Bahram Noshad Department of Electrical Engineering, Bandar Deylam Branch, Islamic Azad University, Bandar Deylam, Iran.

Transcription

1 Journal of Advance in Computer Reearch Quarterly pissn: x eissn: Sari Branch, Ilamic Azad Univerity, Sari, I.R.Iran (Vol. 9, No. 3, Augut 8), Page: - A New Model for Analyi of Unuual Mal-Operation of Differential Protection Due to the Ultra-Saturation Phenomenon during the oaded Power Tranformer Energization Bahram Nohad Department of Electrical Engineering, Bandar Deylam Branch, Ilamic Azad Univerity, Bandar Deylam, Iran bahramnohad@yahoo.com Received: 7//5; Accepted: 8// Abtract In thi paper, a new model baed on the thevenin equivalent circuit for invetigating the ultra-aturation phenomenon during the energization of a loaded power tranformer i preented and it effect on the differential protection of the tranformer i conidered. The ultra-aturation phenomenon caue the maloperation of the power tranformer differential protection. So, the decription and control of the ultra-aturation phenomenon i neceary for preventing of the fale trip of the differential protection. In thi paper, to model the ultra-aturation phenomenon, the nonlinear characteritic of the tranformer core, the effect of current tranformer, and the core loe are taken into account. It i aumed that the load of the tranformer i a reitive and inductive load. Alo, the effect of the reidual flux and inception angle on the ultra-aturation phenomenon are invetigated. The reult how that the ultra-aturation phenomenon poe a great problem for protective relaying of power tranformer. The outcome of thi reearch can further be ued a hint for ubtation operation peronnel a well a for the development of new protection tabilization criteria, which i not dicued further in thi paper. The explanation of the ultra-aturation phenomena i the firt tep toward developing new idea and criteria for more reliable tranformer protection that would better handle uch abnormal cae than currently employed relaying equipment. In thi paper, the fourth-order Runge-Kutta method i ued to olve the equation and imulation of the ultra-aturation phenomenon i done by MATAB program. Keyword: oaded Tranformer Energization, Tranformer Differential Protection, Ultra- Saturation Phenomenon, Mal-Operation. Introduction The power tranformer protection i an important component in power ytem. The effect of magnetizing inruh current ha to take into account in power tranformer protective cheme. Thi i becaue the magnetizing inruh current, which occur when a tranformer i energized on the tranmiion line or an external line fault i cleared, ometime reult in time full load current and hence can caue mal-operation of the differential relay. The differential protective ytem etablihe the main protection againt hort circuit fault on primary and econdary winding of tranformer. Thi

