Considerations in applying symmetrical components for fault current evaluation of an islanded neutral resistance-grounded system

Recognizing that line-to-ground (L-G) faults are the most likely type of faults to occur, many industrial power systems are equipped with resistance grounded neutrals (NGR) to limit L-G fault currents. Limiting, identification and isolation of L-G fault currents prevents the progression of L-G faults into multiple phase faults with large fault currents and considerable damage to power systems. Under L-G fault conditions, power systems however are not symmetrical. Although new generations of commercial power system software packages are equipped to analyze asymmetrical faults (such as L-G), it is necessary for industrial power system engineers to comprehend system asymmetrical behavior under L-G fault conditions and avoid false conclusions that could result from oversimplified system modelling, incorrect inputs to programs or inappropriate selection of calculation methodology. One of the most effective tools in analyzing and verifying L-G fault conditions is the Symmetrical Components Method which was developed about a century ago and still provides valuable analytical information. This paper highlights some of the concerns with existing systems grounding practices, presents a case study, provides a tutorial example on the utilization of symmetrical components as a tool for the verification of system parameters and limitations and discusses tips, tricks and tracks associated with such applications especially for a system with diversified transformer connections.