The Research Method of Prevention Transformer Differential Protection Misoperation Caused by the Sympathetic Inrush

Transcription

1 The Research Method of Preventon Transformer Dfferental Protecton Msoperaton Caused by the Sympathetc Inrush Jngbn Yang 1, Dalong Fu 2, Youwen Tan *3, Yuwe L 4. Shenyang Agrcultural Unversty, Shenyang Cty, Laonng Provnce, 120 Donglng Road, Chna *youwen_tan10@163.com Abstract Dfferental protecton has been the man protecton of transformer for a long tme. The key and dffcult problem s how to prevent the dfferental protecton msoperaton caused by nrush current. In recent years, there has been a lot of sympathetc nrush accdents caused by msoperaton of transformer dfferental protecton, affectng the safe and stable operaton of power system. Ths paper establshed the smulaton model of nrush current and, analyss of sympathetc nrush mechansm. At the same tme, the fundamental reasons of dfferental protecton msoperaton caused by the sympathetc nrush were found out. And relevant methods and measures of preventng the sympathetc nrush msoperaton n the transformer dfferental protecton are put forward. Keywords Transformer; Dfferental; Prodecton; Synpathetc Inrush Current; Measures Introducton Power transformer s the most mportant equpment n power system, whch has extremely mportant sgnfcance for relable transportaton, flexble dstrbuton and safe use of electrcty. Its runnng safety s drectly related to the ablty of the power system s contnuous, relable and stable workng. If power transformer happens fault, t wll not only make the system power supply nterrupton, but also nfluence the ndustral and agrcultural producton and peopleʹs normal lfe (HAYWARD. 1941). Even t can endanger the personal safety and equpment.(sun Hu et al. 2003) Especally because the transformer s expensve and complex structure, once t s damaged due to fault, the mantenance dffculty s more serous and the mantenance tme s longer, whch wll brng enormous loss to natonal economy. For a long tme, dfferental protecton has always been one of the man protecton schemes of the nternal fault of transformer. The reasons of longtudnal dfferental protecton msoperaton are n many aspects. When transformer closes n the removal of no load or voltage recovers after external fault, the nrush current of transformer core saturaton and produce s the most serous problem faced by the transformer dfferental protecton. (Xuesong Zhang 2006, Daqang B, 2007) In recent years, there are many maloperaton of adjacent parallel transformer dfferental protecton when a no load transformer s closed. Ths brngs a lot of harm to the normal operaton of the man equpment. Ths knd of phenomenon has somethng wth sympathetc nrush current (Hongchun Shu et al. 2006). Theoretcal Analyss and Smulaton Verfcaton of Produce Mechansm of Transformer and Sympathetc Inrush Current To research produce mechansm of sympathetc nrush current, we use parallel two transformers operaton as an example. The equvalent crcut model s shown n Fgure 1. The parallel operaton of transformers can produces sympathetc nrush current. When the T1 s n normal operaton, T2 s no load. If the nrush current n T2 s large, T1 wll appear sympathetc nrush current correspondng to nrush current. And great changes wll take place n the lne current waveforms. In Fgure 1, the system voltage s ndcated by US. A resstance s RS. A nductance s LS. Equvalent 12 Internatonal Journal of Engneerng Practcal Research, Vol. 4 No. 1 Aprl /15/ DEStech Publcatons, Inc. do: /jepr

