Cast Resin & Oil Filled Transformer - Core

Core

The core is constructed using thin sheets of cold rolled grain-oriented magnetic silicon steel insulated on both sides.

Conventional grain oriented steel (CGO steel) is used for transformers with normal no-load losses, while transformers with reduced no-load losses are built using higher quality HiB steel.

These steel sheets are 0.30 mm. thick.
The core sheets are cut at an angle of 45°, thus allowing maximum magnetic flux in the rolling direction.

Then the sheets are stacked in layers of either single or multiple overlaps. The multiple overlap or step-lap method offers additional benefits in terms of lowering no-load losses and noise levels.

Once the sheets are stacked, the core is compressed and glued to form a firmly bonded whole.

The majority of OTDS oil filled transformers have an oval-shaped core section which have traditional stepped and a fully filled round shape, with a square mid section.
This method combines the benefits of a rectangular core section (simplicity of production) with those of a round core section (excellent short circuit withstand capability of the windings).
Using simple profiled steel yoke clamping systems and a number of metal tensioning bands, OTDS have eliminated the need for clamping bolts in the yokes.

Cast Resin Transformer - High Voltage Windings

The high voltage windings are almost exclusively of a layered construction.

The copper conductors are made of one or more round or square wires, completely insulated by pure cellulose paper or by double enamel.

The insulation between the layers consists of pre-coated Kraft paper, applied in sheet form.

Cast Resin Transformer - Low Voltage Windings

The low voltage windings are usually made of copper sheet conductor (foil); this reduces the axial stresses produced by short circuit to a minimum.
The sheets and connectors welded onto them are made of electrolytically pure copper with a rigorously guaranteed conductions.
The maximum voltage between each turn is only a few volts. This allows the insulation needed between the turns (foils) to be limited to 1 thermo-hardening epoxy adhesive which cures and bonds during the drying process.

Cast Resin Transformers

Oil Filled Transformer Fittings

Standard Fittings

1. Rating plate
2. H.V. bushings
3. L.V. bushings
4. H.V. tapping switch
5. H.V. double primary voltage switch
6. Oil filling and plug
7. Conservator with oil level indicator
8. Thermometer pocket
9. Lifting lugs
10. Earthing terminal
11. Oil drain valve
12. Rollers or skids

Optional fittings

• Thermometer
• Silica gel breather
• Buchholz relay
• Relief vent valve
• Cable boxes
• Plug-in bushings and connectors
• Box for auxiliary circuits
• Protective relay

Oil Filled Transformers

Resin Transformer Fittings

Standard Fittings

1. Rating plate
2. H.V. terminals
3. L.V. terminals
4. Lifting lugs
5. Box for auxiliary circuits
6. H.V. tapping switch
7. Securing lugs
8. Bidirectional rollers
9. Earthing terminal

Optional Fittings

• Electronic control with display and thermoresistance
• Protection box
• Forced air cooling
• Plug-in bushings and connectors
• Sound insulation supports

Transformer - Tank

The vast majority of distribution transformer tanks are constructed with cooling fins. The demand for panel radiator type is now somewhat limited.

In hermetically sealed transformers, the cooling fin design also can withstand the working pressure or the pressures required during the treatments and filling of the transformers. This allows the tank to be totally filled (and hermetically sealed), thus guaranteeing a long lasting of transformer and reducing maintenance. The conservator type fitted on the top of the transformer tank follows older rigid tank design practice.

This cylindrical conservator acts as an expansion tank for the oil when it expands as the windings heat up. The oil conservator is often fitted with a gauge glass, an air vent and an air dryer for the purpose of ensuring that only dry air can come into contact with the oil, and only at atmospheric pressure.

After welding, the tank is shot-blasted to remove any scale, oil or other surface impurities, leaving a clean prepared surface for maximum adhesion of the paint coating. Air-drying paint is then applied by spraying or flooding.

Several coats of paint are applied, to a total thickness of at least 100 microns, thus guaranteeing adequate protection against corrosion for indoor or outdoor transformers.

The tanks may be galvanised upon request. This is often requested for transformers constantly exposed to the elements, for pole-mounted or platform-mounted transformers.