Very high-resistance fault on a 525 kV transmission line - Case study

This paper analyzes a 300 ohm primary ground fault, which is an unusually high value for a 525 kV transmission line in southeastern Brazil. This case study emphasizes the techniques used by the analysts. Considering that the fault impedance was larger than those usually observed in single-phase faults on extra-high-voltage (EHV) lines, this paper discusses the probable cause of the fault and mentions an analysis technique to evaluate such faults. The protective relaying community lacks information regarding the causes and values of fault resistances to ground on high-voltage (HV) and EHV transmission lines. The objectives of this paper are to stimulate research and contribute to the collection of very high-resistance fault information. The analysis techniques are presented using symmetrical components and fault calculations to arrive at fault parameter values that are very close to the ones provided by protective relays. The performance of the line protection is evaluated for the specific fault conditions, with calculation of the observed impedances and currents. The importance of the ground over- current directional protection on a pilot directional comparison scheme is shown. Speculation on the widespread use of differential protection for transmission lines should stimulate discussions of line protection philosophies and applications. The criteria for the resistive reach setting of the quadrilateral ground distance characteristic are presented to show an evolution of past criteria and to open discussion about the setting limits. The conclusions of this paper highlight the importance of present event report analysis techniques regarding fault calculation software and the need for appropriate settings criteria for the resistive ground distance element threshold. This paper also supports the use of ground directional overcurrent protection with a pilot scheme for HV and EHV transmission line protection, while proposing the widespread use of differential functions fo- r transmission lines, even for the most extensive cases.