Capacitors are widely used in dc links of power electronic converters to balance power, suppress voltage ripple, and store short-term energy. Condition monitoring (CM) of dc-link capacitors has great significance in enhancing the reliability of power converter systems. Over the past few years, many efforts have been made to realize CM of dc-link capacitors. This article gives an overview and a comprehensive comparative evaluation of them with emphasis on the application objectives, implementation methods, and monitoring accuracy when being used. First, the design procedure for the CM of capacitors is introduced. Second, the main capacitor parameters estimation principles are summarized. According to these principles, various possible CM methods are derived in a step-by-step manner. On this basis, a comprehensive review and comparison of CM schemes for different types of dc-link applications are provided. Finally, application recommendations and future research trends are presented.

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An Overview of Condition Monitoring Techniques for Capacitors in DC-Link Applications

Direct current (DC)-link voltage ripple analysis is essential for determining harmonic noise and for DC-link capacitor design and selection in single-phase pulse-width modulation (PWM) inverters. This paper provides an extensive theoretical analysis of DC-link voltage ripple for full-bridge (H-bridge) inverters, with simulation and experimental verifications, considering a DC source impedance (non-ideal DC voltage source). The DC voltage ripple amplitude is theoretically estimated as a function of the output current, both amplitude and phase angle, and the modulation index. It consists of a switching frequency component and a double-fundamental frequency component (i.e., 100 Hz), thereby both components are considered in the analysis. In particular, the peak-to-peak distribution, maximum amplitude, and root mean square (RMS) values of the voltage switching ripple over the fundamental period are obtained. Based on the DC voltage requirements, simple and effective guidelines for designing DC-link capacitors are obtained.

Theoretical and Experimental Investigation of Switching Ripple in the DC-Link Voltage of Single-Phase H-Bridge PWM Inverters

This article proposes a health indicator estimation method based on the digital-twin concept aiming for condition monitoring of power electronic converters. The method is noninvasive, without additional hardware circuits, and calibration requirements. An application for a buck dc–dc converter is demonstrated with theoretical analyses, practical considerations, and experimental verifications. The digital replica of an experimental prototype is established, which includes the power stage, sampling circuit, and close-loop controller. Particle swarm optimization algorithm is applied to estimate the unknown circuit parameters of interest based on the incoming data from both the digital twin and the physical prototype. Cluster-data of the estimated health indicators under different testing conditions of the buck converter is analyzed and used for observing the degradation trends of key components, such as capacitor and MOSFET. The outcomes of this article serve as a key step for achieving noninvasive, cost-effective, and robust condition monitoring for power electronic converters.

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A Digital Twin Based Estimation Method for Health Indicators of DC–DC Converters

Insulated-gate bipolar transistor (IGBT) modules and dc-link capacitors are important parts in the majority of power electronic converters which contribute to cost, size, and failure rate on a considerable scale. This paper presents an online condition monitoring method for both IGBT modules and dc-link capacitors of power converters based on short-circuit current of an IGBT module, which is a good condition indicator according to the theory analysis. The failure prediction of dc-Link capacitor is realized by equivalent series resistance, which can be calculated by short-circuit current and a step voltage, and the bond wires fatigue can also be identified by the short-circuit current. The proposed method is capable for detecting small changes in the failure indicators of IGBT modules and electrolytic capacitors, and its effectiveness is validated by a confirmatory experiment. The novelty of the proposed method is that the degradations of IGBT modules and capacitors can be identified simultaneously, and has the merits of low cost and circuit simplicity

Online Condition Monitoring for Both IGBT Module and DC-Link Capacitor of Power Converter Based on Short-Circuit Current Simultaneously

Aluminium electrolytic capacitors (AECs) are used in dc-link of single-phase grid connected solar photovoltaic (PV) inverters to suppress the dc-link voltage oscillation. With aging its capacitance decreases and equivalent series resistance (ESR) increases, which may lead to increase in voltage ripple, thereby reducing average power extracted from solar PV and/or damage the semiconductor switches due to overvoltage. Further, prolonged use of the aged capacitor could lead to the open circuit of the dc-link due to vaporization of the electrolyte. Therefore, to enhance the reliability of the system by predictive maintenance, it is necessary to monitor the health of AEC. A quasi-online method for condition monitoring of the AECs in solar inverters is proposed in this paper. This method is active during the night when there is no solar irradiance. In this case, the current of various odd harmonic frequencies are injected by the inverter and dc-link capacitor impedances at various frequencies are evaluated. Using these impedance values, least mean square algorithm is used to estimate the ESR and capacitance values. The magnitude of each injected harmonic current is kept within the limits prescribed by IEEE 929-2000 standard. The key advantage of the technique is that it combines the estimation of both ESR and capacitance to determine the health. Further, it does not require significant processing capability and is implemented in the existing digital processor/controller used for inverter control. Detailed simulation studies are carried out using MATLAB-Simulink. Proposed scheme is verified experimentally on a scaled down version of a grid connected solar inverter. Puncturing based technique is used for accelerated aging of the capacitor. Estimated values of ESR and C are in agreement to that measured from LCR meter.

