Round Coil vs. Rectangular Coil for Transformers

by | Aug 14, 2020

Substation transformers feature either a round or rectangular core and coil assembly. A transformer’s iron core and windings are designed to carry magnetic flux through the transformer, and the core is typically constructed in a round, rectangular or oblong shape — although oblong cores are much less common in dry tape transformers.

Round and rectangular core shapes offer unique advantages. Since the core shape determines the type of coil the transformer requires, round iron cores must use round coils, and so on. But which coil configuration is best for dry type transformers?

Primary Differences of Round vs. Rectangular Coils

Under normal operating conditions, round and rectangular coils in dry type transformers behave almost identically. But under abnormal conditions, such as in the event of a short circuit, each type responds differently.

The physical differences between round and rectangular coils are straightforward. Round coils have a circular shape that can withstand radial forces while maintaining its circular shape. Rectangular (or square) coils feature four sides and slightly rounded edges, and they’re generally cheaper to manufacture. Importantly, they handle radial forces less effectively than their circular counterparts, most times losing their rectangular shape during a short circuit.

Another difference between rectangular and round coils in dry type transformers is the negative space within their core configurations. While circular core setups have slight air gaps between the magnetic flux-carrying steel and the windings, rectangular cores fill this gap completely — resulting in slightly less inefficiencies.

Through Fault Capabilities

The most important difference between round and rectangular coils lies in their ability to respond to through faults. A through fault refers to a type of fault that happens outside of the transformer zone. When through faults are not cleared by the downstream components, the transformer will become overloaded, unable to handle the sudden influx of electrical currents running through it.

In through fault scenarios, transformers with round coils tend to fare much better than those with rectangular coils. This is due to the rectangular coils’ loss of shape when encountering short circuit situations. Even if the rectangular coil features restraining or “bracing” devices to help it maintain its shape, a short circuit will likely produce substantial damage.

Which Type Is Better in Dry Type Transformers?

From a cost-benefit perspective, cheaper is not always better. While a rectangular coil is easier and less expensive to produce, circular coils offer more benefits when it comes to handling short circuit incidents. Round coils feature fault protections that substantially reduce the potential for damage due to through faults and similar occurrences, and round coils almost always fare better because of their more uniform stress absorption.

Contact ELSCO Transformers Today

For high-quality dry type transformers, turn to ELSCO Transformers. We provide customers across industries with expert guidance on selecting the right transformer type for their specific applications. We also offer free quotes on our medium voltage transformer products. Contact us online or call 800-232-9002 today.


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