Metallized Film Capacitors Research Articles

Temperature Rise of Metallized Film Capacitors Under AC and DC Superimposed Voltage

Temperature Rise of Metallized Film Capacitors Under AC and DC Superimposed Voltage

Crystallization Morphology-Dependent Breakdown Strength of Polypropylene Films for Converter Valve Capacitor

In this paper, a novel modification method of polypropylene (PP) films is proposed in the field of metallized film capacitors for converter valves. An organic crystallization accelerator with good dispersion improves the micromorphology of PP films. The doping content of the organic crystallization accelerator is optimized based on breakdown experiments under DC voltage. The DC breakdown strength is 584.8 kV/mm, an increase of 19.6%, compared with that of the original film. Under DC, DC superimposed harmonics, and DC superimposed pulse voltages, the modified PP film has successfully withstood the tests, showing stable advantages in breakdown properties without increasing the conduction loss. Trap analysis and theoretical modeling were performed to clarify the mechanism of improving electrical performances based on crystallization control. The entanglement layer introduced by the organic crystallization accelerator helps to expand the crystal area, resulting in deep trap sites and limitation of carrier migration. This research can provide a reference for the PP performance optimization for metallized film capacitors of converter valves.

Self-Healing Processes of Metallized Film Capacitors in Overload Modes—Part II: Theoretical and Computer Modeling

In this article, we present the theoretical models on self-healing (SH) processes in metallized film capacitors (MFCs) in overload modes. Based on the proposed dynamic model of capacitor's SH resistance, the current and voltage of MFC in SH process and the value of dissipated energy, W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SH</sub> , can be calculated. Theoretical model of MFC degradation due to SH processes approves that in the vicinity of the breakdown places of polymer dielectric, a high SH energy leads to their carbonization. The numerous SH events result in the decreasing of the equivalent parallel resistance (EPR). An analysis of MFC electrothermal state shows that at critical values of applied voltage at 4-5 of nominal values, the dangerous local heating of the polymer dielectric can occur. Computer simulation describes the behavior of the EPR of the polymer film capacitor, which is determined by the number and intensity of SH acts. Finally, the numerical model for evaluation of the thermal state of the capacitor is offered. Numerous SH acts (especially of high SH energy level) cause the local heating of MFC section. The surface temperature of the capacitor may seem normal, though the local heating reaches the level dangerous for polymer. The analytic estimations and simulation results are in quite good agreement with experimental data.

Study on microstructure and its correlation with sheet resistance of Zn-Al metallized film

ABSTRACT Zn-Al film was prepared on biaxially oriented polypropylene (BOPP) substrate via vacuum thermal evaporation. The microstructure and its correlation with sheet resistance of Zn-Al film were explored systematically. The results showed that the macromorphology of Zn-Al film was closely related to the surface topography of BOPP substrate. XRD and SEM analysis indicated that Zn-Al film displayed a columnar and strong textured structure of Zn (002). TEM results revealed that the film thickness was in the range of 20–25 nm, of which amorphous alumina was identified on/at the surface and interface, while Zn and a small amount of ZnO were detected in the middle. The amorphous alumina on the film surface prevented the further oxidation of Zn, and therefore helped to improve the stability of sheet resistance of the film. Nevertheless, the insulative alumina significantly decreased the film conductivity, which deteriorated the current handling ability of metallized film capacitors.

Self-Healing Processes of Metallized Film Capacitors in Overload Modes—Part 1: Experimental Observations

Metallized film capacitors (MFCs) are used in many applications requiring high volumetric energy characteristics. Along with an increase in the dielectric permittivity of the polymer film, operating in overload mode is a simple way to dramatically increase the energy density of capacitors for relatively short periods. The unique feature of MFC, a self-healing (SH) ability, makes this possible. However, the task is complicated by the lack of information on the energy characteristics of SH in real capacitors, especially when they are voltage overstressed. Part 1 of this article contains the results of experimental investigations of SH processes in overload modes under the soft-training test. At the level of voltage up to 8 of nominal values, three types of SH were discovered: single, repetitive, and multiple. The energy characteristics of SH processes obtained in real MFCs varied widely from those in the literature data. The cumulative SH energy characteristic was introduced and proposed for diagnostic purposes. The reasons for repetitive SH were investigated and analyzed. Equivalent parallel resistance as a sensitive diagnostic parameter of the overloaded MFC was chosen and applied.