2 A New Model for Analyi of Unuual B. Nohad protective ytem hould operate rapidly during internal fault. However, it hould not operate under non-internal fault condition uch a inruh current. The mot common technique ued to prevent fale trip during the energization of power tranformer i harmonic retraint relay. The principle of the harmonic retraint relay i baed on that the econd harmonic and ometime the fifth harmonic component of the inruh current i coniderably larger than in a typical fault current [,]. Normally, in order to dicriminate between the internal fault and inruh current, an algorithm i ued in which the differential protection operate when the magnitude of the fundamental component of differential current tabilize above.5p.u and the ratio of the econd harmonic to fundamental harmonic of the differential current tabilize below 5% [3-7]. But it ha been reported that in certain condition the mal-operation of differential protection under inruh current ha caued tripping of healthy tranformer [5-7]. In [5-7], the maloperation of differential protection during the energization of the loaded tranformer wa reported whoe origin i known a ultra-aturation phenomenon. The author oberved that energization of a loaded tranformer may caue the ituation, called ultraaturation, when the DC flux in the core in the initial tage of the proce increae rather than decreaing [5]. Hence, the ditortion of the current wave hape get maller, and the percentage of the econd harmonic decreae below the relay retraining level [5-7]. In thi cae, the current ac wave hape i approximately unditorted and the level of the econd harmonic i negligible. For the analytical tudy of the ultra-aturation phenomenon, a preliminary loaded tranformer energization model i uggeted [5]. Uing thi model, the delayed mal-operation of differential protection can be decribed [8-]. In [5], many implification during the imulation are carried out which take the magnetizing reactance of the time-variant characteritic a an equivalent inductance, neglecting the core model of the tranformer, without conidering the tranferring effect of current tranformer to the primary inruh, and conider only the reitive load, can be mentioned which doe not coincide with the real ituation. Hanli Weng, Xiangning in and Pei iu revied their previou model in 7. According to them the previou theory cannot be utilized directly to analyze thi phenomenon. Therefore, a new model for analyzing the tranient behavior of the loaded tranformer energization, together with the current tranformer model involving the magnetic hyterei effect, taking the nonlinear magnetizing reactance, and no conidering only the reitive load i propoed [6]. But in [6], the core loe of the tranformer are neglected and a difficult model for current tranformer in primary ide i conidered whoe main difficulty in current tranformer modeling i the imulation of the hyterei loop. Andrzej Wizniewki et al. the condition which mut be met to make ultra-aturation and exceive ultraaturation poible are preented in 8 [7]. The ATP-EMTP program i ued for imulation. In [7], the core model of the tranformer and the magnetizing reactance are neglected and thi doe not coincide with the real ituation. In thi paper, a new model baed on the thevenin equivalent circuit for invetigating the ultra-aturation phenomenon during the energization of a loaded power tranformer i preented and it effect on the differential protection of the tranformer i conidered. In thi model, the nonlinear characteritic of the tranformer core, the effect of current tranformer, and the core loe are taken into account. It i aumed that the load of the tranformer i a reitive and inductive load. In addition to a new model for power tranformer, the effective model for current tranformer i preented in thi paper. The main advantage of thi propoed model in compare with previou model are a following:

3 Journal of Advance in Computer Reearch (Vol. 9, No. 3, Augut 8) - ) The core loe are taken into account. ) A new model ha been conidered for current tranformer. In thi model, information of B-H curve for magnetic branch in t required. Alo, ince hyterei effect in t taken into account, reult can be compared with the IEEE model conidering hyterei effect. 3) It involve proper computing peed and accuracy. 4) Alo, the mal-operation of the differential protection depend on a variety of factor the mot important parameter of which are reidual flux and inception angle. In thi paper the parameter mentioned will be tudied in variou cenario. The explanation of the ultra-aturation phenomena i the firt tep toward developing new idea and criteria for more reliable tranformer protection that would better handle uch abnormal cae than currently employed relaying equipment. In thi paper, the fourth-order Runge- Kutta method i ued to olve equation and imulation of the ultra-aturation phenomenon i done by MATAB. The paper i organized a follow. Section introduce the modeling of the loaded power tranformer to tudy the ultra-aturation phenomenon. Section 3 preent a new model for current tranformer. The propoed algorithm i preented in ection 4. Simulation of the ultra-aturation phenomenon i preented in ection 5. Finally, Concluion and future work are given in ection 6.. Modeling of the oaded Power Tranformer The loaded tranformer energization can be decribed by the equivalent circuit a illutrated in Figure. i R R i i m i R b U + R C b Figure. Circuit of the loaded tranformer energization In thi model, U i the electromotive force of the ource, and R repreent the inductive and the reitive component of the equivalent impedance compriing of ytem impedance and leakage impedance of tranformer primary winding. and R repreent the inductive and reitive component of the equivalent impedance compriing of the leakage impedance of tranformer econdary winding and the impedance of burden. The magnetizing nonlinear branch of tranformer core i illutrated by an equivalent inductance. For determining the new model, according to Figure, the nonlinear magnetizing branch of tranformer i removed and the thevenin equivalent circuit i obtained from point A and B. 3

