Computation of Lightning Impulse Backflashover Outages Rates on High Voltage Transmission Lines

Transcription

1 International Journal of Automation and Power Engineering (IJAPE) Volume Issue, January DOI:./ijape... omputation of Lightning Impulse Backflashover Outages Rates on High Voltage Transmission Lines M. H. Shwehdi *, S. Raja Mohammad * Electrical Engineering Department/ King Faisal University Hofuf Saudi Arabia * mshwehdi@kfu.edu.sa; rsumsudeen@kfu.edu.sa Abstract Lightning impulse 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. 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. This paper presents the effects of many tower, ground parameters on the backfloshover voltage level. Also demonstrate the results used to determine the lightning backflashovers level on KV transmission lines utilized by Saudi Electric ompany (SE) in Saudi Arabia, using two well known approaches IGRE, 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 Keywords Lightning Flashovers; Backflash Analysis; Lightning Arresters Introduction 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 wave shape, 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. 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 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. Field tests have shown that the parameters of the first stroke are different from that of the subsequent strokes. Lightning Flashes 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. 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

2 International Journal of Automation and Power Engineering (IJAPE) Volume Issue, January 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 crossarm 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. 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 it s 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 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. The significance of lightning parameters on power systems is gauged by the severity of the transient overvoltage s they create and the consequent damages to the power system. As mentioned before, these overvoltages are generated by three different ways. omputation 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 forces current to flow down the tower and out on the ground wires. Thus voltage is built up across the line insulation. If these voltages equal or exceed the line FO, flashover occurs. This event is called a backflash. By referring to figure, 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: VTT KspKTT I VTA KspKTA I (.) V R I F e FIG. SURGE VOLTAGES AT THE TOWER AND AROSS THE INSULATION And the current through the footing resistance is Re IR I (.) Ri Where TT KTT Re T ZT t f TA KTA Re T ZT t f (.) KSP R T T S T S T S R T R T... t f t f t f For these equations: Z gri Z T R Z g R i i Re T Ri ZT Ri Ri R (.) Ri Also, the tail of the voltages can be conservatively approximated by a time constant τ: T S (.) Ri That is, the equation for the tail of the surge is ( tt f )/ ett VF e (.) To be complete the definition of the variables are: tf= time to crest of the stroke current, μs

3 International Journal of Automation and Power Engineering (IJAPE) Volume Issue, January = coupling factor ZT= surge impedance of the tower, ohms = 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 estimate of the backflash rate, the BFR, examine figure. The surge voltage on the ground wires produces a surge voltage on the phase conductor equal to the coupling factor times the voltage on the ground wires, or VTT. Also note that the voltage VTA is located on the tower opposite the phase conductor. Therefore, the crest voltage across the insulation V is V IKTA KTT KSP (.) Also, note that the crest voltage VIF across the insulation caused by the footing resistance is VIF Re I (.) For a flashover to occur, the voltage across the insulator V, must be equal to or greater than the FO of the insulation. Replacing V of Eq. (.) with FO, the current obtained is the critical current I at and above which flashover occurs, i.e., FO I (.) KTA KTT KSP Since KTT is in many cases approximately equal to KTA, then approximately, FO I (.) KTT KSP The probability of a flashover is the probability that the stroke current I equals or exceeds the critical current I, or Pr ob I I P I f I di (.) The backflash rate BFR is this probability times the number of strokes, NL, that terminate on the ground wires, or BFR= P I (.) L N Where. h Sg NL Ng (.) Where h is the tower height (meters), Sg is the I horizontal distance between the ground wires (meters), and Ng is the ground flash density (flashes/km year), thus the BFR is in terms of flashovers per 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=. NL P(I) (.) alculations & Results The KV HV line of figure whose characteristics are given in table, are used to calculate the backflash rate using different methods. Also, this case study will include the following.. The effect of decrease of resistance from Ro versus Ri.. One versus two shield wires. The effect of under built shield or ground wire FIG. KV TOWER DIMENSIONS As shown in the figure & the backflash rate for the above mentioned high voltage lines with span length of meters and FO of KV has been calculated by using IGRE method software and simplified method. The comparison appears acceptable for the line with tower height of meters, but for tower height of meters the simplified method is inadequate. So, the IGRE method is always the proper tool. BFR, Flashovers/ km-yrs X Ro, ohms IGRE Simplified FIG. OMPARISON OF BFR S FOR IGRE METHOD AND SIMPLIFIED METHOD, KV DOUBLE IRUIT TOWERS WITH TWO GROUND WIRES AND HEIGHT OF METERS

