The effect of DC current on power transformers

Abstract

Power transformers are an integral part of almost all electrical transmission and distribution networks. Their reliable service is of the utmost importance in modern society which is dependent on a constant electricity supply. There are a range of factors that can hinder the operation of a power transformer. This dissertation presents the results of investigations into one of these factors through an analysis of the effect that direct current has on the operational characteristics of a power transformer. There are a host of adverse effects that can accompany the presence of a direct current in a transformer's windings. The predominant effect that is witnessed is half cycle saturation. This leads to increased harmonic distortion, increased reactive power losses, overheating and elevated acoustic noise emissions. Direct current can be found in a transformer's windings as a result of imperfections in connected equipment and also due to magnetic disturbances of the earth's field. Tests conducted indicate that personal computers are a potentially significant source of DC when a large number of units are connected to a common point of coupling. Similarly the possibility exists for AC and DC induction motor drives to contribute sizeable quantities of DC Bias.