In a high voltage current transformer (HV CT) the insulation system is limited to off-line testing, and oil sampling during operation is forbidden. Therefore a comprehensive, simple approach to evaluating the condition of low voltage, medium voltage and high voltage CTs is paramount for testing service companies and utility operators.
CT saturation, incorrect polarity, ratio inaccuracies, wrong sizing of burden and insulation failure can all lead to malfunction, or failure of protection schemes. Unreliable, unpredictable CT performance could increase the risk of damage to major devices, so a complete evaluation of a CT should include electrical tests that will verify the function and performance of the CT in relation to nameplate data and industry recommendations. Standards and guidelines referenced in such an evaluation would include IEEE C57.13.1, C57.13.5, IEC 61869 and NETA ATS and MTS standards.
Moreover, assessment of the electrical insulation of a CT would involve common and modern techniques such as insulation resistance, power factor (tan) and dielectric frequency response (DFR) tests. These tests provide a better understanding of the condition of electrical insulation through the increasingly important interpretation of test data collected in the performance of such tests.
Throughout this paper, a simple approach to testing CT in the field is presented in the form of a logical sequence of testing best practise. The theory and purpose of functional and dielectric tests are reviewed and new measurement techniques like simultaneous multiple saturation curves and advanced frequency based insulation diagnostic techniques are analyzed.
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