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Constraints on the use of surge arresters for improving the backflashover rate of transmission lines
Electric Power Systems Research 180 (2020) 106064
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Constraints on the use of surge arresters for improving the backflashover rate of transmission lines
T
Silverio Visacro*, Fernando H. Silveira, Barbara Pereira, Rafael M. Gomes LRC – Lightning Research Center, Graduate Program in Electrical Engineering, Federal University of Minas Gerais, 6627 Antônio Carlos Avenue, Belo Horizonte, MG, 31270-901, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Transmission line surge arresters Backflashover Lightning performance of transmission lines Grounding Tower-footing resistance
The effectiveness of using transmission line surge arresters (TLSA) under different conditions for improving the lightning performance of lines is assessed by means of computational simulation. These conditions comprise using TLSA only in critical towers and/or using devices only in certain phases, leaving one or two-phases unprotected. In addition to assess the effect on stricken towers, the transfer of overvoltage to unprotected adjacent towers is also addressed, considering different values of tower-footing impedance. In the analyses, lightning overvoltages across insulator of typical single-circuit 138-kV and double-circuit 230-kV transmission lines were calculated by using the Hybrid Electromagnetic Model (HEM) and flashover occurrence was assessed by using the Disruptive Effect method (DE), in each simulated condition. The results indicate that the operation of TLSA protects the own insulator and diminishes the backflashover risk at unprotected insulators. However, this risk remains high, depending on tower-footing impedance. Also, the risk of flashover at the unprotected adjacent towers due to the operation of surge arresters at the stricken tower was found to be significant in the case of all phases of the stricken tower protected, notably when the tower-footing impedance of the protected tower is high and that of the adjacent tower is low.
1. Introduction Backflashover is the mechanism governing the lightning performance of transmission lines (TL) below 500 kV, installed in regions of unfavorable soil resistivity [1,2]. There is a set of alternatives for reducing backflashover rates to improve the lightning performance of TLs. The definition of the one to apply is determined considering technical and economic aspects, which take the characteristics of the line and the environment where it is installed into account. Usually the preferential and less expensive practice for reducing backflashover rates consists of decreasing tower-footing impedance ZP (or corresponding grounding resistance) [3,4]. For those lines installed over high resistivity soils, this can be reached by installing long counterpoise wires [5]. A second alternative consists of installing underbuilt wires along those spans that flanks towers with values of tower-footing impedance still exceeding the threshold values that leads to an acceptable number of outages. As discussed in [6], depending on the number of underbuilt wires (two or one) and of the tower-footing impedance value, the outage number can be significantly reduced by using this alternative,
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reaching in certain cases a reduction comparable to that yielded by decreasing the tower-footing grounding impedance to the half-value. For those cases, in which the two practices mentioned above do not guarantee the desired lightning performance of the line, the installation of transmission line surge arresters in parallel with insulators can be adopted [7]. A surge arrester device has a V × I characteristic that exhibits very high impedance for steady state voltages (similar to an open switch for voltages amplitudes lower than a certain threshold) and a low impedance for overvoltages exceeding this threshold. TLSAs are applied to limit overvoltages between phase conductors and tower structure, preventing them to exceed the insulation withstand, that would lead to the occurrence of flashover across insulators [7,8]. The use of TLSA in all phases of a tower is extremely effective for preventing backflashover at that tower. Nevertheless, the cost of this practice is usually higher, as, in addition to the devices’ cost, it involves the installation and maintenance of the devices at the towers, and still requires a certain level of investment on tower-footing electrodes to ensure this practice to be effective. In some cases, this cost makes the large-scale application of such measure unfeasible. Thus, using TLSA only in a few “critical” towers along the line or using it only in one or two phases of the tower have been a relatively common practice [9].
Corresponding author. E-mail address: [email protected] (S. Visacro).
https://doi.org/10.1016/j.epsr.2019.106064 Received 5 April 2019; Received in revised form 9 August 2019; Accepted 9 October 2019 Available online 06 December 2019 0378-7796/ © 2019 Elsevier B.V. All rights reserved.
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Unfortunately, the technical literature does not properly address the constraints on applying these practices [10–12]. Installing TLSA to protect only one or two phases of a tower can let the unprotected insulator string of the stricken tower still susceptible to backflashover, depending on the tower-footing impedance. It is presumable that, even when TLSA are installed in all phases of a stricken tower, during their operation, overvoltages are transferred to adjacent towers and, if they are not protected with TLSA, under determined conditions of tower-footing impedance of the stricken and adjacent towers, the overvoltages can yield flashover there [8,9]. The literature does not discuss these constraints for providing the users with guidelines. This work addresses the effectiveness of using TLSA for improving the lightning performance of transmission lines by means of computational simulation, considering the mentioned questions and taking as reference the parameters of two typical real transmission lines. Fig. 2. Geometric parameters of the towers of the (a) 138-kV and (b) 230-kV transmission lines (average parameters).
2. Methodology of development To determine the constraints of TLSAs application, this work considered different schemes of installation of surge arresters and different conditions of tower footing-impedance. To develop a quantitative evaluation that allows illustrating better these constraints, the parameters of two real transmission lines were considered, a 138-kV singlecircuit and a 230-kV double-circuit transmission lines, represented in Figs. 1 and 2. Note that the quantitative results are valid specifically for these two lines. In the simulations, the lightning strike to the tower was represented by the direct impression of the current wave on the tower top. No lightning channel was represented, as it does not affect the insulators’ overvoltages calculated by using an electromagnetic model (like in this
work), different from approaches based on distributed circuit parameters [13]. The strike to the tower was assumed at the instant of zero power-frequency voltage at the critical unprotected phase. Attenuation and distortion of the voltage wave propagating along shield wires due to Corona effect were not considered. The overvoltages developed across insulators were calculated using the Hybrid Electromagnetic Model (HEM), widely applied to determine current and voltage distributions in lightning-related studies. Details of the HEM model, whose formulation is derived from electromagnetic field integral equations and results are provided in circuital quantities, are found in [14,15]. In particular, reference [14] validate the use of this model for calculation of lightning overvoltages, by exhibiting the very good agreement between calculated and measured overvoltages across the insulators of a transmission line. The voltage across insulators is determined as the difference of electric potentials at the two terminals of the insulator, which, in an electromagnetic model, is intrinsically equal to the integral of the electric field along the path between these terminals, across the insulator surface. A flashover criterion, consisting of the Disruptive Effect (DE) method, was applied for determining the critical currents IC (peak current leading the insulator to flashover). This method, which integrates the instantaneous value of the overvoltages during the interval it exceeds a certain threshold value related to the insulation withstand of the line (expressed by means of the line CFO) to assess whether this integral reaches the value required for flashover the insulator, is described in detail in [16,17]. The percentage of peak currents exceeding the critical current was calculated from the IEEE cumulative first-stroke peak current distribution [18]. Note that this percentage corresponds to the backflashover probability of that tower when it is subject to a lightning strike. Thus, multiplying this percentage by the number of strikes to the line allows promptly determining the expected number of outages. Reference [19] deeply discusses the application of the HEM model and of the DE method to determine the critical currents and the backflashover probability. The so-called double-peaked waveform [20], shown in Fig. 3, was adopted for representing the lightning current in simulations. This waveform exhibits the typical features of measured first return stroke currents, consisting of an initial concavity, an abrupt rise about the half peak that lasts until the first peak and the presence of a second peak, usually higher than the first. References [21,22] shows that this waveform is representative of the first return stroke currents measured at Monte San Salvatore and at Morro do Cachimbo stations. A discussion on the effect of the lightning current waveform on the lightning overvoltage and backflashover probability is presented in [23]. In this work, the simulated current wave reproduces the median values of current and time parameters of the first-stroke currents measured at Mount San Salvatore Station [24].
