Back-To-Back Operation Of Active Line Modules - Siemens SINAMICS S120 System Manual

6.4

Back-to-back operation of Active Line Modules

f Frequency of the power grid
1
f Frequency of the island grid
1
F Line filter of the power grid
1
F Line filter of the island grid
2
Figure 6-6 Back-to-back operation - diagram showing the principle of operation
Back-to-back operation of Active Line Modules designates the connection between two
Active Line Modules in a DC link. In this case, the DC link is supplied from an Active Line
Module (ALM
the AC side, the two Active Line Modules are operated in two separate grids. Typical
applications include the supply of island grids from the public power grid as well as
establishing back-to-back station couplings (e.g. for frequency conversion). Inadmissible
ground connections between the two separate grids (e.g. via capacitively grounded neutral
points) result in overcurrents, instability and overvoltages, which can also damage other grid
components (e.g. power supply units).
Further, it is not permissible that there is any other low ohmic ground connection between
the AC connection of the Active Line Module at the island grid and the primary of an island
grid transformer (e.g. no neutral point grounding). This is also especially true for capacitive
connections of the PE conductor to the grid neutral point. As a consequence, in most
situations, the grounding bar of the Active Interface Module must be removed when used in
an island grid.
A typical configuration with 2 Active Line Modules (first Active Line Module in the supply grid,
DC link, second Active Line Module for forming the island grid) can therefore look like this:
● Line filter of the first Active Line Module (supply grid) with grounding bar in the Active
Interface Module (
● Line filter of the second Active Line Module (island grid) without grounding bar in the
Active Interface Module (
● Further, it is not permissible that there is any other low ohmic ground connection between
the AC connection of the Active Line Module at the island grid and the primary of an
island grid transformer (e.g. no neutral point grounding between the Active Line Module
and the island grid transformer,
● It is permissible to ground the neutral point at the secondary of the island grid transformer
(e.g. neutral point grounding of the transformer, connecting EMC filters with respect to
ground, etc.).
Grid infeed
System Manual, 04/2015, A5E03347436A
), the ALM generates a decoupled grid (regarding voltage and frequency). On
1 2
①
).
②
).
③
Detailed descriptions of the function modules
6.4 Back-to-back operation of Active Line Modules
).
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