High AC current calibration system based on Rogowski coil

MIKES is the national metrology institute (NMI) of Finland and one of its key tasks is to provide calibration services to both domestic and international customers. The high AC current calibration method of MIKES is based on a Rogowski coil used as a current transducer. This paper discusses the structure and properties of the current calibration system and the theory behind it. Typically Rogowski coils are not used in accurate high AC current measurements because their accuracy is sensitive to changes in conductor geometry and in measurement arrangements. However, we have improved the measuring accuracy of Rogowski coil by installing a precision coil on a custom coaxial busbar and compensating its temperature dependence. Rogowski coil is also routinely calibrated against a current shunt in order to ensure the claimed performance. The measuring instruments consist of two digitizing voltmeters which are synchronized using an external trigger generator, and controlled by a computer. No integrator is used. High current for the calibration is generated using a synthesizing power source and a transformer. Capacitors are used in the primary side of the transformer to compensate the inductance of the high current circuit which consists of copper busbars. Periodical calibration for high current measuring devices such as current transformers is vital in order to know how accurate and stable these devices are. To verify that current transformers are very stable, calibration results for current transformers that have been calibrated with this setup during the years are also investigated. In addition to current transformers' stability, results give information about the performance of the calibration system at MIKES.