DC-link Electrolytic Capacitor Research Articles

바이폴라 스위칭을 적용한 인버터의 직류단 전해 커패시터 진단

바이폴라 스위칭을 적용한 인버터의 직류단 전해 커패시터 진단

A reliability prediction method considering degradation self-acceleration effect in DC-link electrolytic capacitor

The reliability of DC-link electrolytic capacitors is crucial to ensure the quality of power supply systems. The degradation of capacitor parameters may lead to a higher temperature and thus accelerate degradation as a self-accelerating effect. In this article, an improved reliability prediction method for DC-link electrolytic capacitors is proposed, as existing methods have not adequately accounted for the self-acceleration effect. The degradation under dynamic stress is obtained by cumulative computations and the stress is updated according to the degraded parameters. The degradation models are converted into degradation rate models to overcome the computational challenges associated with small-step iterations that may make traditional methods unaffordable. The proposed method for developing the degradation rate model is widely applicable and achieves satisfactory accuracy. To demonstrate the practicality of the proposed method, a case study of a boost motor drive system is presented. The appropriate iteration step can be determined by comparing the results of the lifetime distributions obtained using different iteration steps. Degradation paths considering self-acceleration effects can be obtained, enabling more precise system quality analysis and reliability prediction.

An Online DC-Link Capacitance Estimation Method for Motor Drive Systems Based on an Intermittent Reverse-Charging Control Strategy

The dc-link capacitor in the three-phase ac–dc–ac motor drive system is a critical component. The degradation of dc-link electrolytic capacitors is a common failure that has a significant impact on system reliability, which can often lead to system breakdown or damage. It is significant to provide monitoring and prognostic techniques for dc-link capacitors to assist in preventive maintenance. To realize online estimation of the dc-link capacitance, a reverse-charging mode of dc-link capacitors is actively introduced during the normal operation, by actively altering the control of the power switches in the converter in an intermittent manner. During the proposed reverse-charging mode, the dc-link capacitor bank is charged by the motor, and the charging current exactly originates from the current in one particular phase, which can be designated and measurable in typical motor drive systems. Based on the reverse-charging mode, the capacitance can be calculated through the voltage rise and charging current. Simulation and experimental tests are implemented to verify the accuracy and feasibility of the proposed method. It is shown that this method is capable of providing online monitoring of dc-link capacitors with high accuracy.

Bidirectional Buck-Boost converter as an Active Power Decoupling schema for Single-Phase Power Inverters

The power ripple in the DC-link of photovoltaic and energy storage systems interconnected to the AC mains can drastically impact the lifespan of the photovoltaic panel and the system in general. This problem can be easily solved by using DC-link electrolytic capacitors as a passive decoupling method; however, this implies significant drawbacks such as increased volume and cost of the capacitor and a significant reduction in efficiency and reliability. This can be compensated by replacing electrolytic capacitors with active power decoupling methods based on power electronic converters. Thus, a Bidirectional Buck-Boost converter as an active power decoupling scheme is proposed. First, a power decoupling overview is addressed. Afterward, the operation of the Buck-Boost converter is detailed. Therefore, the control strategy is also explained. Finally, some simulations and preliminary experimental results to verify the integrated operation of the power converter are given.

Comparative Efficiency Analysis between a Conventional Single-Phase Inverter and an Inverter with a Minimalist Active Power Decoupling Circuit

This paper focuses on a comparative analysis between the conventional single-phase H-bridge inverter and an inverter implementing a minimalist active power decoupling method, which is used to eliminate the power ripple of twice the fundamental frequency at the DC link. This method allows minimising the need for large electrolytic DC link capacitors, with only two smaller film capacitors being required on the hardware side. The implemented 1 kW inverter makes use of silicon carbide (SiC) MOSFETs, offering the main advantage of high switching frequencies, reduced size, and improved efficiency. The performance analysis, including switching and conduction losses and the inverter efficiency, is done through the results obtained from simulations and experiments.

Condition Monitoring of DC-Link Electrolytic Capacitor in Back-to-Back Converters Based on Dissipation Factor

Condition-monitoring systems are used to determine the best timing for predictive maintenance of power electronics converters. These monitoring systems can significantly reduce the converter repair time and increase system availability. In this article, an investigation has been conducted on the possibility of using the dissipation factor as a lifetime indicator of electrolytic capacitors. A criterion is also proposed based on the dissipation factor to detect electrolytic capacitors’ end of useful life. A method has been presented for measuring the dc-link capacitor dissipation factor in a back-to-back converter. Using this technique, it is possible to estimate the switching component of the capacitor current by measuring the output currents of the converter without a need to measure the capacitor current directly. The proposed method is simulated, and the impact of various factors, such as switching frequency, capacitor series inductor, and the network filter on the accuracy of the dissipation factor measurement, is discussed. Experimental results are also provided to validate the acceptable accuracy of the proposed method.

