Critical stress of oval foil winding with epoxy coated insulation determined using measured equivalent modulus of elasticity

In the distribution transformers design oval windings are used due to economic advantages. On the other hand, such windings are more susceptible to radial forces in a short circuit. A diamond dotted paper with an epoxy coating is used in order to increase the stiffness of the winding. Despite that, winding failure may occur during the short circuit, e.g. buckling of inner winding. Because of a very thin foil conductor (typically 0.5–2 mm), the most critical is inner low voltage foil winding which can collapse due to radial forces at stresses far below the elastic limit of conductor material. This paper shows an analytical approach to the calculation of critical stress in inner oval foil winding with epoxy coated insulation. Critical stress was calculated using the equation for free buckling of round winding. Equivalent Young\u27s modulus of elasticity was obtained experimentally from the testing of the sample model loaded with bending force on a tensile test machine. A total of 12 test samples were formed from aluminium foil conductor and diamond dotted paper and cured at the temperature of 105°C. The results were successfully verified on distribution transformers subjected to short circuit withstand tests

DESIGN IMPROVEMENTS IN MODERN DISTRIBUTION TRANSFORMERS

In the paper design improvements of distribution transformers related to improved energy efficiency and environmental awareness are discussed. Eco design of transformers, amorphous transformers, voltage regulated transformers and transformers filled with ester liquids are analyzed. As a consequence of growing energy efficiency importance, European Commission has adopted new regulation which defines maximum permissible levels of load and no-load losses of transformers with rated power ≤ 3150 kVA, and minimum peak efficiency index for transformers with rated power > 3150 kVA up to 40 MVA. The impact of new regulation on the design and economy of transformer is presented. Amorphous transformers, with up to 70 % lower no-load losses in comparison to the conventional transformers, could be an alternative with respect to energy efficiency. Although their initial price is higher than the price of conventional transformers, some studies show that they might have economic advantages. The increasing penetration of distributed energy sources can cause an increase in voltage variations in low voltage networks. To keep the voltage within limits defined by EN50160, voltage regulated distribution transformers could be used. Although mineral oil has been used as a dielectric fluid in transformers for many years, there are some environmentally friendlier alternatives – natural and synthetic ester-based fluids

Short circuit withstand capability of oval foil windings in distribution transformers

Transformator je vrlo važan dio elektroenergetskog sustava pa je samim time jedan od ključnih zahtjeva tijekom projektiranja dimenzioniranje na vanjski kratki spoj kako bi se osigurao nesmetani rad. Budući je kratki spoj jedan od češćih uzroka kvarova transformatora, nužno je imati kriterije čijim bi se zadovoljavanjem u fazi projektiranja transformatora omogućilo da transformator izdrži kratki spoj. Tema otpornosti i dimenzioniranja energetskih transformatora detaljno je obrađena u literaturi, dok za suvremene distribucijske transformatore s ovalnim folijskim namotima i dijamantnim papirom nema dovoljno literature. Upravo je to tema ove disertacije. Ova disertacija bavi se određivanjem numeričkih kriterija za dimenzioniranje ovalnih folijskih namota distribucijskih transformatora na kratki spoj. Objašnjeno je ispitivanje koje transformator mora zadovoljiti zajedno s izrazima za izračun udarne (dinamičke) komponente struje kratkog spoja, kao i uvjeti za prolaznost ispitivanja prema standardu IEC 60076-5. Prolazak ispitivanja transformatora na kratki spoj potvrđuje otpornost transformatora na kratke spojeve u mreži i osigurava nesmetan rad transformatora. Uz analitičke izraze za djelovanje sile na namote u transformatoru, obrađena su i posljedična radijalna naprezanja koja se javljaju tijekom kratkog spoja. Uz naprezanje na vlak i tlak te savijanje, posebna pozornost je dana naprezanju na izvijanje. Napravljeno je opsežno ispitivanje Youngovog modula elastičnosti kompozita čiji rezultati su korišteni za definiranje kriterija za dimenzioniranje transformatora na kratki spoj. Verifikacija samih kriterija napravljena je na 43 transformatora u rasponu snaga 160 1000 kVA koji su ispitani prema standardu IEC 60076-5. Neki od ispitanih transformatora bili su iz redovne proizvodnje dok su neki napravljeni isključivo u svrhu verifikacije novih kriterija.The transformer is an important part of the power system and must be designed to be able to withhold an external short circuit. External short-circuits in the power systems are still one of the most common causes of the transformer malfunction. Therefore, it is crucial to have criteria during the designing phase the fulfillment of which would allow the transformer to withstand a short-circuit. There is a lot of published literature about power transformers and how they can be made short-circuit proof while there is not enough literature for modern distribution transformers with oval foil windings and diamond dotted paper. That is the focus of this dissertation. This dissertation describes the numerical criteria for the oval foil windings designing to be able to withstand short-circuit in the power system. The short-circuit testing procedure together with the expressions for calculating short-circuit current, as well as the conditions for passing the test according to the IEC 60076-5 standard are given. Short-circuit forces and stress in the windings is explained. Along with the analytical expressions for the forces in the windings, the radial stresses that occur during a short circuit are described. In addition to tensile and compressive stress and bending, special attention is paid to buckling stress and Young's modulus of elasticity. The verification of the criteria was done on 43 transformers in the power range 160 1000 kVA that were tested according to the IEC 60076-5 standard

