Circular Vs. Rectangular Windings | Part 2

March 25, 2014

In Part 1 of this two-part series on Circular vs. Rectangular Windings, we stated that most substation transformers are either circular or rectangular in core and coil assembly. The magnetic circuit is one of the most important parts of a transformer and it consists of a laminated iron core which carries flux linked to the windings. The type of iron core determines the shape of the transformer coil or windings.

For substation transformers larger than 5,000kVA or industrial applications which have loads that require more rigorous duty, a circular core and coil assembly is more common. The iron core has multiple widths of core laminations to form a circular shape, or what it called a “cruciform” type of core. The most common type of circular winding done for substation transformers utilizes a continuous disk or “helical” type of winding. The turns of the coil are stacked up to form round disks that are separated on the coil by radial spacer columns. The coils are assembled in the core and frame assembly and compacted under pressure to hold the coil winding in place during a short circuit or fault.

A rectangular core and coil assembly is used in transformers 5000kVA and smaller with standard distribution type of loads. In a rectangular core and coil assembly, the core utilizes only one width of core lamination. The rectangular core is much easier to assemble than circular or cruciform type of core. A rectangular core is designed to accept a rectangular or square shaped coil only. Rectangular coils also utilize a “layer type” of winding, where the turns of each coil are wound on the flat coil in layers and separated by insulation and cooling ducts. The coils are then assembled on the core and frame assembly. The coil sides are blocked as tight as possible to keep coils from deforming under stresses during a short circuit or fault.

During a fault, an incredible amount of radial and axial forces are generated, which causes stress on the transformer coil. The current in the primary and secondary windings flow in opposite directions, this creates the repulsive forces between the two windings. The result is the coil tends to expand outwards and assume a circular shape under the influences of short-circuit stresses. As such, the coil that is originally circular in shape is fundamentally the most durable and least likely to distort during a fault. A rectangular coil construction will try to “round itself out” under these same repulsive forces between the two windings. This can cause the rectangular shaped coil to dramatically change shape, resulting in damage to the coil’s insulation.

The axial force in transformer coils occurs when the electrical centers of the primary and secondary windings are unbalanced. This unbalance causes a force to be exerted from primary to secondary windings. The force tends to make the primary and secondary windings slide past one another in what is called “telescoping” of the coil. This causes substantial damage to a transformer coil which typically results in transformer failure.

Faults or short circuits happen fairly often. When a tree branch hits a power line or a car hits a pole, or during a particularly severe ice storm, faults do occur. As such, it is vital that the design of substation transformer greater than 5,000kVA or industrial applications which have loads that require more rigorous duty have designs which incorporate a circular core and coil assembly.  The coil tends to assume a circular shape under the influence of short circuits or faults. The coil which is originally circular is fundamentally the best shape and resists distortion.

When designing a transformer coil it is important to keep the electrical centers of the primary and secondary windings balanced. The best way to balance the electrical centers is to utilize the circular design of a continuous disk or helical type of winding. The axial forces that are present can be maintained by the core clamping structure which secures the coil under pressure to prevent the coil from telescoping. The core clamping structure utilizes clamping rings and Jack-Bolts which apply pressure to the coils.

The electrical centers of a rectangular shaped layer type of coil cannot be balanced very well. The radial and axial forces generated during a short circuit or fault can distort the shape of the coil. This results in the primary and secondary windings sliding past each other as the coil telescopes. A rectangular shaped layer type of coil is very difficult to clamp and keep from distorting and is more subset able to failure during a short circuit or fault.