IEC 61850 in the industrial substation: protection without copper wires

In the classical substation every signal is a wire. Breaker position — a wire. Interlock between incomer and bus coupler — a wire. Trip command from the protection — a wire. A large substation has thousands of them, each routed, labelled, terminated and — on any change — rewired. IEC 61850 replaced that model with one Ethernet cable and a standardised language in which devices talk to each other.

The standard dates from 2004, its second edition from 2010–2013, and in transmission and distribution substations in Bulgaria it is now common practice. Industrial 20 kV substations — the factory transformer station with two incomers, a bus coupler and a dozen feeders — are still mostly built the old way. And that is exactly where the benefit is most tangible.

The three things the standard defines

A data model. Every device — protection relay, meter, controller — describes itself through standardised “logical nodes”. The breaker is XCBR, overcurrent protection is PTOC, measurement is MMXU — regardless of manufacturer. One manufacturer’s device knows what “breaker position” means on another’s, without translation.

Communication services. Three kinds, by speed. MMS is client-server exchange for SCADA — commands, readings, events. GOOSE is fast device-to-device exchange by multicast with a guaranteed time under 4 milliseconds — for trips and interlocks. Sampled Values carry the measured current and voltage values from instrument transformers to the relays as a stream of 80 samples per cycle, instead of over copper secondary circuits.

A configuration language. SCL — XML files that describe the whole substation: devices, connections, signals. The engineer configures the system in one file rather than device by device. On a change the file is edited and reloaded.

What that means for the factory substation

• Interlocks become messages. The condition “the bus coupler cannot close while both incomers are closed” is traditionally a chain of auxiliary contacts through several terminal strips. With GOOSE it is one message from each relay to the others, with integrity checking — if a message does not arrive on time, the system knows and reacts.
• Busbar protection gets simpler. The blocking scheme — feeders tell the incomer “the fault is on me, do not trip” — is hard to set up over copper. Over GOOSE it is a standard function of every modern relay.
• Copper disappears. Not all of it, but a large part. Secondary circuits remain only between the instrument transformers and the nearest relay, and with non-conventional instrument transformers — Rogowski coils, optical — those become digital too.
• Self-supervision. Devices send GOOSE messages even when nothing happens — every few seconds. A broken cable is detected immediately, not at the first short circuit.
• Integration with the plant SCADA is one MMS connection, not development for every signal.

What it demands

The network is part of the protection and is designed as such. Managed switches, traffic separation by VLAN, redundancy — by PRP or HSR (IEC 62439-3), in which every message is sent over two paths simultaneously. Time synchronisation by IEEE 1588 to sub-microsecond accuracy is mandatory for Sampled Values. None of this is exotic, but none of it is part of the typical transformer station either.

Testing changes. You cannot “inject current at a terminal” — you inject a GOOSE or SV stream from a test set, and the devices have a test mode and simulation flags, introduced in the second edition precisely so that the real breaker does not trip during a test.

Cybersecurity stops being someone else’s problem. A network on which trip commands travel is a network someone may try to use. IEC 62351 describes the measures; in practice that means a separate network, controlled access and no link to the office network “for convenience”.

When it pays

For a new substation with more than two or three feeders, for one with two supplies and automatic transfer, for a site whose SCADA needs to see its own power supply — there the price difference in the relays comes back from saved copper, shorter installation and the ability to change logic without rewiring. For a station with one incomer and two feeders the standard is superfluous. The line runs somewhere between, and it is seen most clearly when you draw how many wires the old version would have had.

Related area: Automation systems

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