Choosing the correct coils for transformers

When purchasing a transformer, several factors must be evaluated carefully to ensure that the correct choice is made. One question that is frequently asked by those investing in this technology is whether it is best to opt forround, rectangular or disc wound coils. There is however no simple answer to this question other than "it depends".

The reason rectangular and elliptical coils have been around for so long is that they work for certain standard applications and their construction favors high volume manufacturing processes.

If your need is truly for a standard distribution transformer subjected only to limited available fault currents, equipped with fusing or protection devices, subject to stable loading with little variation, or is a small to medium sized transformer with star connected primary and secondary windings, then rectangular windings might work for you.

If on the other hand, your application is more "exposed" to higher levels of fault currents, repeated events, or high current requirements with lower designed impedances, like motor starting, the answer is yes you should buy a layer wound, round coil design and preferably one with robust bracing incorporating heavy duty pressure plates to add axial stability to counteract the strong physical forces experienced during faults.

Unlike wound cores, as are typically used with rectangular or elliptical coils, round coils allow for the utilization of miter cut, cruciform cores. Cruciform cores are comprised of multiple width laminations which, when assembled present a round cross-section and largely fill the core window in the round coils. Round coils with a cruciform core provide superior resistance to radial forces to which the transformer is subjected during faults and severe duty cycles.

The improved cooling available with the round coil construction also helps dissipate short circuit thermal effects after the fault has cleared and the transformer resumes normal loading. The improved cooling is possible because, unlike wound cores, cruciform cores in conjunction with specially designed upper and lower pressure plates allow the cooling fluid to flow throughout the coils thereby substantially reducing hot spots in the insulation system, which in turn leads to longer transformer life expectancy.

If your purchases include larger transformers, 10 MVA and above, or higher BIL levels, 350 kV BIL and above, you should also consider buying disc-wound, round coils. The disc-wound design and construction gives all the benefits of layer wound round coils with miter cut cruciform cores and even more mechanical strength and transient fault withstand capability.

Furthermore, the disc construction minimizes electrical stresses within the coil caused by switching surges, lightening strikes, and voltage disturbances frequently found in the network at higher voltage levels and seen by larger transformers.

It is our recommendation that for most substation applications, for essentially all station transformer applications, and for those high-risk distribution transformer applications, you should buy wisely and purchase the round coil design for lower total cost of ownership.