54. IWK Internationales Wissenschaftliches Kolloquium International Scientific Colloquium

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1 7-1 September 29 PROEEDINGS 54. IWK Interntionles Wissenschftliches Kolloquium Interntionl Scientific olloquium Informtion Technology nd Electricl Engineering - Devices nd Systems, Mterils nd Technologies for the Future Fculty of Electricl Engineering nd Informtion Technology Strtseite / Inde:

2 Impressum Herusgeber: Redktion: Der Rektor der Technischen Universität llmenu Univ.-Prof. Dr. rer. nt. hbil. Dr. h. c. Prof. h. c. Peter Schrff Refert Mrketing Andre Schneider Redktionsschluss: 17. August 29 Fkultät für Elektrotechnik und Informtionstechnik Univ.-Prof. Dr.-Ing. Frnk Berger Technische Relisierung (USB-Flsh-Ausgbe): Institut für Medientechnik n der TU Ilmenu Dipl.-Ing. hristin Weigel Dipl.-Ing. Helge Drumm Technische Relisierung (Online-Ausgbe): Universitätsbibliothek Ilmenu Postfch Ilmenu Verlg: Verlg ISLE, Betriebsstätte des ISLE e.v. Werner-von-Siemens-Str llmenu Technische Universität llmenu (Thür.) 29 Diese Publiktionen und lle in ihr enthltenen Beiträge und Abbildungen sind urheberrechtlich geschützt. ISBN (USB-Flsh-Ausgbe): ISBN (Druckusgbe der Kurzfssungen): Strtseite / Inde:

3 TEMPERATURE PROBLEMS IN THE FURNAE TRANSFORMER AGGREGATES, ONSISTING OF THE MAIN AND THE VOLTAGE-ADDING TRANSFORMER Kiril Stoikov 1, Simon Filipov-Petrkiev 2, Vleri Mldenov 3 1 TU-Sofi; 8, Kl. Ohridski St., Deprtment Bsics of Electricl Engineering; k_stoykov@tu-sofi.bg 2 TU-Sofi; 8, Kl. Ohridski St., Deprtment Theory of Electricl Engineering; petrkievs-te@tu-sofi.bg, vlerim@tu-sofi.bg ABSTRAT The pper discusses the distribution of the therml field in the mgnetic core of mono-phse trnsformer ggregte including the min nd the voltge-dding trnsformer. The min im of the nlysis is to locte the most overheting section of the mgnetic system studied nd to investigte this overhet in the different overlods. The eperimentl dt re compred with the simultion ones nd the resulting error is less thn 5 %. Inde Terms therml field, mono-phse trnsformer ggregte, overheting 1. INTRODUTION The furnce trnsformers nd trnsformer ggregtes work continuously under vrible lod which chnges from open-circuit to short-circuit. In ddition to this the lod is not symmetricl. Under these circumstnces this type of trnsformers requires complete nlysis of electrodynmic nd het problems. Modern ppliction of electrothermics in industry is connected with development n improvement of new more powerful furnce trnsformers nd ggregtes. With incresing of power the het problems increse, too, so the trnsformers must be thermlly designed is such wy tht the temperture of every prt of them to meet stndrd vlues under given gurnteed opencircuit to short-circuit looses. Otherwise locl overhetings could be risen. The investigtions show tht these overhetings frequently tke plce in mgnetic cores. The trnsformers losses relese het in trnsformer windings. According to effective stndrd mimum llowed temperture vlues of mgnetic system is 115 (on surfce) nd 95 (for trnsformer oil). If temperture rises with 8 then the life of trnsformers decreses up to two times. Trnsformer oil conducts the het of ctive prts of trnsformer to the wlls of the tnk nd from there to the ir. The hot lyers of the oil move to the upper prt of the tnk while colder ones move to lower prt. Since the temperture difference between tnk wlls nd ir is dozens of degrees the crrying of the het is fulfilled both by convection nd rdition. The high temperture on the surfce of the core could deteriorte the performnce of both trnsformer oil nd the hrd insultion (electro insultion cylinders). A possible result could be fire in the steel. 2. PROBLEM STATEMENET The trnsformer is the non-homogeneous object with respect to the heting processes. The problem connected with heting could be solved if the following is given: quntity of het relese for unit time, its loction nd possibility of its tking wy. It is known tht there re open-circuit losses in operting mode of trnsformer, nd the losses depending on property of mterils (steel, cooper, luminum, etc.) nd lso of the design of the trnsformer. Open-circuit losses re connected with hysteresis losses nd eddy current losses while short-current losses include windings losses nd dditionl losses of leking field. Serious overheting problems rise with incresing the power of trnsformers. In prctice, for decresing the totl losses nd tking wy the temperture, suitble coolers re used. In spite of ll tht hs been told, it is very difficult temperture of rel trnsformer to be mesured. In this pper n ttempt for n nlyticl investigtion of the temperture distribution in the core of n oil trnsformer ggregte is mde. A single-phse furnce trnsformer ggregte with min trnsformer (MT) nd voltge-dding trnsformer (VT) is considered [1]. A common mgnetic circuit for both trnsformers is used (Fig. 1). Mgnetic system for both trnsformers (MT nd VT) consists of three cores. The three windings of the MT (1, 2 nd 3) re on the one of the cores (b). The two windings of VT (4 nd 5) re on the other core (). The third core (c) is without ny windings. This core usully is clled mgnetic shunt. The sum of mgnetic flues b " Φ + Φ nd their difference b Φ Φ of mgnetic flues re closed through it th Interntionles Wissenschftliches Kolloquium

