35 kV Cast Resin Dry-Type Transformer

To do away with the oil pit. Underground substations, plant-room levels of high-rise buildings, retrofits inside existing works and industrial projects short of space often cannot build a pit and oil-retention arrangement, which shuts an oil-immersed unit out. A solid-insulated dry-type unit needs no pit, no retention and no oil fire-protection system, can sit indoors close to the load centre, and saves the fire design for the oil system, the periodic oil testing and the leak inspections. The price is indoor use, a lower ambient limit of −5 °C, and an installation environment free of significant pollution.

No. This class is LI170 AC70/AC5 and a 10 kV dry-type unit is LI75 AC35/AC5; the two sets differ by more than a factor of two, so surge arrester arrangement, insulation distances and the high-voltage connection all have to be recalculated for this class. The solid insulation of a cast coil has no self-restoring capability under overvoltage, so the overvoltage protection on the incoming side is settled at the design stage together with the protection scheme.

By reading three things together. High impedance means a low prospective short-circuit current on the low-voltage side, which relaxes the breaking capacity and busbar dynamic strength required of the switchgear below — carrying a low impedance over to a large rating pushes the switchgear selection into a very high band. The cost in the other direction is poorer voltage regulation, with larger swings as load changes. So low-voltage switchgear ratings, the short-circuit calculation and the voltage-quality requirement are checked together, with on-load tap changing added as compensation where needed.

Only no-load loss, each level about 10% below the last: 3130, 2820 and 2540 W at 1250 kVA, and 27100, 24400 and 21960 W at 25000 kVA. Load loss shares one column across the three performance levels and does not change with generation. The saving follows directly from the annual energised hours and is independent of load factor. Send the annual energised hours and the tariff and the payback for a specific generation can be calculated.

It follows the voltage swing and how often taps are adjusted. The on-load type has ±4×2.5% taps and switches under load, covering 2000–25000 kVA; the off-circuit type has to be de-energised to switch. The price is slightly higher losses (at 2000 kVA, 4500 W no-load against 4230 W and 19000 W load against 18200 W) and a tap changer that needs periodic inspection and contact-life management. It is worth it where voltage swings widely or voltage quality matters; where the supply side is steady and taps move once or twice a year, off-circuit is the better buy.

Yes, and this class suits it particularly well. 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 35 kV insulation distances, the ventilation and cooling air path, the clear width of maintenance aisles and the high-voltage entry arrangement are all settled in one pass at the module layout drawing stage, leaving only the connection on site.