1- High-resistance faults can occur on EHV systems, and normally such faults are cleared from the system by backup directional ground time overcurrent relays. These types of faults offer a challenge to relaying systems that are based on operating principles using impedance (distance) measurements. In this case, the delayed clearing is acceptable since these high-resistance faults have less impact on the transient stability of the system.
2- Protection of EHV lines should be based on applying redundant ground directional overcurrent protection to provide a guarantee to clear high-resistance ground faults and to sense open-phase conditions.
3-The communication schemes are used nowadays to provide faster tripping for the high resistive fault and open phase (AIDED SCHEME).
4- for the communication scheme, practically I think negative sequence voltage polarization to identify the forward direction of the fault, in many cases more reliable than zero sequence polarization voltage especially in parallel lines since the zero sequence can be affected by mutual coupling and cause the relay to mal-operate during external faults resulting in cascade trippings.
What do you think is better to be used in the DEF-aided scheme, negative sequence voltage polarization or zero sequence?
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