LIMM study of space charge in crosslinked polyethylene

Electric field/space charge profiles obtained using the laser-intensity-modulation-method (LIMM) are presented for XLPE samples machined from power cable insulation cross-linked using dicumyl peroxide. The samples, 110 Pin thick with vacuum-evaporated aluminium electrodes, were subjected to de fields of 48 kV/mm and both polarities at room temperature for at least 24 hr. The LIMM measurements were made at room temperature, and at various times after the end of poling. Electric field and space charge profiles obtained using the scale-transformation and polynomial regularization methods were in fair quantitative agreement over much of the sample thickness, except when the electric field showed a maximum or minimum. Simulation data for assumed electric field profiles strongly suggested that the polynomial regularization method is more accurate. Most of the space charge was concentrated near the electrodes, with densities of order 5-10 C m(-3) in those regions. Charge of the same sign tended to accumulate adjacent to a given physical electrode, for both poling polarities.