SCL Files: ICD, SCD and CID Demystified
A junior engineer joins a UK digital substation project on a Monday morning, opens the project folder, and finds a bewildering collection of files: .icd, .scd, .cid, plus a few XML variants nobody has explained yet. The senior engineer says “just load the CID into the relay” and walks off to a meeting. Sound familiar? If you have ever stared at those three-letter extensions and wondered which one does what, you are not alone. We have seen this exact scenario play out on projects up and down the country, from National Grid’s digital substation programme to local DNO upgrades. This article breaks down what each SCL file type actually is, when it is used, and why getting them right matters for substation automation and smart grid interoperability.
What Are SCL Files and Why They Matter in IEC 61850?
SCL stands for Substation Configuration Language. It is an XML-based format defined in IEC 61850-6, the part of the standard that governs how we describe the configuration of intelligent electronic devices (IEDs) and their communication systems in substation automation. Think of SCL as the shared language that lets a relay from one vendor talk to a merging unit from another, and lets our engineering tools understand both.
In the UK, SCL files are not optional extras. Transmission operators like National Grid and distribution network operators such as UK Power Networks and SSEN rely on them to guarantee interoperability across multi-vendor substations. Without a consistent SCL approach, every project becomes a bespoke integration exercise, and nobody wants that.
The Role of XML and IEC 61850-6
SCL uses XML because XML is structured, human-readable and machine-parsable. IEC 61850-6 defines the schema, so a valid SCL file must conform to strict rules about logical nodes, data objects, communication parameters and namespaces. This is why a file that opens in one vendor’s configuration tool may be rejected by another if it has been hand-edited badly. We always recommend treating SCL as structured engineering data, not as a text file to hack.
How SCL Supports Smart Grid Interoperability
Smart grid functions depend on devices sharing data reliably. SCL captures the data model, the communication configuration and the GOOSE publishing and subscribing relationships in one place. When a new bay is added or a protection scheme is changed, the SCL file is where that change is expressed. That is what makes interoperability achievable rather than aspirational.
ICD Files: The IED Capability Description
An ICD file is the IED Capability Description. It is supplied by the manufacturer and describes what a specific device can do: which logical nodes it supports, which data objects it exposes, and what GOOSE messages it can publish. Major IED vendors whose ICD files appear routinely in UK projects include ABB, Siemens, GE and Schneider Electric. Each ICD is device-specific, so a Siemens relay ICD will not match an ABB relay ICD, even if they serve similar functions.
What an ICD Contains: Logical Nodes and Data Sets
Inside an ICD you will find the device’s logical nodes (for example, PDIS for distance protection or XCBR for circuit breaker control), the data objects under each node, the data sets used for reporting and GOOSE, and the control blocks that manage them. It is essentially the vendor’s declaration of capability, frozen at a point in time.
Using ICDs in Early Design Stages
During the design phase of a digital substation, we use ICDs to understand what each device can deliver before any real configuration exists. This informs signal lists, protection philosophies and network loading calculations. If the ICD says a device cannot publish a particular GOOSE message, we need to know that early, not during commissioning.
SCD Files: The Substation Configuration Description
The SCD file is the Substation Configuration Description. It represents the whole substation: multiple IEDs combined, with communication parameters added, GOOSE control blocks mapped, and network topology described. System integrators working on National Grid’s digital substation projects treat the SCD as the single source of truth for IEC 61850 engineering. Everything downstream is derived from it.
Building an SCD from Multiple ICDs
Building an SCD means importing the relevant ICDs into a system configuration tool, assigning each device an instantiated name and IP address, and defining the communication relationships between them. It is a project-wide activity and requires coordination between protection, control, telecoms and the vendor teams.
GOOSE and MMS Configuration in SCD
GOOSE (Generic Object Oriented Substation Event) is the peer-to-peer messaging used for fast protection signalling. MMS handles client-server communication for SCADA and engineering access. Both are configured within the SCD, including the datasets, control blocks, VLAN priorities and subscription mappings. Getting GOOSE wrong can mean a protection scheme that looks fine on paper but fails to trip in practice.
Version Control and Change Management
An SCD that is edited by three people without version control is a recipe for disaster. We insist on a central repository, a clear revision numbering scheme, and a documented approval process for every change. Treat the SCD like source code, because functionally, that is what it is.
