IEC 61850 is the protocol for communication between devices inside the substation: a common data model, GOOSE between bays and file-based engineering. IEC 60870-5-104 and DNP3 are telecontrol protocols that carry data and commands to the control center; 104 dominates in Europe and much of Latin America and DNP3 in North America. Modbus is the simplest and is used with meters, inverters and auxiliary equipment.
Why there are so many protocols
Each protocol was born to solve a specific problem at a specific time. Modbus, so that one controller could read registers from another in a plant. IEC 60870-5-101 and DNP3, so that a control center could poll distant RTUs over slow links. IEC 60870-5-104, to carry the same over IP networks. IEC 61850, so that devices inside a substation could share one data model and send fast messages to each other.
None is better in the abstract: the right question is on which segment of the network it will be used and what that segment needs.
The protocols one by one
| Protocol | Standard | Born for | Medium | Where it is used today |
|---|---|---|---|---|
| IEC 61850 | IEC 61850 series | Communication between devices inside the substation | Ethernet: MMS over TCP/IP, GOOSE and SV at layer 2 | Station bus and process bus |
| IEC 60870-5-104 | IEC 60870-5-104 | Telecontrol over IP networks | TCP/IP, port 2404 | Substation to control center (Europe, much of Latin America) |
| IEC 60870-5-101 | IEC 60870-5-101 | Telecontrol over serial links | Serial | Older links to the control center |
| DNP3 | IEEE 1815 | Telecontrol between SCADA master and outstations | Serial or TCP/IP (port 20000) | Control center, distribution, reclosers (North America and many distribution utilities) |
| Modbus | Modbus Organization specification | Reading and writing registers between controllers | RS-485/RS-232 (RTU) or TCP, port 502 | Meters, inverters, station service, industry |
| IEC 60870-5-103 | IEC 60870-5-103 | Protective relays to the control system | Serial | Substations with earlier-generation numerical relays |
Each telecontrol protocol has its own guide with the details: IEC 60870-5-104, DNP3 and Modbus RTU and TCP.
What really sets them apart
Data model. IEC 61850 describes each data item with a name that says what it is (a circuit breaker logical node, its position, its quality and its time stamp). In 104 and DNP3 each data item is a numeric address whose meaning lives in a signal list agreed between the parties. In Modbus it is a register, with no quality or time stamp in the basic version.
Events. IEC 104 and DNP3 are designed to report changes with a time stamp without waiting to be polled, and DNP3 sorts its events into priority classes. Modbus has no events: the client polls and compares. IEC 61850 sends reports over MMS and GOOSE messages when a data item changes.
Device-to-device messages. Only IEC 61850 has a message designed for one IED to trip or interlock another within a few milliseconds: GOOSE. How work is split between GOOSE and MMS inside IEC 61850 is covered in GOOSE vs MMS in IEC 61850. The others are master-slave or client-server protocols, made for supervision and control.
Engineering. In IEC 61850, the configuration of the whole system is described in SCL files that each brand’s tools can read. In 104, DNP3 and Modbus, each end is configured point by point from a signal list.
Security. None of the telecontrol protocols includes security in its basic version. DNP3 added secure authentication in IEEE 1815-2012; IEC 104 and IEC 61850 are protected with the IEC 62351 series, and Modbus has a separate security specification. In practice protection also depends on the network: segmentation, firewall and controlled access.
Which protocol on each segment
- Is the communication between devices of the same substation?
- YesDoes it need trips or interlocks between bays?
- YesIEC 61850 GOOSE
- NoIEC 61850 MMS to the HMI and the gateway
- NoDoes it go to the control center?
- YesWhatever the utility requires: IEC 60870-5-104 or DNP3
- NoModbus for nearby meters, inverters or auxiliaries
Toward the control center there is hardly ever a choice: the utility or the SCADA owner sets it in its specification. Inside the substation there is room to choose, and in new high-voltage projects the de facto standard is IEC 61850, as the basis of the substation automation system. In distribution networks with reclosers and radio or cellular communications, DNP3 is still very common because of its event handling and its tolerance of slow links.
