Exploring the Possibility of the Power Transmission Towers Shield Wires Removal with considering Tower Footing Resistance via EMTP-RV Software

Transcription

1 Volume 6.1, January 2016 ISSN Exploring the Possibility of the Power Transmission Towers Shield Wires Removal with considering Tower Footing Resistance via EMTP-RV Software Ali Mahmoudian, Mohsen Niasati Department of Electrical and Computer Engineering, Semnan University ABSTRACT Lightning is the main cause of reduced reliability of power transmission overhead lines so that studies show most of the interruption in transmission lines are caused by lightning. Mostly, the shield wire and tower footing resistance reduction techniques are used to improve the performance of transmission lines against lightning. However, the application of shield wire increases construction costs as well as the risk of occurring short circuits between phases and shield wire due to galloping fluctuation or tearing wires. The tower footing resistance can be very variable according to changes in environmental conditions. In this paper, with the aid of simulation in EMTP-RV software, transmission lines performance against lightning is studied and evaluated in case of the elimination of shield wire and line arrester usage for different conditions of the system. The results show if eliminating shield wire in case of using line arrester in land with lower resistance is possible. Index Terms Back flashover; Line arrester; Shield wire; Tower footing resistance. I. INTRODUCTION Tower transmission lines are as one of the main parts of the power system which have a significant impact on the reliability of the power network. Studies have shown that more than 70% of the faults and interruptions that are created on power lines are caused by lightning transient waves. Transmission lines are always subjected to the lightning transient waves risks due to large extent and crossing different regions in terms of climate and environmental characteristics. Lightning with two general form of a direct strike (strike the phases) and indirect strike (strike to the shield wire, the body of the tower, the ground and adjacent trees,...) can produce transient voltage in transmission lines conductors and cause flashover or back flash over on both ends of the insulator chains or between phases and tower body and eventually leads to an interruption in the transmission lines[1,2]. Factors such as area lightning density, towers dimension, weather conditions, presence or absence of shield wires, and tower footing resistance changes effect on the of lightning strike power lines. The number of strike by lightning to the line during a year can be account into different formulas, including Anderson Formula according to Equation (1) [3]. Model of traveling waves depending on frequency is used to model transmission line conductors in EMTP-RV software N = T d (b+4h ) kv (1) untransposed transmission line conductors data is presented in Table I. T d: region Keraunic level h: towers average height(meter) b: shield wires distance from each other, or the width of the tower(meter) Generally, the use of shield wires with tower footing resistance reduction is the most effective method to reduce the risks of lightning on transmission lines. Although, shield wire provides relatively high protection for power transmission lines against direct strike of lightning, cloud electromagnetic and electrostatic voltage, it increases the cost of transmission line implementation and increases the risk of short-circuit lines (due to breakage and fall shield wire on the phase conductors or phase conductors in contact or close to the shield wire caused by galloping phenomenon). So, it should be avoided as much as possible to install a shield wire for overhead transmission lines (except in areas with high lightning density, high importance lines and also near the substation). This paper will examine the possibility of eliminating the shield wire in transmission lines and optimal use of line arresters instead of that, in different conditions. II. TRANSMISSION LINE MODEL FOR LIGHTNING TRANSIENT STUDIES To study the transmission lines performance against lightning, a 400 kv bundle single circuit transmission line with two shield wire is considered as shown in Fig.1, and it is simulated in EMTP-RV software. Each of the main sections of the line are modeled as followed. A. Transmission Line Conductors Model Table I: Transmission Line Conductors Data Conductor data Number of conductor Number of bundles Conductor type Phase conductor 9 3 Curlew Shield conductor 2 - Curlew Core

2 Volume 6.1, January 2016 ISSN nominal Cross section(mm 2 ) Conductor diameter(mm) Dc resistance(ω/km) B. Power Transmission Line Towers Model Towers model and specification are presented in Fig.1 and Table II respectively. - 2 Z.ln g R = ti i h + h 1 2 ( i = 1,2) (2) R =- 2 Z.ln 3 3 g (3) 2H L = a. R. i i V t (i=1,2,3) (4) H = h + h + h (5) In the above equations γ is Diffusion coefficient, α is Attenuation coefficient and both are equal to 0.8,1 respectively. V t is Wave velocity and is equal to 300(m/ms). Table III: Tower Surge Impedance Model Values R L Zt1 220 R L Zt2 220 R L3 7 Zt3 150 Fig.1. Towers Model Table II: Transmission towers geometric specification H h1 h2 h3 A B 2r *R(Ω) and L(µH) D. Lightning Strike Current Waveform Model Lightning strike current is modeled as an ideal current source parallel with a large resistance (1000 to 2000 ohms), according to CIGRE model as it is shown in Fig.3. C. Towers Surge Impedance Model Surge impedance model for towers that represented in Fig.1 is according to Fig.2. [4]. The parameters values of model that are represented in Fig.2 are obtained with relations (2) to (5). Fig.3. Lightning current modeling according to CIGRE model Mathematical modeling of lightning current waveform is as followed:[5]. t t i ( t ) I 0 *( e e ) (6) I 0: lightning current peak α: before-wave attenuation coefficient β: after-wave attenuation coefficient Fig.2. Tower Surge Impedance Model In this paper, the effect of the lightning waveforms on performance of transmission lines including the main wave and sub-wave lightning is studied. E. Line Arrester Model Regarding the nature of lightning, dynamic model of arrester that is suitable for fast transient waves studies is used [6,7].

