In the foregoing, it has also been pointed out that CCVTs have difficulty retaining accuracy during transient conditions. In particular, the fundamental frequency filtering included in these devices prevents the accurate representation of step changes in voltage as may occur at the instant a short circuit happens. The filtering also prevents accurate measurement of harmonics and low frequency or dc components of a voltage waveform. Recently introduced optical voltage transducers (OVTs) provide better wide band performance and also provide most of the benefits previously mentioned for optical CTs.
In the case of OVTs, the voltage is usually measured by the Pockels effect (modulation of a light beam by an electric field). It is based on a characteristic of lithium niobate (LiNbO3) crystal. The crystal is called an electro-optic crystal. When exposed to an electric field, the index of refraction of one of its axes changes in proportion to the strength of the field, a characteristic called birefringence. The shift of axis can be analyzed by passing a beam of polarized light through the crystal, permitting accurate calculation of the voltage producing the field. The electric field is integrated over the length of the sensing crystal to produce the voltage.
Two types of OVT as shown in Figure.
Type 1 uses “line-to-ground continuous integration” in which the electric field strength is integrated over the length of a single electro-optic crystal.
Type 2 uses “line-to-ground distributed integration,” whereby multiple crystals are used, and their outputs are combined to determine the total line-to-ground voltage.
Similar to the case of OCTs, the OVT will include monitoring facilities that monitor the integrity of the measurement and raise an alarm if the measurement is found to be outside certain tolerances. Configuration and sensitivity of the monitoring facility settings will need to be carefully considered by the user in a similar fashion to the OCT.
