Effective Placement of Surge Arrester During Lightning

Transcription

1 Effective Placemet of Surge Arrester Durig Lightig 1 G. Radhika, 2 Dr.M.Suryakalavathi ad 3 G.Soujaya 1 Sr. Assistat Professor, VNR VJIET, radlalitha.g@gmail.com 2 HOD-EEE, JNTUiversity, muagala12@yahoo.co.i 3 EEE Dept, VNR VJIET, soujayavemula@yahoo.co.i Abstract Lightig iterferece occurs maily o overhead lies ad has bee a problem sice the early days of electricity supply. Over voltages which occur o the lies, travel towards the termial or substatio, ad ca damage, particularly the expesive equipmet such as power trasformers. The lightig surge causes the travellig waves ad the temporary icrease i voltage i the trasmissio lie system. Lightig is the most harmful for destroyig the trasmissio lie ad settig devices so it is ecessary to study ad aalyse the temporary icrease i voltage for desigig ad settig the surge arrester. Arresters provide protectio to equipmet by limitig the voltage across the equipmet termials i the presece of a surge o the system. This voltage limitig characteristic of arresters is the primary feature of a arrester i most cases, ad the reaso for its existece. The voltage limitig characteristic has several ames ad has eve chaged over the years. I this work Metal-Oxide Surge Arrester is used. This project presets the PSCAD/EMTDC based trasiet modellig of a three phase trasmissio lie circuit for aalysig their performace durig lightig. The method used to aalyze the icrease i voltage due to lightig was doe by usig the applicatio called PSCAD/EMTDC. This software could be used to aalyze electric system i differet aspects, icludig trasitio state ad steady state. Therefore, it could be used to aalyze the trasmissio lie system. Circuit model is developed for a three phase model ad the same is used for simulatio studies. This study geeralizes the modellig details ad performig the aalysis of lightig surge o surge arrester placemet usig the PSCAD/EMTDC software. The outcome of this paper would be the effect of surge arrester placemet i terms of voltage level measured at particular poits. Keywords Lightig, Surge Arrester, Isulatio Co-ordiatio, PSCAD/EMTDC. I. INTRODUCTION Lightig is a major cause of overhead lie faults. Betwee 5% to 10% of the lightig-caused faults are thought to result i permaet damage to power system equipmet. Lightig has bee oe of the importat problems for isulatio desig of power systems ad it is still the mai cause of outages of trasmissio ad distributio lies [4]. A complete awareess of the parameters of lightig strokes is essetial for the predictio of the severity of the trasiet voltages geerated across power apparatus either by a direct stroke to the power lie/apparatus, or by a idirect stroke. However, o two lightig strokes are the same. Because of lightig travelig waves are produced i the system. These travelig waves cause the temporary icrease i voltage i the trasmissio lie system. The icrease i voltage is harmful for the isulator of lies ad devices coected to the trasmissio lie. Therefore, it is ecessary to aalyze such icrease i voltage i order to desig the surge arrester (or the isulator) suitable for the ivestmet, the good performace of the system ad the reliability of trasmissio lie system. Durig the seve years betwee 1998 ad 2004, 81 direct lightig strokes to phase coductors were observed. The umber is based o the first stroke for multistroke evets ad excludes some data for which parameters could ot be estimated. All of the lightig strokes were observed i the summer. It is essetial for electrical power egieerig to reduce the umber of outages & services &electric supply. There fore it is ecessary to protect the power apparatus from over volts i electric system, amely lightig over voltages & switchig over voltages. Lightig over voltages is a ature atural pheomeo while switchig over voltages origiates i the system itself by the coectio &discoectio of circuit breakers cotacts or due to iitiatio or iterruptio of faults. Switchig over voltages is highly damped over voltages. The magitude over voltage s appearig o trasmissio lies does ot deped o the lie desig & hece lightig performace teds to improve with icreasig isulatio level,i.e. with system voltage o the other had, switchig over voltage s are proportioal to operatig voltage. Lightig strokes hittig towers, coductors or ay object lie the eighborig of a trasmissio lie ca produce abormal curret/voltage waves alog coductors (phase ad/or shield wires) as well as alog 167

