MV-514 · Phase 4 Advanced

Digital Substation, SCADA & IEC 61850

Trigger a fault and follow it through a digital substation in slow motion: sampled values stream from the merging units, the protection IED decides, a GOOSE message trips the breaker, and the event reaches SCADA. Break a network link and see what redundancy does.

In short

In a digital substation, merging units digitise CT and VT signals at the switchgear and publish sampled values (SV) on a process bus, replacing much of the copper wiring.
Protection-class SV (IEC 61850-9-2 LE) uses 80 samples per cycle: 4,000 frames per second at 50 Hz, about 5 Mbit/s per stream.
GOOSE messages carry trips and interlocks between IEDs. Trip messages must arrive within about 3 ms (IEC 61850-5 transfer time class for trips).
PRP sends every frame over two independent networks and HSR sends it both ways round a ring, so one failure loses no data and needs no recovery time.
The station sends events and measurements to the control centre through a gateway, typically using IEC 60870-5-104 or DNP3.

By Eng. Mannan Maqsood, Electrical EngineerPart of the MV Hub

Digital Substation Fault Timeline 2D ms timeline

Press Fault to play the event in slow motion (1 ms of real time takes about 50 ms on screen). Change the redundancy and break LAN A to see the difference.

0.0 ms

Normal: sampled values streaming

Process Bus Bandwidth Calc

Estimate the network load from sampled value streams and GOOSE, and check it against the link speed.

Merging units

Samples per cycle

Frame size on wire (bytes)

Link speed

GOOSE publishers

What makes a substation digital?

In a conventional substation, every CT, VT, breaker status and trip signal is a pair of copper wires from the switchgear to the relay panels. In an IEC 61850 digital substation, merging units at the switchgear convert analogue signals into sampled values, and switchgear controllers publish breaker positions and receive trip commands as GOOSE messages, all over Ethernet. This cuts copper, makes the design testable in software, and allows protection to be reconfigured without rewiring.

Station bus and process bus

Process bus: between merging units, switchgear controllers and protection IEDs. Carries sampled values and GOOSE. Needs very accurate time sync (IEEE 1588 PTP) so samples from different merging units line up.
Station bus: between IEDs, the HMI and the gateway. Carries MMS reports and controls, and GOOSE for interlocking.

PractitionerThe substation configuration language (SCL) files describe every IED, dataset and message: ICD (IED capability), SCD (whole substation) and CID (configured IED). Keeping the SCD under version control is as important as keeping the drawings up to date.

ExpertTest with GOOSE and SV simulation tools and the IEC 61850 test/simulation flags so you can isolate a live IED from the process during testing. Cyber security to IEC 62351 and IEC 62443 is part of the design, not an add-on.

Distribution automation and SCADA

At distribution level, smart RMUs with motorised switches, FPIs and an RTU let the control centre switch remotely and run FLISR (see MV-501). The RTU or gateway usually talks IEC 60870-5-104 or DNP3 over fibre, radio or cellular links.

Regional notes

GCC

Utilities are rolling out IEC 61850 at transmission and primary substations and remote-controlled RMUs on 11 kV networks, with strong cyber security requirements.

Australia and New Zealand

Transmission companies and distributors use IEC 61850 widely, with DNP3 common for telecontrol to distribution assets.

Asia

IEC 61850 is standard for new primary substations in many Asian utilities, with distribution automation programmes in major cities.

Test yourself

0 / 6

6 questions. Pick an answer to see the explanation. Practise about 50 more MV-514 questions in the MV Question Bank.

Frequently asked questions

What is a GOOSE message?

Generic Object Oriented Substation Event: a fast multicast Ethernet message used in IEC 61850 to send trips, interlocks and status between IEDs. It is repeated at shrinking then growing intervals after a change so it is not lost.

What are sampled values?

Digitised CT and VT measurements published by merging units on the process bus, typically 80 samples per cycle for protection under IEC 61850-9-2 LE.

What is the difference between PRP and HSR?

PRP (Parallel Redundancy Protocol) sends each frame over two separate networks. HSR (High-availability Seamless Redundancy) sends each frame both ways round a ring. Both give zero recovery time for a single failure.

How fast must a GOOSE trip be?

IEC 61850-5 sets a transfer time of about 3 ms for trip messages in the fastest performance class, measured from the publishing application to the receiving application.

Related MEPVirtual labs

MV-501 MV Systems & Network Explorer MV-507 MV Protection & Relay Coordination MV-515 Condition Monitoring & Failure Forensics ELV-304 Structured Cabling & Data Networks

Standards referenced

IEC 61850 series
IEC 62439-3 PRP and HSR
IEC 60870-5-104 / IEEE 1815 DNP3
IEC 62351 and IEC 62443 Cyber security
IEC 61850-9-3 Precision time protocol