Capacitance Loss Mechanism and Prediction Based on Electrochemical Corrosion in Metallized Film Capacitors

For the metallized film capacitors (MFCs) designed to maximize the self-healing performance in the event of overvoltage, electrochemical corrosion is the major capacitance loss (CL) mechanism especially under high temperature and/or high humidity. In this paper, the electrode corrosion evolution is analyzed by dividing the process into anode and cathode reaction. In order to investigate the CL characteristics, three types of MFCs are stressed with 305 VAC under different temperature and humidity. Based on the corrosion mechanism and distribution analysis, a staged CL prediction method is proposed. The results indicate that the typical CL curve can be divided into 4 stages which are characterized by the reaction rate coefficient and the capacitance loss exponent derived from the experimental data.

Combined Fuzzy Time Series Prediction Method for Fault Prediction of EML Pulse Capacitors

Metallized film capacitors are an important part of the electromagnetic rail launch system. It is of great significance to predict its failure. In engineering, the failure law of capacitor is obtained by cyclic pulse accelerated life test of capacitor module with a small number of samples, and is used to predict the actual failure of capacitor in electromagnetic launch (EML) system. This method has the following problems: first, there are few samples of capacitor in life test at affordable cost, which cannot include all capacitor characteristics; second, the time interval of actual EML missions and the voltage of the capacitors are both changing and inconsistent with the conditions used in the life test; third, the statistical law is affected by the measurement and other errors. For these reasons, the prediction accuracy of the existing methods is limited. In this article, a new combined time series prediction method is proposed by using the high-order fuzzy time series prediction theory. Based on the capacitor used in the prototype of an electromagnetic railgun, the life test data of 20 pulse capacitors are used as the prior knowledge input, and the prediction model is obtained. Five groups of capacitor life test data simulating real launch conditions are used as test data to verify that the prediction accuracy of the proposed method is better than that of the traditional method, and the prediction accuracy of capacitor capacitance, which made by different processes, is much higher than that of the traditional method.

Application of Numerical Simulation for Metallized Film Capacitors Electrodes Design

Polymer film capacitors are widely used in modern power equipment, because of excellent volumetric characteristics in combination with high-voltage application ability. Using segmented electrodes of nanometer thickness increased the capacitor's performance and reliability because of the self-healing feature. In this paper, we present the results of the experimental investigation and numerical simulation of electrothermal destruction of the metallized film capacitors segmented electrodes during the self-healing process. The destruction processes were investigated for both a single gate and single segment, comprising four parallel gates connected to the segment. The numerical simulation was conducted by means of COMSOL Multiphysics software. The model takes into account the heat flow from metal layer to polymer film since it has significant influence on the destruction process. Based on a good agreement of experimental and numerical results, the simulation model was proposed for the real metallized film capacitor segmented electrodes design. The model allows evaluating the single segment isolating time during self-healing, the energy required for the isolating, effective value of segmented electrodes surface resistance.

Residual Lifetime Prediction with Multistage Stochastic Degradation for Equipment

Residual useful lifetime (RUL) prediction plays a key role of failure prediction and health management (PHM) in equipment. Aiming at the problems of residual life prediction without comprehensively considering multistage and individual differences in equipment performance degradation at present, we explore a prediction model that can fit the multistage random performance degradation. Degradation modeling is based on the random Wiener process. Moreover, according to the degradation monitoring data of the same batch of equipment, we apply the expectation maximization (EM) algorithm to estimate the prior distribution of the model. The real-time remaining life distribution of the equipment is acquired by merging prior information of real-time degradation data and historical degradation monitoring data. The accuracy of the proposed model is demonstrated by analyzing a practical case of metalized film capacitors, and the result shows that a better RUL estimation accuracy can be provided by our model compared with existing models.

