ABB REL670 2.2 IEC, Applications Manual

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21.3 IEC/UCA 61850-9-2LE communication protocol
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21.3.1 Introduction
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Every IED can be provided with communication interfaces enabling it to connect to
the process buses in order to get data from analog data acquisition units close to the
process (primary apparatus), commonly known as Merging Units (MU). The
protocol used in this case is the IEC/UCA 61850-9-2LE communication protocol.
The IEC/UCA 61850-9-2LE standard does not specify the quality of the sampled
values. 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 the 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, will have the same quality as the direct input of currents and
voltages.
The process bus physical layout can be arranged in several ways, described in
Annex B of the standard, depending on what are the needs for sampled data in a
substation.
1MRK 506 369-UEN B
Section 21
Station communication
Line distance protection REL670 2.2 IEC 813
Application manual