Rogowski coil with reduced immunity to conductor location and stray magnetic fields

Rogowski coil is a potential replacement for traditional instrument transformers for power frequency measurement on medium and high voltage power lines. It shows great promise in enabling lightweight, accurate measurement system for current, both for fundamental and harmonic frequencies. However, at present the traceability for the measurement of harmonics is missing. The basic advantage of a Rogowski coil over a traditional current transformer is that compared with traditional current transformers, which have an iron core with non-linear and hysteretic characteristics, the air core Rogowski coil has linear behaviour with respect to both the applied current and the frequency. The weak point of Rogowski coil is the effects produced by external currents (or external varying magnetic fields) in the vicinity of the coil, and sensitivity to primary conductor position. The fields arising from currents flowing in neighbouring phases and ground conductors are one of the main sources for systematic measurement errors of both traditional current transformer and Rogowski coil. This paper describes a shielding to improve immunity of the Rogowski coil against coupling of stray magnetic fields arising either from external currents, or caused by noncentering of the conductor through the coil. A hollow toroidal magnetic shield which is combined with windings inside and outside of the shield to prevent its saturation was designed and manufactured. The metrological performance of the developed Rogowski coil is characterized. Preliminary results show that the immunity to the primary conductor position, and pick-up from nearby current carrying conductors or from other interference sources has been reduced by an order of magnitude to less than 10 µA/A. Together with temperature compensation techniques, the measurement uncertainty has been reduced to less than 100 µA/A. The drawbacks of the new design are increased weight and reduced dynamic range.