High‐voltage surge protection by a varistor‐filled air gap

Abstract

The electric discharge across a varistor granule filled air gap under a fast-rising voltage pulse was investigated for surge protection applications. The effects of temperature and pressure on the arc and the electrical conduction were analyzed by the characteristic changes in voltage waveforms triggered by a fast-rising high voltage pulse.In addition to the gap size, experimental results show that competing mechanisms among arc conduction, conduction through the varistor granule network, thermionic emission from Joule heating at granule-to-granule contact points, and the magnitude of the switching voltage dictate the maximum surge protection voltage for the filled air gap. Experimental evidence indicated that accumulated degradation was created at small contact points between varistor granules by repetitive assaults from longer duration, high voltage pulses. The uniqueness of using varistor over other dielectric granules in an air gap for surge protection is identified and discussed.

Uncovering microstructure and composition susceptibility of high permittivity ceramic granules to plasma‐induced arc breakdown

“…Additional details of FRB behaviors associated with PMN–PT granules and dependences on physical variables (viz., temperature and pressure) may be found in Ref. [10].…”

“…Cursory DC resistance measurements have in fact revealed large decreases (>11 GΩ to ∼100 kΩ) in some spark‐gap test fixtures after extensive testing consistent with this type of performance outcome, although microscopic examination of granules was not possible at the time. Experimental results indicate that localized arc breakdown induced granule damage can be alleviated by redistribution from shaking the test fixture which alters granule contact points 10 . Although dielectric stimulated arc breakdown is intrinsically stochastic in nature at ambient pressures (i.e., breakdown pathways vary spatially), this type of damage can lead to irreversible alteration in material electrical properties and, consequently, become unreliable for even passing low voltage signals.…”

“…PMN–PT granules were loaded into a single pin test fixture (Figure 1) and subjected to repeated electrical impulses of ∼500 ns in duration and applied voltage greater than 1500 V. Details of the experimental setup and procedure were described in detail elsewhere 10 . Briefly, the single pin fixture consists of two conically shaped insulating parts that hold and center a stainless‐steel pin inside a cylindrical shell from both ends with a 300 μm spark gap 10 . The voltage ramp rate was adjusted by a pulse shaping network to be as close as possible to 10 kV/μs to simulate a fast‐rising surge condition.…”

“…Unlike more common and broadly used varistor systems, relatively little is currently known about the ability of PMN–PT and similar dielectric materials to withstand repeated arc discharges and associated energy deposition. Although it is tempting to infer similar damage outcomes as observed in varistors, the manners in which surge currents are diverted in both types of materials are fundamentally different (i.e., nonlinear electric field dependent resistance vs. dielectric stimulated arcing) 10 . Moreover, the common assumption that dielectric stimulated arc breakdown in air pockets is self‐healing has not been thoroughly tested and may be compromised if significant chemical change occurs that reduces large electric field splitting 10 needed to induce arcing or creates permanent conduction path through contacts between granules.…”

“…Although it is tempting to infer similar damage outcomes as observed in varistors, the manners in which surge currents are diverted in both types of materials are fundamentally different (i.e., nonlinear electric field dependent resistance vs. dielectric stimulated arcing) 10 . Moreover, the common assumption that dielectric stimulated arc breakdown in air pockets is self‐healing has not been thoroughly tested and may be compromised if significant chemical change occurs that reduces large electric field splitting 10 needed to induce arcing or creates permanent conduction path through contacts between granules. We use a combination of scanning electron microscopy (SEM) and energy‐dispersive spectroscopy (EDS) mapping to characterize the effect of arc discharge on local regions of PMN–PT granules subjected to repeated high voltage electrical impulses.…”