software. The SNTP server should be stable, that is, either synchronized from a stable
source like GPS, or local without synchronization. Using a local SNTP server without
synchronization as primary or secondary server in a redundant configuration is not
recommended.
2.4 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 shall the accuracy of the current and voltage transformers, together with
the merging unit, have the same quality as direct input of currents and voltages.
Section 2 1MRK504116-UUS C
Requirements
30
Application manual
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