Substation Telecommunications Architecture Design Report

Short answer

A substation telecommunications architecture describes how its services reach the rest of the power system: teleprotection, SCADA, metering, synchrophasors, voice, video and remote access. It sets the medium of each link, route diversity, capacity per service, power for the equipment, the demarcation point with the utility and the acceptance tests. It is the external part of the network; the internal part is the substation LAN.

What it is and what it is not

When people talk about substation “communications,” two different things tend to get mixed. The first is the internal network: switches, rings, VLANs and the protocols between IEDs, explained in the substation communications network. The second is what ties the substation to the rest of the power system and keeps those links alive: fibers, multiplexers, radios, power supplies and agreements on who is responsible for each segment.

This second part is the telecommunications architecture. In a project it is delivered as a design report: a document that justifies the decisions and turns them into diagrams, equipment lists and test criteria. Reviewing it before buying equipment is one of the tasks with the best return in avoided problems, because correcting a route or a medium on paper costs nothing compared with correcting it after the fiber has been laid.

The services that leave the substation

Everything is organized around services. Each one asks something different of the link, and the architecture comes out of that difference.

Service	Main requirement	Media that usually serve it
Teleprotection and line differential protection	Low, stable delay and, for differential, symmetric in both directions	Dedicated fiber or multiplexed channel with controlled delay
SCADA to the control center	Availability and a backup path	Multiservice network with primary and alternate route
Revenue and power quality metering	Integrity and time stamping	Multiservice network
Synchrophasors	Common time and constant throughput	Multiservice network with priority
Operational voice	Low latency and availability	Multiservice network or dedicated channel
Video and access control	High throughput and tolerance to pauses	Network with reserved bandwidth
Remote management and engineering	Security and access logging	Controlled connection, never direct

The guide on WAN networks for substations details what each traffic type asks for in terms of quality of service. One practical consequence matters here: the medium and route of each critical service are decided before deciding how much bandwidth the general link has.

Transmission media

Optical fiber. It is the main medium at medium and high voltage. On overhead lines it runs inside the shield wire (OPGW) or is attached to the conductors (ADSS); inside the substation it runs through its own ducts. The choice between single-mode and multimode fiber depends on distance and equipment.

Multiplexers and transport networks. They bring several services onto one fiber pair: synchronous technologies, packet transport with traffic engineering, or IP/MPLS. Multiplexer and fiber optic network explains their role, and teleprotection over IP/MPLS networks the conditions for carrying protection over a packet network.

Radio and cellular. They serve remote substations or act as backup for services that tolerate more delay. Their limits are covered in radio and cellular links.

Dedicated teleprotection channels. Teleprotection equipment connects to the communications channel through specific interfaces; the channel design must consider its redundancy.

Route diversity

A backup that shares the same trench, the same span or the same duct as the main link is not a backup: an excavation or a fallen structure cuts both at once. The design report must show, on a plan or a schematic, where each route runs and where they cross.

Diversity is reviewed at three levels:

Physical: different fiber routes, or different media.
Equipment: independent multiplexers, cards and ports for the main and the backup service.
Power: supplies and feeds that do not depend on a single breaker or a single DC bus.

For differential protection, direct or multiplexed fiber changes the list of failure points and the behavior during a route switchover.

Capacity

Capacity is estimated per service and added up per segment, with margin for growth. A common mistake is to size the link on average SCADA traffic and forget the peaks: an event burst after a fault, the download of disturbance records, or a simultaneous firmware update on several devices. Capacity is shared with priorities so that critical traffic does not compete with management traffic.

Power for the telecommunications equipment

Multiplexers, radios and converters must keep working when the substation supply fails, precisely because they are the ones that report the failure. The design report must state what source they are powered from, what voltage they use, how many independent supplies they have and how long they must run without the grid. This ties in with the sizing of the substation battery charger and with station service by voltage level: the telecom autonomy adds to that of the rest of the DC loads.

Demarcation point and responsibilities

Many projects involve several parties: the substation owner, the utility or control center, and a transport provider. The design report must set where each party’s responsibility ends, normally at a fiber distribution frame or at a specific port of a device. Without that point, a link failure turns into a weeks-long dispute. The components, redundancy, backup center and operator screens are treated in Control Center Design.

Security at the boundary

The link leaving the site is also the path through which unauthorized access could enter. The design report must show where the perimeter firewall is, which demilitarized zone (DMZ) separates the control network from the outside, and how access is logged. This is developed in substation and plant cybersecurity and in OT network segmentation. The domains and the placement of security controls are explained in IEC TR 62351-10.

How the telecommunications design report is built

Service inventory. What leaves the substation and where it goes
Requirement of each service. Latency, availability, symmetry and throughput
Medium and route per service. Fiber, multiplexer, radio or cellular
Route diversity. Physical, equipment and power
Capacity and priorities. Sum per segment with margin
Power and autonomy. Supplies and time without the grid
Demarcation and secure boundary. Responsibilities and perimeter firewall
Acceptance tests. Each service end to end and each expected failure

Acceptance tests

The design report ends in a list of checks run when the system is received: physical quality of each fiber link (received power and reflectometry), end-to-end services with their latency, switchover to the backup route when a link is disconnected, and behavior on loss of power. For the fiber segment, the guide on fiber network tests shows the general criteria, and for the internal network, communication network testing.

Common mistakes

Designing by equipment instead of by service. A multiplexer is bought and only later does it turn out not to meet the differential protection requirement.
Backup on the same route. Two fibers in the same cable are not two routes.
Forgetting power. The link is redundant, but both devices hang from the same supply.
No demarcation point. Nobody knows who owns the fault.
A report that is never updated. When the substation is expanded, services are added without reviewing capacity or priorities.

How we help

At Tiza Group we prepare and review the communications engineering of substations, from defining services and routes to acceptance tests, and we carry out the integration with SCADA and gateways. If you have a telecommunications design report to review or a substation to design, we can quote the scope.

References

IEC TR 61850-90-12: wide area network engineering guidelines for power utility automation.
IEC TR 61850-90-4: network engineering guidelines for substation automation.
IEC 60834-1: teleprotection equipment of power systems, performance and testing.
IEEE C37.94: interface between teleprotection equipment and multiplexers over fiber.
IEC 62443: security for industrial automation and control systems.

Frequently asked questions

What is the difference between the substation network and the telecommunications architecture?

The substation network connects the IEDs, the local SCADA and the gateway inside the site: switches, rings, VLANs. The telecommunications architecture covers what leaves the site and what supports both: links to other substations and to the control center, multiplexers, transmission media, equipment power and the split of responsibilities. They are designed separately but must fit together at the gateway and multiplexer interface.

Is one fiber link enough if there is a radio or cellular backup?

It depends on the service. For SCADA, a lower-capacity backup is usually acceptable if the utility's requirements allow it. For teleprotection, a medium with variable or asymmetric delay may not be suitable for certain schemes, such as line differential protection. Each service must validate its backup against its own latency, availability and symmetry requirement, not against the SCADA requirement.

What is tested when the telecommunications system is accepted?

The quality of each physical link is measured, each service is checked end to end for latency and behavior under packet loss, and the expected failures are simulated: fiber cut, multiplexer failure, loss of power. The result is that every service recovers within the required time and that alarms reach the people who need to see them.