Iec 61850-9-2Le Merging Unit Requirements - ABB RELION 670 Series Applications Manual

1MRK 511 364-UUS A
IED with GPS clock
• Independent of asymmetry.
22.5

IEC 61850-9-2LE Merging unit requirements

The merging units that supply the IED with measured values via the process bus must fulfill the
IEC61850-9-2LE standard.
This part of the IEC61850 is specifying "Communication Service Mapping (SCSM) – Sampled values
over ISO/IEC 8802", in other words – sampled data over Ethernet. The 9-2 part of the IEC61850
protocol uses also definitions from 7-2, "Basic communication structure for substation and feeder
equipment – Abstract communication service interface (ACSI)". The set of functionality
implemented in the IED (IEC61850-9-2LE) is a subset of the IEC61850-9-2. For example the IED
covers the client part of the standard, not the server part.
The standard does not define the sample rate for data, but in the UCA users group
recommendations there are indicated sample rates that are adopted, by consensus, in the
industry.
There are two sample rates defined: 80 samples/cycle (4000 samples/sec. at 50Hz or 4800
samples/sec. at 60 Hz) for a merging unit "type1" and 256 samples/cycle for a merging unit
"type2". The IED can receive data rates of 80 samples/cycle.
Note that the IEC 61850-9-2 LE standard does not specify the quality of the sampled values, only
the transportation. Thus, the accuracy of the current and voltage inputs to the merging unit and
the inaccuracy added by the merging unit must be coordinated with the requirement for actual
type of protection function.
Factors influencing the accuracy of the sampled values from the merging unit are for example anti
aliasing filters, frequency range, step response, truncating, A/D conversion inaccuracy, time
tagging accuracy etc.
In principle the accuracy of the current and voltage transformers, together with the merging unit,
shall have the same quality as direct input of currents and voltages.
Application manual
Section 22
Requirements
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