In a modern substation or industrial plant, dozens of intelligent electronic devices (IEDs) from different manufacturers must exchange measurements, status, alarms and commands with each other and with the SCADA or DCS. A communication protocol defines message format, addressing, timing and error detection. Choosing the right one directly affects project cost, response speed, reliability and future expansion.
Communication levels in a substation automation system
- Process level: instrument transformers, breakers and sensors. In modern designs, sampled values (SV) and commands travel over a process bus.
- Bay level: protection relays, bay controllers and meters. Fast protection and interlocking signals are exchanged here and on the station bus.
- Station level: local HMI, gateway/RTU, time server and event recorder.
- Control centre: the SCADA or EMS, usually reached over a WAN using telecontrol protocols such as IEC 104 or DNP3.
The main protocols
Modbus (RTU, ASCII and TCP)
The oldest and simplest widely used industrial protocol. RTU runs over RS-485 serial lines and TCP over Ethernet (port 502). It is master/slave and data is mapped into four register groups (coils, discrete inputs, input registers and holding registers). Its simplicity and near-universal support are its strengths; however it has no time stamps, no sequence-of-events recording and no spontaneous reporting, so the master must poll continuously.
DNP3 (IEEE 1815)
Designed for unreliable, long-distance communication links. It groups points into classes 0–3, buffers time-stamped events and supports unsolicited responses. It runs over serial and TCP/UDP (port 20000), and Secure Authentication adds message authentication.
IEC 60870-5-101 and IEC 60870-5-104
The IEC telecontrol standard. 101 is serial, 104 runs over TCP/IP (port 2404). Features include spontaneous transmission, general interrogation, select-before-operate (SBO) commands and CP56Time2a time tags. It is the most common substation-to-control-centre protocol in Europe and the Middle East.
IEC 60870-5-103
A companion standard for connecting protection relays to a monitoring system over fibre or RS-485. It is mostly found in older substations and has been replaced by IEC 61850 in new projects.
IEC 61850
More than a protocol: it comprises an object-oriented data model, a configuration language (SCL), communication services and conformance testing. Its main services are MMS for client–server communication (SCADA), GOOSE for fast protection and interlocking signals, and Sampled Values for digital transmission of current and voltage samples. See our article "The IEC 61850 Architecture" for details.
PRP and HSR (IEC 62439-3)
These deliver seamless redundancy over Ethernet. In PRP each device connects to two independent LANs and frames are sent on both at the same time; in HSR the topology is a ring and frames circulate in both directions. Recovery time is zero in both, while RSTP can interrupt traffic for tens of milliseconds up to several seconds when a link fails.
OPC UA (IEC 62541)
An integration layer between OT systems (SCADA, DCS) and IT systems (databases, MES, analytics dashboards). Strengths are a rich information model, built-in security (encryption and certificates) and platform independence. It is not suitable for fast protection signalling.
Industrial fieldbus protocols
Profibus DP, Profinet, EtherNet/IP and CANopen are used mainly in machine automation, drives and PLC systems and are less common at substation level.
Comparison table
| Protocol | Medium | Typical use | Events and time | Speed |
|---|---|---|---|---|
| Modbus RTU/TCP | Serial / Ethernet | Meters, PLCs, auxiliaries | None (polling) | Low to medium |
| DNP3 | Serial / TCP | Control-centre link | Yes (SOE) | Medium |
| IEC 60870-5-104 | TCP/IP | Substation to control centre | Yes (CP56Time2a) | Medium |
| IEC 61850 MMS | Ethernet | In-substation SCADA | Yes (reports) | Medium to high |
| IEC 61850 GOOSE | Ethernet layer 2 | Protection and interlocking | Yes | Very high (milliseconds) |
| IEC 61850 SV | Ethernet layer 2 | Process bus | Continuous samples | Very high |
| OPC UA | TCP/IP | OT/IT integration | Yes | Medium |
Criteria for choosing a protocol
- Installed equipment: protocols that the project's relays and meters support natively avoid the cost of converters.
- Speed requirement: protection and interlocking signals need GOOSE; monitoring can tolerate seconds of delay.
- Accurate event recording: for SOE and disturbance analysis the IED itself must time-stamp events.
- Point count and bandwidth: a 9600 bit/s serial link is a bottleneck for several hundred points.
- Redundancy and availability: PRP/HSR or RSTP, dual gateways, two channels to the control centre.
- Cyber security: support for authentication and encryption (IEC 62351, IEC 62443).
- Expandability and interoperability: prefer open standards over proprietary protocols.
A common architecture today
Most substations use IEC 61850 inside the substation (between relays, HMI and gateway) and IEC 104 or DNP3 towards the control centre. The gateway or RTU converts and maps signals. Legacy devices using Modbus or IEC 103 are added through the same gateway, and meters and auxiliary equipment are normally Modbus.
Frequent commissioning mistakes
- Wrong register base addressing (starting at 0 or 1) and wrong byte/word order for 32-bit floating-point values
- Mismatch between single-point and double-point signal types
- No time synchronisation, giving wrong event time stamps
- No suitable deadband for measurements, which floods the event buffer
- Ignoring quality bits and test/blocked status
- Networks with no VLAN separation and uncontrolled multicast traffic
Cyber security
Connecting a substation to corporate networks and the WAN enlarges the attack surface. Minimum measures: network segmentation with an industrial firewall, closing unnecessary ports and services, user and device authentication, log monitoring, controlled firmware updates and configuration backups. IEC 62351 covers security for power-system protocols and IEC 62443 provides the framework for industrial control system security.
Conclusion
No single protocol is best for every need. Project success depends on layered design, an accurate IO list, time synchronisation and integration testing (FAT/SAT). ENP supports you from communication architecture and gateway configuration to end-to-end signal testing from relay to SCADA.