Single-Phase PFC Rectifier With Integrated Flying Capacitor Power Pulsation Buffer

Single-phase Power Factor Correction (PFC) rectifiers with sinusoidal grid currents are inherently subject to an input power fluctuating at twice the mains frequency. In order to potentially mitigate bulky, heavy and failure-prone electrolytic dc-link capacitors, active Power Pulsation Buffer (PPB) concepts are proposed in the literature. For converter systems employing Flying Capacitor (FC) multilevel bridge-legs, the FCs can be utilized as a twice-mains frequency energy storage, i.e., as an integrated active PPB without the need for additional power components. Such ac-dc-stage-integrated FC PPBs capable of buffering the complete input power variation are known in literature which, however, require sophisticated control strategies with varying switching frequency and discontinuous conduction mode. This paper presents a novel ac-dc-stage-integrated FC-PPB approach which is compatible with standard PFC control concepts and enables a significant reduction in dc-link voltage variation. First, a control concept cycling the FC voltage in a wide range without interfering with the grid current controller is derived step by step and verified by means of circuit simulations. Design guidelines for the calculation of suitable FC capacitance values are presented and the limits in buffering capability are discussed. The concept is then experimentally verified with a 2.2kW three-level FC single-phase PFC rectifier employing 600V GaN power semiconductors where the dc-link voltage variation is reduced by 28% compared to conventional operation. Last, the applicability of the ac-dc-stage-integrated FC-PPB concept to other FC converter topologies is discussed.

PV String-Level Isolated DC–DC Power Optimizer with Wide Voltage Range

This paper proposes a photovoltaic (PV) string-level isolated DC–DC power optimizer with wide voltage range. A hybrid control scheme in which pulse frequency modulation (PFM) control and pulse width modulation (PWM) control are combined with a variable switching frequency is employed to regulate the wide PV voltage range. By adjusting the switching frequency in the above region during the PWM control process, the circulating current period can be eliminated and the turn-on period of the bidirectional switch of the dual-bridge LLC (DBLLC) resonant converter is reduced compared to that with a conventional PWM control scheme with a fixed switching frequency, resulting in better switching and conduction loss. Soft start-up control under a no-load condition is proposed to charge the DC-link electrolytic capacitor from 0 V. A laboratory prototype of a 6.25 kW DBLLC resonant converter with a transformer, including integrated resonant inductance, is built and tested in order to verify the performance and theoretical claims.

On-Line Fault Diagnosis of DC-Link Electrolytic Capacitors in Boost Converters Using the STFT Technique

Faults in aluminum electrolytic capacitors (AECs) are classified as the major cause for power electronics equipment breakdown, mainly due to AECs wear out through the vaporization of the electrolyte, as a result of both aging and temperature effects. The majority of manufacturers identify the end-of-life threshold of such capacitors as their equivalent series resistance (ESR) increases by 80% or their capacitance (C) decreases by 20%, relative to their initial values. Therefore, the online estimation of these two parameters can provide valuable and timely information that allows detecting any potential capacitor failure. Changes in C and ESR parameters influence the relationship between the capacitor voltage and current ripples. This ratio is dominated by C at low frequencies, and by ESR in the high-frequency range. The aim of this article is to propose an algorithm allowing a continuous estimation and tracking of both parameters (ESR and C). This algorithm should be simple, fast, and suitable for online implementation. Therefore, this article introduces the use of the short-time Fourier transform (STFT) technique to determine and track C and ESR values, starting from some always-existing harmonics at low and high frequencies. The STFT is based on applying the discrete Fourier transform algorithm on a sliding window, which makes it more appropriate for online implementation. The effectiveness of the proposed method is proved by simulation and laboratory experiments, using a boost dc-dc converter.

Direct AC–AC Active-Clamped Half-Bridge Converter for Inductive Charging Applications

In this article, a new direct ac-ac active-clamped half-bridge converter feeding a series-series compensated inductive charging system is proposed. The proposed converter eliminates the rectification stage and bulky life-limited electrolytic dc-link capacitors, resulting in an actual single-stage (ac-to-ac) conversion with low component. The operation principles, steady-state analysis, and design considerations of the proposed converter are presented in detail. Moreover, a new predictive dead-beat technique for input current control and a linear average charging current controller are developed for the proposed converter, which enables the unity power factor and charging current regulation. The performance of the proposed battery charger is validated through experimental results on a 1-kW field programmable gate array (FPGA)-controlled silicon carbide based laboratory prototype.

