STUDIUL SUPRATENSIUNILOR CARE APAR LA DECONECTAREATRANSFORMATORULUI DE LA REEA

OVERVOLTAGES STUDY ON TRANSFORMER TURN-OF

T. IONICUniversitatea Politehnica Timioara, Romania

Facultatea Automatic i Calculatoare, Bd. Prvan 2Tel:+40-(256)-403245, e-mail: tionica@aut.utt.ro

L. BJANUniversitatea Politehnica Timioara, Romania

Facultatea Electrotehnic, Bd. Prvan 2,Tel:+40-(256)-403464, e-mail:uvu@lselinux.utt.ro

M. SVOBODAUniversitatea Politehnica Timioara, Romania

Facultatea Electrotehnic, Bd. Prvan 2,Tel:+40-(256)-403447, Fax:+40-(256)-403452

V. GIURGIUUniversitatea Politehnica Timioara, Romania

Facultatea Electrotehnic, Bd. Prvan 2,Tel:+40-(256)-403455, Fax:+40-(256)-403452

Rezumat: Lucrarea se ocup de studiul regimului tranzitoriu care areloc la deconectarea transformatorului de la reea. Obiectivul de bazal lucrrii este dimensionarea corect a circuitelor de protecie lasupratensiune, utilizate pentru protecia echipamentelor cu electro-nic de putere i a altor aparate sensibile la supratensiuni. Studiulsupratensiunilor se realizeaz att cu ajutorul schemelor de simularenumeric (Pspice, OrCad, Matlab mediul Simulink), ct i prinncercri experimentale. Comanda deconectrii la momente de timpbine precizate s-a realizat cu ajutorul unui sistem de dezvoltare cumicrocontroller de tip 80C552. n urma simulrilor i a testelorrealizate, s-au tras concluzii interesante legate de dimensionareacircuitelor de protecie cu elemente de tip RC la supratensiuni.

Keywords: deconectarea transformatorului, supratensiuni, simularenumeric, microcontroler 80C552.

Abstract: Presented paper study the overvoltages that appear incase of transformer turn-off transients. The main objective is properselection of overvoltage protections, used in power electronicsequipments and other electronically parts. Overvoltages studyis perform both by digital simulation Pspice OrCad and Matlab-Simulink) and by experimental tests. In case of experimentaltests an electronic circuit based on 80C552 microcontrollerdevelopment system was build. This experimental model facilitatethe synchronization of turn-off moment with zero cross voltage.The experimental and simulation results from this study areusefully to properly selection of RC overvoltages protectionelements.

Keywords: turn-off transformer, overvoltages, numerical simulations80C552 microcontroller.

1. IntroducereCereterea gradului de tehnologizare a proceselor indus-

triale are ca efect utilizarea pe scar tot mai larg a echipa-mentelor cu electronic de putere n reglarea proceselor.O problem foarte delicat o reprezint protecia acestorechipamente la supratensiuni de comutaie externe cum arfi deconectarea transformatorului de la reea.

n cadrul acestei lucrri se analizeaz cauzele apariieisupratensiunilor i se ncearc gsirea unor soluii deatenuare a efectelor acestora.

2. Consideraii teoreticePrincipala cauz a apariiei supratensiuilor la deconectare

este tierea[1] undei de curent nainte de trecerea ei prinzero n cazul sarcinilor puternic inductive. Acest lucrurezultz destul de clar din legea induciei electromagnetice:

dtdiLue = (1),

n care ue este tensiunea indus, L inductana circuitului idi/dt panta de descretere a curentului. Dac momentuldeconectrii coincide cu trecerea curentului prin zeroregimul teranzitoriu lipsete.

3. ncercri experimentaleStudiul supratensiunilor la deconectarea transformatorului

de la reea s-a realizat pe cale experimental i prin simulare

1. IntroductionThe continue increase of industrial technological pro-

cesses complexity lead to a very large increasing of powerelectronically equipments demand, for processes control.A very delicate problem is represented by protection fromovervoltages generated on transformer turn-off.

In this paper will be analyzed the cause of overvoltageproduction and tray to found methods to attenuate them.

