IEC 61850 is the international standard for communication in substations. Its goal is interoperability between devices of different manufacturers while reducing copper wiring and engineering cost. Unlike traditional protocols that only expose "register addresses", it describes the device and its functions as a named, self-describing model.
Main parts of the standard
| Part | Topic |
|---|---|
| 61850-5 | Communication requirements and message transfer times |
| 61850-6 | SCL configuration language |
| 61850-7-x | Data model structure and abstract services |
| 61850-8-1 | Mapping to MMS and GOOSE |
| 61850-9-2 | Sampled Values mapping |
| 61850-10 | Conformance testing |
| 61850-90-x | Technical reports (e.g. inter-substation communication) |
The data model
The model is hierarchical:
- a server (IED) contains one or more logical devices
- each logical device contains logical nodes such as PTOC (time overcurrent), PDIF (differential), PDIS (distance), XCBR (circuit breaker), CSWI (switch controller), MMXU (measurement) and PTRC (trip conditioning)
- each node contains data objects (DO) such as Op (operate), Str (start) and Pos (position)
- each data object contains data attributes (DA) such as value, quality (q) and time stamp (t)
A full reference looks like `IED1/XCBR1.Pos.stVal`: the position (stVal) of the circuit breaker in node XCBR1 of device IED1. Every attribute has a functional constraint (FC), such as ST for status, MX for measurement, CO for control and SP/CF for settings.
The SCL configuration language
System engineering is done with standard XML files:
- ICD / IID: capabilities of an IED type (from the manufacturer)
- SSD: single-line and function specification of the substation
- SCD: configuration of the whole substation, including all IEDs, the network, data sets, reports and GOOSE links
- CID: the final configuration of each IED that is downloaded to it
Typical workflow: import the ICD files into the engineering tool, define data sets and GOOSE links, generate the SCD, export the CID of every device, download and test. SCD version control matters: changing a single data set can alter the configuration revision (confRev) and break communication.
Communication services
MMS: client–server communication
Runs over TCP/IP on port 102 and serves SCADA and HMI. Key capabilities:
- Read/write of values and settings
- Reporting using buffered (BRCB) and unbuffered (URCB) report control blocks. Triggers can be data change (dchg), quality change (qchg), data update (dupd), periodic (integrity) or general interrogation (GI). Buffering ensures events are not lost during a brief communication outage.
- Control: direct or select-before-operate (SBO), in normal and enhanced security modes
- Logs and files: access to event logs and disturbance files (COMTRADE)
GOOSE: fast messages between relays
A publisher/subscriber model on Ethernet layer 2 (Ethertype 0x88B8) with multicast addressing and no IP routing. The goal is a transfer time below 3 ms for trip messages (performance classes P2/P3 with transfer-time class TT6 in Part 5). For reliability the message is retransmitted several times at increasing intervals after a change and is also sent periodically in steady state (heartbeat). The stNum (state change count) and sqNum (sequence counter) let a receiver detect lost messages.
Practical notes: use a VLAN with priority 4 or higher (IEEE 802.1Q), multicast filtering in switches, and supervise GOOSE loss (loss of subscription) in the relay logic.
Sampled Values (SV)
Digital transmission of current and voltage samples from a merging unit to relays, based on IEC 61850-9-2 (and the 9-2LE profile). On a 50 Hz system, 80 samples per cycle (4000 samples/s) is common for protection and 256 samples for power quality and accurate metering. SV requires very accurate time synchronisation (about 1 µs) and is usually combined with PTP and PRP/HSR redundant networks. A process bus replaces the copper CT/VT wiring with fibre.
Testing and commissioning
- Conformance: check the device's conformance certificate against 61850-10 and review the ICD and PICS/PIXIT
- Service testing: connect a test client to the relay and verify reports, data sets and commands
- GOOSE testing: simulate GOOSE publishing and subscription, measure transfer time and check APPID, MAC and VLAN settings
- Network analysis: capture traffic with Wireshark or a dedicated tool and check stNum/sqNum and confRev
- End-to-end protection test: inject current into the publishing relay and observe the subscribing relay and breaker
- Test mode (Mod/Beh): place the IED in Test mode while testing so that commands and signals carry the test flag and in-service equipment is not affected
Common problems
- confRev or data set mismatch between publisher and subscriber
- multicast flooding caused by missing VLANs and filters
- loss of time synchronisation on the SV network
- no SCD version management, leaving uploaded files inconsistent
- downloading a CID to a relay without first saving its original protection settings
Conclusion
IEC 61850 is not just "another protocol". It is a different engineering and testing methodology that, with precise documentation, suitable tools and end-to-end testing, reduces whole-life substation cost. ENP provides SCL configuration, GOOSE/MMS testing and SCADA integration.