2 The Research Method of Preventon Transformer Dfferental Protecton Msoperaton Caused by the Sympathetc Inrush 13 resstor of the prmary sde of the transformer T1 s R1. The nductance s L1. Equvalent resstance of the prmary sde of the transformer T2 s R2. Inductance s L2. B s a common pont. When the swtch S closes, transformer T2 wll produce nrush current 2, whch s completely nclned to one sde of the tmelne. There are a lot of non perodc components. Nonperodc components of the current generate voltage drop, whch make bus voltage of T1 and T2 occur DC excurson through the system resstance. Snce the magnetc flux n the transformer s the ntegral of the voltage. Magnetc flux of the transformer T1 wll be nclned to the sde as the new bus voltage shft, the supermposton of the ncremental offsets magnetc flux and components of the cycle magnetc flux,whch wll make the core of T1 saturated and occur sympathetc nrush current. Set S, 1,respectvely,as system current and the current that flows through T1, S = The whole process can be descrbed as follows. FIGURE 1. TWO PARALLEL RUNNING TRANSFORMER EQUIVALENT CIRCUIT DIAGRAM. The Occurrence of Sympathetc Inrush Current As shown n Fgure 1, before T2 s closed, T1 s on operaton normally, the transformer T1 core s not saturated. So 1 equals 0. After T2 s closed, T1 has not yet saturated n tme, 1 s stll 0. At the moment, S equals 2, the voltage balance equaton of transformer T1 s as follow: d 1 d 2 us Ls Rs2 (1) dt dt Consderng a cycle, the result of ntegral of both sdes s as follow: s s 2 2 s 0 2 (2 ) (0) u ( ) d L [ (2 ) (0)] R ( ) d The us (θ) s a snusodal voltage source, a cycle of (2) ntegraton s zero. 2 s as nrush current. Accordng to the waveform of nrush current, we know when θ equals 0 and θ equals 2π, 2 (0) 2 (2π) 0.By above formulas, we can get: 1 1 s 0 2 (2 ) (0) R ( ) d (3) Accordng to formula (3), t can be analyzed that the changes of the T1 flux are determned manly by perodc ntegraton of the 2 n T2. Each cycle ncrement of the flux lnkage of T1 s: 1 s 2 R 0 ( ) d (4) To consder t easly, let s assume that the magnetzng nrush current 2 of T2 s nclned to the tme axs postve sde.(smlarly,t does not affect the results f assumng t s nclned to the tme axs negatve sde) From the formula (4), t can be seen that as Δψ1 s negatve, flux of T1 s ncreased n the opposte drecton, gradually reachng saturaton pont. Before reachng saturaton pont, 1 can be substantally consdered to be zero. Because of the role of the AC component of the flux ψ1, before the non perodc component of flux Ψ1 does not fully reach the saturaton pont, part flux of each cycle may exceed the saturaton pont at some pont. And therefore, flow wll generate at the peak of the flux, wth ntermttent angle. Wth T1 flux ncreasng n the opposte drecton and nrush current 1 growng, ntermttent angle decreases. Due to flux of T1 ncreases n the negatve drecton, flux s not saturated n T1, when there s a postve saturaton of flux n T2. So negatve saturaton only occurs n T1.That s, 1 and 2 are n contrary drecton and staggered n tme. Development Stage of the Sympathetc Inrush Current When sympathetc nrush current appears n T1, change of flux ψ1 s formed jontly by 1 and 2, then: After fnshng: 1 s 1 1 s 2 [( R R ) R ] d (5) 1 s [ R ( ) R ] d (6) Because ntermttent angle exsts n sympathetc nrush current 1 and nrush current 2, 1 s nclned to one sde of the tmelne, both of 1 and 2 contan nonperodc components. Assumng 1.f or 2.f are nonperodc component of 1 or 2and sf s non perodc component, we can know: (7) sf 1f 2f

