Use of Current Transformers in Calibrations of Rogowski Coils at High Pulsed Currents

Abstract

Rogowski coils (RCs) are widely used for measurements of high AC currents, transient currents and impulse currents. In AC resistance welding, they are used as current sensing coils (CSCs) in conjunction with weld-current meters/monitors (WCMs) for monitoring/controlling the weld currents, and thus ensuring the quality of welds. The CSCs are calibrated stand-alone or with WCMs at high pulsed currents at conditions close to those encountered in the field. These ´gated’ sinusoidal currents with variable angle of conduction deviate from sinusoidal waveform, and their harmonic content is large. This requires high sampling rate and integration, which affects negatively measurement uncertainty. In spite of the CT core-saturation, size, weight and cost constraints for measurements at large currents, they are used as reference devices in the calibrations of RCs at steady-state AC currents. A special electronically-enhanced CT was developed for the calibration of RCs in a copper coaxial cage at high pulsed currents of up to 35 kA. The implementation of this CT as the reference in a calibration system for Rogowski coils at high pulsed currents, which requires no integration, will be discussed in the paper, including traceability to SI units. Preliminary estimates of the measurement uncertainty will be given.