ABB describes a bottom-tank hairpin CT and an inverted/top-core design [1]. Compare their actual sections: moving the active assembly changes primary length, insulation boundaries and the mechanical load path.
Use a section drawing to compare the designs. A photograph or the word ‘inverted’ does not establish which internal metal parts are at primary potential, grounded or electrically screened.
Locate the primary-to-secondary insulation
Draw the primary conductor, graded boundaries, secondary route and supports in both sections. A core’s location does not establish its potential; a high-mounted head can contain a screened low-voltage assembly.
The core’s physical location is not its electrical potential. In particular, a head near a high-voltage primary terminal can contain a low-voltage screened assembly. Mark the intended potentials and insulation boundaries before comparing clearances.
| Cross-section feature | Hairpin example | Top-core example |
|---|---|---|
| Core location | Bottom section around insulated primary route | Head section around a short primary route |
| Main graded insulation | Around the hairpin primary conductor | Around screened active parts and secondary descent |
| Primary heat route | Long conductor and its surrounding insulation | Shorter conductor with head-region heat sources |
| Mechanical mass | More active mass near the support base | More active mass above the insulator/support |
Trace losses into the surrounding insulation
ABB identifies longer hairpin primary length as a conductor-loss consideration and shorter top-core length as an advantage [1]. Heat transfer still depends on joints and local oil/solid paths [2]; length alone cannot rank an internal temperature.
Separate primary-conductor heating from secondary-winding heating. A design that cools the primary well can retain a difficult thermal path from a secondary winding embedded in thick insulation. Compare the actual heat-run conditions and identified limiting locations, rather than turning conductor length into a complete thermal ranking.
Compare the load path to the foundation
Core mass in the head increases the importance of support and insulator loading. A bottom-tank arrangement changes the centre of gravity and can accommodate a different core volume. Neither observation alone qualifies seismic performance; the assembly, foundation, terminal loads and response spectrum must match the proposed installation.
For maintenance, mark the oil-compensation device, test tap, secondary box and service access on the same sections. Their positions affect accessibility without implying that the internal active parts are intended for field disassembly.
For an independent comparison, assume primary lengths 2.0/0.50 m, equal section 1,000 mm², resistivity 2.0 × 10⁻⁸ Ω·m at one evaluated temperature, and current 1,000 A RMS. R = ρL/A and I²R give hairpin/top-core losses 40/10 W. Assume respective local thermal resistances 1/4 K/W: both rise 40 K [2]. If the head then has an assumed 5 W joint source into that same path, its rise becomes 60 K. Against an assumed 50 K local-rise cap, hairpin passes and that top-core case fails despite lower primary-body loss. Resolve joint/cooling geometry before selection; this fictional model supplies neither a universal design ranking nor insulation coordination.
A top-core design’s shorter primary does not offset an unqualified secondary heat path or excessive support loading. A hairpin design’s lower core position does not remove the losses of its longer primary. Selection has to follow the section drawing, local thermal evidence and installed structural duty, with accuracy requirements checked for the actual ordered cores.
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.
- U.S. Department of Energy, DOE-HDBK-1012/2-92, Thermodynamics, Heat Transfer, and Fluid Flow, Volume 2, 1992.