An investigation into voltage transformers used for tariff metering of medium and high voltage loads and standard guidelines for its application

For certain VT connections and unknown consumer earthing practices, metering errors will occur during unbalanced loading. Because of the small load offered by modern numerical tariff meters and relays, VT burden ratings may be inappropriate and the VT may not operate within its accuracy range. This can give rise to the ferroresonance phenomenon and neutral inversion for certain system and VT conditions. VTs used for tariff metering were investigated based on limb design, parameters, earthing and connection methods. To compare the accuracy, case studies were created and calculations performed with the aid of an Excel iterative program. The calculations compared the actual real power supplied to a load with the real power seen by the voltage and current transformers. From the results and theoretical research, optimum VT earthing, connections, ratings and standards were determined and compiled in a concise Code of Practice.