Electromagnetic simulator accelerates the design optimisation of custom transformers

Kolektor Etra manufactures a wide range of power and generator transformers with ratings up to 500 MVA and voltages up to 420 kV, as well as distribution transformers rated up to 3150 kVA. The company has been in existence for 78 years, and in addition to its long-term domestic markets, it now sells extensively in mainland Europe and Scandinavia.

Before investing in Opera-3D, Kolektor Etra used some free finite element software to perform tasks such as flux density and other electro-magnetic calculations. However, the lack of any 3D modelling capabilities meant that the company found it difficult to optimise their design in cases of unsymmetrical geometry, such as winding exits, core clamping structures, etc.. It therefore took the decision to move to a professional finite element package that offered 3D modelling.

After evaluating electromagnetic design software from several vendors, Kolektor Etra chose Opera-3D from the Vector Fields Software product line of Cobham Technical Services. Aside from its powerful 3D electromagnetic modelling capabilities, the principal reasons for the choice were that the software provides a simple means of calculating eddy current losses and has an intuitive user interface. Kolektor Etra was also impressed by Cobham's technical support.

Like most power transformer manufacturers, Kolektor Etra is under constant pressure to improve product efficiency by optimising insulation design and reducing electrical losses, which involves a host of design trade-offs such as insulating distances, insulation thickness, choice of electromagnetic shields, and the placement and geometry of internal and external structural elements. These issues are compounded by the desire to create ever more compact designs, in order to save on materials.

According to Andrej Jurman, Head of Kolektor Etra's electrical design department, We use 3D Opera models to optimise the insulation design of transformer winding exit structures, and also make use of 2D models for designing the insulation between windings. But perhaps the most significant benefit is that we can now calculate and estimate the value of eddy current losses caused by stray magnetic flux. Although it is almost impossible to obtain the exact results (mostly because of the unknown magnetising curves of ferromagnetic materials and consequently an unknown value of surface impedance), our engineers now have a very powerful means of evaluating and comparing the performance of different design concepts.

Kolektor Etra recently conducted some research into eddy current losses due to stray magnetic fields that occur in the conductive structural parts of a three-phase oil-filled power transformer, particularly in the tank walls. After constructing a three dimensional geometric model, Kolektor Etra used Opera and a time harmonic 3D model at power supply frequency to investigate the effect of different configurations of magnetic screening shield and the shape of core clamping structures on performance. Analysis showed that the stray flux losses which are part of transformer short circuit losses were significantly reduced by approximately 30%.

As Andrej Jurman points out, Gaining a clear picture of the effect of design changes can provide significant cost-savings, which is why we regard the Opera electromagnetic field simulator as such a useful tool. Although the losses caused by stray fields are very hard to separate out from measured short circuit losses in a transformer accurately, modelling enables us to see the shape of stray flux and the places with the highest local loss generation visually. In this way we can evaluate and improve the efficiency of our designs and perform certain measurements. Because a large proportion of our transformer products are one-off designs, we do not model every transformer in detail - but our simulation results heavily influence our general design strategy, helping to maximise the efficiency of every product we make and providing us with a tool for continuous optimisation.