Problems associated with zero sequence compensation applied to distance protection

Zero sequence compensation is one of the essential functions found within distance protection and is widely used all over the world. However, the application of this function can be problematic as it can cause complex and subtle effects. Often investigations into problems found with zero sequence compensation have not clarified whether or not a problem would have caused unwanted operation under very severe conditions. In this paper the undesirable effect of zero sequence compensation on the measurement of resistance and on the performance of Mho elements is investigated and described. These undesirable effects are explained using vector diagrams and equations in order that a general tendency of the effect can be understood. The explanation is confirmed by simulation using ATP. Finally, the problems are summarised and possible solutions are suggested