Detection of Self-Healing Discharge in Metallized Film Capacitors Using an Ultrasonic Method

Benefiting from self-healing features, metallized film capacitors (MFCs) are widely employed to compensate reactive power (VAR) and thus improve the performance of AC systems. To ensure the aforementioned functions, self-healing testing is a compulsory quality inspection for every type of MFC. In 2014, the International Electrotechnical Commission (IEC) issued a standard that recommended a general and instructive test procedure based on audible noise or ultrasound signals. However, more details relevant to this high voltage (HV) test were not provided. In this paper, we focused on the ultrasonic detection technique to reveal the self-healing characteristics of two typical MFCs. By launching a series of HV tests with star and delta MFCs, the waveform features, discharge energy, and spectrum distributions were analyzed. It was observed that the partial discharge always occurs before self-healing discharge with the same spectrum features ranging above 40 kHz. To solve the entanglement of these two discharge processes, a relative amplitude difference method is proposed. Based on the experimental observations, a detection algorithm incorporated with the ultrasonic emission sensors, preamplifier, and high-speed A/D converter was developed to assist the self-healing performance test.

An improved lifetime prediction method for metallized film capacitor considering harmonics and degradation process

Metallized film capacitor (MFC) is one of the stand-out components in terms of failure rate in power electronic converters. However, the influence of harmonics and degradation process on MFC are not well described by the conventional lifetime prediction method, causing a large gap between prediction result and engineering practice. Therefore, this paper further explores the aging failure mechanisms of MFC and proposes an improved lifetime prediction method. The function mechanism that the harmonics change the partial discharge inside MFC to affect lifetime is expounded, which is modelled by several influence factors. Moreover, the coupling relationship of hot-spot temperature and equivalent series resistor (ESR) aging is discussed by an improved aging model based on the parameter drift of ESR. The capacitance degradation curve obtained from this model are consistent with the experimental results, verifying the validity of this method.

Accelerated Life Testing Method of Metallized Film Capacitors for Inverter Applications

DC-link capacitors are one kind of reliability-critical component in traction inverters. As such, it is essential to evaluate the degradation characteristics of dc-link capacitors. Accelerated life testing is an effective and standard method to evaluate the capacitors’ lifetime and degradation mechanisms. Therefore, an accelerated life testing method for dc-link capacitors in three-phase inverter applications is proposed in this article. In order to reproduce the same encountered stress, exactly reflecting the real condition for the capacitor under test, a three-phase inverter with the capacitor under test and a three-phase balanced inductor load are used as the evaluation circuit. Due to the employment of the inductor load, the power consumption of the inverter can be reduced, while the full-scale current rating can be maintained so that the capacitor under test is subjected to the practical ripple current stress. In order to estimate a capacitor’s capacitance with high accuracy, an effective and direct condition monitoring method, which is based on the proposed evaluation circuit, is also proposed in this article. By injecting an ac voltage into the dc link, the ac current and voltage components in the dc-link capacitor are generated. Then, the capacitance mathematical model can be developed by analyzing the injected voltage component and the resultant current component. As the proposed method only requires estimating the parameters from the input voltage and current, there is no need to install additional capacitor current sensors, which are usually difficult to install. Moreover, the amount of monitored and processed data can be reduced. Thus, the proposed method is easy to implement for accelerated life testing in practice. In addition, experiments are carried out to verify the analysis and effectiveness of the proposed solution.

Reliability Modeling for Metallized Film Capacitors Based on Time-Varying Stress Mission Profile and Aging of ESR

Metallized film capacitor (MFC) is one of the key components in power electronic converters, accounting for a large proportion of failures. However, the time-varying external stress in long-term mission profile and time-varying internal stress due to the degradation of MFC are not well described by the conventional reliability evaluation method, which leads to a large deviation between the evaluation results and the engineering practices. Therefore, an improved reliability evaluation method considering time-varying stress mission profile and aging process for MFC is proposed. First, this article analyzes the influence of harmonics and provides a more accurate lifetime model under constant stress condition, compared with the conventional method. Second, the subsection integral accumulated damage model is proposed to transform the long-term time-varying stress mission profile to the short-term ideal constant stress condition, solving the reliability evaluation under time-varying external stress condition. And then, an improved aging model is proposed to describe the nonlinear variation of equivalent series resistance (ESR) and internal temperature rise due to the degradation process of MFC. Furthermore, Monte Carlo simulation is applied to analyze the difference of individual capacitor, obtaining lifetime distribution. Finally, a practical application example of modular multilevel converter (MMC) system is implemented to verify the superiority of the improved method.

