Increase In Equivalent Series Resistance Research Articles

Local outlier factor based condition monitoring for sub-module capacitors in modular multilevel converters

Modular multilevel converters (MMC) as a key technology for future power grids is rapidly developing. The failure rate of sub-module (SM) capacitors is only lower than that of power devices in MMC, so accurate evaluation of the reliability of capacitors is crucial for the safe operation of MMC. Both the increase of equivalent series resistance (ESR) and decrease of equivalent series capacitance (ESC) are features for capacitor deteriorating. ESC or ESR of the capacitor reaching to limited values indicates that the end of its lifespan and it must be replaced. Most of contemporary researches only focus on ESC or ESR. This article considering the characteristics of both ESC and ESR, proposes a method for monitoring the status of SM capacitors based on machine learning algorithm: local outlier factor (LOF). By utilizing the existing SM switching function and capacitor voltage signals within a fundamental frequency cycle in the control system of MMC, the proposed strategy can not only monitor the faulty capacitors, but also can sort the degrees of abnormal states of capacitors, and the strategy is applicable to MMC with single and multiple types of SM topologies. The proposed method is verified by the hybrid MMC model on Matlab/Simulink.

Online DC-Link Capacitance Monitoring for Digital-Controlled Boost PFC Converters Without Additional Sampling Devices

Aluminum electrolytic capacitors (Al-Caps) are widely used in the dc link of ac/dc applications; however, they are one of the weakest components in power electronic converters. With the degradation of Al-Cap, its capacitance ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> ) decreases, and the equivalent series resistance increases. Based on this, many efforts have been made to realize condition monitoring (CM) of Al-Caps. Unfortunately, additional hardware (including current sensors and sampling circuits) and/or powerful data processing tools are usually required, which increases the cost and would bring new reliability risks. Considering this issue, this article presents a transient-based CM scheme for dc-link capacitors in digital-controlled boost power factor correction (PFC) converters, which does not need additional sampling devices. By analyzing the relationship between large-signal transient voltage profile and dc-link capacitance, a calculation model is established. Based on the sampled voltage and current signals used for digital control, the dc-link capacitance can be online estimated. Its feasibility is validated in simulation with a boost PFC operating in continuous conduction mode (CCM) and discontinuous conduction mode. Furthermore, taking a 1-kW boost PFC operating in CCM as a case study, experimental results illustrate that the estimation errors of the dc-link capacitance are less than 3.5% (error of a single test) and 1.5% (average error of multiple tests).

Plasticized H+ ion-conducting PVA:CS-based polymer blend electrolytes for energy storage EDLC application

In this work, plasticized proton-conducting polymer blend electrolytes based on poly (vinyl alcohol): chitosan were synthesized, characterized, and utilized in an electric double-layer capacitor (EDLC). A structural study was performed using X-ray diffraction. The scarification and decrease of intensity of the crystalline peak were detected upon the addition of glycerol plasticizer (GP). The morphological appearance of the plasticized blend electrolyte films was examined through scanning electron microscopy, in which the role of GP on ion dissociation was indicated to be important. Electrochemical impedance spectroscopy was performed to estimate the conductivity of the films. Before the EDLC application, other electrochemical techniques, such as transference number measurement (TNM), linear sweep voltammetry (LSV), and cyclic voltammetry (CV), were carried out. To evaluate the performance of the EDLC, the charge–discharge profile was then examined. The EDLC has been cycled 250 times at a current density of 0.75 mA cm−2. The specific capacitance (Cspe) of the EDLC, which is known as an essential parameter, was found to be constant after the first cycle. The reduction in Cspe value was correlated with the increase of equivalent series resistance. Finally, the manners of energy (E) and power (P) densities as a function of the number of cycles were studied.

Failure Prediction of Submodule Capacitors in Modular Multilevel Converter by Monitoring the Intrinsic Capacitor Voltage Fluctuations

Modular multilevel converters (MMCs) are emerging as a promising topology for medium- and high-power applications. Aluminum electrolytic capacitors (AECs) are usually employed in MMCs as floating capacitors due to their high volumetric efficiency and low price. AECs gradually deteriorate over time due to electrolyte vaporization, and for this reason, they have been recognized as one of the most fragile components in the converter. As they continue to age, the AEC's capacitance decreases and its equivalent series resistance increases, which can result in an increase of submodule (SM) capacitor voltage ripple, power loss and could damage the operation of the MMC with prolonged use of aged capacitors. To prevent such damage, monitoring the health of capacitors is an important step to enhance the reliability of the MMC by predictive maintenance. This paper presents a failure prediction scheme for SM capacitors in the MMC by monitoring the SM capacitor voltage oscillations. Detailed simulation studies are carried out for a five-level MMC in the PLECS® platform and verified experimentally. The estimated capacitance through simulations and experiments is in close agreement to that value measured using the LCR meter.

