Proposal for extension of routine tests of the inductive current transformers to evaluation of transformation accuracy of higher harmonics

Abstract

In the paper conditions and evaluation criteria for testing transformation accuracy of inductive current transformers for distorted current are proposed. Moreover, testing procedure and developed measuring system with its calibration method are described in detail. To ensure high accuracy of results the values of ratio and phase errors at harmonic are calculated from the harmonic components of the composite error measured by the differential circuit. Necessary equations are presented. Proposed solution enables possibility for evaluation of accuracy class extension for quality metering of active and passive instrument current transformers regardless of the types of the primary connection and the primary converter. Moreover, rated values of secondary currents of tested CT and reference transducer may be different. Presented implementation of developed methodology justifies the need for extension of routine tests as for multi-range reference inductive current transformers designed for transformation of sinusoidal current of frequency 50 Hz positive results for 0.1 and 0.2 accuracy classes are obtained. In the paper short description of other measuring systems and standard requirements for testing transformation accuracy of passive and active instrument transformers are presented. Furthermore, possible conditions are discussed that may cause decrease of transformation accuracy of the inductive current transformers for higher harmonics of distorted current in relation to sinusoidal current of their rated frequency.