Space Charge Effects in Dielectric Liquids

Abstract

Insulating liquid dielectrics used in power apparatus, such as transformers and cables, and in pulse power technology, such as Marx generators and pulse forming lines, have their performance affected by injected space charge. The space charge distorts the electric field distribution and introduces the charge migration time between electrodes as an additional time constant over the usual dielectric relaxation and fluid transport times. Pipe flows and a Couette flow system of coaxial cylinders where the inner cylinder can rotate are described which measure the streaming current for flow electrification where the mobile part of the electrical double layer is entrained in the flow. Charge injection from high voltage stressed electrodes is measured by this Couette apparatus as well as by Kerr electrooptic field and charge mapping measurements. Such optical measurements are described between parallel plate and coaxial cylindrical electrodes for nitrobenzene, ethylene carbonate, highly purified water, water/ethylene glycol mixtures, transformer oil, liquid and gaseous SF6, and high voltage stressed and electron beam irradiated polymethylmethacrylate. Specific attention is directed to highly purified water because Kerr electrooptic measurements, voltage-current terminal measurements, and electrical breakdown tests have shown that the magnitude and polarity of injected charge and the electrical breakdown strength depend strongly on electrode material combinations and voltage polarity.