LIGHTNING PROTECTION OF MEDIUM VOLTAGE OVERHEAD LINES WITH COVERED CONDUCTORS BY ANTENNA-TYPE LONG FLASHOVER ARRESTERS

Transcription

1 C I R E D 17 th Internationa Conference on Eectricity Distribution Barceona, May 23 LIGHTNING PROTECTION OF MEDIUM VOLTAGE OVERHEAD LINES WITH COVERED CONDUCTORS BY ANTENNA-TYPE LONG FLASHOVER ARRESTERS Georgij PODPORKIN, Vadimir PILSHIKOV, Aeksander SIVAEV, Michai YARMARKIN Streamer Eectric Company, Russia georgij.podporkin@streamer.ru INTRODUCTION covered A new, so caed, antenna type of ong fashover arrester (ALFA) is presented in this report. With the use of ALFA it is possibe to protect MV ines with covered conductors in a very simpe way using the covered conductor with its insuation as ong fashover arrester. An antenna is connected to an eectrode instaed over the surface of the covered conductor. When ightning channe approaches to the overhead ine high vaue potentia is induced at the antenna. Under this induced votage creeping discharge deveops into both directions from the eectrode thus bridging the gap between cut-through camp and the insuator. LONG FLASHOVER ARRESTER PRINCIPLE Uness specia ightning protection steps are taken to safeguard medium votage overhead power ines with covered conductors, a ightning overvotage eads first to a fashover of a ine insuator and next to a breakdown of the soid conductor insuation. With a high probabiity such a ightning fashover brings about a power frequency arc which keeps burning at the insuation breakdown point unti the ine is disconnected. The arc can easiy burn the insuating covering and, with heavy faut currents, met the conductor. MV ines can be effectivey protected against both ightning overvotages and conductor fusion by ong fashover arresters (LFA) whose ength is much greater than that of the insuator which it protects [1-3]. Due to a specia inner structure of the LFA, its impuse fashover votage is ess than that of the insuator which it protects ; so when subjected to ightning overvotage the LFA gets fashed over whie the insuator withstands. Advantages of the LFA are a simpe construction and thus a ow cost and a good reiabiity because ightning discharges deveop aong the LFA without causing a power arc foow (PAF). It appears highy promising to make the covered conductor itsef perform functions of a ong fashover arrester (Fig. 1) [1, 2, 4]. To this end, a cut-through camp is to be mounted at a definite distance from the end of the conductor s binding; this camp maintains contact with the core of the conductor. A ightning overvotage between the conductor and the poe first fashes over the ine insuator and next causes a creeping discharge, which deveops on the surface of the Fig. 1. Lightning protection diagram of overhead ine with covered conductors 1 thunderstorm coud; 2 ightning channe; 3 space charge; 4 poe; 5 insuator; 6 insuator fashover channe; 7 binding; 8 insuating cover; 9 conductor s core; 1 surface fashover channe; 11 cutthrough camp. conductor starting from the end of its binding. On reaching the cut-through camp, the creeping discharge connects the overhead ine conductor to the ine poe via a fairy ong discharge channe formed by the conductor and the insuator fashover paths. Because of a arge tota ength of the discharge channe thus formed, the ightning fashover does not give rise to an AC power arc, which assures uninterrupted operation of the overhead ine. However, the breakdown strength of a covered conductor s insuation is reativey ow, ranging from 15 to 2 kv according to the thickness and materia of the insuation and the insuator and binding construction [5]. Besides, as an insuator gets fashed over votage is appied to the insuation of a covered conductor abrupty, with a high rate of rise. This prevents the creeping discharge from reaching the cut-through camp and shunting the conductor insuation, which eads to a breakdown of the insuation. Because induced overvotages can be as heavy as 3 kv the insuation of covered conductors can be broken down, particuary with direct ightning strokes on ine conductors resuting in much heavier overvotages. ANTENNA-TYPE LFA PRINCIPLE A covered conductor with a reativey weak insuation can be used to buid a ightning protection system provided the votage appied to the insuation rises smoothy, permitting the creeping discharge to reach the cut-through camp and to shunt the conductor insuation before the votage rises to the breakdown eve. The idea of the antenna-type ong fashover arrester (ALFA) is to use an antenna connected STR_Podporkin_A1 Session 2 Paper No