2 steel towers. Due to these pheomea, over voltages are produced ad electric arcs (betwee two or more coductors as well as betwee phase coductors ad the tower structure) may appear, if isulatio breakdow voltage is reached. Back flashover occurs whe lightig stroke termiates o the overhead groud wire or tower. A stroke that so termiates forces currets to flow dow the tower ad out o the groud wires. Thus voltages are built up across the lie isulatio. If these voltages equal or exceed the lie critical flashover voltage (CFO), flashover occurs [4]. Study o back flashover is very importat to is very importat to evaluate lightig performace as majority of lightig strokes termiate o shield wire tha phase coductor. The objectives of this study are to protect the power system equipmets from lightig ad determie the effect of surge arrester placemet. This will be doe by comparig the voltage level measured close to the trasformer with the suggested basic isulatio level (BIL) value used by the utility. The ideal approach to study the trasiet pheomea i a power system is to capture ad record the trasiets usig wide badwidth trasducers ad recordig equipmet ad the aalyse these waveforms However, capturig trasiet sigals this way represetig all possible scearios is ot realistic. A alterative techique is to simulate the power system usig a suitable electromagetic trasiet program PSCAD/EMTDC. The Power System Computer Aided Desig (PSCAD/ EMTDC) programs available i the market today preset the power systems compoets while keepig the complexity of the models to a miimum. II. MODELLING OF THE SYSTEM Fig.1. Selected Model Lightig o phase lie of trasmissio lie system might cause isulatio breakdow of importat devices. Therefore, this occurrece must be avoided as much as possible. The way to reduce the risk is the use of Lightig arresters. The proposed model is a log trasmissio lie icludes, trasmissio lies T1,T2,T3, trasmissio lie iterfaces, lightig surge circuit, lie impedace, stray capacitace of the equipmets, trasformer. Figure 1 shows the modelig arragemets of the system. A 10KA surge with impulse of 8/20μs is take. I geeral lightig is represeted by a curret source of egative polarity. The trasmissio of a travellig wave at juctio poits of equal impedaces i a trasmissio lie is of great importace i trasmissio systems. Sometimes a voltage rise or build up of voltage ca occur depedig o the type of the impedace at trasitio poits. I the above proposed model statio 2 was termiated with a trasformer, whose solutio ca be obtaied usig the Laplace trasforms rather tha usig operatioal calculus. The trasmitted wave reachig the trasformer will be either a double expoetial or a damped siusoidal wave ad the steepess of the wave frot gets reduced. Herei the lie termiated with a trasformer is take as a parallel L-C combiatio [6]. Over voltages could be calculated by lightig curret (I 0) ad lie surge impedace (Z s). The circuit is show i Fig. 1. Therefore over voltage due to lightig (V 0) i fig. 1 is as follows [1]. Over voltage = i z V 0 = 2 V 0 = 1750kV *350 System with over voltage is simulated usig PSCAD/EMTDC software, ad circuit with surge arrester is also simulated for fidig the best positio of the surge arrester by chagig the legth of trasmissio lie T3 to 90m, 60m, ad 30m. III. ARRESTER MODELLING Whe specifyig a metal-oxide arrester model for a study, the accuracy of the simulatio is improved whe the arrester characteristic is chose to be cosistet with the frequecy or time-to-crest of the voltage ad curret expected durig system perturbatios [5].This cocept is key to modellig because metal-oxide arresters are frequecy-depedet devices, i.e., the voltage across the arrester is a fuctio of both the rate of rise ad the magitude of the curret coducted by the arrester. I order to obtai metal-oxide arrester characteristics for a wide rage of wave shapes a umber of differet curret test wave shapes have bee used [2]. 168

3 R 0 = 100 d ohm = 145 ohm 100 C = pf = pf d Fig.2. Arrester Model The iductor L 0 i the model represets the iductace associated with magetic fields i the immediate viciity of the arrester. The resister R 0 is used to stabilize the umerical itegratio whe the model is implemeted o a digital computer program. Capacitor c represets the termial-to-termial capacitace of the arrester. The iductace L 1 ad resister R 1 of the model comprise the filter betwee two o-liear resisters. This sectio details the determiatio of the metal oxide model parameters for a oe colum arrester with a overall legth of 1.45 meters. The discharge voltage, V 10, for this arrester is 248kV for a 10kA, 8 x 20µs curret wave shape. A. Arrester Iformatio Required d - Legth of arrester colum i meters (use overall dimesios from catalog data) - Number of parallel colums of metal-oxide disks v 10 - discharge voltage for a l0ka, 8 x 20µS curret, i kv B. Lumped Parameter Elemets Usig the equatios preseted here, the iitial values for Lo, Ro, L1, RI & C are determied as follows[2] L 1 = 15 d µh Here, d = legth of arrester colum i meters =1.45m oxide disks L 1 = = umber of parallel colum of metal 15 d µh = µh 15 d R 1 = ohm = ohm 0.2d L 0 = µh = 0.29 µh Fig.3. V-I Relatioships for No-Liear Resistor Part of Model Figure 3 is used to determie the iitial characteristics of the oliear resistors A 0, ad A 1. Each of the V-I poits for the oliear resistors is foud by selectig a curret poit ad the readig the relative IR i pu from the plot. This value is the multiplied by (V 10/l.6) to determie the model discharge voltage i kv for the associated curret. This scalig from pu to actual voltage is doe by the applicatio of the followig formula to the "Relative IR" pu voltage foud for that curret as give i Figure 3[2]. For A 0, the Discharge KV = [Relative IR i pu for A 0 (i)] * [V 10/1.6] For the above arrester, the associated V-I voltage for a l0ka curret for the oliear resistor, A 0, is determied by readig the "Relative IR" for a 10kA curret from Figure.3 Examiatio of the plot shows that the "Relative IR" for a 10 ka curret is 1.9pu. Therefore the discharge kv for A 0 associated with l0ka is [2] 248 Discharge KV = 1.9* 1.6 Discharge KV = Similarly, For A 1, the Discharge KV = [Relative IR i pu for A 1 (i)]*[v10/1.6] Discharge KV = 1.55*(248/1.6) Discharge KV = TABLE 1 : V-I CHARACTERISTICS FOR A0 169

