Electro-optic voltage measuring system with temperature compensation

Abstract

As the dielectric constant and the halfwave voltage of electro-optic crystals having a fourfold axis of rotary inversion are oppositely dependent on the absolute temperature, but their product is nearly independent of temperature, temperature compensation in an ac voltage measuring system utilizing such an electro-optic crystal is provided by measuring the time averaged current through the crystal. Good electric isolation is achieved by a current sensing circuit which produces a pulsed light signal having a pulse rate proportional to the time averaged current through the crystal. The pulsed light signal is converted to an electrical signal for input to a digital computer which calculates therefrom, and from a reference current and voltage measured at a reference temperature, and a time averaged voltage measurement, a temperature correction factor which is applied to the crystal halfwave voltage used by the computer in calculating the instantaneous value of the voltage to be measured. A look up table containing empirically derived values related to the calculated correction factor can be used to generate a correction factor adjusted for leakage through the crystal, fringing of the field produced by the voltage to be measured, and residual temperature dependence of the electro-optic crystal. In a practical embodiment of the invention, shield rings surround both ends of the electro-optic crystal to reduce fringing of the electric field.