4 A New Model for Analyi of Unuual B. Nohad i R R i U + R C i m A + V - oc B R b b Figure. Equivalent circuit model for determination the thevenin voltage and impedance The power upply i defined a: U( t) = Um in( ω t + θ) () According to equivalent circuit in Figure, the thevenin voltage i: ( B + B ) bt Voc = ( A + A )coωt + in ωt + ( C + C ) e co ω Where a ( a ω ) A 3 =, a Aa B B R ( a ω a ) a 3 A =, ( a ω ) + ω B R a a ( a ω ) A ( D Cb + D Cb) at + e a 4 =, C A a bt in at () R ( ω a + ( a ω )) a 4 A =, ( a ω ) + a ω =, C = A, D =, D = a 4 A a B, R R R RC R RC R R RC RC R R R RC a = + + ( ), b = ( + + C + ), 4 a RC + RC + R + R =, RCU m inθ a 4 =. And alo, the thevenin impedance i: Z = ) + th R RC + R RC + R R =, a RCU mω coθ a 3 =, ( R + jx) ( R + jx ) ( RC = Rth jxth (3) A RC + A RC + B RC B RC Where Rth =, X th =, ( A + R ) + B ( A + R ) + B C R R + R R + R X + R X X X + X X + R X + XR A =, B =, Z Z Xth Z = ( X + X ) + ( R + R ), th =, X = ω, X = ω. ω C 4

5 Journal of Advance in Computer Reearch (Vol. 9, No. 3, Augut 8) - Due to (3), the thevenin impedance i an equivalent reitance and inductance. After obtaining the thevenin model of the nonlinear magnetizing branch from point A and B, the nonlinear magnetization branch i returned to the circuit a hown in Figure 3. i Rth th V oc + d - Figure 3. Simplified circuit model of loaded power tranformer According to Figure 3, the nonlinear magnetizing branch current i i a function of. The accurate curve of i hould be hown a a multi-valued curve if taking the hyterei into account. For the convenience of olving the equation, the magnetization curve can be implified, a hown in Figure. It can be aumed that the aturation point i ( i, ). The inductance in aturation region and no aturation region are and, repectively. It hould be emphaized that the inductance of the magnetizing branch of tranformer i till nonlinear, even if the above implification i ued. Hence, according to Figure i can be defined a follow: i = i + + i i > < Where i the flux linkage, S i the flux linkage at the knee point of the magnetization curve, i i the magnetization current at the knee point of the magnetization curve and i the lope of the aturation curve. (4) Figure 4. The approximate magnetizing characteritic of tranformer core 5

6 A New Model for Analyi of Unuual B. Nohad The magnetization curve hown in Figure 4 i divided into three region. Region : > In thi cae the magnetization current i defined: ( ) i = + i (5) According to equivalent circuit in Figure 3: V oc V d + R i + th th di d + Subtitute (5) into (6): oc + R ( th According to (7): = d d th + i ) + ( ) + = V R oc th = ( + i ) (8) th th + + Region : < S In thi cae the magnetization current i defined: i = ( + ) i (9) V d i According to (6) and (9): oc + R ( th Due to (): + d d th i ) + ( ) + = (6) (7) () V R + oc th = ( i ) () th th + + Region 3: In thi cae the magnetization current i defined: = i () According to (6) and (): 6

7 Journal of Advance in Computer Reearch (Vol. 9, No. 3, Augut 8) - V d oc i i d d + Rth ( ) + th ( ) + = (3) Due to (3): Voc R i th = ( ) (4) thi thi + + According to equivalent hown in Figure : d d + R i + = U i + = i + i i m (6) d d = Where i + Ri m d =. Due to (5) and i = : R C d = U R ( ) d 3. Current Tranformer Modeling The equivalent circuit of a current tranformer i hown in Figure 5. In thi circuit R and are the reitive and the inductive component of the equivalent impedance compriing of ytem impedance and leakage impedance of tranformer primary winding. R and are reitance and inductance of the econdary ide of the current tranformer. R b and b are reitance and inductance burden. Since the core lo doen t affect the behavior of the current tranformer aturation, it i neglected []. The equivalent circuit of the current tranformer, referred to the econdary ide, i hown in Figure 6. (5) (7) (8) R i p N p : N i p R i i R C R b b Figure 5. Equivalent circuit of current tranformer R i N p ip = ip N i R b b Figure 6. Equivalent circuit of current tranformer referred to econdary ide 7

