Understanding the effect of pre-fault load flow on the effect of fault resistance on the impedance seen by the traditional ground distance function

Summary This article describes the effect of pre-fault load flow on the effect of fault resistance on the impedance seen by the traditional ground distance function (i.e., with the traditional use of the residual compensation factor, K0). The fault resistance is seen by this distance function as an additional apparent impedance (AAI), whose Tilt (T) in the R–X plane is not constant. There is a widespread misunderstanding about this subject because several documents indicate that the sign of T is directly determined by the sign of the pre-fault active power, which is not always true. This article clarifies this point by including detailed explanations, which were not available in the literature. First, an analytical deduction shows that the locus of the AAI is a circle, for a given pre-fault load flow and for a given fault location. Subsequently, diverse loci are numerically computed for a power system taken as an example, in order to show that the sign of T can change, when the fault resistance increases. For a given pre-fault load flow, the sign of T could change for relatively low values of fault resistance because the effect of fault resistance is amplified on the AAI due to the current from the remote line end. The proper knowledge of the loci of these apparent impedances is important for the setting of distance relays and for some possible applications of the concept of adaptive protection. Copyright © 2014 John Wiley & Sons, Ltd.