Photovoltaic inverter direct current bus capacitor state monitoring method

Abstract

The invention provides a photovoltaic inverter direct current bus capacitor state monitoring method, and belongs to the technical field of capacitor state monitoring. The method is realized based on amplitude-frequency response of a capacitor equivalent circuit, and an equivalent circuit of a direct-current bus capacitor is established, and an impedance characteristic equation of the direct-current bus capacitor is constructed. DC bus current and power grid side current are reconstructed to obtain to-be-monitored DC bus capacitance current of the photovoltaic inverter, and then DC bus capacitance voltage and reconstructed capacitance current are collected and processed by a filter. Based on the filtered voltage and current signals, corresponding amplitude and phase angle are extracted through Fourier transform, and according to the impedance characteristic of a direct-current bus capacitor equivalent circuit and in combination with amplitude-frequency response, an equivalent series resistance value ESR and an ideal capacitance value C of a capacitor are solved. According to the method, only existing elements of the system are utilized, additional sensors do not need to be added, the capacitor state is jointly monitored through two parameters, high reliability is achieved, and the method can be applied to other converters.

Description

Technical Field

[0001] The invention belongs to the technical field of capacitor status monitoring, and in particular relates to a method for monitoring the status of a DC bus capacitor of a photovoltaic inverter. Background Art

[0002] Aluminum Electrolytic Capacitor (AEC) is widely used in photovoltaic inverters due to its large capacitance, higher volumetric efficiency and high cost performance. Aluminum electrolytic capacitors provide smooth DC bus voltage and energy buffering for load transients, but are one of the most fragile components in power converters in terms of service life and reliability. Among all power electronic devices, capacitors and power devices have the highest failure rate, and about 30% of failures are caused by capacitor degradation. Once an aluminum electrolytic capacitor fails, the DC bus voltage will fluctuate significantly, which will make the system unstable and even affect the normal operation of the photovoltaic inverter. Therefore, the health of aluminum electrolytic capacitors is an important indicator for determining the reliability of photovoltaic inverters.

[0003] As aluminum electrolytic capacitors age, the electrolyte will gradually evaporate, resulting in a decrease in capacitance and an increase in equivalent series resistance. It is generally believed that the two changes occur together. In order to more timely and accurately determine the aging state of electrolytic capacitors, the capacitance value C and equivalent series resistance ESR It is generally believed that when the capacitance value C Reduced to 80% of its initial value or equivalent series resistance ESR When it increases to 200% of its initial value, the aluminum electrolytic capacitor fails.

[0004] At present, the capacitor status monitoring methods at home and abroad are mainly divided into offline methods, online methods and quasi-online methods. The offline method requires the capacitor to be removed from the circuit for measurement, which not only affects the normal working state, but also requires high-precision instruments. The online method can monitor the condition of the capacitor without interrupting the normal operation of the instrument, but some methods require additional sensors, external circuits or injected signals during implementation, which may increase additional costs or affect the normal operation of the system. Some methods have good monitoring effects, do not add additional hardware and are cost-effective. The quasi-online method monitors the capacitor status under specific working conditions of the circuit, mainly including charging and discharging characteristics, no-load conditions and external signal injection. It cannot achieve real-time online monitoring and has a certain time interval. Summary of the invention

[0005] The purpose of the present invention is to provide a method for monitoring the state of a DC bus capacitor of a photovoltaic inverter. Based on the equivalent circuit of the DC bus capacitor, the impedance characteristics and amplitude-frequency response thereof are analyzed to obtain the equivalent series resistance value. ESR and ideal capacitance value C , thereby monitoring the capacitor status online in real time. This is a non-invasive, more reliable and more widely used capacitor aging status monitoring method, which is implemented using existing sensors and does not affect the normal operation of the system or increase additional costs.

