Top down engineering with SCL

Owned by Davood Sooran (Deactivated)
Sept 11, 2024
5 min read

1. General remarks

There are many definitions on what top-down means. One is describing it with Put information into SCL as soon as possible within your processes. This definition of top down is aiming to make decisions early in the process and reuse these from there on, and by doing so to decrease the amount of duplicate decisions being made along the way and increase the efficiency of the overall process. It has then also the nice side effect of having a lot - ideally all information - in one file.

The benefits of top down using SCL is depending on the usage of IEC 61850 in the grid. If you want to use a lot of services including sampled values and build digital substations with IEC 61850, the use of SCL make a lot of sense. In fact, it then becomes difficult not use top down. If you are using only dynamic reporting communicating between field level and station level devices, top down could be too complicated compared to the benefits.


To better compare the various suggestions for SCL-workflows, four basic process steps are introduced with specific outcome that can vary from workflow to workflow.


2. Typical specification

The earliest process you could engage with SCL and formalize decisions in a SCL is a typical specification.

This process step is the preparation step for a tender and can be reused over multiple tenders. Here you can

  • define your functions amount and functions structure

    • e.g. decide to have a measurement function including three phases and the neutral phase

    • e.g. define that your overcurrent protection PTOC should be configurable though IEC 61850 by adding the configuration inputs to the function

    • …

  • group functions to function schemes

    • e.g. YOUR distance protection scheme has 7 zones (seven PDIS in distance protection function) and an auto recloser function

  • allocate functions or functions schemes to substation elements

    • e.g. define that your bay has three types of protections schemes. One overcurrent, one under- and one over frequency

    • e.g. you want to say that YOUR circuit breaker contains a control function CSWI and an interlocking function CILO and has point on wave switching

  • design a virtual IED

    • allocate logical nodes to an IED that is virtual and define the structure of this IED (logical devices)

    • define what services this IED must support


All of the above is a serious time investment. We are speaking about years not weeks. The reason is the many and many decisions that are moving to the SCL. Every single one is a little agreement between all concerned parties in the entity, say Alliander.

The potential advantages however are massive!!!

  • Reusable: Most of the above is long-lasting or in other words they do not change and are reusable in the next tenders.

  • Standardized: All of it formalized in an open and supported standard. Vendors can read and interpret the file. There is little need for a word document expressing the rest of the requirements that cannot be added to the SCL.

  • Automated: The vendor and the user can add automated checks on exchanging files. E.g. Alliander could fully automatically check back whether the proposed IEDs meet the requirements (Services and data structure).


Cigré B5.68 Survey results

As part of Cigré B5.68 a survey was sent out to multiple users, to ask them about their vision about digital substation engineering. Following questions were asked:

      • Question 1: What are the root causes blocking deployment of IEC 61850 top down engineering process today?

      • Question 2: Today, what are the kind of documents used all along the engineering process of an IEC 61850 system?

      • Question 3: Do your company expect to use improved IEC 61850 engineering process (with specification, a.k.a. 90-30 and 7-6) also to express their specification in machine-readable format?

      • Question 4: What is the current process to deploy IEC 61850 systems with legacy systems?

      • Question 5: How your teams are organized to execute engineering of systems? number of people, responsibilities of different teams, interactions between them, …

Benchmarking results

In 2021 meetings with other utilities (RTE, EDF, Hydro Quebec, Amprion, SPEN, Red Electrica) were organized, in order to get feedback and insights on the decisions they made and the solutions they apply. Please find below most important conclusions: -

      • There is a market evolution towards top-down engineering with machine-readable IED specifications -

      • The more the data-modeling and IED allocation is specified and mandatory, the more harmonized and automated the engineering process can be. But the offer from the market becomes smaller and some large suppliers will not compete. -

      • Most utilities applying top-down engineering use private developed tools, tailored to their specific needs and processes -

      • No good examples of automated digital substation documentation were found, understanding the available documentation required: o

        • Deep knowledge of IEC61850 (reading tabular formats) o

        • Large manual effort to draw user-friendly low level representations of digital communication

3. Tendering

In this step the utility communicates with the vendors mainly though the exchange of SCL files. Both the vendor and the utility can use the fact that SCL files are standardized and check requirements or the fulfillment of requirements in an automated process. The outcome is so-called IED specification description files that are used in the later system configuration step.

4. System specification

This process starts when designing a real substation. This has in rough shape and form the following two steps:

  • Drawing of the single line diagram and defining the substation structure

  • Defining the substation purpose. E.g. what type of feeder do I want to have and which functionality is needed for this.


The outcome is depending strongly on the shape and form of the atomic parts the utility has been able to standardize. Its outcome could be a System Specification Description *.ssd file or already a partly filled System Configuration Description *.scd file. One without the communication description.

Also, depending on the already existing software solutions and established processes, this and the next step could be combined to one single step.

5. System Configuration

Create System Configuration Description *.scd file that feeds back to the IEDs and configures the IEC 61850 part automatically. Here all the communication related information is being added to the SCL file such as:

  • structure of the communication network

  • communication configuration e.g. IP addresses

  • communication mappings

    • structure, amount and subscriber (user) of GOOSE messages

    • structure, amount and subscriber (user) of Sampled Value messages

    • structure, amount and client (user) of Report messages

Depending on the vendor tools capability, additional configuration settings such as

  • protection settings

  • 104 to IEC 61850 mapping

is added to the SCL.


As an IED, not only edge devices should be considered, but all entities that want to retrieve data online. E.g. a database storing measurement data could be an IED communicating to IEDs in the field.

Control center applications can also be considered as an IED and can communicate over IEC 61850 directly.