Short circuit withstand capability of oval foil windings in distribution transformers

Transformator je vrlo važan dio elektroenergetskog sustava pa je samim time jedan od ključnih zahtjeva tijekom projektiranja dimenzioniranje na vanjski kratki spoj kako bi se osigurao nesmetani rad. Budući je kratki spoj jedan od češćih uzroka kvarova transformatora, nužno je imati kriterije čijim bi se zadovoljavanjem u fazi projektiranja transformatora omogućilo da transformator izdrži kratki spoj. Tema otpornosti i dimenzioniranja energetskih transformatora detaljno je obrađena u literaturi, dok za suvremene distribucijske transformatore s ovalnim folijskim namotima i dijamantnim papirom nema dovoljno literature. Upravo je to tema ove disertacije. Ova disertacija bavi se određivanjem numeričkih kriterija za dimenzioniranje ovalnih folijskih namota distribucijskih transformatora na kratki spoj. Objašnjeno je ispitivanje koje transformator mora zadovoljiti zajedno s izrazima za izračun udarne (dinamičke) komponente struje kratkog spoja, kao i uvjeti za prolaznost ispitivanja prema standardu IEC 60076-5. Prolazak ispitivanja transformatora na kratki spoj potvrđuje otpornost transformatora na kratke spojeve u mreži i osigurava nesmetan rad transformatora. Uz analitičke izraze za djelovanje sile na namote u transformatoru, obrađena su i posljedična radijalna naprezanja koja se javljaju tijekom kratkog spoja. Uz naprezanje na vlak i tlak te savijanje, posebna pozornost je dana naprezanju na izvijanje. Napravljeno je opsežno ispitivanje Youngovog modula elastičnosti kompozita čiji rezultati su korišteni za definiranje kriterija za dimenzioniranje transformatora na kratki spoj. Verifikacija samih kriterija napravljena je na 43 transformatora u rasponu snaga 160 1000 kVA koji su ispitani prema standardu IEC 60076-5. Neki od ispitanih transformatora bili su iz redovne proizvodnje dok su neki napravljeni isključivo u svrhu verifikacije novih kriterija.The transformer is an important part of the power system and must be designed to be able to withhold an external short circuit. External short-circuits in the power systems are still one of the most common causes of the transformer malfunction. Therefore, it is crucial to have criteria during the designing phase the fulfillment of which would allow the transformer to withstand a short-circuit. There is a lot of published literature about power transformers and how they can be made short-circuit proof while there is not enough literature for modern distribution transformers with oval foil windings and diamond dotted paper. That is the focus of this dissertation. This dissertation describes the numerical criteria for the oval foil windings designing to be able to withstand short-circuit in the power system. The short-circuit testing procedure together with the expressions for calculating short-circuit current, as well as the conditions for passing the test according to the IEC 60076-5 standard are given. Short-circuit forces and stress in the windings is explained. Along with the analytical expressions for the forces in the windings, the radial stresses that occur during a short circuit are described. In addition to tensile and compressive stress and bending, special attention is paid to buckling stress and Young's modulus of elasticity. The verification of the criteria was done on 43 transformers in the power range 160 1000 kVA that were tested according to the IEC 60076-5 standard

