The industry has a bad habit of treating communication protocols like fashion choices. You see it in procurement specs: a laundry list of protocols requested for a project, often without any understanding of the underlying data models or the overhead involved. If you are still trying to force a legacy SCADA mindset onto a modern digital substation, you are setting yourself up for a world of pain.
I once consulted on a site where the integration team attempted to bridge a process-bus-heavy digital substation to a legacy regional control center using IEC 60870-5-104 (IEC 104). They spent three months troubleshooting why their “real-time” monitoring was lagging by several seconds during fault conditions. The issue wasn’t the bandwidth of the fiber; it was the protocol conversion bottleneck at the gateway, where the rich, object-oriented data model of IEC 61850 was being flattened into the rigid, address-based structure of IEC 104. They were trying to pour a gallon of data into a thimble-sized pipe.
The Problem Nobody Talks About
The fundamental disconnect between IEC 61850 and IEC 104 is architectural, not just syntactical. IEC 104 is a telecontrol protocol. It was designed to move status points and analog values from a Remote Terminal Unit (RTU) to a Master Station. It is point-to-point, polling-based (or report-by-exception), and relies on a static mapping of data addresses.
IEC 61850 is a substation automation ecosystem. It is object-oriented, self-describing, and designed for peer-to-peer communication. When you implement substation-automation-iec-61850 correctly, you aren’t just moving bits; you are moving structured information about the physical state of the grid. The “problem” is that engineers often treat IEC 61850 as if it were just a fancy version of Modbus or 104, ignoring the complexity of the Substation Configuration Language (SCL) and the overhead of the Manufacturing Message Specification (MMS).
Technical Deep-Dive
To understand why these protocols behave differently, we have to look at the data structure.
The IEC 104 Approach
IEC 104 uses a simple structure: an Information Object Address (IOA) corresponds to a value. The Master station sends a request, or the RTU sends a spontaneous report. The protocol is inherently “dumb”—it doesn’t know what the data represents. It only knows that address 4001 equals “Breaker Status.” If you change the breaker, you have to manually update the mapping in the Master Station database.
The IEC 61850 Approach
IEC 61850 uses Logical Nodes (LN) and Data Objects (DO). A breaker status is not an address; it is a path: Substation/Bay/Breaker/Pos. The device “knows” it is a breaker, it knows its position, and it knows its quality attributes (e.g., whether the data is valid, blocked, or substituted). This metadata is carried with the data.
| Feature | IEC 60870-5-104 | IEC 61850 (MMS) |
|---|---|---|
| Model | Address-based (Flat) | Object-oriented (Hierarchical) |
| Configuration | Static mapping | SCL (XML-based) |
| Interoperability | Limited (requires mapping) | High (self-describing) |
| Data Types | Fixed/Limited | Extensible/Complex |
| Primary Use | SCADA-to-RTU | Substation Bus/Process Bus |
Implementation Guide
When integrating these protocols, the goal should be to keep IEC 61850 within the substation environment and use IEC 104 only for the wide-area network (WAN) uplink to the control center.
- Keep the Substation Flat: Use IEC 61850 for all internal communications. Utilize GOOSE (Generic Object Oriented Substation Event) for inter-relay tripping and interlocking. Do not attempt to route GOOSE messages over a WAN; the timing requirements are too strict, and the multicast nature of the protocol will saturate your routers.
- Gateway Selection: Your gateway is your protocol translator. Ensure it supports robust SCL import. If you have to manually map every point in your gateway, you have failed to utilize the self-describing nature of the protocol.
- Quality Bits: IEC 61850 provides detailed quality flags. When mapping to IEC 104, you often lose this granularity. Ensure your mapping logic handles “Invalid” or “Test” flags so that the SCADA operator isn’t looking at “stuck” data that is actually just in a maintenance state.
Failure Modes and How to Avoid Them
The most common failure mode occurs during commissioning. Because IEC 104 is point-based, it is easy to accidentally map the wrong signal to the wrong address, leading to “phantom” alarms. In IEC 61850, the failure mode is usually related to SCL file versioning.
The “SCL Mismatch” Scenario
Imagine you update a relay’s firmware or change a protection setting that alters the Logical Node structure. If you do not update the CID (Configured IED Description) file and push it to the gateway, the gateway will continue to look for data objects that no longer exist or have shifted in the hierarchy. The system won’t crash; it will simply show “Communication Error” or “Invalid Data.” This is significantly harder to troubleshoot than a simple protocol timeout.
To avoid this, enforce a strict version control policy for your SCD (Substation Configuration Description) files. Treat your SCL files like source code. If the SCL isn’t versioned in your repository, you aren’t managing your substation; you are just hoping it keeps working.
When NOT to Use This Approach
Do not force IEC 61850 on legacy RTUs that lack the processing power to handle the MMS stack. You will see high CPU utilization, packet drops, and intermittent connection resets. If your hardware is 15 years old, stick to DNP3 or IEC 104.
Furthermore, avoid using IEC 61850 for low-bandwidth, high-latency satellite or cellular backhaul links. The overhead of the protocol stack can lead to excessive retransmissions, which will kill your throughput. In these cases, a lean, polled IEC 104 implementation is technically superior.
Conclusion
IEC 61850 is not a magic bullet. It is a powerful, complex standard that demands a high level of engineering rigor. IEC 104 is a workhorse—limited, aging, but reliable for the specific task of moving data from the substation to the control room.
The “right” way is to stop viewing them as competitors and start viewing them as layers in your communication architecture. Use IEC 61850 to build an intelligent, self-describing substation environment, and use IEC 104 as the bridge to the outside world. If you find yourself spending more time fighting the protocol than analyzing the power system, you’ve likely over-engineered the connection or ignored the physical limitations of your transport medium.
This article is intended for informational purposes only for experienced electrical engineers and equipment procurement professionals. All specific technical parameters, protocol compliance thresholds, and performance specifications mentioned must be independently verified against the applicable standard revision, equipment datasheet, and site-specific engineering studies before any design, procurement, or operational decision is made. GridHacker and its authors accept no liability for misapplication of the content herein.
*This article is intended for informational purposes only for experienced electrical engineers and equipment procurement professionals. All specific technical parameters, protocol compliance thresholds, and performance specifications mentioned must be independently verified against the applicable standard revision, equipment datasheet, and site-specific engineering studies before any design, procurement, or operational decision is made. GridHacker and its authors accept no liability for misapplication of the content herein.*
Hero image: Jet black iphone.. Generated via GridHacker Engine.
