Selection Of The Current Transformers - ABB PQFS Installation, Operation And Maintenance Instructions

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 PQFS standard accuracy CTs with the following
minimum specifications have to be used:
It is strongly recommended that the three CTs have the same characteristics.
WARNING: The connection of different filter units in a PQFS system, as well as
other loads, on the same CT must be in series.
In order to determine the suitable CTs for your application, please refer to the chart in
Figure
Remark: in some applications two or more power supplies exist (e.g. a network
transformer supply and a generator supply). When the current into both supplies has to
be filtered, summing CTs have to be used. All summing CTs must have the same ratio.
More information on how to install the summing CTs is given in next section.
shows the admitted 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
Figure 38: Incorrect connection
in the case of 4Q-loads
· 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 Figure
VA burden shall be adapted accordingly. Note that the burden requirement for a
complete filter system (consisting of up to 4 filter units) is 5 VA, excluding
connecting cables.
· Class 1 accuracy
· Primary side current rating sufficient to monitor the total line current (including
transient phenomena such as drive/motor starts ...)
40.
4Q
load
40. In case the CTs are shared with other loads, the
Manual Power Quality Filter PQFS ç Electrical design and installation 47
4Q
PQF
load
Figure 39: Correct connection
in the case of 4Q-loads