition of nonlinear and non-stationary signals. Unlike traditional Fourier transforms or wavelet transforms, which rely on pre-defined basis functions, EMD can decompose complex original signals step-by-step into several intrinsic mode functions (IMFs) arranged from high to low frequencies, along with a residual component, based on the signal's own time-scale characteristics. These IMF components contain local feature information of the original signal at different time scales and have clear physical meaning.

[0036] In the technical solution provided by this invention, to address the problem of overlapping ripple current components and difficulty in directly extracting effective characteristic quantities under complex operating conditions of photovoltaic inverters, the output current of the photovoltaic array, the inductor current of the boost converter, and the inductor current of the grid-connected inverter all contain multiple frequency components caused by maximum power point tracking (MPPT) control, switching actions, and grid disturbances. By decomposing the above current signals using an EMD unit and selecting several IMF AC components with the largest effective values, noise and minor components in the signal can be effectively removed, and the dominant ripple component with the highest correlation to capacitor aging characteristics can be extracted. This processing provides a clean data foundation for subsequent accurate selection of input capacitor characteristic IMF1 and bus characteristic IMF2, thereby achieving decoupled calculation of the equivalent series resistance (ESR) of the input capacitor and DC bus capacitor without adding complex hardware circuitry, improving the accuracy of condition monitoring and the system integration.

[0037] One embodiment of the present invention provides a photovoltaic inverter capacitor ESR calculation system based on EMD, such as... Figure 1As shown, the photovoltaic inverter includes a boost converter and a grid-connected inverter. The input terminal of the boost converter is connected to the photovoltaic array, and the output terminal is connected to the grid-connected inverter. The output terminal of the grid-connected inverter is connected to the power grid, completing the grid connection of the photovoltaic system. The input capacitor is the capacitor at the input terminal of the boost converter. The calculation system includes a photovoltaic array current EMD unit, a boost converter inductor current EMD unit, a grid-connected inverter inductor current EMD unit, a grid-connected inverter inductor current rectification unit, an input capacitor characteristic IMF screening unit (IMF screening unit 1), a DC bus capacitor characteristic IMF screening unit (IMF screening unit 2), an input capacitor ESR calculation unit (ESR calculation unit 1), and a DC bus capacitor ESR calculation unit (ESR calculation unit 2).

[0038] The photovoltaic array current EMD unit is used to perform empirical mode decomposition on the input photovoltaic array current signal and output the n AC components with the largest effective values. Its input terminal is connected to the photovoltaic array current detection terminal, and its output terminal is connected to the input capacitance characteristic IMF screening unit.

[0039] The boost converter inductor current EMD unit is used to perform empirical mode decomposition on the input boost converter inductor current signal and output the n AC components with the largest effective values. Its input terminal is connected to the boost converter inductor current detection terminal, and its output terminal is connected to the DC bus capacitor characteristic IMF screening unit.

[0040] The grid-connected inverter inductor current rectifier unit is used to receive the grid-connected inverter inductor current signal, rectify it into a DC signal and output it to the grid-connected inverter inductor current EMD unit. Its input terminal is connected to the grid-connected inverter inductor current detection terminal and its output terminal is connected to the grid-connected inverter inductor current EMD unit.

[0041] The grid-connected inverter inductor current EMD unit is used to perform empirical mode decomposition on the DC signal of the input grid-connected inverter inductor current and output the n AC components with the largest effective values. Its output terminal is connected to the DC bus capacitor characteristic IMF screening unit.

[0042] The input terminal of the input capacitance feature IMF screening unit is connected to the output terminal of the photovoltaic array current EMD unit and the output terminal of the boost converter inductor current EMD unit, respectively. It is used to screen the AC component of the boost converter inductor current IMF based on the AC component of the photovoltaic array current IMF, and output the AC component of the boost converter inductor current IMF with the least correlation with the AC component of the photovoltaic array current IMF as the input capacitance feature IMF1.

[0043] The input terminal of the DC bus capacitor characteristic IMF screening unit is connected to the output terminal of the boost converter inductor current EMD unit and the output terminal of the grid-connected inverter inductor current EMD unit, respectively. It is used to screen the AC component of the grid-connected inverter inductor current IMF based on the AC component of the boost converter inductor current IMF, and output the AC component of the grid-connected inverter inductor current IMF with the least correlation with the AC component of the boost converter inductor current IMF as the DC bus capacitor characteristic IMF2.

[0044] The input terminal of the input capacitance ESR calculation unit is connected to the output terminal of the input capacitance feature IMF screening unit and the input capacitance voltage detection terminal, respectively, and is used to receive the input feature IMF1 and the input capacitance voltage signal and calculate the ESR1 of the input capacitance.

[0045] The input terminal of the DC bus capacitor ESR calculation unit is connected to the output terminal of the DC bus capacitor characteristic IMF screening unit and the DC bus capacitor voltage detection terminal, respectively, to receive the DC bus capacitor characteristic IMF2 and the DC bus capacitor voltage signal, and to calculate the ESR2 of the DC bus capacitor.