Online Condition Monitoring System for DC-Link Capacitor in Industrial Power Converters

This paper proposes an online condition monitoring system for aluminum electrolytic capacitors used as dc-link capacitors in industrial power converters. Electrolytic capacitors are one of the most fragile components of a power converter system because of their wear-out failures and short lifespan. Therefore, the use of condition monitoring systems for aluminum electrolytic capacitors allow preventive maintenance to be performed, thereby avoiding unexpected shutdown and downtime. The dominant wear-out mechanisms in aluminum electrolytic capacitors are characterized by the increase in equivalent series resistance. Hence, the condition monitoring system proposed in this paper is based on the online estimation of the capacitor's equivalent series resistance using the switching frequency components of the dc-link capacitor voltage and current. The equivalent series resistance estimation method is tested using the PSpice circuit simulator and is validated experimentally using a custom-made inverter. Finally, the equivalent series resistance estimation method is adapted for the condition monitoring of capacitors in industrial power converters and the proposed condition monitoring system is implemented in a commercial ac drive system to prove the applicability of the proposed method to industrial power converters.

ESR and capacitance monitoring of a dc-link capacitor used in a three-phase PWM inverter with a front-end diode rectifier

Condition monitoring plays an important role in estimating health condition of capacitors because the ageing of the capacitors is usually accompanied by an increase in equivalent series resistance (ESR) and a decrease in capacitance. Either capacitance or ESR cannot be a unique indicator of the lifetime of capacitors in some cases. This paper presents a condition monitoring method of a dc-link capacitor used in a three-phase PWM inverter with a front-end diode rectifier intended for motor drives. The monitoring method extracts both the ESR and capacitance of a capacitor under test from the actual ripple current and voltage without disconnecting the capacitor nor injecting an additional current. The monitoring method, therefore, can be implemented online. Experimental results verify that the monitoring method independently obtains the ESR and capacitance changes of the capacitor under test. This contributes to accurate lifetime estimation of dc-link capacitors.

Effect of viscoelasticity of ethylene vinyl acetate encapsulants on photovoltaic module solder joint degradation due to thermomechanical fatigue

The solder joint degradation due to thermomechanical fatigue is investigated in this paper for photovoltaic (PV) mini-modules with ethylene vinyl acetate (EVA) of different viscoelastic properties. The mini-modules were laminated at different curing temperatures in order to obtain EVA encapsulation with different viscoelastic properties. The influence of viscoelasticity of EVA on the thermomechanical fatigue generated on solder joint is analyzed based on a two-dimensional (2D) finite-element model. Based on simulation of thermomechanical stresses accumulation, mini-modules with EVA cured at lower temperatures accumulated approximately 40% more stresses during the thermal cycle testing than mini-modules with optimal cured EVA. The tested mini-modules with EVA cured at lower temperature showed greater power degradation than the optimal cured mini-modules. An apparent increase in equivalent series resistance is the primary factor the power loss. A good correlation between the accumulated thermomechanical fatigue and the increase in equivalent series resistance is demonstrated with the tested samples.

Symmetric Supercapacitor Electrodes from KOH Activation of Pristine, Carbonized, and Hydrothermally Treated Melia azedarach Stones.

Waste biomass-derived activated carbons (ACs) are promising materials for supercapacitor electrodes due to their abundance and low cost. In this study, we investigated the potential use of Melia azedarach (MA) stones to prepare ACs for supercapacitors. The ash content was considerably lower in MA stones (0.7% ash) than that found in other lignocellulosic wastes. ACs were prepared by KOH activation of pristine, carbonized, and hydrothermally-treated MA stones. The morphology, composition, surface area, porosity, and surface chemistry of the ACs were determined. Electrochemical measurements were carried out in three- and two-electrode cells, 3EC and 2EC, respectively, using 1 M H2SO4 as the electrolyte. The highest capacitance from galvanostatic charge-discharge (GCD) in 2EC ranged between 232 and 240 F·g−1 at 1 A·g−1. The maximum energy density reached was 27.4 Wh·kg−1 at a power density of 110 W·kg−1. Electrochemical impedance spectroscopy (EIS) revealed an increase in equivalent series resistance (ESR) and charge transfer resistance (RCT) with greater ash content. Electrochemical performance of MA stone-derived ACs was compared with that of other ACs described in the recent literature that were prepared from different biomass wastes and results showed that they are among the best ACs for supercapacitor applications.

Failure of Polymer Aluminum Electrolytic Capacitors Under Elevated Temperature Humidity Environments

Polymer aluminum (PA) electrolytic capacitors have solid conductive polymers as electrolyte. Reliability of polymer capacitors at elevated temperature humidity is not well understood. Elevated temperature-humidity testing was performed on PEDOT PA electrolytic capacitors by two manufacturers, Nichicon and Nippon Chemi-Con. Both the companies are among the top suppliers of aluminum electrolytic capacitors worldwide. Two different dominant failure modes of increase in equivalent series resistance and increase in leakage current (LC) were found in the PA capacitors from the two different capacitor manufacturers. The capacitance value of the polymer capacitors remained stable during the test. In the subsequent analysis, a new failure mechanism that has not been previously reported in the literature was found that leads to the high LC failure of PA electrolytic capacitors. The criteria for evaluation for this new failure mechanism should be included during the part and manufacturer selection process.

