Oil-Immersed Distribution Transformer

It follows the maintenance regime and the oil-testing requirement. In the sealed unit the oil does not meet air, no topping-up or filtering is needed in service, and there is no conservator or breather to look after, which suits lightly attended and widely scattered sites. The conventional unit keeps the conservator, the oil level is visible and oil samples are easy to draw, which is common on network projects with a scheduled oil-testing regime. Note that the overall dimensions differ — close to 260 mm in length at 630 kVA — so set out the foundation for whichever is chosen.

First establish which stretch applies. S11 to S13 lowers only no-load loss (810 W to 570 W at 630 kVA) while load loss stays at 6200 W, so the saving is independent of load factor and follows directly from the annual hours. From S20 no-load and load loss fall together (510 W and 4960 W; S22 gives 460 W and 4460 W), so heavily loaded sites gain a further share. Carrying earlier experience over to S20 understates the saving. Send the annual running hours, the average load factor and the tariff and the generation that pays can be calculated.

It follows the installation position and the fire-protection conditions. The oil-immersed unit can be arranged outdoors, has a lower ambient limit of −25 °C, and at the same capacity has lower no-load loss and usually a lower unit price; the price is an oil pit or oil-retention arrangement and the fire-protection requirements that come with oil. The dry-type unit is an indoor machine, can be placed directly at the load centre and needs no oil pit, which favours damp environments and underground substations, but its low-temperature limit is only −5 °C. Inside a prefabricated power module no oil pit is provided, which is why the great majority there are dry-type.

The number of tap positions and the step size. ±5% gives two positions in 5% steps, ±2×2.5% gives four positions in 2.5% steps. The first suits a line end whose voltage swings widely but where fine accuracy is not required; the second suits finer regulation. Both are off-circuit: the transformer has to be de-energised to change taps, and they cannot be switched on load.

From the unbalanced-load capability required on the low-voltage side and the neutral earthing requirement. All three are available; at 500 kVA and below the parameter table separates load loss by vector group, and from 630 kVA up it no longer does. On a specific project the phase relationship with the equipment upstream and downstream, the protection configuration and the conditions for parallel operation all have to be checked alongside, and the choice is confirmed before ordering.

Yes. 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. What matters for an oil-immersed unit is the oil pit or oil-retention arrangement — sized on the oil volume and settled in one pass at the layout drawing stage together with ventilation opening area, clear cooling space, maintenance aisle width and fire separation. Supply of the transformer on its own is equally possible.

On the actual total weight of the capacity chosen, not on a proportional extrapolation. At 630 kVA the total is 1860 kg (core and windings 1240 kg, oil 280 kg) and at 2500 kVA it is 5470 kg, with the wheel gauge widening from 660×660 mm to 1070×1070 mm. Radiating surface grows with capacity, so weight does not grow in proportion to it. Whether the oil is drained for long-distance transport is settled separately.