Page 858 - Industrial Power Engineering and Applications Handbook

25/812 Industrial Power Engineering and Applications Handbook

iapacitorc;_l$
elements

(a) Use of expulsion type external fuses (b) Use of internal fuses (c) Finished capacitor units
(For self-healing type (MPP) capacitor units (For non-healing type (MO or PP)
capacitor units)
Figure 25.2 Arrangement of capacitor elements for making a capacitor unit (Courtesy: Savin Capacitors)

Such capacitor units require careful meticulous checks protected capacitor units, while the capacity of the unit
by measuring the capacitance of the unit every six months may have deteriorated below acceptable limits over a
or even less, depending upon the duty and the switching period of time. It is also possible that failure of a few
operations it is performing. Repair is not possible at site elements on a minor fault may lead to a cascade failure
as it requires a re-impregnation under vacuum and hermetic of the healthy elements connected in parallel, due to an
sealing once again which is possible only at the manu- overvoltage across them (Figure 25.3). This is a feature
facturer’s works. Since it may not be possible to exercise that is almost absent in series-protected units (see also
close monitoring of the health* of the capacitor unit Section 25.4.2). But self-healing types of capacitors have
when it is in operation it is possible that there have occurred
progressive isolation of elements after blowing off the During a severe fault
internal fuses. This may cause a reduction in the capacity bellows expand and it
of the unit and hence an undercompensation. The internal J snap opens
fuses cannot be replaced at site, being installed inside a Bellow
sealed unit.
different types of dielectrics in use for impregnation and A - 4
Be low
We provide in Table 25.1 a brief comparison of the
insulation while forming capacitor elements. The table I
will also help make a proper choice of the dielectric and
the type of capacitors for a particular application.

25.2 A critical review of internally
protected capacitor units

While it is true that in the absence of external fuses it is
not possible to assess the health of a unit, it is equally true
that an intact external fuse is also no guarantee of a healthy
unit. In fact, internal fuses isolate only small elements
and the capacitance of the unit remains almost intact over
long periods. Periodic checks, say, once every six months,
are important to assess thd. health of the unit and this is
applicable to all types of capacitor units. Generally, external
fuses show a fault between the line and the shell only.
They stay immune to mild immense faults occurring inside
the shell. Minor faults therefore do not show up in externally i_ Elements _I
in parallel
*with the availability of microprocessor-based p.f. correction relays * Failure of one element will cause an ovetvoltage across the others in
(Section 23.17) which measure the health of a unit while performing parallel and may lead to a cascade failure.
switching operations, and indicate this through a display, blinker or
alarm, it is now possible to monitor the health of a unit while in Figure 25.3 Cascade failure of healthy elements in the
operation and to take prompt preventive or corrective measures. absence of an internal protection