CONCERNING THE NO LOAD HIGH VOLTAGE TRANSFORMERS DISCONNEING Mara D Brojbou and Vrgna I Ivanov Faculty o Electrcal engneerng Unversty o Craova, 7 Decebal Blv, Craova, Romana E-mal: mbrojbou@elthucvro, vvanov@elthucvro KEYWORDS no load transormer, overvoltage, Matlab ABSTRA The computaton program o the transent recovery voltages that appears across the termnals o a crcut breaker, ater the low nductve current has been nterrupted at the dsconnectng no load hgh voltage transormers, usng the routnes set developed n Matlab s presented The computaton algorthm s based on the peculartes o operaton o the transormer The computaton program has been appled or transormers rom the hgh voltage electrc statons ST North Craova and ST Urechest The computed level o the overvoltage ampltude allows the proper selecton o the rated nsulaton level o the electrcal equpment rom the electrc statons INTRODUION The breakng o a low nductve current (around the A value) causes overvoltages The selecton o the crcut breaker wll be made so that the overvoltages that occurrng do not damage the nsulaton o the consumers such as transormer or motors The computaton o the overvoltages that appear at the dsconnecton o a no load transormer (the current nterrupted s the transormer magnetzng current) s deeply developed n the lterature (Dragan et all, 7) The evaluaton methods o the overvoltages are also well developed, startng rom analytcal ones, based on a mathematcal model o the system obtaned usng a smpled dagram and endng wth the drect methods, based on drect test on the system By the analytcal methods, the ntegral transormaton method and the Laplace transormaton respectvely, are most common In ths paper an automatc computaton algorthm o the overvoltages due to the no load transormers dsconnecton, when appearng the cuttng phenomena o the nductve current, s presented The low nductve current cuttng phenomena appears due to the electrc arc nstablty From analyss o the expermental results (Gusa ), or the hgh voltage transormer havng a large range o rated current, the value o the cuttng current s approxmately constant and depends manly on the characterstcs o power crcut breaker and less on the characterstcs o the dsconnected transormer The computaton o the overvoltages s made by means o the monophase equvalent dagram, Fgure, neglectng o the electrcal resstances o the electrcal connectons between hgh voltage source and transormer ~ e s R s L s C s a b Fgures : The monophase equvalent dagram o the low nductve current dsconnectng where: L S s the equvalent nductvty, C S s the equvalent capacty and R S s the equvalent electrc resstance o the hgh voltage source; L T s the transormer magnetzng nductvty, C T s the transormer equvalent capacty and R T s the resstance correspondng to the core losses o the transormer; L k s the nductvty o the electrcal connecton between voltage source and transormer At the tme when the crcut breaker contacts are openng, the electrc current contnues to pass through the electrc arc ormed between the contacts Due to the ntensve deonzaton, the quenchng arc occurs beore natural zero nductve current swtchng Due to the energy stored n these two crcuts, they begn to oscllate ndependently The oscllatons ampltude rom the crcut () depends on the stored energy n the L S nductvty Thereore, ater dsconnecton, the hgh requency oscllatons occur, havng the small ampltude that overlap wth the source voltage so that t may be consdered that n the crcut () remans the snusodal voltage Meanwhle, n the dsconnected crcut () the oscllatons pulsaton T s less than the oscllatons pulsaton S and hgher then ndustral pulsaton : T = / LT, but the ampltude overvoltage s greater The ampltude o the overvoltage n the dsconnected crcut () s determned usng the equaton o the energy balance, takng nto account the energy loss on the resstance crcut by ntroducng a subunt actor (=,,) I I t s the current through the L T nductvty and U s the voltage over the C T capacty at the tme moment o the nductve C T () () L k L T R T Proceedngs th European Conerence on Modellng and Smulaton ECMS Klaus G Trotzsch, Mchael Möhrng, Ul Lotzmann (Edtors) ISBN: 78---- / ISBN: 78---- (CD)