8 A New Model for Analyi of Unuual B. Nohad The magnetization curve illutrated in Figure 4 i ued for current tranformer core that and i are different for power tranformer and current tranformer. To model the current tranformer, equivalent circuit hown in Figure 6 i conidered. In thi circuit, we defined: R = R + R = + b b According to the equivalent circuit hown in Figure 6: i = i + () p i di e = Ri + () N p i p = i p () N In thee equation, i p i the primary current referred to econdary ide, i i the magnetizing current, i i the econdary current, N i the number of econdary turn, and winding. According to (): i (9) N p i the number of primary turn, e i the induced voltage in the econdary i = i (3) p According to Figure 4, the magnetization curve i divided into three region. Region : > ( ) i = + i (4) According to (3) and (4): i = i p ( ) i (5) Differentiate from (5): di di d p = (6) d Due to d = e from (), (5) and (6): R ( = ( i p + Region : < S ) di p i ) + ( ) + (7) 8

9 Journal of Advance in Computer Reearch (Vol. 9, No. 3, Augut 8) - i = ( + ) i (8) According to (3) and (8): i = i p ( + ) + i (9) Differentiate from (9): di di d p = (3) d Due to d = e from (), (9) and (3): R ( + = ( i p + Region 3: ) di p + i ) + ( ) + (3) i = i (3) According to (3) and (3): i = i p i (33) Differentiate from (33): di di i d p = (34) d Due to d = e from (), (33) and (34): R N p N di p p = ( i p i ) + ( ) (35) + i N + i N 4. Propoed Algorithm The differential protection hould be able to ditinguih the internal fault from the external fault, the magnetizing inruh current and the ultra-aturation phenomenon and it hould only operate under internal fault. Normally, in order to ditinguih between the external fault, internal fault and magnetizing inruh current, an algorithm i ued in which the differential protection operate when the amplitude of the baic component of the differential current fixe upper than.5 p.u and the level of the econd harmonic to baic harmonic of the differential current fixe lower than 5%. But it ha been decribed that in certain condition the fale trip of differential protection under 9

10 A New Model for Analyi of Unuual B. Nohad magnetizing inruh current ha led to tripping of healthy tranformer. One of tranient phenomenon that lead to the fale trip of the power tranformer differential protection during the energization of a loaded power tranformer i the ultra-aturation phenomenon. In thi paper, a new model baed on the thevenin equivalent circuit for invetigating the ultra-aturation phenomenon during the energization of a loaded power tranformer i preented. To model the ultra-aturation phenomenon, firt the differential current of phae are obtained from ubtraction of econdary current of current tranformer on the primary and econdary ide of the power tranformer. Then, the following tep are performed: a) The teady tate amplitude of the baic component of the differential current ( i d ) are calculated by the ue of the Dicrete Fourier Tranform (DFT) algorithm. If the teady tate amplitude fixe lower than.5 p.u., the normal condition will be occurred. Otherwie, the ultra-aturation phenomenon may be occurred and mut calculate the econd harmonic to the baic harmonic. H b) The ratio of the econd harmonic to the baic harmonic ( ) of the differential H bae current i calculated by the ue of the DFT algorithm. If the ratio fixe lower than 5%, the mal-operation of the power tranformer differential protection due to the ultraaturation i occurred. So, according to above tep, if the differential protection ue.5 p.u. a the operating threhold for the amplitude of the baic component of differential current and 5% a the econd harmonic retraint ratio, the fale trip occur due to the ultraaturation. The flowchart of propoed algorithm i hown in Figure 7.

