Haoyuan Li

Lifetime investigation and prediction of metallized polypropylene film capacitors

Abstract Metallized polypropylene film capacitors (MPPFCs) possess characteristics of high reliabilities and high energy densities, so they are widely used in the pulse power systems. MPPFC prototypes with high voltage and large capacitance are composed of a number of cylindrical MPPFC elements connecting in series or in parallel. The experimental data show that the lifetime of MPPFC prototypes is far shorter than that of MPPFC elements under the same voltage stress. This paper analyses operational factors that affect influence the lifetime of MPPFCs, and predicts the lifetime under various operational factors based on the experimental results. The relationship between MPPFC elements and MPPFC prototypes in terms of lifetime is presented with a reliability analysis of the Weibull distribution, and is validated through experiments. Finally this paper presents a lifetime prediction model. The results suggest that the predicted data could match well with experimental results. This paper is devoted more to the lifetime analysis under various operational factors and less to theoretical physical analysis.

Temperature Rise of Metallized Film Capacitors in Repetitive Pulse Applications

Metallized film capacitors (MFCs) enjoy characteristics of high energy density and high reliability due to the self-healing capability, and thus are commonly used as energy storage devices in pulsed power systems. With expanding of pulsed power applications, high repetition pulsed power technology has become an essential topic. Research to explore the application of the MFC to repetitive pulse power systems is of great importance. This paper focuses on high-energy-density capacitors in repetitive pulse applications in a repetition rate less than 100 Hz. A heat transfer model is established to analyze the temperature distribution in the MFC. Temperature rise of the MFC is investigated based on the repetition pulse lifetime test platform. The simulation results are accordant with the experimental temperature rise. This model can be used to assess the temperature rise of the MFC applied in repetitive pulsed discharges.

T pattern fuse construction in segment metallized film capacitors based on self-healing characteristics

Abstract The breakdown happens in metallized polypropylene film (MPPF) capacitor can be classified into two cases: the first one is self-healing, which means that the insulation will recover after the breakdown; the other one is self-healing failure, which means that the capacitor will fail because of short-circuit fault. In this paper, the MPPF capacitor applied in DC filtering which adopt the T pattern segment film technology is investigated. To simulate the two cases mentioned above, a model based on self-healing experiment data is built by Power Systems Computer Aided Design (PSCAD). The current density flowing through the fuse and fuse energy is calculated and analyzed. Meanwhile, the fuse burn-out criteria are investigated according to electrical explosion theory and phase transition energy of segment metallized film fuse. The fuse design methodology of T pattern segment film is presented and a design case is provided.

Lifetime Improvement of Metallized Film Capacitors by Inner Pressure Strengthening

Metallized polypropylene film capacitors (MPPFCs) are widely used in pulse power systems for their characteristics of high reliabilities, high energy densities, and excellent pulse power handling capabilities. The teardown showed that the capacitance loss decreased gradually from the outer layers to the inner layers in a cylindrical MPPFC winding. This paper concentrates on the inner pressure in the MPPFC winding. First, the effects of the inner pressure on the self-healing and the lifetime are investigated. The lifetime can be extended by the inner pressure strengthening. Second, an inner pressure calculation formula is presented. The effects of the heat treatment and wrapped film on the pressure are considered. In addition, the heat treatment and wrapped films are chosen to strengthen the pressure. Finally, the experiments are designed to validate the effect of the lifetime improvement by the methods of the inner pressure strengthening. Results show that the lifetime can be extended by 30% with rational heat treatment and by 38% with rational design of the wrapped film.

Polarization characteristics of metallized polypropylene film capacitors at different temperatures

When the metallized polypropylene film capacitor (MPPFC) is used under high electric field in pulsed power systems, an obvious voltage decay phenomenon will occur. The voltage decay is mainly resulted from the relaxation polarization in dielectric applied under high electric field. This paper focuses on the relaxation polarization in metallized biaxially oriented polypropylene (BOPP) film capacitors. The extended Debye model of relaxation polarization is discussed and the parameters of each RC branch are obtained. Meanwhile, the relationship between the increasing proportion of polarization charge (ΔQa/Qh) and temperature is investigated. The experimental results show that under 1kV, ΔQa/Qh varies from 0.018 to 6.868 with the temperature varing from 10 to 70 /spl deg. Moreover, the equivalent capacitance of capacitors in frequency domain is analyzed with consideration of slow polarization. The analysis shows that both a lower frequency and a higher temperature can accelerate the relaxation polarization and increase the voltage decay. Based on the extended Debye model developed from the polarization charge test, the voltage drop is calculated and the results can match well with those of the voltage maintaining test.

