CALCULATION OF LIGHTNING FLASHOVERS AND BACKFLASH LEVEL ON 230kV TRANSMISSION LINES Bander J. Al-Qahtani * SAOO-NGPD-TSU Saudi Aramco Abqaiq, Saudi Arabia Bander.qahtani@aramco.com ABSTRACT Lightning has been one of the important problems for insulation design of power systems and it is still the main cause of outages of transmission and distribution lines. Lightning caused outages can be reduced by lightning protection devices such as ground wires and lightning arresters. This paper presents a comparative studies used to determine the lightning backflashovers level on 230kV transmission lines utilized by Saudi Electric Company (SEC) in Saudi Arabia, using two well known approaches CIGRE, and the simplified method. The studies include lightning flashovers, backflash analysis, as dependent on the tower design parameters which is considered the main parameters that reduce the rate of lightning bachflashovers in the transmission lines. The study results can be applied to reduce the number lightning flashovers and therefore reduce the transmission lines outages. KEY WORDS Lightning flashovers, backflashovers, simulation and ground wires 1. Introduction A complete awareness of the parameters of lightning strokes is essential for the prediction of the severity of the transient voltages generated across power apparatus either by a direct stroke to the power line/apparatus, or by an P.O.Box 66467, Dammam 31576, Kingdom of Saudi Arabia M. H. Shwehdi Electrical Engineering Department King Fahd University of Petroleum & Minerals Dhahran, Saudi Arabia mshwehdi@kfupm.edu.sa indirect stroke. However, no two lightning strokes are the same. Therefore, the statistical variations of the lightningstroke parameters must be taken into account in assessing the severity of lightning strokes on the specific design of a power line or apparatus. The lightning return-stroke current and the charge delivered by the stroke are the most important parameters to assess the severity of lightning strokes to power lines and apparatus. The return-stroke current is characterized by a rapid rise to the peak, Ip, within a few microseconds and then a relatively slow decay, reaching half of the peak value in tens of microseconds. The return-stroke current is specified by its peak value and its waveshape. The waveshape, in turn, is specified by the time from zero to the peak value (tf, front time) and by the time to its subsequent decay to its half value (th, tail time). The tail time being several orders of magnitude longer than the front time, its statistical variation is of lesser importance in the computation of the generated voltage. The generated voltage is a function of the peak current for both the direct and indirect strokes. For backflashes in direct strokes and for indirect strokes the generated voltage is higher the shorter the front time of the return-stroke current [1]. The front time (and the tail time, to a lesser extent), influence the withstand capability (volt-time characteristics) of the power apparatus. The charge in a stroke signifies the energy transferred to the struck object. The ancillary equipment (e.g., surge protectors) connected near the struck point will be damaged if the charge content of the stroke 2 exceeds the withstand capability of the equipment. The return-stroke velocity will affect the component of the voltage which is generated by the induction field of the lightning stroke [1]. Field tests have shown that the parameters of the first stroke are different from that of the subsequent strokes. 2. Lightning Flashes the system. Thus, assuming the lightning channel to be a current source, the transient voltages across the insulator of a phase conductor are generated in three ways: (i) lightning striking the phase conductor (shielding failure), (ii) lightning striking the tower or the shield wire (backflash), and (iii) lightning striking the nearby ground (indirect stroke). The severity of these three types of transient voltages is influenced by different lightning parameters [2, 3]. Lightning damages a power apparatus in two ways: (i) it raises the voltage across an apparatus such that the terminals across the struck apparatus spark over causing a short circuit of the system or the voltage punctures through the apparatus electrical insulation, causing permanent damage. (ii) The energy of the lightning stroke may exceed the energy handling capability of the apparatus, causing meltdown or fracture. The significance of lightning parameters on power systems is gauged by the severity of the transient overvoltages they create and the consequent damages to the power system. As mentioned before, these overvoltages are generated