Low-Voltage windings for rectifier transformers
Low voltage, very high current – with significantly elevated harmonic content. Cordex paperless transposed cable limits losses and overheating under non-sinusoidal load conditions inherent to rectifier operation.
LV-RT
Design Rationale
Like furnace transformer LV windings, rectifier LV windings are characterised by voltages below 1.5 kV and currents in the range of tens of kiloamperes. However, the two applications present a fundamentally different electromagnetic challenge: in rectifier transformers, the harmonic content of the current is significantly higher. The non-sinusoidal current drawn by rectifier bridges injects dominant harmonic components – 5th, 7th, 11th, 13th and higher – into the winding conductors, increasing skin effect and proximity effect losses as the square of harmonic order.
To limit losses and overheating under these conditions, the adopted solution is the use of paperless transposed cables (Cordex). Cordex eliminates inter-strand paper insulation, reducing the total radial build of each coil, while continuous transposition of the individual strands equalises flux linkage across all sub-conductors – suppressing both circulating currents and skin effect losses at harmonic frequencies.
The coils are helically wound one above the other and separated by oil ducts created through spacers, forming multiple coils arranged axially and connected in parallel. This construction combines the compactness of helical geometry with the current-carrying capacity required for rectifier duty – and provides direct oil access to each coil’s conductor surface for effective harmonic-loss thermal management.
Key Engineering Features
Harmonic Loss Suppression
Cordex continuous transposition equalises strand flux linkage, cancelling circulating currents and suppressing skin effect losses at rectifier harmonic frequencies – the dominant additional loss mechanism in this application.
Parallel Coil Architecture
Cordex Paperless CTC
Elimination of inter-strand paper insulation reduces the winding’s radial build and lowers thermal resistance between conductor strands and the cooling oil – providing a direct thermal path for harmonic-generated heat.
Oil Duct Cooling
Spacers between axial coils create defined oil ducts, providing direct cooling of each coil’s conductor surface – essential for managing the elevated thermal load produced by harmonic losses under continuous rectifier duty.
Technical Note
Harmonic Thermal Assessment – All rectifier LV windings are thermally evaluated under the actual harmonic spectrum of the rectifier circuit, applying the K-factor or FHLmethodology per IEC 60076-1 / IEEE C57.110. The design confirms that temperature rise under the combined fundamental and harmonic load remains within the rated thermal class of the insulation system. Phase-shifted winding variants for 12-pulse and 24-pulse rectifier configurations are available, reducing harmonic injection into the supply network while maintaining full current-carrying capacity across the parallel coil stack.
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