Manual Power Quality Filter PQFM
Electrical design and installation 67
6.10.7 CT connections for the case that plain capacitors are present in the
network
In some installations plain capacitors (without detuning reactors) coexist with harmonic
producing loads. This situation is not advisable given that the harmonics impose a very
high stress on the capacitors as a result of which their lifetime is greatly reduced.
Moreover, due to the resonance condition created (by the capacitor and the
predominantly inductive transformer and line impedance) high voltage distortion may
be introduced which can cause other equipment in the plant to malfunction. Also a
resonance amplifies the harmonic current created by the loads as a result of which the
feeders and transformers may be overloaded. For these reasons ABB suggests to replace
the plain capacitor by a detuned capacitor bank when harmonics are present in the
network.
In installations where plain capacitors are present and cannot be changed to detuned
capacitor banks, the connection diagram of
is recommended as opposed to
the diagram shown in
Feeding
transformer
Filter CTs
Linear and
non-linear
loads
PQFx
Plain capacitor
bank
Figure 48: Recommended connection diagram for PQFM and plain capacitors
Feeding
transformer Filter CTs
Linear and
non-linear
loads
PQFx
Plain capacitor
bank
Figure 49: Alternative for
when the connection approach of
cannot be
implemented (solution to be avoided)
When it is not possible to implement the connection diagram shown in
the
alternative diagram given in
may be used.
NOTE: In order to be able to operate in installations where plain capacitors are present,
the PQF active filters incorporate a dedicated advanced SDP control algorithm. This
algorithm will, if needed, stop filtering harmonics momentarily to optimize the harmonic
control parameters for the changing network conditions (e.g. due to capacitor bank
switching). As a result, the overall filtering efficiency of the filter may be lower when plain
capacitors are present in the network. Therefore ABB advises for best performance to
always replace plain capacitor banks by appropriately sized detuned capacitor banks in
installations where harmonics are present and where filtering devices are used.
