On high- and extra-high-voltage lines two independent main protections, main 1 and main 2, are installed so that the failure of a relay, a CT, a VT, a channel or a battery does not leave the line unprotected. Independence is achieved by separating CT cores, VT windings, DC supplies, trip coils and teleprotection channels.
What the main 1 and main 2 scheme is
A transmission line cannot be left unprotected while it is energised. If its only relay fails, the fault is cleared late and from far away, with more equipment affected and a risk to stability. That is why at high and extra-high voltage two main protections are installed, called main 1 (M1) and main 2 (M2), each able to clear any line fault on its own at high speed. In some utilities they are known as system A and system B.
The scheme is part of the substation protection schemes, and it is not just a matter of installing two relays. Redundancy is only worth something if nothing the two share can fail and blind both at once.
The protection functions and how they are coordinated
- Main 1 and main 2: distance 21 with teleprotection, line differential 87L, or one of each. Both take CTs and VTs on the line side.
- Teleprotection: each main protection has its own channel to the remote terminal, ideally over physically separate routes. See teleprotection schemes.
- Local backup: directional ground overcurrent 67N for high-resistance faults, and the time-delayed distance zones.
- Breaker failure 50BF: initiated by both main protections.
- Reclosing 79: in one of the two, or in both with coordination logic so that they do not close twice. See single-pole and three-pole reclosing.
- Auxiliary functions: power swing blocking 68, switch-onto-fault, VT fuse failure supervision, trip circuit supervision.
What is separated between M1 and M2
| Element | Usual separation |
|---|---|
| Current transformers | separate protection cores |
| Voltage transformers | separate secondary windings or miniature circuit breakers |
| DC supply | two circuits, often from two battery banks |
| Tripping | two breaker trip coils |
| Teleprotection | two channels, preferably over separate routes |
| Relays | separate devices; often of a different brand or principle |
Anything shared becomes a single point of failure. A classic example is a single VT miniature circuit breaker for both distance relays: if it trips, both lose voltage at the same time.
Logic and sequence
For a single-phase fault on the line:
- M1 and M2 detect the fault in their zone 1 or with permission from the remote terminal.
- Each one trips its coil; the breaker opens the faulted phase (single-pole) or all three.
- Both initiate 50BF and reclosing.
- After the dead time, the recloser closes. If the fault persists, the main protections trip again and reclosing is locked out.
- If the breaker does not open, 50BF trips the local bus and sends a transfer trip to the remote terminal.
In breaker-and-a-half or ring bus configurations, M1 and M2 sum the currents of the two breakers of the bay and trip both.
Variants by voltage level
- Extra-high voltage: M1 and M2 with redundant teleprotection, single-pole reclosing, two batteries and two trip coils. It is the usual practice.
- High voltage: it depends on the owner and on how critical the line is. Many transmission owners require M1 and M2; others accept one main protection with backup.
- Medium voltage and industry: a single main protection (overcurrent, or differential on short cables) with upstream backup. Duplication is reserved for critical plant links.
Design criteria
- Real independence along the whole path: measurement, logic, channel, supply and tripping.
- Reaches and times defined in the protection coordination study, considering mutual coupling, series compensation and infeed.
- Choice of teleprotection scheme according to the available channel and the line length.
- Coordination between the two reclosers to avoid double closing.
- Requirements of the transmission owner and of the system operator, which are reviewed for each new connection.
There are no universal reach or time values: they depend on the line, on the adjacent lines and on the studies.
How each manufacturer implements it
The most common line relay families are Siemens SIPROTEC 5 (configured in DIGSI 5), SEL (AcSELerator QuickSet), Hitachi Energy, formerly ABB, Relion 670 (PCM600), GE Vernova Multilin UR (EnerVista) and MiCOM, a name used today by both GE Vernova and Schneider Electric. What changes is how the zones are defined, how distance and differential are combined in one device, how two reclosers are coordinated and how signals are exchanged: contacts, a proprietary fibre protocol or GOOSE. When M1 and M2 are of different brands, each logic is written in its own tool and both must achieve the same behaviour.
Why it fails or misoperates
- Both lose voltage at once: shared VT or miniature circuit breaker.
- One trips and the other does not: settings different from those studied, wrong settings group, the channel of only one main protection down.
- Three-pole trip when single-pole was expected: one main protection is not enabled for single-pole tripping, or the other sends it a three-pole trip.
- Double reclosing: both 79 functions active with no coordination logic.
- Overreach or underreach due to zero-sequence mutual coupling on double-circuit lines, which affects ground distance.
- Fault between breaker and CT, which neither sees as internal unless there is dead-zone logic.
The first step is to compare the oscillography of M1 and M2 with a common time reference.
How it is tested
Each main protection is tested by secondary injection with a three-phase test set (for example an Omicron CMC 356): zones, times, teleprotection logic, 50BF and reclosing. Then the complete scheme is tested: each main protection must trip its own coil, losing one DC supply must not take out the other, and the channels of both must reach the remote terminal, with end-to-end tests.
Do you have a line where M1 and M2 do not behave the same, or a new scheme to configure? At Tiza Group we review the events and configure the relays remotely. See our remote relay configuration service or contact us.
References
- IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
- IEEE C37.119-2025, Guide for Power System Circuit Breaker Failure Protection.
- IEEE C37.104-2022, Guide for Automatic Reclosing of Circuit Breakers for AC Distribution and Transmission Lines.
- CIGRE TB 140 (1999), Reliable fault clearance and back-up protection.
- S. H. Horowitz and A. G. Phadke, Power System Relaying.
Frequently asked questions
Do main 1 and main 2 have to be from different manufacturers?
It is not mandatory everywhere, but it is common practice because it prevents the same design or firmware error from affecting both. Some owners also require different measuring principles, for example distance in one and line differential in the other. It is defined by the transmission owner or the system operator.
If both protections operate at the same time, does the breaker receive two trips?
Yes, and that is intended. Each main protection energises its own trip coil, or both, depending on the design. The breaker opens on the first command that arrives; the second is redundant.
Is remote backup still needed with two main protections?
Yes. The two main protections cover the failure of a relay, a CT, a VT or a channel, but not the failure of the breaker, which both share. That is the job of local 50BF and, as a last resort, the time-delayed zones of the relays at the remote terminals.