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I'm doing a research on long three phase transmission line protection systems and I found many schemes such as distance protection, merz-price voltage balance system, split-conductor, "translay" protection system, etc.

However, I am not sure protection systems are specific for the number of circuits per transmission lines, that is some circuits work only on single circuit lines other work only double circuit lines.

If there's a difference between the two types of protection systems, is it simply a question of installation, meaning that a protection system is installed on each circuit independently? Or do double circuit transmission lines have their own protection systems?

Thank you!

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    \$\begingroup\$ Does this help? Or maybe this \$\endgroup\$ – Andy aka Apr 19 '20 at 17:48
  • \$\begingroup\$ It does say that we have to account for the mutual inductance effect between the two circuits, how can I know which protection systems support mutual inductance effects (excuse the question)? \$\endgroup\$ – OmarAI Apr 19 '20 at 18:14
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No, double circuit transmission lines do not have a special protection system. Each circuit has its own protection system like any other transmission line. Typical protection schemes being POTT, PUTT, DCB, current differential, or regular old step distance non-pilot.

Yes, mutual coupling is an issue but it’s also an issue on many single circuit lines that just happen to share right-of-way for a portion of their distance with other line(s). Plus, mutual coupling is really only an issue for directional ground elements that operate on zero sequence. Using negative sequence directional elements substantially alleviates the issue. The effects of mutual coupling on reach elements (ground distance) are fairly easily accommodated as long as you properly model it in short circuit studies. J. Lewis Blackburn covers this well in his textbook, “Protective Relaying Principles and Applications”.

A more troublesome problem with double circuit lines is the possibility of back flash after a lightning strike producing an inter circuit fault. e.g. An AB-ground fault involving A-ph on one circuit and B-ph on the other circuit. This can “flip out” protection logic in the line relays.

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    \$\begingroup\$ Thank you, just the answer I was looking for! :) \$\endgroup\$ – OmarAI Apr 19 '20 at 19:25
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    \$\begingroup\$ +1 good answer. I would also add that Alstom's /Network Protection and Automation Guide/, s13 /Protection of complex transmission circuits/, also discusses mutual induction effects on protection systems. (NPAG is a free download, Blackburn is harder to find.) \$\endgroup\$ – Li-aung Yip Apr 23 '20 at 22:02
  • \$\begingroup\$ Hi Li-aung - i agree, thank you. Lots of good material on mutual coupling for free on net. Here is link to Mason's old book, the Art & Science of Protective Relaying. He calls it "mutual induction" whereas most relay engineers in U.S. call it mutual coupling. Anyway, page 286 talks about neg seq dir etc. \$\endgroup\$ – relayman357 Apr 23 '20 at 22:18

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