Electro-optic measurement of electric stress in oil-barrier insulating systems

The development of an electrooptic probe system to measure stress and charge conditions near a barrier in an oil-insulated system working at 300-kV AC or 600-kV impulse is described. A cylindrical channel, filled with a high Kerr coefficient liquid, was used within the barrier. Cross polarizer/analyzer techniques were used to measure the stress and its direction. The system is nonperturbing and gives protection against interference. Since the method relies on the calculation of the barrier surface stress from the measured probe stress, the permittivity-deduced AC stresses are acceptable, but the calculation for the slowly-changing stresses is dependent on the variable and ill-defined resistivities. A closed probe purification system had to be used. The results of the stress measurements are consistent with the Kerr theory