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

Figure 30: Incorrect connection in the case of
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:
−
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
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 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 32.
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.
48 Electrical design and installation  Manual Power Quality Filter PQFS
4Q
PQF
load
4Q-loads
Figure 32. In case the CTs are shared with other loads, the VA
Figure 31: Correct connection in the case of
4Q-loads
4Q
PQF
load