A method for on-line identification of health state of hybrid capacitor bank with double functions

Abstract

The application discloses a kind of mixed capacitor group health state on-line identification method with dual function, determine DC bus capacitor arrangement mode is: using electrolytic capacitor series-parallel connection, provide voltage support;Thin film capacitor is connected in series and then connected in parallel, provide switching frequency current and switching time high-frequency charge-discharge current path;Circuit board insulation plane is wrapped between positive bus and negative bus plane, offset parasitic parameter;Series additional inductance on electrolytic capacitor branch, to improve electrolytic capacitor life;The additional inductance is used as secondary excitation inductance, constructs voltage transformer, collects voltage signal on the primary side of voltage transformer;The open-circuit voltage of secondary side is matched with monitoring table, and the ESR aging degree of capacitor is obtained.The application achieves life improvement and health state monitoring through high-frequency current component without affecting the main function of converter and increasing the complexity of calculation, and the result shows that the error is less than 2%, to realize the reliability improvement of DC bus capacitor.

Description

Technical Field

[0001] The present invention relates to the technical field of power equipment status monitoring, and is applied in solid-state transformer DC bus scenarios. Specifically, it is a method for online identification of the health status of a hybrid capacitor group with dual functions. Background Art

[0002] Power electronic converters have a strong ability to accommodate new energy sources, but their operational reliability is relatively poor. Aging of the electrolytic capacitors in hybrid capacitor banks is a significant factor in converter performance degradation. This paper develops a design method that combines the dual functions of "capacitor life extension" and "condition monitoring." This approach offers advantages such as simplicity, dual functionality, and no interference with the main circuit.

[0003] First, the aging mechanism and mutual influence of electrolytic capacitors and film capacitors in the hybrid capacitor group are analyzed in detail. By connecting a special current acquisition device with a certain excitation inductance in series with the electrolytic branch that has less impact on the overall performance, the characteristic that the impedance of the electrolytic capacitor increases after aging, which leads to changes in the current distribution in the hybrid capacitor group, is fully utilized. On the one hand, the ripple current flowing through the electrolytic capacitor is limited, thereby improving the life of the electrolytic capacitor; on the other hand, the "voltage transformer + rectifier bridge" monitoring device can detect changes in the loop ripple current in real time and stably, thereby realizing capacitor status monitoring.

[0004] Research and related surveys have shown that nearly 30% of power converter failures are caused by faulty DC bus capacitors. Real-time online monitoring of components with high failure rates in power converters, and the timely identification and replacement of defective components, can significantly reduce the failure rate of solid-state transformers and improve equipment reliability. Compared to film capacitors, aluminum electrolytic capacitors offer advantages such as large capacity, high power density, and a cost-effective price-performance ratio, making them widely used in power converters. However, they suffer from lower reliability, are prone to parameter drift, and offer limited safety and stability. Capacitance (C) or equivalent series resistance (ESR) is widely used as a characteristic aging parameter for electrolytic capacitors for online monitoring. Generally, under the same test conditions, an electrolytic capacitor is considered to have aged when its capacitance (C) drops to 80% of its initial value or its ESR increases to 2–3 times its initial value. Factors affecting capacitor reliability primarily include environmental factors and operating factors. Environmental factors primarily include ambient temperature and humidity, while operating factors primarily include the capacitor's operating voltage, ripple current, and charge / discharge cycles. The failure rate of electrolytic capacitors, an important branch of capacitors, is even higher. In a certain survey, the failure rate of electrolytic capacitors in electronic devices reached 60%, which is twice that of power transistors.

[0005] The DC bus capacitor is one of the key components of the solid-state transformer, and its safety and reliability are the basis for the normal operation of the solid-state transformer. In order to suppress the DC bus voltage fluctuation and cope with different frequency ripple current, a mixed capacitor group structure of electrolytic capacitor and film capacitor is usually adopted, in which the two different types of capacitors will play different roles:

[0006] 1) The electrolytic capacitor has a higher capacitance value density and higher economy, and mainly plays a role in supporting voltage and bearing low-frequency ripple current in the DC bus capacitor group.

[0007] 2) The film capacitor has higher reliability, longer life and stronger ripple current capacity, but its capacitance value is smaller, and its main role is to share high-frequency ripple current.

