The wide area network (WAN) links substations with each other and with the control centers, and carries services with very different demands: teleprotection and 87L need low, stable and symmetrical delay; SCADA needs availability; synchrophasors need volume and timing; voice and video need bandwidth. IEC TR 61850-90-12 is the engineering guideline for these networks: topology, redundancy, latency, quality of service, traffic management, synchronization, security and maintenance.
What a utility WAN is
Inside the substation, the communication network links IEDs, controllers and gateway. Outside it, the wide area network (WAN) links the substation with other substations and with the control centers. It is usually built on the utility’s own fiber, often in the shield wire of the lines, with multiplexers and a fiber optic network, complemented by radio, leased links or cellular at smaller sites.
Historically each service had its own channel: teleprotection over power line carrier or a dedicated channel of the SDH multiplexer, SCADA over a serial link, voice over another channel. Today the trend is a single packet network (IP/MPLS or MPLS-TP) that carries everything. That convergence saves equipment, but it forces the network to be designed for the most demanding service.
What each traffic type needs
| Traffic | What matters to it | Comment |
|---|---|---|
| Teleprotection | Low, predictable delay, availability, command security | Permissive, blocking or trip commands between line terminals |
| Line differential 87L | Low, stable and symmetrical delay | Asymmetry turns into an angle error |
| GOOSE and SV between substations | Low delay; transport of layer 2 frames or routable versions | Interlocks, wide area protection schemes |
| SCADA (IEC 104, DNP3, ICCP) | Availability, redundant routes | Tolerates more delay than protection |
| Synchrophasors | Volume, multicast delivery and accurate time stamps | Many continuous streams toward the concentrators |
| Management, remote access and records | Security and separation | Includes oscillography downloads |
| Operational voice and surveillance video | Bandwidth | Video uses the most and is the least time-critical |
IEC TR 61850-90-12 focuses on protection data sent by GOOSE and sampled values and on the multicast transport of large volumes of synchrophasors, because they constrain the design the most. The guide on synchrophasors, PMUs and PDCs describes that traffic.
What IEC TR 61850-90-12 covers
The technical report is aimed at data network engineers and integrators working with IEC 61850, and takes them from understanding the technologies to specification, component selection and testing. It covers three communication scopes: substation to substation, substation to control center, and control center to control center. The engineering topics it covers are:
- Topology and redundancy of the transport network.
- Latency and quality of service (QoS), so that each traffic type gets the treatment it needs.
- Traffic management and service separation.
- Clock synchronization across the WAN.
- Network security.
- Network maintenance.
- Utility use cases.
It complements IEC TR 61850-90-4, which covers the network inside the substation, and IEC TR 61850-90-1, which covers IEC 61850 communication between substations from the application point of view.
Design decisions
Service separation. In a converged network, each traffic family goes in its own virtual service (layer 2 or layer 3 VPN, or emulated circuits). That way a storm on the video network does not reach teleprotection, and security rules are applied per service.
Quality of service. Protection traffic gets the highest priority and reserved bandwidth; video gets the lowest. Without QoS, a burst of low-importance traffic can delay a trip command.
Deterministic routes. For 87L it is not enough for the packet to arrive: it must arrive over the same path in both directions. Connection-oriented technologies, such as MPLS-TP, make it possible to fix symmetrical working and protection routes; in IP/MPLS it is achieved with traffic engineering. The guide on IEEE C37.94 explains how asymmetry between go and return paths becomes an 87L angle error.
Redundancy. Routes over different physical paths, duplicated equipment at critical nodes and switchover times compatible with each service.
Time. The WAN can distribute time between substations as a backup to local GNSS, as long as the network is designed for it; time synchronization in substations explains why the backup matters.
Security. The WAN is the conduit that joins different security zones. Each entry into a substation goes through an industrial router and firewall, and management of the network equipment is kept separate from operational traffic.
- 01Service inventoryWhich traffic goes from where to where, with what demands
- 02Requirements per serviceDelay, symmetry, availability and bandwidth
- 03ArchitectureTopology, transport technology and service separation
- 04QoS and routesPriorities, reservations, working and protection routes
- 05TestingDelay, switchover and load before moving protection onto the network
Transport technologies
- SDH: the classic technology of utility multiplexers. It offers fixed-capacity channels with stable delay, with interfaces such as C37.94 and G.703; many utilities plan to replace it with packet networks.
- MPLS-TP: a connection-oriented variant of MPLS, with routes defined by management and path protection, designed to behave as predictably as SDH did.
- IP/MPLS: more flexible and widespread among telecom carriers; for protection it requires careful traffic engineering and QoS.
The choice depends on the services, the existing network and the experience of the team that will operate it.
Common mistakes in the migration
- Migrating teleprotection from SDH to packets without measuring delay and asymmetry on each protection route.
- Configuring QoS on some nodes and not on others: a single node without priorities is enough to lose the guarantee.
- Carrying maintenance remote access over the same service as SCADA.
- Sizing the network for current traffic without allowing for synchrophasors, video or new substations.
- Leaving the WAN out of the commissioning tests of the line protection.
How we help
A WAN is designed from the services it carries, and the most demanding one is protection. At Tiza Group we do the communications engineering for high and extra-high voltage substations and for generation plants and industry: service inventory, requirements, architecture, teleprotection and 87L routes, and equipment specification. We do this work remotely, worldwide. Tell us about your project.
Related guides
- Substation communication network
- Multiplexer and fiber optic network
- Teleprotection schemes
- IEEE C37.94: optical interface for teleprotection and 87L
- Synchrophasor network, PMU and PDC
- Radio and cellular links
- GOOSE between substations: TR 61850-90-1 and R-GOOSE
- Redundant gateway and dual links to the control center
- OPGW and ADSS cable: fiber optics on transmission lines
- Substation communications by voltage level
References
- IEC TR 61850-90-12:2020, Communication networks and systems for power utility automation – Part 90-12: Wide area network engineering guidelines.
- IEC TR 61850-90-4:2020, Network engineering guidelines.
- IEC TR 61850-90-1:2010, Use of IEC 61850 for the communication between substations.
- IEC 60834-1, Teleprotection equipment of power systems – Performance and testing – Part 1: Command systems.
- IEEE C37.94, N Times 64 Kilobit Per Second Optical Fiber Interfaces Between Teleprotection and Multiplexer Equipment.
Frequently asked questions
What is IEC TR 61850-90-12?
It is an IEC technical report with engineering guidelines for wide area networks in power utility automation systems. It covers communication between substations, between substation and control center, and between control centers. Its second edition is from 2020 and replaced the first, from 2015.
Can teleprotection share the same network as SCADA and video?
Yes, if the network separates the services and guarantees protection traffic its delay and availability with quality of service, deterministic routes and adequate redundancy. What cannot be done is to treat it as just another traffic type competing for bandwidth with video or web browsing.
Why do utilities have their own network instead of using only a telecom carrier's?
For control of delay, availability during power system events, and security. A leased service may be enough for SCADA at smaller sites, but transmission teleprotection requires guarantees that many utilities prefer to secure with their own fiber and equipment.