by three different ways. A lightning flash generally consists of several strokes which lower charges, negative or positive, from the cloud to the ground. The first stroke is most often more severe than the subsequent strokes. Low current continues to flow between two strokes, thus increasing the total energy injected to the struck object. The transient voltage from the lightning strike is generated by: (i) direct stroke and (ii) indirect stroke. For direct strike, it can strike an apparatus. In that case, the apparatus will be permanently damaged. Most often, lightning strikes the phase conductor of the power line. In that case, a traveling voltage wave is generated on the line; it travels along the line and is impressed across the terminals of an apparatus or most often the insulator between the phase conductor and the cross-arm of the tower at the end of the span. If the voltage is high enough, the insulator flashes over causing a short circuit of the system. The lightning return-stroke current is the most significant parameter in the estimation of the response of electrical apparatus and systems to lightning strikes. The return stroke current rises to its peak in a few microseconds and then decays to the half value in a few tens of microseconds [4]. The return-stroke current is identified by three parameters: peak value Ip, front time tf and time to half value th. The difficulty with the exponential function representing a return-stroke current is that it is not easy to select the parameters of these analytical expressions to fit the three parameters (Ip, tf and th). However, this problem does not arise if the return-stroke current is represented as linearly rising and linearly falling functions [4]: Many overhead power lines are equipped with shield wires to shield the phase conductors. Even then, shielding failures occur when lightning bypasses the shield wires and strikes a phase conductor. When lightning strikes a tower, a traveling voltage is generated which travels back and forth along the tower, being reflected at the tower footing and at the tower top, thus raising the voltages at the cross-arms and stressing the insulators. The insulator will flash over if this transient voltage exceeds its withstand level (backflash). Even if lightning strikes a shield wire, the generated traveling voltage wave will travel to the nearest tower, produce multiple reflections along the tower, causing backflash across an insulator. When lightning hits the ground several hundred meters away from the line (indirect stroke), the electric and magnetic fields of the lightning channel can induce high voltage on the line for the insulators of the low-voltage distribution lines to spark over causing a short circuit of 3. Computation of Insulator Voltage I (t ) 1tu(t ) 2 (t t f )u (t t f ) (1) Where α1 = Ip/tf, and α2 = (2th–If)Ip/2tf (th–If). For short tf in the order of a few microseconds, eqn. 1 seems to work very well. With eqn. 1, the three parameters of the returnstroke current can be varied very easily. Starting with the return-stroke current, the various voltage components across the insulator were computed. 3.1 First and Second Voltage Components To compute the first voltage component, i.e. the crossarm voltage Vca, the tower was assumed to be a vertical transmission line of a fixed surge impedance Zt. The voltage and current waves were assumed to travel along the tower with a constant p.u. velocity of βt. The first reflections from the adjacent towers for the shield-wire voltages were also included in the computation. The tower footing resistance was assumed to be constant, Rtf. The tower-top was terminated by shield wire(s) and the 3 lightning channel of constant surge impedance Zch. The cross-arm voltage due to the multiply reflected voltage waves along the struck tower was computed by following a previous method as shown in [4]. Although Zt, βt, Rtf and Zch were assumed as constant, they were used as input variables which could be changed for parametric analysis. The second voltage component is the voltage induced on the phase conductor due to electromagnetic coupling with the shield wire. This voltage is equal to kcf Vt, where Vt is the tower-top/shield-wire voltage and the coupling factor kcf is equal to Zps/Zsh. Zps is the mutual surge impedance between the phase conductor and the shield wire; Zsh is the shield-wire surge impedance [4]. The tower-top voltage was computed following the same procedure as for Vca. The insulator-string voltage due to the first and second voltage components is: Vins Vca kcf Vt (2) 3.2 Third Voltage Component The third voltage component is the voltage induced on the phase conductor