COSEM Object Model (IEC 62056) carried with IEC 61850 Data Model

IEC TC 57 has just published a very interesting new draft (57/1521/CD):

IEC 61850-80-4 TS:
Communication networks and systems for power utility automation - Part 80-4:
Translation from COSEM object model (IEC 62056) to the IEC 61850 data model

The IEC 61850 is already THE international Standard series when it comes to (electric) power applications and information modeling and exchange. Metering Information is quite often exchanged with DLMS and COSEM:

COSEM (Companion Specification for Energy Metering) is part of the DLMS (Device Language Message Specification)

This information needs to be “fed” into the IEC 61850 world. That is done by defining how the corresponding COSEM information can be wrapped with IEC 61850 Logical Nodes and Data Objects. The title says: Translated … which means the same.

Here is an example:

IEC 61850 Data	COSEM OBIS Code	Explanation
TotVAh	(1-b:9.8.0.255) – (1-b:10.8.0.255)	Net apparent energy
TotWh	1-b:1.8.0.255 - 1-b:2.8.0.255 (¦+A¦ - ¦-A¦)	Net real energy
TotVArh	1-b:3.8.0.255 - 1-b:4.8.0.255 (¦+R¦ - ¦-R¦)	Net reactive energy
SupWh	1-b:1.8.0.255 (+A)	Real energy supply (default supply direction: energy flow towards busbar and is equivalent to Energy Export[+])
SupVArh	1-b:3.8.0.255 (+R)	Reactive energy supply (default supply direction: energy flow towards busbar and is equivalent to Energy Export[+])
DmdWh	1-b:2.8.0.255 (-A)	Real energy demand (default demand direction: energy flow from busbar away and is equivalent to Energy Import[-])
DmdVArh	1-b:4.8.0.255 (-R)	Reactive energy demand (default demand direction: energy flow from busbar away and is equivalent to Energy Import[-])

These IEC 61850 Logical Nodes are of interest for the translation:

MMTR - Metering 3 Phase
MMTN - Metering Single Phase
MMXU - Measurement
MMXN - Non-phase-related measurement
MMDC - DC measurement
MSQI - Sequence and imbalance
MHAN - Non-phase-related harmonics or interharmonics
MHAI - Harmonics or interharmonics
MFLK - Flicker measurement

Closing date for comments: 2015-02-20

Mapping between DLMS/COSEM and IEC 61850

A first draft has been written describing the mapping between DLMS/COSEM and IEC 61850:

80-4: Mapping between the DLMS/COSEM (IEC 62056) data models and the IEC 61850 data models

It defines a one-to-one relationship between IEC 62056 OBIS codes and IEC 61850 Logical Nodes and Data Objects.

New Work Item Proposal – Mapping between DLMS/COSEM (IEC 62056) data models and IEC 61850 data models

It was just a matter of time before an official work item was suggested to look into some kind of “co-operation” of information models developed for metering data (DLMS/COSEM, IEC 62056) and power system automation (IEC 61850).

Here is the proposal: IEC TC 57/1276/NP with the title:

“Mapping between the DLMS/COSEM (IEC 62056) data models and the IEC 61850 data models”

The ballot closes 2012-11-23.

Data derived from current and voltage and other sensors play an important role in the electric delivery system: on all voltage levels and all over!

With more needs for monitoring and controlling the system, substation automation functionality will be needed at any voltage level. It would be an advantage if automation functions could tap information provided by revenue meters.

A crucial objective is to find out and document which data objects can be used from the revenue meter and how these data objects map into the IEC 61850 information model.

This activity shows that IEC 61850 (originally positioned at medium and high voltage substations) is a kind of a core technology for more and more applications throughout the electric power delivery system. It has the potential to uniquely bridge the gap between islands of information found in systems providing electric power (utilities, factories, power plants, …).

Models like the IEC 61850 MMXU are universally applicable to any voltage level of an A.C. electrical system.

Comparison of Message Exchange DLMS/COSEM, SML and IEC 61850

The paper “Comparison of the Communication Protocols DLMS/COSEM, SML and IEC 61850 for Smart Metering Applications” analyzes these protocols … especially to figure out the message lengths. The application the authors have in mind is for metering application. Independent of the application it is interesting what they figured out.

The major result is summarized as “In this paper the most significant qualitative features of a smart metering application layer protocol have been identified. The comparison of DLMS/COSEM, SML, and IEC 61850 has shown that no single protocol is superior in all aspects. The analysis and comparison of the message size has shown that DLMS and the MMS IEC 61850 clearly outperform the rest. …”

The message length of the protocols is one aspect. The more crucial impact on the needed resources is determined by the system behavior:

• Is TLS used? TLS adds a lot of bytes ... especially for the hand-shake for opening a secure connection. By the way, IEC 61850 has security measures: defined in IEC 62351-4. Encryption eats a lot of the computing resources! 
• Is cyclic sending of the payload supported issued by the server (cyclic reports in IEC 61850 do not need a request message)?
• Frequency to exchange the metered values. How often are the metered values read? Will the TCP connection closed/resumed between two transmissions? In case of permanent TCP connections, there may be more bandwidth consumed for TCP keep-alive than for the payload.

The paper can be downloaded [PDF, 200 KB].

It is recommended to focus on the system – message encoding should have a lower priority when building future power automation systems!