High Resistance Faults in EHV/HV grids: EF protections and detection logics

This paper describes protection systems currently used by Terna on National Transmission Grid. It deals with distance protections, line differential protections and protections against high resistive faults (in the following GAR from Italian acronym of Guasti Altamente Resistivi), with reference to issues related with high resistance single-phase to ground faults. The characteristics of the latter are analyzed, showing the range of compensated fault resistance and illustrating Terna's current application on some critical 132 kV and 220 kV lines. A possible evolution, which overcomes some current limitations of GAR protection (e.g. application to double circuit lines) are described in last part