due to the electromagnetic fields of the lightning channel. The computation of the phaseconductor voltage followed previous analysis [4], with the difference that, in the present case, the stroke hits the tower top instead of the ground. This difference is manifested in the inducing voltage Vi, which is the voltage in space (in the absence of the phase conductor) caused by the residual charge in the upper part and the return-stroke current in the lower part of the lightning channel. Vi is: increases as a function of time and the return-stroke velocity, with its lower and upper limits 0 and hc. For a stroke to tower of height hc, the lower and the upper limits of z ׳are ht and hc. Thus, for a stroke to tower, the voltages induced on the phase conductor were computed for two different cloud heights (hc and ht), and then the second induced voltage (for ht) was subtracted from the first induced voltage (for hc). 4. Computation of Backflash Rate The overhead ground wires or shield wires have been located so as to minimize the number of lightning strokes that terminate on the phase conductor. The remaining and vast majority of strokes and flashes now terminate on the overhead ground wires. A stroke that so terminate forces current to flow down the tower and out on the ground wires. Thus voltage are built up across the line insulation. If these voltages equal or exceed the line CFO, flashover occurs. This event is called a backflash. By referring to figure 1, equations for the crest voltage, the voltage at the tower top prior to any reflections from the footing resistance, and the final voltage can be derived as follows hp A (3) ) dz t 0 Where Φ is the scalar potential due to the residual charge in the upper part of the lightning channel, and A is the vector potential due to the return-stroke current in the lower part of the channel. For stroke to ground, Φ and A are Vi ( r r hc q ( r , t ) 0 1 c (r , t ) dr 40 z r r A( r , t ) 0 4 z 0 r r ) c dr r r I ( r , t (4) Figure 1. Surge voltages at the tower and across the insulation [5] VTT K sp K TT I VTA K sp K TA I (4.1) VF Re I And the current through the footing resistance is (4.2) Re (5) where r and r ׳are field and source points, respectively: I is the return-stroke current: q0 is the constant linear charge density of the leader stroke: hc is the cloud height: and is the instantaneous height of the upward-moving head of the return stroke above ground. For a stroke to ground, z׳ IR Where Ri I 4 KTT Re T ZT TT tf IC T KTA Re T ZT A tf (4.3) For these equations: (4.9) Since KTT is in many cases approximately equal to KTA, then approximately, CFO (4.10) I C T T T 2 K SP 1 R 1 T 1 2 S R T 1 4 S R T 1 6 S ..... t t t f f f CFO KTA CKTT K SP 1 C KTT K SP The probability of a flashover is the probability that the stroke current I equals or exceeds the critical current IC, or Pr obI I C PI C f I dI (4.11) IC Re T Z g Ri Z g 2 Ri R Z g 2 Ri Z Ri T ZT Ri Z g 2 Ri Zg (4.4) Z g 2 Ri Also, the tail of the voltages can be conservatively approximated by a time constant τ: Zg Ri (4.5) TS That is, the equation for the tail of the surge is eTT VF e (t t f ) / (4.6) To be complete the definition of the variables are: tf = time to crest of the stroke current, μs C = coupling factor ZT = surge impedance of the tower, ohms Zg = surge impedance of the ground wires, ohms TT = tower travel time, μs TA = tower travel time to any location on the tower A, μs TS = travel time of a span, μs I = stroke current, KA IR = current through footing of struck tower, KA Ro = measured or low-current footing resistance, ohms Ri = impulse or high-current footing resistance, ohms = time constant of tail, μs Now, to provide first estmate of the backflash rate, the BFR, examine figure 6. The surge voltage on the ground wires produces a surge voltage on the phase conductor equal to the coupling factor C times the voltage on the ground wires, or CVTT. Also note that the voltage VTA is located on the tower opposite the phase conductor. Therefore, the crest voltage across the insulation V1 is V1 I KTA CKTT K SP (4.7) Also, note that the crest voltage VIF across the insulation caused by the footing resistance is VIF 1 C Re I (4.8) For a flashover to occur, the voltage across the insulator V1, must be equal to or greater than the CFO of the insulation. Replacing V1 of Eq. (7) with CFO, the current obtained is the critical current IC at and above which flashover occur, i.e., The backflash rate BFR is this probability times the number of strokes, NL, that terminate on the ground wires, or BFR= N L PI C (4.12) Where NL Ng 28h 0.6 Sg (4.13) 10 Where h is the tower height (meters), Sg is the horizontal distance between the