Investigation of the Effects of DC Current at the Point of Common Coupling on the Operation of Distribution Transformers

Abstract

Non-linear loads, such as ac and dc motor drives, computers etc. connected to the power distribution network inject a DC current component back to the network. Individual currents from different loads add at the point of common coupling (PCC) and the net value sometimes exceeds the upper limits specified in Standards. Heavy proliferation of power electronic intensive loads is mainly responsible for this unusual situation. The presence of DC current at the PCC creates operational issues in the distribution transformers and other components in the system, leading to significant power quality issues and long term reliability issues. This paper presents the findings of an investigation on the DC current injected by loads on a sample distribution system, and the effects of DC current on the operation of a power transformer. Both the experimental results and the simulation results with PSCAD are presented and discussed.