Trihal cast resin dry type transformers (ENG) - Trinet

3. The effects of harmonics on a transformer’s behaviourA transformer is defined by its ratedpower which is equal to the productof no load voltage and the ratedcurrent (times √3 for three-phase)and is calculated as a function of thenetwork frequency.However, as we have seen in theprevious chapter, a harmonic generatorsignificantly changes the transformer’sdimensioning characteristics:• Ohmic lossesif no account is taken of harmoniccurrents, the power that the transformershould deliver will be greaterthan its rated power, leading to anincrease in Ohmic losses (RI 2 ) andtherefore to a risk of deteriorating thedielectric qualities of the transformerdue to over-heating.• Eddy current lossesEddy currents are proportional to the square of the frequency. However, a harmoniccurrent is of high frequency and could thus, even with a low factor, causeadditional losses which can not be ignored.• the magnetic corein extreme cases, two phenomena can disrupt the correct operation of a transformer:⇒ at high frequency harmonic currents there are corresponding harmonic fluxeswhich are superposed on the fundamental flux. Depending on the upstream impedance,these fluxes can increase the resulting peak flux value.⇒ a possible continuous component will create a shift in the hysteresis cyclerelative to zero. For half a cycle, the magnetic core is subjected to a flux whichis too high.These two cases can result in the induction level reaching the saturation peakfor the magnetic core, in an increase in the ferromagnetic losses of magnetis-F:Dcial/MK/MKT/InfoMK/Anglais/GENERAL/TIGEA09 4