Choosing grid communications: IEC 101, IEC 104 and IEC 61850

A solar farm can have Modbus at its inverters, IEC 104 at its grid connection and IEC 61850 in its high-voltage substation. Those interfaces can all be appropriate. The useful question is which equipment owns each measurement or action, and what the receiving system needs to do with it.

Use this guide to write a communications brief before selecting an RTU or protocol converter. The detailed IEC 101 and IEC 104 comparison and IEC 61850 guide cover the encodings and services.

Choose the function before the protocol

The following are common arrangements, not restrictions on where a standard can be used. A grid operator's connection requirements and the selected equipment determine the actual interface.

IEC describes 61850 as a set of technical principles, covering semantics, engineering language, services and security. Asking for an “IEC 61850 port” leaves most of that specification unanswered. ABB's service overview shows MMS, GOOSE and sampled values as separate communication paths.

A practical solar and battery site

Consider an illustrative site with PV inverters, a battery, a connection-point meter, an RTU and switchgear protection IEDs. The grid operator asks for measured active power, available battery discharge power, breaker position and an active-power limit command.

Give each signal a clear owner:

The RTU may expose these points through IEC 104 while reading local equipment through other interfaces. That does not make the upstream and downstream records interchangeable. If the battery permits 60 kW now, a 100 kW nameplate must not be sent as its present capability. For generation and demand boundaries, see solar PV monitoring.

Specify the information that crosses a converter

IEC TS 61850-80-1 addresses exchange from a common-data-class model through IEC 101 or IEC 104. It is a mapping problem as well as a connection problem.

For one measured point, record the source object, destination address and meaning in the same row. An illustrative active-power entry might say: “connection-point active power, kW, positive for export, source timestamp retained, invalid when the source declares invalid or exceeds the agreed freshness limit”. The exact object name, common address, information object address and freshness limit are project choices, not universal defaults.

For every row, agree:

• Units, scaling, sign and any deadband.
• Destination type and the source values it can represent.
• Which source quality conditions map to invalid, substituted, blocked or stale data at the receiver.
• Whether the timestamp is measurement time, event time or gateway receipt time, and its time-zone convention.
• The event buffer, recovery order and behaviour after communications loss.

Do not hold the last number indefinitely while presenting it as a current reading. A Modbus register can carry a value, but quality and measurement time need separately documented registers or another explicit mechanism. See timestamps and quality and protocol gateways versus transparent bridges.

Keep supervisory control separate from protection

An instruction accepted by a protocol stack has not necessarily moved a breaker or changed measured power. The controller can reject it, an interlock can inhibit it, or the equipment can fail to reach the requested state.

Specify the required command model, including selection where applicable, acceptance, completion and independent feedback. MZ Automation's IEC 61850 control API exposes select, operate and command-termination handling separately. The IEC command confirmation guide explains the corresponding telecontrol distinctions.

For the example active-power limit, record the requested limit, the controller's accepted setting and the measured connection-point power. A controller can accept a limit while the plant is already below it. That is different from proving that a requested reduction was delivered.

Protection and interlocking require the engineered scheme's timing and failure behaviour. A successful MMS read or an IEC 104 session does not qualify a GOOSE protection path. Test that path with its own engineering and commissioning procedure.

Ask suppliers for a usable handover

IEC 61850-10 defines conformance test techniques for different device and tool roles. A test report must match the role you need.

Request the exact product and firmware, supported edition and services, interoperability documentation, and the configuration files needed by the receiving system. For IEC 61850, ask who owns the SCL engineering project and how changes to datasets or subscriptions reach every affected IED. For IEC 101 or IEC 104, request the agreed point list and command procedure, not only a protocol checkbox.

Security is another explicit part of the brief. IEC 62351 covers different profiles for TCP/IP, MMS, telecontrol and peer-to-peer traffic. Ask which parts the actual products implement and enable, who manages certificates and keys, and how permissions and security events are handled. “Ethernet” and “private network” are not descriptions of those capabilities.

Prove the complete path

Use agreed test signals and a controlled commissioning procedure. For each representative point, check the value, units, quality and time at both ends. Then test a lost source, a stale value, a reconnect and buffered events. For commands, record rejection as well as acceptance and verify the resulting process state separately.

Keep the site monitoring system's scope clear. Where a project requires utility telecontrol or a protection interface, select and qualify the RTU, converter or IED for that role. A connection through a protocol supported by Edge still needs a documented mapping at the converter; it does not establish native IEC 101, IEC 104 or IEC 61850 support in Edge.