Electricity + Control February 2019

TRANSFORMERS + SUBSTATIONS

Transformer design: Changing for a better tomorrow

by Aaron Rost, Trafo Power Solutions

Electrical transformers have been around for well over 130 years, and although the operating principle has remained the same, with time, design refinements have en- hanced transformers’ efficiency and reliability.

Take Note! The two main losses in any transformer are core losses and copper losses. Design enhancements have improved the effi- ciency of transformers.

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T raditional oil filled, paper insulated transform- ers are predominately used in the power grid systems we know today.They use paper sat- urated in oil wrapped around the winding material as an insulation medium. If not maintained correct- ly, insulation degradation will occur. For instance, if an oil filled transformer has a leaking drain tap which is left for a period of time the natural heating and cooling cycles promote moisture ingress. This will lead to a breakdown of the cellulose structure of the paper insulation and will reduce the dielec- tric strength of the oil. With these two conditions present, partial discharge will inevitably occur and further erode the insulation properties of the paper and oil. Over time, an arc will begin to flow, leading eventually to complete transformer failure. If not caught in time, transformer failures can lead to extensive damage to property due to oil being ignited. Oil fires are notoriously difficult to extinguish and the potential for these introduces additional requirements for oil filled transformers, including fire suppressing systems and room de- sign considerations such as bund walls. To eliminate the drawbacks of oil filled trans- formers, several solutions have been created in the form of OpenWoundTransformers (OWT), Vac- uum Pressured Impregnated Transformers (VPI) and Cast Resin Transformers (CRT). OWTs are constructed by dipping preheated windings into a high temperature varnish bath and then baking the high temperature varnish. This re- places the need for oil and paper, and only a small amount of material is flammable. In the VPI construction technique, layers of poly-

ester resin are applied to the windings, which are subjected to interchanging cycles of pressure and vacuum that ensure deeper penetration of the resin within the windings. This reduces the chance of air voids occurring. The windings are cured in an oven. With CRT construction the windings are placed in a mould that is filled under vacuum with resin epoxy. Within the mould, fibreglass reinforc- ing mesh is used to further strengthen the wind- ings, which are then cured in a heat-controlled oven. The process prevents the formation of air voids and the resin specified for CRTs is non-flam- mable. With the addition of fibreglass mesh, the medium voltage windings can handle shock loads such as sudden load changes, with less chance of the medium voltage windings deforming. Losses of a transformer Transformers are simple electrical devices that work off the principle of electromagnetic induction.There are no moving parts, so where do the losses come from? Unfortunately, there is no ideal transformer and the losses arise from the electrical character- istics of the materials from which a transformer is constructed. The two main forms of losses found in any transformer are core losses and copper losses. Core losses, often referred to as ‘no load’ loss- es, pertain to the core steel losses. They stem from eddy currents and hysteresis losses. Eddy currents are caused by the magnetic flux of the pri- mary windings inducing a current within the core. The current flows in circles within the core and, since the core has an ohmic value, wasted power in the form of heat is generated. Hysteresis loss-

Cross-section of a medium voltage winding.

32 Electricity + Control

FEBRUARY 2019