High-impedance differential applications with mismatched CTs

Applying high-impedance differential schemes with mismatched-ratio current transformers (CTs) is generally discouraged. However, in instances where facilities are being expanded, new circuit breakers and CTs may have different ratings than the existing equipment, making it necessary to modify purchase standards to match the existing equipment. Techniques have been developed to allow this application when necessary. Each of the methods has advantages and drawbacks that must be considered in determining the best solution for the application. One easy solution is to tap a higher-ratio CT at a matching tap. For this solution, general guidance has cautioned that those considering applying a high-impedance scheme across a partial CT secondary winding tap must evaluate the effect of the higher voltage on the insulation of the circuit components connected to the other terminals of the CT. This paper investigates the issue in detail, including results of testing CTs in a high-current test facility, to provide practical guidance to practicing engineers in determining the risks and application considerations.