Case Study about the Energy Absorption Capacity of Metal Oxide Varistors with Thermal Coupling

Abstract

Metal oxide varistors are applied today inside modern surge arresters for overvoltage protection for all voltage levels. Their main issue is the thermal activation of their crossing current, which could lead to complete destruction by thermal runaway. This article presents a new technological solution developed in order to increase the thermal stability of metal oxide varistors. It consists in connecting in parallel two or more similar varistors (for dividing their current), having a thermal coupling between them (for equalizing their temperatures and forcing them to act together and simultaneously as much as possible). Starting from a finite element computer model performed for each situation (varistor standalone or parallel), up to real measurements, the thermal stability of the equipment was analyzed in permanent and impulse regime. Experiments were carried out in the same conditions. Experimental data obtain from two disk varistors corresponds very well to simulations, proving that parallel connection of varistors, combined with a thermal exchange between them is an efficient technical solution for thermal stability improvement, even if not apparently economically justified.

Service Limits for Metal Oxide Varistors Having Cylindrical Symmetry as Function of the Ambient Temperature

“…The interspace between the grains acts like a diode junction, which allows the current to flow only in a single direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs [3].…”

“…When a higher voltage is applied, the diode junction opens due to a combination of thermionic emissions and electron tunneling, causing a higher current flow. The result of this behavior is an extremely nonlinear current characteristic, in which the MOV has a high resistance to low voltages and a low resistance to high voltages [3].…”

“…• the current-voltage characteristic has a high non-linearity, the non-linearity coefficient α (the slope of the curve) having values between 20 and 70; • most constructive variants do not require fuses in series with varistors; • the volume of the MOV surge arrester (mass and size) is smaller; • the crossing current flow in the permanent service regime (in the absence of overvoltages) has extremely low values; • their response time is very short (between 10 and 25 ns); • they have an increased energy absorption capacity (specific energy E > 150 J/cm 3 ), and allow currents with a density J > 5 kA/cm 2 to pass; • the protection level is lower (the equipment is more sensitive to voltage increases).…”

Efficient replacement of the radioactive sources in the gas-filled surge arresters construction for the insulation co-ordination at the low voltage level

“…Since the mass of ions (or at oms) of a gas is at least 2000 times greater than the mass of an elec tron, it is log i cal that the response speed of the gas-filled surge ar rest ers is sig nif icantly lon ger than the speed of the semi con duc tor surge ar rest ers based only on free or bound elec trons. In ad di tion to the slow-mo tion of the ions, as an important element in the functioning of the gas-filled surge arresters, there is the initiation of the electrical breakdown that will end in a self-sustaining process [6][7][8].…”

Possibility of achieving an acceptable response rate of gas-filled surge arresters by substitution of alpha radiation sources by selection of electrode material and the electrode surface topography

“…Their re sponse time is lon ger than the drain di ode response time. The varistor can con duct an overvoltage of higher power than the drain di ode, but at high overvoltage pow ers, it leads to per ma nent dam age or de struc tion [16,17].…”