A single-stage AC-DC-AC buck-boost converter with variable frequency operations for power quality

ABSTRACT This paper presents a new direct ac-ac converter topology with new switching techniques and step-changed frequency operations. However, the existing traditional ac-ac converters suffer from two-stage power processing, ac-dc-ac, large dc-link electrolytic capacitor, modulation indices constraints, and complex control. The proposed converter provides symmetric non-inverting and inverting buck-boost operating modes with bipolar voltage and step-change frequency which can compensate broad range of voltage sag and swell without commutation and shoot through problem. Furthermore, the proposed operations minimise the dc-link capacitance requirement (<10 μF), and thus, film capacitor can be employed with high reliability and low loss and less switches. The proposed operating mode when use with dynamic voltage restorer work under entire power factor range and is suitable for reactive load. A detailed analysis using Sabre simulation and experimental results is presented.

Condition Monitoring of DC-Link Electrolytic Capacitors in PWM Power Converters Using OBL Method

Since the lifespan of an electrolytic capacitor is relatively short compared to other power semiconductor devices, the failure rate accounts for 60% and, thus, it is the most vulnerable component of the power conversion device. Therefore, the accurate measurement of the lifetime of an electrolytic capacitor is very important in ensuring the reliability of the entire system, including the capacitor. In this paper, an online failure detection method for a DC-link electrolytic capacitor in a back-to-back Pulse width Modulation (PWM) converter using the opposition-based learning particle swarm optimization-based Support Vector Regression (OPSO-SVR) technique is proposed. In this method, the capacitance and the DC-link capacitor power have been used in offline mode for SVR training and testing. During the offline mode, the SVR parameters have been optimized with the OPSO algorithm to use online to estimate the real value of the DC-link capacitor. The experimental results prove the superiority of the proposed technique over the SVR.

Current Source AC-Side Clamped Inverter for Leakage Current Reduction in Grid-Connected PV System

For the grid-connected photovoltaic inverters, the switching-frequency common-mode voltage brings the leakage current, which should be eliminated. So far, many kinds of single-phase inverters have been published for this purpose, but most of them are the conventional voltage-type ones, which have the disadvantages of poor reliability due to the DC-link electrolytic capacitor and the risk of short-through of the bridge switches. To solve this technical issue, a novel current source inverter with AC-side clamping is proposed to mitigate the switching-frequency common-mode voltage. Meanwhile, a novel modulation method is proposed for the new single-phase inverter to achieve low-frequency operation of the main switches, which reduces the switching losses. Finally, the proposed method is implemented on the TMS320F28335DSP + XC6SLX9FPGA digital hardware platform. Also, the performance comparisons are done with the traditional solution. The results prove the proposed solution.

A Soft-Switching Single-Stage Converter With High Efficiency for a 3.3-kW On-Board Charger

This paper presents a highly efficient 3.3-kW single-stage on-board charger (OBC). The proposed OBC consists of two interleaved cells sharing a full-bridge diode rectifier. Each cell is composed of an active clamp circuit and a series resonant circuit. The proposed OBC provides zero-voltage switching of all switches and reduces the reverse-recovery problem by zero-current switching of the output diodes. The interleaved technique maintains the proper power level with high efficiency. The proposed control algorithm enables power factor correction by the OBC without any auxiliary circuit and balances the input current between the two cells. In addition, the proposed converter requires no electrolytic dc-link capacitor, ensuring compact size and a long lifetime. Owing to these features, the proposed OBC efficiently outputs a high power factor over a wide power range and achieves reliability with a simply structured, single-power processing step. Therefore, the proposed OBC is potentially feasible for automotive applications. Finally, the theoretical analysis is confirmed in a 3.3-kW prototype and validated in experimental results.

Current Harmonic Reduction Based on Space Vector PWM for DC-Link Capacitors in Three-Phase VSIs Operating Over a Wide Range of Power Factor

This paper proposes a novel space vector pulsewidth modulation (SVPWM) strategy to reduce the switching-frequency-order inverter input current harmonics for a two-level three-phase voltage-source inverter operating over a wide range of the load power factor. The fluctuation of the inverter input current around its average value is minimized by optimization of the utilized voltage space vectors to reduce the inverter input current harmonics, which extends the lifetime of the dc-link electrolytic capacitors. Furthermore, the proposed strategy can be adapted to a motor drive system, where a wide variation of the load power factor is desired, by changing the optimized combinations of the voltage space vectors according to the polarities of the output phase currents. Experiments with the application of the novel SVPWM confirm that the inverter input current harmonics is reduced by up to 29.5% compared to that with the conventional SVPWM strategy. Moreover, the analytical and experimental results confirm that the proposed SVPWM reduces the inverter input current harmonics in both driving mode and regenerative braking mode, as well as with any load power factor.

