Friday, July 5, 2019

IEC 61850-8-2 Versus IEC 61850-8-1

Many people have complained that IEC 61850 is far too complex ... especially because of the mapping defined in IEC 61850-8-1 as the SCSM (Specific Communication Service Mapping) using ISO 9506 MMS as the carrier to exchange IEC 61850 client/server messages. MMS (Manufacturing Message Specification) offers generic objects (NamedVariables, NamedVariableLists, ...) and services (Read, Write, InformationReport, ...). The application of MMS for IEC 61850 requires to define very tricky mappings ...

Click HERE for downloading the FDIS of ISO 9506-1.
Click HERE for downloading the FDIS of ISO 9506-2.

As the convenor of ISO TC 184/SC WG2 (responsible for MMS for many years) I have supported MMS allover ... and I still do it. It is a solution that works well for years ... especially in the domain of substation automation. Other application domains, e.g., DER devices connected directly to a control center (without the need of horizontal communication with up to hundreds of devices), could make use of a lighter message service and mapping concept. The idea of using webservices was discussed many years ago. The IEC TC 57 set up a team to look into it.

The following document written in 2012 discusses the:

IEC 61850-8-2 Web Services Justification

"IEC 61850/MMS is an open scalable suite of protocols that can support real-time operation. However, these protocols are not well understood by typical IT professionals that work in these stakeholder environments, even though the rich information models of IEC 61850 meets the semantic needs of the distributed applications. ...
Therefore it makes economic sense to map the rich abstract IEC 61850 information models to the more ubiquitously deployed and understood communication and security profiles.   This requirement can be satisfied by mapping IEC 61850 to web services, which are the most commonly implemented technologies.
IEC 61850/Web Services is not a replacement for MMS used in the substation.  Rather, IEC 61850/Web Services would be targeted to customer environments where information is used to interact with customer-owned equipment, including Distributed Energy Resources (DER) systems and facility energy management systems. In these situations, information flows between utility systems, customer systems, and third party systems, straddling utility and customer ownership, sometimes within a single application deployment.
The stakeholders deploying DER systems understand that IEC 61850 over web services provide least cost protocols to interact with their DER devices because:
Web Services provide greater compatibility with widely deployed IT infrastructures, tools and skills, including cyber security.

Finally the work ended in the following standard:

Communication networks and systems for power utility automation – Part 8-2: Specific communication service mapping (SCSM) – Mapping to Extensible Messaging Presence Protocol (XMPP)

The published document 8-2 uses principles of IEC 61850-8-1 as well as MMS services and protocols! Really? Yes. The main difference regarding message exchange is in the encoding of the MMS messages: 8-1 uses ASN.1 BER and 8-2 uses XML messages (with the structure of MMS services defined as XML Schema). Strange? Yes!

You can find the XML schema for the MMS messages using XML encoding here:

Code component of the IEC 61850-8-2, reflecting the XML namespace described in this document. It includes as well the virtual API with IEC 62351-4.

Summary on the protocol issues: IEC 61850-8-2 does NOT provide a direct mapping of IEC 61850-7-2 ACSI models and services to webservices!

I am involved in defining a third mapping (of a subset of services) ... to use JSON schema (and objects) for the models and the messages ... in order to offer really light weight messaging carrying a subset the original semantic of IEC 61850 models!!

We have successfully implemented client and server using http Get and Set services that carry JSON objects representing IEC 61850 Objects.

Example of DataSet and Report (showing the basic idea of mapping to JSON):

The decision to propose the JSON mapping officially to IEC TC 57 is expected later in 2019.

By the way, our granddaughter (20, Student in EE and IT) has implemented this example in Python running on Windows and on two Raspberry Pi3 (one playing the role of server and the other one as client).

We expect that this mapping will push the application of IEC 61850 models and services in domains that need simple solutions that can be programmed by many engineers and programmers.

The JSON mapping could easily be used as an interface from an original IEC 61850 Server (e.g., a bay controller or protection device) to underlying systems:

Sample JSON response from our Fronius PV Inverter with some values:


Response (JavaScript):

Stay tuned to learn more ...

No comments: