Rockwell Automation 1519 MV DRIVE HARMONIC FILTER PF CORRECTION UNIT - Manual (Page 8)

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1-4 Introduction
Protective Features (cont.) Short-circuit Protection for Capacitors
Short-circuit protection for harmonic filter capacitors is addressed by
individual fusing of each capacitor. The intent of the capacitor fusing is to
clear a short-circuit-type fault condition on a failed capacitor. The fuses are
chosen to prevent a capacitor from rupturing. The fuses are not designed to
prevent the capacitor from failing. These fuses are generally chosen at 175%
to 200% of the capacitor rms current rating. The thermal overload described
above prevents operation of the fuse in the overload region.
Blown Capacitor Fuse Detection
Blown capacitor fuse detection is performed by an Allen-Bradley SMP-3 relay.
This relay is also fed by window-type current transformers which are used on
each phase for every capacitor. The phase loss function of this relay is used to
detect a blown capacitor fuse. If a capacitor fuse has blown then there will be
no current flow in that particular phase. It is undesirable for a harmonic filter
to continue to operate with a blown fuse on one or more capacitors since this
significantly alters the tuned frequency of the filter. A contact from the SMP-3
relay is wired to open a dedicated harmonic filter contactor and/or shut down
the associated drive. Note that the SMP-3 relay is not used as an overload relay
in this application since it is designed for a sinusoidal waveform. It determines
the rms value of a current waveform by repetitively sampling to find the peak
of the waveform. It then assumes that the rms current is the peak value divided
by the square root of two. This relationship between peak and rms current is
not correct when there is significant harmonic content in the waveform. For
this reason the DIP switches for overload protection are set substantially above
the current ratings of the filter components to prevent nuisance tripping due to
overload and allow phase loss detection to reliably take place.