Fig. 1. Simulated condition: lightning striking the tower top of the (a) 138-kV single-circuit transmission line exhibiting 400-m long spans between adjacent towers, a single shield wire and a critical flashover overvoltage (CFO) of 650 kV; (b) 230-kV double-circuit transmission line exhibiting 500-m long spans, double shield wires and CFO of 1200 kV. Overvoltages calculated across the insulators of the stricken and adjacent towers. 2
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Fig. 3. Double-peaked current waveform used in simulations [20], exhibiting the median parameters of amplitude and time of the first-stroke currents measured at Mount San Salvatore Station: Ip = 31.1 kA, Ip1 = 27.7 kA, Td30 = 3.8 μs, Td10 = 5.6 μs, T50 = 75 μs. (as defined in [23]).
In simulations the tower footing was represented by its first-returnstroke impulse impedance (ZP), varying in a range from 10 to 40 Ω. As shown in [4,19], using this representation of electrodes practically leads to the same critical current and backflashover probability obtained under their physical representation. Using this ZP representation releases the need to consider the soil resistivity, which is the value that results in the assumed impulse impedance, considering the geometry of tower-footing electrodes in each case. The presence of the surge arrester was taken into account by positioning a variable resistance in parallel with the insulator, according to the device’s V × I curve. Such representation aims to resemble the characteristics of 132-kV and 198-kV-rated voltage surge arresters described in [25]. The effect of installing 3, 2 and 1 TLSAs (single circuit TL) and 6, 4 and 2 TLSAs (double-circuit TL) on the protected stricken tower and on the unprotected adjacent towers was assessed. This representation of surge arresters allows obtaining accurate results of overvoltages calculated across the insulators of the stricken tower, but impoverishes the quality of the results at adjacent towers.
Fig. 4. Reduction of the overvoltage across unprotected insulator strings of the stricken tower due to the use of 2-1 and 4-2 TLSAs, respectively in the singlecircuit and double-circuit transmission lines. Impressed current waveform of Fig. 3 and tower-footing impedance ZP = 20 Ω for the stricken and adjacent towers. Table 1 Improvement of the lightning performance of tower by using TLSAs under different tower-impedance values — 138-kV single-circuit transmission line.
3. Results and analysis 3.1. The effect on the stricken tower Using surge arresters to protect all phases of a TL tower clamps all the transient overvoltages resulting across insulator strings during a lightning strike to the tower and prevents the occurrence of backflashover at that tower. Using less surge arresters still decreases the overvoltage across the unprotected phase(s). However, it is required to assess whether this decrease is sufficient to prevent the backflashover occurrence. Fig. 4 illustrates the reduction of the overvoltage amplitude resulting from the use of two and one TLSA in the single circuit TL and of 4 and 2 TLSA in the double-circuit TL. The analyses of Table 1 reveal that using a single TLSA promotes a significant improvement of the lightning performance of the tower and this improvement tends to decrease with increasing tower-footing impedance. Improvements of about 29%, 27% and 19% were reached for tower-footing impedance of 10, 20 and 40 Ω. Much significant improvements of the tower performance were achieved using 2 TLSAs, respectively of about 45%, 42%, and 33%. On the other hand, the analyses also reveal that, even using 1 and 2 surge arresters per tower, the percentage of currents exceeding the critical current is still high, notably for high values of tower-footing impedance. It is about 15% and 37% for a single TLSA and 12% and 31% of the lightning strikes to the line, respectively for ZP of 20 and 40 Ω. This means that, depending on the rate of lightning incidence, the line can still present a significant number of outages.
ZP (Ω) Stricken towera
Condition
Ic (kA)
%I > Ic
Variation (%)
10
Without TLSA 1 TLSA 2 TLSA
83.7 96.1 106.3
7.0 5.0 3.9
– −28.6 −44.5
20
Without TLSA 1 TLSA 2 TLSA
51.8 59.8 66.3
20.9 15.3 12.2
– −26.7 −41.9
40
Without TLSA 1 TLSA 2 TLSA
32.9 37.8 42.2
46.2 37.4 31.0
– −19.1 −32.9
a
Same of the adjacent towers tower-footing grounding impedance.
As expected, the results of Table 2 show a much better performance, due to the higher CFO of 1.2 MV of the 230-kV TL relative to 0.65 MV of the 138-kV TL. They also reveal the significant increase of the critical current due to the use of surge arresters. For a 10-Ω tower-footing grounding impedance, IC increases from 134.2 kA to 150.7 kA and 175.2 kA, respectively for 2 and 4 installed TLSAs. This corresponds to decreases of the currents exceeding IC to 1.6% and 1.1% and respective improvements of the tower performance of about 26% and 50%. For a 20-Ω tower-footing impedance, IC increases from 101 kA to 127 kA and 148 kA for 2 and 4 TLSAs, decreasing the currents exceeding IC from 3
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improvement of the lightning performance of the stricken protected tower, their use can worsen the lightning performance of adjacent towers. The mechanism that yields this condition is simple. Surge arresters are usually installed at towers exhibiting high values of tower-footing impedance. Thus, even with the operation of surge arresters, high voltages are developed at the top of the stricken tower, resulting from the significant tower-footing grounding potential rise (GPR) added to the potential rise along the tower caused by the flow of the lightning current towards the ground. As the surge arrester operates, this high voltage is transferred by the phase conductor to the energized side of the insulators of the adjacent towers. On the other hand, a minor part of the incident lightning current is transferred to the adjacent tower by the shield wire(s). The flow of this current towards the ground produces a potential rise at this tower top. This corresponds to the sum of the GPR caused by the current at the tower-footing to the potential rise the current causes along the tower. Thus, overvoltages are established across the insulators of the unprotected phases of the adjacent towers, given approximately by the difference between the electric potential developed at the top and the potential transferred from the stricken tower due to the operation of the surge arresters. If the tower-footing impedance of the stricken tower is very high and that of the adjacent tower is very low, the overvoltage across the insulator can be also very high, leading it to flashover, if it is not protected by a surge arrester. Differently, if the two tower-footing impedance values are comparable, the overvoltage tends to be low, preventing the flashover occurrence. The occurrence of this mechanism was assessed in this work by determining the percentage of currents exceeding IC at the adjacent towers when the arrester(s) of the stricken tower operate(s). To obtain this percentage, the lightning overvoltage across the adjacent towers’ insulators was determined in the same simulated conditions of Table 1 and 2. One more condition was simulated, consisting of surge arresters protecting all phases of the stricken tower. The corresponding results are presented in Tables 4 and 5. Only for providing quantitative references, the percentages of currents exceeding IC for the unprotected insulators of the stricken tower was included in the tables (into brackets). The results in Tables 4 and 5 are all consistent with the analysis of the mechanism of overvoltage transfer described in this section. As expected, these percentages of peak currents exceeding IC are significantly lower for the double-circuit TL. On the other hand, the tables reveal some trends valid for both TLs. First, the percentage of currents exceeding IC in the adjacent towers increases with increasing tower-footing impedance of the stricken tower and decreasing towerfooting impedance of the adjacent tower. This percentage also increases with the increasing number of surge arrester installed at the stricken tower. Following this trend, the critical simulated condition consists of
Table 2 Improvement of the lightning performance of tower by using TLSAs under different tower-impedance values — 230-kV double-circuit transmission line. ZP (Ω) Stricken towera
Condition
Ic (kA)
%I > Ic
Variation (%)
10
Without TLSA 2 TLSA 4 TLSA
134.2 150.7 175.2
2.2 1.6 1.1
– −25.8 −49.8
20
Without TLSA 2 TLSA 4 TLSA
101.1 126.8 148.2
4.4 2.5 1.7
– −43.6 −62.1
40
Without TLSA 2 TLSA 4 TLSA
64.8 92.7 111.0
12.8 5.5 3.5
– −57.3 −72.7
a
Same of the adjacent towers tower-footing grounding impedance.
