56 Electrical design and installation
Manual Power Quality Filter PQFM
WARNING:
CE versions of filter system with common cable entry have to be
equipped with the optional fuse box in each panel.
Remarks:
−
In case of regenerative loads (e.g. loads that may inject active energy to the
network, usually called 4Q-loads), it is very important to connect the PQF outside
the protection of this load. Indeed, consider
where a common
protection is installed for both the regenerative load and for the PQF. When the
load re-injects energy to the network and the mains protection trips, the whole
energy may be pushed into the PQF, which may become severely damaged.
shows the correct protection scheme for regenerative loads. In this case, if the
breaker of the load trips, the PQF is isolated from the energy fed back by the drive.
PQF
4Q
load
Figure 33: Incorrect connection in the case of
4Q-loads
PQF
4Q
load
Figure 34: Correct connection in the case of
4Q-loads
6.9 Selection of the current transformers
Each filter unit in a filter system has to monitor the line current in order to determine the
harmonic load and function correctly. This is done by three current transformers (CTs).
For proper operation of the PQFM standard accuracy CTs with the following minimum
specifications have to be used:
−
5 A secondary current rating.
−
15 VA burden for up to 30 meters of 2.5 mm² cable. For longer cables lengths refer
to the chart in
.
In case the CTs are shared with other loads, the VA
burden shall be adapted accordingly. Note that the burden requirement for a
complete filter system (consisting of up to 8 filter units) is 5 VA, excluding
connecting cables.
−
Class 1 accuracy or better.
−
Primary side current rating sufficient to monitor the total line current (including
transient phenomena such as drive/motor starts …).
It is strongly recommended that the three CTs have the same characteristics.
WARNING:
The connection of different filter units in a PQFM system, as well as
other loads, on the same CT must be in series.
