Phase shifting transformers

The bulk transfer of power is mostly a passive affair, where the power flows to the load by the lowest impedance path. To influence this flow, the phase angle of the sending or receiving end must be modified – this may be done by the use of Phase Shifting Transformers. The phase shifting transformer introduces a circulating current into the system which acts to either reduce (when operating in buck mode) or increase (when operating in boost mode) the load current in the line where the phase shifting transformer is located.

The actual phase shift is achieved by the interconnection of two phases of the star-connected regulating windings across one phase of the delta connected “booster” winding. By on-load tap changing, the magnitude and polarity of the phase shift can be modified, which in turn determines the circulating current in the system. For lower power ratings the two active parts may be placed in a single tank. 

However, in higher power phase shifting transformers, the two active parts (the exciter and booster transformers) are mounted in separate tanks and the two active parts are connected together. This connection between the exciter and booster transformers is done at full system voltage and must normally be done inside oil-filled trunking, to guard against the risk of a damaging internal short circuit occurring.

SGB-SMIT Group has excellent experience of large phase shifting transformers, including the capability to analyse the complex transient voltages that may occur during external switching or lightning events. 

Exciter transformer, with current-steering transformers of dual tank (exciter and booster transformer)
3 phase 700 MVA; 230/230 kV; +/- 30º
Single tank 3 phase 350 MVA; 115/115 kV; +/- 60°, view from HV side with on-load tapchangers
Single tank 3 phase 350 MVA; 115/115 kV; +/- 60°, on transport to site

Technical Information

Design

Royal SMIT Transformers B.V. has an ongoing engineering evaluation process for continuous improvement and upgrade of its design programs.

Royal SMIT engineering is making full use of a high end 3D-CAD system for the complete transformer design. Active parts and tanks can be combined in one model allowing the best possible check on correct dimensioning.

Almost all engineers use 3D-CAD stations. The introduction of 3D-CAD technology was thoroughly prepared over the past years. A team of experts worked fulltime on the required tank- and active part models.

The great advantages for SMIT as well as its customers are:

  • constant quality because of modular design
  • large reduction of engineering time for standardised transformer designs
  • reduced engineering lead time for even the most complicated transformers

Unique design features

Clamping of windings

Detail of independent coil clamping

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.

Smit style windings

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.

Tank design

Left: measured tank vibrations
Right: FEM model of tank vibrations

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.

Downloads

References

Power Generation

Reliable Transformers with the unique single winding clamping construction are shipped wordwide to customers producing power. We have extensive experience with requirements for the nuclear industry and the renewable energy. The single winding clamping construction gives additional strength regarding short circuits and does not influence the core clamping. 

Solar power stations
Nuclear Power plants
Oil and Gas Power plants

Power Distribution

Large Power Transformers have been delivered for customers choosing for Transformers with a long life span, high reliability and with the approved unique single winding clamping construction.

High Voltage distribution
Medium Voltage distribution

Industry

Reliable energy supply in all production processes is of main importance for our customers. Reliable Transformers are part of this philosophy. Our Transformers have been delivered all over the world for multiple industries chosing for quality and reliability.

Chemical
Oil and Gas
Steel

Railways

Short Circuit Resistance is a major concern for customers within the railway industry. That is why our Transformers are chosen, being very reliable due to the unique single winding clamping construction and the high quality of our Transformers. 

High Speed
Mass Transit Systems

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