Banc automatique d’étalonnage des transformateurs de courant

¹ LNE – 29 avenue Roger Hennequin – 78197 Trappes – France
² IUT de Cergy Pontoise – 95 Rue Valère Collas, 95100 Argenteuil – France

^a Denis Bélières: denis.belieres@lne.fr

Abstract

Current transformers allow the measurement of high AC currents (several kA) by reducing the value of this current to normalised levels of 1 A or 5 A. They can therefore standardize current measurement equipements while providing dielectric isolation of the primary circuit. The parameter to be calibrated is a complex number representing the error on the ratio and the phase shift between the primary and secondary currents, which depends on the load of the transformer. This calibration can be done using a current comparator, and this method provides the best overall uncertainty (some 10^-6 in relative value). Two current comparators with magnetic flux cancelling have been made in the 1970s by the Laboratoire Central des Industries Electriques. The first covers a range of primary current from 5 A to 1000 A, and the second from 25 A to 25000 A. They are associated with manually operated electronic equipments: an adjustable dummy load or “burden” and an injection system for the error current. In those years, the development of the electronic devices has been very difficult due to the poor performance of the components used, and the desire to achieve a closed loop controlled system for direct reading of the current error, and direct controlling of the load. These electronic devices have been entirely redefined by abandoning the idea of a servo system for direct reading, thus complicating their operation. But they are now fully controllable by a remote computer via an USB connection, allowing full automation of the procedure: there is no more manual control, either switch or potentiometer, they all have been replaced by electronic circuits. This radical modernisation also resulted in a significant reduction of the dimensions: everything fits in a single 19" × 3U standard cabinet.