Online diagnosis method of DC bus capacitor of DCDC converter in photovoltaic system

Abstract

The present invention provides an online diagnosis method for the DC bus capacitor of a photovoltaic system DCDC converter, and relates to the technical field of electrical equipment monitoring. The method aims at the aging fault problem of the DC bus capacitor of the DCDC converter in the photovoltaic power generation system, and provides a non-invasive online diagnosis solution based on the estimation of capacitive equivalent parameters. In the locking link of the DCDC converter after the daily sunlight intensity decreases, the DC bus voltage and the output current of the DCDC converter are collected by using the configured sensor unit to estimate the equivalent capacitance of the DC bus capacitor. When the estimated value decreases to 80% of the initial value, a fault protection signal is issued. The present invention does not require the entire system to be powered off, nor does it require the installation of an invasive current sensor on the DC bus. The online estimation result has high accuracy and high diagnostic reliability.

Description

Technical Field

[0001] The invention relates to the technical field of electrical equipment monitoring, and in particular to an online diagnosis method for a DC bus capacitor of a DCDC converter of a photovoltaic system. Background Art

[0002] Distributed photovoltaic power generation is connected to the power grid in large quantities, with the characteristics of scattered access points and uncertain output. Photovoltaic array structures include: centralized, AC module, string and multi-branch structures. For any array structure, in order to ensure the maximum output power, each photovoltaic power generation branch needs to be connected to a DCDC converter, and the maximum power tracking of photovoltaic power generation is completed through real-time control and adjustment of its duty cycle.

[0003] Aluminum electrolytic capacitors are widely used in power electronic converters of photovoltaic systems and are one of the components most susceptible to aging and failure. Capacitor aging and failure are mainly caused by electrical and thermal stresses during operation, which lead to electrolyte evaporation and electrochemical reactions (degradation of the oxide layer). After the capacitor capacitance decreases, the output voltage of the DCDC converter becomes unstable, affecting the maximum power tracking, further reducing the power generation efficiency of the photovoltaic system and the power quality of the back-end power grid. In serious cases, after a capacitor fails, the entire photovoltaic power generation branch will be out of operation. Therefore, it is very important to perform online status monitoring and aging diagnosis on the DC bus capacitor of the photovoltaic system DCDC converter.

[0004] Most of the existing power electronic converter DC bus capacitance monitoring methods are based on electrical parameter estimation, specifically, the DC bus capacitance is equivalent to a series resistance parameter (denoted as R x ) and capacitance parameters (denoted as C x ), by measuring the voltage and current, estimate R x and C x , thus determining the aging state of the DC bus capacitor. A series of empirical experiments show that during the aging process of the DC bus capacitor, R x Increase and C x When R x Increase to twice its initial value or C x When it decreases to 80% of the initial value, the DC bus capacitor can be considered to have failed.

[0005] This type of R-based x and C x The estimation methods are divided into direct estimation method and indirect estimation method. The direct estimation method is that when the current and voltage on the DC bus capacitor are available, R can be directly calculated. x and C xThese two electrical parameters. In actual industrial applications, DCDC converters are usually required to minimize size, increase power density and integration. Current sensors are generally not installed on the internal DC bus, so the current flowing through the DC bus capacitor cannot be measured online. Therefore, the direct estimation method is usually applied offline, that is, when the DCDC converter is deactivated, an excitation signal is applied, the voltage and current on the DC bus capacitor are measured, and then R is calculated. x and C x In general, the offline direct estimation method has higher accuracy, but it requires the entire system to be powered off and cannot be kept in normal operation for testing.