2 C I R E D 17 th Internationa Conference on Eectricity Distribution Barceona, May 23 to an arrester for causing its fashover even before the ightning eader comes in direct contact with the power ine (see Fig. 2). Whie the ightning eader sti moves from the thunderstorm coud to the overhead ine a high potentia is induced on the arrester s antenna. The antenna is connected to an eectrode on the surface of the covered conductor. A difference of potentias between the eectrode and the grounded core of the conductor causes formation of a creeping discharge deveoping both ways from the eectrode. Even before the ightning eader strikes the ine such a creeping discharge fashes over the surface of the covered conductor, shunting the covered conductor s insuation by the creeping discharge channe and thereby protecting it from breakdown. A ightning stroke on a ine conductor or cose to the ine causes a overvotage both on the conductor and the binding, which gets connected to the conductor via the discharge channe. As the overvotage reaches the insuator fashover eve the insuator gets fashed over making the ightning overvotage current fow from the conductor via the cutthrough camp down a ightning fashover channe aong the conductor, as we as down a ightning fashover channe over the insuator (not shown in Fig. 2, see Fig. 1) and on to the ground down the body of the conducting poe. The ightning overvotage current is foowed by the AC foow current fowing down the fashover channe. The arc gets extinguished when the foow current crosses the zero, and the power ine continues it uninterrupted operation without an outage. For items 1 to 11, see Fig. 1; 12 eectrode; 13 antenna. distance from certain the ends of the meta binding of the conductor. In the midde of the conductor sections between the cut-through camp and the end of the cose-by binding, the eectrodes are mounted on the surface of the conductor insuation. Antennas are stretched between each eectrode and the tip of the stand and secured to the stand via composite tension insuators. As the ightning channe approaches the ine a high potentia gets induced on an antenna and thus on its phase conductor eectrode. A high votage taking rise between the eectrode and the conductor core causes a fashover of the covered conductor by a creeping discharge. A fina ightning stroke on the poe body or on one of the conductors resuts in a fashover of a the three ine insuators (Fig. 3a) and thus in a three-phase short circuit with fairy ong fashover channes. The tota ength L of a fashover channe between two phases is found from the formua: where: L = +h+h+ (1) EMBODIMENT OF LIGHTNING PROTECTION SYSTEM Shown in Fig. 3 is an aternative embodiment of such a ightning protection system for a medium votage (MV) overhead ine with covered conductors. The ine poe is fitted with a stee stand, such as a piece of pipe. Cutthrough camps are mounted on each phase conductor of the ine at a a) b) Fig. 2. Iustration of the operating principe of ALFA: Fig. 3. Protection of MV overhead ine by ALFA. a) Eectric schematic; b) Line mock -up testing STR_Podporkin_A1 Session 2 Paper No

3 C I R E D 17 th Internationa Conference on Eectricity Distribution Barceona, May 23 For items 1 to 13 see Figs. 1 and 2; 14 composite insuator; 15 stee pipe. = 1 + 2, the fashover ength over the covered conductor s surface; 1, the fashover ength from the antenna eectrode to the binding; 2, the fashover ength from the antenna eectrode to the cut-through camp; h, the fashover ength over the insuator surface. The foow current is a two-phase short circuit current. It can be as heavy as 1 ka. For such current a critica eectric fied strength, which prevents formation of a power arc is E cr = 4 kv/m [3]. The tota fashover ength necessary to rue out a power arc is found from the equation (2) L = U /. E cr E.g., the fashover ength for 12 kv ines is L = 12/4 = 3 m and for 6.6 kv ones, L = 6.6/4 = 1.65 m. The fashover ength over the surface of a covered conductor is found from (1) as foows: =(L-2h)/2, (3) making = 1.3 m for 12 kv ines (at h =.18 m) and =.7 m for 6.6 kv ines (at h =.15 m). With the antenna eectrode instaed about midway between the binding end and the cut-through camp, m and m for 12 and 6.6 kv ines, respectivey. To start deveopment of creeping discharges on the surface of a.35 to.75 m ong covered conductor, an impuse votage of a 6 to 8 kv crest vaue is sufficient [2,6]. CALCULATION OF ANTENNA VOLTAGE INDUCED BY APPROACHING LIGHTNING LEADER The cacuation was performed for the embodiment aternative shown in Fig. 3. The induced votage is directy proportiona to the inear charge aong the approaching ightning channe. In its turn, the inear charge can be approximatey reated to the ightning current with the hep of reationship [7] as foows: I τ = 1, (4) 1.56 where I is ightning current, ka; τ, inear charge, µc/m. It is thus cear that the heavier is the ightning current, the higher is the induced votage, i. e. the higher is a ightning above the ine, the earier wi the ALFA be triggered. By contrast, the ess heavy is the ightning