[0006] In order to achieve the above object, the present invention adopts the following technical solution: A method for monitoring the state of a DC bus capacitor of a photovoltaic inverter comprises the following steps: Step S1, establishing an equivalent circuit model of the DC bus capacitor to be monitored in the photovoltaic inverter, and establishing its impedance characteristic equation; Step S2, according to the operating principle of the photovoltaic inverter, the DC bus capacitor current to be monitored of the photovoltaic inverter is obtained by current reconstruction; Step S3, collecting the voltage signal of the DC bus capacitor to be monitored and the reconstructed current signal, and processing them using the designed filter; Step S4, based on the voltage and current signals after filtering, extract the corresponding amplitude and phase angle by Fourier transform; Step S5, analyzing the impedance characteristics and amplitude-frequency response of the DC bus capacitor, and solving the equation to obtain the equivalent series resistance value ESR and ideal capacitance value C .

[0007] Furthermore, in step S1, the equivalent circuit model of the DC bus capacitor to be monitored is composed of an equivalent series resistor and an ideal capacitor, and the impedance characteristic equation established is: (2) In the formula, Z is the DC bus capacitor impedance, j is an imaginary unit, ω is the angular frequency of the capacitor voltage and current signals, ESR is the equivalent series resistance value, C is the ideal capacitance value.

[0008] Furthermore, in step S2, the DC bus current of the photovoltaic inverter, the grid-side current and the modulation signal of the power switch tube are used to reconstruct the current signal of the DC bus capacitor.

[0009] 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 collected by the sensor, and the DC component and high-frequency component in the voltage and current signals are filtered out.

[0010] Furthermore, in step S4, based on the filtered capacitor voltage and current signals, the amplitudes of the capacitor voltage and current signals are extracted by Fourier transform. and , phase angle and , and further obtain their amplitude ratio and phase difference .

[0011] Furthermore, in step S5, based on the equivalent circuit model of the DC bus capacitor to be monitored, by analyzing its impedance characteristics and amplitude-frequency response, the amplitude and phase angle of the voltage and current signals obtained in step S4 are used to solve the equivalent series resistance value. ESR and ideal capacitance value C .

[0012] Compared with the prior art, the advantages of the present invention are: (1) Make full use of existing components in the system without adding additional sensors and external circuits, and without injecting external signals.

[0013] (2) Simultaneously monitor the ideal capacitance value C and equivalent series resistance ESR , compared to monitoring only one parameter (ideal capacitance value C Or equivalent series resistance ESR ) method is more reliable.

[0014] (3) In the experimental results, the ideal capacitance value C and equivalent series resistance ESR The identification results have high stability and effectiveness, and the maximum errors of both are within the acceptable range.

[0015] (4) Compared with other methods, this monitoring method is simple and efficient, and can be widely extended to other application scenarios (such as Buck, Boost, and UPS). BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a flow chart of a method for monitoring the state of a DC bus capacitor of a photovoltaic inverter in one embodiment of the present invention; Figure 2 is an equivalent circuit diagram of a DC bus capacitor of a photovoltaic inverter in one embodiment of the present invention; Figure 3This is a schematic diagram of the structure of a photovoltaic inverter system in one embodiment of the present invention; Figure 4 Schematic diagram of AC fundamental wave components of voltage and current signals after filtering in one embodiment of the present invention; Figure 5 FIG. 4 is a specific flow chart of a capacitor status monitoring method in one embodiment of the present invention. DETAILED DESCRIPTION

[0017] The technical solution of the present invention is further described below with reference to the accompanying drawings and through specific implementation methods.

[0018] like Figure 1 As shown, a method for monitoring the state of a DC bus capacitor of a photovoltaic inverter includes the following steps: 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 impedance characteristic equation established is: (3) In the formula, Z is the DC bus capacitor impedance, j is an imaginary unit, ω is the angular frequency of the capacitor voltage and current signals, ESR is the equivalent series resistance value, C is the ideal capacitance value.

[0019] Step S2, according to the operating principle of the photovoltaic inverter, the DC bus capacitor current to be monitored of the photovoltaic inverter is obtained by current reconstruction. The photovoltaic inverter system structure adopted by the present invention is as follows: Figure 3 shown.

[0020] Specific reconstruction process: Using voltage sensor to sample DC bus capacitor voltage , the current sensor samples the DC bus current and grid-side current , capture the power switch tube pulse through the pulse trigger circuit , , and The above collected data is converted into AD through the switch conversion chip, and the inverter side input current is first reconstructed through the grid side current and the modulation signal of the power switch tube. , as shown in formula (4), the capacitor current can then be calculated by the DC bus current and the inverter input current , as shown in formula (5): (4) (5) Step S3, collecting the voltage signal of the DC bus capacitor to be monitored and the reconstructed current signal, using the designed filter to filter the capacitor voltage signal and the reconstructed current signal, and filtering out the DC component and high-frequency component in the voltage and current signals.

