A method for monitoring the state of DC bus capacitors in a photovoltaic inverter

Abstract

This invention provides a method for monitoring the state of the DC bus capacitor in a photovoltaic inverter, belonging to the field of capacitor state monitoring technology. This method is based on the amplitude-frequency response of the capacitor's equivalent circuit. It establishes the equivalent circuit of the DC bus capacitor and constructs its impedance characteristic equation. The DC bus capacitor current to be monitored in the photovoltaic inverter is reconstructed using the DC bus current and the grid-side current. Then, the DC bus capacitor voltage and the reconstructed capacitor current are collected and processed using a filter. Based on the filtered voltage and current signals, the corresponding amplitude and phase angle are extracted using Fourier transform. According to the impedance characteristics of the DC bus capacitor's equivalent circuit and combined with the amplitude-frequency response, the equivalent series resistance value of the capacitor is calculated. ESR and ideal capacitance value C This method utilizes only existing system components, requires no additional sensors, and monitors the capacitor state using two parameters, exhibiting high reliability and scalability to other converter applications.

Description

Technical Field

[0001] This invention belongs to the field of capacitor condition monitoring technology, specifically relating to a method for monitoring the condition of DC bus capacitors in a photovoltaic inverter. Background Technology

[0002] Aluminum electrolytic capacitors (AECs) are widely used in photovoltaic inverters due to their large capacitance, higher volumetric efficiency, and cost-effectiveness. AECs provide smooth DC bus voltage and energy buffering against load transients; however, in terms of lifespan and reliability, they are among the most vulnerable components in power converters. Among all power electronic devices, capacitors and power devices have the highest failure rates, with approximately 30% of failures caused by capacitor degradation. Once an AEC fails, the DC bus voltage will fluctuate significantly, leading to system instability and potentially affecting the normal operation of the photovoltaic inverter. Therefore, the health of the AEC is a crucial indicator of the reliability of a photovoltaic inverter.

[0003] As aluminum electrolytic capacitors age, the electrolyte gradually evaporates, leading to a decrease in capacitance and an increase in equivalent series resistance. These two changes are generally considered to occur simultaneously. To more timely and accurately determine the aging state of electrolytic capacitors, capacitance value... C and equivalent series resistance value ESR Both need to be monitored. It is generally believed that when the capacitance value... C Reduce to 80% of its initial value or equivalent series resistance value ESR When the value is increased to 200% of its initial value, the aluminum electrolytic capacitor fails.

[0004] Currently, domestic and international methods for monitoring capacitor status are mainly divided into offline methods, online methods, and quasi-online methods. Offline methods require removing the capacitor from the circuit for measurement, which not only affects normal operation but also requires high-precision instruments. Online methods can monitor the capacitor's condition without interrupting normal instrument operation; however, some methods require additional sensors, external circuits, or injected signals, which may increase costs or affect the system's normal operation. Some online methods, however, offer good monitoring results, do not require additional hardware, and are cost-effective. Quasi-online methods monitor the capacitor's status under specific circuit operating conditions, mainly including charging and discharging characteristics, no-load conditions, and external signal injection. They cannot achieve real-time online monitoring and involve a certain time interval. Summary of the Invention

[0005] The purpose of this invention is to provide a method for monitoring the state of the DC bus capacitor in a photovoltaic inverter. Based on the equivalent circuit of the DC bus capacitor, the equivalent series resistance value is obtained by analyzing its impedance characteristics and amplitude-frequency response. ESR and ideal capacitance value C This allows for real-time online monitoring of capacitor status. It is a non-invasive, more reliable, and more widely applicable method for monitoring capacitor aging, utilizing existing sensors without affecting normal system operation or incurring additional costs.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for monitoring the state of DC bus capacitors in a photovoltaic inverter includes the following steps:

[0008] Step S1: Establish the equivalent circuit model of the DC bus capacitor to be monitored in the photovoltaic inverter, and establish its impedance characteristic equation.

[0009] Step S2: Based on the operating principle of the photovoltaic inverter, the DC bus capacitor current to be monitored in the photovoltaic inverter is obtained through current reconstruction.

[0010] Step S3: Acquire the voltage signal and reconstructed current signal of the DC bus capacitor to be monitored, and process them using the designed filter;

[0011] Step S4: Based on the filtered voltage and current signals, extract the corresponding amplitude and phase angle using Fourier transform;

[0012] Step S5: Analyze the impedance characteristics and amplitude-frequency response of the DC bus capacitor, and solve the equations to obtain the equivalent series resistance value. ESR and ideal capacitance value C .

[0013] Furthermore, in step S1, the equivalent circuit model of the DC bus capacitor to be monitored consists of an equivalent series resistance and an ideal capacitor, and the established impedance characteristic equation is:

[0014] (2)

[0015] In the formula, Z The DC bus capacitance impedance. j The imaginary unit, ω The angular frequency of the capacitor voltage and current signals. ESR This is the equivalent series resistance value. C This is the ideal capacitance value.