current cuttng, the equaton o the energy balance s obtaned: LT It U U max + = () where U max s the ampltude value o the overvoltage Neglectng the current through the R T resstance the U max can be obtaned: LT U max U = + max U max cos sn () LT Where U max s the ampltude o the voltage source u(t) and s the current cuttng angle I t = Iomax sn Consderng the expressons o the pulsatons t s obtaned: T U = + max U max cos sn () Fgures : The varaton o the u ( t), ( t) In realty, the phenomenon s nluenced by the appearance o the arc regnton between contacts a b o the crcut breaker Takng nto account the successve regntons the varatons o the u ( t), u ( t) are shown n Fgure u In Fgure the varatons o the voltage source u(t), the nductve current o the transormer (t) and the recovery voltage u (t) across the crcut breaker termnals at the moment o current cuttng are shown Usng the above presented ormulas, the ampltude actor o the overvoltage can be computed: u U O u u u u+u T u U max T = = cos + sn () U max Analyzng the last ormula, can be observed that the worst dsconnected case occurs when the cuttng angle has the value = /, that means the cuttng current reaches the maxmum value o the nductve current and the maxmum value o the ampltude actor s obtaned: I m I t t t t t t a t a t t t t t a Fgures : The varaton n tme o the u(t), (t) and u (t) succesve regntons t t u-u T t t = T () The varatons o the source voltage, nductve current and transent recovery voltage u ( t), u ( t) are shown n Fgure From ormula () t s noted that maxmum value o the overvoltage depends on the rato between the requency o the overvoltage oscllatons rom the dsconnected crcut () and the ndustral requency oscllatons Theoretcally, ths rato can reach hgher values ( or, even more) I the wthstand voltage o the quenchng medum o the crcut breaker s exceeded by the transent recovery voltage, then t happens that arc to regnte In the gure are dsplayed the varaton o the voltages and the current through the analyzed crcut, consderng the successve regntons o the arc between the contacts a-b o the crcut breakers The appearance o the successve regntons leadng to reducton o the transent recovery voltage THE COMPUTATION PROGRAM The Matlab applcaton has been developed n order to compute the electrcal overvoltages whch appear at the dsconnecton o the no load hgh voltage transormers rom the hgh voltage electrcal statons o the Oltena dstrct network, named ST North Craova and ST
Urechest The parameters o the AT transormer rom ST North Craova are: rated power S n = MVA, rated voltageu n = kv, no load current I o =%, the capacty at the nput termnals o the transormer C T = F The parameters o the AT transormer rom ST Urechest are: rated power S n = MVA, rated voltage U n = kv, no load current I o =,%, the capacty at the nput termnals o the transormer C T = F The program was developed n an nteractve manner whch allows the selecton o the hgh voltage network (Brojbou ) The man program runs routnes that allow: the calculus o the parameters o the transent overvoltage or the selected transormer (the ampltude o the overvoltage, the ampltude actor and the requency), the graphcal analyss o the nluence o cuttng angle o the magnetzaton current on the transent overvoltage; graphcal analyss o the nluence o the source s power and voltage The routnes can run wthn the man program or ndependently In Table are presented the computed parameters applyng the desgned program, or electrcal statons ST North Craova and ST Urechest Staton ST North Craova AT Sn= MVA Un= kv ST Urechest AT Sn= MVA Un= kv Table The Computed Parameters I omax [A] H It U max k [rad] [A] [kv] /,,7,8 / 77,, / 8,,77 / 8,7, / 8 78,8, /,,8 The Matlab program has been appled to graphcally observe the nluence o cuttng angle on the overvoltages values By analyzng the computed data, was observed the ncreasng o the recovery transent overvoltage ampltude as well as the ampltude actor value, as the cuttng angle o the nductve current ncreases The graphcal varatons o the source voltage and the recovery transent voltage or the AT transormer rom ST Urechest, consderng the derent cuttng angles o the nductve current are presented n Fgure ( = p / ) and Fgure ( = p / ) u, u [V] x - - Sn = MVA, Un = kv, It = 8 A - 7 8 Fgures : The recovery transent voltage varaton u ( t) Sn=MVA, Un=kV, H=p/ u, u [V] - - - x Sn = MVA, Un = kv It = 8 A - 7 8 Fgures : The recovery transent voltage varaton u ( t) Sn=MVA, Un=kV, H=p/ From the mentoned gures can be notced the ncreasng o the overvoltage ampltude value, as the cuttng angle ncreases The requency o the overvoltage varaton s constant T =Hz, because the transormer parameters are constant, C T = F, L T =, H The graphcal varatons o the source voltage and the recovery transent voltage or the AT transormer rom ST North Craova, consderng the derent cuttng angles o the nductve current are presented n Fgure ( = p / ) and Fgure 7 ( = p /)
