Load Injected DC Current in Distribution Transformers: Investigation and Elimination

Due to heavy proliferation of power electronic and non-linear loads on a present day utility system, the DC current accumulated at the distribution transformer is significant. DC current shifts the operating point of the magnetization characteristic and hence causes the magnetizing current to exhibit severe asymmetry between positive and negative half cycles in magnitude and shape, owing to magnetic saturation. It affects the shape of input current and recreates a DC current at the input with magnitude even greater. This paper describes the work on influence of DC current on input current in a loaded transformer, and a system to divert DC current completely away from the transformer. Implementation was done with a specially designed current sensor and a fast responsive controlled current source. The later was implemented in power electronic with closed loop control. The work was challenging because measuring of small DC current superimposed with a large AC current was not straightforward, and also injecting a small DC current accurately against a large AC voltage was not straightforward. Both these challenges were successfully overcome and an extremely good DC current diversion system was developed. Details of the design and results of investigations are presented.