Study On Application Performance And Its Influencing Factors Of High Energy Density Metallized Film Capacitor

The metallized film capacitor (MFC) is a key device in pulsed power systems. The application performances of the MFCs should affect the output characteristics of the system. This paper mainly focuses on the application performances of the MFC, including the lifetime characteristic and the voltage maintaining performance. Through experimental and theoretical analysis, the effects of the self-healing, the electrical conductivity and relaxation polarization on the application performance are studied.Firstly, this paper carries out the experimental investigation on the application performance of MFCs. The experimental results show that the lifetime of MFCs decreases with the peak value of charged electric field in high pulsed discharge applications, and the decreasing function is (L/L0)∞(E/E0)-7.32. And results also reveal that the lifetime of MFCs decreases with the reversal coefficient in underdamped circuits. The decreasing function is (L/L0)∞[ln(RF0)/ln(RF)]-0.7. These results provide basises for the lifetime prediction of MFCs applied in high pulsed power systems. In addition, the research results show that the electric field, operating temperature, charging rate and hold time all play important roles in the voltage decay process. It can be observed that the general curve of the voltage decay has a rapid initial drop and then the curve can usually be fitted to a pure exponential function. The higher the electric field or the operating temperature is, the more significant the voltage decay is. In addition, the faster the charging rate is, the more significant the voltage decay is. But, if the charged voltage is maintained for a specified period of time, the voltage decay will reduce dramatically. Initial analysis shows that application performances of MFCs are related to the self-healing, electrical conductivity and relaxation polarization.Secondly, this paper verifies the effects of sheet resistance (Rsq>30Ω/□) and pressure between layers on the self-healing characteristic through experiments. The experimental results show that the self-healing energy is approximately inversely proportional to the square of the sheet resistance, and is approximately proportional to the clearing area of the electrode and the square of the breakdown voltage. The self-healing energy decreases with the increase of the pressure. Then, the effect of the interlayer air on the discharge arc in the self-healing process is analyzed. Meanwhile, the paper presents that the lifetime can be extended through excluding or decreasing the interlayer air. At last, four methods are provided:wrap strengthening, heat treatment, end-sealing and impregnation in the vacuum. The experimental results show that the lifetime can be increased up to1.6,1.2,1.6and4-5times respectively through these four methods. Through the linear relationship between the self-healing energy and the clearing area, this paper presents the function of the voltage decay of MFCs on self-healing energy.Thirdly, this paper concentrates on the electrical conduction characteristics of the BOPP films. The conductivity as a function of the electric field and the temperature is deduced based on the Poole-Frenkel effect and the field-enhanced carrier mobility. And experiments are performed to measure the conductivities of the BOPP at different electric fields, temperatures and crystallinities. The results show that conductivities are almost the same at low fields (lower than10V/μm), and then increase rapidly as the electric field increases but the increasing trend slows down as the electric field is up to400V/μm. The influence of the temperature on the conductivity of BOPP film is mainly reflected on the increase of the initial conductivity or the carrier concentration. The results also reveal that conductivity decreases by2-3times as the crystallinity increases from39%to46%. Eventually the voltage decay resulting from the electrical conduction of the BOPP film is derived and calculated on the basis of the electric conduction theory and experimental results.At last, the relaxation polarization characteristic of MFCs under high electric fields is studied. The relaxation polarization circuit model of MFCs is established through theoretical analysis. And then, the relaxation polarization charge is tested. The experimental results show that the time constant of relaxation polarization reaches hundreds or even thousands of seconds under high electric fields. The relaxation polarization strength is related to the electric field. When the polarization process in MFCs is close to the steady-state, the ratio of the relaxation polarization charge and fast polarization charge is about13.5%under400V/μm. On the basis of experimental research and theoretical analysis, this paper presents the effects of the relaxation polarization on the application performance of MFCs: the dielectric loss, aging and breakdown of the BOPP film, voltage decay. Through the circuit model analysis, it is revealed that the relaxation polarization does cause the rapid initial voltage decay. After the static capacitance value has been reached, the voltage decay due to the leakage is at a much smaller pace, and can usually be fitted as a pure exponential decay.