[0016] Furthermore, in step S2, the current signal of the DC bus capacitor is reconstructed using the DC bus current of the photovoltaic inverter, the grid-side current, and the modulation signal of the power switch.

[0017] Furthermore, in step S3, the designed filter is used to filter the voltage signal across the DC bus capacitor to be monitored and the reconstructed current signal acquired by the sensor, thereby filtering out the DC component and high-frequency component in the voltage and current signals.

[0018] Furthermore, in step S4, based on the filtered capacitor voltage and current signals, Fourier transform is used to extract their amplitudes respectively. and Phase angle and And further obtain their amplitude ratios and phase difference .

[0019] Furthermore, in step S5, based on the equivalent circuit model of the DC bus capacitor to be monitored, its impedance characteristics and amplitude-frequency response are analyzed. Using the amplitude and phase angle of the voltage and current signals obtained in step S4, the equivalent series resistance value can be obtained after solving. ESR and ideal capacitance value C .

[0020] Compared with the prior art, the advantages of the present invention are:

[0021] (1) Make full use of the existing components in the system, without adding additional sensors and external circuits, and without injecting external signals.

[0022] (2) Simultaneously monitor the ideal capacitance value C and equivalent series resistance value ESR Compared to monitoring only one parameter (ideal capacitance value) C or equivalent series resistance value ESR The method is more reliable.

[0023] (3) In the experimental results, the ideal capacitance value C and equivalent series resistance value ESR The identification results of both methods have high stability and effectiveness, and the maximum error of both is within an acceptable range.

[0024] (4) This monitoring method is simple and efficient compared to other methods and can be widely extended to other application scenarios (such as Buck, Boost and UPS). Attached Figure Description

[0025] Figure 1This is a flowchart of a method for monitoring the state of a photovoltaic inverter's DC bus capacitor according to an embodiment of the present invention;

[0026] Figure 2 This is an equivalent circuit diagram of the DC bus capacitor of a photovoltaic inverter in one embodiment of the present invention;

[0027] Figure 3 This is a schematic diagram of a photovoltaic inverter system structure according to one embodiment of the present invention;

[0028] Figure 4 This is a schematic diagram of the AC fundamental components of the filtered voltage and current signals in one embodiment of the present invention;

[0029] Figure 5 This is a flowchart illustrating the capacitor state monitoring method in one embodiment of the present invention. Detailed Implementation

[0030] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0031] like Figure 1 As shown, a method for monitoring the state of the DC bus capacitor of a photovoltaic inverter includes the following steps:

[0032] Step S1: Establish an equivalent circuit model of the DC bus capacitor to be monitored in the photovoltaic inverter. The model consists of an equivalent series resistor and an ideal capacitor, such as... Figure 2 As shown, the established impedance characteristic equation is:

[0033] (3)

[0034] In the formula, Z The DC bus capacitance impedance. j The imaginary unit, ω The angular frequency of the capacitor voltage and current signals. ESR This is the equivalent series resistance value. C This is the ideal capacitance value.

[0035] Step S2: Based on the operating principle of the photovoltaic inverter, the DC bus capacitor current to be monitored in the photovoltaic inverter is obtained by current reconstruction. The photovoltaic inverter system structure adopted in this invention is as follows: Figure 3 As shown.

[0036] Specific reconstruction process: The DC bus capacitor voltage is sampled using a voltage sensor. The current sensor samples the DC bus current. and grid-side current The pulse of the power switch is captured by the pulse triggering circuit. , , and The collected data is then converted from analog to digital by a switching converter chip. The inverter-side input current is first obtained by reconstructing the grid-side current and the modulation signal of the power switch. As shown in equation (4), the capacitor current can then be calculated using the DC bus current and the inverter side input current. As shown in equation (5):

[0037] (4)

[0038] (5)

[0039] Step S3: Collect the voltage signal and reconstructed current signal of the DC bus capacitor to be monitored. Use the designed filter to filter the voltage signal and the reconstructed current signal of the capacitor, and filter out the DC component and high-frequency component in the voltage and current signals.

[0040] Step S4: Based on the filtered capacitor voltage and current signals, Fourier transform is used to extract their amplitudes respectively. and Phase angle and And further obtain their amplitude ratios and phase difference As shown in equations (6) and (7), the AC fundamental components of the filtered voltage and current signals are as follows: Figure 4 As shown.

