Page 860 - Industrial Power Engineering and Applications Handbook

251814 Industrial Power Engineering and Applications Handbook
quickly on the smallest fault and prevent the occurrence heating of the dielectric within the shell and endanger
of a major one. The operation is extremely fast as all the safety. To avoid explosion of the shell it is common
parallel elements, which are fully charged when the unit practice to provide pressure relief bellows in the unit.
is in operation, discharge into the faulty element. The These will expand under such conditions to relieve
operation of the capacitor unit is restored within milli- excessive inside pressure, on the one hand, and isolate
seconds. There is no necessity for shell protection in the supply to the unit, on the other, by snap-breaking the
internally protected capacitor units whereas for externally capacitor’s internal connecting wire between the capacitor
protected capacitor units, fuses are required to be closely terminals and its elements, as shown in Figure 25.3.
co-ordinated with the shell rupture characteristics, as The metallized film capacitors have the characteristic
discussed in Section 26.1.1(6). However, internally of self-healing. On a small dielectric failure the capacitor
protected capacitors have some disadvantages too. For element is not rendered completely unserviceable. After
example, when they are used on a power distribution clearing the fault, the affected capacitor element returns
network and mounted on poles they are exposed to external to the circuit and the capacitor unit functions normally.
direct lightning strikes. Similarly, line-switched capacitors Only the punctured area is eliminated from the element
are subject to internal switching surges and inrush currents. and causes a negligibly small reduction in its capacitance
It is possible that all these surges will fuse many, if not value. Such a characteristic is termed ‘self-healing’ and
all. the internal fuses in tandem of such capacitors because such capacitors are known as the self-healing type.
the fuses are of very small ratings and have a low transient The capacitor element in this case is made up of
withstand capability (1*r). Such a fusing may render the extremely thin metallic films. When a dielectric failure
whole unit unusable while the dielectric properties may occurs in any of the elements, the current passes through
be unaffected. It is also not easy to replace or closely the film. The film, being too thin to sustain the current,
monitor the health of the capacitors mounted on poles. fuses only at the point of dielectric puncture clearing the
With external fuses this disadvantage is overcome fault quickly. The external fuses remain intact, and so
automatically as a blown fuse in both events can be easily remains the affected element in service. Some
detected and replaced. manufacturers claim that 10 000 such failures and healings
Nevertheless, unbalance is a major occurrence in either may reduce the rating of the element or the unit made up
case and an unbalanced protection, in addition to short- of such elements by barely 1%.
circuit protection is imperative in all types of capacitor
units. Hence it is also possible to design series protected
capacitor units in larger sizes and for higher voltages, 25.4 Making a capacitor unit from
which are more economical, compared to smaller units. elements
Whether to use internal or external protection has For low-voltage applications these elements may be made
become a matter of debate due to the different practices up to 1 kVAr or so each and designed up to 440 V,
adopted by manufacturers. Users must decide on the depending upon the system requirement, while for high-
protection that best meets their requirements. voltage applications, they may be designed up to 2-3
kV, each rated for 20-100 kVAr. These values are only
Important indicative and may vary from one manufacturer to another
depending upon the dielectric used, the process adopted
For loads, such as on motors where the capacitor unit is and the site requirements.
being switched with the machine, or where close When larger units are required, a number of such
monitoring of the capacitor health is a prerequisite to elements with appropriate voltage and kVAr ratings, may
avoid an eventual outage of the capacitor unit by gradual be connected in parallel, the size of which will depend
depletion of its capacitance, internally protected capacitor upon the requirement and the economic size of the
units may be preferred. container. For an LT system, each unit may be made up
to 50 kVAr, while a more realistic size is found up to 25
25.3 Self-healing capacitors kVAr in steps of 1,2,3,4,5, IO, 15,20 and 25 kVAr. For
HT capacitors, they may be made in larger units, such as
50,56, 84, 100, 150 and 166.7 kVAr. For higher voltages,
The failure of a capacitor element due to a change in the the elements may be connected in series groups say,
system parameters, such as fluctuations in voltage, switch- three or four series groups, to make them suitable for a
ing operations or the presence of harmonic generating 6.6 kV system and four to six elements up to a 15 kV
sources on the system is quite common. The element system. For yet higher voltages, the capacitor units may
may also fail as a result of internal defects in the element be connected in series to achieve almost any operating
itself or its dielectric quality. The dielectric film coating, voltage. These series elements may be connected in parallel
being extremely thin (5-10 ps), may break down quickly to achieve larger ratings as may be required. Figures
under such unfavourable operating conditions and cause 25.4(a) and (b) illustrate some arrangements.
a failure of one or more elements. At excessive fault
currents, however, it may cause a blow up of the external
fuses, rendering the whole unit unserviceable and causing 25.4.1 Restriction in series-parallel combinations
an extra voltage across the healthy units connected in the of capacitor elements
same parallel group of capacitor banks. Such an occurrence While making a capacitor unit, the same precautions
is rare but it may generate excessive pressure due to would be mandatory as discussed in Section 25.5.1 for