Quasi-Online Technique for Health Monitoring of Capacitor in Single-Phase Solar Inverter

To interconnect low-voltage solar photovoltaics (PV) with dc system, a dc–dc boost converter is required. To minimize the switching frequency oscillations in solar PV voltage, aluminum electrolytic capacitors (AECs) are connected between solar PV and the converter. Operational life of AECs depends on electrical and environmental parameters and is less than that of solar PV panels. Equivalent series resistance (ESR) of AEC increases with life, thereby increasing the voltage ripple across PV terminals. This results in less power extraction from solar PV. This paper proposes a method for health monitoring of AEC using the parameters measured for maximum power point tracking (MPPT) (PV voltage and current). The proposed technique is applicable for both continuous conduction mode and discontinuous conduction mode of operation of the converter. Effect of temperature on ESR is incorporated for accurate determination of the health of capacitor. Key advantage is that no additional current or voltage sensor is required to implement this technique, thereby offering a low-cost solution. Further, the proposed technique does not require significant processing capabilities and is implemented in the same digital controller used for MPPT. Detailed simulation studies are carried out and results are included in this paper. A laboratory prototype of dc–dc boost converter is developed for experimentation. Results are in agreement with that obtained from simulation studies.

Online Monitoring Technique for Aluminum Electrolytic Capacitor in Solar PV-Based DC System

Condition monitoring of aluminum electrolytic capacitors (AECs) is essential for predictive maintenance of power electronic converters. The AEC is considered at the end of its life when its capacitance or equivalent series resistance (ESR) reaches corresponding critical values. In the literature, it is found that either of these parameters may reach its critical limit depending on the operating conditions and applications. However, most of the existing health monitoring techniques of the AEC in dc–dc converters are based on the estimation of ESR. To address the aforementioned issue, this paper proposes to estimate the low-frequency impedance of the AEC, which is dominated by its capacitance value, thereby allowing health monitoring based on the capacitance value. The technique is based on injection of a low-frequency current ripple into the AEC using duty ratio control of the dc–dc converter. The parameters of new and aged capacitors are experimentally obtained at various temperatures and are used to establish the failure criteria. Furthermore, the proposed method is applicable for both the continuous conduction mode and the discontinuous conduction mode (DCM) of operation. For DCM operation, a sampling instant to recover a low-frequency waveform is suggested based on mathematical analysis. Detailed simulation studies are performed and results are included in this paper. Experimentation is carried out on a dc–dc boost converter integrating solar photovoltaic with the dc system. Experimental results are found to be in agreement with simulation results.

Low-Frequency Impedance Monitoring and Corresponding Failure Criteria for Aluminum Electrolytic Capacitors

Electrolytic capacitors are popularly used in single-phase grid-feeding solar photovoltaic (PV) inverters to suppress the second harmonic and switching frequency voltage ripples. With aging equivalent series resistance (ESR) of capacitor increases and its capacitance value decreases, which lead to increase in dc-link voltage ripple. Oscillations in PV operating point around its maximum power point (MPP), results in reduction of average output power and revenue generated. To address this, frequent replacement of capacitors are required, which may lead to increased cost. Therefore, capacitors must be replaced at an optimal period to ensure maximum earnings. To realize this, a technique for monitoring of power extraction efficiency (PEE) is proposed in this paper. Further, criteria for replacement of capacitor based on the measured values of PEE is suggested. Mathematical model relating the capacitance and ESR values to the PEE is derived. Effect of variation in temperature and solar radiation on PEE is discussed. Detailed simulation studies are carried out using MATLAB-Simulink. A scaled down laboratory prototype of inverter is developed. The proposed technique is implemented in the existing digital processor/controller used for MPP tracking, thereby avoiding additional circuits/sensors. PEE estimated by simulation and experimentation are found to be within 1% of each other.

Lifetime Monitoring of Electrolytic Capacitor to Maximize Earnings From Grid-Feeding PV System

Grid tied single-phase solar inverters offer less operational life when compared with the Photovoltaic (PV) panels due to the use of aluminium electrolytic capacitor (AEC). With time, AEC degrades, resulting in an increase in ripple voltage on PV panel. This would reduce the power extraction efficiency of the system, thereby a loss of revenue. This study proposes a simple and cost-efficient online technique for monitoring the health of AECs. The proposed technique suggests evaluating the AEC impedance at twice the grid frequency. Sampling of PV voltage, PV current and inductor current is required to determine the second harmonic impedance of AEC. These voltage and current sensors are present in commercially available solar PV inverters for their control operation. The proposed technique could be extended to other single-phase inverter applications provided that voltage and current sensors are available. Suitable mathematical model of the technique is derived to determine the effect of sampling instant on the accuracy of the results. Criteria for the replacement of capacitor based on impedance monitoring is defined. For validation of the proposed technique, detailed simulation studies are carried out for a single-phase PV system. The technique is verified experimentally on a scaled down solar PV inverter prototype.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-pel.2016.0603

Nikunj Agarwal

Papers

Quasi-Online Technique for Health Monitoring of Capacitor in Single-Phase Solar Inverter

Online Monitoring Technique for Aluminum Electrolytic Capacitor in Solar PV-Based DC System

Low-Frequency Impedance Monitoring and Corresponding Failure Criteria for Aluminum Electrolytic Capacitors

Lifetime Monitoring of Electrolytic Capacitor to Maximize Earnings From Grid-Feeding PV System

Capacitor impedance estimation utilizing dc-link voltage oscillations in single phase inverter