Effect of Supply Unbalance on Harmonic Emissions of Uncontrolled SixPulse Rectifiers

Standards dealing with power quality disturbances and emission limits for equipment and installations are constantly evolving as additional knowledge is gained. Often, each type of disturbance, such as harmonics, is considered individually and without regard for the possible effects of other types of disturbances, such as unbalance. The interdependencies between the disturbance phenomena are often disregarded. The work reported in this paper is focused on quantifying and correlating the effect of supply system unbalance on the harmonic emissions of the most basic harmonic-producing converter, the uncontrolled six-pulse rectifier. Six-pulse rectifiers with different loading conditions are supplied by sources with varying levels of unbalance to determine the effect of the unbalance on harmonic emissions via simulation. Different unbalanced supply conditions, all within credible levels, are considered and the effects on the harmonic emissions of the six-pulse rectifier are evaluated. The results of these simulations show that as source unbalance increases, THD and triplen harmonics also increase in a nearly-linear pattern. This type of relationship may be appropriate for consideration in the ongoing efforts in standardization related to both harmonics and unbalance so that more coordinated decisions can be made.