11 Journal of Advance in Computer Reearch (Vol. 9, No. 3, Augut 8) - Figure 7. Flowchart of the propoed algorithm

12 A New Model for Analyi of Unuual B. Nohad 5. Simulation of the Ultra-Saturation Phenomenon It i uppoed that the tranformer ha load connected and i energized from the highvoltage ide at t =. The ource parameter are: U( t) = Um in( ω t + θ) U m = kv ω = π rad = θ Where θ i the phae angle of phae A when the tranformer i connected to the voltage ource. The tranformer parameter are: k = 35 kv =.6 H, =.35 H, R = 5 Ω R = 5 Ω RC = 3. 7 kω m = U m ω m =. = 5H, =.H, i = Parameter for the current tranformer on the high-voltage ide of the power tranformer are: k = 6 / 5, B =.8T, =.7mH, 3 A = 3.47e m, =.75wb * turn, i =.5mA, R =.5Ω Parameter for the current tranformer on the low-voltage ide of the power tranformer are: k = 4 / 5, i =.3mA, B =.9T, R =.5Ω =.7mH, 3 A = 3.53e m, =.34wb * turn, and can be olved from (8), (), (4), and (8) by uing the forth-order Runge-Kutta method with a time tep. The Figure 8 how the curve of the magnetic linkage in the core after tranformer energization. The magnetic current i according to (4), the primary current according to i = and the econdary current according to (6) have been calculated uing the computed and that are hown in Figure 9,, and, repectively.

13 Journal of Advance in Computer Reearch (Vol. 9, No. 3, Augut 8) - Figure 8. The magnetic linkage of tranformer core Figure 9. The current of tranformer magnetizing branch 3

14 A New Model for Analyi of Unuual B. Nohad Figure. The primary current of the tranformer Figure. The econdary current of the tranformer 4

15 Journal of Advance in Computer Reearch (Vol. 9, No. 3, Augut 8) - i i the primary current of current tranformer on the primary ide of power tranformer and i i the primary current of current tranformer on the econdary ide of power tranformer. Now the econdary current of current tranformer on primary and econdary ide of power tranformer hould be achieved. i related to current tranformer on the primary and econdary ide of power tranformer and can be olved from (7), (3) and (35) uing any numerical integration method. In thi analyi, the forth-order Runge-Kutta method ha been ued with a time tep. The magnetic current i according to (4) and the econdary current of current tranformer according to given (5), (9) and (33) are calculated uing the computed. In thee relation, i p i the primary current of power tranformer. In Figure, the primary current refer to econdary ide and econdary current of the current tranformer on the primary ide of the tranformer are hown, that are i and i repectively. In Figure 3, the primary current refer to the econdary ide and econdary current of the current tranformer on the econdary ide of the tranformer are hown, that are i and i repectively. Alo the differential current ( i d ) hown in Figure 4 can be obtained from ubtract i and i. Figure. The primary current refer to econdary ide and the econdary current of the current tranformer on the primary ide of the power tranformer 5

16 A New Model for Analyi of Unuual B. Nohad Figure 3. The primary current refer to econdary ide and the econdary current of the current tranformer on the econdary ide of the power tranformer Figure 4. Waveform of the differential current 6