[0046] based on Figure 1 The photovoltaic inverter capacitor ESR calculation system shown in this invention provides a photovoltaic inverter capacitor ESR calculation method based on EMD, including the following steps:

[0047] S100. Perform empirical mode decomposition on the DC signals of the photovoltaic array output current, the boost converter inductor current and the grid-connected inverter inductor current obtained by sampling, and select n high-content IMF AC components according to the effective value, where n is a natural number greater than 1.

[0048] In some embodiments of the present invention, the selection of n high-content IMF AC components based on their effective values ​​is specifically as follows:

[0049] The effective values ​​of each IMF AC component obtained through empirical mode decomposition are calculated using the following formula. And select the n IMF AC components with the largest effective values ​​based on the calculation results;

[0050]

[0051] Among them, IMF denoted as the instantaneous value of the IMF AC component, and T is the sampling period.

[0052] For example, when n is 3, the AC component of the photovoltaic array output current IMF is the three AC components with the highest effective values, denoted as IMF. p 1. IMF p 2 and IMF p3; The AC component of the inductor current IMF of the boost converter is the three AC components with the highest effective values, denoted as IMF. Lf1 1. IMF Lf1 2 and IMF Lf1 3; The AC components of the DC signal IMF of the grid-connected inverter inductor current are the three AC components with the highest effective values, denoted as IMF. Lf2 1. IMF Lf2 2 and IMF Lf2 3.

[0053] S200. Obtain the input capacitance characteristic IMF1 and the DC bus capacitance characteristic IMF2.

[0054] From the n high-content IMF AC components of the boost converter inductor current, the IMF AC component with the least correlation to the IMF AC component of the photovoltaic array output current is selected as the input capacitor feature IMF1; from the n high-content IMF AC components of the grid-connected inverter inductor current, the IMF AC component with the least correlation to the IMF AC component of the boost converter inductor current is selected as the DC bus capacitor feature IMF2.

[0055] In some embodiments of the present invention, the method for screening input capacitance feature IMF is as follows: for each AC component of the boost converter inductor current IMF, calculate its correlation coefficient with n high-content AC components of the photovoltaic array output current and sum them to obtain the cross-correlation coefficient, and select the AC component of the boost converter inductor current IMF with the smallest cross-correlation coefficient as input capacitance feature IMF1.

[0056] For example, when n is 3.

[0057] r Lf11 =r(IMF Lf1 1, IMF p 1)+ r(IMF Lf1 1, IMF p 2)+ r(IMF Lf1 1, IMF p 3);

[0058] r Lf12 =r(IMF Lf1 2, IMF p 1)+ r(IMF Lf1 2, IMF p 2)+ r(IMF Lf1 2, IMF p 3);

[0059] r Lf13 =r(IMF Lf1 3, IMFp 1)+ r(IMF Lf1 3, IMF p 2)+ r(IMF Lf1 3, IMF p 3);

[0060] Where, r Lf11 r Lf12 r Lf13 These are the cross-correlation coefficients between the three high-content IMF AC components of the boost converter inductor current and the IMF AC components of the photovoltaic array output current.

[0061] In some embodiments of the present invention, the method for screening DC bus capacitor characteristic IMF is as follows: for each grid-connected inverter inductor current IMF AC component, calculate its correlation coefficient with n high-content IMF AC components of the boost converter inductor current and sum them to obtain the cross-correlation coefficient, and select the grid-connected inverter inductor current IMF AC component with the smallest cross-correlation coefficient as DC bus capacitor characteristic IMF2.

[0062] For example, when n is 3.

[0063] r Lf21 =r(IMF Lf2 1, IMF Lf1 1)+ r(IMF Lf2 1, IMF Lf1 2)+ r(IMF Lf2 1, IMF Lf1 3)

[0064] r Lf22 =r(IMF Lf2 2, IMF Lf1 1)+ r(IMF Lf2 2, IMF Lf1 2)+ r(IMF Lf2 2, IMF Lf1 3)

[0065] r Lf23 =r(IMF Lf2 3, IMF Lf1 1)+ r(IMF Lf2 3, IMF Lf1 2)+ r(IMF Lf2 3, IMF Lf1 3)

[0066] Where, r Lf21 r Lf22 r Lf23These are the cross-correlation coefficients between the three high-content IMF AC components of the grid-connected inverter inductor current and the IMF AC components of the boost converter inductor current.

[0067] S300. Calculate the ESR of the photovoltaic inverter capacitor.

[0068] The equivalent series resistance (ESR1) of the input capacitor is calculated based on the input capacitor characteristic IMF. The calculation formula is as follows:

[0069] ;

[0070] Where T1 is the ESR1 calculation period, IMF1(t) is the instantaneous value of the input capacitance characteristic IMF, and v in (t) is the instantaneous value of the input voltage; T1 is determined according to the main frequency of the input capacitor characteristic IMF1. Preferably, T1 = (5~10) / f1, where f1 is the main frequency of the input capacitor characteristic IMF1.