Short circuit withstand capability of oval foil windings in distribution transformers

Transformator je vrlo važan dio elektroenergetskog sustava pa je samim time jedan od ključnih zahtjeva tijekom projektiranja dimenzioniranje na vanjski kratki spoj kako bi se osigurao nesmetani rad. Budući je kratki spoj jedan od češćih uzroka kvarova transformatora, nužno je imati kriterije čijim bi se zadovoljavanjem u fazi projektiranja transformatora omogućilo da transformator izdrži kratki spoj. Tema otpornosti i dimenzioniranja energetskih transformatora detaljno je obrađena u literaturi, dok za suvremene distribucijske transformatore s ovalnim folijskim namotima i dijamantnim papirom nema dovoljno literature. Upravo je to tema ove disertacije. Ova disertacija bavi se određivanjem numeričkih kriterija za dimenzioniranje ovalnih folijskih namota distribucijskih transformatora na kratki spoj. Objašnjeno je ispitivanje koje transformator mora zadovoljiti zajedno s izrazima za izračun udarne (dinamičke) komponente struje kratkog spoja, kao i uvjeti za prolaznost ispitivanja prema standardu IEC 60076-5. Prolazak ispitivanja transformatora na kratki spoj potvrđuje otpornost transformatora na kratke spojeve u mreži i osigurava nesmetan rad transformatora. Uz analitičke izraze za djelovanje sile na namote u transformatoru, obrađena su i posljedična radijalna naprezanja koja se javljaju tijekom kratkog spoja. Uz naprezanje na vlak i tlak te savijanje, posebna pozornost je dana naprezanju na izvijanje. Napravljeno je opsežno ispitivanje Youngovog modula elastičnosti kompozita čiji rezultati su korišteni za definiranje kriterija za dimenzioniranje transformatora na kratki spoj. Verifikacija samih kriterija napravljena je na 43 transformatora u rasponu snaga 160 1000 kVA koji su ispitani prema standardu IEC 60076-5. Neki od ispitanih transformatora bili su iz redovne proizvodnje dok su neki napravljeni isključivo u svrhu verifikacije novih kriterija.The transformer is an important part of the power system and must be designed to be able to withhold an external short circuit. External short-circuits in the power systems are still one of the most common causes of the transformer malfunction. Therefore, it is crucial to have criteria during the designing phase the fulfillment of which would allow the transformer to withstand a short-circuit. There is a lot of published literature about power transformers and how they can be made short-circuit proof while there is not enough literature for modern distribution transformers with oval foil windings and diamond dotted paper. That is the focus of this dissertation. This dissertation describes the numerical criteria for the oval foil windings designing to be able to withstand short-circuit in the power system. The short-circuit testing procedure together with the expressions for calculating short-circuit current, as well as the conditions for passing the test according to the IEC 60076-5 standard are given. Short-circuit forces and stress in the windings is explained. Along with the analytical expressions for the forces in the windings, the radial stresses that occur during a short circuit are described. In addition to tensile and compressive stress and bending, special attention is paid to buckling stress and Young's modulus of elasticity. The verification of the criteria was done on 43 transformers in the power range 160 1000 kVA that were tested according to the IEC 60076-5 standard