17 Journal of Advance in Computer Reearch (Vol. 9, No. 3, Augut 8) - A illutrated in Figure. and 3, the primary current of the power tranformer contain much higher aperiodic component becaue of the nonlinearity of the tranformer core, but the aperiodic component on the econdary current of the power tranformer i very low. In thi cae, the current tranformer of both ide in the tranforming behavior differ o greatly that the fale differential current (Figure 4) with ignificant amplitude and relatively low harmonic content will likely be formed becaue of the tranforming difference of the current tranformer. Figure 5 diplay the change of the magnitude of the fundamental component of i d in Figure 4, which i obtained with the Dicrete Fourier Tranform (DFT) algorithm. In thi Figure the magnitude of the fundamental component of the differential current i normalized according to the econdary current of current tranformer. Figure 5. The normalized magnitude of the fundamental component of the differential current Figure 6 diplay the ratio change of the econd harmonic to fundamental harmonic of the differential current after energization which i obtained with the DFT algorithm. 7

18 A New Model for Analyi of Unuual B. Nohad Figure 6. Ratio of econd harmonic to fundamental harmonic of the differential current with DFT algorithm A illutrated in Figure 5, the fundamental component of differential current i above.5p.u from the beginning of energization and approximately, after 5 cycle (.) it i tabilied on.569p.u. According to Figure 6, a the energization time exceed.98, the ratio of the econd harmonic to fundamental harmonic tabilize below 5%. If the differential protection ue.5 p.u. a the operating threhold and 5% a the econd harmonic retraint ratio, the mal-operation occur at.98. The occurrence of the delayed mal-operation of the differential protection depend on a variety of factor that the mot important which can be noted reidual flux and inception angle. In the previou imulation the inception angle wa degree and the reidual flux wa Webber a hown in Figure 8. In table and, variou cenario for different inception angle and reidual fluxe are preented, repectively. Table. Variou cenario for different inception angle θ m () t a t trip ( p. u.) ( ) ( wb) () ()

19 Journal of Advance in Computer Reearch (Vol. 9, No. 3, Augut 8) - Table. Variou cenario for different reidual fluxe m () θ t a t trip ( p. u.) ( wb) ( ) () () In table and, m t a و t trip are the magnitude of reaching the fundamental component of the differential current to., the time of reaching the ratio of econd harmonic to fundamental harmonic of the differential current to 5% and the time of the differential protection tripping, repectively. According to table, the extreme cae of inruh occur in inception angle of zero and a the inception angle increae, the time of the tripping of the differential protection decreae. A hown in table, if reidual flux increae, the time of the tripping of the differential protection increae. 6. Concluion and Future Work In thi paper, a new model baed on thevenin equivalent circuit for invetigating the ultra-aturation phenomenon during the energization of a loaded power tranformer wa preented and it effect on the differential protection of the tranformer wa conidered. In thi model, the nonlinear characteritic of the tranformer core, the effect of current tranformer, and the core loe were alo taken into account. It wa aumed that the load of the tranformer i a reitive and inductive load. In addition to a new model for power tranformer, the effective model for current tranformer wa preented in thi paper. The primary current of the power tranformer contain much higher aperiodic component becaue of the nonlinearity of the tranformer core but the aperiodic component on the econdary current of the power tranformer i very low. In thi cae, the current tranformer of both ide in the tranforming behavior differ o greatly that the fale differential current with ignificant amplitude and relatively low harmonic content will likely be formed becaue of the tranforming difference of the current tranformer which caue the ultra-aturation phenomenon to occur. The mal-operation of the differential protection depended on a variety of factor the mot important parameter of which were reidual flux and inception angle. Finally, the parameter mentioned were tudied in variou cenario. The reult howed that the ultraaturation wa a likely phenomenon. The main advantage of thi propoed model in compare with previou model were a following: ) The core loe were taken into account. ) A new model ha been conidered for current tranformer. In thi model, information of B-H curve for magnetic branch wan t required. Alo, ince hyterei effect wan t taken into account, reult could be compared with the IEEE model conidering hyterei effect. 3) It involved proper computing peed and accuracy. 4) Alo, the mal-operation of the differential protection depended on a variety of factor the mot important parameter of which were reidual flux and inception angle. In thi paper the parameter mentioned tudied in variou cenario. The explanation of the ultra-aturation phenomena i the firt tep toward developing new idea and 9

