Reliability Model of Power Transformer with ONAN Cooling

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1 Reliability Mol of owe Tansfome with ONAN Cooling M. Sefidgaan*, M. Mizaie* and A. Ebahimzah* Abstact: Reliability of a powe system is consiably influenced by its equipments. owe tansfomes ae one of the most citical and expensive equipments of a powe system and thei pope functions ae vital fo the substations and utilities. Theefoe, eliability mol of powe tansfome is vey impotant in the isk assessment of the engineeing systems. This mol shows the chaacteistics and functions of a tansfome in the powe system. In this pape the eliability mol of the powe tansfome with ONAN cooling is obtained. The tansfome is classified into two subsystems. Reliability mol of each subsystem is achieved. Makov pocess epesentation and the fequency/ duation appoach ae employed to obtain a complete eliability mol of the subsystems. By combining these mols eliability mol of powe tansfome is obtained. The eliability mol associated with the tansfome is then poposed combining the mols of subsystems. The poposed mol contains five states. To make the mol moe applicable, the 5-state mol is alleviated to a -state one. Numeical analysis and sensitivity analysis elevant to the poposed eliability mol ae pefomed fo evaluating the numeical values of the mol paametes and the impact of diffeent components on the eliability of the mol. Keywods: owe Tansfome, ONAN Cooling, Reliability, Makov Mol. Intoduction In electical powe netwok, the tansfome is the most cucial and the most vice and its pope functions is vey citical fo the utility. So, Evaluation and impovement of the eliability of tansfomes ae vey impotant. The fist step fo obtaining this pupose is to find out the causes of tansfome failues and to temine tansfome outage. In [] moe than 000 failues, that had occued between 968 and 978, ae collected. It has been found that, in geneal, the failue ate of a tansfome is about % pe unit-yea. In this pape, failues have been categoized by failed components, failue oigins and failue causes of autotansfomes, powe station and substation tansfomes. In [] the fault tee diagam of tansfome is scibed. The fault tee mol consists of the possible systems that could cause a tansfome failue, subsystems that could fail and consequently leads to a system failue and potential oot causes of the subsystem failues. Ianian Jounal of Electical & Electonic Engineeing, 00. ape fist eceived Nov. 009 and in evised fom 8 May 00. * The Authos ae with the Depatment of Electical Engineeing, Babol Noshivani Univesity of Technology, Babol, Ian. s: mb.sefidgaan@gmail.com, mizaie@nit.ac.i, and e_zah@nit.ac.i Monitoing and diagnostic method is one way fo impoving the eliability of tansfomes. In [] taditional diagnostic methods and nontaditional tansfome monitoing techniques ae investigated. It claes that, monitoing and diagnostic techniques monito and pedict the tansfome failues, so that appopiate actions can be taken befoe the occuence of foced outage. Using this method, eliability of tansfomes could be inceased. Evaluating the life and pedicting the eliability of tansfomes ae altenative, useful techniques fo impoving thei eliability. In [4] novel techniques fo life assessment of the tansfome insulation have been pesented and estimated by using of load and ambient tempeatue. This estimating esidual life of insulation can enhance maintenance and epaiing action of tansfome ownes. In [5], based on the mechanical stength of winding insulation pape (gee of polaization), the eliability of a powe tansfome is temined. In the pape a validation method fo the moling technique has been pesented and also gee of polaization insulation pape has been estimated by load patten and ambient tempeatue. In [6] Fault Tee Analysis (FTA) and Mente Calo algoithm ae combined to analyze the eliability of tansfome. Fault tees ae built. By using the fault tee, Mente calo is applied to quantitatively analysis the mol. Then the eliability inx and cell impotance Ianian Jounal of Electical & Electonic Engineeing, Vol. 6, No., June 00 0