3 Volume 6.1, January 2016 ISSN In this paper, for simulating the line arrester, ieee simplified model that is represented in Fig.4 has been used[8]. V gap(t): applied voltage at the time t, to the terminals of the air gap V 0: minimum voltage to be exceeded before any breakdown process can be started or continued T 0: time which would be Vgap(t) > V 0 k,v 0 and D: constants corresponding to an air gap configuration and overvoltage polarity. Fig.4. Arrester IEEE Simplified model (Pinceti model) In this model R is equal to 1MΩ [9]. L 0 (µh) and L 1(µH) Parameters values are according to (7)and(8) equations. V V r1/ T r8 / 20 1 L. 2. V 0 12 V n r8 / 20 (7) V V r1/ T r8 / 20 1 L. 2. V 1 4 V n r8 / 20 (8) occurs when the integral becomes greater or equal to D. The parameters V 0, k and D are determined by using the voltage time curve. G. Tower footing impedance model In this paper the impedance (capacitive) model for modeling towers ground system for studies of transient state has been used. This model has proper accuracy for soils with high permeability. In this model the effects of lightning is considered by the capacitor in parallel with a resistor [12]. In the above equations, V n is nominal voltage of arrester,v r1/t2 is residual voltage of arrester for fast wave with current amplitude 10kA and V r8/20 is residual voltage for wave with current amplitude 10kA and 8/20(µs) waveform[8,9]. F. Insulator String Model According to the IEC standard for studying of the transient state, insulator strings is modeled as a capacitor in parallel with switch (air gap)[10]. Fig.6. Capacitive impedance model of tower footing ground R 0 is resistance of tower footing ground at low current and frequency. C amount has large range of change (10-12 to 10-3 ) and has A significant Influence on the of back flashover on insulator chain. There is one uncertain parameter, which the ratio of R i and R, where R i is an initial resistance and R is given R 0 minus R i and in most of the cases the ratio is taken as 0.75[12]. III. SIMULATION AND RESULTS Simulation for different cases is done that each corresponding to a different area of the tower footing resistance. Fig.5. Transient model of line insulator string Mathematical model for insulator flashover represented in equation (9).[11]. t T 0 ( V ( t ) V ) dt D (9) gap 0 k Existing effect of the shield wire and line arrester on the voltage is studied across the middle phase insulator chain for different ground resistance and different lightning waveform. Due to greater heights the middle phase, it's more susceptible to lightning rather than other phase. According to line voltage level of 400KV, lightning basic insulation level(bil) is 1425KV approximately [13]. Obviously, a higher voltage rather than the aforementioned level increases possibility of flashover and the back flashover in across

4 Volume 6.1, January 2016 ISSN the insulators. Simulation figures and results for different cases are represented in figures 7 to 11 and tables 4 to 8 respectively. Fig.10. The voltage across the insulator chains in (Volt) with shield wire and with line arrester in all three phases for the lightning current amplitude 30 KA, 8/20µs waveform and the tower footing resistance 10Ω Fig.7. The voltage across the insulator chains in (Volt) without shield wire and line arrester for the lightning current amplitude 30 KA, 8/20µs waveform and the tower footing resistance 10Ω Fig.8. The voltage across the insulator chains in (Volt) with shield wire and without line arrester for the lightning current amplitude of 30 KA, 8/20µs waveform and the tower footing resistance 10Ω Fig.11. The voltage across the insulator chains in (Volt) with shield wire and line arrester in middle phases only for the lightning current amplitude 30 KA, 8/20µs waveform and the tower footing resistance 10Ω Table IV and ground resistance of 10Ω and for the lightning current amplitude 30 KA, 8/20µs waveform in different cases with considering flashover Fig.9. The voltage across the insulator chains in (Volt) without shield wire and with line arrester in all three phases for the lightning current amplitude 30 KA, 8/20µs waveform and the tower footing resistance 10Ω Case arrester without arrester The voltage across the insulator

