Direct or Multiplexed Fiber for 87L Line Differential

The decision

Every line differential 87L needs a digital channel between the relays at the two terminals to exchange the measured currents. There are two ways to give it that channel:

• Direct (dedicated) fiber: the optical port of the relay at one terminal connects through a fiber pair to the relay at the other terminal, with no equipment in between.
• Multiplexed channel: the relay connects to a multiplexer of the telecom network, usually with the IEEE C37.94 optical interface or with a 64 kbit/s G.703 electrical interface, and the network carries the channel to the multiplexer at the other terminal.

The choice affects delay, channel symmetry, reach, availability and who is responsible for what. There is no single answer: it depends on the line length, the fibers available and the utility’s telecom network.

Why the channel matters so much for 87L

To compare currents, 87L needs the samples from both terminals to correspond to the same instant. There are two main methods to align them:

• Round trip (ping-pong): the relay measures how long a message takes to go and come back, and assumes each direction takes half.
• External time reference: each terminal time-stamps its samples with a common reference, usually GNSS, and needs to assume nothing about the channel.

With the first method, any difference between the go and return delays becomes an alignment error equal to half that difference, and the alignment error shows up as an angle error: 21.6 degrees per millisecond at 60 Hz and 18 degrees at 50 Hz. An angle error between the two currents produces a false differential current, which under heavy load can approach the operating threshold.

Direct fiber

With dedicated fiber, the go and return paths run through the same cable with the same length, with no intermediate equipment adding variable delays. The channel is symmetrical and stable by construction, and the round-trip method works without surprises.

Its limits are different:

• Reach: set by the relay’s optical transceiver against the total link loss, including splices, connectors and the aging margin. Multimode interfaces cover short distances and long-reach single-mode ones cover longer lines. The guide on the multiplexer and fiber optic network covers the physical medium.
• Fiber use: each relay takes a fiber pair end to end, which is no longer available for other services.
• No network supervision: the fiber has no management system to report its degradation; the relay itself watches it with its error counters and channel alarms.

Multiplexed channel

With a multiplexer, 87L shares the telecom network with teleprotection, SCADA and voice. It is the natural option when the line is long, when there are no free fibers end to end or when the network already has routes through other substations.

Here the risks that direct fiber does not have appear:

• Longer, variable delay, from passing through the equipment and from the route chosen.
• Route switching: in an SDH ring, when a section fails, the network switches traffic to the protection route. If the switchover affects only one direction, the go and return paths take routes of different length and the channel becomes asymmetrical.
• Packet networks: when transport migrates from SDH to IP or MPLS networks, the 87L channel travels by circuit emulation, and delay variation and symmetry depend on how that network is designed.

Publications from a relay manufacturer indicate that SDH networks have a natural asymmetry of a fraction of a millisecond and that an asymmetry of around 1 to 2 ms can already cause unwanted operations. That is why, when 87L goes through a multiplexer, the network is configured to switch both directions together, the delay and asymmetry alarms are set in the relay, or alignment by external time reference is used.

Many important lines combine both options: one channel over direct fiber and the other through a multiplexer, on different routes, as the second channel of the same 87L or as the channel of the second main protection.

What not to do

• Raise the 87L threshold to “tolerate” asymmetry instead of fixing the channel.
• Accept a multiplexed network without knowing how each service switches.
• Use alignment by external reference without supervising synchronization at both terminals.
• Test 87L only with the channel on its normal route: the problem shows up when the network switches.

How it is checked

In general terms, commissioning an 87L with a multiplexed channel verifies the delay and asymmetry reported by the relay on the normal route and on each protection route, forcing the network switchovers with the function under test. The end-to-end test, with synchronized injection at both terminals, confirms that the protection operates for internal faults and stays stable for external ones. If your 87L already shows channel alarms or tripped with no fault, the guide on 87L tripping on channel failure describes the diagnosis.

How we help

At Tiza Group we review the channel settings, the delay and asymmetry alarms and the alignment method of your 87L relays with our remote relay configuration service, for the main brands, on high and extra-high voltage lines. To check the relay and channel together in the field, we carry out the end-to-end test at both terminals, in Colombia or elsewhere on request. Contact us with the brand and model of your relays.

Related guides

• Line differential protection 87L
• IEEE C37.94: optical interface for teleprotection and 87L
• 87L tripping on channel failure
• Multiplexer and fiber optic network
• Time synchronization in substations
• OPGW and ADSS cable: fiber optics on transmission lines
• Substation communications by voltage level

References

• IEEE C37.94, Standard for N Times 64 Kilobit Per Second Optical Fiber Interfaces Between Teleprotection and Multiplexer Equipment.
• ITU-T G.703, Physical/electrical characteristics of hierarchical digital interfaces.
• IEC 60834-1, Teleprotection equipment of power systems – Performance and testing – Part 1: Command systems.
• IEEE C37.243-2015, Guide for Application of Digital Line Current Differential Relays Using Digital Communication (under revision).
• IEC TR 61850-90-12:2020, Wide area network engineering guidelines.

Frequently asked questions