4.4% to 2.5% and 1.7%. This corresponds to improvements of 44% and 62% of the tower performance. For a 40-Ω tower-footing impedance, the corresponding improvements are of 57% and 73%. In spite of these improvements, depending on the lightning incidence rate, backflashovers across insulators of the unprotected phases of the stricken tower can still cause a significant number of outages, even using 2 or 4 surge arresters per tower. A complementary analysis of practical interest specifically for double-circuit lines consists of assessing the effect of concentrating the TLSAs in a single circuit in relation to their distribution in both circuits. Table 3 exhibits the results corresponding to 1, 2 and 3 TLSAs installed in a single circuit of the 230-kV line. For the sake of comparison, the table includes some results corresponding to the distribution of TLSA in both circuits. Note that using 2 TLSAs in a single circuit is less efficient than distributing them in both circuits in the usual conditions that would require the use of surge arresters (ZP of 20 and 40 Ω). The results is different for a 10 Ω impulse impedance, but this is not a usual condition that requires installing TLSAs. On the other hand, though using 3 TLSAs in a single circuit seems less efficient than distributing 4 of them in both circuits, this arrangement ensures the continuity of the operation of this fully protected circuit. Thus, in certain applications, this could be advantageous. 3.2. The effect on the adjacent towers An additional constraint of using TLSA concerns the effect of their operation on adjacent towers. As discussed next, in spite of the Table 3 Improvement of the lightning performance of tower by using TLSAs in a single circuit of 230-kV double-circuit transmission line. ZP (Ω) Stricken towera
a
TLSA distribution in a single circuit
Table 4 Percentage of currents expected to exceed the critical current able to cause backflashover at the adjacent towers — 138-kV transmission line.
TLSA distribution in both circuits
Number of TLSA
Ic (kA)
%I > Ic
Number of TLSA
Ic (kA)
%I > Ic
10
1 2 3
144.2 159.3 171.1
1.81 1.40 1.16
– 2 4
– 150.7 175.5
– 1.61 1.09
20
1 2 3
111.9 120.0 125.6
3.43 2.88 2.57
– 2 4
– 126.8 148.2
– 2.50 1.68
40
1 2 3
73.2 78.7 82.5
9.69 8.14 7.29
– 2 4
– 92.7 111.0
– 5.48 3.50
ZP (Ω) stricken tower
ZP (Ω) adjacent tower
%I > IC (%) 1 TLSA
%I > IC (%) 2 TLSA
%I > IC (%) 3 TLSA
10
10 20 40
0.70 (5.01)a 0.47 0.21
0.87 (3.9) 0.60 0.30
1.13 (0) 0.79 0.41
20
10 20 40
1.75 1.15 (15.3) 0.53
2.24 1.54 (12.2) 0.76
3.20 2.23 (0) 1.13
40
10 20 40
4.56 3.18 1.63 (37.4)
5.76 4.17 2.27 (31)
8.37 6.16 3.50 (0)
a The values into brackets correspond to the percentage of currents exceeding IC for the unprotected insulators of the stricken tower.
Same of the adjacent towers tower-footing grounding impedance. 4
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Table 5 Percentage of currents expected to exceed the critical current able to cause backflashover at the adjacent towers — 230-kV transmission line. ZP (Ω) stricken tower)
ZP (Ω) adjacent tower
%I > IC (%) 2 TLSA
%I > IC (%) 4 TLSA
%I > IC (%) 6 TLSA
10
10 20 40
0.21(1.6)a > 0.1 > 0.1
0.27 (1.1) > 0.1 > 0.1
0.29 (0) 0.2 0.1
20
10 20 40
0.51 0.31(2.5) 0.12
0.64 0.41 (1.7) 0.17
0.69 0.45 (0) 0.20
40
10 20 40
1.21 0.8 0.36 (5.5)
1.52 1.03 0.51(3.5)
1.62 1.12 0.58 (0)
a
The values into brackets correspond to the percentage of currents exceeding IC for the unprotected insulators of the stricken tower.
the 40 Ω for the tower-footing impedance of stricken protected tower and 10 Ω for that of the adjacent tower. In Table 4 (single-circuit TL), about 4.6%, 5.8% and 8.4% of the lightning strikes to the protected tower would lead the adjacent tower to flashover, respectively for 1, 2 and 3 TLSA installed at the tower. The corresponding percentages decrease to about 1.2%, 1.5% and 1.6% for 2, 4 and 6 TLSA installed at the adjacent tower of the double-circuit TL, as shown in Table 5. The percentage of currents exceeding IC at the adjacent tower is very low in comparison with that at the stricken tower, for the conditions of TLSAs protecting one and two phases (single-circuit TL:1 and 2 installed TLSAs; double circuit TL: 2 and 4 TLSAs). However, this percentage becomes relatively relevant when all phases of the stricken tower are protected by TLSAs, reaching the order of about one third to one fifth of the percentage of peak currents leading insulators of the stricken tower to flashover, in the condition of two and one phases protected. Fig. 5 summarizes the previous results. Note that the percentages are expressed in terms of an equivalent parameter: the probability of flashover at the adjacent towers. It denotes that this probability is significant only for protected towers exhibiting a high tower-footing impedance. Additionally, it shows that the significance is enhanced with decreasing tower-footing impedance of adjacent towers. Both aspects are clearly depicted in the three upper curves of Fig. 5(a) and (b). The results above suggests addressing some questions of interest for applications. First, the probability of flashover at the adjacent towers is much lower than that at the stricken tower when only one or two phases are protected. The peak currents, which would lead insulators of the adjacent towers to flashover, are already included among those peak currents causing backflashover at the stricken tower. Thus, in the analysis of the stricken tower performance, the contribution of flashovers at adjacent towers for the conditions of one and two phases protected can be disregarded. The situation is different for the condition of fully protected stricken tower (all phases protected by TLSAs). This relatively significant flashover probability (of about 8.4%, 3.2% and 1.1% for the singlecircuit TL and 1.6%, 0.7% and 0.3% for the double-circuit TL and 10-Ω grounding impedance of adjacent tower) has to be attributed to the probability of flashover of the stricken tower at the condition of all phases protected. This means that, even in the case of fully protected tower, there will be a probability of outage due the mechanism of surge transfer to adjacent tower, expressed by the percentages indicated above. Thus, concerning the practical aspects related to the impact of lightning strikes to towers protected by surge arresters on the adjacent towers, the only relevant contribution consists of that of flashovers at adjacent towers with low tower-footing impedance flanking fully protected towers with high tower-footing impedance.
Fig. 5. Probability of flashover (in percentage) across insulators of adjacent towers due to strikes to a protected tower exhibiting different tower-footing impedances (10, 20 and 40 Ω) as a function of the tower footing impedance of adjacent towers: (a) 138-kV and (b) 230-kV transmission lines.
