It must be acknowledged that Flat
winding technology is still being used in modern capacitors, in particular by
1. many manufacturers of medium and high voltage
ALLFILM capacitors for power factor correction (i.e. non-self-healing, oil-impregnated
capacitors with aluminium electrodes and double layers of polypropylene
dielectric), and by
2. few manufacturers of power electronics
capacitors (self-healing, metallized polypropylene film, dry dielectric)
It is claimed by the
manufacturers offering flat winding technology that these capacitors can be smaller
and, as a consequence, save material cost. In reality, the flat winding
technology bears serious technical and mechanical disadvantages if compared with
cylindrical windings. These disadvantages may become very essential when
considering capacitors for long operating periods as are required, for example,
in HVDC and traction applications.
Technical Aspects:
Initially, a flat capacitor element is wound
very similar to round elements; however this is made on a core with large
diameter which is removed after the winding process. The element is then
pressed flat to form a stable, quite solid block.
The main technical flaw of this
technique lies in the pressing itself: Even though flattened windings which are
produced on top-modern equipment look absolutely homogenous at first glance,
this smooth impression disappears as soon as the windings are opened and
unwound. By the laws of geometry, the outer film layers on the winding must get
stretched whilst the inner layers get compressed and wrinkled; moreover, the
innermost winding turns get folded with a sharp edge. It is obvious that the
stretching of the outer film layers does not contribute to their long-term stability.
But even worse is the fact that, as opposed to the perfectly uniform electrical
field in round windings, the wrinkles and the sharp folding edges in the flat
winding cause undefined conditions of the electrical field; the field strength
can nearly double in the areas of the bending edges! It is logical that this
must contribute to a higher failure risk.
One
of the most important parameters for the design of polypropylene film
capacitors is a high and homogenous pressure among the film layers. High
pressure ensures the hermetical closure of the winding element which is a very
critical pre-condition for long life-time and stable behaviour during the
entire operating period.Thanks to their compact round shape and the defined
shrink of the film during the thermal treatment
(which is an obligatory part of our production process), round capacitor
windings are self stabilizing perfectly. Their internal pressure remains
constant throughout the operating time, and the windings cannot change their
volume or shape anymore.
Flattened
windings, however, need to be pressed externally, not only during the
production process but also during the entire operating life. It is clear that
the external pressure which can be implemented on the winding is limited by the
capacitor construction, and that it is impossible to maintain this pressure
uniform and unchanged during the entire operating period.
Polypropylene
elements are contact-sprayed (“schooped”) with a zinc layer at both ends in
order to establish electrical contact with the metallization on the film.
Whilst cylindrical windings are covered completely and uniformly at both ends
by this layer, the flat windings cannot be contact-sprayed in the area of the
sharp bend which is – as shown above - the most critical area of the element anyway.
As a consequence, the current for energizing these folded areas has to enter
the winding element through the parts of the shooping layer which are in the
vicinity of the bending, causing higher current density (and potential for
local hotspots) in these areas which increase the risk of failures.
Moreover, the
uniform geometric condition of round-shaped windings allows for an optimum penetration
of the zinc-particles during the shooping process (assuming proper know-how of
the manufacturer, of course), for perfect contact with minimized losses.
Such optimization
is unthinkable with flattened windings which bear a higher risk of poor contact
which may result in more local hotspots.
Anybody
who has seen a rectangular reactor winding after heavy surge charges, knows
about the power of surge currents: under heavy current stress, any winding, no
matter if forced into flat or rectangular shape, strives towards its optimum
shape, which is round. During heavy surge discharges (e.g. external fault
situation causing short circuit discharge of the capacitor), the uneven current
distribution inside the flat windings produces additional mechanical stress
within the elements (also compare pt. 2) which may deteriorate or damage the
link between schooping layer and metallized film. This will have a negative
effect on the operating life (or failure risk) of the capacitor. Obviously,
round windings have no such problem.
Flat windings can only be
efficiently displayed if (a) the outer
housing size fits exactly the width of the (flat-pressed) winding (b) the film
has a considerable width (otherwise, short flat-packs will require a higher
number of windings to be assembled which increases cost).
(a.) will pre-define the size of
these capacitors. They can only be altered in certain size steps because they
must make efficient use of the flat packs. A manufacturer of flat-winding
capacitors will therefore always strive to adjust the customer’s specification
to his “ideal” sizes in order to achieve the optimum usage of space for his
flat windings and make it more difficult for competitors to match. This
explains why such manufacturers often try to “buy” the business by a very low
initial price. This may become a cost trap later when the customer depends
completely on these dimensions (and on the supplier).
Even though it is true that round
windings do not fill a rectangular space as completely as flat windings, manufacturers
with round windings are more flexible in adapting their shape and size to the
customer’s project requirements.
(b.) means a long current path
inside the winding which always contributes to higher series resistance Rs. The
series resistance, however, is responsible for the current power losses of a capacitor;
hence it contributes to the temperature rise inside the capacitor and, as a
consequence, to the failure rate. With round windings, the manufacturer is free
to optimize the winding element depending on current, capacitance, ambient
temperature conditions, and available space. Of course, the length of flattened
windings can be reduced as well in order to improve the series resistance.
However, since the diameter of the original round bobbin for the flat winding
cannot be increased at will, any reduction of the length of flat-packs must in
turn be compensated by a bigger number of windings to be assembled. This would
then increase cost.
From a technical and commercial point of view the flat windings do not offer any significant advantage, it is therefore recommended not to disturb the geometry and stay focussed on the round windings.