[0006] The indirect estimation method mainly solves the problem that it is difficult to measure the DC bus current online. The core idea is to control the switching mode of the DCDC converter, inject high-frequency components, generate voltage ripple on the DC bus, use the harmonic components associated with the injection frequency to reconstruct the DC bus current, and then perform R x and C x The indirect estimation method does not require the installation of an intrusive current sensor on the DC bus. During system operation, the DC bus current is reconstructed by directly measuring the voltage across the DC bus capacitor, so it has the ability to be implemented online. In general, the indirect estimation method does not require the entire system to be powered off to ensure normal operation for testing, but the estimation result is less accurate than the offline method, and controlling the switching mode of the DCDC converter is essentially intrusive, which will affect the voltage and current waveforms and reduce the power quality.

[0007] However, the direct estimation method is usually performed offline and requires the entire system to be powered off; otherwise, an intrusive current sensor needs to be installed on the DC bus of the DCDC converter, which is not conducive to converter integration, will reduce power density, and introduce other operational reliability issues. The indirect estimation method usually requires controlling the switching state of the DCDC converter and using the measured voltage to reconstruct the DC bus current. The sampling accuracy and sampling speed of the voltage measurement unit are high, which will increase the operation and maintenance costs. In addition, controlling the switching mode of the DCDC converter is essentially intrusive, which will affect the voltage and current waveforms, reduce the power quality, and affect the stability of system operation. Both of the above methods require accurate measurement of the voltage across the DC bus capacitor of the DCDC converter, which is difficult to implement in the context of photovoltaic systems. The fundamental reason is that on each photovoltaic power generation branch, the output voltage of the DCDC converter (denoted as u b ) is approximately equal to the common DC bus voltage (u com However, there are usually hundreds of meters to kilometers of connecting cables between the DCDC converter and the public DC bus, which results in u b Not equal to u com .u b and ucom There will be a difference between them, which causes the above use of u com The estimation results of the two methods are inaccurate. Therefore, the traditional parameter estimation method is not actually suitable for the DC bus capacitor status monitoring of the DCDC converter of the photovoltaic system, and its aging status online diagnosis results have the problems of low accuracy and insufficient reliability. Summary of the invention

[0008] Aiming at the problem of aging failure of DC bus capacitor of DCDC converter in photovoltaic power generation system, the present invention provides an online diagnosis method of DC bus capacitor of DCDC converter in photovoltaic system.

[0009] The present invention provides a method for online diagnosis of DC bus capacitance of a photovoltaic system DCDC converter, which is applied to the above photovoltaic system. The method comprises: step S1, measuring the output electrical quantity of the photovoltaic array, and when the output electrical quantity is less than a set threshold, issuing an instruction to lock the DCDC converter; step S2, controlling the switch state of the power device IGBT to be continuously closed, and recording the common DC bus voltage u after the power device IGBT is closed. b and the output current i of the DCDC converter O Step S3, according to the waveform, calculate the DC bus capacitor A in a reverse charging interval i , ΔU b and ΔK i ; Step S4, according to the A i , ΔU b and ΔK i Calculate ΔK i / A i and ΔU b / A i ; In the rectangular coordinate system, ΔK i / A i The data is plotted on the x-axis, and ΔU b / A i The data is plotted on the y-axis to form a discrete point; step S5, repeating the above steps S1 to S4 within a specified monitoring period to form multiple discrete points in the rectangular coordinate system; step S6, performing linear fitting using the multiple discrete points, recording the intersection of the fitting curve and the y-axis, and estimating the equivalent capacitance parameter C of the DC bus capacitor within the monitoring period according to the coordinates of the intersection x Step S7, during the specified photovoltaic system operation time including multiple monitoring cycles, repeat the above steps S1 to S6 to obtain multiple equivalent capacitance parameters C in the DC bus capacitor x ; Step S8, when the equivalent capacitance parameter C xWhen the DC bus capacitor decreases to a preset percentage of the healthy initial value, it is determined that the current DC bus capacitor is faulty.

[0010] Furthermore, in step S1, the output electrical quantity includes the output current i of the photovoltaic array PV , or the output voltage u of the photovoltaic array PV .