4. Relevant practical implications To allow discussing the results developed for the single- and doublecircuit TLs, the lightning performance of their towers was calculated from the results of Section 3A. This performance was translated as the average probability of flashover per tower. As the results of Section 3 corresponds to direct lightning strikes to the tower, the estimated percentages of currents exceeding IC was multiplied by a factor of 0.6, for taking into account the lower overvoltages of strikes to shield wire (s) distributed along the span, in the region of the tower influence [18]. Fig. 6 exhibits the curves expressing the expected performance of protected towers of the TLs as a function of their tower-footing impedance, for the considered conditions of surge arrester installation. Specifically, the curves for 3 and 6 TLSAs in Fig. 6(a) and (b), respectively, correspond to the percentage of flashover at adjacent towers. The figure also includes dashed lines corresponding to the maximum acceptable probability of backflashover per tower. This threshold was calculated for the tower of each line, assuming a defined flash density Ng of 4 flashes/km2/year and the maximum acceptable numbers of 5 and 2 outages/100-km/year for the 138-kV and 230-kV TLs. The respective numbers Ns of 86 and 107 strikes/ 100-km/year to the lines was estimated by using the following expression Ns = Ng⋅[2⋅14(h0.6) + D]/10, given in IEEE Standard 1243 [18]. In this expression, whose unit is outages/100-km/year, h and D correspond to the average height of shield wire(s) and distance between them, respectively. This resulted in maximum acceptable flashover probabilities of 5.8% (5/ 86) and 1.4% (2/142) for the 138-kV and 230-kV TLs, respectively. The curves of Fig. 6 reveal some important practical aspects. Each tower exhibits a performance governed by the value of its tower-footing 5
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great influence on these limits. 5. Conclusions By developing quantitative evaluations of the lightning performance of towers of a 138-kV and a 230-KV transmission lines, this work discussed the constraints and the effectiveness of installing TLSAs for improving the lightning performance of transmission lines. The impact on the unprotected insulators of the stricken tower and of the transfer of the risk of flashover to adjacent towers due to surge arresters’ operation was addressed. The results of this work clearly denote the need to observe the minimum tower-footing conditions for an efficient application of TLSA to improve the lightning performance of TLs, notably when some of the phases are not protected. In the conditions considered in this work, for the 138-kV TL, the limiting tower-footing impedance values were found to be about 14, 17 and 40 Ω, respectively for 1, 2 and 3 installed TLSAs. The corresponding values for 2, 4 and 6 TLSAs installed at the 230-kV TL were about 18, 27 and 48 Ω. It is remarkable that, even when all the phases of the tower are protected, a maximum value of its tower-footing impedance has to be observed to prevent the tower to develop a lightning performance worse than the acceptable. This shakes the impression that simply using TLSAs allows achieving a satisfactory performance of a TL. The qualitative results and analyses developed in this work have general validity. The quantitative results depend on the environment and TL characteristics, notably the local flash density and TL geometry. This leads to the need of developing specific evaluations for defining the efficient use of TLSAs for the particular conditions of the transmission line under consideration. This can be done using procedures similar to that developed in this work. Declaration of Competing Interest
Fig. 6. Lightning performance of the transmission line (backflashover probability in percentage), considering different conditions of TLSA installation and tower-footing impedance: (a) 138-kV and (b) 230-kV. Assumed conditions: Ng = 4 flashes/km2/year. Maximum acceptable outages of 5 and 2 outages/ 100-km/year, respectively.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment
impedance and the condition of surge arrester installation. In general, the performance of the 230-kV TL is much better, exhibiting backflashover probabilities for ZP of 40 Ω ranging from 1% to 7.7% in relation to the values of about 5% to 27% for the 138-kV TL. In order to prevent developing a performance worse than that given by the dashed line, the tower-footing impedance of each tower has to be lower than a specific value, corresponding to the condition of TLSAs installation. The arrows in Fig. 6(a) indicate maximum acceptable tower-footing impedances of 12, 14, 17 and 41 Ω, for the conditions of without, with 1, 2, and 3 TLSAs installed at the tower of the 138-kV TL. These impedances correspond to low-frequency grounding resistances of about 18, 22, 26 and 60 Ω, respectively. The corresponding impedance values given in Fig. 6(b) for the 230-kV TL are higher, respectively of 11, 18, 27 and 48 Ω. Their respective associated low-frequency resistance are about 17, 28, 41 and 70 Ω. In particular, it is interesting mentioning the limiting values of about 60 and 70 Ω for the tower-footing impedance of the 138-kV and 230-kV TLs in the condition of all phases protected by surge arresters, as these maximum values were defined from flashovers occurring at adjacent towers. The qualitative results of the analyses developed in this section have general validity. Nevertheless, the values of the threshold backflashover probability is highly dependent on the flash density along the line. Densities above the assumed 4 flashes/km2/year would decrease this threshold probability and, therefore, would decrease the acceptable limits of tower-footing impedance in each condition of TLSAs installation. The characteristics of the line, notably the tower height, has also
Silverio Visacro and Fernando H. Silveira would like to acknowledge the financial support provided by the Brazilian National Council of Technological and Scientific Development (CNPq) for their participation in this work, processes 310564/2016-6 and 308351/2018-5, respectively. References [1] S. Visacro, Direct strokes to transmission lines: considerations on the mechanisms of overvoltage formation and their influence on the lightning performance of lines, J. Lightning Res. 1 (2007) 60–68. [2] S. Visacro, ArtLiber (Ed.), Lightning: an Engineering Approach, (in Portuguese), 2005, pp. 1–272 São Paulo, Brazil. [3] S. Visacro, A comprehensive approach to the grounding response to lightning currents, IEEE Trans. Power Deliv. 22 (January (1)) (2007) 381–386. [4] S. Visacro, The use of the impulse impedance as a concise representation of grounding electrodes in lightning protection applications, IEEE Trans. Electromagn. Compat. 60 (5) (2018) 1602–1605, https://doi.org/10.1109/TEMC.2017.2788565. [5] S. Visacro, F.H. Silveira, Lightning performance of transmission lines: methodology to design grounding electrodes to ensure an expected outage rate, IEEE Trans. Power Deliv. 30 (2015) 237–245. [6] S. Visacro, F.H. Silveira, A. De Conti, The use of underbuilt wires to improve the lightning performance of transmission lines, IEEE Trans. Power Deliv. 27 (2012) 205–213. [7] Outline of Guide for Application of Transmission Line Surge Arresters—42 to 765 kV Extended Outline – 1012313, Oct.2006. [8] EPRI - Handbook for Improving Overhead Transmission Line Lightning Performance (technical report). [9] S. Visacro, F.H. Silveira, B. Pereira, R.M. Gomes, Concerns on surge-arrester application for improving the backflashover rate of transmission lines, Proceedings of International Conference on Lightning Protection (ICLP2018), Rzeszow, Poland,
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S. Visacro, et al. 2018. [10] R. Shariatinasab, J. Gholinezhad, K. Sheshyekani, Estimation of energy stress of surge arresters considering the high-frequency behavior of grounding systems, IEEE Trans. Electromagn. Compat. 60 (August (4)) (2018) 917–925. [11] K. Munukutla, V. Vittal, G.T. Heydt, D. Chipman, B. Keel, A practical evaluation of surge arrester placement for transmission line lightning protection, IEEE Trans. Power Deliv. 25 (July (3)) (2010) 1742–1748. [12] J. He, J. Hu, Y. Chen, S. Chen, R. Zeng, Minimum distance of lightning protection between insulator string and line surge arrester in parallel, IEEE Trans. Power Deliv. 24 (April (2)) (2009) 656–663. [13] Z.G. Datsios, P.N. Mikropoulos, T.E. Tsovilis, Effects of lightning channel equivalent impedance on lightning performance of overhead transmission lines, IEEE Trans. Electromagn. Compat. 61 (June (3)) (2019) 623–630. [14] S. Visacro, A. Soares Jr., HEM: a model for simulation of lightning-related engineering problems, IEEE Trans. Power Deliv. 20 (April (2)) (2005) 1026–1028. [15] S. Visacro, F.H. Silveira, Evaluation of current distribution along the lightning discharge channel by a hybrid electromagnetic model, J. Electrostat. 60 (2–4) (2004) 111–120. [16] H. Hileman, Insulation Coordination for Power Systems, CRC, Boca Raton, FL, 1999, pp. 627–640. [17] R.O. Caldwell, M. Darveniza, Experimental and analytical studies of he effect of non-standard waveshapes of the impulse strength of external insulations, IEEE
Trans. Power App. Syst. PAS-92 (July (4)) (1973) 1420–1428. [18] IEEE Std 1243, Guide for Improving the Lightning Performance of Transmission Lines, (1997). [19] S. Visacro, F.H. Silveira, Lightning performance of transmission lines: requirements of tower-footing electrodes consisting of long counterpoise wires, IEEE Trans. Power Deliv. 31 (2016) 1524–1532. [20] A. De Conti, S. Visacro, Analytical representation of single- and double-peaked lightning current waveforms, IEEE Trans. Electromagn. Compat. 49 (May (2)) (2007) 448–451. [21] S. Visacro, A representative curve for lightning current waveshape of first negative stroke, Geophys. Res. Lett. 31 (April (L07112)) (2004) 1–3. [22] F.H. Silveira, S. Visacro, Lightning parameters of a tropical region for engineering application: statistics of 51 flashes measured at Morro do Cachimbo and expressions for peak current distributions, IEEE Trans. Electromagn. Compat. (2019), https:// doi.org/10.1109/TEMC.2019.2926665. [23] F.H. Silveira, S. Visacro, Lightning performance of transmission lines: impact of current waveform and front time on Backflashover occurrence, IEEE Trans. Power DelIV (2019), https://doi.org/10.1109/TPWRD.2019.2897892. [24] R.B. Anderson, A.J. Eriksson, Lightning parameters for engineering application, Electra 69 (1980) 65–102. [25] Siemens, High-Voltage Surge Arresters — Product Guide, (2019).