2 paamete help to find out the weak link of the system. esualt show thatthe method has guiding significance fo System Reliability Analysis. One of the impotant methods in eliability investigation and the fist step in system isk evaluation is to temine the components outage mols [7]. This mol is also known as Makov mol and achieved fo many equipment. In [8], [9] and [0] Makov mols of potection vice, tansfome potection and unified powe flow contolle (UFC) have been achieved. [] esents new pobabilistic methodologies fo computing the optimal numbe of tansfome spaes fo powe distibution substations. Two new mols ae poposed fo the eliability evaluation step. In the fist one, the system opeational states ae epesented by a Makov pocess. The second one uses a chonological Monte Calo simulation mol to assess the eliability pefomance of a system with inventoy of spaes. Sensitivity analyses showed that the poposed mols ae capable of satisfactoily intifying the optimal inventoy, consiing paametes such as failue and eplenishment ates of the inventoies, besis costs associated with system opeation and investment in the pocuement of new equipment. But no wok has been done on makov mol of powe tansfome yet. In this pape eliability mol of the powe tansfome with ONAN (Oil Natual Ai Natual) cooling is pesented. The tansfome is categoized into two subsystems. Fist, a eliability mol associated with each subsystem achived. The eliability mol of the powe tansfome is then poposed by combining the mols of subsystems. Outage mol and fequency-duation appoach applied to achieve a complete mol of powe tansfome. The pape is oganized as follows. In Section stuctue of a powe tansfome is analyzed and it is classified it into two subsystems. In Section and 4 eliability mol of each of two subsystems have been obtained. The complete eliability mol of a tansfome is pesented in Section 5. Section 6 epesents with the numeical study on eliability data and finally the pape is conclud in Section 7. owe Tansfome owe tansfomes consist of diffeent pats. Coe and winding ae two main components of the tansfome, belonging to the pat known as the active pat of the tansfome. The coe has the ole of cay magnetic flux. The windings ae aanged as cylindical shells aound the coe, each one is coveed with insulation pape. The function of the windings is to cay cuent. Active pat of the tansfome is contained insi the tank which physically potects it. The tank is also the containe of the oil. On-load tap-change (OLTC), by adding o subtacting tuns of the windings, egulates the voltage level of the tansfome. Bushings, one of the impotant components of the tansfome, connect the windings and the powe system. Cooling system is used to cool the active pat of the tansfome. owe tansfomes utilize diffeent class of cooling systems. In this pape the tansfome with ONAF cooling is consied. Components of the powe tansfome ae gouped into thee subsystems; the coe, windings, tank and dielectic fluid as the subsystem, bushing, tap-change and cooling system as the subsystem. At fist, mols fo the two subsystems ae poposed, then using these mols, complete eliability mol fo the powe tansfome is obtained. It should be noted that the loading type affects the eliability mol. In this pape, loading is assumed to be the nomal life expectancy loading. The main challenge of the nomal life expectancy loading is to pefom continuous loading at the ated output un the usual conditions. Opeation un usual conditions is equivalent to the opeation in the aveage ambient tempeatue of 0 o C fo cooling ai o 5 o C fo cooling wate []. Reliability Mol of Subsystem Subsystem consists of the windings, coe, tank and dielectic fluid. To achieve a mol fo this subsystem, two mols fo windings and the esting components ae poposed in the following subsections.. Winding State-Space Mol Lage amount of failues occu in winding, the most pevalent one is shot cicuit. If shot cicuit is sevee, it will damage the tansfome. In this situation, the system fails o it must be immediately emoved fom sevice. But some faults do not cause the outage of the tansfome and just distub the nomal opeation of the system. eventive tests and on-line monitoing help us to diagnose these types of faults. The most successful technique fo on-line fault diagnosis is dissolved gas-inoil analysis (DGA). By the aid of the DGA esults, seveal types of faults in the tansfome can be tected. One of these fault is themal fault with tempeatue in the ange of 00 and 700 (T). When this fault occus while tansfoeme in opeation, discission should be taken based on the equied loading of the powe netwok and type and location of the fault. If the utility does not need the tansfome focedly, it is emoved fom sevice fo epaiing actions. Even if the utility tend is to hold it in the netwok, only in some cases it is possible to keep it in the netwok. It is possible when fault is placed in the aea, in which by educing load can be esult in diminishing tempeatue of the hot spot. In [], typical locations of powe tansfome faults ae mentioned. Abad insulation between adjacent paallel conductos in windings is one of the faults that lead to T. When this fault occus, by educing load to about 0-50 pecent of ated powe, the tansfome can be emained in sevice. Accoding to above statements, state-space eliability mol of windings contain thee state is 04 Ianian Jounal of Electical & Electonic Engineeing, Vol. 6, No., June 00

