Optimization of the Accuracy of Semi-Indirect Energy Metering System

Difference between a measurement reading and the true value of that measurement is known as accuracy. The measuring device or system should have the capability to measure and match the actual value to the quantity that is being measured to an acceptable accuracy declared by respective standards. Measurements of electrical energy in various demand categories of different utilities have their own limits. The employed measuring system is designed according to the demand category. Semi indirect metering system is being used to measure energy consumed between 42 kVA to 1 MVA by the governing Sri Lankan utility, Ceylon Electricity Board (CEB). It requires a current transformer (CT), an energy meter and lead wires only. In general, such systems are installed after certain standard checkups carried out for individual accuracy of CTs and energy meters in a standard testing laboratory. However, it has been observed that there is a considerable deviation of accuracy than expected when considering the accuracy of the whole system setup. Replacing of analogue meters by digital meters in semi indirect measuring environment to establish an automated meter reading system by a Utility has exclusive advantages such as immunity tamper up to certain extent and features supported by enhance data communication facilities etc. However, careful study is required to omit the potential of getting deviated the total system error beyond the desired limits due to the burden mismatch owing to CTs, energy meters and associated lead wires. In this research study, it was observed that considerable percentage of semi indirect metering systems installed are operating at under burden condition. The study focused to minimize the error deviation of semi indirect metering system owing to the above mentioned factors with a new concept of appropriate burden matching. The study also concentrated on to finding the factors affecting the accuracy and how to eliminate the error through a simple error compensation method. In here all the experiments were done in the laboratory for 100/5 semi indirect metering model and it describes the methodology for improving the accuracy clearly by appropriate component selection using developed charts for different contract demands operating at different power factors depending on the application. ENGINEER, Vol. 47, No. 03, pp. 1-11, 2014