4 In prctice there re different designs of mgnetic cores. In spite of the design ech mgnetic system consists of core nd yoke. The shpe of the crosssection of the core nd yoke of power trnsformer ggregte is given on Fig. 2. Steel lmelle re suitble rrnged in order to obtin necessry form (Fig. 3). M M Figure 1 Electricl circuit of trnsformer ggregte with three-core mgnetic system with reversion of control winding H B B B D B The following equlities for mgnetic shunt re vlid: Φ = Φ + Φ (1) c b " " Φ c = Φb Φ. (2) Here, the winding 1 is primry one tht ecites constnt mgnetic flu. The windings 2 nd 5 re secondry ones for the MT nd VT. They re connected in series nd in this wy they form the secondry windings of the entire trnsformer ggregte. Winding 3 of MT ctivtes the VT winding 4 which ecites mgnetic flu in its core. The mgnitude of the flu depends on the stte of the control winding. In the first cse in the core (c) the inductnce nd open-circuit losses increse while in the second cse they highly decrese - s result the dimension of the cross-section of the core could be smller. If the trnsformer works in hevy mode nd of the sme time there is n overloding, i.e. shortcircuit stte in which prcticlly ggregtes operte longer, then both the inductnce nd open-circuit losses considerbly increse. Ectly in this cse is quite possible locl overheting of the core to occur. As result both the core lmelle nd yoke insultion deteriortes, lso the insultion properties of the trnsformer oil worsen. The purpose of this pper is to nlyze the distribution of temperture field in given mgnetic circuit, i.e. to loclize the most thermlly loded prts nd to investigte the overheting in different cses of overlodings. Figure 2 The shpe of the cross-section of the core nd yoke of the power trnsformer ggregte B B A A Figure 3 Steel lmelle of the core nd yoke of the power trnsformer ggregte Overheting could be decresed with the so-clled cooling grooves. During the design process of the core usully one ims t mimum possible effective cross-section re becuse of better usge of the spce in trnsformer volume. Tightening the core cn be done in two wys with wooden pltes nd cotters when the dimeters re no so big (Fig. 4) [3] nd with bndges, otherwise. Figure 4 The cross-section of the core tightened with wooden pltes nd cotters Tightening the yokes is done with yoke bems which hve very sophisticted shpes. Hving in mind everything tht hs been sid it is cler tht ll the tightening elements (pltes, cotters, bems, etc.) covered nd thermlly isolted prts of the surfce of the core which frequently leds to locl overhetings fire in the steel. D B B A

5 3. MATHEMATI MODEL OF THE PROBLEM Suppose tht the temperture field t the core is twodimensionl one nd specific mgnetizing losses in ll prts of the core nd the yoke re constnt. The nonlinerity of the therml process is not tken into ccount. Generlly, the core cross-section re is divided into rectngulr res which coincide with the pproprite cross-cooling groove mde of electro insulting mteril. In this wy the problem is independently solved for ech re. If one uses the symmetry of the core cross-section, resp. yoke then the problem could be solved for only ¼ re. The process of het trnsferring from seprte prts of the trnsformer to the ir is ccording to the lws of het-echnge, i.e. it is subject to non-reversible processes for het distribution in spce. Actully this distribution is echnge if internl energy between seprte prts of trnsformer. The process of het distribution cn be given by the following eqution: with boundry conditions: T λ T q y λ 2 y + = 2 T = n (3) (4) λ T T n y ny ( T T ) λ + + y α = (5) where: T, is the temperture of the nlyzing re; α is the coefficient of therml conductivity over the lmell length; T is the temperture of the oil nd it is constnt, over the whole boundry; n, n re unit vectors of the boundry res. y In the process of investigtion the coefficient of therml conductivity for different points of the core is different. In plces with woolen cotters it is thought to be zero, otherwise it is nonzero. 4. PROBLEM SOLUTION The mthemticl model considered is pplied to the rel trnsformer with power 2 MVA nd voltge 2kV. The dimeter nd the verticl section of the mgnetic core is 28 mm nd 512 cm 2, respectively for the core nd yoke with two cooling chnnels with 7 steps. The investigtion process shows tht the overhetings in the most eternl lyer of the mgnetic core led to deteriorte of the electro insolting properties of the trnsformer oil. In this reson, it ws mde the mesurements of the core temperture on this re. They re relized with thermocouples. The het trnsfer in the trnsformer is simulted by use of QuickField. The temperture distribution in the mgnetic system is shown in Fig. 5. T, deg y, mm 5 Temperture distribution in the mgnetic system 2 4 6, mm Figure 5 The temperture distribution in the mgnetic system of the trnsformer considered The mimum difference between the mesured nd simulted results is 5%. 5. ONLUSIONS The results bout the therml field distribution confirm the theoreticl ones tht the overheting prts of the trnsformer ggregte considered re the most eternl lyer of the mgnetic core. These results, helps designer to decide whether to use cooling grooves or not. 6. REFERENES [1] Dtchev, A., K. Stoikov, R. Dchev, Single phse trnsformer ggregte, Bulgrin ptent 42655, Sofi, Bulgri, 1986 (in Bulgrin). [2] Preiningerov, V., M., Krizek, V. Khoun, Temperture Distribution in Lrge Trnsformer ores, Proceedings of the Scientific Jubilee onference The trnsformer production is 1 yers old, Budpest, [3] Аншин, В., А. Крайз, В. Мейксон, Tрансформаторы для промышленных электропечей, энергоиздат, Москва, [4] Florov, V., et ll, Energy equipments prophylis, Sofi, 23, (in Bulgri). [5] Tарле, Г., Ремонт и модернизация систем охлаждения трансформаторов, энергия, Москва,