Intelligent Electronic Devices (IEDs) are widely used in modern substations and power systems. They collect electrical data, monitor equipment status, run protection functions, and exchange information with other devices.

But an IED does not work alone. It needs a communication protocol to send measurements, alarms, status changes, and control commands to other systems.

So, what is an IED, and which communication protocols are commonly used with it? This guide explains the basics and compares four common protocols: Modbus RTU, DNP3, IEC 60870-5-104, and IEC 61850.

What Is an IED?

An IED is a microprocessor-based electrical device used to monitor, control, measure, or protect power system equipment.

Typical IEDs include:

• Protection relays
• Intelligent meters
• Feeder terminals
• Bay controllers
• Circuit breaker controllers
• Power quality monitors

An IED can collect values such as voltage, current, frequency, and power. It can also detect equipment status and faults. Depending on the device, it may send alarms, record events, or issue control commands.

The key difference from traditional electrical equipment is the built-in processing and communication capability. An IED can process field data and exchange it with SCADA, substation automation systems, and other IEDs.

Why Do IEDs Need Communication Protocols?

IEDs from different devices or manufacturers may use different ways to organize and transmit data.

A communication protocol defines how this data is exchanged. It determines things such as:

• What data can be transmitted
• How devices identify each other
• How commands and responses work
• How events are reported
• How data is represented

For example, a simple IED may use Modbus RTU to expose voltage and current through register addresses. A newer substation IED may use IEC 61850 logical nodes and standardized data models.

The protocol affects how easily the IED can connect to the rest of the system.

Common IED Communication Protocols

Modbus RTU

Modbus RTU is one of the simplest protocols used with IEDs and other power equipment.

It commonly runs over RS-485 and uses a master-slave model. A master device, such as a PLC or SCADA system, requests data from slave devices.

Modbus RTU uses registers to represent data. This makes it easy to implement and widely supported.

However, the master must normally poll the slave for data. The protocol also does not define the meaning of each register. Engineers need to check the device documentation and map the correct addresses.

Use Modbus RTU when simple device communication and wide device compatibility matter more than advanced substation functions.

DNP3

DNP3 was developed for power system applications. It is commonly used between RTUs, IEDs, SCADA systems, and control centers.

Unlike basic polling protocols, DNP3 supports event data and timestamped information. It also supports unsolicited responses, so an IED can report important changes without waiting for a new request.

DNP3 supports both serial communication and TCP/IP.

It is a good fit for power system monitoring, distribution automation, and remote control applications where event reporting and communication reliability matter.

IEC 60870-5-104

IEC 60870-5-104, usually called IEC 104, extends IEC 60870-5-101 communication to TCP/IP networks.

It is widely used for communication between substations and remote control or dispatch centers. Ethernet and IP networks make it suitable for geographically distributed power systems.

IEC 104 also supports event-driven data transmission. Important status changes can be reported without relying only on continuous polling.

Use IEC 104 when the main requirement is IP-based communication between substations and higher-level control systems.

IEC 61850

IEC 61850 was developed for substation automation. It uses a different approach from register-based protocols such as Modbus.

Instead of simply assigning values to register addresses, IEC 61850 uses standardized data models based on concepts such as Logical Nodes (LN) and Common Data Classes (CDC).

IEC 61850 also defines several important communication services:

• MMS: Used for monitoring, data exchange, and control.
• GOOSE: Used for fast event communication, such as interlocking and protection signals.
• SV: Used to transmit sampled electrical values.

This model gives IED data a more standardized structure. It can also improve interoperability between devices from different manufacturers.

The trade-off is configuration complexity. IEC 61850 projects require careful network design, data modeling, and engineering.

IED Protocol Comparison

There is no single protocol that fits every IED project.

Check the existing devices first. Then consider the required response time, network type, data model, interoperability, and control functions.

How Do You Integrate IEDs Using Different Protocols?

A common problem appears during substation upgrades.

A new system may use IEC 61850, while existing IEDs still use Modbus RTU, DNP3, or IEC 104. Replacing every legacy device is often unnecessary and expensive.

A protocol gateway can sit between these systems and translate data from one protocol to another.

For example:

Legacy IED → Protocol Gateway → IEC 61850 Network → SCADA

The gateway reads data from the legacy IED, maps the required data points, and presents them to the new system in the required protocol format.

For IEC 61850-based projects, an IEC 61850 gateway can be used to integrate legacy IEDs into an IEC 61850 network. The exact functions depend on the gateway, but typical work includes protocol conversion, data mapping, and IEC 61850 data modeling.

Before choosing a gateway, check the actual protocols supported by the IEDs. Also confirm the serial interface, Ethernet interface, data points, event requirements, and required IEC 61850 services.

Do not assume that two devices can communicate just because both use Ethernet. The protocol and data model still need to match.

How to Choose an IED Communication Protocol

A few basic questions can help narrow the choice:

What does the existing IED support?
Start with the device manual. The available protocol often determines the practical options.

Is the communication local or remote?
Modbus RTU may be enough for local device communication. DNP3 and IEC 104 are often used for remote monitoring and control.

Do you need event-driven reporting?
If the IED needs to report important changes without waiting for polling, consider protocols such as DNP3, IEC 104, or IEC 61850.

Do you need standardized device data?
IEC 61850 is more suitable when standardized data models and multi-vendor substation integration are important.

Are legacy devices part of the project?
If old and new protocols need to coexist, a protocol gateway may be more practical than replacing every device.

Conclusion

An IED is more than a measurement or protection device. It can collect data, process information, perform control functions, and communicate with other systems.

The communication protocol determines how this information moves through the power system.

Modbus RTU is simple and widely supported. DNP3 is well suited to power system monitoring and event reporting. IEC 104 provides IP-based communication for remote applications. IEC 61850 adds standardized data models and communication services for modern digital substations.

For a new project, select the protocol based on the IED, network, data requirements, and system architecture. For an upgrade project, protocol gateways and IEC 61850 gateways can help connect legacy IEDs to newer systems without replacing every existing device.

Source: Adapted from COME-STAR Industrial Networking Technical Blog → https://www.come-star.com/blog/ied-protocol/