Auto-transformers - SGB-SMIT Group (2024)

At higher voltages, large auto-transformers are used to interconnect the 400kV-500kV systems with either the 230kV-275kV systems or 120kV-145kV systems (the exact values depend on the country being considered).

The HV and MV windings in an auto-transformer are connected together unlike in other transformers. The result is that the MV winding (common winding) is shared between the MV and HV systems. This results in a smaller and less expensive transformer than if the windings were separated as in a conventional fully rated two winding transformer.

Royal SMIT Transformers B.V. produces large power transformers with independent clamping of the core and windings. This clamping system is unique amongst manufacturers of large power transformers.

The independent clamping arrangement has many advantages:

• The windings are clamped independently from the core, so no stresses are transferred to the core steel material, resulting in improved core losses in service.
• Unlike a traditional clamping system, with the Royal SMIT style of clamping each phase of a 3-phase transformer may be clamped individually. This means that the clamping pressure can be easily adapted for small variations between the different phases, resulting in better supported windings and greatly improved short-circuit performance of the transformer.
• The coil clamping pressure is maintained by the steel tie-rods connecting the upper and lower steel clamping plates, meaning the winding pressure is maintained for the life of the transformer.

Royal SMIT style windings differ from most of our competitors in having axial cooling ducts without pressboard radial spacers. This type of winding style has several advantages:

• As there are no pressboard axial spacers present in the winding, there is less axial shrinkage of the windings, so that the winding clamping pressure is maintained throughout the life of the transformer, resulting in improved short-circuit withstand capabilities.
• The mechanically robust windings that result from the Royal SMIT style windings give excellent load noise performance in service.
• The axial cooling channels allow excellent oil flow through the windings. Most importantly, their width is independent of the axial clamping pressure. This is an advantage over traditional radial cooling ducts, which may greatly reduce in size due to the shrinkage of the axial pressboard spacers during drying and clamping.

Royal SMIT Transformers B.V. has developed finite element models of the transformer tank design allowing the tank stresses and vibration levels to be accurately predicted:

• The tank design may be optimised in terms of cost and weight, reducing the overall shipping dimensions and weight.
• The vibration levels on the tank surfaces can be greatly reduced, giving increased reliability in service.
• A stiffer tank design transmits less of the internal noise generated by the core (the main source of no-load noise) and windings (the main source of load noise) to the outside, resulting in a quieter transformer in service. This is especially important given the tighter environmental controls now in place for noise pollution.