Current Reversal Guard on Parallel Lines

The problem it solves

On two parallel lines between substations A and B, a fault on one of them also drives current through the other. Call L1 the healthy line, protected by a POTT scheme, and L2 the faulted line, with the fault near B.

1. During the fault: on L1 the current flows from A to B and on to the fault through bus B. The L1 relay at A sees the fault forward (within its overreach) and sends permission. The L1 relay at B sees it reverse and blocks. L1 does not trip: correct.
2. The L2 breaker at B opens first. The fault is now fed only from the L2 terminal at A. The contribution from substation B reaches it by travelling along L1 to bus A, so the current on L1 reverses: it flows from B to A.
3. The race: the L1 relay at B now sees the fault forward, while the permission sent by A has not yet reset in the channel. If its overreach picks up before the permission drops, it trips the healthy L1.

This race between the reset at one terminal and the pickup at the other is current reversal. It occurs in permissive overreaching schemes and in blocking schemes, and it is one of the complementary logics described in teleprotection schemes.

How the guard works

The guard relies on the relay’s reverse elements:

• If a reverse element has been picked up for longer than a pickup time (of the order of one cycle, according to an SEL technical paper), the logic concludes that there is an external fault.
• While it is active, and for an additional dropout time (of the order of several cycles) after the reverse element resets, the guard blocks the permissive send and the scheme trip.

In the example, the L1 relay at B saw the fault in the reverse direction throughout the fault. When the current reverses, the guard prevents it from tripping or sending permission for a few cycles, long enough for the permission from A to reset in the channel and for A, which now sees the fault in reverse, to stop sending.

Some designs use the reverse element to override the forward element instantaneously, even phase by phase, before the pickup time expires. And for faults that evolve from external to internal, some reset the guard when distance zone 1 sees the fault, so that the trip is not lost.

In DCB the problem is the same with the opposite sign: the terminal that was sending a block stops seeing the fault in reverse and stops blocking, while the other terminal starts seeing it forward. The solution is to extend the block for a time after the reverse element resets, combined with the coordination timer of the DCB scheme.

Siemens calls this logic transient blocking, with adjustable waiting and blocking times, and applies it to both permissive and blocking schemes; other manufacturers call it current reversal logic or current reversal guard.

Channel and timing

The guard setting depends on the channel: the dropout time must exceed the time the permission takes to stop arriving at the other terminal, plus the reset of the remote forward element and a margin. A channel with variable delay, or with signal extension in the teleprotection equipment, lengthens that time. If the channel is lost, the scheme stops accelerating and the guard has no effect.

Advantages and limitations

Advantages: it prevents false trips of the healthy line on double circuits and in meshed networks with parallel paths, without reducing the reaches.

Limitations: it delays by a few cycles the trip for an internal fault that appears right after an external one, unless the logic resets the guard with zone 1. It depends on the reverse element seeing every external fault that the remote overreach sees.

Where it is used

On every high- or extra-high-voltage line with an overreaching scheme and a parallel path: double circuits on the same tower, parallel lines on different corridors and closed rings. It is not usually needed in PUTT, because the signal is keyed by a zone 1 that does not see external faults.

How each manufacturer implements it

SEL (AcSELerator QuickSet) implements the guard with pickup and dropout timers inside its scheme logic. Siemens SIPROTEC 4 7SA (DIGSI) includes it as transient blocking associated with teleprotection. Hitachi Energy Relion 670 (PCM600) groups it with the weak-end infeed logic in a dedicated block (ZCRWPSCH). GE Vernova (Multilin, EnerVista) offers current reversal logic in its pilot schemes. The MiCOM P44x (GE Vernova MiCOM Agile and Schneider Electric Easergy MiCOM) call it current reversal guard within their aided schemes. Names, the elements that start it and the default times differ.

Why it fails or misoperates

• Healthy line trips: guard disabled, dropout time shorter than the channel reset, or a reverse element with less reach than the remote overreach.
• Slow trip for an evolving fault: guard without zone 1 reset.
• Trip after reclosing on the neighbouring line: sequences in which the guard has already reset.

Check the oscillography of both terminals of the healthy line and the opening order of the faulted line’s breakers.

How it is tested

Secondary injection simulates the complete sequence: a reverse fault for a time and then the same fault forward, verifying that there is no send and no trip during the dropout time. A GPS-synchronised end-to-end test reproduces the reversal with both terminals and the real channel.

Did your healthy line trip when the parallel one faulted? At Tiza Group we analyse the events and the current reversal logic remotely. See our remote relay configuration service or contact us.

References

• Kasztenny, Mynam, Fischer and Guzmán, Permissive or Blocking Pilot Protection Schemes? How to Have It Both Ways, SEL, Western Protective Relay Conference, 2020.
• IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
• CIGRE, Technical Brochure 192, Protection using telecommunications (2001).
• Blackburn and Domin, Protective Relaying: Principles and Applications.

Frequently asked questions