Ct Connections For The Case That Plain Capacitors Are Present In The Network - ABB PQFM Instruction Manual

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
the diagram shown in
Figure 48: Recommended connection diagram for PQFM and plain capacitors
Feeding
transformer Filter CTs
Figure 49: Alternative for
When it is not possible to implement the connection diagram shown in
alternative diagram given in
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.
Figure 49.
Feeding
transformer
Filter CTs
Plain capacitor
bank
PQFx
Figure 48 when the connection approach of
implemented (solution to be avoided)
Figure 49 may be used.
Manual Power Quality Filter PQFM ç Electrical design and installation 67
Figure 48 is recommended as opposed to
Linear and
PQFx
non-linear
loads
Linear and
non-linear
Plain capacitor
loads
bank
Figure 48 cannot be
Figure 48 the