[0021] Step S4, based on the filtered capacitor voltage and current signals, use Fourier transform to extract their amplitudes respectively and , phase angle and , and further obtain their amplitude ratio 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 shown.

[0022] (6) (7) Step S5, based on the equivalent circuit model of the DC bus capacitor to be monitored, by analyzing its impedance characteristics and amplitude-frequency response, 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 The specific flow chart of the capacitor state monitoring method proposed in the present invention is as follows: Figure 5 shown.

[0023] According to the impedance characteristics of aluminum electrolytic capacitors, the transfer function of the DC bus capacitor equivalent circuit model is As shown in formula (8): (8) In the formula, and represent the voltage and current signals in the complex domain, respectively. Z(AEC) is the impedance of the DC bus capacitor, s is a complex variable.

[0024] Therefore, the amplitude ratio You can also pass the function The modulus calculation is as shown in formula (9): (9) in, ω=2πf , f= 100Hz. In the above formula, ω is a known quantity, , C and ESR is an unknown quantity. However, It can be calculated by formula (6), so only C and ESR is an unknown quantity, and the relationship between them can be expressed by formula (10): (10) For the transfer function (8), its phase angle is the phase difference , The phase angle can be expressed by formula (11): (11) In the above formula, ω is a known quantity, and It can be calculated by formula (7). Therefore, C and ESR The relationship can be expressed as: (12) By combining equations (10) and (12), we can solve the unknown quantity C and ESR : (13) (14) A certain type of photovoltaic inverter is used as the verification object of the method proposed in the present invention. The ideal capacitance value of the aluminum electrolytic capacitor used is 9mF, and the equivalent series resistance is 50mΩ. The equivalent series resistance value of the DC bus capacitor is monitored in real time during the S1 to S5 process proposed in the present invention. ESR and capacitance value C ,The estimation results of capacitance parameters and the corresponding relative errors are shown in Table 1.

[0025] Table 1

[0026] It can be seen from Table 1 that the method proposed in the present invention has high accuracy and the equivalent series resistance value is ESR and capacitance value C Can effectively monitor the status of DC bus capacitor.

[0027] The above description is intended to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail through the aforementioned embodiments, those skilled in the art should understand that the technical solution described in the aforementioned embodiments may still be modified, or some of the technical features therein may be replaced by equivalents. These modifications or replacements will not cause the corresponding technical solution to deviate from the spirit and scope of the technical solution of each embodiment of the present invention.

Claims

1. A method for monitoring the state of a DC bus capacitor of a photovoltaic inverter, characterized in that: The following steps are involved: Step S1, establishing an equivalent circuit model of the DC bus capacitor to be monitored in the photovoltaic inverter, and constructing its impedance characteristic equation; Step S2, according to the operating principle of the photovoltaic inverter, the DC bus capacitor current to be monitored of the photovoltaic inverter is obtained by current reconstruction; Step S3, collecting the voltage signal of the DC bus capacitor to be monitored and the reconstructed current signal, and processing them using the designed filter; Step S4, based on the voltage and current signals after filtering, extract the corresponding amplitude and phase angle by Fourier transform; Step S5, analyzing the impedance characteristics and amplitude-frequency response of the DC bus capacitor, and solving the equation to obtain the equivalent series resistance value ESR and ideal capacitance value C .

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

3. A photovoltaic inverter DC bus capacitor state monitoring method as claimed 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 switch tube.

4. A photovoltaic inverter DC bus capacitor state monitoring method as claimed in claim 1, characterized in that: In step S3: The voltage signal at both ends of the DC bus capacitor to be monitored and the reconstructed current signal measured by the sensor are filtered by a filter to remove the DC component and high-frequency component in the voltage and current signals.

5. A photovoltaic inverter DC bus capacitor state monitoring method as claimed in claim 1, characterized in that: 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 ratio and phase difference .

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

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