Multiplexers and Fiber Networks: OPGW, SDH and MPLS-TP

Short answer

The fiber-optic network between substations uses OPGW cables on transmission lines or ADSS on distribution structures, and multiplexers that share the capacity among teleprotection, line differential, SCADA, voice and data. SDH multiplexers assign fixed channels; MPLS-TP packet multiplexers keep the deterministic behavior of a transport network. For line differential protection, latency matters and, above all, so does having the same delay in both directions.

What it is and where it sits

A utility’s fiber-optic network links its substations and control centers. It carries teleprotection, line differential (87L), SCADA, operational voice, video and corporate data. It has two layers: the fiber cable, which runs along the transmission or distribution lines, and the multiplexers, which in each substation share the capacity among services.

In the control house, the cable arrives at an optical distribution frame (ODF), and from there patch cords run to the multiplexer. From the multiplexer the interfaces go out to the teleprotection equipment, the line relays, the RTU or gateway and the substation communications network.

Main parts

OPGW (optical ground wire): replaces the shield wire of the line and carries the fibers inside a metallic conductor. IEEE 1138 sets its performance and tests.
ADSS (all-dielectric self-supporting): a metal-free, self-supporting cable for distribution structures or lines where the shield wire is not replaced. IEEE 1222 covers its performance and tests.
Splice boxes on the towers and at the substation entrance, where the OPGW transitions to a dielectric cable that enters the control house.
Optical distribution frame and patch cords.
Multiplexer with interface cards: E1 and 64 kbit/s per ITU-T G.703, X.21, IEEE C37.94, Ethernet and, in some units, built-in teleprotection cards.
Centralized network management.

How it works

Time-division multiplexing (SDH/PDH). Synchronous digital hierarchy (ITU-T G.707) assigns each service a fixed time slot within a frame. A 64 kbit/s channel for teleprotection always has the same capacity and a fixed, low latency. SDH rings are protected by mechanisms such as SNCP or MS-SPRing (ITU-T G.841): if a fiber is cut, traffic moves to the other direction of the ring.

Packet multiplexing (MPLS-TP). The MPLS transport profile (requirements in RFC 5654, architecture in RFC 5921) carries everything in packets, but with preconfigured paths, and the supervision and protection of a transport network. E1 or C37.94 services are carried by circuit emulation. RFC 5654 requires support for co-routed bidirectional paths, in which forward and return follow the same route.

Technology	How it shares capacity	Latency	Protection against a cut
SDH (ITU-T G.707)	One fixed time slot per service	Fixed and low	SNCP or MS-SPRing (ITU-T G.841): traffic moves to the other direction of the ring
MPLS-TP	Packets with preconfigured paths; E1 or C37.94 by circuit emulation	Needs careful latency and symmetry engineering for protection	Supervision and protection of a transport network

Interfaces for protection. IEEE C37.94 defines the optical interface between teleprotection and multiplexer at N × 64 kbit/s (N from 1 to 12), over multimode fiber up to 2 km and, since the 2017 edition, single-mode up to 20 km. G.703 defines electrical interfaces, which require care with shielding and grounding in the wiring.

Latency and asymmetry for 87L. The line differential relay compares samples from both ends, so it must know how long they took to arrive. The channel-based (ping-pong) method measures the round-trip time and assumes each direction takes half. If the delays differ, the alignment error is half the difference. At 60 Hz, each millisecond of error equals 21.6 degrees; at 50 Hz, 18 degrees. This shows up as a false differential current. The risk grows when a ring switches only one transmission direction. The solutions are to switch both directions together, use symmetric paths, or synchronize the relays to an external time source (GNSS or PTP), which allows each direction to be measured separately (see time synchronization).

Variants and selection criteria

Dedicated fiber relay to relay: the simplest and fastest option for 87L on short lines; it consumes fibers.
SDH: fixed, deterministic latency, with a large installed base in utilities.
MPLS-TP or packets with quality of service: more capacity for Ethernet and video; needs careful latency and symmetry engineering for protection.
OPGW or ADSS depending on the line, the span and whether a shield wire is needed.
Single-mode (ITU-T G.652) for long links; multimode only for short runs inside the substation.

Key specification data

Number of fibers in the cable and reserve for growth.
Optical budget: transmitter power minus receiver sensitivity, which must exceed the sum of the losses of the fiber, splices and connectors plus a margin. The values come from the datasheets of each device.
Maximum latency allowed for each protection service and ring protection switching time.
Type and number of C37.94, G.703, E1 and Ethernet interfaces.
Network synchronization (reference clock) and, if required, time distribution.
Battery power supply and card redundancy.

Integration with protection and control

There is no IEC 61850 logical node for the multiplexer as such. In the relays, channel supervision is reflected in communication alarms and, in IEC 61850, in the LCCH node for physical channel supervision. The alarms of the multiplexer management system must reach the control center, because a channel degradation affects the protection.

Commissioning and maintenance tests

OTDR on each fiber: length, attenuation, splices and connectors.
Insertion loss with a source and power meter. The same fiber tests, applied to the ring of a plant, are in fiber network tests for a solar plant.
Bit error rate (BER) on each E1 or 64 kbit/s channel.
Latency of each protection service, over both paths of the ring.
Ring switching with the 87L in test service, checking the delay and asymmetry the relay reports.
Complete scheme tests in end-to-end tests.

Common failures and what to check first

Dirty connectors or bent patch cords: clean and inspect first.
Moisture in OPGW splice boxes: attenuation that grows over time.
One-way ring switching: asymmetry and false 87L operations.
Multiplexer clock errors: slips and intermittent errors.
Wrong cross-connect of a time slot or a path.
Transmit and receive fibers swapped on C37.94.

Protection communications with Tiza Group

Is your 87L alarming on channel or asymmetry, or do you need to integrate teleprotection into a new network? At Tiza Group we do the communications integration with our IEC 61850 engineering, at high and extra-high voltage, in substations and industry.

References

IEEE 1138: testing and performance of optical ground wire (OPGW).
IEEE 1222: testing and performance of ADSS cable for power lines.
IEEE C37.94: N × 64 kbit/s optical fiber interfaces between teleprotection and multiplexer equipment.
ITU-T G.703, G.707, G.841 and G.652.
IETF RFC 5654 and RFC 5921: MPLS-TP requirements and architecture.
IEC TR 61850-90-12: wide area network engineering guidelines.

Frequently asked questions

Why did the line differential trip when the multiplexer switched the ring?

A known cause is asymmetry: if only one direction moves to the backup path, the forward delay is no longer equal to the return delay. A relay that estimates the delay from the round trip (ping-pong) misaligns the samples and sees a false differential current.

What is the difference between OPGW and ADSS?

OPGW replaces the shield wire of an overhead line and carries the fibers inside a metallic conductor. ADSS is fully dielectric and self-supporting; it is installed on existing structures, mainly distribution, with no shield wire function.

Can 87L run over a packet network?

Yes, if the network guarantees bounded latency and symmetric paths. MPLS-TP allows bidirectional paths that follow the same route in both directions. It also helps to synchronize the relays to an external time source, which lets them measure each direction separately.