3 shown in Fig.. State is elated to the condition in which no fault occus in windings. State shows the status that windings failue leads to emoval of the tansfome fom sevice. State monstates the condition that windings have fault but by educing the load, tansfome will emain in sevice. aametes in Fig. ae as follows: λ : winding failue ate λ : winding fault ate that does not cause emoval of the tansfome fom the sevice λ : faulty winding failue ate μ : winding epai ate. Coe, Tank and Delectic Fuid Sate-Sace Ml In this section, eliability mol of coe, tank and oil of the tansfome is obtained. Coe has two states, up and down. Tank and oil togethe have two simila states, and they ae consied as one component. Also, by inceasing viscosity of oil, cooling educes and subsequently load of tansfome must be educed, but this situation aely happens and so it is negligible. If faults occus in one of the coe o tank and dielectic fluid, the tansfome fails, theefoe these ae seies components. The set of equations fo these components ae [7]: λ = λ + λ () ctd coe ta& ctd = () λcoecoe + λta& ta & + λcoecoeλta& ta & λcoe + λta& Uctd = Ucoe + Uta& UcoeU () ta& whee λ coe and λ ta & ae failue ates of the coe and tank & dielectic fluid. coe, ta&, U coe and U ta& ae epai times of coe and tank& dielectic fluid and unavailability of coe and tank & dielectic fluid, espectively. Space-state of these components ae shown in Fig.. Fig. State-space mol of coe, tank and dielectic fluid.. Complete Reliability Mol of Subsystem Reliability mols of winding and othe components of this subsystem (coe, tank and dielectic fluid) ae combined to establish the eliability mol associated with the entie subsystem (Fig. ). In this Makov epesentation, it is assumed that when subsystem fails as a failue of one component, no subsequent fail occus in othe components until the subsystem comes back to sevice. Anothe emak in Fig. is about tansfeing fom the state C to D. In state C, although fault has been occued in windings, subsystem still woks. Howeve due to this fault, tansfome opeates in a ated state. Othe components of this subsystem ae healthy. As the fault occus in the healthy components of this subsystem, the subsystem entes into the state D. This state is a zeo capacity state fo the subsystem. Theefoe, the tansfome will be emoved fom sevice. Although the fault occuing in state C is a kind of fault that can emove subsystem fom sevice, but fo foce need it still emains in sevice. Thus when subsystem is emoved fom sevice, winding is completely failed, too and epaiing actions on this winding ae simila to fail winding. The complete state-space mol of the subsystem can be educed to thee states as shown in Fig. 4. States, and epesent full, zeo and ated capacity, espectively. Indices of Fig. 4 can be by achieved using the fequency balance appoaches follows [4]: Fig. State-space mol of winding. Fig. Complete state-space mol of subsystem. Sefidgaan et al: Reliability Mol of owe Tansfome with ONAN Cooling 05

4 = A (4) = + + B E D (5) = C (6) whee, and ae the pobabilities of full, zeo and ated capacity. The fequencies of tansfeing fom one state to anothe of Fig. 4 ae: f = + fab f AE (7) f f (8) = AC f = + fba f EA (9) f = + fcb f CD (0) The failue ate and epaiing ate of the complete eliability mol of the subsystem ae: f fab + fae λ = = = A () λ A + λctd A = λ + λctd λ A f f + f CB CD = = = C λ + λ C ctd C C = λ f f λ = = = λ AC A μ f f + f + λ BA EA = = = B + E + D μ + μ + + B ctd E B E D ctd () () (4) 4 Reliability Mol of Subsystem This subsystem inclus bushings, tap-change and cooling system. Bushings may damage because of seveal easons. Its state is consied as up and down. The faults of tap-change ae one of the majo poblems in the tansfome. Two states ae consied fo tapchange, too. It can be assume that two components ae seies. The othe pat of this subsystem is the cooling system. Although cooling system influences on the loading, but because of ou assumption in this pape that the tansfome utilizes ONAN cooling, this pat of subsystem has two states, too. Theefoe thee sections of this subsystem ae seies components as shown in Fig. 5. In this figue λ bsh, μ bsh λ tc, μ tc, λ clg and μ clg ae failue ate and epai ate of bushing, tapchange and cooling system espectively. Fig. 4 Equivalent thee-state mol of subsystem. Fig. 5 seies components of subsystem. In o to achieve equivalent indices fo this subsystem, fist by eplacing coe and tank with bushings and tao-change in equations and, failue ate and epai time of these two components will be achieved. Then assume that we have two components: cooling system and the equivalent of bushings and tapchange. By the same pocedue, equivalent paametes of Fig. 6 will be achieved. 5 Complete Reliability Mol of Tansfome In o to achieve complete eliability mol of the powe tansfome with ONAN cooling, the thee subsystems must be combined. Space-state mol of the powe tansfome is shown in Fig. 7. In this figue when system tansfes fom the state C to D, a pocess simila to Fig. occus. The complete eliabilityvmol of the tansfome, shown in Fig. 7, can be educed to thee states as shown in Fig. 8. The equivalent paametes ae obtained by applying the fequency balanced appoach. This pocess is simila to the Eqs. (4)-(4) because the same constuctions of Figs. and 7. Fig. 6 Equivalent two-state mol of subsystem. 06 Ianian Jounal of Electical & Electonic Engineeing, Vol. 6, No., June 00