A Life Model for Supercapacitors

Supercapacitors provide high-power energy storage for electrical systems. The expected useful life of a supercapacitor is related to the oxidation of functional groups on the graphite electrode surface during usage, and it is highly dependent on operational voltage and temperature. In this paper, a life model is developed for commercial supercapacitors. The model incorporates a new voltage multiplier to describe the combined effects of temperature and voltage on supercapacitor life. Accelerated testing was conducted to obtain the time to failure of supercapacitors over a range of voltage and temperature conditions, validate the life model, and compare the model with two previously established capacitor life models. Failure was defined by a 30% decrease in capacitance or a 100% increase in equivalent series resistance.

Rate and cycle performances of supercapacitors with different electrode thickness using non-aqueous electrolyte

Activated carbons (AC) powder was coated onto Al current collectors with different thickness (∼30 and ∼80μm) for use as electrodes of supercapacitors. The relationship between the electrode thickness and the specific capacitance of the supercapacitor cells using a non-aqueous electrolyte was investigated under different charge–discharge current density and for charge–discharge cycling at constant current density. Two types of microporous AC powders of which specific surface areas were 1454 and 2587m2/g were deposited using a binary binder composed of styrene-butadiene rubber and sodium carboxymethyl cellulose. The rate stability of the specific capacitance was shown to be independent of the electrode thickness, and thinner electrodes could maintain higher specific capacitance regardless of the AC type. Thicker electrodes allowed a larger increase in equivalent series resistance with cycling, decreasing the voltage allocated to the formation of the electrical double-layer and thus reducing the specific capacitance. Microscopic surface observation showed that the thicker electrodes exhibited voids resulting from reduced adhesion among AC particles. The reduced adhesion and the electrolyte decomposition in the thicker electrodes were likely to be connected to their performance deterioration.

N-Type Hyperbranched Polymers for Supercapacitor Cathodes with Variable Porosity and Excellent Electrochemical Stability

A series of n-type hyperbranched polymers exhibiting variable porosity and excellent electrochemical stability are presented for use as the cathodes of asymmetric supercapacitors. The polymers are designed with triphenylamine (TPA) cores and naphthalene diimide (NDI) terminal units, with NDI being chosen for its electrochemical stability under reduction. A different number of thiophene rings between the TPA and NDI units are used to alter the porosity. These devices show very good stability over 500 cycles as a result of the stable NDI unit, and nitrogen adsorption experiments confirm that the addition of thiophene spacers results in a concomitant increase in the polymer matrix pore size. Electrochemical impedance spectroscopy (EIS) characterization reveals that as the porosity of the polymer increases, the diffusion resistance decreases. However, as the pore size increases, the charge transfer resistance and equivalent series resistance increases. Finally, one of the polymers is used to fabricate proof-o...

Effect of accelerated ageing on the performance of high voltage carbon/carbon electrochemical capacitors in salt aqueous electrolyte

An accelerated ageing protocol has been developed and validated to determine the factors influencing the life-time of high voltage carbon/carbon electrochemical capacitors (ECs) in aqueous lithium sulfate electrolyte, and to offer solutions reducing ageing of the cells. The study allowed the internal overpressure, increase of equivalent series resistance and corrosion of the positive current collector to be distinguished as the main failures which can appear during operation of these ECs at high voltage. To assure stability during cycling, the capacitor should not operate above 1.5V at room temperature. Adding sodium molybdate (Na2MoO4) to the electrolyte reduces the corrosion of the positive collector and lowers the equivalent series resistance which remains constant during floating at 1.5V; additionally, the initial capacitance is enhanced due to a pseudo-faradaic contribution of the molybdate additive. The internal resistance is further lowered when the pellet electrodes are glued to the current collectors with a carbon conductive adhesive.

An Accelerated Test Method for Predicting the Useful Life of an LED Driver

This paper proposes an accelerated life-test method for LED drivers that use electrolytic capacitors at the output stage. The type of failure considered here is parametric. Estimated failure rates of power components suggest the electrolytic capacitor used at the driver output stage is the weakest link. As an electrolytic capacitor degrades, its capacitance decreases and its equivalent series resistance increases, contributing to an increase of output current ripple. Thus, the amplitude of the output current ripple is a good indicator of the degradation level of the electrolytic capacitor. Temperature was selected as the acceleration factor because LED drivers are usually exposed to elevated temperatures in typical applications. The current ripple was experimentally found to have a negative impact on light output and efficacy; therefore, the useful life of an LED driver is proposed as the time it takes for the output current ripple to reach its maximum rate of change. The relationship between the capacitor's positive pin temperature and the useful life of the LED driver was then established using the proposed definition. The accelerated life test shows that the LED driver's useful life is shortened exponentially, as the output electrolytic capacitor's operating temperature increases.