Control of Motor Drives Fed by PFC Circuits without DC-Link Electrolytic Capacitors

Control of Motor Drives Fed by PFC Circuits without DC-Link Electrolytic Capacitors

Wear-Out Failure Analysis of an Impedance-Source PV Microinverter Based on System-Level Electrothermal Modeling

In this paper, the wear-out performance of an impedance-source photovoltaic (PV) microinverter (MI) is evaluated and improved based on two different mission profiles. The operating principle and hardware implementation of the MI are first described. With the experimental measurements on a 300-W MI prototype and system-level finite-element method simulations, the electrothermal models are built for the most reliability-critical components, i.e., power semiconductor devices and capacitors. The dependence of the power loss on the junction/hotspot temperature is considered, the enclosure temperature is taken into account, and the thermal cross-coupling effect between components is modeled. Then, the long-term junction/hotspot temperature profiles are derived and further translated into components’ annual damages with the lifetime and damage accumulation models. After that, the Monte Carlo simulation and Weibull analysis are conducted to obtain the system wear-out failure probability over time. It reveals that both the mission profile and the thermal cross-coupling effect have a significant impact on the prediction of system wear-out failure, and the dc-link electrolytic capacitor is the bottleneck of long-term reliability. Finally, the multimode control with a variable dc-link voltage is proposed, and a more reliable dc-link electrolytic capacitor is employed, which results in a remarkable reliability improvement for the studied PV MI.

Hybrid-Modulation-Based Bidirectional Electrolytic Capacitor-Less Three-Phase Inverter for Fuel Cell Vehicles: Analysis, Design, and Experimental Results

This paper presents a novel six-pulse low-frequency (LF) fluctuating high-voltage dc-bus-based power system architecture for fuel cell vehicles (FCVs) application. A novel hybrid modulation scheme consisting of six-pulse modulation at LF scale and modified secondary modulation and 33% pulse width modulation at high-frequency (HF) scale is proposed. Three-phase ac waveforms for the propulsion system of FCVs are generated from LF fluctuating high-voltage dc bus. The proposed modulation technique significantly reduces the switching losses of the bidirectional dual-stage inverter: 1) soft-switching of both sides of the front-end current-fed full-bridge converter is realized; and 2) at any moment, only one leg of back-end three-phase inverter is switched at HF, while the other two legs are kept in on or off states. This tremendously reduces the bidirectional inverter's switching losses and improves the system efficiency. The LF fluctuating high-voltage dc bus allows the elimination of large electrolytic dc-link capacitor, which contributes to a more reliable and compact design. This paper presents the operation, analysis, and design of a bidirectional inverter implementing the proposed hybrid modulation technique. Simulation results obtained from power electronics simulation software PSIM and experimental results from the lab prototype clearly validate the effectiveness of the proposed modulation technique.

Inverter Power Control Based on DC-Link Voltage Regulation for IPMSM Drives Without Electrolytic Capacitors

DC-link electrolytic capacitor critically affects the lifetime of the motor drive system. This paper proposes an inverter power control strategy based on dc-link voltage regulation for the electrolytic capacitor-less interior permanent magnet synchronous motor drive system. The dc-link electrolytic capacitor is replaced by a small film capacitor and the power factor correction circuit is also eliminated. Hence, the inverter power should be regulated effectively to achieve high input power factor and low grid current harmonics. The inverter power control loop based on proportional resonant (PR) controller is established to regulate the inverter power into sinusoidal wave. The PR controller can be designed to achieve a high gain at the desired frequency and the parameters are easy to design due to the simple structure. In addition, a power compensation method based on the dc-link voltage regulation is used to diminish the error of inverter power control, which does not depend on the precise calculation and is easy to realize. The effectiveness of the proposed method is demonstrated by the experimental results on an air conditioner. The input power factor can reach 0.992 and the harmonics of grid current are considerably lower than the requirement of EN61000-3-2 standard.

Stability Analysis of AC–DC Full-Bridge Converters With Reduced DC-Link Capacitance

Voltage ripple in single-phase ac–dc converters is usually disregarded to design the dc-link voltage control system, since large dc-link electrolytic capacitors are typically employed. However, replacement of electrolytic capacitors by film capacitors has been widely considered for increasing reliability. Consequently, due to cost constraints, such replacement usually employ low capacitance and may be combined with fast dc-bus voltage controllers. This paper shows that the conventional linear time-invariant (LTI) model may not represent the real converter behavior for reduced capacitance and/or faster controllers, leading to poor system performance or even instability. In this way, for any of these cases, the linear time-periodic (LTP) model is highly encouraged for stability and transient analysis as a tool for the controller design. Experimental results confirm that stability margins are precisely obtained from the LTP model but may not from the LTI model. Finally, this paper also compares both LTI and LTP models while considering the control gain and the dc-bus capacitance size. It is graphically revealed that the phase margin of the LTI model diverges from the LTP model for low dc-bus capacitances and high control gains.