Optical high voltage measurement transformer using white light interferometry

A new approach to perform measurement of potentials in high voltage levels using electrooptical Pockels sensors is presented here. This paper describes an application of the White Light Interferometry technique to a high voltage optical fiber measurement system. In this system the information is encoded in the spectrum of the light, allowing the measurement to be independent of the optical power transmitted by the optical fiber link. A prototype was built and tested under excitation of a.c. voltages up to 60 kV in 60 Hz showing good response and demonstrating the feasibility of this method. Introduction High voltage measurements have been made using electromagnetic voltage transformers (VTs) and, in some cases, capacitive or resistive dividers. The application of recently developed new technologies provides alternatives to the conventional VTs having improved performances of insensitivity to electromagnetic interference (EMI), wider frequency response, etc. Electrooptic techniques combined with optical fiber links can be used in optical voltage sensors (OVTs) which have many advantages in replacing the conventional VTs, such as: the possibility of a totally dielectric construction, electromagnetic noise immunity, complete electric insulation, wide frequency bandwidth, small size, light weight, etc. The optical sensor using a Pockels micro single crystal, which is a crystal with a long and thin rod shape, was developed to overcome the effect of oscillatory signals and to realize an ideal high voltage optical sensor having the wide bandwidth of dc to high frequency. However, the maximum voltage that is measurable in such a kind of sensor is limited to less than 80 kV, which is approximately the half-wave voltage (Vπ) of this sensor. Additionally, since the Pockels cell used can be seem as a polarimetric interferometer, the voltage measured is traduced in the optical intensity at its output. Therefore, the measurement becomes dependent on the losses in the link, which can change unpredictably in time. In the present work an application of White Light Interferometry (WLI) to the Pockels cell used to built OVTs is proposed as an option to overcome such problem. Since in a system based on the WLI method the information is encoded in the spectrum of the light, it can provides a measurement which is independent of the optical power present in the optical fiber link output. Experimental Setup The linear electro-optic effect, known as Pockels effect, has been extensively used to built modulators that are the core of high voltage optical sensors. The authors have developed a polarimetric Pockels sensor operating in a longitudinal configuration which is able to measure high voltages up to about 70 kV. In a latter development, this high voltage sensor is used as birefringent sensor interferometer in a WLI sensor system which is the main part of an optical high voltage transformer. These developments, together with their fundamentals, are following described. The refractive index of some materials changes when an electric field is applied to them. This is the electrooptic effect. A refractive index can be expressed using a power series with respect to an applied electric field E as [1]: n = no + rE + sE 2 + ......... (1) where no is the ordinary refractive index exhibited by the material in case of no applied field and r and s are the coefficients for the electro-optic effect. The second term in the right hand side of (1) shows a linear dependence on the electric field, corresponding to the Pockels effect, and the third term shows a square dependence, corresponding to the Kerr effect. In optical modulators the applied electric field and the light beam directions can be parallel or perpendicular to each other. These two cases are called longitudinal and transverse modulators, respectively. ANNALS OF OPTICS XXV ENFMC 2002 205 For a polarimetric modulator in longitudinal configuration, as shown in Fig. 1, with polarizer and analyzer whose polarization directions are crossed to each other, between the two orthogonal light components is induced a phase retardation Γ.