[0011] Further, in step S3, after the power device IGBT is closed, a DC bus capacitor is within the first reverse charging interval. i , ΔU b and ΔK i Calculate using the following formula:

[0012]

[0013]

[0014]

[0015] In the formula, t 1 and t 2 They respectively represent the starting time and the ending time of the first reverse charging interval of the DC bus capacitor after the DCDC converter is locked.

[0016] Further, in step S6, linear fitting is performed using the plurality of discrete points, including: according to the A i , ΔU b and ΔK i A quantitative relationship is formed between the discrete points, and a binary linear fitting is performed using the multiple discrete points.

[0017] Furthermore, the A i , ΔU b and ΔK i The quantitative relationship between them is:

[0018]

[0019] In the formula, l 0 Indicates the equivalent inductance of the connecting cable; C x Represents the equivalent capacitance parameter in the DC bus capacitor.

[0020] Further, in step S6, the equivalent capacitance parameter C of the DC bus capacitor in the monitoring period is x =1 / b, b is the coordinate of the intersection of the fitting curve and the y-axis.

[0021] Furthermore, the preset percentage is 80%.

[0022] Compared with the prior art, the online diagnosis method for the DC bus capacitor of the photovoltaic system DCDC converter provided by the present invention has at least the following beneficial effects:

[0023] (1) There is no need to shut down the entire system, nor is there any need to additionally control the switching signal of the DCDC converter. Instead, the DCDC converter's lockout link, which will inevitably occur when the sunlight intensity decreases every day, is utilized. This is simple, efficient, and requires no additional operations.

[0024] (2) The method proposed in the present invention does not require the installation of an intrusive current sensor on the DC bus, and only uses the sensor unit configured in the system to measure and obtain the required electrical quantity.

[0025] (3) The method proposed in the present invention avoids the influence of resistive equivalent parameters, and the estimation result directly reflects the capacitive equivalent parameters of the DC bus capacitor of the DCDC converter. Compared with the traditional method, the sensitivity of online diagnosis of aging status is improved. Under different load conditions, the estimation accuracy of parameter Cx is high, and the error is less than 2%. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0027] Figure 1 The following schematically shows an application scenario of the method provided in an embodiment of the present invention in a photovoltaic system.

[0028] Figure 2 The equivalent circuit diagram of a photovoltaic power generation branch in a photovoltaic system according to the method of an embodiment of the present invention is schematically shown.

[0029] Figure 3 The flowchart of the online diagnosis method of the DC bus capacitor of the DCDC converter of the photovoltaic system according to the embodiment of the present invention is schematically shown.

[0030] Figure 4 The method of the embodiment of the present invention is schematically shown in an application case of a photovoltaic system. b and i O Waveform diagram of .

[0031] Figure 5 The figure schematically shows the discrete data and fitting result of an application case of the method according to the embodiment of the present invention in a photovoltaic system. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and / or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

[0034] All terms (including technical and scientific terms) used herein have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification, and should not be interpreted in an idealized or overly rigid manner.

[0035] Figure 1 The following schematically shows an application scenario of the method provided in an embodiment of the present invention in a photovoltaic system. It should be noted that: Figure 1 The examples shown are merely examples of application scenarios to which the embodiments of the present invention can be applied, to help those skilled in the art understand the technical content of the present disclosure, but do not mean that the embodiments of the present invention cannot be used in other devices, systems, environments or scenarios.

[0036] like Figure 1 As shown, the application scenario of this embodiment is a photovoltaic system, which includes a photovoltaic power generation branch, which includes a photovoltaic array, a DCDC converter and a connecting cable connected in series, and the rear end of the connecting cable is connected to a common DC bus. The DCDC converter includes a power device IGBT (Insulated Gate Bipolar Transistor) and a diode.

[0037] The controller is connected to the photovoltaic array and the DCDC converter respectively, and is used to collect the electrical parameters of the direct current emitted by the photovoltaic array, and is also used to output a PWM signal to the DCDC converter according to the switching state of the power device IGBT to control the opening and closing of the DCDC converter.

