In top-down IEC 61850 engineering you start from the substation specification (SSD file, with the single-line diagram and the functions), choose the IEDs that fulfill them and build the SCD, from which the configuration of each device is derived. In bottom-up engineering you start from IEDs already chosen or configured (ICD or IID) and integrate them into the SCD. Most projects mix both approaches; what does not change is that the SCD is the central document handed over to the owner.
Why the engineering workflow matters
In an IEC 61850 substation, the communication configuration is not written device by device: it is described in SCL files, the IEC 61850-6 configuration language, and exchanged between tools from different manufacturers. The order in which those files are created, who creates them and who has the right to modify them is the engineering workflow.
There are two approaches: top-down, which starts from the system specification, and bottom-up, which starts from the devices. The difference is not academic: it defines how much rework there will be when a bay is added or an IED is replaced.
Files and tools
We explain SCL files in detail in IEC 61850 SCL files. Their role in the engineering workflow:
| File | What it describes | Who produces it |
|---|---|---|
| SSD | The specification: single-line diagram and functions (logical nodes) tied to the switchyard, with no IEDs defined | Specification tool |
| ICD | The capability of an IED type | IED manufacturer |
| IID | One IED already configured for the project | Manufacturer’s tool |
| SCD | The complete system: substation, communication, IEDs, datasets and subscriptions | System configuration tool |
| CID | The configuration loaded into one IED | Manufacturer’s tool, from the SCD |
| SED | Part of the system for exchange with another project | System configuration tool |
IEC 61850-6 distinguishes three types of tool: the specification tool, the system configuration tool (vendor-independent) and the IED configuration tool (each manufacturer’s own). Edition 2 added the IID and SED files and a clause on the engineering rights of each tool.
Top-down engineering
- 01Specification (SSD)Single-line diagram and functions of each bay, vendor-independent
- 02IED selection (ICD)Devices are chosen whose capabilities cover the functions
- 03System configuration (SCD)Functions assigned to IEDs, addresses, datasets, GOOSE, reports and subscriptions
- 04Configuration of each IED (CID or IID)The manufacturer's tool imports the SCD and completes the device-specific part
- 05Back to the SCDChanges made in the IED return to the SCD so that it remains the reference
Its advantages:
- The specification is vendor-independent. The owner describes what it needs, not a relay model.
- The SCD is born from the single-line diagram, with functions tied to the switchyard equipment. That makes traceability and reuse in other substations easier.
- Extensions are orderly: the new bay is specified like the others and added to the existing SCD.
Its difficulty is that it needs a specification tool and a system tool that support it well, and a team that knows how to use them. It also takes discipline: if a change is made only in the IED and does not return to the SCD, the workflow breaks.
Bottom-up engineering
The workflow starts from the devices:
- IEDs chosen or preconfigured. Each manufacturer delivers its ICD, or an IID with its template already applied.
- Integration into the SCD. The system tool brings the IEDs together and defines the communication between them.
- Configuration of each IED. The manufacturer’s tool imports the SCD and generates the CID.
It is the most common workflow in practice, because:
- IEDs are bought early and each manufacturer brings its templates.
- Many integrators work with proven solutions from one manufacturer and add the other devices around them.
- Not every specification is delivered as an SSD.
The risk is that the SCD ends up describing what was configured rather than what was specified, and that each substation turns out different even though it performs the same function. Without the substation section of the SCD, the relationship between functions and the single-line diagram lives only in the drawings.
Roles and engineering rights
In a multi-vendor project, each manufacturer owns the internal configuration of its IEDs and the integrator owns the SCD. The workflow works when these are clear:
- Who may modify what. The integrator defines the communication between IEDs; the manufacturer, the internal logic of its device.
- How changes come back. What the manufacturer’s tool changes in an IED (for example, a dataset) returns to the SCD as an IID.
- How it is versioned. Each SCD delivery carries its revision number and a change history.
- How parts of the system are shared between projects, for example with another substation or the control center, through SED files.
What to hand over to the owner
- The final SCD, matching what was installed and tested.
- The ICD of each IED type and the CID or IID loaded in each device.
- The device-specific configuration of each IED, exported from its manufacturer’s tool, because the SCD contains neither the protection settings nor all of the internal logic.
- The version table of firmware, tools and files.
You can open and review an SCD with our free tool, the IEC 61850 SCL viewer.
How to choose
- Does the owner have a standard functional specification for its substations?
- YesTop-down with an SSD, and require the final SCD to keep the substation section
- NoAre the IEDs already bought or preconfigured by the manufacturer?
- YesBottom-up, making sure every change returns to the SCD
- NoWrite the functional specification first and proceed top-down
We help with your project
Tiza Group does IEC 61850 engineering for high and extra-high voltage substations and for industry as part of our secondary engineering: specification, SCD, documented handover and test plan. Multi-vendor IED configuration is done through remote relay configuration. If you inherited a substation without an up-to-date SCD, we rebuild it. Contact us.
Related guides
- IEC 61850 SCL files: ICD, IID, SSD, SCD, CID and SED
- IEC 61850: what it is, parts of the standard and data model
- IEC 61850 GOOSE
- IEC 61850 digital substation
- Standards for substation communications and synchronization
- Configuration management: settings and SCD versions
- Substation signal list for SCADA
- Substation IP addressing and VLAN plan
- Substation communications by voltage level
- Migrating from 103, 101 or Modbus to IEC 61850: how to plan it
References
- IEC 61850-6:2009 + Amd 1:2018 + Amd 2:2024, Configuration description language for communication in electrical substations related to IEDs.
- IEC 61850-4, System and project management.
- IEC 61850-7-4, Compatible logical node classes and data object classes.
Frequently asked questions
What is top-down engineering in IEC 61850?
It is the workflow that starts from the system specification: the substation single-line diagram and the functions each bay needs, described in a vendor-independent SSD file. From there the IEDs are chosen, integrated into the SCD with the system configurator, and the configuration of each device is generated.
Why are most projects done bottom-up?
Because many tools and specifications do not produce a complete SSD, IEDs are bought before system engineering is closed, and integrators work with each manufacturer's templates. The result works, but the SCD describes what was configured and not always what was specified.
Which files should the integrator hand over to the substation owner?
At least the final SCD matching what is installed, the ICD of each IED type and the CID or IID loaded in each device, together with version records and the export of each IED's configuration from its manufacturer's tool. Without an up-to-date SCD, any extension starts by rebuilding it.