Fast rise breakdown in dielectric filled air gap for surge protection

Abstract

Electric discharge across an air gap can be self-healing, providing a unique capability for repetitive, fast, high-voltage/current switching applications through arc conduction. Furthermore, incorporating dielectric granules in the air gap stimulates gas ionization, which lowers the breakdown voltage and narrows breakdown voltage distribution, thereby enabling engineered surge protection from multiple lightning strikes on aerospace vehicles and sensitive solid-state electronics in critical systems. This study investigates the effect of the permittivity of dielectric granules, gap filling, surface roughness, and metal work function on fast-rising, high-voltage breakdowns. In addition to the air gap width, these factors play important roles in gas ionization, field concentration, and initiation of electrical discharge and arcing. Therefore, they could potentially be used to control and narrow operational breakdown voltages for practical applications. Additionally, a modified Langevin–Debye model is developed to correlate the breakdown voltage and the permittivity of the dielectric filler. These investigations identify and highlight key underpinning mechanisms governing the gas discharge behavior across a dielectric filled air gap during voltage surge events.

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

“…These results indicate at ambient conditions the breakdown across a varistor‐filled air gap is controlled by the transient “thermionic‐assisted air breakdown” mechanism. Two important features are also immediately recognized, including: (1) the standard deviation of the regulating voltage at each pressure condition from ambient pressure to 1200 torr is below 6.6 V (not shown) which is significantly narrower than the standard deviation observed in an empty (~40 V) or a high permittivity granule filled air gap (> 35 V), 13 and (2) although breakdown mechanism may change with pressure, the maximum surge protection voltage remains the same. The former is manifested by the superior low intrinsic variations in switching voltage of varistor materials in comparison to a stochastic gas discharge mechanism 11,12 .…”

“…The insert transmission electron images show the presence of a glassy grain boundary phase between ZnO grains room temperature from 10 −2 torr to 1200 torr at 50 torr per step (with more data collected using smaller pressure steps at the lower pressure range). In light of a previous investigation, 13 the air gap was fully filled with varistor granules because an underfilled air gap could affect measurement consistency on the maximum surge protection voltage. To rule out the possibility of degradation or damage due to repetitive FRVPs, fresh granules were used at the beginning of each set of measurements.…”

“…Precautions are needed when designing surge protection for high-temperature applications as the conduction mechanism across the air gap changes. These breakdown mechanisms, particularly the dielectric induced arcing, 9,13 have important implications for dielectric barrier discharge devices for generating plasma resources. [33][34][35]…”

“…The mechanism for degradation due to accumulated damages 7,[14][15][16] from high voltage pulses is discussed with experimental evidence. A collection of these fundamental understandings helps identify the uniqueness of varistor over other dielectrics 13 in surge protection design and applications.…”

“…Self‐healing arc conduction across air 9,10 pockets between the varistor granule network can also help divert surge current to ground during consecutive strikes, thereby protecting critical assets 11 . The introduction of dielectric granules perturbs the uniform electric field across the air gap and further enhances the local voltage gradient in the air pockets, thereby reducing the gas breakdown voltage—a key concept behind “dielectric stimulated arcing” 12,13 for high‐power surge protection. Furthermore, the intrinsic nonlinear conduction of the varistor materials may further narrow the maximum surge protection voltage distribution and provide additional engineering control to extend its use for high‐current power switching tube and fault spark gas protection applications.…”