Comparing Round Winding Technology with Flat Winding for Power Applications

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.