Liquid water stressed with HV impulses: Effect of polarity on transient pre-breakdown processes

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 electrical conductivity of water 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. The paper presents a study of the dielectric behaviour of water stressed with high voltage impulses in a point-plane electrode topology. Water with different conductivities including distilled water, tap water and a water based ionic solution was investigated. The volt-time breakdown characteristic of water is discussed and the pre-breakdown time and nominal breakdown velocity have been obtained for both positive and negative polarity impulses. 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.