20 A New Model for Analyi of Unuual B. Nohad criteria for more reliable tranformer protection that would better handle uch abnormal cae than currently employed relaying equipment. In thi paper, the fourth-order Runge-Kutta method wa ued to olve equation. Thi work preented here concentrate on the phenomenon itelf, trying to explain when exceive flux without zero croing may appear. Knowing why and when the ultra-aturation may occur, one may better undertand the poible cae of the differential protection mal-operation. The outcome of thi paper may be alo treated a hint for ubtation peronnel, defining the ytem configuration, and parameter that hould be avoided in order to be on the afe ide hould unfavorable energization occur. It eem that improvement of the protection operation would be poible with the introduction of new, modified, or extended criteria. The protection criteria hould, on one hand, carry enough information on the event to be ditinguihed and, on the other hand, enure appropriate tabilization for other event for which protection operation i undeirable. Certain propoal of uch new criteria can be found in the literature, uch a a complex econd harmonic retraint, flux retraint (etimated on bai of voltage), or current wave hape analyi; other till wait for their inventor. It i believed that a lot of improvement can be reached with the introduction of adaptivity in the differential protection, a imple example of which i to ue adaptive threhold a well a adaptive meaurement procedure. It ha alo been proved that coniderable improvement of the operation and quite imple achievement of adaptive feature of protection function may be obtained with the ue of variou artificial-intelligence technique. There i hope that an appropriate combination of claical and intelligent technique hould bring additional benefit; therefore, further invetigation on the ubject are deirable. Reference [] P. iu, O. P. Malik, D. Chen, Hope GS, Guo Y, Improved operation of differential protection of power tranformer for internal fault, IEEE Tran Power Deliver, 99. [] A. Kunakorn, Application of dicrete wavelet tranform for tranformer inruh current detection in protective control cheme, In: Proceeding of the international ympoium on communication and information technologie conference, 4. [3] A.Wizniewki, H. Ungrad, and W.Winkler, Protection Technique in Electrical Energy Sytem, New York: Marcel Dekker, 995. [4] Numerical Differential Protection Relay for Tranformer, Generator, Motor and Mini Bu bar, SIEMENS AG, 7UT63/63x V.4.6 Intruction Manual, Order. C53-G76-C6-, 6. [5] X. in and P. iu, The Ultra-Saturation Phenomenon of oaded Tranformer Energization and It Impact on Differential Protection, IEEE Tranaction on power delivery, 5. [6] H. Weng, X. in, and P. iu, Studie on the Operation Behavior of Differential Protection during a oaded Tranformer Energization, IEEE Tranaction on power delivery, 7. [7] A. Wizniewki, W. Rebizant, D. Bejmert, and. Schiel, Ultra aturation Phenomenon in Power Tranformer Myth and Reality, IEEE Tranaction on power delivery, 8. [8] M. Taghipour, S. M. Razavi, and M. A. Shaminejad, Intelligent Determining Amount of Inter- Turn Stator Synchronou Motor Uing an Artificial Neural Network Trained by Improved Gravitational Search Algorithm, Journal of Advance in Computer Reearch, 4. [9] M. Taghipour, M. Yazdani, S. A. Gholamian, and M. Razavi, A Novel Approach for Dicrimination Magnetizing Inruh Current and Internal Fault in Power Tranformer Baed on Neural Network, Journal of Advance in Computer Reearch, 4.

21 Journal of Advance in Computer Reearch (Vol. 9, No. 3, Augut 8) - [] S. M. B. Sadati, J. Mohtagh, A. Ratgou, oad and Harmonic Forecating for Optimal Tranformer oading and ife Time by Artificial Neural Network, Journal of Advance in Computer Reearch, 5. [] M. Naidu and G.W. Swift, Dynamic Analyi of a current Tranformer during Fault, Electric Power Sytem Reearch, 986.

22 A New Model for Analyi of Unuual B. Nohad