ground wires (meters), and Ng is the ground flash density (flashes/km2-year), thus the BFR is in terms of flashovers per 100 km-years. The equations for KTT and KI show that the voltage across the insulation increases as the time to crest of the stroke current decreases. This is caused by the tower component of voltage. Thus the critical current increases as the time to crest increases. Therefore, theoretically, all fronts should be considered. To do this, the equation for BFR should be changed to the following: BFR=0.6 NL P(IC) (4.14) 5. Simulation & Results The 230 kV HV line of figure 2 whose characteristics are given in table I, are used to calculate the backflash rate using different methods. Also, this case study will include the following 1. The effect of decrease of resistance from Ro versus Ri 2. One versus two shield wires 3. The effect of underbuilt shield or ground wire As shown in the figure 3 & 4 the backflash rate for the above mentioned high voltage lines with span length of 300 meters and CFO of 1200kV has been calculated by using CIGRE method software and simplified method. The comparison appears acceptable for the line with tower height of 35 meters, but for tower height of 70 meters the simplified method is inadequate. So, the CIGRE method is always the proper tool. 5 10 BFR, Flashovers/100 km-yrs Using the CIGRE method, the BFR of the single circuit 230 kV is shown in Fig. 5 as a function of RO with the ratio ρ/RO as a parameter. To illustrate the effect of the decrease of resistance with current, a curve labeled Ri=RO for which the footing resistance is not decreased is also presented. 9 8 7 CIGRE Method 6 5 Simplified Method 4 3 2 1 0 1 2 3 4 5 6 X10 Ro, ohms Figure 3. Comparasion of BFRs for CIGRE method and simplified method, 230kV double circuit towers with two ground wires and height of 35 meters BFR, Flashovers/100km-yrs 12 10 8 Simplified Method 6 CIGRE Method 4 2 0 1 2 3 4 5 6 X10 Ro, ohms A ground wire located below the phase conductors cannot truthfully be called a shield wire, since it has no shielding function. Rather, its function is to increase the coupling factor to the lower phases, those phases that are most likely to flashover. For example, for the 230-kV doublecircuit, two-ground-wire line with a shield wire height of 35 meters and coupling factor to the top, middle, and bottom phase of 0.350, 0.248, and 0.183, respectively, installing a ground wire at 12 meters above ground at the center of the tower increases these coupling factors to 0.441, 0.347, and 0.307, respectively. Thus all coupling factors are increased and are more uniform. Figure 7 shows the dramatic decrease in BFR for this case. 12 BFR, Flashovers/100km-yrs For some applications, where the cost of two shield wires is not economically and technically justified, or where there is low ground flash density, a single shield wire can be used. The single wire increases the value of Re, decreases the coupling factor, and thus increase the BFR. To illustrate, the curves of Fig. 6 have been constructed to compare one and two shield wires for a 230 kV doublecircuit line and two shield wires for a single-circuit 230 kV line. Using one shield wire on the double-circuit line essentially doubles the BFR as compared to the twoshield-wire case. Figure 4. Comparasion of BFRs for CIGRE method and simplified method, 230kV double circuit towers with two ground wires and height of 70 meters 10 8 p/Ro=40 6 p/Ro=20 p/Ro=10 4 2 0 1 2 3 4 5 6 7 8 X10 Ro, ohms Figure 5. Effect of decrease to high-current footing resistance 9 BFR, Flashovers/100km-yrs Figure 2. 230 kV Tower Dimensions 8 7 6 5 2 Grd Wire 4 1 Grd Wire 3 2 1 0 1 2 3 4 5 6 X10, ohms Figure 6. Tow shield wires for the 230kV double circuit line with height of 35 m decrease the BFR, p/Ro=20 6 BFR, Flashovers/100km-yrs 10 9 8 7 2 Grd Wires 6 5 2 Grd Wires+under built grd wire 4 3 2 [1] P. Chowdhuri, J. G. Anderson, W. A. Chisholm, T. E. Field, M. Ishii, J. A. Martinez, M. B. Marz, J. McDaniel, T. R. McDermott, A. M. Mousa,T. Narita, D. K. Nichols, & T. A. Short, Parameters of Lightning Strokes: A Review, IEEE Transactions and Power delivery, March 28, 2003. 1 0 1 2 3 4 5 6 7 8 X10 Ro, ohms Figure 7. An underbuilt ground wire decreases the BFR, 230kV double circuit line with height of 35 m, p/Ro=20. 