2. Theoretical backgroundsThe main cause for overvoltage production is the current

wave cutting[1] in case of inductive loads, like powertransformers, turn-off. This already results fromelectromagnetic induction low:

dtdiLue = (1),

where ue is the emf L is the circuit inductance and di/dtthe current speed decreasing. If the turn-off match withcurrent zero crossing then the transients is not present.

3. Experimental testsTransformer turn-off overvoltage study was by

experimental and by digital simulation performs. Two

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numeric. Pentru acesta s-au utilizat dou transformatoareunul trifazat avnd puterea S = 12300 [VA], tensiunile nprimar respectiv secundar U1N/U2N = 380V/260V (conexi-une D/y) iar frecvena f = 50Hz i unul monofazat cuS = 250[VA], U1N/U2N = 220V/24V respectiv f = 50Hz.

Pentru a putea stabili momentele deconectrii s-a con-struit un model experimental bazat pe un sistem de dez-voltare[2] realizat cu microcontrolerul 80C552. Schemade principiu a circuitului este prezentat n lucrarea Studiulregimului tranzitoriu n cazul conectarii transformatoruluin gol la reea din acela volum.

n Fig.1 este prezentat forma de und a tensinii la de-conectarea de la reea a transformatorului monofazat. Tensi-unea a fost nregistrat n secundarul transformatorului.

different transformers were used to realize this purpose.One of them is an 12300[VA], tri phase transformer withprimary and secondary voltages U1N/U2N = 380V/260V(D/y conections) and frequency f = 50Hz. The other one is asingle phase transformer with S = 250 [VA], U1N/U2N =220V/24V respectively f = 50Hz.

An experimental model based on 80C552 developmentsystem[2] was build for precisely define the transformerturn-off moments. The block diagram of this experimentalmodel is presented in paper Turn-on transients oftransformer on no-load from the same volume.

In Fig.1 the voltage diagram in case of single phasetransformer turn-off is presented. The wave was taken fromthe secondary winding of the transformer.

Fig.1. Forma de und a tensunii la deconectarea de la reea.a transformatorului monofazat.obinut pe cale experimentalFig.1. The voltage diagram in case of single-phase transformer turn-off by experimental way

4. Simulare digitalSimularea deconectrii s-a realizat pentru transforma-

torul trifazat a crui date nominale au fost prezentate maisus. n cazul deconectrii se utilizeaz schema echivalentde mers gol a transformatorului[3].

S-a considerat c n acest situaie nu este necesar sse in seama de fenomenul de saturaie magnetic, modelulfiind astfel mult mai simplu.

n Fig.2. este perzentat schema circuitului echivalentpentru simularea deconectrii transformatorului. Schemaconine sursele de tensiune sinusoildal, intreruptoarelepentru deconectare grupurile de protecie RC conectatentre faze, i schema echivalent a transformatorului cuelemente RL n conexiune delta.

Se observ c supratensiunile sunt de cteva ori maimari dect valoarea maxim a undei sinusoidale.

Forma de und a tensiunii la deconectarea transforma-torului trifazat, obinut prin simulare numeric, fr parti-ciparea grupurilor de protecie este prezentat n Fig.3.

Ca i n cazul ncercrilor experimentale se constatfaptul c supratensiunea este de cel puin dou ori maimare dect amplitudinea undei sinusoidale.

4. Digital simulationDigital simulation was realized for tri phase transformer

with the specified rated value.In case of turn-off the no load transformer equivalent

circuit was used[3]. Also taking in to account of magneticallysaturations was considered unnecessarily in this case so,the simulations model will be simplified.

In Fig.2 the digital simulation equivalent circuit incase of transformer turn-off is represented. It contains thesinusoidal power supplies, circuit breakers, RC protectioncircuits connected between phases, and equivalenttransformer circuit realized with rl elements in deltaconnection.

It may be seen that overvoltages are more that twotimes greater than maximum value of sinusoidal wave.

The turn-off tri phase transformer voltage diagram, incase of numerical simulation, without protection circuitenclose is represented in Fig.3

Like in the case of experimental tests it may be seenthat overvoltages its more that two times bigger thanmaximum value of sinusoidal wave.