4 International Journal of Automation and Power Engineering (IJAPE) Volume Issue, January BFR, Flashovers/km-yrs X Ro, ohms Simplified IGRE FIG. OMPARISON OF BFR S FOR IGRE METHOD AND SIMPLIFIED METHOD, KV DOUBLE IRUIT TOWERS WITH TWO GROUND WIRES AND HEIGHT OF METERS Using the IGRE method, the BFR of the single circuit KV is shown in Fig. 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. BFR, Flashovers/km-yrs X Ro, ohms p/ro= p/ro= p/ro= FIG. EFFET OF DEREASE TO HIGH URRENT FOOTING RESISTANE 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. have been constructed to compare one and two shield wires for a KV double circuit line and two shield wires for a single circuit KV line. Using one shield wire on the double circuit line essentially doubles the BFR as compared to the two shield wire case. BFR, Flashovers/km-yrs X, ohms Grd Wire Grd Wire FIG. TWO SHIELD WIRES FOR THE KV DOUBLE IRUIT LINE WITH HEIGHT OF M DEREASE THE BFR, P/RO= 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 KV double circuit, two ground wire line with a shield wire height of meters and coupling factor to the top, middle, and bottom phase of.,., and., respectively, installing a ground wire at meters above ground at the center of the tower increases these coupling factors to.,., and., respectively. Thus all coupling factors are increased and are more uniform. Figure shows the dramatic decrease in BFR for this case. BFR, Flashovers/km-yrs X Ro, ohms Grd Wires Grd Wires+under built grd wire FIG. AN UNDERBUILT GROUND WIRE DEREASES THE BFR, KV DOUBLE IRUIT LINE WITH HEIGHT OF M, P/RO= onclusion 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. insulator string voltage, and hence the outage rate due to backflash. 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 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. Equations were developed to estimate the BFR that include the tower component of voltage; using the IGRE method. This method is sufficiently 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 under built shield or ground wire were highlighted. The KV line design from utility is considered very highly engineered, using two ground shield wires with. meter span at each side made almost a full cover for both circuits. This tower can be considered as lightning proof.

5 International Journal of Automation and Power Engineering (IJAPE) Volume Issue, January AKNOWLEDGMENT The authors express appreciation to The Deanship of Research of King Faisal University for continued facilities and financial support. REFERENES Andrew R. Hileman, Insulation oordination for Power Systems, Eastern Hemisphere Distribution, New York,. Dennis W. Lenk, F. Richard Stockum & David E. Grimes, A new approach to distribution arrester design, IEEE Transactions on power delivery, vol., No., April. P. howdhuri, A.K. Mishra and B.W. Mconnell, ʺVolt time characteristics of short air gaps under nonstandard lightning voltage wavesʺ, ibid., Vol., No., pp.,. P. howdhuri, A.K., ʺParameters of Lighting Strokes and Their effect on Power Systemsʺ, Vol., No., pp.,. P. howdhuri, A.K., S. Li and P. Yan ʺRigorous analysis of back flashover outages caused by direct lightning strokes to overhead power linesʺ, IEEE Proceedings,. P. howdhuri, J. G. Anderson, W. A. hisholm, T. E. Field, M. Ishii, J. A. Martinez, M. B. Marz, J. McDaniel, T. R. McDermott, A. M. Mousa,T. Narita, D. K. Nichols, and T. A. Short, Parameters of Lightning Strokes: A Review, IEEE Transactions and Power delivery, March,. R. Thottappillil and M. A. Uman, omparison of lightning return stroke models, J. Geophys. Res., vol., pp.,. V. ooray and 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., pp.,.