6. Conclusion The most significant parameters of the lightning return stroke to estimate the severity on the power system are: (i) peak current, (ii) current front time, (iii) velocity and (iv) total charge of the flash. The electromagnetic fields of the lightning channel and the magnetic fields of the traveling current waves along the power-line tower will significantly affect the insulator-string voltage, and hence the outage rate due to backflash. Analytical methods to estimate the backflash outage rate have been proposed, which should result in better prediction of the lightning performance of overhead power lines. In this report, equations were developed to estimate the BFR that include the tower component of voltage; their use is called CIGRE method. This method is suffiently complex so that the use of the computer program is suggested. The effect of decrease of the concentrated grounds value on the BFR was addressed. Also, the effect of the number of shield wires as well as adding underbuilt shield or ground wire were highlighted. The 230 kV line design from SEC is considered very highly engineered, using two ground shield wires with 7.3 meter span at each side made almost a full cover for both circuits. This tower can be considered as lightning proof. 7. Acknowledgment The authors express appreciation to Saudi Electric Company engineers for thier time and support also their gratitude to KFUPM for educational, studies facilities and support. References: [2] P. Chowdhuri, A.K. Mishra & B.W. McConnell, Volt-time characteristics of short air gaps under nonstandard lightning voltage waves, ibid., Vol. 12, No. 1, pp. 470-476, 1997. [3] P. Chowdhuri, A.K., Parameters of Lighting Strokes and Their effect on Power Systems, Vol. 12, No. 1, pp. 1047-1051, 2001 [4] P. Chowdhuri, A.K., S. Li & P. Yan Rigorous analysis of back-flashover outages caused by direct lightning strokes to overhead power lines, IEEE Proceedings, 2002 [5] Andrew R. Hileman, Insulation Coordination for Power Systems, (Eastern Hemisphere Distribution, New York, 1999) [6] R. Thottappillil & M. A. Uman, Comparison of lightning return stroke models, J. Geophys. Res., vol. 98, pp. 22 903–22 914, 1993. [7] V. Cooray & R. E. Orville, The effect of the variation of current amplitude, current rise time and return stroke velocity along the return stroke channel on the electromagnetic fields generated by the return stroke, J. Geophys. Res., vol. 95, pp. 18 617–18 630, 1990. [8] Dennis W. Lenk, F. Richard Stockum & David E. Grimes,A new approach to distribution arrester design, IEEE Transactions on power delivery, vol. 3, No. 2, April 1988. [9] P. Pinceti & M. Giannettoni, A simplified model for zinc oxide surge arrester, IEEE Transactions on power delivery, Vol. 14, No. 2, April 1999. Biographies Bander J. Qahtani; Born in Al-Khobar 1979. He obtained his B.Sc. degree in electrical engineering with honors from King Fahd University of Petroleum & Minerals (KFUPM) in 2002. In the year 2000 he was selected as distinguished student for Saudi Aramco Scholarship program. During his studies at KFUPM he has conducted several term projects and studies dealing with Industrial power systems. Upon graduation, Bandar was employed by Aramco as instrument engineer with Southern Area Producing Engineering Department (SAPED) in Abqaiq. He is enrolled in the Msc. Program at KFUPM. Bandar has 7 published and presented many technical papers and reports to region, and international conferences. includes, power system analysis, Power Quality & Harmonics, overvoltages analysis on Power Systems, Transmission and Distribution Systems. Dr. Shwehdi is active in IEEE activities. He is listed as a distinguished lecturer with the DLP of the IEEE/PES DLP upon the Board selection, was named and awarded the 2001 IEEE/PES outstanding chapter engineer,. He was named and awarded the 1999 IEEE WG for standard award, the GCC-CIGRE 1998 best applied research award, IEEE/IAS Outstanding Supervisor for Student Research 1989, 1990, and the IEEE outstanding student advisor in 1990. M. H. Shwehdi (S'74, M'85, SM 90) received the B. SC. degree from University of Tripoli, Libya in 1972. He obtained the M. Sc. Degree from the University of Southern California and Ph.D. degree from Mississippi State University in 1975 and 1985 respectively all in electrical engineering. He was a consultant to A.B. Chance Company, and Flood Engineering. Dr. Shwehdi held teaching positions with the University of MissouriColumbia, Texas A & I University, University of Florida and Penn. State University from 1991-1993. At present he is associate professor with the King Fahd University of Petroleum & Minerals (KFUPM), Saudi Arabia. His research interest Table I Characteristics of Lines, Distances in meters System Voltage h yA yB yC Sg Sa Sb Zg ZT CA CB CC 230 35 29 24 18 5 8 11 379 190 .35 .25 .18 a230 35 29 24 18 0 8 11 600 190 .22 .16 .12 230 70 64 59 53 5 8 11 421 210 .42 .34 .28 35 29 24 18 5 8 11 239 190 .44 .35 .31 b230 a b Single ground wire. Underbuilt ground wire at h=12 m at center of tower