The gateway: where they meet
Several protocols coexist in almost every substation. The gateway or RTU reads the IEDs over IEC 61850, Modbus or 103 and delivers the data to the control center over 104 or DNP3. In that translation each data item changes its name, type and quality and time format, and select-before-operate commands move from one model to the other. The details are in IEC 61850 to IEC 104 and DNP3 mapping.
A well-configured gateway keeps the protocol choice on one segment from limiting the other. A poorly configured one produces the typical symptoms: signals arriving without quality, events carrying the gateway’s time instead of the relay’s, or commands rejected with no explanation.
What about OPC UA?
A fifth candidate shows up in industrial plants: OPC UA, standardized as IEC 62541. It is not a telecontrol protocol of the power sector but an architecture for exchanging data between automation and information systems, with a built-in data model and security. It is used mainly to bring substation data to the DCS, the energy management system or analytics platforms, usually from the gateway. Inside the substation it does not replace IEC 61850, nor IEC 104 or DNP3 toward the utility.
Common mistakes when choosing
- Requiring IEC 61850 to the control center when the utility’s SCADA does not support it. The result is an extra gateway nobody had planned for.
- Using Modbus for functions that need time-stamped events. The sequence of events ends up with the resolution of the polling cycle.
- Choosing DNP3 or 104 without agreeing on the profile: data types, event classes, time synchronization and behavior on loss of link.
- Migrating without a plan. Moving from 103 or Modbus to IEC 61850 in a substation in service is planned by scenarios, as explained in migrating legacy protocols to IEC 61850.
We help you with your communications
At Tiza Group we configure the communications of relays, bay controllers and gateways in IEC 61850, IEC 60870-5-104, DNP3 and Modbus, for substations and industry. If you need to set up the protocol of a relay or a recloser, remote relay configuration solves it remotely. For on-site IEC 61850 engineering and integration, see our IEC 61850 configuration service, in Colombia, or elsewhere on request.
Related guides
- IEC 61850: what it is, parts of the standard and data model
- IEC 60870-5-104
- DNP3
- Modbus RTU and TCP
- IEC 60870-5-103
- IEC 61850 to the control center
- IEC 61850 to IEC 104 and DNP3 mapping in the gateway
- SCADA, SAS, DCS and HMI: differences in power systems
- IEC 61850 GOOSE: how it works, stNum, sqNum and testing
- IEC 60870-5-101: serial telecontrol protocol and moving to 104
- IEC 62351: cybersecurity for IEC 61850, 104 and DNP3
- Substation RTU and gateway: functions, protocols and testing
References
- IEC 61850-8-1, mapping to MMS (ISO 9506) and to ISO/IEC 8802-3.
- IEC 60870-5-101 ed. 2 (2003) + Amd 1 (2015), basic telecontrol tasks.
- IEC 60870-5-104 ed. 2 (2006) + Amd 1 (2016), network access for IEC 60870-5-101 using standard transport profiles.
- IEEE Std 1815-2012, Standard for Electric Power Systems Communications – Distributed Network Protocol (DNP3).
- Modbus Organization, Modbus Application Protocol Specification V1.1b3 and Modbus Messaging on TCP/IP Implementation Guide.
- IEC 62351, communication security for power systems.
- IANA, Service Name and Transport Protocol Port Number Registry (ports 102, 502, 2404 and 20000).
Frequently asked questions
Does IEC 61850 replace IEC 104 and DNP3?
Inside the substation, yes, in most new projects. Toward the control center, not in general yet: most control centers receive IEC 60870-5-104 or DNP3, and the substation gateway translates. IEC 61850 has profiles for communicating with control centers, but they are less common.
What is the difference between IEC 101 and IEC 104?
They share the same application layer: the same data types, causes of transmission and addresses. 101 travels over serial links; 104 over TCP/IP, with its own header that numbers the frames and supervises the link. That is why moving from 101 to 104 keeps the signal list.
DNP3 or Modbus for a recloser or a meter?
DNP3 if the data goes to SCADA and you need time-stamped events, quality and report by exception. Modbus if the job is simply reading registers from a nearby device, such as a meter or an inverter, and the time stamp is not critical.