The effect of space charge on d.c. conduction in transformer oil under non-uniform field

Electric field distribution including space charge effect is calculated by the Charge Simulation Method. From the analysis of conduction current expression, the conduction current in liquids is found to be caused by Schottky emission affected by space charge. Experiments and simulations are performed with needle electrodes of various materials (W, Cu, Mo and Ni) with various tip radius (3, 6, 10, 25 and 50 µm). The simulated conduction current is in good agreement with the experimental one when the mobility of the charge carrier is 6.0 × 10⁻⁶ m²/V.s, and the work functions of W, Cu, Mo and Ni are 1.05, 1.07, 1.06 and 1.08 eV, respectively. Light emitting region observed at the negative point in transformer oil, is divided into two; one caused by the electronic excitation, and the other caused by the photoluminescence.