5 Volume 6.1, January 2016 ISSN Table V with ground resistance 10Ω and for the lightning current amplitude 30 KA, 8/20µs waveform in different cases with considering back flashover Case arrester without arrester The voltage across the insulator Table VI expressed the effect of lightning waveform on the voltage across the insulator chain. Table VI with ground resistance 10Ω and for the lightning current amplitude 30 KA and different waveforms in (First stroke and Subsequent stroke) Waveform type 0.1/1µs 2/5 µs 4/10 µs 8/20 µs 10/350 µs The voltage across the insulator Table VII with ground resistance 10Ω and for the lightning with 8/20µs waveform in different current amplitude cases Lightning current amplitude(ka) The voltage across the insulator Table VIII with ground resistance 10Ω and for the lightning current amplitude 30 KA, 8/20µs waveform with considering back flashover for different ground resistance in different cases Case Without shield wire and arrester With shield wire and without arrester Without shield wire and with arrester(only in middle phase) With shield wire and arrester(only in middle phase) The voltage across the insulator 30Ω 50Ω 100Ω As it is observed, the greater lightning current amplitude increase the of insulator chain insulation failure. On the other hand, according to the Table VI, by reducing the slope of the front wave and increase time of the back wave, the output voltage across the insulator chain increases. Also, when the tower footing resistance value is low, it can reduce the voltage across the line insulators by installing the line arrester and in this condition that would be possible to remove the shield wire. Therefore, using line arrester can have a significant effect in reducing the lightning over voltage and thus increase the reliability of line. IV. CONCLUSION In this paper, the arrester usage in order to prevent flash over (direct and return) on the line and removing the shield wire was studied. The simulation results for different conditions showed that the arrester has a significant effect in reducing the risk of a flashover across the line insulators. As well as If the tower footing resistance is low, it can remove the shield wire and with installation surge arresters in the appropriate places, the reliability of the line against lightning maintain, and even increased. Hence, tower footing resistance has important contribution in reducing the line flashover and back flashover. Therefore, the tower footing resistance reduction is an effective way to reduce interruption caused by lightning and increase the reliability of the line. For areas that are impossible or costly tower footing resistance reduction, optimal installation of line arrester on line phases is recommended. Then, for towers that are located in areas with high soil resistance (rocky mountainous

6 Volume 6.1, January 2016 ISSN areas) or in areas with high lightning density, the first priority should be optimal installation of arrester. REFERENCES [1]. Chowdhuri, P. (2001). Parameters of lightning strokes and their effects on power systems. In Transmission and Distribution Conference and Exposition, 2001 IEEE/PES (Vol. 2, pp ). IEEE. [2]. Radhika, G., & Suryakalavathi, M. (2013, September). Back flashover analysis improvement of a 220 KV double circuit transmission line. In Communication and Computing (ARTCom 2013), Fifth International Conference on Advances in Recent Technologies in (pp ). IET. [3]. IEEE Working Group. (1985). A simplified method for estimating lightning performance of transmission lines. IEEE Trans. Power App. Syst, 104(4), [4]. Ishii, M., Kawamura, T., Kouno, T., Ohsaki, E., Shiokawa, K., Murotani, K., & Higuchi, T. (1991). Multistory transmission tower model for lightning surge analysis. Power Delivery, IEEE Transactions on, 6(3), [5]. Liu, X. (2012, April). The Lightning Current Measurement Based on Wavelet Transform. In Advanced Materials Research (Vol. 490, pp ). [6]. Swindler, D. L., Schwartz, P., Hamer, P. A. U. L., & Lambert, S. R. (1997). Transient recovery voltage considerations in the application of medium-voltage circuit breakers. Industry Applications, IEEE Transactions on, 33(2), [7]. bre, D. M., Neves, W. L. A., & Souza, B. A. (2001). An Alternative to Reduce Medium-Voltage Transient Recovery Voltage Peaks. In IPST International Conference on Power Systems Transients.. [8]. Pinceti, P., & Giannettoni, M. (1999). A simplified model for zinc oxide surge arresters. Power Delivery, IEEE Transactions on, 14(2), [9]. Fernandez, F., & Diaz, R. (2001, June). Metal oxide surge arrester model for fast transient simulations. In The Int. Conf. on Power System Transients IPAT(Vol. 1). [10]. IEC : Insulation co-ordination Computational guide to insulation co-ordination and model [11]. IEC : Insulation co-ordination Part 4: Computational guide to insulation co-ordination and modeling of electrical networks, 2004 [12]. Radhika, G., & KALAVATHI, M. S. (2010). LIGHTNING SURGE ANALYSIS ON GROUNDING MODELS OF A TRANSMISSION LINES. Journal of Theoretical & Applied Information Technology, 12. [13]. Probert, S. A., Song, Y. H., Basak, P. K., & Ferguson, C. P. (2003). Review of the basic insulation level for 400 kv oil filled cable systems: Switching and temporary overvoltages (TOV). European transactions on electrical power, 13(5),