Trihal cast resin dry type transformers (ENG) - Trinet

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5. Dimensioning the transformer• To prevent the windings overheating :☞ The first solutioninvolves oversizing the transformer or limiting its load if it is a piece of equipmentwhich is already installed.Standards organisations UTE, CENELEC, ...) have adopted or are currently studyingderating equations such as :K = [1 + 0,1 * (Ì Hi 1,6 * Ti 2 )] -0,5See chapter 1 Hi = harmonic numberTi = harmonic factor expressed as a % of the transformer’srated intensi ty• Example 1A 1000 kVA transformer delivers a current which includes the following harmonics :H5 = 25 % H7 = 14 % H11 = 9 % H13 = 8 %K = [1 + 0,1 * (5 1,6 * 0,25 2 + 7 1,6 * 0,14 2 + ...)] -0,5The calculation gives k = 0,91 : the transformer will only supply 910 kVA.• Exemple 2We want to determine the size of a transformer needed to supply 1000 kVA withharmonics of number H = 6 x n Ù 1, with n = 1,2, ...etc and with a factor of 1/H(typical case of a six-phase rectifier).By limiting the calculation to number 29, the coefficient K will be 0,87.K = [1 + 0,1 * (5 1,6 *0,2 2 + 7 1,6 * 0,14 2 + 11 1,6 * 0,09 2 ...)] -0,5The minimum power for the transformer becomes 1000 / 0,87 = 1150 kVAF:Dcial/MK/MKT/InfoMK/Anglais/GENERAL/TIGEA09 6
☞ The second solutionThis involves dimensioning of a transformer which is specially adapted to supplya harmonic generator, subject to prior agreement between the user and the manufacturer.• Against saturation of the magnetic core :when the extreme conditions described in § 3 are specified, it may be necessaryto :⇒ specify a transformer operating with a low-flux density to account for the highharmonic flux.⇒ specify a gapped magnetic core to limit the influence of any possible continuouscomponent.6. ConclusionsWhatever the configuration and the solution chosen, it is essential that there isdialogue between the user and the manufacturer.This concise analysis of the influence of harmonic waves shows that the problemis complex :• the consequences on elements of the network due to the overloads and overvoltageswhich the harmonics produce can be dangerous.• attention must be paid to controlling and attenuating harmonic interferenceeffects.• the influence of harmonics makes it necessary to increase the size of equipmentand results in a large increase in cost (up to 50 % in extreme cases).BibliographyEDFMerlin GerinLes Cahiers de l'Ingénierie (Engineering Manuals)Qualité de l'alimentation électrique (Quality of electrical supply)Cahiers techniquesF:Dcial/MK/MKT/InfoMK/Anglais/GENERAL/TIGEA09 7
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Trihal cast resin dry type transformers (ENG) - Trinet

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