3 14 Jngbn Yang, Dalong Fu, Youwen Tan, Yuwe L After sympathetc nrush current occurrng, the varaton of flux n transformer T1 s: 1 s 1f 2f 1 1f FIGURE 2. A PARALLEL TRANSFORMER SIMULATION MODEL. 2 [ R ( ) R ] (8) As 1 and 2 are n the opposte drecton, we assume that the ampltude of 2 s postve and the magntude 1 s negatve. Wth appearance of 1, the flux ψ 1 of T1 ncreases slowly n reverse drecton. As the flux of T1 cumulates negatvely and constantly, sympathetc nrush current 1 ncreases rapdly. We can conclude the followng equaton: R s 1 f f Rs R1 R s 1 f f Rs R1 Changes of T1 flux n each cycle s as follows: (9) Rd (10) Snce 1 s negatve, Δψ1 changes n sgn, ndcatng that the flux of transformer T1 decreases gradually shfts. At ths pont, there s a postve and negatve half cycle symmetry current. The magnetc chan ψ1 of T1 ncreases to the maxmum n the opposte drecton, whle sympathetc nrush current 1 ncreases to the maxmum, the absolute value of the magnetc chan n T1 decreases, then, 1 begns to decrease slowly (Yanyun Zou 2009, Wanca Wu 2011, Hongchun Shu 2006, Gong We et al 2012, Daqang B et al 2005, Xaofan Shen et al 2011, Xnchen L 2014). Smulaton Establshng smulaton shown based on Fgure 2 n Matlab, man parameters of the transformer model are: the wndng wrng, voltage, leakage nductance n resstance, saturaton characterstc n core, hysteress loop, etc. The transformer characterstcs can be easly set. In the study, we use tral and error methods n the hysteress loop to determne the hysteress loop by comparng smulaton waveforms, the theoretcal waveforms and actual waveforms. Parameters of transformer T1 are: capacty 150MVA, frequency 50Hz, rated voltage (500/ 3 ) kv / (220 3 ) kv. Hgh voltage and low voltage wndng resstance and leakage nductance are the same, whch are respectvely and Exctaton resstor s 450 (per unt), wth two lnear mtatng saturaton characterstcs φ: 0,0; 0,1.2; 1.0,1.45. Parameters of transformer T2 are consstent wth the T1 s. After settng parameters, the above theoretcal analyss are smulated.smulaton results are shown below. FIGURE 3. WAVEFORM OF SYMPATHETIC INRUSH I2 THROUGH THE TRANSFORMER T2. The fgures show that the transformer produces nrush current rapdly, after the unloaded transformer s closed.inrush current decreases wth the passage of tme.after Inrush current appears, the runnng transformer wll generate sympathetc nrush current.

4 The Research Method of Preventon Transformer Dfferental Protecton Msoperaton Caused by the Sympathetc Inrush 15 And frst sympathetc nrush current ncreases and then decreases. Smulaton results concde wth the theoretcal analyss. FIGURE 6. PHI 2 (0) =1.2 SYMPATHETIC INRUSH WAVEFORM. FIGURE 4. WAVEFORM OF SYMPATHETIC INRUSH I1THROUGH THE TRANSFORMER T1. Factors that Affect Sympathetc Inrush After analyzng the generaton mechansm of sympathetc nrush, ths study further analyzes how the resdual magnetsm of no load swtchng transformer and system mpedance parameters nfluence on ampltude of sympathetc nrush current and saturaton velocty by numercal smulaton. We can sum up characterstcs of the parallel transformer sympathetc nrush. FIGURE 7. THE SYSTEM RESISTANCE IS 6 OHM SYMPATHETIC INRUSH WAVEFORM. The Effect of Sympathetc Inrush from T2 Dfferent Resdual Magnetsm Fx closed tme of T2 and change the ntal value of magnetc chan ψ2(0). As the remanence ψ2 (0) ncreases gradually, ψ2 (t) also ncreases, sympathetc nrush current generates rapdly and ampltude ncreases n negatve drecton. In other words, when closng angle must be fxed, the greater the remanent magnetzaton of the transformer, larger sympathetc nrush,the speed of reachng the maxmum s faster. FIGURE 8. THE SYSTEM RESISTANCE IS 10 OHM SYMPATHETIC INRUSH WAVEFORM. Fgure 5 for φ2(0) = 0.8 sympathetc nrush waveform, Fgure 6 for φ2 (0) = 1.2 sympathetc nrush waveform. Compared wth φ2 (0) = 0.8, φ2 (0) = 1.2, the remanence of T2 transformer s bgger, sympathetc nrush also s bgger, and the speed of reachng the maxmum s faster. FIGURE 9. SYSTEM REACTANCE 0.2H FOR THE SYMPATHETIC INRUSH WAVEFORM. The Influence of the System Resstance on Sympathetc Inrush FIGURE 5. PHI 2 (0) =0.8 SYMPATHETIC INRUSH WAVEFORM. System resstance can affect the ampltude and attenuaton of sympathetc nrush. The depth of nfluence depends on ts proporton n the whole crcut. The producton process of sympathetc nrush

