It is well known that fast fault clearance is of vital importance for the High Voltage Power Systems, but unfortunately in Distance Relay protection, the faults on the protected HVTL near the remote end substation postpone to the 2nd zone which includes time delayed.
Zone over lapping and three zones to have distance approximation as the impedance observed variation due to fault reactance changing is more with distances from relay....
As the distance increases severity of fault is less and the effect on affecting the system conditions and damaging the equipment is less. Instead of immediately switching off we can wait and see whether fault is temporary and gets cleared.
Because of the errors in impedance measurements, (CT and VT), because of errors in HV transmission lines parameters (R. L. G. C), so the 100% zone can cover busbars at the second (end) substation and cause the distance protection to act in 1.th Zone and caused false switch of the line, (but the fault is at the busbar at the second substation!
I would suggest you take a look at the following link. It mentions the reasons briefly, which are more related to the effect of a power swing, behavior of transient current characteristic which may look like a fault. It might cause the relay to detect the current abnormality as a fault, and cause false tripping and disconnection of healthy lines. : https://www.typhoon-hil.com/documentation/typhoon-hil-application-notes/References/distance_protection_relay.html#:~:text=A%20distance%20relay%20is%20a,of%20measured%20current%20and%20voltage.
At my school of line distance protection for HV lines it has always been said: "an underreaching zone shall never overreach, an overreaching zone shall never underreach". The first zone is considered an underreaching zone. It shall never operate for faults outside the protected line.
The zone may overreach for many reasons. Some are explained with not accurated power line data, CT/VT errors. But one, very important cause of overreaching, is the presence of the load over the fault quantities on a fault with fault resistance, with "double infeed" condition, which means the fault current flows from both sides of the line (typical situation for meshed high voltage networks).
Even with perfect power line model and perfect CT/VT measurement, also during transients, the distance protection relay may overreach when there is a fault with fault resistance, and exporting load. More the exporting load, more the overreaching. In some cases 80% of the line impedance is not enough to avoid overreaching. I have set some distance protection zones to 65% of the line length, to avoid overreaching. This needs calculations (fault calculations) where the load is superimposed to the fault currents (often this is not done), and also good competence on the relay protection algorithm (which is not a commonly shared information), but some generic reasonable assumptions can be done anyway. The standard IEC 60255-121 proposes standardized tests to measure this overreaching, which is a protection function performance. There are special tests with double infeed, load flow and faults with fault resistance.
Some simplified information about this is here: Conference Paper The IEC 60255-121:2014 standard and its impact on performanc...
I have tried to bring the discussion here, but it seems it continues to LinkedIn instead.. so here the link to the post...https://www.linkedin.com/posts/activity-6816639459350638592-BhdB
Don't worry about a fault at the remote end bus. It has already a main protection i.e. busbar protection (no time delay).
Because of errors in CT a VT and in z-measurement I- zone can reach busbars in substations so all TL connected to busbars can be switched off by distance protection od the TL.
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