Triangular Wound Core Dry-Type Transformer

On the auxiliary busbar of a captive power station or cogeneration plant, on the incomer of a rectifier transformer for a high-power variable-speed drive, and on industrial medium-voltage busbars carried over from earlier designs. 3.15 kV is not in the standard distribution voltage series, and such cases previously had to be handled by a special order or an oil-immersed unit. This series carries it as a regular high-voltage step alongside 6 to 11 kV, taken from the same table, with the insulation level still calculated as LI75 AC35/AC5.

It cannot be compared that way. This series is given at 75 °C and the cast resin dry-type series at 120 °C. The load loss of one and the same transformer is about a sixth higher at 120 °C than at 75 °C, so the two numbers are not on the same basis. Convert to a common reference before comparing losses across the series or calculating annual energy cost; where a tender document specifies a reference temperature, convert to it before filling the figure in.

Far smaller. On the oil-immersed triangular unit no-load current goes from 1.1% down to 0.15%, close to an order of magnitude; here it goes from 0.5% down to 0.3%. The reason is that the cast windings and core window design of a dry-type unit already hold the magnetising current low, leaving less room to improve. Expecting the oil-immersed gain here will overstate it.

There is only one variable. Capacity range, impedance, no-load current and mounting dimensions are identical, and load loss is the same except in the 80 kVA band; the difference sits in no-load loss — 1300 W against 1170 W at 630 kVA and 3600 W against 3240 W at 2500 kVA, about a tenth lower. Work it out directly from the no-load difference, the annual energised hours, the tariff and the price difference; no other parameter is traded off.

Less magnetising reactive power drawn at no load, a lower inrush multiple, and less harmonic content carried in by the magnetising current. In a distribution system with many transformers at a low individual load factor the reactive power adds up, and where switching is frequent a smaller inrush is easier on upstream protection settings. Note that this is not a loss saving — losses come from the figures for the type selected, and excitation behaviour is a separate matter.

Yes. No oil pit is provided inside a module, which matches a dry-type unit naturally; where it travels with the module, the prefabricated power module line takes on the enclosure, the internal arrangement and the foundation fixing, and the transformer leaves the works as equipment inside it. The enclosure outline of a triangular core differs from a conventional rectangle, so the internal arrangement is set out from the actual shape and maintenance aisles and cooling air paths are checked against the largest diagonal dimension.