x 7 u, u [V] - Sn= MVA, Un = kv, It =, A Ampltude actor - - 7 8 Fgures : The recovery transent voltage varaton Sn=MVA, Un=kV,H=p/ u, u [V] x - - Sn = MVA, Un = kv, It= A - 7 8 7 8 It/Imax Fgure 8 The varaton o the ampltude actor vs It / Io max rato, T / as parameter: - T / =, - T / =, - T / =, - T / =, - T / = 8, - / = T In the Fgure, the varatons o the ampltude actor dependng on the cuttng angle, as parameter are shown From ths gure, the same concluson can be drown: the ampltude actor value ncreases as the cuttng angle value ncreases or the same value o the rato 7 Fgures 7: The recovery transent voltage varaton Sn=MVA, Un=kV, H=p/ The requency o the overvoltage oscllatons s constant T = Hz, because the transormer 8 parameters are constant, C T = F, L T =, 7H Regardng the nluence o the rato It / Io max on the ampltude actor or derent values o the rato, the graphcal varatons are shown n Fgure 8 As expected, the maxmum values o the ampltude actor are obtaned or the maxmum value o the rato, as result rom () ormula In the same tme, or the same value o the rato, the ampltude actor ncreases as the cuttng value o the nductve current ncreases; consequently, the cuttng o the current at one value near the maxmum value led to the hgher value o the recovery transent overvoltage In the Fgure, the varatons o the ampltude actor dependng on the cuttng angle, as parameter are shown From ths gure, the same concluson can be drown: the ampltude actor value ncreases as the cuttng angle value ncreases or the same value o the rato Ampltude actor 7 8 Cuttng angle [gr] Fgure The varaton o the ampltude actor vs cuttng angle, T / as parameter: - T / =, - T / =, - T / =, - T / =, - T / = 8, - / = T The maxmum values o the recovery transent voltage have been computed wthout takng nto account the lmtng nluence o the wthstand voltage o the crcut breaker due to the speed o the recovery o the quenchng delectrc medum rgdty (ol or SF ) The dmnshng o the overvoltage level can be also obtaned by ncreasng o the nput capacty o the transormer The lmtng o the delectrc stresses o the equpment can be done through the proper selecton o the surge arresters parameters
CONCLUSIONS The Matlab computaton program presented n ths paper allows the evaluaton o the maxmum values o the recovery transent voltages appearng at the no-load hgh voltage transormer dsconnectng The nluence o the cuttng angle values o the nductve current on the overvoltage ampltude as well as the nluence o the rato T / has been also studed The program has been appled or the case o the hgh voltage transormers rom ST North Craova and ST Urechest, or whch the rated parameters are known The computed level o the overvoltages can be compared wth the wthstand perormances o the swtchng equpments wthn the statons The analyss [Brojbou, M, ] emphases the correctness o the chosen equpments o the staton REFERENCES Dragan, G et all, 7 Swtchng surges n power systems Techncal Ed, BucureJt Gusa, M,, The transent regme o the network, Gheorghe Asach Ed, Ias Brojbou, M, The computaton o the electrcal stresses n the hgh voltage network rom the ST Craova Research Report nr/, Benecary Transelectrca SA Craova Brojbou, M; Ivanov, V; Leoveanu, MS, Computaton program or short-crcut current rom Craova substaton, Buletnul Insttutulu Poltehnc IaJ Tom l (LIV), Fasc, Sera ElectrotehncN, EnergetcN, ElectroncN AUTHOR BIOGRAPHIES MARIA D BROJBOIU was born n n the town Pucoasa Romana She s currently workng as Proessor at the Unversty o Craova, Electrcal Engneerng Faculty, Department o Electrcal Engneerng snce 8 Beore that, she worked as desng engneer at the Electroputere holdng the Research and Development Center She s Doctor n Scence Technque Electrcal Engneerng VIRGINIA IVANOV was born n Vela, Dolj, Romana, She was graduated n Electrcal Engneerng at Unversty o Craova, Romana, n 8 and Doctor n Electrcal Engneerng n From 8 to 8 she worked as researcher wth the Researchng Insttute or Motors, Transormers and Electrc Equpment Craova In 8 she joned the Faculty or Electrcal Engneerng, Department o Electrcal Engneerng