- Core Form Transformer
Winding
Each coil is designed and manufactured to withstand lightning and switching impulses and short-circuit forces. Copper conductors are insulated with thermally-upgraded paper and transposed in regular intervals to minimize circulating currents ensuring uniform temperature distribution and controlled voltage stresses along the winding.
Insulation Material
High density pressboard is formed into parts such as cylinders, washers and end rings and used along with spacers and blocks to make up the insulation system. The material is kept at very low moisture levels during manufacturing and dried further then impregnated with oil when the fully assembled transformer is processed.
Core
The core consists to 2, 3, 4 or 5 column structures, depending on the number of phases, capacity and transport restrictions. The core material is a cold-rolled, non-ageing silicon-steel with high permeability and low hysteresis loss. Each sheet is coated with an inorganic coating to insulate the sheets and reduce eddy current levels. Core legs are secured with epoxy glass bands and yokes are secured with insulated stainless steel bands to provide structural rigidity during transport and operation.
Core and Coil Assembly
After the windings have been constructed, they are clamped and dried under iso-static pressure to remove moisture from the insulation and shrink the windings to their clamped, in-service height. The individual windings are nested into a phase assembly in a concentric manner over each core leg followed by installation of the leads and support structures. The windings are then clamped under pressure to the core frame and the complete active part is dried using a vapor-phase process. After the final vapor-phase process, all windings are re-compressed to their designed values and all bolted connections are re-tightened. Transformers rated 80 MVA and greater are typically equipped with dash-pots to provide constant clamping pressure to the windings under all conditions. Kian Tajhiz’s assembly rooms are controlled-access positive-pressure clean rooms designed to minimize dust and other contaminants during the assembly.
Tank
The transformer is provided with a suitable steel tank of substantial construction which is oil-tight and gas-tight. The tank has sufficient strength to withstand an internal pressure of full vacuum and vacuum drying without damage or permanent deformation. The transformer tank is provided with manholes and hand holes of suitable size to afford easy access to the lower ends of bushing terminals, upper portions of the coils and side position of the tap changer.
Final Assembly
All components like bushings, radiators, conservator and accessories are assembled on the transformer tank. Gaskets are made of materials which do not deteriorate under service conditions and used for sealing between flanges to prevent leakage. Valves made of bronze, brass and stainless steel are used according to customer requirements for oil filing, draining, vacuuming and shut off. After complete assembly, the transformer is filled with insulating oil under vacuum.
- Shell Form Transformer
Winding
High dielectric strength paper is spirally machine-wound in overlapping layers over each strand and layer of subdivided conductors. The outermost tape layer has a thermoset coating to bond the coils after clamping and during drying to ensure high mechanical strength.
Insulation Material
High density pressboard sheets are formed into channels, washers and other insulation parts to make up the insulation system. These parts are interleaved around and between the coils during stacking to ensure that proper electrical clearances are maintained. Pressboard parts are only used in compression.
Core and Coil Assembly
The coil assemblies are lowered into the form-fit tank bottom and wedged securely in place. Hipersil laminations, stacked on fill blocks around the coils and on a T-beam through the coil openings, are applied with overlapped joints to increase mechanical strength and keep exciting current at a minimum. The core section in the coil openings are wedged at the top to compress the core and to hold the coil assembly rigid. Wedges are driven between phases, top fill blocks are positioned and vertical wood pieces, with pressboard channels are connected between the top and bottom fill blocks.
Tank
The form-fit shell form tank is designed and manufactured into a complete unit with high mechanical strength to withstand the effects of transport, vacuum and vibration. The transformer tank is provided with manholes and hand holes of suitable size to afford easy access to the lower ends of bushing terminals, upper portions of the coils and side position of the tap changer.
Final Assembly
All components like bushings, radiators, conservator and accessories are assembled on the transformer tank. Gaskets are made of materials which do not deteriorate under service conditions and used between two separate flanges to prevent leakage. Valves made of bronze, brass and stainless steel are used for oil filing, draining, vacuuming and shut off. After complete assembly, the transformer is filled with insulating oil under vacuum.