[0038] Exemplarily, the rear end of the photovoltaic power generation branch is connected to a three-phase inverter, and then through LC filtering (passive filter), connected to the AC power grid through the PCC (point of common coupling).

[0039] In the DC side of the photovoltaic grid-connected system, the electrical quantities that can be directly measured include: the output voltage u of the photovoltaic array PV , the output current i of the photovoltaic array PV , the output current i of the DCDC converter O and the common DC bus voltage u b The output voltage of the DCDC converter cannot be measured directly, but is approximately equal to u b .u PV and i PV Determined by the operating conditions of the photovoltaic panels, as the intensity of sunlight decreases every day, the output of the photovoltaic panels gradually decreases, u PV and i PV Gradually decrease. In engineering, u PV or PV When it is less than the set threshold, the photovoltaic power generation branch will be shut down, that is, the DCDC converter will be locked.

[0040] Figure 2 The equivalent circuit diagram of a photovoltaic power generation branch in a photovoltaic system according to the method of an embodiment of the present invention is schematically shown.

[0041] like Figure 2 As shown, in this photovoltaic power generation branch, the DC bus capacitor is equivalent to a series resistor R x And the series capacitor C x ; The equivalent resistance and inductance of the connecting cable are denoted as r 0 and l 0 ; The voltage across the DC bus capacitor is denoted as u x ; The current flowing through the DC bus capacitor is denoted as i x .

[0042] It should be noted that the method provided by the present invention is only for the series capacitor C in the DC bus capacitor. x To estimate, the series resistance R x It is not included in the estimation.

[0043] For example, Figure 2 As shown, the DCDC converter in the photovoltaic power generation branch includes a voltage stabilizing capacitor, an inductor, a power device IGBT (Insulated Gate Bipolar Transistor), a diode and a DC bus capacitor.

[0044] The following is based on Figure 1 and Figure 2 The application scenario of the photovoltaic system described herein is described in detail with respect to the online diagnosis method of the DC bus capacitor of the DCDC converter of the photovoltaic system provided by the embodiment of the present invention.

[0045] Figure 3The flowchart of the online diagnosis method of the DC bus capacitor of the DCDC converter of the photovoltaic system according to the embodiment of the present invention is schematically shown.

[0046] like Figure 3 As shown, the photovoltaic system DCDC converter DC bus capacitor online diagnosis method according to this embodiment is applied to the above Figure 1 and Figure 2 The photovoltaic system shown in the figure comprises steps S1 to S8.

[0047] Step S1, measuring the output electrical quantity of the photovoltaic array, when the output electrical quantity is less than a set threshold, issuing an instruction to lock the DCDC converter.

[0048] Specifically, the output electrical quantity includes the output current i of the photovoltaic array PV , or the output voltage u of the photovoltaic array PV The output electrical quantity is the output current i of the photovoltaic array. PV For example, in i PV Less than the set threshold I lock When , a command to lock the DCDC converter is issued.

[0049] Step S2, controlling the switch state of the power device IGBT to remain closed, and recording the common DC bus voltage u after the power device IGBT is closed b and the output current i of the DCDC converter O waveform.

[0050] In the locking phase of the DCDC converter after the daily sunlight intensity decreases, the common DC bus voltage and the output current of the DCDC converter in the first reverse charging interval are collected to calculate A i , ΔU b and ΔK i Three variables.

[0051] Step S3, according to the waveform, calculate the A of the DC bus capacitor in a reverse charging interval i , ΔU b and ΔK i .

