FDTD Electromagnetic and Thermal Simulation of a Metal Oxide Varistor Element Considering the Temperature Dependence of Its Resistivity

Electromagnetic fields and the heat of a metal oxide varistor (MOV), in which a lightning impulse current flows, are calculated using the finite-difference time-domain (FDTD) method. The MOV is represented with small rectangular parallelepiped cells, each of which has a resistivity dependent on electric field and temperature. For this purpose, the expression of resistivity as a function of the electric field, proposed previously, is extended to include the dependence on temperature. The temperature dependence is based on voltages across an MOV for impulse currents of 0.5 to 10 kA at temperatures in a range from about 300 to 900 K, measured by Andoh et al. (2000). FDTD-calculated waveform of voltage across the MOV agrees well with the corresponding measured one for a short impulse current with a magnitude of about 4 kA and a duration of about 30 μs. In addition, the temperature on the surface of the MOV agrees well with the corresponding measured one. Further, calculations are carried out for the MOV with a nonuniform resistivity distribution, which roughly simulates deterioration or degradation of the MOV, for a long duration current having a magnitude of 5 kA. The proposed expression of resistivity, given as a function of electric field and temperature, is useful in studying electro-thermal calculations, which can provide insights into causes of MOV damages.