CID Files: The Configured IED Description
The CID file is the Configured IED Description: the final, device-specific configuration exported from the SCD and loaded into each IED. It contains all the instantiated data, communication addresses, GOOSE subscriptions and settings the device needs to operate on the network. UK commissioning teams rely on CIDs during Factory Acceptance Testing (FAT) and Site Acceptance Testing (SAT).
From SCD to CID: The Engineering Workflow
The workflow is straightforward in principle: vendor provides ICD → integrator builds SCD → integrator exports a CID per device → CID is loaded into the IED. In practice, changes ripple. If the SCD is updated, affected CIDs must be regenerated and reloaded. We keep a matrix mapping each IED to its current CID revision.
CIDs in Commissioning and Testing
During FAT, we verify the CID matches the approved SCD and that GOOSE publications and subscriptions behave as expected. During SAT, we confirm the same on site, often using network analysers to capture GOOSE traffic. A mismatch between CID revision and SCD revision is one of the most common causes of commissioning delays.
ICD vs SCD vs CID: Key Differences and When to Use Each
Here is the short version we give to every new starter:
- ICD — vendor-supplied, device capability, used in design. Ownership: manufacturer.
- SCD — project-wide, whole-substation configuration, used as the engineering master. Ownership: system integrator.
- CID — device-specific, exported from the SCD, loaded into the IED. Ownership: commissioning/integration team.
UK DNOs including SP Energy Networks and SSEN specify SCL file requirements in their IEC 61850 standards, so it is worth reading those documents before starting any project.
File Scope and Ownership
Scope drives ownership. ICDs belong to the vendor and should not be edited. SCDs belong to the project and must be controlled centrally. CIDs belong to the device and must be traceable back to a specific SCD revision.
Common Pitfalls in SCL Management
The usual suspects: editing ICDs locally, losing track of SCD revisions, mismatched CID versions, inconsistent naming conventions, and hand-editing XML without validation. Each of these has caused real commissioning delays on UK projects we have been involved with.
Best Practices for Managing SCL Files in UK Substation Projects
If we could give only a handful of rules, they would be these:
- Maintain a single, central SCD repository with strict access control.
- Enforce naming conventions for IEDs, logical devices and datasets from day one.
- Validate every SCL file with proper tools before it leaves the office.
- Involve vendors early so ICD limitations are known before design freeze.
- Log every change with a reason, an approver and a revision number.
The UK’s transition to digital substations, driven by the Smart Grid Forum and ENA Engineering Recommendations such as ER G100 for smart grid connections, makes SCL proficiency essential rather than optional. G100 in particular pushes more intelligent, communicating devices onto the network, which means more SCL files to manage.
Tools and Validation
Helinks and Triangle MicroWorks are the SCL validation tools we see most often used by UK system integrators. They catch schema errors, unresolved references and GOOSE configuration mistakes before they reach a relay. Do not skip validation.
Collaboration Between Utilities and Vendors
Good SCL outcomes come from early, structured collaboration. Utilities define requirements, vendors supply accurate ICDs, integrators build and maintain the SCD, and commissioning teams verify CIDs. When one party works in isolation, the file set drifts and the project pays for it.
FAQ
What is the difference between an ICD and a CID?
An ICD describes what an IED can do and is supplied by the manufacturer. A CID describes how a specific IED is configured for a specific project and is exported from the SCD for loading into the device.
Can I edit an ICD file?
No. ICDs are vendor-controlled capability descriptions. If something is missing from an ICD, raise it with the vendor rather than editing the file locally, as any changes will be lost on the next firmware or tool update.
How often should the SCD be revised?
Whenever the substation configuration changes: new bays, changed GOOSE mappings, updated IP addressing, firmware-related model changes. Each revision should be numbered, approved and communicated to everyone holding a downstream CID.
Which tools validate SCL files in the UK?
Helinks and Triangle MicroWorks are widely used by UK system integrators. Most IED vendor configuration tools also include some level of SCL checking, but dedicated validators are more thorough.
Are SCL files required for ER G100 compliance?
ENA Engineering Recommendation G100 addresses smart grid connections and their communication and control requirements. Where IEC 61850 devices are used to meet those requirements, SCL files are the mechanism by which the configuration is defined and maintained.
Mastering ICD, SCD and CID files is not a niche skill reserved for a few specialists; it is a core competency for anyone working on IEC 61850-based substation automation and smart grid projects in the UK. The SCD is not a document you produce once and file away — it is a living engineering artefact that evolves with the substation. Treat it that way, and the rest of your SCL workflow will follow.
