Capacitor Disconnect - Danfoss AHF005 Design Manual

Introduction to Advanced Ha... AHF005/010 Design Guide

4.1.2 Capacitor Disconnect

If the specific application requires a higher power factor at no-
load and the reduction of the capacitive current in stand-by, a
capacitor disconnect should be used. A contactor disconnects
the capacitor at loads below 20 %. It is important to note that
the capacitors may not be connected at full load or disconnec-
ted at no load.
4 4
It is very important to consider the capacitive current in the
design of applications where the harmonic filter is supplied by
a generator. The capacitive current can overexcite the genera-
tor in no-load and low-load condition. The over-excitation
causes an increase of the voltage that can exceed the allowed
voltage for the AHF and the frequency converter. Therefore a
capacitor disconnect should always be used in generator
applications and the design carefully considered.
Compared to multi-pulse rectifiers, passive harmonic filter
(such as AHF) are more robust against background distortion
and supply imbalance. However, the performance of passive
filters is inferior to the performance of active filters when it
comes to partial load performance and power factor. For
details about the performance positioning of the various
harmonic mitigation solutions offered by Danfoss, please
consult the relevant harmonic mitigation literature.
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