[0008] The above structure of the DC bus capacitor group of the converter mainly comes from the inverter and the DC-DC link, which are the network-side double-frequency ripple and the switching frequency ripple of the DC-DC link, as shown in Figure 1 .

[0009] The ripple current on the DC bus capacitor can be mainly divided into parts:

[0010] 1) 100Hz double-frequency current and multiple switching frequency currents (15kHz, 30kHz, 45kHz,…) generated by the previous AC / DC conversion;

[0011] 2) 40kHz switching frequency current ripple generated by DC / DC conversion.

[0012] The increase of ESR will affect the output voltage ripple and the ripple current in the capacitor circuit, and since the increase of ESR will directly lead to the increase of the impedance in the circuit, the changes on the output voltage and current are relatively large. At the same time, since the increase of ESR will lead to the change of impedance distribution in the electrolytic capacitor circuit and the film capacitor circuit, the distribution of the main ripple current (40kHz) in the two circuits will change greatly, as shown in Figure 2 , the ripple current changes with ESR, and is independent of the capacitance value. This time, the on-line monitoring of the capacitor state is intended to utilize this shunt characteristic to monitor the electrolytic circuit ripple current, and compare it with the effective value of the electrolytic circuit current before the aging of the capacitor to evaluate the health status of the electrolytic capacitor.

[0013] At present, the integration of functions has not been realized in the field of reliability and state monitoring of capacitors. The similar DC bus capacitor reliability improvement and monitoring method of the present application is as follows:

[0014] (1) Life improvement

[0015] The current main method to improve the life of capacitors is to optimize the design. Reference [1] optimizes the ratio of electrolytic capacitors to film capacitors in a hybrid capacitor group and reasonably distributes the current stress of the capacitors to maximize the overall performance.

[0016] (2) Capacitance monitoring

[0017] Capacitor condition monitoring can be categorized into two main areas: capacitance and ESR. During the aging process, a capacitor's lifespan is considered complete when its capacitance drops by 20% or its ESR increases to twice its initial value.

[0018] According to the description in the literature [2], the existing method needs to collect the capacitor current and ripple voltage, and calculate the capacitance or ESR based on the relationship between current and voltage.

[0019] Literature [1] Z. Zhao, P. Davari, W. Lu, H. Wang and F. Blaabjerg, "An Overview of Condition Monitoring Techniques for Capacitors in DC-Link Applications," in IEEE Trans. Power Electron., vol.36, no.4, pp.3692-3716, April 2021.

[0020] Literature [2] H.Wang, C.Li, G.Zhu, Y.Liu and H.Wang, "Model-Based Design and Optimization of Hybrid DC-Link Capacitor Banks," in IEEE Trans.PowerElectron., vol.35, no.9, pp.8910-8925, Sept.2020.

[0021] Disadvantages of existing technology:

[0022] 1. The ripple current components in the DC bus capacitors are complex and require analog or digital filtering, which increases the complexity of the device itself and is detrimental to its safe operation and reliability. The present invention uses a table lookup to identify parameter changes online based on the voltage generated on the secondary side by the high-frequency ripple components of the electrolytic capacitors in the hybrid capacitor bank. This method is simple to implement and does not require complex current sensors or calculations.

[0023] 2. Existing technical solutions focus on the low-frequency components of electrolytic capacitors, ignoring the impact of high-frequency current components on their reliability and their role in condition monitoring, making existing methods unsuitable for practical application. The present invention uses the natural frequency difference between low-frequency and high-frequency currents to reduce the high-frequency current component and extend its service life. It also monitors ESR by tracking the voltage changes caused by the high-frequency current.

[0024] 3. Current technical solutions all have a single function, which inevitably affects the main circuit, resulting in a low overall cost-effectiveness of condition monitoring. This invention focuses on the lifespan and monitoring issues of capacitors. By adding a high-frequency current suppression and monitoring device to the electrolytic capacitor circuit, this dual function is achieved through a simple hardware design.

[0025] Explanation of terms:

[0026] DC-Link capacitors: DC-Link capacitors are key components of solid-state transformers. As the link between the AC / DC and DC / DC links, they primarily provide voltage support. Their safety and reliability are essential for the proper operation of solid-state transformers.

