Large Three Phase Oil immersed Distribution Transformer

Product Overview

Large Three Phase Oil immersed power transformers arecustom-built pieces of equipment that are crucial links tothe bulk transmission grid. Usually, they link a generator-transmission line and/or linking lines of different voltages.
CNHK’s Large Three Phase Oil immersed power transformer aremanufactured and tested in accordance the IEC60076

Type Designation

Construction features

The transformers are of the breathing, conventional type provided with an oil conservator.
Two types of transformers are manufactured:
Transformers with off-load tap changer
Transformers with on-load tap changer

Core

The three legged bolt-less core is constructed of grain oriented steel laminations. The joints between the leg and yokeare mitred at 45° and interleaved. The cross section of the leg is circular.

Windings

The material of the windings is either copper or aluminum. The windings are made of paper insulated rectangular wire inthe form of multilayer, disc or helical windings. Transformers provided with on-load tap-changer usually have a separate tap winding.

Off-load tap-changer

The transformers are normally equipped with a 5-position off load tap-changer on the High Voltage side, with its handle located on the cover. When changing the tap position the transformer has to be un-energized. Position 1 of the tap-changer (+ position) corresponds with the highest ratio, which gives the lowest voltage on the Low Voltage side. Position5 of the tap-changer (- position) corresponds with the lowest ratio which consequently gives the highest voltage on the Low Voltage side.

On-load tap-changer

The on-load tap-changer is mounted on the cover of the main transformer tank. The tap changer works as a selectorswitch combining the features of a diverter switch and a tap selector. The selector switch is located in a separate oilcompartment which is connected to its own conservator. The selector switch is of the high-speed spring operated typewith resistive transition impedances. The tap-changer is equipped with a protective relay. The selector switch can belifted for inspection and the oil in the compartment can be changed separately. The on-load tap -changer is providedwith a motor drive mechanism for remote and local control. Normally the motor drive is three-phase and the controland the heating circuits are for single phase AC supply. When re-quested, DC circuits can be supplied.

Insulating oil

The mineral oil used in the transformers complies with IEC 296 and several national standards. The oil slows down the ageing process.

Tank

The transformer tank is usually of the rigid type provided with removable radiators connected to the tank by means of ashut-off valve. On request, the tank can be made vacuum proof.
Transformers up to 5 MVA can be supplied with a compact tank with corrugated cooling fins.
Painting and surface treatment Prior to the painting of the tank, the cover and oil conservator are sandblasted. External surfaces are painted immediately after pre-treatment with a two-pack epoxy primer coat.
Corrugated tanks are flow painted with alkyd primer and acryic finish to a total thickness of 80 um. The conservator and the bottom part of the tank are painted internally.

Options and accessories

The transformers are fitted with the following

standard accessories:

-Bushings
– Oil conservator with oil level indicator
-Filling and drain valves
-Oil sampling device
-Earthing terminals
– Lifting lugs
-Jacking pads (radiator tanks)
-Rating plate
-Silica-gel breather
-Buchholz relay
-Top oil thermometer
Additional accessories can be fitted in accordance with the customer’s specifications.

Vacuum treatment

Drying and vacuum treatment takes place in a special oven, where the windings are heated electrically while simultaneously the air is evacuated from the oven, reaching an absolute pressure of 1 mbar. The transformer is filled with oil while it is still under vacuum. In this way moisture and gases are effectively remo ved from the windings and oil.

Bushings

The bushings are of a solid porcelain type and located on the cover. Bushing insulators canbushings comply with IEC 137. be changed without opening the transformer. The
The terminals can be provided with an air insulatedenclosure. The enclosure can be on the primary and/or onthe secondary side and may be constructed for cable orbus duct connection. Alternatively the transformers can  be provided with plug-in bushings.