A Novel Sensor to Measure the DC Bias at the Point of Common Coupling of Distribution Transformers

With the proliferation of solar PV inverters and other power electronic intensive loads on the power utility system the accumulated DC current at the point of common coupling can be significant. This DC current disturbs the normal operation of the distribution transformer due to the change of symmetrical magnetization, leading to significant power quality issues upstream in addition to the effects on the transformer itself. Therefore, measures need to be adopted to divert DC current away from the secondary of the transformer. Accurate sensing of DC current at the transformer secondary is a prime need in any implementation of a DC current diversion scheme. This is a challenging task because the DC current is superimposed with a large AC current. In this paper, a novel design of a current sensor that can accurately detect small DC current superimposed on a large AC current at the 400 V secondary of a distribution transformer is presented. The sensor is fast responsive and suitable for closed loop control of DC current. The details of sensor design and modelling are presented. The sensor performance is simulated in MATLAB and the simulation results are also given.