[0027] Electrolytic capacitor (E-cap): An electrolytic capacitor is a type of capacitor with a metal foil as the positive electrode. The oxide film in close contact with the metal is the dielectric. The cathode is composed of a conductive material, an electrolyte, and other materials. The electrolyte is the main component of the cathode, hence the name. Because the electrolyte evaporates over time, electrolytic capacitors are generally less reliable.

[0028] Film capacitors (F-caps): Film capacitors use metal foil as electrodes, overlapped with a plastic film such as polyethylene, polypropylene, polystyrene, or polycarbonate, and then wound into a cylindrical shape. They offer high reliability, long life, and low ESR, but are also larger and more expensive.

[0029] Equivalent series resistance (ESR): An important parameter in the capacitor equivalent circuit, caused by the parasitic resistance of the electrode leads and dielectric, which determines the performance of the capacitor.

[0030] Lifetime extension (LE): Increase the effective use time of a key component.

[0031] Condition monitoring (CM): Real-time monitoring of the operating status of certain key components to ensure their reliability and guide operation and maintenance. Summary of the Invention

[0032] To address the above issues, the present invention aims to provide a dual-function online health status identification method for hybrid capacitor banks. This method, without affecting the primary functions of solid-state transformers or increasing computational complexity, uses high-frequency current components to achieve both life extension and health status monitoring, thereby improving the reliability of DC bus capacitors. The technical solution is as follows:

[0033] A dual-function online identification method for the health status of a hybrid capacitor bank includes the following steps:

[0034] Step 1: Determine the hybrid DC busbar capacitor arrangement

[0035] Electrolytic capacitors are used to provide voltage support; film capacitors and electrolytic capacitors are used in combination to bear the switching frequency current and the high-frequency charging and discharging current at the switching moment;

[0036] Step 2: Add an additional inductor in series with the electrolytic capacitor branch to increase the electrolytic circuit impedance, reduce ripple current, and extend the life of the electrolytic capacitor;

[0037] Step 3: Constructing the Condition Monitoring Voltage Sensor

[0038] The additional inductor connected in series to the electrolytic capacitor branch serves as the primary winding of the sensing transformer to construct a voltage transformer with a predetermined ratio. The AC voltage signal on the additional inductor is converted through a rectifier bridge and a filter capacitor to obtain a DC voltage, which is used as an indicator of the capacitor state.

[0039] Step 4: Establish a state parameter monitoring model

[0040] Based on the specific harmonic currents generated by the preceding AC / DC converter and DC / DC isolation converter, and the impedance differences between electrolytic capacitors and film capacitors, the effective value of the current in the electrolytic capacitor circuit is calculated using Ohm's law. The effect of changes in the electrolytic capacitor ESR on the effective value of the electrolytic capacitor circuit current is scanned to obtain the electrolytic capacitor current corresponding to different electrolytic capacitor ESRs.

[0041] The relationship between the effective value of the primary inductor current and the secondary output DC voltage is as follows:

[0042]

[0043] Where, I L+ is the effective value of the primary inductor current, V T is the collected DC voltage signal, V fis the initial conduction voltage drop of the rectifier bridge diode, n is the voltage transformer ratio; ω 2fs L + is the inductive impedance, L + is the additional inductance value;

[0044] At this point, a state parameter monitoring model can be established to reflect the state of the electrolytic capacitor ESR by collecting the output DC voltage and matching it;

[0045] Step 5: Online collection and identification of state parameters

[0046] The open circuit voltage of the secondary side of the voltage transformer is collected and matched with the parameter monitoring model to obtain the ESR aging degree of the electrolytic capacitor.

[0047] It should be noted that when selecting the inductance value in step 2 above, the resonant inductance value should be avoided to ensure that the impedance at a frequency of 40kHz increases to a preset range to reduce the ripple current of the electrolysis circuit;

[0048] The recommended inductor calculation process is as follows: the capacitor ripple current and recommended inductance at each frequency are calculated, and the degree of suppression and ripple current weight at each frequency are calculated. In combination with the principle of multi-objective optimization, the knee inductor at different frequencies is selected as the inductor. The final additional inductance is determined by combining the ripple current weight at each frequency.