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Constraints on the use of surge arresters for improving the backflashover rate of transmission lines
T
Silverio Visacro*, Fernando H. Silveira, Barbara Pereira, Rafael M. Gomes LRC – Lightning Research Center, Graduate Program in Electrical Engineering, Federal University of Minas Gerais, 6627 Antônio Carlos Avenue, Belo Horizonte, MG, 31270-901, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Transmission line surge arresters Backflashover Lightning performance of transmission lines Grounding Tower-footing resistance
The effectiveness of using transmission line surge arresters (TLSA) under different conditions for improving the lightning performance of lines is assessed by means of computational simulation. These conditions comprise using TLSA only in critical towers and/or using devices only in certain phases, leaving one or two-phases unprotected. In addition to assess the effect on stricken towers, the transfer of overvoltage to unprotected adjacent towers is also addressed, considering different values of tower-footing impedance. In the analyses, lightning overvoltages across insulator of typical single-circuit 138-kV and double-circuit 230-kV transmission lines were calculated by using the Hybrid Electromagnetic Model (HEM) and flashover occurrence was assessed by using the Disruptive Effect method (DE), in each simulated condition. The results indicate that the operation of TLSA protects the own insulator and diminishes the backflashover risk at unprotected insulators. However, this risk remains high, depending on tower-footing impedance. Also, the risk of flashover at the unprotected adjacent towers due to the operation of surge arresters at the stricken tower was found to be significant in the case of all phases of the stricken tower protected, notably when the tower-footing impedance of the protected tower is high and that of the adjacent tower is low.
1. Introduction Backflashover is the mechanism governing the lightning performance of transmission lines (TL) below 500 kV, installed in regions of unfavorable soil resistivity [1,2]. There is a set of alternatives for reducing backflashover rates to improve the lightning performance of TLs. The definition of the one to apply is determined considering technical and economic aspects, which take the characteristics of the line and the environment where it is installed into account. Usually the preferential and less expensive practice for reducing backflashover rates consists of decreasing tower-footing impedance ZP (or corresponding grounding resistance) [3,4]. For those lines installed over high resistivity soils, this can be reached by installing long counterpoise wires [5]. A second alternative consists of installing underbuilt wires along those spans that flanks towers with values of tower-footing impedance still exceeding the threshold values that leads to an acceptable number of outages. As discussed in [6], depending on the number of underbuilt wires (two or one) and of the tower-footing impedance value, the outage number can be significantly reduced by using this alternative,
⁎
reaching in certain cases a reduction comparable to that yielded by decreasing the tower-footing grounding impedance to the half-value. For those cases, in which the two practices mentioned above do not guarantee the desired lightning performance of the line, the installation of transmission line surge arresters in parallel with insulators can be adopted [7]. A surge arrester device has a V × I characteristic that exhibits very high impedance for steady state voltages (similar to an open switch for voltages amplitudes lower than a certain threshold) and a low impedance for overvoltages exceeding this threshold. TLSAs are applied to limit overvoltages between phase conductors and tower structure, preventing them to exceed the insulation withstand, that would lead to the occurrence of flashover across insulators [7,8]. The use of TLSA in all phases of a tower is extremely effective for preventing backflashover at that tower. Nevertheless, the cost of this practice is usually higher, as, in addition to the devices’ cost, it involves the installation and maintenance of the devices at the towers, and still requires a certain level of investment on tower-footing electrodes to ensure this practice to be effective. In some cases, this cost makes the large-scale application of such measure unfeasible. Thus, using TLSA only in a few “critical” towers along the line or using it only in one or two phases of the tower have been a relatively common practice [9].
Corresponding author. E-mail address: [email protected] (S. Visacro).
https://doi.org/10.1016/j.epsr.2019.106064 Received 5 April 2019; Received in revised form 9 August 2019; Accepted 9 October 2019 Available online 06 December 2019 0378-7796/ © 2019 Elsevier B.V. All rights reserved.
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Unfortunately, the technical literature does not properly address the constraints on applying these practices [10–12]. Installing TLSA to protect only one or two phases of a tower can let the unprotected insulator string of the stricken tower still susceptible to backflashover, depending on the tower-footing impedance. It is presumable that, even when TLSA are installed in all phases of a stricken tower, during their operation, overvoltages are transferred to adjacent towers and, if they are not protected with TLSA, under determined conditions of tower-footing impedance of the stricken and adjacent towers, the overvoltages can yield flashover there [8,9]. The literature does not discuss these constraints for providing the users with guidelines. This work addresses the effectiveness of using TLSA for improving the lightning performance of transmission lines by means of computational simulation, considering the mentioned questions and taking as reference the parameters of two typical real transmission lines. Fig. 2. Geometric parameters of the towers of the (a) 138-kV and (b) 230-kV transmission lines (average parameters).
2. Methodology of development To determine the constraints of TLSAs application, this work considered different schemes of installation of surge arresters and different conditions of tower footing-impedance. To develop a quantitative evaluation that allows illustrating better these constraints, the parameters of two real transmission lines were considered, a 138-kV singlecircuit and a 230-kV double-circuit transmission lines, represented in Figs. 1 and 2. Note that the quantitative results are valid specifically for these two lines. In the simulations, the lightning strike to the tower was represented by the direct impression of the current wave on the tower top. No lightning channel was represented, as it does not affect the insulators’ overvoltages calculated by using an electromagnetic model (like in this
work), different from approaches based on distributed circuit parameters [13]. The strike to the tower was assumed at the instant of zero power-frequency voltage at the critical unprotected phase. Attenuation and distortion of the voltage wave propagating along shield wires due to Corona effect were not considered. The overvoltages developed across insulators were calculated using the Hybrid Electromagnetic Model (HEM), widely applied to determine current and voltage distributions in lightning-related studies. Details of the HEM model, whose formulation is derived from electromagnetic field integral equations and results are provided in circuital quantities, are found in [14,15]. In particular, reference [14] validate the use of this model for calculation of lightning overvoltages, by exhibiting the very good agreement between calculated and measured overvoltages across the insulators of a transmission line. The voltage across insulators is determined as the difference of electric potentials at the two terminals of the insulator, which, in an electromagnetic model, is intrinsically equal to the integral of the electric field along the path between these terminals, across the insulator surface. A flashover criterion, consisting of the Disruptive Effect (DE) method, was applied for determining the critical currents IC (peak current leading the insulator to flashover). This method, which integrates the instantaneous value of the overvoltages during the interval it exceeds a certain threshold value related to the insulation withstand of the line (expressed by means of the line CFO) to assess whether this integral reaches the value required for flashover the insulator, is described in detail in [16,17]. The percentage of peak currents exceeding the critical current was calculated from the IEEE cumulative first-stroke peak current distribution [18]. Note that this percentage corresponds to the backflashover probability of that tower when it is subject to a lightning strike. Thus, multiplying this percentage by the number of strikes to the line allows promptly determining the expected number of outages. Reference [19] deeply discusses the application of the HEM model and of the DE method to determine the critical currents and the backflashover probability. The so-called double-peaked waveform [20], shown in Fig. 3, was adopted for representing the lightning current in simulations. This waveform exhibits the typical features of measured first return stroke currents, consisting of an initial concavity, an abrupt rise about the half peak that lasts until the first peak and the presence of a second peak, usually higher than the first. References [21,22] shows that this waveform is representative of the first return stroke currents measured at Monte San Salvatore and at Morro do Cachimbo stations. A discussion on the effect of the lightning current waveform on the lightning overvoltage and backflashover probability is presented in [23]. In this work, the simulated current wave reproduces the median values of current and time parameters of the first-stroke currents measured at Mount San Salvatore Station [24].
Fig. 1. Simulated condition: lightning striking the tower top of the (a) 138-kV single-circuit transmission line exhibiting 400-m long spans between adjacent towers, a single shield wire and a critical flashover overvoltage (CFO) of 650 kV; (b) 230-kV double-circuit transmission line exhibiting 500-m long spans, double shield wires and CFO of 1200 kV. Overvoltages calculated across the insulators of the stricken and adjacent towers. 2
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Fig. 3. Double-peaked current waveform used in simulations [20], exhibiting the median parameters of amplitude and time of the first-stroke currents measured at Mount San Salvatore Station: Ip = 31.1 kA, Ip1 = 27.7 kA, Td30 = 3.8 μs, Td10 = 5.6 μs, T50 = 75 μs. (as defined in [23]).