current, the ower is the induced votage. According to CIGRE (K. Berger), 95% of ightnings feature a current in excess of 5 ka [8]. A ightning stroke with a current of this magnitude on a conductor causes an overvotage of about 8-9 kv, which exceeds consideraby the insuation eve of a medium votage (MV) power ine. Thus a direct ightning stroke on such a ine inevitaby resuts in an insuation faiure even at a minimum magnitude of current. This is why the efficiency of ALFA shoud be checked at minimum ightning current magnitudes. If the arrester performs we at a minimum ightning current, it wi be even more efficient at any heavier ightning current. With a this taken into account, cacuations were made for the minimum magnitude of ightning current I = 5 ka. Using the equivaent charge method, the ightning channe was presented as a 5 m ong vertica cyindrica conductor. In accordance with (4), the inear charge was taken to be 18 µc/m, i. e. the tota charge of the channe-simuating cyinder was.9 C. The radius of the channe-simuating cyinder was found from the formua r τ 2πε E = = 12 6 str m, (5) where E str =.8 MV/m is the eectric fied strength in the streamer zone of the negative-poarity ightning channe. The tip of the ightning channe was simuated by an 8 m ong cyinder of a 4 m radius. The tota charge of the tip was assumed to be, according to the recommendation of [7], q = 9 τr = = 648 C (6) t µ The charge of the thunderstorm coud was not taken into account. The cyinder was assumed to go down at a constant speed v =3 1 5 m/s=.3 m/µs, the eevation of its ower end above the ground, further referred to as the ightning height, varying from 3 m to 15 m. The cacuation showed the channe potentia to vary inconsideraby, from 2.3 MV to 19.7 MV, in other words, to stay within around U 2 MV. As the ightning channe approaches the ground, partia capacitance C a between the channe and the antenna increases (see Fig. 4). Capacitance to the ground of the antenna and its eectrode on the conductor surface incudes aso capacitance of the eectrode about the earthed conductor core. As a creeping discharge takes rise and deveops the discharge channe together with streamers gets couped to the antenna-eectrode system forming an antenna-eectrode-streamers system with an even arger capacitance; this joint antenna-ground capacitance C ag grows with the progress and deveopment of the creeping discharge channes. Connected in parae with C ag is a ground eakage resistance R whose vaue is function of the resistance of two wet pouted parts of the conductor 1 and 2 and that of the ine insuators and the antenna (Fig. 2). The equivaent circuit diagram used to cacuate the antenna potentia vs. time is shown in Fig. 4. The inputs STR_Podporkin_A1 Session 2 Paper No

4 C I R E D 17 th Internationa Conference on Eectricity Distribution Barceona, May 23 for cacuation with the hep of the circuit diagram shown in Fig. 4 are the vaues of its components C a, C ag and R. The equivaent EMF can be assumed to be equa to the potentia of the ightning channe U 2 MV. Fig. 4. Equivaent circuit diagram used to cacuate the antenna potentia vs. time The vaues of partia capacitance between the ightning channe and the antenna C a and between the antenna and the ground C ag (with aowance for the eectrode and creeping discharge streamers) are cacuated using the equivaent charge procedure. The cacuation was made for two cases: 1) ightning stroke on the poe, with the ightning channe assumed to descend verticay in ine with the poe; 2) ightning stroke on the conductor in the mid-span, with the ightning channe assumed to be removed 35 m from the poe. Partia capacitance between the ightning channe and the antenna C a was found for a standard 1 m high 12 kv poe at ightning heights H varying from 3 to 15 m and streamer engths str varying from to 1. m. Capacitance C a was shown by the cacuation to range from.4 to 2.5 pf for Case 1 and from.25 to.5 pf for Case 2. Capacitance C a increases rapidy as the ightning height H decreases but shows a sight variation ony with a changing streamer ength str. Cacuated vaues of capacitance C a are we approximated by power functions of the form By substituting (8) in (7), the channe-antenna partia capacitance vs. time reationship is as foows: C а a H n a ( τ ) n It was aso shown by cacuation for severa fixed creeping discharge streamer engths that partia capacitance C ag between the antenna (with aowance for the eectrode and creeping discharge streamers) and the ground depends heaviy on the streamer ength. Therefore the antenna potentia vs. time reationship was cacuated using the circuit diagram in Fig. 4 with aowance for variation of capacitance between ightning and antenna according to (9) severa fixed creeping discharge streamer engths. Computation was carried out with the hep of the Microcup