Page 857 - Industrial Power Engineering and Applications Handbook

Making capacitor units and ratings of switching devices 25/81 1
25.1 Making a capacitor element Making use of this, some manufacturers have introduced
an improvized version by increasing the thickness of the
dielectric coating of individual elements and impregnating
A capacitor unit is made up of a number of capacitor the entire capacitor unit so formed with a non-oil dielectric,
elements stacked together in combination of series and such as epoxy resin. Epoxy resin is devoid of leakage,
parallel configurations to add up to the required voltage while leakage is possible in an oil dielectric. The unit,
rating and VAr size. A capacitor element can be formed called a double dielectric unit, is now suitable to sustain
with any of the available dielectric materials, as noted
below, depending upon the voltage rating of the capacitor higher voltage fluctuations and the presence of system
harmonics, as required by fluctuating and non-linear loads.
unit, the likely voltage variations, harmonic contents in But provision of an inductor coil is now essential in each
the supply system and switching operations.
capacitor unit to suppress the inrush current. The repair
of such units is simple and can be carried out at site if
25.1.1 Metallized polypropylene (MPP): spare elements are available.
self-healing type
25.1.3 Mixed Dielectric (MD)
Each capacitor element is made up of two layers of thin
PP dielectric films, 5-10 pm thick. Each film is metallized This is the non-healing type, and is normally provided
with an Al, Zn-A1 or Ag-Zn-A1 alloy on one side (Figure with internal fuses. In this type, two layers of A1 foil are
25.l(a)) by a vacuum deposition process. To ensure used and coated with a mixed dielectric, formed of tissue
accuracy of process in metallizing the film, the process or kraft paper and polypropylene (Figure 25.1(b)).
is carried out under highly controlled environmental Elements so formed are not self-healing and are normally
conditions of temperature, moisture and dust. The provided with internal fuses for their individual protection
thickness of the metal film is extremely thin, in the range (Figure 25.2(b)). Their life, of approximately 90 000
of 0.2-5 pm. These two layers are then interleaved and operating hours is better than an MPP element, and can
wound on a mandrel (Figure 25.1 (c)) to form an element. withstand around 20 000 switching operations per year.
The size of the element will depend upon the design, The elements are suitable for sustaining a higher voltage
voltage rating and size of the capacitor unit to be produced. variation and the presence of harmonics, as required by
These elements are the self-healing type and require no fluctuating and non-linear loads.
internal fuses (Figure 25.2(a)). During an internal fault, The elements thus formed are then inserted into a sheet
only the metallized film is fused, removing a negligible steel container, vacuum dried and impregnated, with
area of the metal film which is not replenished. The PP suitable non-PCB dielectric, which may be an oil dielectric
films remain almost intact, with small localized damage or epoxy resin. The capacitor shell is then hermetically
with a few pinholes. Their life may be considered to be sealed, in oil dielectrics, to avoid any leakage of dielectric
around 60 000 operating hours suitable for approximately during operation.
5000 switching operations per year. But they are suitable Until the 1970s the chemical used as the impregnating
only for linear loads. The elements are stacked together and dielectric medium for capacitor units was PCB
to form the required size of capacitor unit and impregnated (polychlorinated biphenyl) liquid. It was found to be toxic
with a suitable dielectric, which may be a synthetic oil and unsafe for humans as well as contamination of the
or epoxy resin. environment. For this reason, it is no longer used. The
latest trend is to use a non-PCB, non-toxic, phenyl xylyl
25.1.2 Double dielectric process ethane (PXE-oil), which is a synthetic dielectric liquid of
extremely low loss for insulation and impregnation of the
By altering the dielectric mix and its thickness it is possible capacitor elements or to use mixed polypropylene or all-
to change the characteristics of an element to make it polypropylene (PP) liquids as the dielectric. A non-oil
suitable for higher voltage variations and non-linear loads. dielectric, such as epoxy resin, is also used.

Dielectric PP film
Dielectric
PP film
PP film Alternate
Alternate
Layer of
Film coating of tissue paper electrodes
Film coating of
electrodes
AI, Zn-AI or
AI, Zn-AI or protruding
protruding
at the ends
Ag-Zn-AI alloy
Ag-Zn-AI alloy AI foil at the ends
conductor
Tinned copper tabs
(a) An MPP element (b) An MD or PP element. For PP element (c) MPP, MD or PP plates wound on a mandrel
there is no layer of tissue paper
Figure 25.1 Making an element