[0071] The equivalent series resistance (ESR2) of the DC bus capacitor is calculated based on the IMF characteristic of the DC bus capacitor. The calculation formula is as follows:

[0072] ;

[0073] Where T2 is the calculation period of ESR1, IMF2(t) is the instantaneous value of the DC bus capacitance characteristic IMF, and v Cf (t) represents the instantaneous value of the DC bus voltage. T2 is determined based on the dominant frequency of the DC bus capacitance characteristic IMF2. Preferably, T2 = (5~10) / f2, where f2 is the dominant frequency of the DC bus capacitance characteristic IMF2.

[0074] To verify the effectiveness of the ESR calculation method for photovoltaic inverter capacitors based on EMD provided in this invention, simulations were performed. At the start of the simulation, the ESRs of the input-side and DC bus-side capacitors were set to 0.2Ω and 0.3Ω, respectively. Some waveforms are shown below. Figure 2 As shown. After the simulation stops, the photovoltaic array output current i p Boost converter inductor current i Lf1 Inductor current i of grid-connected inverter Lf2 Input voltage v in DC bus voltage v Cf The data was imported into the Matlab Workspace for data processing. The calculation results of the input and DC bus capacitance ESR are shown in Table 1. It can be seen that the unified calculation method for input and DC bus capacitance provided by this invention has high tracking accuracy.

[0075] Table 1. Calculation results of input and DC bus capacitance ESR

[0076]

[0077] As can be seen from Table 1, the method proposed in this invention can accurately calculate the ESR of the photovoltaic inverter input and DC bus capacitor online.

[0078] The advantage of the ESR calculation method for photovoltaic inverter capacitors based on EMD provided by this invention is that it does not require the introduction of new measurement points, the method is simple and easy to implement, it can accurately identify the ESR of the input and DC bus capacitors without changing the normal operating mode, and it can also realize online real-time calculation.

[0079] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0080] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for calculating the ESR of a photovoltaic inverter capacitor based on EMD, wherein the photovoltaic inverter includes a boost converter and a grid-connected inverter, the input terminal of the boost converter is connected to a photovoltaic array, and the output terminal is connected to the grid-connected inverter, the input capacitor is the capacitor at the input terminal of the boost converter, and the capacitor ESR calculation method is as follows: Empirical mode decomposition was performed on the DC signals of the photovoltaic array output current, the boost converter inductor current and the grid-connected inverter inductor current obtained from sampling, and n high-content IMF AC components were selected according to the effective value of each component, where n is a natural number greater than 1. From the n high-content IMF AC components of the boost converter inductor current, the IMF AC component with the least correlation to the IMF AC component of the photovoltaic array output current is selected as the input capacitor feature IMF1. From the n high-content IMF AC components of the grid-connected inverter inductor current, the IMF AC component with the least correlation to the IMF AC component of the boost converter inductor current is selected as the DC bus capacitor characteristic IMF2. The equivalent series resistance (ESR1) of the input capacitor is calculated based on the input capacitor characteristic IMF1, using the following formula: ； Where T1 is the ESR1 calculation period, IMF1(t) is the instantaneous value of the input capacitance characteristic IMF, and v in (t) represents the instantaneous value of the input voltage; The equivalent series resistance (ESR2) of the DC bus capacitor is calculated based on the DC bus capacitor characteristic IMF2. The calculation formula is as follows: ； Where T2 is the ESR2 calculation period, IMF2(t) is the instantaneous value of the DC bus capacitance characteristic IMF, and v Cf (t) represents the instantaneous value of the DC bus voltage.

2. The method for calculating the ESR of a photovoltaic inverter capacitor according to claim 1, characterized in that, T1 = (5~10) / f1, where f1 is the main frequency of the input capacitor characteristic IMF1; T2 = (5~10) / f2, where f2 is the main frequency of the DC bus capacitor characteristic IMF2.

3. The method for calculating the ESR of a photovoltaic inverter capacitor according to claim 1, characterized in that, The specific steps for selecting n high-content IMF communication components based on their effective values ​​are as follows: The effective values ​​of each IMF AC component obtained through empirical mode decomposition are calculated using the following formula. And select the n IMF AC components with the largest effective values ​​based on the calculation results; Among them, IMF denoted as the instantaneous value of the IMF AC component, and T is the sampling period.

4. The method for calculating the ESR of a photovoltaic inverter capacitor according to claim 1, characterized in that, The screening method for input capacitance characteristic IMF is as follows: For each boost converter inductor current IMF AC component, calculate its correlation coefficient with the n high-content IMF AC components of the photovoltaic array output current and sum them to obtain the cross-correlation coefficient. Select the boost converter inductor current IMF AC component with the smallest cross-correlation coefficient as the input capacitor characteristic IMF1.

5. The method for calculating the ESR of a photovoltaic inverter capacitor according to claim 1, characterized in that, The screening method for DC bus capacitance characteristic IMF is as follows: For each grid-connected inverter inductor current IMF AC component, calculate its correlation coefficient with the n high-content IMF AC components of the boost converter inductor current and sum them to obtain the cross-correlation coefficient. Select the grid-connected inverter inductor current IMF AC component with the smallest cross-correlation coefficient as the DC bus capacitor characteristic IMF2.

6. The method for calculating the ESR of a photovoltaic inverter capacitor according to claim 1, characterized in that, n=3。

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