Cryogenic Power Electronics: Capacitors and Inductors

Capacitors and inductors that are suitable for cryogenic use are presented in this study. With the long-term goal of developing power electronic converters for cryogenic use, we studied various off-the-shelf metalized polypropylene film capacitors at cryogenic and ambient conditions. Capacitance and breakdown voltage of the film capacitors were the main parameters measured at room temperature and in liquid nitrogen. The results show that the material of dielectric film and the method of packaging play a role in the characteristics of breakdown voltage and capacitance in cryogenic conditions. In general, both capacitance and voltage rating of the capacitors were comparable if not better at cryogenic conditions. Moreover, with the long-term goal of developing inductors for cryogenic applications, we built and tested inductors with and without a magnetic core. The resistance, inductance, maximum current, and energy density were measured and compared. According to the results, the energy density of the cryogenic inductor without a magnetic core can be designed to be much higher than its room temperature counterpart mainly due to the superior cooling power of liquid nitrogen and the reduced resistivity of the windings at cryogenic temperatures.

Polymer nanocomposites for high-energy-density capacitor dielectrics: Fundamentals and recent progress

Polymer dielectrics are the key component in film capacitors, which are one of the most fundamental elements in modern electronics and power systems [1–3]. Film capacitors are capable of storing energy when voltage is applied, in the form of electric charges separated by a dielectric material sandwiched by a pair of metal electrodes. Film capacitors possess the advantages of high breakdown strength, low power loss and processing flexibility compared with their counterparts in competition such as electrolytic capacitors and ceramic capacitors [4], meaning they can sustain high voltage, have great efficiency in electrical energy storage and can be manufactured in low-cost and efficient methods. Moreover, the unique self-clearing (sometimes referred to as self-healing) characteristic of polymer dielectrics ensures reliable operation of film capacitors [5], which is of paramount significance since capacitors storing a large amount of electrical energy can explode when catastrophic failure occurs. To be specific, the self-clearing is a process that when localized breakdown occurs at a spot of metallized film capacitors (i.e., a film capacitor using dielectric polymer films with metallic electrodes deposited on the surface [5], [6]), the energy released in the breakdown process can clear the breakdown-site, forming an open circuit. This process protects the capacitor from catastrophic failure (Figure 1) [7]. Serving as a device for charge storage and control, film capacitors are commonly used in many types of circuits such as snubbers, phase shifters, filters and AC/DC converters/inverters. In particular, film capacitors are capable of releasing the stored energy in an extremely short period of time to supply a high power density [8–10], which is desirable for the pulsed power application [1]. However, sizeable energy storage has not yet been achievable with film capacitors due to the limited volumetric energy density (usually measured in Joule per cubic centimeter at the material level). This shortcoming has caused a major issue in the electrical apparatus and electrified systems where capacitors constitute a large volume and weight fraction, i.e., a bulky, and sometimes even cumbersome size. For instance, in the 800 kV voltage source converter-based high voltage direct current transmission system, each converter tower has a volume of approximately 480 m3, where capacitors are ∼60% of the converter weight, and ∼50% of the volume.

Surface modification of thermoplastics by low-pressure microwave O2 plasma treatment for enhancement of the adhesion of the interface box/encapsulating resin and the influence on film capacitors operating under extreme humidity conditions

A low-pressure microwave O2 plasma is applied on the plastic boxes of the metallized film capacitors to strengthen the interfacial adhesion of thermoplastic box/encapsulating resin. The effectiveness of the plasma is reported by analyzing Scanning Electron Microscopy (SEM) micrographs, X-Ray Photoelectron Spectroscopy (XPS) spectra, and contact angle and roughness measurements. An enhancement of the polar character and total free surface energy in both thermoplastics (polybutylene terephthalate (PBT) and polycarbonate (PC)) are determined. Hence, a better wettability of the plastic boxes with the encapsulating epoxy resin is achieved after the plasma application. During the Temperature-Humidity Bias Test (THB), the metallized film capacitors with PC box improves the performance after being plasma treated. Additional capacitor interfaces seem to play a key role in the performance of the device according to the loss of capacitance determined during the THB Test.