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Fig.2. Schema circuitului echivalent pentru simularea deconectrii transformatoruluiFig.2. Equivalent circuit in case of transformer turn-off

Fig.3. Forma de und a tensunii la deconectarea transformatorului trifazat obinut prin simulare numeric, fr circuit de protecieFig.3. The voltage diagram in case of tri phase transformer turn-off by numerical simulation without protection circuit

Fig.4. Tensinuea la deconectare obinut prin simulare numeric, cu circuit de protecie dimensionat conform [4]Fig.4. The voltage diagram in case of turn-off by numerical simulation with protection circuit, choices from [4] recommendation

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Pentru atenuarea supartensiunilor la deconectare sauintrodus n simulare grupurile de protecie calculate conformliteraturii de specialitate[4] i indicaiilor din filele de catalogale productorilor de dispozitive semiconductoare de putere.Pentru R = 180 i C = 1F se obine forma de undprezentat n Fig.4. Dup cum se observ supratensiunileau valori mai reduse dar n circuit apar oscilaii care seamortizeaz cu o constant de timp relativ mare.

n Fig.5 este reprezentat forma de und a tensiunii ladeconectare cu valorile elementelor din circuitul deprotecie optimizate prin simulari repetate.

Valorile componentelor din circuitul de protecie sunt,R = 500 i C = 2 F. Rezultatele obinute n acest cazsunt mult mbuntite.

For overvoltages suppression protection circuits,calculated from technical literature[4] and from powerdevices purchaser data sheets, was introduced betweenphases in simulation process. The diagram from Fig. 4 wasobtained for R = 180 i C = 1F. The overvoltages inthis case are a little bet smaller but amortized oscillationwith relatively high value of time constant is present.

In Fig.5 the turn-off voltage diagram, with optimizedprotection circuit, is represented.

The values of circuits elements obtained by repeatedsimulations are, R = 500 i C = 2F. More improvedresults are available in this case.

Fig.5. Tensinuea la deconectare, cu circuit de protecie optimizat prin simulare numericFig.5. The voltage diagram in case of turn-off by numerical simulation with optimized protection circuit

5. ConcluziiStudiul fenomenelor tranzitorii utiliznd simularea

numeric ne ofer o imagine clar asupra fenomenului ncauz. Cu modelul odat stabilit i corect parametrizatputem simula oricte variante de deconectare dorim fras apelm la apartur de laborator sofisticat i cu osbstanial economie de timp.

5. ConclusionsTransient processes study using digital simulation may

give as a very clearly image about certain phenomena. Withthe model already defined and good parameterization we areable to simulate a great number of cases without anysophisticated lab equipment and with substantial timeeconomy.

Cu ajutorul modelelor implementate se poate optimizacalculul grupurilor RC care se monteaz n amonte deechipamentul cu electronic de putere n vederea protecieila suparatensiuni. Circuitul electronic prezentat la parteaexperimental ofer posibilitatea selectrii momentuluideconectrii astfel nct s putem reproduce oricare dinsituaiile practice care ar putea s apar. Acest lucru nuera realizabil cu aparatura clasic de laborator undemomentele conectrii/ /deconectrii sunt aleatoare fa detrecerea prin zero a tensiunii/curentului.

Bibliogarfie (References)

With the implemented models we can optimize the RCcircuits for power electronically equipments overvoltageprotection. The electronic circuit presented in thirdparagraph (Experimental tests) offers the possibility toconnect/ /disconnect the transformer on the prcisedmoments. This kind of job is impossible to realized withclassical equipments when the connection/disconnectionmoments are randomly.

1. ***** OVERVOLTAGES Measurement and Statistical Simulation, TAVRIDA ELECTRIC INDUSTRIAL GROUP, www.tavrida-electric.com.au

2. TIBERIU, I.: Microprocesoare i Microcontrolere (Microprocessors and Microcontrollers), Editura Solness, Timioara 20013. BOLDEA I.,: Transformatoare i Maini Electrice (Tansformers and Electrical Machines), Editura Editura Didactic i

Pedagogic, Bucureti 19944. Kelemen, A., Imecs, M.,: Electronic de putere (Power electronics), Editura Editura Didactic i Pedagogic, Bucureti 1983