Extension to 230 kA with uncertainty reduction for calibration of shortcircuit currents at INRiM

The Istituto Nazionale di Ricerca Metrologica (INRiM) is the only National Metrology Institute (NMI) holding the Calibration Measurement Capability (CMC) for calibrating short-circuit currents up to 170 kA under the framework of the Mutual Recognition Arrangement of the Comité international des poids et mesures (CIPM MRA). This paper presents the extension of this CMC up to 230 kA and the improvement of the measurement uncertainty of the INRiM system for calibrating sensors and measurement systems for high short-circuit currents and for the Joule integral. The extension has been possible by means of a linearity verification of the INRiM system carried out by comparing it with an intrinsically linear measurement system based on a Rogowski coil. This comparison allowed the update and refinement of the uncertainty budget, specifically the component associated with the linearity of the INRiM system, for both the peak current and the Joule integral. An innovative approach has been adopted by applying a median filter to the voltage measurement of the Rogowski coil to remove noise. This filter has been integrated into the calibration software for high short-circuit currents and optimized for the use with Rogowski coils without an integrator.The uncertainties of the INRiM measurement system have been then reviewed and improved for currents up to 170 kA and 230 kA, resulting, in relative parts 4.2 × 10−3 and 5.6 ×10−3, respectively. Consequently, the extension of the INRiM CMC for calibration of short-circuit currents and for Joule integral up to 230 kA can be considered feasible with the achieved uncertainties