Core is the part of the transformer where the magnetic field oscillates.
Core material is a high grade super oriented steel with a small percentage of silicone. All this is to make sure that the energy losses that result in heat are minimized. The core steel is rolled into thin sheets, say 20 gauge, and each side is coated with insulating material. The purpose of these thin sheets is to further minimize losses. For power transformers, these sheets are stacked together with mitered joints, by varying the width of steel laminations in the leg of mitered core, a circular cross section can be approximated. The process involves core cutting and stacking equipment to construct core frame. Automatic core cutting machines have become standard now for power transformer cores. It’s the stacking of core that requires extra care and precision. One has to be careful about:
+ gap at the step lap needs to be small, under 2mm.
+ Control of burrs, thin, steel threads that break loose in oil and create partial discharge later.
+ Control of square and perpendicularity of magnetic circuit.
These are some of the few we’re naming. In summary, manual stacking a common practice in the industry, is cumbersome, time consuming and prone to errors in case of a large power transformer construction.
At GT, we use state of the art Astronics core cutting assembly unit. Astronics automated, core cutting equipment allows precise cutting of single and multi-leg transformer core.
Integrated with the core cutter is an automated E-stacker. The E-stacker stacks the core steel into final position as it is cut. This technique practically eliminates human errors, and lowers losses The E-stacker is a major investment, but then the payoff is in the form of higher reliability and of course higher efficiency. At other VTC plants a different version of core stacking is used for medium power transformers.