Current transducers for DC circuit breakers require a wide measurement range, low sensitivity to temperature, small size, high reliability, and low cost. Many alternatives to the shunt resistor have been studied, however none of them complies with these requirements. The aim is to develop a sensor by integrating different technologies. Two solutions will be investigated. The first solution is suitable for currents with amplitude up to the rated value, and it is based on the measurement of the voltage drop across a portion of the current carrying conductor of the circuit breaker. The research will investigate how to compensate for the skin effect, proximity effect and thermal drift through proper temperature measurement and digital signal processing techniques. The measurement of high intensity currents, such as those produced by a short circuit, is instead addressed by means of a solution based on an array of Hall-effect magnetic sensors placed around the conductor.
The arrangement of the sensors will be optimized through FEM analysis, while the effect of temperature and the nonlinearities will be experimentally evaluated. The research activity is based on the assumption that, to date, it has not been possible to use a single transducer to cover the whole current range with adequate accuracy while maintaining reasonably low cost. Therefore, the decision was made to split the problem: current measurement during normal operations and the measurement of surge currents. This approach will be followed by the development of a hybrid current sensor, ready to be employed in a relatively short time span. However, it cannot be excluded that the improvements in the two employed technologies will allow using just one of them to be used to cover the entire measurement range. The idea of covering the wide current range with several sensing technologies is not new. For example, several circuit breakers use a manganin shunt resistor for current measurements during regular operations and a Rogowski coil to measure surge currents. However, the related cost is quite high. The proposed solution uses a portion of the current carrying conductor instead of a low thermal drift shunt resistor, and a low cost Hall effect sensor array instead of a Rogowski coil. Therefore, the result of the experimental activity will be a hybrid current transducer with the same (or better) performance as conventional ones, but at a considerably lower cost.