In simulations the tower footing was represented by its first-returnstroke impulse impedance (ZP), varying in a range from 10 to 40 Ω. As shown in [4,19], using this representation of electrodes practically leads to the same critical current and backflashover probability obtained under their physical representation. Using this ZP representation releases the need to consider the soil resistivity, which is the value that results in the assumed impulse impedance, considering the geometry of tower-footing electrodes in each case. The presence of the surge arrester was taken into account by positioning a variable resistance in parallel with the insulator, according to the device’s V × I curve. Such representation aims to resemble the characteristics of 132-kV and 198-kV-rated voltage surge arresters described in [25]. The effect of installing 3, 2 and 1 TLSAs (single circuit TL) and 6, 4 and 2 TLSAs (double-circuit TL) on the protected stricken tower and on the unprotected adjacent towers was assessed. This representation of surge arresters allows obtaining accurate results of overvoltages calculated across the insulators of the stricken tower, but impoverishes the quality of the results at adjacent towers.
Fig. 4. Reduction of the overvoltage across unprotected insulator strings of the stricken tower due to the use of 2-1 and 4-2 TLSAs, respectively in the singlecircuit and double-circuit transmission lines. Impressed current waveform of Fig. 3 and tower-footing impedance ZP = 20 Ω for the stricken and adjacent towers. Table 1 Improvement of the lightning performance of tower by using TLSAs under different tower-impedance values — 138-kV single-circuit transmission line.
3. Results and analysis 3.1. The effect on the stricken tower Using surge arresters to protect all phases of a TL tower clamps all the transient overvoltages resulting across insulator strings during a lightning strike to the tower and prevents the occurrence of backflashover at that tower. Using less surge arresters still decreases the overvoltage across the unprotected phase(s). However, it is required to assess whether this decrease is sufficient to prevent the backflashover occurrence. Fig. 4 illustrates the reduction of the overvoltage amplitude resulting from the use of two and one TLSA in the single circuit TL and of 4 and 2 TLSA in the double-circuit TL. The analyses of Table 1 reveal that using a single TLSA promotes a significant improvement of the lightning performance of the tower and this improvement tends to decrease with increasing tower-footing impedance. Improvements of about 29%, 27% and 19% were reached for tower-footing impedance of 10, 20 and 40 Ω. Much significant improvements of the tower performance were achieved using 2 TLSAs, respectively of about 45%, 42%, and 33%. On the other hand, the analyses also reveal that, even using 1 and 2 surge arresters per tower, the percentage of currents exceeding the critical current is still high, notably for high values of tower-footing impedance. It is about 15% and 37% for a single TLSA and 12% and 31% of the lightning strikes to the line, respectively for ZP of 20 and 40 Ω. This means that, depending on the rate of lightning incidence, the line can still present a significant number of outages.
ZP (Ω) Stricken towera
Condition
Ic (kA)
%I > Ic
Variation (%)
10
Without TLSA 1 TLSA 2 TLSA
83.7 96.1 106.3
7.0 5.0 3.9
– −28.6 −44.5
20
Without TLSA 1 TLSA 2 TLSA
51.8 59.8 66.3
20.9 15.3 12.2
– −26.7 −41.9
40
Without TLSA 1 TLSA 2 TLSA
32.9 37.8 42.2
46.2 37.4 31.0
– −19.1 −32.9
a
Same of the adjacent towers tower-footing grounding impedance.
As expected, the results of Table 2 show a much better performance, due to the higher CFO of 1.2 MV of the 230-kV TL relative to 0.65 MV of the 138-kV TL. They also reveal the significant increase of the critical current due to the use of surge arresters. For a 10-Ω tower-footing grounding impedance, IC increases from 134.2 kA to 150.7 kA and 175.2 kA, respectively for 2 and 4 installed TLSAs. This corresponds to decreases of the currents exceeding IC to 1.6% and 1.1% and respective improvements of the tower performance of about 26% and 50%. For a 20-Ω tower-footing impedance, IC increases from 101 kA to 127 kA and 148 kA for 2 and 4 TLSAs, decreasing the currents exceeding IC from 3
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improvement of the lightning performance of the stricken protected tower, their use can worsen the lightning performance of adjacent towers. The mechanism that yields this condition is simple. Surge arresters are usually installed at towers exhibiting high values of tower-footing impedance. Thus, even with the operation of surge arresters, high voltages are developed at the top of the stricken tower, resulting from the significant tower-footing grounding potential rise (GPR) added to the potential rise along the tower caused by the flow of the lightning current towards the ground. As the surge arrester operates, this high voltage is transferred by the phase conductor to the energized side of the insulators of the adjacent towers. On the other hand, a minor part of the incident lightning current is transferred to the adjacent tower by the shield wire(s). The flow of this current towards the ground produces a potential rise at this tower top. This corresponds to the sum of the GPR caused by the current at the tower-footing to the potential rise the current causes along the tower. Thus, overvoltages are established across the insulators of the unprotected phases of the adjacent towers, given approximately by the difference between the electric potential developed at the top and the potential transferred from the stricken tower due to the operation of the surge arresters. If the tower-footing impedance of the stricken tower is very high and that of the adjacent tower is very low, the overvoltage across the insulator can be also very high, leading it to flashover, if it is not protected by a surge arrester. Differently, if the two tower-footing impedance values are comparable, the overvoltage tends to be low, preventing the flashover occurrence. The occurrence of this mechanism was assessed in this work by determining the percentage of currents exceeding IC at the adjacent towers when the arrester(s) of the stricken tower operate(s). To obtain this percentage, the lightning overvoltage across the adjacent towers’ insulators was determined in the same simulated conditions of Table 1 and 2. One more condition was simulated, consisting of surge arresters protecting all phases of the stricken tower. The corresponding results are presented in Tables 4 and 5. Only for providing quantitative references, the percentages of currents exceeding IC for the unprotected insulators of the stricken tower was included in the tables (into brackets). The results in Tables 4 and 5 are all consistent with the analysis of the mechanism of overvoltage transfer described in this section. As expected, these percentages of peak currents exceeding IC are significantly lower for the double-circuit TL. On the other hand, the tables reveal some trends valid for both TLs. First, the percentage of currents exceeding IC in the adjacent towers increases with increasing tower-footing impedance of the stricken tower and decreasing towerfooting impedance of the adjacent tower. This percentage also increases with the increasing number of surge arrester installed at the stricken tower. Following this trend, the critical simulated condition consists of
Table 2 Improvement of the lightning performance of tower by using TLSAs under different tower-impedance values — 230-kV double-circuit transmission line. ZP (Ω) Stricken towera
Condition
Ic (kA)
%I > Ic
Variation (%)
10
Without TLSA 2 TLSA 4 TLSA
134.2 150.7 175.2
2.2 1.6 1.1
– −25.8 −49.8
20
Without TLSA 2 TLSA 4 TLSA
101.1 126.8 148.2
4.4 2.5 1.7
– −43.6 −62.1
40
Without TLSA 2 TLSA 4 TLSA
64.8 92.7 111.0
12.8 5.5 3.5
– −57.3 −72.7
a
Same of the adjacent towers tower-footing grounding impedance.
