On high voltage and especially extra-high voltage lines, teleprotection usually has two independent channels so that losing one does not leave the line without accelerated tripping. Redundancy only works if the channels do not share a point of failure: different physical routes, different media such as fiber and power line carrier, and separate equipment and power supplies. How the two signals are combined depends on what the scheme must protect: dependability or security.
Why the channel is duplicated
Teleprotection clears a fault anywhere on the line without delay, but it depends on a communication channel. If the channel fails, permissive schemes lose their acceleration and faults near the remote terminal are cleared in zone 2 time; blocking schemes may trip on external faults; the line differential 87L is blocked. The guide on teleprotection channel failure covers each case.
In distribution and on much of the high voltage network, that risk is accepted with a single channel and the backup protections. On extra-high voltage lines, which carry large blocks of power and on which system stability depends, slow clearing can have consequences across the whole grid. That is why each main protection usually has its own channel, and why the two channels must not be able to fail at the same time from the same cause.
Redundancy is not the same as diversity
Two channels are only redundant if they do not share a point of failure. The list of common points is longer than it looks:
| Common point | Why both fail together |
|---|---|
| Same fiber cable (for example, the line’s own OPGW) | A lightning strike, a damaged splice or a fallen tower cuts all its fibers |
| Same multiplexer | A failure of the device or of its synchronization takes out all its channels |
| Same route in the transport network | A switchover or an outage on one section affects every service that goes through it |
| Same DC supply | One open breaker shuts down both devices |
| Same physical medium | Power line carrier depends on the line itself, which is precisely the one with the fault |
Diversity seeks to avoid those common points. It is achieved with different physical routes (the protected line’s OPGW for one channel and a path through other substations for the other), different media (fiber and power line carrier, or fiber and radio), separate communication equipment and independent power supplies.
Power line carrier has a known limitation as a second channel: it travels over the protected line, and an internal fault can attenuate it just when it is needed. Even so, because its failure mode differs from that of fiber, it remains a diversity option on many lines, especially with schemes that tolerate that limitation.
How two channels are combined
With two channels, the relay or the teleprotection equipment receives the same command twice and must decide how to combine them. IEC 60834-1 rates teleprotection by three competing parameters: security (not receiving false commands), dependability (not losing true commands) and transmission time. Combining channels shifts the balance between the first two:
- “1 out of 2” logic (OR): the command is valid if it arrives over either channel. It improves dependability; a false command on a single channel is enough to act.
- “2 out of 2” logic (AND): the command is valid only if it arrives over both. It improves security; losing one channel prevents action.
- Is the received command supervised by a local measurement of the receiving relay?
- Yes, permissive or blocking scheme1 out of 2 is generally used: local measurement provides security and two channels provide dependability
- No, direct transfer tripWhich risk weighs more in that application, a false trip or a lost command?
- False trip2 out of 2, or 1 out of 2 with added local supervision
- Lost command1 out of 2, with a high-security channel
In permissive schemes the relay only trips if it also sees the fault in the forward direction; that local supervision provides security, and combining the channels with OR improves dependability at no significant cost. In direct transfer trip, by contrast, the command trips with no further conditions; there a false command is a false trip, which is why high channel security or logic that does not depend on a single channel is required. The specific settings are defined in each project’s protection philosophy.
87L with two channels
The line differential does not exchange simple commands but the measured currents, so it needs a digital channel. Many 87L relays accept two channels and switch from one to the other if the main one fails. Here redundancy has a nuance: if the two channels have different delays, switching changes the propagation time, and the relay must recalculate the sample alignment. The guide on IEEE C37.94 explains why asymmetry between the go and return paths is the critical point.
Monitoring each channel
A redundant channel that fails silently leaves the line with a single channel without anyone knowing, just like a PRP network that loses one of its LANs. That is why each channel is monitored separately: the teleprotection equipment watches the continuity of its link with guard signals or test frames, the multiplexer reports the quality of its route and the 87L relay reports the delay and errors of each channel. Each of those alarms must reach SCADA with a text that says which channel and which line are affected, so the repair is scheduled before the second one fails.
What is checked in the engineering
- That each channel has an identified physical route end to end, including the sections inside each substation.
- That no device, cable, power supply or transport network section is common to both.
- That the channel combination is consistent with the scheme of each protection.
- That loss of each channel produces a separate alarm in SCADA.
- That commissioning tests cover operation on each channel separately and switching between them.
How we help
Channel diversity is decided in the communications engineering, while changing a route is still cheap. At Tiza Group we do the communications engineering of high and extra-high voltage substations remotely: teleprotection architecture, fiber routes, multiplexers and power line carrier, with the protection philosophy as the starting point. If your line relays are already in service and you need to review the channel logic, we do it with remote relay configuration. Tell us about your project.
Related guides
- Teleprotection schemes
- Teleprotection equipment
- Teleprotection channel failure
- Line trap and power line carrier
- Multiplexer and fiber optic network
- Line differential protection 87L
- Teleprotection and 87L over IP/MPLS networks: what changes
- Direct or multiplexed fiber for line differential 87L
- WAN networks for substations: IEC TR 61850-90-12
- Substation communications by voltage level
References
- IEC 60834-1, Teleprotection equipment of power systems – Performance and testing – Part 1: Command systems.
- IEC TR 61850-90-1:2010, Use of IEC 61850 for the communication between substations.
- IEEE C37.94, Standard for N Times 64 Kilobit Per Second Optical Fiber Interfaces Between Teleprotection and Multiplexer Equipment.
- IEEE C37.113, Guide for Protective Relay Applications to Transmission Lines.
Frequently asked questions
Are two fibers in the same OPGW cable two redundant channels?
They protect against the failure of one fiber or one device, but not against failure of the cable. A lightning strike, a damaged splice or a fallen tower affects both at once. For real diversity, the second channel must take another physical route or another medium.
How are the signals of two teleprotection channels combined?
It depends on the scheme. In a permissive or blocking scheme it is enough for the command to arrive over either channel, which improves dependability. In a direct transfer trip without local supervision, arrival over both may be required, which improves security at the cost of dependability. The choice is documented in the protection philosophy.
Should each main protection of the line have its own channel?
It is the usual practice when the line has two main protections: each with its own channel, its own communication equipment and, preferably, its own route, so that no single failure takes out both. The exact requirements are set by the utility or the owner's protection philosophy.