Transformer insulation glossary

Plain definitions of the terms used on this site and on transformer drawings.

Angle ring

A pressboard ring with an L-shaped cross section, fitted at the ends of a winding to support the insulation and guide oil flow.

Axial duct

A vertical oil channel along the height of a winding, formed by strips placed between the winding and the cylinder.

Bakelite

Trade name that has become the common word for phenolic laminate sheet. See also Hylam.

Clack band

A round press-paper band used to hold winding sections and leads in position.

Cleat

A block, often of Permawood, that clamps and supports leads inside the transformer tank.

Clamping ring / pressure ring

A strong ring (pressboard or Permawood) at the top and bottom of a winding through which clamping force is applied.

Cooling duct

Any oil channel inside the winding that lets oil circulate and carry heat away.

Dovetail spacer

A small pressboard block with a dovetail-shaped key. It slots into a dovetail strip to form radial cooling ducts.

Dovetail strip

A pressboard strip with dovetail slots along its length, into which dovetail spacers lock.

Distribution transformer

A transformer that steps voltage down for local supply, typically 11 kV or 33 kV to 415 V, up to a few MVA.

Hylam

Another trade name for phenolic laminate sheet, widely used in India.

Insulation kit

A complete set of insulation parts for one transformer, cut and packed together.

kVA

Kilovolt-ampere, the unit used to rate transformer capacity.

Oil impregnation

Filling the pores of dried insulation with transformer oil, usually under vacuum, to raise its insulating strength.

OEM

Original equipment manufacturer. In this context, a company that builds transformers.

Permawood

Densified laminated wood used for load-bearing insulation parts. Also called transformer wood or UDEL wood.

Power transformer

A large transformer used in transmission and generation, generally above a few MVA.

Pressboard

Dense kraft board used for most solid insulation and spacing inside oil-filled transformers.

Press paper

Thin pressboard, typically under 0.8 mm, used for layer insulation and wrapping.

Radial duct

A horizontal oil channel between winding discs, formed by dovetail spacers.

Static ring / static plate

A shielded ring at the line end of a winding that evens out voltage stress.

Thermal class A

An insulation rating for continuous operation up to 105 °C.

Winding cylinder

A pressboard tube on which a winding is wound, insulating it from the core or the next winding.