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

Abstract

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.

Detailed analysis of the effect of parallel lines on the impedance seen by the traditional ground distance function

“…Only one case of system impedances and P PF is shown, for the sake of simplicity. The locus of the apparent impedance is always a circle, as in the case without parallel lines (which has been shown in the literature [27]). Values of R F < 0 do not have physical meaning; however, they are necessary in order to obtain the whole shape of the circles.…”

“…Therefore, if the reactive limit of the distance function has a tilt, the proper setting of the angle of this tilt should be influenced by the connectivity of the parallel line. Furthermore, the proper setting of the angle of this tilt should be also influenced by the pre-fault flow condition, as it was highlighted in [27] for the case without parallel lines. On the other hand, if the reactive limit of the distance function does not have a tilt (i.e.…”

“…A similar analysis was not available in the literature. The effect of the pre-fault load flow on the effect of fault resistance (R F ) on the impedance seen by the T-21G was previously analysed for cases without parallel lines [27]; here, those cases are complemented by considering a parallel line. On the other hand, the relative importance of each relevant factor (pre-fault load flow, system impedances, fault resistances, parallel line connectivity, and K M0 compensation) is studied, and a similar analysis had not been previously shown in the literature.…”

A new ground impedance measuring element for digital distance relaying of long parallel lines in high fault resistance conditions

“…It is well-documented in the literature [4][5][6][7] that non-zero fault resistances give rise to a discrepancy between the impedance seen by the ground elements and its expected value, namely the positive-sequence series impedance of the line from the relaying to the fault point. The discrepancy is directly proportional to the fault resistance value and is dependent on prior-fault system operating conditions.…”

“…In order to provide the protection of each phase of transmission lines against SLG short circuits, full-scheme (non-switched) conventional digital distance relays comprise three impedance measuring elements, each assigned to one phase and is known as ground element [2,3]. It is well-documented in the literature [4][5][6][7] that non-zero fault resistances give rise to a discrepancy between the impedance seen by the ground elements and its expected value, namely the positive-sequence series impedance of the line from the relaying to the fault point. The discrepancy is directly proportional to the fault resistance value and is dependent on prior-fault system operating conditions.…”