Implementation Guideline

The Implementation guideline for digital interface to instrument transformers using IEC 61850-9-2 is a document that was produced by a group of experts from all major suppliers and a few utilities which participated in the working group “Digital IT” of the UCA International Users Group.

This profile of the standard is commonly referred to as IEC 61850 9-2 LE. It gives additional information on how to implement a digital communication interface to non-conventional instrument transformers according to IEC 61850-9-2 and IEC 60044-7/8.
The purpose of the document is to define a subset of IEC 61850-9-2 that shall support a fast market introduction of this standard. The subset facilitates first implementations, especially in existing products. It further clarifies uncertainties with respect to the interpretation of the standards and/or to precisely define what options to choose in case the standards permit a choice of options.

The document defines all the generally binding issues and is intended to be used together with Protocol Implementation Documents (PID’s) that are established for all equipment supporting this digital interface. The PID’s of the individual equipment then define what functions a unit supports and how it reacts to error conditions.

The purpose of the definitions in the document is to ensure that equipment designed accordingly is interoperable within the limits defined in the PID documents of the connected units.
The subset of IEC 61850-9-2 defined in this document supports the service SendMSVMessage only. As a consequence, the communication is unidirectional from the merging unit to the bay level devices and does not need to support the MMS stack. Therefore, implementation in existing bay level devices is straightforward.
The document further defines a logical device merging unit and a dataset used for the transmission of the sampled values (SV). See Figure 1.

The Physical Layer is recommended as fiber optic transmission system 100Base-FX full duplex with ST connectors is. The only allowed alternate solutions are 100Base-FX with MT-RJ connectors, or electrical transmission using 100Base-TX full duplex with RJ-45 connectors.
IEC 61850 does not specify logical devices. Logical devices may be described in SCL and configured through several services of IEC 61850. To reduce the first implementations to a minimum of required services without losing interoperability, this guideline provides a detailed specification of the logical device merging unit as used within the scope of this guideline.

The merging unit (MU) shall have the capability to accept an external synchronizing signal, so that its sampling can be synchronized both between MUs and to an external time reference.
The synchronization signal shall be a 1PPS optical input. The source for time synchronization shall have an accuracy of ± 1 µsec. If neutral current and / or voltage are not measured, the merging unit has to calculate these values as a sum of the phase values. However, since the receiver needs to know, if the values are calculated or measured, the merging unit has to indicate that in the respective quality field.

No specifications are made with regard to physical devices (IEDs). An IED may consist of more than one logical device merging unit sharing the same communication interface.

Two multicast sampled value control blocks "MSVCB01" and "MSVCB02" are defined and configured. MSVCB01 has 1 ASDU and sampling rate 80, while MSVCB02 has 8 ASDU and sampling rate 256. The sampling rates are in samples/cycle at the nominal system frequency.

Five appendixes that provide details of the implementation guidelines are included at the end of the document:
Appendix A: Content of an Ethernet frame
Appendix B: The XML file included is an .icd file for a merging unit with 80 samples per period.
Appendix C: Tables with measurement accuracy requirements
Appendix D: Nomograms for checking of dynamic ranges
Appendix E: Reference arrow orientation system

It can be downloaded from the UCA International Users Group web site at :



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