program. Leakage resistance R (Fig. 4) was varied from 1 kohm to 1 MOhm. Shown in Tabe 1 are cacuated vaues of the antenna potentia at eakage resistance R=1 MOhm. It can be seen from Tabe 1 that the antenna potentia depends heaviy on the streamer ength str. The arger is str, the arger is capacitance C ag of the antenna- ground system and, in accordance with the Fig. 4 diagram, the ower is the antenna potentia. For exampe, with the ightning channe aigned with the poe 3 m above the ground, the antenna potentia woud be 28 kv without streamers and 19 kv with.5 m ong streamers, i. e. one third ess. Thus, as streamers increase, the antenna potentia and, accordingy, the votage on the LFA cabe insuation go down. (9) Tabe 1 shows aso that in Study Case 2 (with the ightning TABLE 1. Antenna potentia U a, kv vs. ightning height and ocation at various streamer engths str H, τ, Streamer ength str, m m µs C a а =, (7) H n where a, n are parameters; H, ightning height, m Partia capacitance C a between the channe and the antenna is approximated by (7) with vaues of the parameters a = 862 pf and n = 1.95 for Case 1 and a = 2.37 pf and n=.53 for Case 2. With a count-down approach, i. e. with the zero time taken to be the instant of the ightning striking the ground and τ assumed to be the ightning channe propagation time from the ightning ower end to the ground, the ightning height can be expressed as H v τ 3 1 τ. (8) = STR_Podporkin_A1 Session 2 Paper No

5 C I R E D 17 th Internationa Conference on Eectricity Distribution Barceona, May connect to a ine conductor via the fashover channe, and the antenna potentia drops consideraby. Note: Numerator and denominator show antenna potentia for Case 1 and 2 ightning channe ocations, respectivey. channe shifted 35 m from the poe) the antenna potentia is a few times ower than in Study Case 1. The difference is particuary marked at sma ightning heights (H = m): when the ightning channe is aigned with the poe the distance to the antenna is function of the ightning height ony, whie with a mid-span position of the the ightning it depends on the distance to the poe as we (here 35 m). Sti, even with the ightning channe 35 m away from the poe and at a 15 m ightning height (Tabe 1), the antenna potentia is about 8 kv with no streamers, which exceeds the creeping discharge breakdown votage on a.75 m ong cabe [2]. This assures a fashover on the surface of a covered conductor of a 12 kv ine (see Section 2). The antenna potentia depends on the eakage resistance as we. Fig. 5 quotes cacuated antenna potentia vs. time for.25 m ong streamers and for various eakage resistance vaues. At surface conductivity γ of 1 µs, characteristic of wet pouted surface of conductors and ine and antenna insuators, eakage resistance is about 1 MOhm. With the ightning descending more or ess in ine with the poe, the conductor surface is fashed over at an 8 kv antenna potentia when the ightning is about 45 m above the ground (see Fig. 5, curve 2); with the ightning channe ocated in the midde of a 7 m span, i. e. 35 m away from the poe, a covered conductor is fashed over when the ightning is some 15 m above the ground (see Tabe 1). It has thus been shown that the surface of a covered conductor is fashed over we before the ightning channe comes in contact with a power ine, both with the ightning channe above the poe and the ightning striking the poe, and with the ightning channe above a span of the ine and the ightning striking a mid-span point. PRELIMINARY EXPERIMENTAL CHECK Shown in Fig. 6 is a test arrangement permitting to simuate operating conditions of an antenna-type ightning protection system according to Fig. 2. Capacitance between the ightning channe and the antenna was simuated by an air capacitor consisting of a high votage doughnut eectrode (1.2 m externa diameter and.3 m diameter of the tube) and a 6 m square pane of.5 m wire mesh, with a 2 m insuating gap between the eectrode and the wire mesh. An impuse generator was used to appy positive and negative poarity 1.2/5 µs impuses. Votage was raised in 2 kv steps. At either poarity, incompete discharges were observed near the eectrode at a 6 kv output votage; at about 64 kv output votage the cabe surface was competey fashed over (Fig. 6, a), i. e. the cut-through camp got connected to the binding via creeping discharge channes and the eectrode on the conductor surface. The procedure simuated the stage of a ightning approaching a power ine. Fig. 5. Antenna potentia vs. time to breakdown of the ightningground gap τ at different vaues of eakage resistance R (streamer ength,.25 m; ightning channe aigned with the poe): 1. R = 1 MOhm; 2. R = 1 MOhm; 3. R=5 kohm; 4. R = 3 kohm; 5. R = 1 kohm; 6. R = 1kOhm. It can be seen from Fig. 5 that, as the ightning channe approaches the overhead ine, the potentia of