[0052] Specifically, after the power device IGBT is closed, a DC bus capacitor is in the first reverse charging interval. i , ΔU b and ΔK i Calculate using the following formula:

[0053]

[0054]

[0055]

[0056] In the formula, t 1 and t 2 They respectively represent the starting time and ending time of the DC bus capacitor in the first reverse charging interval after the DCDC converter is locked; A i Indicates the negative value of the area between the current and the time axis; ΔU b Indicates the voltage change; ΔK i Indicates the change in the rate of change of current.

[0057] Step S4, according to A i , ΔU b and ΔK i Calculate ΔK i / A i and ΔU b / A i ; In the rectangular coordinate system, ΔK i / A i The data is plotted on the x-axis, and ΔU b / A i The data is plotted on the y-axis to form a discrete point.

[0058] Specifically, A i , ΔU b and ΔK i The following quantitative relationship is satisfied:

[0059]

[0060] In the formula, l 0 Indicates the equivalent inductance of the connecting cable; C x Represents the equivalent capacitance parameter in the DC bus capacitor.

[0061] It should be noted that formula (4) is used to describe the measured data and the calculation results of the capacitive equivalent parameters of the DC bus capacitor, avoiding the influence of the resistive equivalent parameters on the aging state judgment.

[0062] Step S5, repeating the above steps S1 to S4 within a specified monitoring period to form a plurality of discrete points in a rectangular coordinate system.

[0063] For example, a monitoring period may be N days, N ≥ 2, and N groups (ΔK i / A i , ΔU b / A i ) data, that is, there are N discrete points in the rectangular coordinate system.

[0064] Step S6, using multiple discrete points to perform linear fitting, record the intersection of the fitting curve and the y-axis, and estimate the equivalent capacitance parameter C of the DC bus capacitor within the monitoring period according to the coordinates of the intersection. x .

[0065] Further, using multiple discrete points to perform linear fitting includes: according to the above A i , ΔU b and ΔK i The quantitative relationship between them is calculated by using multiple discrete points for binary linear fitting, and the intersection of the fitting curve and the y-axis is recorded as (0, b).

[0066] Next, the equivalent capacitance parameter C of the DC bus capacitor within the monitoring period is estimated. x =1 / b.

[0067] Thus, after obtaining a plurality of discrete points, a fitting curve is obtained by fitting, which can depict the quantitative relationship between the measured data and the capacitive equivalent parameter of the DC bus capacitor. The inverse of the intersection of the fitting curve and the y-axis is the estimated capacitive equivalent parameter of the DC bus capacitor.

[0068] Step S7, repeating the above steps S1 to S6 within the specified photovoltaic system operation time including multiple monitoring cycles to obtain multiple equivalent capacitance parameters C in the DC bus capacitor. x .

[0069] This step is used to obtain multiple C in different monitoring cycles during the longer operation time of the photovoltaic system. x Estimation results.

[0070] Step S8, when the equivalent capacitance parameter C x When the DC bus capacitor decreases to a preset percentage of the healthy initial value, it is determined that the current DC bus capacitor is faulty.

[0071] For example, the preset percentage is 80%. x When it decreases to 80% of the healthy initial value, the DC bus capacitor is determined to be faulty.

[0072] Through the embodiments of the present invention, the present invention proposes a non-intrusive online diagnosis solution based on the estimation of capacitive equivalent parameters for the aging fault problem of the DC bus capacitor of the DCDC converter in the photovoltaic power generation system. In the locking link of the DCDC converter after the daily sunlight intensity decreases, the configured sensor unit is used to collect the DC bus voltage and the output current of the DCDC converter, and the equivalent capacitance of the DC bus capacitor is estimated. When the estimated value is reduced to 80% of the initial value, a fault protection signal is issued.

[0073] Traditional offline methods cannot be implemented online, cannot guarantee real-time monitoring of the aging state of the DC bus capacitor, and cannot provide fault warning. Traditional online methods require the installation of non-intrusive current sensors on the DC bus, reducing the integration of the converter; otherwise, if no sensors are installed, the high-frequency injection current reconstruction method will affect the voltage and current waveforms, reduce the power quality, and the accuracy of the estimation results cannot be guaranteed.