5 Table Reliability data of components. aametes Value Unit Desciption Component λ f/y Failue ate Winding Fig. 7 Complete state-space mol of powe tansfome. Fig. 8 Equivalent thee-state mol of powe tansfome. The obtained mol can be used by both manufactues and powe system plannes/opeatos. Using this mol, manufactues will be able to ecognize the citical components and could offe a system with a specific level of eliability. System plannes also must evaluate the eliability of the system and maintain a specific level of powe system eliability. Reliability mol of components is equied fo this pupose as pats of the system [0]. 6 Numeical Study In this section numeical analysis and selected sensitivity analysis elevant to the eliability mol of the tansfome ae pefomed. Using these analyzes, the impacts on tansfome availability of vaiations in the components failue ates will be illustated. The eliability data fo the powe tansfome components is given in Table. In Table the calculated patue ate fo the Makov mols of subsystems and thei elated mols is monstated. By using these paametes the pobability, fequency and duation of the diffeent states can be calculated fo each mol. λ f/y Failue ate λ f/y Failue ate 400 hous Repai time λ coe f/y Failue ate Coe coe 480 hous Repai time λ tank 0.00 f/y Failue ate Tank& dielectic fluid tank 85 hous Repai time λ bsh 0.00 f/y Failue ate Bushings bsh 48 hous Repai time λ tc f/y Failue ate tc 960 hous Repai time λ clg 0.00 f/y Failue ate clg 7 hous Repai time Tapchange Cooling system Table Failue and epai ates of Makov mols. aamete Value (occ/y) aamete Value (occ/y) λ ctd λ 0.05 μ ctd 8.4 μ.07 λ λ μ 5.54 λ λ μ 8.55 λ λ 0.06 The obtained pobability and fequency values fo the tansfome mol shown in Fig. 8 ae pesented in Table. This table shows that, in most of the time the tansfome woks in full capacity state. λ in Fig. 8 is the failue ate of tansfome. It can be seen that this failue ate is close to the failue ate claed in []. Compaative studies ae pefomed to illustate the impact of the failue ates of diffeent components on the eliability of the tansfome. The esult is shown in Table4 fo two diffeent failue ates coesponnt to the winding,tank and oil and tap-change. In each case, the failue ate of only one component is changed, both to one-thousandth and to ten times of the base value pesented in Table. Vaiations in failue ates of tank and oil lead to elatively little change in the pobability and fequency values. But the impacts of vaying the failue ates of the winding and tap-change on the state pobabilities and fequencies ae significant. Sefidgaan et al: Reliability Mol of owe Tansfome with ONAN Cooling 07