4.4% to 2.5% and 1.7%. This corresponds to improvements of 44% and 62% of the tower performance. For a 40-Ω tower-footing impedance, the corresponding improvements are of 57% and 73%. In spite of these improvements, depending on the lightning incidence rate, backflashovers across insulators of the unprotected phases of the stricken tower can still cause a significant number of outages, even using 2 or 4 surge arresters per tower. A complementary analysis of practical interest specifically for double-circuit lines consists of assessing the effect of concentrating the TLSAs in a single circuit in relation to their distribution in both circuits. Table 3 exhibits the results corresponding to 1, 2 and 3 TLSAs installed in a single circuit of the 230-kV line. For the sake of comparison, the table includes some results corresponding to the distribution of TLSA in both circuits. Note that using 2 TLSAs in a single circuit is less efficient than distributing them in both circuits in the usual conditions that would require the use of surge arresters (ZP of 20 and 40 Ω). The results is different for a 10 Ω impulse impedance, but this is not a usual condition that requires installing TLSAs. On the other hand, though using 3 TLSAs in a single circuit seems less efficient than distributing 4 of them in both circuits, this arrangement ensures the continuity of the operation of this fully protected circuit. Thus, in certain applications, this could be advantageous. 3.2. The effect on the adjacent towers An additional constraint of using TLSA concerns the effect of their operation on adjacent towers. As discussed next, in spite of the Table 3 Improvement of the lightning performance of tower by using TLSAs in a single circuit of 230-kV double-circuit transmission line. ZP (Ω) Stricken towera
a
TLSA distribution in a single circuit
Table 4 Percentage of currents expected to exceed the critical current able to cause backflashover at the adjacent towers — 138-kV transmission line.
TLSA distribution in both circuits
Number of TLSA
Ic (kA)
%I > Ic
Number of TLSA
Ic (kA)
%I > Ic
10
1 2 3
144.2 159.3 171.1
1.81 1.40 1.16
– 2 4
– 150.7 175.5
– 1.61 1.09
20
1 2 3
111.9 120.0 125.6
3.43 2.88 2.57
– 2 4
– 126.8 148.2
– 2.50 1.68
40
1 2 3
73.2 78.7 82.5
9.69 8.14 7.29
– 2 4
– 92.7 111.0
– 5.48 3.50
ZP (Ω) stricken tower
ZP (Ω) adjacent tower
%I > IC (%) 1 TLSA
%I > IC (%) 2 TLSA
%I > IC (%) 3 TLSA
10
10 20 40
0.70 (5.01)a 0.47 0.21
0.87 (3.9) 0.60 0.30
1.13 (0) 0.79 0.41
20
10 20 40
1.75 1.15 (15.3) 0.53
2.24 1.54 (12.2) 0.76
3.20 2.23 (0) 1.13
40
10 20 40
4.56 3.18 1.63 (37.4)
5.76 4.17 2.27 (31)
8.37 6.16 3.50 (0)
a The values into brackets correspond to the percentage of currents exceeding IC for the unprotected insulators of the stricken tower.
Same of the adjacent towers tower-footing grounding impedance. 4
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Table 5 Percentage of currents expected to exceed the critical current able to cause backflashover at the adjacent towers — 230-kV transmission line. ZP (Ω) stricken tower)
ZP (Ω) adjacent tower
%I > IC (%) 2 TLSA
%I > IC (%) 4 TLSA
%I > IC (%) 6 TLSA
10
10 20 40
0.21(1.6)a > 0.1 > 0.1
0.27 (1.1) > 0.1 > 0.1
0.29 (0) 0.2 0.1
20
10 20 40
0.51 0.31(2.5) 0.12
0.64 0.41 (1.7) 0.17
0.69 0.45 (0) 0.20
40
10 20 40
1.21 0.8 0.36 (5.5)
1.52 1.03 0.51(3.5)
1.62 1.12 0.58 (0)
a
The values into brackets correspond to the percentage of currents exceeding IC for the unprotected insulators of the stricken tower.
the 40 Ω for the tower-footing impedance of stricken protected tower and 10 Ω for that of the adjacent tower. In Table 4 (single-circuit TL), about 4.6%, 5.8% and 8.4% of the lightning strikes to the protected tower would lead the adjacent tower to flashover, respectively for 1, 2 and 3 TLSA installed at the tower. The corresponding percentages decrease to about 1.2%, 1.5% and 1.6% for 2, 4 and 6 TLSA installed at the adjacent tower of the double-circuit TL, as shown in Table 5. The percentage of currents exceeding IC at the adjacent tower is very low in comparison with that at the stricken tower, for the conditions of TLSAs protecting one and two phases (single-circuit TL:1 and 2 installed TLSAs; double circuit TL: 2 and 4 TLSAs). However, this percentage becomes relatively relevant when all phases of the stricken tower are protected by TLSAs, reaching the order of about one third to one fifth of the percentage of peak currents leading insulators of the stricken tower to flashover, in the condition of two and one phases protected. Fig. 5 summarizes the previous results. Note that the percentages are expressed in terms of an equivalent parameter: the probability of flashover at the adjacent towers. It denotes that this probability is significant only for protected towers exhibiting a high tower-footing impedance. Additionally, it shows that the significance is enhanced with decreasing tower-footing impedance of adjacent towers. Both aspects are clearly depicted in the three upper curves of Fig. 5(a) and (b). The results above suggests addressing some questions of interest for applications. First, the probability of flashover at the adjacent towers is much lower than that at the stricken tower when only one or two phases are protected. The peak currents, which would lead insulators of the adjacent towers to flashover, are already included among those peak currents causing backflashover at the stricken tower. Thus, in the analysis of the stricken tower performance, the contribution of flashovers at adjacent towers for the conditions of one and two phases protected can be disregarded. The situation is different for the condition of fully protected stricken tower (all phases protected by TLSAs). This relatively significant flashover probability (of about 8.4%, 3.2% and 1.1% for the singlecircuit TL and 1.6%, 0.7% and 0.3% for the double-circuit TL and 10-Ω grounding impedance of adjacent tower) has to be attributed to the probability of flashover of the stricken tower at the condition of all phases protected. This means that, even in the case of fully protected tower, there will be a probability of outage due the mechanism of surge transfer to adjacent tower, expressed by the percentages indicated above. Thus, concerning the practical aspects related to the impact of lightning strikes to towers protected by surge arresters on the adjacent towers, the only relevant contribution consists of that of flashovers at adjacent towers with low tower-footing impedance flanking fully protected towers with high tower-footing impedance.
Fig. 5. Probability of flashover (in percentage) across insulators of adjacent towers due to strikes to a protected tower exhibiting different tower-footing impedances (10, 20 and 40 Ω) as a function of the tower footing impedance of adjacent towers: (a) 138-kV and (b) 230-kV transmission lines.