the antenna increases sharpy, the rate of increase growing with time. The antenna potentia depends heaviy on the eakage resistance: the ower is resistance, the ower is potentia. Theoreticay, the antenna potentia coud be as high as 2 kv. However, it cannot happen in rea ife because, at a 6 to 8 kv votage between the eectrode and the core, a fashover occurs in the gap 2 on the surface of the covered conductor between the eectrode and the cutthrough camp (see Fig. 3 a); this makes the antenna Appication of a 9 kv positive or a 115 kv negative impuse ed to a breakdown of the air gap between the doughnut and the pane of the air capacitor, a the output votage from the impuse generator coming abrupty to the eectrode on the surface of a covered conductor (Fig. 6 b). Whie the ine insuator was fashed over the insuation of the covered conductor was not damaged. With the air capacitor shunted by a piece of wire the test votage was appied directy to the conductor core (Fig. 1) without invoving the antenna; this resuted in an insuator fashover and a conductor insuation breakdown. The tests have thus ampy demonstrated and proved efficiency of the antenna which assures a creeping discharge fashover over the surface of the conductor insuation we before a ightning stroke on the ine. Such a preventive fashover STR_Podporkin_A1 Session 2 Paper No

6 C I R E D 17 th Internationa Conference on Eectricity Distribution Barceona, May 23 protects the conductor insuation against a breakdown by a ightning stroke, at the same time creating a fairy ong ightning fashover path which rues out a power arc. An important feature of the suggested ightning protection conductor or poe of the ine. 2. Such a fashover prepares a fairy ong path for a fashover occurring during a ightning stroke on the ine, and thus prevents a breakdown of the conductor insuation. 3. A ong-fashover arrester fitted with an antenna protects the ine against both induced overvotages and direct ightning strokes. 4. A simpe design makes ong-fashover arresters consideraby ess expensive than conventiona meta oxide arresters. 5. The suggested ightning protection system appears highy promising and encouraging further research and deveopment studies covering, among other things, its higher votage appications. REFERENCES a) [1] G. V. Podporkin, A. D. Sivaev, 1995, Eectric Power Transmission Line With Protection Devices Against Lightning Overvotages", US Patent No. 6,18,187, Date of Patent Aug. 22, 2, PCT Pub. No. WO 97/19456, Russian Federation Appication Priority Data Nov. 17, [2] G. V. Podporkin, A. D. Sivaev, 1998,"Lightning Protection of Overhead Distribution Lines by Long Fashover Arresters", IEEE Transactions on Power Deivery, Vo. 13,, No. 3, Juy, pp b) Fig. 6 Simpified test outine a) simuation of ightning eader approaching a ine; b) simuation of ightning fina strike. system is its simpicity; as seen from Fig. 3, it comprises a meta stand (one per three phases on a poe), three wire rope antennas, three ight-weight composite tension insuators, three eectrodes, and three cut-through camps. Again, this system protects the ine against both induced overvotages and direct ightning strokes. Reported here is ony one of many possibe configurations of an antenna-type ong-fashover arrester. In the authors opinion, the above ightning protection system can be aso deveoped for higher votages. CONCLUSIONS 1. An antenna connected to a ong-fashover arrester on a covered conductor assures its fashover as the ightning channe is approaching the power ine, we before a direct contact of the channe with a [3] G. V. Podporkin, V.E. Pishikov, A. D. Sivaev, 22, Lightning Protection of Medium Votage Overhead Lines by Moduar Long-Fashover Arresters", IEEE Transactions on Power Deivery,, paper PE-137PRD (4-22), accepted for pubication. [4] Y. Morooka et a., 1997, Line Breakage Prevention Capitaizing on Creeping Discharge Characteristics of Insuated Power Wires, 1 th ISH, August, Vo. 5, pp [5] K. Nakamura et a., 1996, Impuse Breakdown Characteristics of 13.2 Covered Conductor Insuator/Tie Configurations ", IEEE Transactions on Power Deivery, Vo. 1,, No. 4, October, pp [6] Т. Yamashita et a., 1997, Propagation Characteristics of a Surface Discharge on Covered Conductors and Cyindrica Dieectric Materias, 1 th Internationa Symposium on High Votage Engineering (ISH 97), August [7] V. P. Larionov, E. S. Koechitskiy, V. N. Shugin, 1981, Cacuation of Lightning Break Through Wire Protection, Eektrichestvo, No. 5,, pp , (in Russian). STR_Podporkin_A1 Session 2 Paper No

7 C I R E D 17 th Internationa Conference on Eectricity Distribution Barceona, May 23 [8] С. Bouquegneau, 22, Lightning Phenomenoogy, 26 th Internationa Conference on Lightning Protection (ICLP 22), Cracow, Poand, September 2-6,, invited ecture 1. STR_Podporkin_A1 Session 2 Paper No