DG Core

How much different are the interlayer and impulse stresses in the rectangular windings compared to the stacked (cruc.) core?

This question has come up because of the stresses on the external part of the winding around the corners of the paper.

The stresses in the corners would be slightly greater that on the sides but not necessarily any greater than those that would be found on the cruciform arrangement. The reasons for this are two- fold: The corner shapes (especially on the inner layers) create more stresses and secondly, the winding in the corners would generally be tighter than on the sides. So the bulge on the sides, which would generally be composed of insulation and films of oil (with about the same dielectric constant), would contribute to slightly longer dielectric paths and therefore less stresses.

As far as I can remember there should be some extended insulation to overlap in the window of the wound core!

Right or wrong?  This we gather would be put in place due to creepage and clearance required!? Please clarify.

Generally, all overlaps are in the ends both to reduce space and provide insulation uniformity in the window.

Is the phase to phase clearance in our design have sufficient space for cooling?

Yes. One caveat: This distance will also have to be re-considered for different BIL values.

Does the phase to phase space have a solid board inserted?

If you can assure the accuracy of the space, it would not. However, to assure spacing a board would be necessary but at the expense of reducing the cooling surface of the coils at that location.

Does the oil temperature rise differ in the circular and rectangular winding.

The difference would not be discernible.

Does the oil temperature rise differ in the wound and cruc. core?

For given similar total losses, no. Individual winding gradients may be slightly different depending on overall construction.

With regards to production manufacture, does the inner part of the rectangular form of the coil round a lot on the corners and edges once removed off the former when winding is finished?

The general answer is generally no. The rounding magnitude depends on the size of the coil and obviously how tightly it is wound.

If so, what means can be used to keep and maintain this form specially when the wound core needs to be assembled in the winding window?

Some people use spreader bars on the corners as they are assembling the coil. Some others –    with larger coils – make a form on the inside of the coil and varnish and bake (corners only) to assure dimensional compliance. To repeat, this rounding is not an overwhelming occurrence.