Prebreakdown and Breakdown Phenomena in HFO Gas Under Impulse Voltage—Part I: Divergent Field

Prebreakdown and breakdown phenomena in hydrofluoroolefin (HFO)1234ze(E) gas are studied with a wide range of electrode geometries, characterized by a field enhancement factor
$\eta $
ranging from 1.7 (quasi-uniform field) to 90 (very divergent field). As in sulphul hexa-fluoride (SF
$_{{6}}{)}$
, breakdown in HFO results from the inception and propagation of leaders. Depending on the field geometry, the measured breakdown voltage represents either the voltage required for leader complete propagation (in divergent field,
$\eta >10$
), or for inception (in quasi-uniform field,
$\eta < 10$
). This article mostly concerns results obtained with divergent fields. Leader propagation in HFO is barely influenced by pressure, and the minimum voltage required for propagation to breakdown is slightly lower compared to SF6, explaining that lower breakdown voltages are recorded with HFO in divergent field. In such conditions, breakdown voltage is mainly determined by the gap distance. HFO degradation, evidenced by the formation of black soot after breakdown, has no influence on leader propagation and thus on breakdown properties in divergent fields.