6 Table obability and fequency of the powe tansfome mol states. obability Value (%) Fequency Value (occ/y) 99.4 F F F Conclusion This pape pesents an a new eliability mol fo a powe tansfomes with ONAN cooling. Makov mols have been applied fo moling the two subsystems of the tansfome. By combining these two subsystems, the thee-state Makov mol of the powe tansfome is achieved. This -states mol can be used to calculate the pobability, fequency and duation of the states. In o to evaluate numeical values of the mol paametes and the impact of diffeent components on the eliability of the mol, a numeical analysis and sensitivity analysis ae pesented. Numeical study illustates that the tansfome woks most pobably in up state. Results also show that failue ate of tansfome is which is close to pactical esults. Sensitive analysis show that winding and tapchange have significant effect on the pobability and fequency indices of a powe tansfome. Table 4 obability and fequency of tansfome mol states against diffeent component failue ates. Component Winding Tank and oil Tap-change Failue ate F F F Refeences [] Dietich W., ''An intenational suvey on failues in lage powe tansfomes in sevice'', Electa, No. 88, pp. -4, May 98. [] IEEE Standad C57.40, IEEE Gui fo the Evaluation and Reconditioning of Liquid Immesed owe Tansfomes, 006. [] Wang M., Vanmaa A. J. and Sivastava K. D., ''Review of condition assessment of powe tansfomes in sevice'', IEEE Electical Insulation Magazine, Vol. 8, Issue. 6, pp. - 5, Nov/Dec. 00. [4] Muthanna K. T., Saka A., Das K. and Waldne K., ''Tansfome insulation life assessment, IEEE Tans. on owe Delivey, Vol., No., pp , Januay 006. [5] van Schijnl A., Wetze J. M. and Woutes. A. A. F., ''Validation of eliability foecasting fo powe tansfomes'', oceedings of the 0th intenational Confeence on obabilistic Methods Applied to owe Systems, MAS '08, pp. -4, May 008. [6] Shuping L. and Zhengqing H., ''Reliability Analysis of tansfome Based on FTA And Mente Calo Method'', 008 Intenational Confeence on High Voltage Engineeing and Application, Chongqing, China, Novembe, 008. [7] Li W., Risk assessment of powe systems: mols, methods, and applications, Wiley-IEEE, Januay 005. [8] Billinton R., Fotuhi-Fiuzabad M. and Sidhu T. S., ''Detemination of theoptimum outine test and self checking intevals in potective elayingusing a eliability mol'', IEEE Tans. on owe Systems, Vol. 7, No., pp , Aug. 00. [9] Seyedi H., Fotuhi M. and Sanaye-asand M., ''An extend Makov mol to temine the eliability of potective system'', owe India Confeence, IEEE, Apil 006. [0] Aminifa F., Fiuzabad M. F. and Billinton R., ''Extend eliability mol of a unified powe flow contolle'', Geneation, Tansmission and Distibution, IET, Vol., Issue. 6, pp , Nov [] Leite da Silva A. M., Cavalho Costa J. G. and Chohuy A. A., obabilistic Methodologies fo Detemining the Optimal Numbe of Substation Spae Tansfomes, IEEE Tansactions On owe Systems, Vol. 5, No., pp , Febuay 00. [] IEEE Standad C57.9, IEEE Gui fo Loading Mineal-Oil- Immesed Tansfomes, 995. [] IEC Standads 60599, Mineal oil-impegnated electical equipment in sevice-gui to the intepetation of dissolved and fee gases analysis, second edition, Mach 999. [4] Billinton R. and Allan R. N., Reliability Evaluation of Engineeing System, nd edition, lenum, New Yok, Ianian Jounal of Electical & Electonic Engineeing, Vol. 6, No., June 00

Moin Sefidgaan was bon in babol, Ian, in 98. He eceived the B.Sc. gee fom the Babol Noshivani Univesity of Technology in 005.Now he study M.Sc in Noshivani Univesity of Technology. Mohammad.

Sc Degees in Electical Engineeing fom Univesity of Shahid Chaman,Ahvaz,Ian and Ian Univesity of Science and Technology,Tehan,Ian in 997 and 000 espectively and hd Degee in Electical Engineeing fom

His eseach inteests inclu life management of high voltage equipments, high voltage engineeing, intelligence netwoks fo intenal faults assessment in equipments and studying of insulation systems in

7 Moin Sefidgaan was bon in babol, Ian, in 98. He eceived the B.Sc. gee fom the Babol Noshivani Univesity of Technology in 005.Now he study M.Sc in Noshivani Univesity of Technology. Mohammad. Mizaie was bon in GhaemShah, Ian in 975. Obtained B.Sc and M.Sc Degees in Electical Engineeing fom Univesity of Shahid Chaman,Ahvaz,Ian and Ian Univesity of Science and Technology,Tehan,Ian in 997 and 000 espectively and hd Degee in Electical Engineeing fom the Ian Univesity of Science and Technology in 007. He woked as an Assistant ofesso in the electical and compute engineeing patment of Babol Univesity of technology fom 007. His eseach inteests inclu life management of high voltage equipments, high voltage engineeing, intelligence netwoks fo intenal faults assessment in equipments and studying of insulation systems in tansfomes, cables, geneatos, beakes, insulatos, electical motos. Ataollah Ebahimzah eceived his h.d. gee in electical engineeing in 006. Now he is an assistant pofesso in the Faculty of Electical and Compute Engineeing at Babol Univesity of Technology. His cuent scientific inteests ae in geneal aea of signal pocessing, biomedical engineeing and atificial intelligence. He is the eviewe of intenational confeences and jounals. Sefidgaan et al: Reliability Mol of owe Tansfome with ONAN Cooling 09

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