Selection Of The Current Transformers - ABB PQFM Instruction Manual

WARNING:
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
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.
34 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.
Figure 33: 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 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
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:
other loads, on the same CT must be in series.
56 Electrical design and installation ç Manual Power Quality Filter PQFM
CE versions of filter system with common cable entry have to be
4Q
PQF
load
4Q-loads
Figure 35. In case the CTs are shared with other loads, the VA
The connection of different filter units in a PQFM system, as well as
Figure 33
Figure 34: Correct connection in the case of
4Q-loads
where a common
Figure
4Q
PQF
load