4. Relevant practical implications To allow discussing the results developed for the single- and doublecircuit TLs, the lightning performance of their towers was calculated from the results of Section 3A. This performance was translated as the average probability of flashover per tower. As the results of Section 3 corresponds to direct lightning strikes to the tower, the estimated percentages of currents exceeding IC was multiplied by a factor of 0.6, for taking into account the lower overvoltages of strikes to shield wire (s) distributed along the span, in the region of the tower influence [18]. Fig. 6 exhibits the curves expressing the expected performance of protected towers of the TLs as a function of their tower-footing impedance, for the considered conditions of surge arrester installation. Specifically, the curves for 3 and 6 TLSAs in Fig. 6(a) and (b), respectively, correspond to the percentage of flashover at adjacent towers. The figure also includes dashed lines corresponding to the maximum acceptable probability of backflashover per tower. This threshold was calculated for the tower of each line, assuming a defined flash density Ng of 4 flashes/km2/year and the maximum acceptable numbers of 5 and 2 outages/100-km/year for the 138-kV and 230-kV TLs. The respective numbers Ns of 86 and 107 strikes/ 100-km/year to the lines was estimated by using the following expression Ns = Ng⋅[2⋅14(h0.6) + D]/10, given in IEEE Standard 1243 [18]. In this expression, whose unit is outages/100-km/year, h and D correspond to the average height of shield wire(s) and distance between them, respectively. This resulted in maximum acceptable flashover probabilities of 5.8% (5/ 86) and 1.4% (2/142) for the 138-kV and 230-kV TLs, respectively. The curves of Fig. 6 reveal some important practical aspects. Each tower exhibits a performance governed by the value of its tower-footing 5
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great influence on these limits. 5. Conclusions By developing quantitative evaluations of the lightning performance of towers of a 138-kV and a 230-KV transmission lines, this work discussed the constraints and the effectiveness of installing TLSAs for improving the lightning performance of transmission lines. The impact on the unprotected insulators of the stricken tower and of the transfer of the risk of flashover to adjacent towers due to surge arresters’ operation was addressed. The results of this work clearly denote the need to observe the minimum tower-footing conditions for an efficient application of TLSA to improve the lightning performance of TLs, notably when some of the phases are not protected. In the conditions considered in this work, for the 138-kV TL, the limiting tower-footing impedance values were found to be about 14, 17 and 40 Ω, respectively for 1, 2 and 3 installed TLSAs. The corresponding values for 2, 4 and 6 TLSAs installed at the 230-kV TL were about 18, 27 and 48 Ω. It is remarkable that, even when all the phases of the tower are protected, a maximum value of its tower-footing impedance has to be observed to prevent the tower to develop a lightning performance worse than the acceptable. This shakes the impression that simply using TLSAs allows achieving a satisfactory performance of a TL. The qualitative results and analyses developed in this work have general validity. The quantitative results depend on the environment and TL characteristics, notably the local flash density and TL geometry. This leads to the need of developing specific evaluations for defining the efficient use of TLSAs for the particular conditions of the transmission line under consideration. This can be done using procedures similar to that developed in this work. Declaration of Competing Interest
Fig. 6. Lightning performance of the transmission line (backflashover probability in percentage), considering different conditions of TLSA installation and tower-footing impedance: (a) 138-kV and (b) 230-kV. Assumed conditions: Ng = 4 flashes/km2/year. Maximum acceptable outages of 5 and 2 outages/ 100-km/year, respectively.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment
impedance and the condition of surge arrester installation. In general, the performance of the 230-kV TL is much better, exhibiting backflashover probabilities for ZP of 40 Ω ranging from 1% to 7.7% in relation to the values of about 5% to 27% for the 138-kV TL. In order to prevent developing a performance worse than that given by the dashed line, the tower-footing impedance of each tower has to be lower than a specific value, corresponding to the condition of TLSAs installation. The arrows in Fig. 6(a) indicate maximum acceptable tower-footing impedances of 12, 14, 17 and 41 Ω, for the conditions of without, with 1, 2, and 3 TLSAs installed at the tower of the 138-kV TL. These impedances correspond to low-frequency grounding resistances of about 18, 22, 26 and 60 Ω, respectively. The corresponding impedance values given in Fig. 6(b) for the 230-kV TL are higher, respectively of 11, 18, 27 and 48 Ω. Their respective associated low-frequency resistance are about 17, 28, 41 and 70 Ω. In particular, it is interesting mentioning the limiting values of about 60 and 70 Ω for the tower-footing impedance of the 138-kV and 230-kV TLs in the condition of all phases protected by surge arresters, as these maximum values were defined from flashovers occurring at adjacent towers. The qualitative results of the analyses developed in this section have general validity. Nevertheless, the values of the threshold backflashover probability is highly dependent on the flash density along the line. Densities above the assumed 4 flashes/km2/year would decrease this threshold probability and, therefore, would decrease the acceptable limits of tower-footing impedance in each condition of TLSAs installation. The characteristics of the line, notably the tower height, has also
Silverio Visacro and Fernando H. Silveira would like to acknowledge the financial support provided by the Brazilian National Council of Technological and Scientific Development (CNPq) for their participation in this work, processes 310564/2016-6 and 308351/2018-5, respectively. References [1] S. Visacro, Direct strokes to transmission lines: considerations on the mechanisms of overvoltage formation and their influence on the lightning performance of lines, J. Lightning Res. 1 (2007) 60–68. [2] S. Visacro, ArtLiber (Ed.), Lightning: an Engineering Approach, (in Portuguese), 2005, pp. 1–272 São Paulo, Brazil. [3] S. Visacro, A comprehensive approach to the grounding response to lightning currents, IEEE Trans. Power Deliv. 22 (January (1)) (2007) 381–386. [4] S. Visacro, The use of the impulse impedance as a concise representation of grounding electrodes in lightning protection applications, IEEE Trans. Electromagn. Compat. 60 (5) (2018) 1602–1605, https://doi.org/10.1109/TEMC.2017.2788565. [5] S. Visacro, F.H. Silveira, Lightning performance of transmission lines: methodology to design grounding electrodes to ensure an expected outage rate, IEEE Trans. Power Deliv. 30 (2015) 237–245. [6] S. Visacro, F.H. Silveira, A. De Conti, The use of underbuilt wires to improve the lightning performance of transmission lines, IEEE Trans. Power Deliv. 27 (2012) 205–213. [7] Outline of Guide for Application of Transmission Line Surge Arresters—42 to 765 kV Extended Outline – 1012313, Oct.2006. [8] EPRI - Handbook for Improving Overhead Transmission Line Lightning Performance (technical report). [9] S. Visacro, F.H. Silveira, B. Pereira, R.M. Gomes, Concerns on surge-arrester application for improving the backflashover rate of transmission lines, Proceedings of International Conference on Lightning Protection (ICLP2018), Rzeszow, Poland,
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S. Visacro, et al. 2018. [10] R. Shariatinasab, J. Gholinezhad, K. Sheshyekani, Estimation of energy stress of surge arresters considering the high-frequency behavior of grounding systems, IEEE Trans. Electromagn. Compat. 60 (August (4)) (2018) 917–925. [11] K. Munukutla, V. Vittal, G.T. Heydt, D. Chipman, B. Keel, A practical evaluation of surge arrester placement for transmission line lightning protection, IEEE Trans. Power Deliv. 25 (July (3)) (2010) 1742–1748. [12] J. He, J. Hu, Y. Chen, S. Chen, R. Zeng, Minimum distance of lightning protection between insulator string and line surge arrester in parallel, IEEE Trans. Power Deliv. 24 (April (2)) (2009) 656–663. [13] Z.G. Datsios, P.N. Mikropoulos, T.E. Tsovilis, Effects of lightning channel equivalent impedance on lightning performance of overhead transmission lines, IEEE Trans. Electromagn. Compat. 61 (June (3)) (2019) 623–630. [14] S. Visacro, A. Soares Jr., HEM: a model for simulation of lightning-related engineering problems, IEEE Trans. Power Deliv. 20 (April (2)) (2005) 1026–1028. [15] S. Visacro, F.H. Silveira, Evaluation of current distribution along the lightning discharge channel by a hybrid electromagnetic model, J. Electrostat. 60 (2–4) (2004) 111–120. [16] H. Hileman, Insulation Coordination for Power Systems, CRC, Boca Raton, FL, 1999, pp. 627–640. [17] R.O. Caldwell, M. Darveniza, Experimental and analytical studies of he effect of non-standard waveshapes of the impulse strength of external insulations, IEEE
Trans. Power App. Syst. PAS-92 (July (4)) (1973) 1420–1428. [18] IEEE Std 1243, Guide for Improving the Lightning Performance of Transmission Lines, (1997). [19] S. Visacro, F.H. Silveira, Lightning performance of transmission lines: requirements of tower-footing electrodes consisting of long counterpoise wires, IEEE Trans. Power Deliv. 31 (2016) 1524–1532. [20] A. De Conti, S. Visacro, Analytical representation of single- and double-peaked lightning current waveforms, IEEE Trans. Electromagn. Compat. 49 (May (2)) (2007) 448–451. [21] S. Visacro, A representative curve for lightning current waveshape of first negative stroke, Geophys. Res. Lett. 31 (April (L07112)) (2004) 1–3. [22] F.H. Silveira, S. Visacro, Lightning parameters of a tropical region for engineering application: statistics of 51 flashes measured at Morro do Cachimbo and expressions for peak current distributions, IEEE Trans. Electromagn. Compat. (2019), https:// doi.org/10.1109/TEMC.2019.2926665. [23] F.H. Silveira, S. Visacro, Lightning performance of transmission lines: impact of current waveform and front time on Backflashover occurrence, IEEE Trans. Power DelIV (2019), https://doi.org/10.1109/TPWRD.2019.2897892. [24] R.B. Anderson, A.J. Eriksson, Lightning parameters for engineering application, Electra 69 (1980) 65–102. [25] Siemens, High-Voltage Surge Arresters — Product Guide, (2019).
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