Relay Misoperation Analysis for Parallel Transmission Lines

Did you know a parallel line going in or out of service can make your 21N underreach or overreach, and can cause your 67N to point the wrong way? It comes from zero-sequence mutual coupling between parallel lines. The residual voltage and current your relay reads aren’t only from your own line... they also include components induced by the neighboring circuit! Directional elements and ground distance functions act on these combined signals, so when the neighbor’s status changes, the relay is interpreting a mix of local and coupled quantities. This can shift apparent reach or even flip the directional decision. A few typical symptoms to watch are: - Wrong way 67N on the healthy line during external faults. - 21N reach shifts (underreach when the neighbor is in-service and overreach when the neighbor is out-of-service and grounded at both ends). - Single-ended fault location biased along the coupled section. How to avoid these problems? Start modeling the line so it only includes mutual coupling where the circuits actually run in parallel. Break the model at the points where the coupling starts and stops. Run short-circuit cases for every real operating state you might see: neighbor energized, neighbor de-energized but grounded at one end, neighbor grounded at both ends, sequential tripping, open ties, and heavy outfeed. This shows how reach and direction will shift before you choose settings. Use 67Q or adaptive ground direction whenever possible. If you keep a 67N, supervise it with 67Q or a sensitive 50Q so it will not misoperate when negative-sequence current is missing. Ground distance (21N) reach changes as the neighbor’s status changes. Use different setting groups when you can: one tuned for when the neighbor is energized (to avoid underreach) and one for when it is out of service and grounded at both ends (to avoid overreach). If switching happens often, keep Zone1 conservative. Line current differential (87L) avoids most mutual-coupling issues and gives fast, reliable clearing. If you must use a pilot scheme like POTT or DCB, set the reverse-looking ground zones long enough to cover any forward overreach on the other line and test them under sequential tripping conditions. When a double-circuit line is temporarily jumpered to run as one circuit, distance zones often underreach. Adding mid-span jumpers or switching to pilot protection during this condition can keep your protection dependable. References: IEEEC37.113 (line protection), IEEEC37.114 (fault location), vendor guides, and your utility’s protection philosophy. Tools: ASPEN Inc., ETAP Software, or PSCAD™  with mutual coupling models and relay event playback to verify logic and settings. Questions to the community: - How do you choose between adaptive k₀ and just shortening Zone1 when lines share a ROW? - Have you seen a wrong way 67N in the field? how did you fix it? - Are you shifting to negative sequence or adaptive ground direction in new relays?

Here's an interesting event from a number of years ago. Two fairly short, parallel 230 kV lines. A-G fault on one of them. Relay at one end sees the fault via its 67G1 element, trips and transfer trips the other end, so the two ends clear about a cycle apart timewise. The relay at one terminal of the other line initially sees the fault in the forward direction (32GF asserts), but at a low current (so 50G1 doesn’t pick up). When the parallel line opens at the first terminal, the fault current increases, but also reverses direction. The 50G1 element picks up quickly, the 32GF element drops out slowly, and the parallel line protection misoperates as 67G1 blips briefly for the reverse ground fault.