Surface Engineering of 2D Dielectric Polymer Films for Scalable Production of High-Energy-Density Films

Metallized polypropylene capacitors (MPPCs) are widely used in the modular multilevel converter (MMC) for high-voltage direct-current transmission systems because of their lower power losses and self-healing capability. The performance of MPPCs in MMC deteriorates with time due to the increase in the equivalent series resistance and a decrease in the capacitance. Therefore, the reliability analysis of MPPC is critical. This article proposes a finite element method (FEM) to analyze the reliability of MPPC by considering corrosion failure. First, the equivalent electric model and actual thermal model of the MPPC are established to calculate power losses and temperature distribution of the MPPC. Second, the corrosion failure of the MPPC is analyzed and simulated by the FEM model, and the lifetime model of MPPC is established by the aging model of polypropylene film and verified by the traditional lifetime model of corrosion failure and the floating aging tests. Finally, the voltage of each submodule (SM) is extracted in the MMC model, and the lifetime of MPPCs in each SM is analyzed by combining the FEM model and the lifetime model. The results show that in the MMC, the SMs in each arm near the dc line or the middle part have a lower lifetime of MPPCs.

Lifetime Analysis of Metallized Polypropylene Capacitors in Modular Multilevel Converter Based on Finite Element Method

An analytical circuit based nonlinear thermal model for capacitor banks

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.

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

Metallized film capacitor's (MFC's) reliability directly affects the stable operation of MMC valve. Conventional reliability assessment method based on data statistics is limited. This paper studies the reliability modeling of MFC considering aging for the application condition of MMC valve. Firstly, the temperature field and electric field model of MFC is established, and the temperature and electric stress distribution are explored. Secondly, the lifetime model of MFC is established based on the electrical-thermal aging of dielectric films. Finally, the aging model of MFC is established from the aspect of equivalent series resistance. Results show that the aging rate of MFC increase with the service time going by, and increase sharply after 21 years. The failure rate of MFC is small at the beginning of operation, and there is a turning point in the 23rd year. Since then, the failure rate of MFC increases sharply. The lifetime of MFC is concentrated.

Reliability Modeling and Analysis on Metallized Film Capacitors for MMC

Temperature field simulation for self-healing power capacitor makes sense to the capacitor optimization and improvement of capacitor’s rated voltage and capacity. On the basis of reasonable simplifications and assumptions for capacitor structure, a 3-D temperature field numerical simulation model for a self-healing power capacitor is formulated in Fluent 15.0. The ambient temperature is set to 55 °C, and the applied voltage is ac 400 V. The temperature distributions of the shell and core of the self-healing capacitor are obtained. The results reveal that the temperature of the core is 2 °C–3 °C higher than that of the shell. The highest temperature of the shell locates on the large side surface, and the temperature of the large side surface is higher than that of the small side surface. To validate the above simulation, a self-healing capacitor is manufactured and thermal resistance sensors are used to measure the temperature in the thermal stability test. The simulation is validated by the experiment. Finally, the influences of the ambient temperature and applied voltage on the temperature rise are investigated. This paper can be taken a reference to the self-healing capacitor operation, maintenance, and even the optimization of its internal structure and material.

Influence of External Factors on Self-healing Capacitor Temperature Field Distribution and Its Validation

Geometric optimization of self-healing power capacitor with consideration of multiple factors

The invention provides a protection performance verification method for internal fuses of a high voltage capacitor unit. The verification method comprises the steps: establishing an equivalent circuit when one capacitor element is broken down in a capacitor unit; solving the equivalent circuit, and acquiring the current expression flowing through a capacitor element subcircuit; according to the current expression and the internal fuse resistance, calculating the energy of the tandem internal fuses, wherein the energy is injected into the capacitor element and breaks down the capacitor element; and determining whether the internal fuses used in the high voltage capacitor unit can be reliably fused by means of combination of the parameters of the whole capacitor device and the material and the size of the internal fuses. The protection performance verification method for internal fuses of a high voltage capacitor unit can accurately determine whether the internal fuses selected by the high voltage capacitor unit can be reliably fused, and can provide reliable guarantee for selecting suitable internal fuses for the high voltage capacitor unit.

Zhiniu Xu

Papers

Influence of External Factors on Self-healing Capacitor Temperature Field Distribution and Its Validation

Geometric optimization of self-healing power capacitor with consideration of multiple factors

Protection performance verification method for internal fuses of high voltage capacitor unit