Oil-paper insulation around a hairpin CT primary must follow a curved conductor. The inner bend offers a shorter surface path than the outer bend. Paper placement, overlap and compaction can therefore vary through the curve even when the wrapping machine maintains the same nominal tension. [1]
Keep the actual instrument-transformer construction, winding arrangement and insulation boundaries explicit in the review. Qualify the represented assembly rather than inferring an internal feature from its external appearance.
Inspect the bend as a section
Mark inner radius, outer radius and the side surfaces. Record local paper thickness and the route of each wrap or tape edge. A thickness measurement on the straight primary section cannot represent a bend where the material may wrinkle, spread or shift.
| Bend region | Geometric concern | Evidence to compare |
|---|---|---|
| Inner surface | Compressed path and possible folds | Local build, wrinkles and layer seating |
| Outer surface | Longer path and possible overlap reduction | Coverage and local thickness |
| Side surfaces | Transition between inner and outer paths | Edge tracking and asymmetric build |
| Shield-containing layer | Paper movement around a stiffer insert | Shield position and insulation at its end |
| Bend-to-straight transition | Change in curvature and wrapping support | Local thickness continuity and springback |
Tension does more than tighten the outer surface
Higher tension can improve seating in one region while stretching paper or concentrating pressure elsewhere. Lower tension can preserve material strain while permitting loose contact. Neither direction is universally preferable. The suitable process depends on paper condition, conductor radius, wrapping method and the support available during application.
Retain tension history where the method provides it, but correlate it with actual geometry. A machine setting is not a direct measurement of local pressure inside the paper build. Guide friction and changes in contact geometry can alter the load transmitted to the bend.
Compare constrained and released geometry
Wrapping tools or fixtures may hold the conductor and paper in position. When restraint is removed, elastic recovery or material relaxation can change the apparent thickness and seating. Record the condition in which dimensional measurements were made. A result taken under temporary pressure should not silently become the finished insulation dimension.
Handling, final restraint and impregnation can change internal seating. ABB’s insulation discussion links voids/discontinuities with local stress and PD mechanisms [2]; a smooth outer surface cannot establish that hidden layers are void-free.
Assume a 180° bend has inner/outer wrapping paths at radii 100/110 mm. Arc lengths πR are 314.16/345.58 mm: the outer route is 10% longer. A common feed/tension setting therefore cannot establish equal local overlap or compaction. For a separate assumed 20 mm-wide, 0.10 mm-thick tape, 20/40 N gives nominal tensile stress 10/20 MPa; these are not paper allowables. Qualify inner, outer and transition witnesses with unloaded geometry/coating/void evidence. Straight-section thickness or doubling tension cannot certify impregnation of the curved HV insulation.
At the bend, thickness, overlap and shield position have to be assessed together in the final restraint state. Meeting the straight-section thickness leaves inner folds, outer overlap loss and displaced grading ends unresolved.
References
- ABB, Instrument Transformers Application Guide, publication 1HSM 9543 40-00en, Edition 4, February 2015, sections 5.1–5.6, 6.1 and 7.1.
- ABB, Instrument Transformers: Technical Information and Application Guide, 1VAP420003-TG, revision A, December 2004.