The complexity of the impulse breakdown of liquid water is reflected by the dependency of
pre-breakdown processes on the polarity, rise-time and wave-shape of the applied impulses as
well as on physical properties, such as the electrical conductivity ofwater itself. Further
understanding of the mechanisms of formation and propagation of impulse discharges in
water and water solutions is therefore required to enable the development of pulsed power
and plasma technologies. This paper presents a study of the dielectric behaviour of water
stressed with positive and negative high voltage impulses in a point-plane electrode topology.
Water with different conductivities including distilled water and tap water was investigated.
The volt-time breakdown characteristic of water is discussed and the total pre-breakdown
time has been obtained for both positive and negative polarity impulses for 2 mm, 5 mm and
10 mm inter-electrode distances. The Laue statistical analysis has been used in order to
calculate the statistical and formative time lags. Using the formative time data the nominal
breakdown velocity in water for positive and negative impulses has been obtained. It has been
shown that the anode discharges propagate with supersonic velocities, up to ~ 20×10
5
cm/s,
and the cathode discharges are sub-sonic with a maximal velocity of ~ 1×10
5
cm/s. Discharge
velocities are slightly higher in distilled water as compared with tap water. Potential
breakdown mechanisms which can explain the observed polarity effects in the transient pre-breakdown processes in liquid water stressed with high voltage impulses are discussed.
Index Terms — Impulse breakdown, water, pulsed power systems.