ABB Relion 670 Series Applications Manual page 529

1MRK 511 401-UUS F
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.
SDM600
IEC18000011 V1 EN-US
Figure 235: Example of a station configuration with separated process bus and station bus
The IED can get analog values simultaneously from a classical CT or VT and from a Merging Unit,
like in this example:
The merging units (MU) are called so because they can gather analog values from one or more
measuring transformers, sample the data and send the data over process bus to other clients (or
subscribers) in the system. Some merging units are able to get data from classical measuring
transformers, others from non-conventional measuring transducers and yet others can pick up
data from both types. The electronic part of a non-conventional measuring transducer (like a
Rogowski coil or a capacitive divider) can represent a MU by itself as long as it can send sampled
data over process bus.
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Section 20
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