Threshold Current Density of Metallized Film Under Multiple Current Pulses

For metallized film capacitors (MFCs), the pulsed current would lead to the electrode fracture, the process might be accelerated under the repetitive pulsed current, and finally leading to the failure of MFCs. In order to study the mechanism of electrode fracture and its influencing factors, an experiment platform based on metallized film is established to observe electrode fracture by using a microscope. The experimental results show that for electrode fracture of metallized film, the threshold current density (TCD) is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.20 \times 10^{10}$ </tex-math></inline-formula> A/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> when the period is 310 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> and the reverse factor is 10%. Theoretical results are also derived based on the analysis of electromagnetic force and Joule heat. The results indicate that, the fracture development is caused by the melting process of the electrode, together with the liquid metal flow driving by the electromagnetic force under the effect of multiple current pulses. Neither the electromagnetic force individually nor the direct vaporization would cause the fracture to develop. With respect to current distortion and heat conversion efficiency, the calculated peak current density is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$j_{\mathrm {max}} = 2.17 \times 10^{10}$ </tex-math></inline-formula> A/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which agrees with the TCD in the experiment. Thinner electrode has a larger TCD, when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d_{m}$ </tex-math></inline-formula> decreases from 20 to 4 nm; TCD increases from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.07 \times 10^{10}$ </tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.25 \times 10^{10}$ </tex-math></inline-formula> A/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Performance metrics for a modern BOPP capacitor film

In this paper, a set of performance metrics for modern biaxially oriented polypropylene (BOPP) capacitor films is established. The fundamental and applied properties of BOPP films required for application in state-of-the-art DC metallized film capacitors are reviewed, highlighting aspects related to high temperature operation, base PP properties and film processing. Commercial BOPP films - both base films and metallized films based on classic isotactic PP - are studied comprehensively, encompassing structural-morphological characterization and short- to medium-term dielectric characterization. Dielectric spectroscopy results demonstrate the negligible dielectric losses of BOPP, being in the range of 10- <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> or less in the expected operation temperature regime. Thermally stimulated depolarization current (TSDC) measurements indicated a modest density of shallow traps (~0.75 eV) and a high density of deep traps (~1.08 eV) in the 5 μm and 10 μm film variants showing differences presumably arising from film processing. Such an electronic structure was found to be connected with ultra-low conductivity (in the range of 10- <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> -10- <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> S/m), high breakdown strength (~700 V/μm) and negligible space charge accumulation up to temperatures of ~70 °C. It is shown that at current design stresses (~200 V/μm at ~60 °C) BOPP is operated close to its fundamental thermal and electrical limitations. Voltage endurance tests at higher fields revealed the onset of high-field degradation and drastically reduced insulation life, and thermal activation of deep traps in the high temperature region (~100 °C) was found to result in reduced dielectric performance.

Optimization of Metalized Film Capacitor Connection to Reduce Stray Inductance

In power electronic applications, due to switching components, the inverter dc-bus generates current pulses at low frequencies and high frequencies (HFs). Usually, a dc-bus capacitor is sized to absorb the current peaks at the switching frequency and to stabilize the dc voltage. For decoupling the HF harmonics, an additional capacitor is required. This decoupling capacitor needs to have a low impedance over a wide range of frequencies. However, their performances for the HF range are highly limited by their stray inductance. In order to minimize the need for an additional decoupling capacitor at HFs, this paper proposes to improve the connections of the dc bus capacitor. Busbar configuration and printed circuit board allow reducing parasitics of the cabling. Several types of interconnections are proposed and characterized in this paper. Both two-terminal and four-terminal capacitors are evaluated and compared regarding operating frequency ranges. The measurement and modeling results show a significant improvement for HF decoupling by using quadrupole coaxial connections.

Energy characteristics of self-healing process in metallized film capacitors

Metallized film capacitors widely used in energy applications were studied. The experimental method for investigation of energy and dynamic characteristics of self-healing processes in real metal-film capacitors was developed. The commercial PET and PP MFCs of 0.22 – 1 μF capacitance and 63–250 V voltage were tested. Depending on applied voltage, 3 types of SH processes in MFC were discovered: single, repetitive, and multiple SH. Dependencies of self-healing energy on breakdown voltage were obtained. These dependencies are described by power law with the exponent n = 2.2–2.4 that significantly differs from literature data. The obtained data will be used for degradation and aging laws formulation for capacitors’ energy storage capability improvement.