[0074] Compared with the traditional method, the method provided by the present invention does not require the whole system to be powered off, nor does it require the installation of an intrusive current sensor on the DC bus. The online estimation result has high accuracy and high diagnostic reliability. In the locking link of the DCDC converter after the daily sunlight intensity decreases, the configured sensor unit is used to collect voltage and current, estimate the equivalent capacitance of the DC bus capacitor, and then judge its aging state, to ensure that an early warning signal can be issued before a fault occurs.

[0075] Based on the above disclosed method, specific application cases are given below. Figure 4 The method of the embodiment of the present invention is schematically shown in an application case of a photovoltaic system. b and i O Waveform diagram of .

[0076] according to Figure 1 A simulation model of a typical photovoltaic system was built in MATLAB / Simulink. The photovoltaic power generation capacity is 100kW, the rated output voltage is 580V (environmental conditions are 1000W / m 2 &25℃). In the simulation model, the equivalent parameter of the DC bus capacitor is denoted as R xo and C x0 , which are equal to 32mΩ and 2500uF respectively. The equivalent resistance and inductance of the connecting cable are r 0 and l 0 The reference value of the common DC bus voltage setting is 800V. The back side of the grid connection point is connected to the large grid and contains adjustable local loads. Its rated active power P load and reactive power Q load Equal to 30kW and 6kVar respectively.

[0077] The light intensity set by the photovoltaic array gradually decreases, i PV In this case, I is set lock =3A, when i PV <I lock When the DCDC converter is locked, the command is issued, and the time is 0.2s. b and i O The waveforms of the two electrical quantities are as follows Figure 4 As shown. Figure 4 It can be seen that iO Oscillation occurs until it is equal to 0, and the starting time of the first reverse charge is t 1 and the end time t 2 Equal to 0.2015s and 0.2042s respectively.

[0078] Using [t 1 , t 2 ] b and i O Data, according to formula (1), formula (2), formula (3) to calculate the three variables, we can get A i =0.0116, ΔU b =-2.2986 and ΔK i =3.7013e4.

[0079] According to formula (4), ΔK i / A i =3.2045e6, ΔU b / A i =-199.0090.

[0080] Figure 5 The figure schematically shows the discrete data and fitting result of an application case of the method according to the embodiment of the present invention in a photovoltaic system.

[0081] Change the local load P load and Q load , random disturbances are added near the rated value to simulate different system operating conditions when the DCDC converter is deactivated. The local load is changed 41 times, and a total of 42 sets of discrete point data are obtained. All discrete points are drawn in the rectangular coordinate system, such as Figure 5 shown.

[0082] According to formula (4), a binary linear fit is performed, the slope of the fitted curve is equal to -2.03e-4, and the ordinate b of the intersection with the y-axis is equal to 406.85. x =1 / b, estimated DC bus capacitor equivalent capacitance parameter C x Equal to 2458uF, and C x0 The true value error is equal to 1.68%. Therefore, the above application case proves that the method proposed in the present invention can accurately estimate the equivalent capacitance parameters of the DC bus capacitor. x The estimation results enable online diagnosis of its aging status.

[0083] In summary, the present invention provides an online diagnosis method for the DC bus capacitor of a photovoltaic system DCDC converter, which does not require the entire system to be shut down, nor does it require additional control of the switching signal of the DCDC converter. It utilizes the locking link of the DCDC converter that will definitely appear after the daily sunlight intensity decreases, and is simple, efficient, and does not require any additional operations.

[0084] The method proposed in the present invention does not require the installation of an intrusive current sensor on the DC bus, and only utilizes the sensor unit configured in the system to measure and obtain the required electrical quantity.

