Analysis and Evaluation of CT Transformation Error for Non Self- Contained Smart Meters Installed in Smart Cities
Current transformers, commonly abbreviated as CTs, are electrical instruments that are designed to produce an alternate current at secondary windings which is proportional to the current at the primary winding. CTs are mainly used in energy metering and for system protection. In the former, CTs play two key roles, namely; to offer an electrical isolation between measuring devices and high voltage conductors, and as current sensor for a transformer operated (non self-contained) energy meter. Upcoming smart cities are characterized by large power consumers. To be able to accurately measure the energy consumed, non self-contained energy meters are installed at large power consumers. It is important to determine the current transformation errors at various customers’ loadings (primary current) and connected CTs’ burden; connected loads of CTs, that is, energy meters and respective wire effective resistance. This paper presents the CT errors at different primary current and burdens. In simple terms, this paper describes how CT’s error correlates with primary current and burden. The research was conducted using CPC 100, a universal primary injection set, where different current transformer turn ratios (100/5 A, 200/5 A, .. 2000/5 A) were used. It also consisted copper wire of various lengths (16 m, 12 m…4 m) and cross sectional areas (2.5 mm2 and 6.0 mm2) to vary the CT connected load. From the research study, the take away points were that provided the CT is not saturated or near saturation, the turns ratio errors are almost equal on all the CTs under study. Further, high burden results noted to be negative transformation errors. From this study, it is recommended that for accurate energy measurement in smart cities where non-self-contained energy meters are installed, the key factors to consider are the saturation point of CT and the connected burden.
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