Fracture behavior of metallized electrode for capacitor under pulsed current

The fracture behavior of metallized electrode (ME) under pulsed current with current density of 1011~1012 A/m2 and pulse with width of microseconds is studied in this paper. High-speed camera is applied to record the surface state of metallized electrode. Based on photos of the surface state and waveforms of the current and voltage, the fracture behavior under the stages of melting, liquefaction, vaporization and plasma are acquired. The process of the exploding phenomenon is discussed in detail. The photos show that the fracture direction is perpendicular to the current. Joule heating and electromagnetic force is the reason for the development direction of the fracture. The influencing factors of electric explosion are experimentally analyzed. Results indicate that the specific action integral (h) can not be considered as a constant. The specific action integral (h) and energy transfer efficiency (η) will decrease with the increasing of di/dt (ratio of peak current value to peak time) and current density (j). The energy transfer efficiency (η) increases from 36% to 71% when the current density ranges from 2.3×1011 A/m2 to 1.1×1011 A/m2, which means the threshold value of electric explosion of 5 nm ME is closely to 1×1011 A/m2. The research results indicate that the high current density (1011~1012 A/m2) and high di/dt may lead to the electrode fracture in the metallized film capacitor. The safe-use current density of capacitors must be below 1×1011 A/m2.

Study on the lifetime characteristics of pulsed capacitors based on the theory of space charge

As a key influence factor on the lifetime of pulsed capacitors, the repetitive frequency (fR) of charge/discharge is studied in this paper based on the theory of space charge. A fact that in the absence of heat accumulation, from 0.1 to 10 Hz, the lifetime of capacitors increases with the fR is found. The measurement of space charge induced by charge/discharge at various frequencies is conducted. The measurement result indicates that the amount of space charge in capacitors experienced charge/discharge at 0.1 Hz is 29% higher than that of 10 Hz, which can reasonably explain why the lifetime of capacitors tested at 10 Hz is twice of that at 0.1Hz. Besides, a simplified TSDC is carried out to qualitatively measure the trap density of capacitors experienced lifetime tests, and found that the traps formed during charge/discharge tests at 0.1 Hz is more than 7x that of 10 Hz. The dielectric loss angle tangent (tanθ) of capacitors which have experienced charge/discharge at various frequencies is also tested, and from the tanθ tested at 0.001 Hz, (3.7 when fR is 0.1 Hz and 2.4 when fR is 10 Hz), it is revealed that the aging of pulsed capacitors can be slowed by enhancing the fR of charge/discharge, which matches well with the results of lifetime test.

Simulation of volatge-maintaining performance of high energy density capacitors based on the measurement of charging current

When a high energy density (HED) capacitor is charged to a certain voltage, the voltage of the capacitor will decay gradually. In this paper, an extended Debye model of relaxation polarization is established and the parameters of the model are obtained by analysis of polarization current. The simulation of the voltage decay of HED capacitors under different field strength, temperature and charging time is conducted, which can match well with the results of the measured voltage decay. And the results indicate that increases of temperature or field strength result in great increases of the voltage decay by altering the parameters of the extended Debye model. Nevertheless, charging time has no impact on these parameters, it affects the voltage decay by affecting the charges absorbed from the power supply by relaxation polarization branches of the capacitor.

Metallized polypropylene film capacitors in repetitive pulse applications

With the expanding of the pulse application, high repetition pulse power technology has become an essential topic. Research to explore the application of self-healing capacitors to repetitive pulse power systems is of great importance. This paper focus on high energy density capacitors applied in repetitive pulse applications less than 100 Hz. Firstly, a heat transfer model based on the finite element method is developed to analyze the distribution of temperature rise in the capacitor. The simulation results are accordant with that of the experimental temperature rise. Secondly, lifetime characteristic of pulse capacitors applied in milliseconds pulse applications is investigated through the establishment of the repetition lifetime test platform. The results demonstrate that the lifetime will increase with the increase of the repetition rate. Moreover, failure mechanisms of pulse capacitor applied in high repetition rate must to be paid special attention. Excessive temperature rise combined with high electric field may result in the disastrous breakdown of the capacitors. Further research must be conducted to determine the failure mechanisms.

Experimental study on the current-carrying test of metallized electrode

In this paper, the current-carrying capacity of metalized electrode is studied. The sheet resistance of tested metalized aluminum electrode is 15Ω/□. High-speed camera is applied to record the state of the metalized aluminum electrode under pulsed current. The results show that the metalized aluminum electrode will fracture perpendicular to the direction of current. According to simulation analyses, joule heating and the electrodynamic force induced by magnetic pressure are the main causes for the fracture of metallized electrode.