[0041] (6)

[0042] (7)

[0043] Step S5: Based on the equivalent circuit model of the DC bus capacitor to be monitored, its impedance characteristics and amplitude-frequency response are analyzed. Using the amplitude and phase angle of the voltage and current signals obtained in step S4, the equivalent series resistance value can be obtained by solving the equation. ESR and ideal capacitance value C The specific flowchart of the capacitor state monitoring method proposed in this invention is as follows: Figure 5 As shown.

[0044] Based on the impedance characteristics of aluminum electrolytic capacitors, the transfer function of the equivalent circuit model of the DC bus capacitor is... As shown in equation (8):

[0045] (8)

[0046] In the formula, and These represent the voltage and current signals in the complex domain, respectively. Z(AEC) The impedance of the DC bus capacitor is... s It is a complex variable.

[0047] Therefore, amplitude ratio It can also be done by passing functions The modulus calculation is shown in equation (9):

[0048] (9)

[0049] in, ω=2πf , f= 100Hz. In the above formula, ω It is a known quantity. , C and ESR It is an unknown quantity. However, It can be calculated from equation (6), therefore only C and ESR The unknown quantity is the relationship between the two, which can be expressed by equation (10):

[0050] (10)

[0051] For the transfer function (8), its phase angle is the phase difference. , The phase angle can be expressed by equation (11):

[0052] (11)

[0053] In the above formula, ω It is a known quantity, and It can be calculated using equation (7). Therefore, C and ESR The relationship can be represented as:

[0054] (12)

[0055] The unknown quantity can be obtained by solving equations (10) and (12) simultaneously. C and ESR :

[0056] (13)

[0057] (14)

[0058] A photovoltaic inverter of a certain model was used as the verification object for the method proposed in this invention. The ideal capacitance of the aluminum electrolytic capacitor used was 9mF, and the equivalent series resistance was 50mΩ. The equivalent series resistance of the DC bus capacitor was monitored in real time during processes S1 to S5 as proposed in this invention. ESR and capacitance value C The estimation results of the capacitance parameters and the corresponding relative errors are shown in Table 1.

[0059] Table 1

[0060]

[0061] As shown in Table 1, the method proposed in this invention has high accuracy, and the equivalent series resistance value... ESR and capacitance value C It can effectively monitor the status of DC bus capacitors.

[0062] The above description is intended to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail through the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. These modifications or substitutions will not cause the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for monitoring the state of the DC bus capacitor in a photovoltaic inverter, characterized in that, Includes the following steps: Step S1: Establish an equivalent circuit model of the DC bus capacitor to be monitored in the photovoltaic inverter and construct its impedance characteristic equation. Step S2: Based on the operating principle of the photovoltaic inverter, the DC bus capacitor current to be monitored in the photovoltaic inverter is obtained through current reconstruction. Step S3: Acquire the voltage signal and reconstructed current signal of the DC bus capacitor to be monitored, and process them using the designed filter; Step S4: Based on the filtered voltage and current signals, extract the corresponding amplitude and phase angle using Fourier transform; Step S5: Analyze the impedance characteristics and amplitude-frequency response of the DC bus capacitor, and solve the equations to obtain the equivalent series resistance value. ESR and ideal capacitance value C .

2. The method for monitoring the state of the DC bus capacitor of a photovoltaic inverter as described in claim 1, characterized in that, In step S1: The equivalent circuit model of the DC bus capacitance to be monitored consists of an equivalent series resistance and an ideal capacitor, and its impedance characteristic equation is constructed as follows: (1) In the formula, Z The DC bus capacitance impedance. j The imaginary unit, ω The angular frequency of the capacitor voltage and current signals. ESR This is the equivalent series resistance value. C This is the ideal capacitance value.

3. The method for monitoring the state of the DC bus capacitor of a photovoltaic inverter as described in claim 1, characterized in that, In step S2: The current signal of the DC bus capacitor is reconstructed using the DC bus current of the photovoltaic inverter, the grid-side current, and the modulation signal of the power switching transistor.

4. The method for monitoring the state of the DC bus capacitor of a photovoltaic inverter as described in claim 1, characterized in that, In step S3: The voltage signal across the DC bus capacitor measured by the sensor and the reconstructed current signal are filtered to remove the DC and high-frequency components from the voltage and current signals.

5. The method for monitoring the state of the DC bus capacitor of a photovoltaic inverter as described in claim 1, characterized in that, In step S4: Based on the filtered capacitor voltage and current signals, their amplitudes are extracted using Fourier transform. and Phase angle and And further obtain their amplitude ratios and phase difference .

6. The method for monitoring the state of the DC bus capacitor of a photovoltaic inverter as described in claim 2, characterized in that, In step S5: Based on the equivalent circuit model of the DC bus capacitor to be monitored, its impedance characteristics and amplitude-frequency response are analyzed. Using the amplitude and phase angle of the voltage and current signals obtained in step S4, the equivalent series resistance value can be obtained by solving the equation. ESR and ideal capacitance value C .

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