A steady state model for prediction of amplitude and phase errors in measuring current transformers

Foreseen expansion and increasing complexity of power distribution networks will increase demand for accurate metering of electric energy flow. The current transformer (CT) is widely used in measurement systems and has direct effect on the overall accuracy of electric power measurement. Therefore, its design and performance are of great importance. A steady state model of a CT is proposed to relate its performance to basic magnetic properties of its core. It enables a CT amplitude and phase errors to be predicted from the magnetic permeability and power loss of its core. Therefore it can be easily implemented at the design stage of these devices. The accuracy of the model has been verified against experimental data and the predicted CT errors were found to be in a good agreement with measured values. A negligible leakage reactance design CT is considered in this work, but an additional parameter would have to be considered for gapped cores or non– uniformly distributed windings. CT errors are determined by magnetic properties of its core in a wide flux density range. Measurement of magnetic properties at very low flux densities can be extremely challenging due to low signal-to-noise ratio. An accurate, low flux density measurement system has been developed for the investigation of CTs. It features digital triggering, cycles and moving averaging techniques, innovative digital compensation, customised digital feedback algorithm and is capable of measuring magnetic properties of materials at flux densities as low as few µT. This setup can be used for testing variety of ferromagnetic materials for other low flux density applications such as magnetic shielding.