[0085] The proposed method avoids the influence of resistive equivalent parameters, and the estimation result directly reflects the capacitive equivalent parameters of the DC bus capacitor of the DCDC converter. Compared with the traditional method, the sensitivity of online diagnosis of aging status is improved. Under different load conditions, the parameter C x The estimation accuracy is high, with an error of less than 2%.

[0086] Some block diagrams and / or flow charts are shown in the accompanying drawings. It should be understood that some blocks or combinations thereof in the block diagrams and / or flow charts may be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing device, so that these instructions, when executed by the processor, may create a device for implementing the functions / operations described in these block diagrams and / or flow charts.

[0087] In addition, the terms "first" and "second" are used only for descriptive purposes and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the feature. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined. In addition, the word "one" or "an" preceding an element does not exclude the presence of multiple such elements.

[0088] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. 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 in the protection scope of the present invention.

Claims

1. A method for online diagnosis of DC bus capacitance of a photovoltaic system DCDC converter, characterized in that: include: Step S1, measuring the output electrical quantity of the photovoltaic array, and when the output electrical quantity is less than a set threshold, issuing an instruction to lock the DCDC converter; Step S2, controlling the switch state of the power device IGBT to remain closed, and recording the common DC bus voltage u after the power device IGBT is closed b and the waveform of the output current i0 of the DCDC converter; Step S3, calculating the A of the DC bus capacitor in a reverse charging interval according to the waveform. i , ΔU b and ΔK i ; Step S4, according to the A i , ΔU b and ΔK i Calculate ΔK i / A i and ΔU b / A i ; In the rectangular coordinate system, ΔK i / A i The data is plotted on the x-axis, and ΔU b / A i The data is plotted on the y-axis to form a discrete point; Step S5, repeating the above steps S1 to S4 within a specified monitoring period to form a plurality of discrete points in the rectangular coordinate system; Step S6, performing linear fitting using the plurality of discrete points, recording the intersection of the fitting curve and the y-axis, and estimating the equivalent capacitance parameter C of the DC bus capacitor within the monitoring period according to the coordinates of the intersection. x ; Step S7, repeating the above steps S1 to S6 within the specified photovoltaic system operation time including multiple monitoring cycles to obtain multiple equivalent capacitance parameters C in the DC bus capacitor. x ; Step S8, when the equivalent capacitance parameter C x When it decreases to a preset percentage of the healthy initial value, it is determined that the current DC bus capacitor is faulty; In step S3, after the power device IGBT is closed, a DC bus capacitor is within the first reverse charging interval. i , ΔU b and ΔK i Calculate using the following formula: Wherein, t1 and t2 represent the starting time and ending time of the first reverse charging interval of the DC bus capacitor after the DCDC converter is locked.

2. The photovoltaic system DCDC converter DC bus capacitor online diagnosis method according to claim 1, characterized in that: In step S1, the output electrical quantity includes the output current i of the photovoltaic array. PV , or the output voltage u of the photovoltaic array PV .

3. The photovoltaic system DCDC converter DC bus capacitor online diagnosis method according to claim 1, characterized in that: In step S6, linear fitting is performed using the plurality of discrete points, including: According to the A i , ΔU b and ΔK i A quantitative relationship is formed between the discrete points, and a binary linear fitting is performed using the multiple discrete points.

4. The photovoltaic system DCDC converter DC bus capacitor online diagnosis method according to claim 3 is characterized in that: The A i , ΔU b and ΔK i The quantitative relationship between them is: Where l0 represents the equivalent inductance of the connecting cable; C x Represents the equivalent capacitance parameter in the DC bus capacitor.

5. The photovoltaic system DCDC converter DC bus capacitor online diagnosis method according to claim 1, characterized in that: In step S6, the DC bus capacitor equivalent capacitance parameter C in the monitoring period is x =1 / b, b is the coordinate of the intersection of the fitting curve and the y-axis.

6. The photovoltaic system DCDC converter DC bus capacitor online diagnosis method according to claim 1, characterized in that: The preset percentage is 80%.

Images