4 Is(KA) V(pu) V 0(KV) TABLE 2 : V-I CHARACTERISTICS FOR A1 Is(KA) V(pu) V 1(KV) V 0, V 1 are the discharge voltages of A 0 ad A 1 No- Liear resistors respectively. Discharge voltage is othig, but the voltage that appears across the termials of a arrester durig passage of discharge curret. IV. SIMULATION RESULTS The selected model is coected with trasmissio lies, lightig surge circuit ad trasformer. Digital simulatio is doe by usig PSCAD/EMTDC software ad the results were preseted here. System with over voltage is show i Figure 4. The output over voltage of the system is show i Figure 5. The Over Voltage is 1784KV. To reduce the over voltage ad to protect the system equipmets Metal-Oxide Surge Arrester is used. The system with surge arrester ad waveforms for arc curret ad waveform of discharge voltage are show 170 i Figures 6, 7, 8 respectively. The discharge voltage of the arrester is 303KV. Fig.4. System with Over Voltage Fig.5. Waveform of Over voltage Fig.6. System with Surge Arrester

5 Fig.7. Waveform of Arc Curret Fig.11. Output Voltage Waveform for placig the Surge Arrester 60m away from the Trasformer Fig.8. Output Voltage Waveform whe Arrester is set Fig.12. Output Voltage Waveform for placig the Surge Arrester 30m away from the Trasformer By movig the surge arrester the arc curret ad voltages before trasformer is show i Table 3. TABLE 3 : SIMULATION RESULTS Surge arrester is set at Before the trasformer 30m away from the trasformer 60m away from the trasformer Arc curret (ka) Discharge Voltage (kv) Fig.9. Model for placig the Surge Arrester 90m away from the Trasformer Fig.10. Output Voltage Waveform for placig the Surge Arrester 90m away from the Trasformer 90m away from the trasformer Table3 shows the results for differet arrester positios. Surge Arrester is placed 30m, 60m ad 90m away from the trasformer. The output voltages 387.5KV, KV ad KV respectively. V. CONCLUSION A detailed proposed model was successfully preseted. As system over voltage is very high, it shows that the lightig surge ca be very dagerous eve at low value of curret if there is o surge arrester is i operatig or used for protectio. This over voltage is reduced by usig the Metal-Oxide Surge Arrester. I the proposed model the distace betwee arrester ad trasformer was set at 90m, 60m ad 30m. The respective output voltages are KV, KV 171

6 ad 387.5KV. Accordig to the simulatio results, at such distaces the surge arrester did ot work best because these distaces were too far from the equipmet to be protected, makig the travelig waves resoud due to the icrease i travelig rage. Hece overall results have demostrated the importace of havig a right locatio of surge arrester placemet. The best way to prevet surge is to set the best distace, that is to say, to set the surge arrester as close as possible to the equipmet to be protected, as fail to do so will cause a sigificat damage to the equipmet. REFERENCES [1] N. Mugkug, S. Wogcharoe., Taes Taitteerapa, C. Saejao, D. Aruyasot, Aalysis of Lightig Surge Coditio Effect o Surge Arrester i Electrical Power System by usig ATP/EMTP Program, World Academy of Sciece, Egieerig ad Techology [2] IEEE workig group Applicatio of surge protective devices subcommittee Surge protective devices committee, Modelig of metal oxide surge arresters, Trasactios o Power Delivery, Vol. 7 No.1, Jauary 1992 [3] M.Z.A. Ab Kadir ad A.M. Azmi Impact of Lightig Surge o Surge Arrester placemet i high voltage substatio, Joural of Applied Sciece 8 (18): ,2008 [4] M. Z. A. Ab Kadir, J. Sardi, W. F. Wa Ahmad, H. Hizam, J. Jasi, Evaluatio of a 132 kv Trasmissio Lie Performace via Trasiet Modellig Approach, Europea Joural of Scietific Research ISSN X Vol.29 No.4 (2009), pp [5] Ja Takami, member, IEEE, Shigemitsu Okabe, Member, IEEE, ad Eiichi Zaima, Member, IEEE, Study of Lightig Surge Overvoltages at Substatios Due to Direct Lightig Strokes to Phase Coductors, IEEE Trasactios o Power Delivery, Vol 25, No.1, Jauary [6] Extra High Voltage AC Trasmissio Egieerig, Rakosh Das Begamudre 172