Development of an optical instrument transformer for DC voltage measurement

This paper deals with the principles, design, assembly, testing results and practical application of a new type of optical voltage instrument transformer based on the Pockels effect, developed for the measurement of DC voltage, with a view to the protection and control of DC power transmission systems including frequency changer power equipment. In order to develop more accurate devices, the authors conceived a new scheme capable of overcoming the particular problems of DC voltage measurement which cause measurement errors. In this scheme, the DC voltage to be measured is chopped and applied to the Pockels crystal. An optical DC voltage instrument transformer was developed and tested using this scheme. The results of the factory tests showed that the characteristics of the new device conform with the required specifications of DC voltage instrument transformers for the protection and control of frequency changer power equipment. The results of field tests conducted for 1 year at an operating frequency conversion substation confirmed that the device is of adequate reliability