OPGW is the shield wire of a transmission line with optical fibers inside: it protects the line against lightning and, at the same time, carries communications between substations. ADSS is an all-dielectric, self-supporting optical cable hung from the structures with no shielding function. At the substation, the OPGW ends in a splice box on the line gantry and continues as dielectric cable to the fiber distribution frame in the control house.
Why the fiber runs on the line
The substations of a transmission system need to communicate with each other for teleprotection, 87L line differential, SCADA, operational voice and management data. The natural path between two substations is the transmission line that joins them. That is why utilities install optical fiber on their lines, in one of two cable types: OPGW or ADSS.
What OPGW is
OPGW (optical ground wire) is a shield wire with optical fibers. It takes the place of the shield wire at the top of the towers and does its electrical job: it shields the phase conductors against lightning and carries lightning currents and part of the fault current to ground.
Inside, the fibers run in one or more metal tubes, surrounded by an armor of metal wires that gives mechanical strength and current-carrying capacity. Its mechanical and electrical design is done together with the line: stringing tension, sag, the short-circuit current it must withstand and the lightning level of the area.
The fiber count is decided when the line is specified, with its whole service life in mind, because changing it later means replacing the cable.
What ADSS is
ADSS (all-dielectric self-supporting) is an all-dielectric, self-supporting optical cable. It has no metal, does no shielding and hangs from the structures on its own hardware, below the conductors or at another position.
It is used mainly to add fiber to existing lines without replacing the shield wire, and on distribution networks. On high-voltage lines, the electric field at the hanging point can degrade the cable jacket over time, so the position on the structure and the jacket type are chosen according to the field level.
| OPGW | ADSS | |
|---|---|---|
| Electrical function | Shield wire | None |
| Material | Metallic, with fibers in tubes | All-dielectric |
| Typical installation | New lines or shield wire replacement | Existing lines and distribution |
| IEEE standard | IEEE 1138 | IEEE 1222 |
| IEC family specification | IEC 60794-4-10 | IEC 60794-4-20 |
Reference standards
- IEEE 1138 sets the performance requirements and tests of OPGW for power lines.
- IEEE 1222 covers the performance and testing of ADSS.
- IEC 60794-4 is the sectional specification for aerial optical cables along power lines; its 2018 edition covers OPGW and ADSS, among others. The family specification IEC 60794-4-10 deals with OPGW (construction, test methods and optical, mechanical, environmental and electrical performance) and IEC 60794-4-20 with ADSS.
How the fiber reaches the substation
- 01OPGW on the lineSplice boxes at towers along the route
- 02Splice box on the gantryThe OPGW is spliced to a dielectric optical cable
- 03Dielectric cable in trenchesReaches the control house without bringing switchyard potentials
- 04Fiber distribution frame (ODF)Each fiber is identified at a port
- 05Patch cords to the equipmentMultiplexer, teleprotection, 87L relays and switches
- Splice boxes at the towers. OPGW is made in lengths, and a sealed splice box is installed at the towers where they join. The same happens at tap-offs.
- Splice box on the gantry. On arrival at the substation, the OPGW ends at the line gantry and is spliced there to a dielectric optical cable. The OPGW stays grounded with the structure.
- Dielectric cable to the control house through the cable trenches, with no metallic elements that could bring switchyard potentials.
- Fiber distribution frame (ODF). Each fiber ends at an identified connector. From there, short patch cords take it to the multiplexer, the teleprotection equipment, relays with a direct channel or the switches.
Records of which fiber in which tube reaches which ODF port, at each end, are what allow a fault to be repaired without spending days searching.
What the fibers are used for
- Teleprotection and line differential, with dedicated fibers or multiplexer channels over the IEEE C37.94 interface.
- SCADA and the link to the control center.
- The utility transport network (SDH or MPLS-TP), which shares capacity among services.
- Time synchronization between substations and operational voice.
- Fiber leasing to third parties, at some utilities.
The fibers are single-mode, because of the distances between substations.
Typical failures
- Lightning strikes that damage armor wires of the OPGW at the point of impact.
- Short-circuit currents that heat the OPGW beyond what was planned if it was not sized for the real fault current.
- Moisture in splice boxes, with attenuation that grows over time.
- Mechanical damage during stringing, from vibration or from conductor galloping.
- ADSS jacket degradation in areas of high electric field.
- Poor splices in the gantry box, or fibers mixed up at the ODF.
Measuring each fiber with an OTDR when the line is handed over leaves a baseline to compare against later. When a teleprotection channel goes down, the fiber is one of the first suspects; the diagnosis is in teleprotection channel failure.
We help with your project
Tiza Group does the communications engineering of high and extra-high voltage substations: fiber entry, distribution frame, fiber assignment and channels for teleprotection, line differential and SCADA. Contact us.
Related guides
- Multiplexer and fiber optic network: OPGW, SDH and MPLS-TP
- Teleprotection equipment
- Teleprotection schemes
- Lightning shielding
- Line trap and power line carrier
- Redundant teleprotection channels: routes and diversity
- WAN for substations: IEC TR 61850-90-12
- Direct or multiplexed fiber for line differential protection
- Substation communications by voltage level
References
- IEEE 1138: testing and performance of optical ground wire (OPGW) for use on electric utility power lines.
- IEEE 1222: testing and performance of all-dielectric self-supporting (ADSS) fiber optic cable for use on electric utility power lines.
- IEC 60794-4:2018, Optical fibre cables – Part 4: Sectional specification – Aerial optical cables along electrical power lines.
- IEC 60794-4-10:2014, OPGW family specification.
- IEC 60794-4-20, ADSS family specification.
Frequently asked questions
What does OPGW stand for?
Optical ground wire: a shield wire with optical fiber. It does the electrical job of the shield wire (shielding the line and carrying lightning and fault currents to ground) and carries optical fibers inside, protected by metal tubes.
When is ADSS used instead of OPGW?
When the line already exists and the shield wire is not to be replaced, on distribution structures without a shield wire, or when fiber has to be added without taking the line out of service for long. ADSS is installed at a position on the structure where the electric field is compatible with its jacket.
Why doesn't the OPGW go straight into the control house?
Because it is a metallic conductor grounded at the towers and at the gantry, and it can carry lightning and fault currents. It is spliced in a box on the gantry to a dielectric optical cable, which runs through the cable trenches to the fiber distribution frame without bringing switchyard potentials into the control house.