[0049] The beneficial effects of the present invention are:

[0050] 1) Because the ripple current components in the DC bus capacitor are complex, analog or digital filtering is required to address them, increasing the complexity of the device itself and hindering its safe operation and practicality. The present invention uses a passive approach for life extension and condition monitoring, which is highly practical and robust.

[0051] 2) Existing electrolytic capacitor condition monitoring technologies focus on low-frequency components, ignoring the high-frequency harmonic components in actual applications, which also have a certain impact on the reliability of electrolytic capacitors under actual working conditions. This invention uses the natural frequency difference between low-frequency and high-frequency currents to reduce the high-frequency current component and extend its service life. At the same time, it monitors ESR by tracking the voltage changes caused by high-frequency current.

[0052] 3) The current technical solutions for life extension and status monitoring are single-function and have low overall cost-effectiveness. The present invention focuses on the life and monitoring issues of capacitors. By adding a high-frequency current suppression and monitoring device to the electrolytic capacitor circuit, through a simple hardware design, utilizing the high-frequency component in the capacitor ripple current and the difference in reliability between thin film, electrolytic capacitors and thin film capacitors, the status monitoring of the electrolytic capacitor is integrated into the life extension impedance, achieving the dual functions of "life extension" and "online identification of health status". BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is the main ripple current distribution of the capacitor bank.

[0054] Figure 2 is the relationship between capacitor current, ESR and capacitance.

[0055] Figure 3 Flowchart of the method for online identification of the health status of a hybrid DC bus capacitor of the present invention.

[0056] Figure 4 This is the recommended inductance calculation flow chart.

[0057] Figure 5 This is the condition monitoring system diagram.

[0058] Figure 6 This is the capacitor status monitoring diagram. DETAILED DESCRIPTION

[0059] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0060] like Figure 3 As shown, the overall technical solution of the present invention is divided into three main parts, namely, solution basic design, life extension design and state parameter online identification integrated design.

[0061] (1) Solution Basis - DC Capacitor Busbar Design

[0062] The full-bridge circuit formed by the medium-voltage IGBTs needs to withstand high voltages, and the parasitic inductance of the capacitor busbars must be controlled to prevent overvoltage during IGBT switching. The figure below shows the busbar capacitor arrangement used: electrolytic capacitors are connected in series and then two groups are connected in parallel to provide voltage support.

[0063] The midpoint (M) plane is wrapped between the positive and negative busbar planes. This configuration ensures that the currents in the DC+ plane and the DC- plane flow in opposite directions, resulting in magnetic fields canceling each other out. The currents in adjacent pins of two series-connected film capacitors also flow in opposite directions, canceling out the inductances. Furthermore, the currents in adjacent series capacitor groups flow in opposite directions within the M plane, canceling out the inductances of the traces within the plane. This multi-layered magnetic field reduction strategy reduces the parasitic inductance of the capacitor busbar.

[0064] (2) Life extension design

[0065] When the converter switches, the parasitic inductance of the commutation circuit will cause high-frequency oscillations and excessive voltage stress on the device. Therefore, the parasitic parameters of the loops of the film capacitor and the switching device are required to be as small as possible. The electrolytic capacitor mainly plays a voltage support role. Its loop layout is more flexible, and the parasitic parameters have little effect on the voltage overshoot at the commutation moment. Therefore, the impedance of the electrolytic loop can be increased by adding resistance or inductance, reducing the ripple current, allowing more ripple current to flow to the film loop, and improving the life of the electrolytic capacitor. Considering that adding resistance will increase the overall loss of the capacitor group, this article adopts the method of adding inductance. The addition of magnetic components also facilitates the subsequent capacitor current acquisition. The inductance selection strategy is as follows: Figure 4 shown.

[0066] After adding a small inductor to the electrolytic circuit, its impedance is:

[0067]

[0068] Among them, L + is the additional series inductor.

[0069] When the inductive impedance and capacitive impedance are equal at a frequency of 40kHz, that is, when When the electrolysis circuit impedance is the smallest, it is Z E =ESR E Therefore, when selecting the inductance value, it is necessary to avoid the resonant inductance value and ensure that the impedance at the 40kHz frequency increases to an appropriate value to reduce the ripple current of the electrolytic circuit.

[0070] According to the calculation results, the current of the electrolytic circuit decreases with the increase of additional inductance, which is beneficial to improving the continuous operating life of the electrolytic capacitor.