5 16 Jngbn Yang, Dalong Fu, Youwen Tan, Yuwe L actual s the process that the swtchng transformer transent flux s redstrbuted to other transformer, n whch system resstance plays an mportant role. The sze of system resstance determnes the speed of redstrbuton. The greater the System resstance s, the faster the T1 saturates, and the occurrence of sympathetc nrush and the saturated velocty are also faster. Fgure 7 s sympathetc nrush waveform wth system resstance of 6 Ω. Fgure 8 s sympathetc nrush waveform wth the system resstance of 10 Ω. Compared wth the system resstance for 6 Ω, the system resstance of 10 Ω accounts for a larger proporton n the whole crcut. And the faster sympathetc nrush occurs and saturats, the bgger sympathetc nrush current wll be. The Effects of the Reactance of the System on Sympathetc Inrush The greater the system reactance, the slower sympathetc nrush saturats and the peak of sympathetc nrush s smaller. But the system reactance s smaller than transformer short crcut reactance, therefore, t s lmted to ncrease the system reactance for reducng peak of nrush current. Smlarly, t s also lmted to reduce the attenuaton velocty of nrush current. Fgure 9 s nrush current waveform wth the system reactance for 0.2 H. Fgure 10 s nrush current waveform wth the system reactance for 0.15 H. Compared to the system reactance for 0.2H,when the system reactance s 0.15H, the faster nrush current generates and saturates, the larger the peak of nrush current s. FIGURE 10 SYSTEM REACTANCE 0.15H FOR THE SYMPATHETIC INRUSH WAVEFORM. Measures for Preventng the Sympathetc Inrush of Transformer Msoperaton Through the above analyss, we can see that the man cause of the dfferental protecton msoperaton are that local transent saturaton of current transformer s caused by aperodc components of sympathetc nrush and second harmonc content n the dfferental current becomes low. Therefore, we shall take correspondng measuresto prevent the phenomenon. In order to prevent transent saturaton of current transformer, f condton permts, we can replace the P TA wth TP TA. To meet the senstvty requrements, we ncrease the fxed value of the dfferental protecton of generator and transformer approprately. Before closng no load Transformer, all capactors should be exted. We also should close the no load transformer at the peak perod of load. These measures can reduce sympathetc nrush effcently. Before closng the swtch of no load transformer, neutral pont should not ground.whch makes t generate exctaton nrush current and do not produce sympathetc nrush. It should be noted that, f the swtch transformer neutral pont ungrounded, to prevent overvoltage problem when closng the noload transformer, clearance protecton should be nstalled n the transformer neutral pont, such as gapless metal oxde arrester: the specal protecton of transformer neutral pont,or rod gaps parallel arrester protecton. Ths can prevent mpulsve overvoltage from damagng transformer core and nsulaton. Conclusons Formaton of sympathetc nrush s related to many factors. Its essental reason s that voltage fluctuatons are caused by exctaton nrush current of swtchng transformer, whch flows through the system resstance. On the bass of summarzng predecessorsʹ work, we set up smulaton models of parallel transformers, and analyze the generaton mechansm of sympathetc nrush, nfluence of the parameters n the model and the causes of the dfferental protecton msoperaton n ths paper. Fnally, related methods and measures of preventng the transformer dfferental protecton msoperaton are put forward, hopng that they wll be helpful to the relable operaton of the transformer protecton. REFERENCES BI Da qang et al. Theory Analyss of the Sympathetc nrusn Operaton Transformers. Automaton of Electrc

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