Ratio Correction

Selection of HV and LV CTs of the transformer is done to approximately match the rated primary and secondary currents. Hence, the current from the HV & LV side CTs may not match correctly under load and through fault conditions. This unbalance may lead to operation of differential protection. Therefore, for correct operation, HV & LV replica currents entering the differential relay should match. In case of digital / numeric relays this is done by providing ratio correction factors for both HV & LV CTs. Depending upon the Digital relay type, either the correction factor needs to be computed and entered during settings or the relay may calculate the same on furnishing the required details. This was done by providing Interposing CTs incase of electromechanical relays.