[0071] Combining the principle of Pareto optimization, the knee inductance at different frequencies is selected as the inductor, and then the final additional inductance is determined by combining the ripple current weight at each frequency.

[0072] (3) Integrated design of online identification of state parameters

[0073] The electrolysis circuit current is an AC quantity, which can be collected by constructing a voltage transformer with a certain ratio by adding magnetic elements (inductance). The additional inductance value added again is the primary excitation inductance of the voltage transformer, such as Figure 5 As shown in the figure, after collecting the AC voltage signal on the primary side of the additional sensor transformer, the easily collected DC voltage can be obtained through the rectifier bridge and filter capacitor. The calculation formula is as follows:

[0074]

[0075] Where, I L+ is the effective value of the primary inductor current, V T is the collected DC voltage signal, V fis the initial conduction voltage drop of the diode, and n is the voltage and transformer ratio.

[0076] After collecting the open circuit voltage of the secondary side of the sensor transformer, it is matched with the monitoring table calculated in advance to obtain the ESR aging degree of the capacitor. Figure 6 As shown in the figure, the recognition accuracy of ESR is at least within 2%, achieving a relatively accurate monitoring effect.

[0077] In summary, this invention extends the lifespan of electrolytic capacitors by adding passive components to the electrolytic capacitor circuit of a hybrid capacitor bank without increasing system complexity. By leveraging the high-frequency components in capacitor ripple current and the reliability differences between thin-film, electrolytic, and film capacitors, the state monitoring of electrolytic capacitors is integrated into the lifespan extension impedance, achieving the dual functions of "lifespan extension" and "online health status identification."

Claims

1. A dual-function online identification method for the health status of a hybrid capacitor bank, characterized in that It has dual functions, is simple, and does not affect the main circuit. It includes the following steps: Step 1: Determine the hybrid DC busbar capacitor arrangement Electrolytic capacitors are used to provide voltage support; film capacitors and electrolytic capacitors are used in combination to bear the switching frequency current and the high-frequency charging and discharging current at the switching moment; Step 2: Add an additional inductor in series with the electrolytic capacitor branch to increase the electrolytic circuit impedance, reduce ripple current, and extend the life of the electrolytic capacitor; Step 3: Constructing the Condition Monitoring Voltage Sensor The additional inductor connected in series to the electrolytic capacitor branch serves as the primary winding of the sensing transformer to construct a voltage transformer with a predetermined ratio. The AC voltage signal on the additional inductor is converted through a rectifier bridge and a filter capacitor to obtain a DC voltage, which is used as an indicator of the capacitor state. Step 4: Establish a state parameter monitoring model Based on the specific harmonic currents generated by the preceding AC / DC converter and DC / DC isolation converter, and the impedance differences between electrolytic capacitors and film capacitors, the effective value of the current in the electrolytic capacitor circuit is calculated using Ohm's law. The effect of changes in the electrolytic capacitor ESR on the effective value of the electrolytic capacitor circuit current is scanned to obtain the electrolytic capacitor current corresponding to different electrolytic capacitor ESRs. The relationship between the effective value of the primary inductor current and the secondary output DC voltage is then calculated as follows: Where, I L+ is the effective value of the primary inductor current, V T is the collected DC voltage signal, V f is the initial conduction voltage drop of the rectifier bridge diode, n is the voltage transformer ratio; ω 2fs L + is the inductive impedance, L + is the additional inductance value; At this point, a state parameter monitoring model can be established to reflect the state of the electrolytic capacitor ESR by collecting the output DC voltage and matching it; Step 5: Online collection and identification of state parameters The open circuit voltage of the secondary side of the voltage transformer is collected and matched with the parameter monitoring model to obtain the ESR aging degree of the electrolytic capacitor.

2. The method for online identification of the health status of a hybrid capacitor bank with dual functions according to claim 1 is characterized in that A dual function is achieved by combining high-frequency current suppression and high-frequency current acquisition. The process of calculating the final recommended additional inductance is as follows: the capacitor ripple current and the recommended inductance at each frequency are calculated, and the suppression degree and ripple current weight at each frequency are calculated. In combination with the principle of multi-objective optimization, the knee inductance at different frequencies is selected as the inductor, and then the ripple current weight at each frequency is combined to determine the final additional inductance.

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