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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14) ISSN 1843-6188 SOME ASPECTS CONCERNING THE INFLUENCES OF SURGE ARRESTERS UPON LIGHTNING BEHAVIOR OF OVERHEAD LINES Iuliana HRISCU, M. GUSA, M. ISTRATE E.ON Moldova Distributie Iasi, [email protected] Technical University Iasi [email protected], [email protected] handling, less sealing problems, better overload performance, reduced cost and market prices – are very important when thousands of arresters shall be widely distributed over a utility's distribution system. Furthermore, distribution systems have the highest need for line arresters, as they are usually not protected by shield wires and their pole footing impedances are high. The situation is different for transmission lines. In the most cases they are reasonably shielded and have acceptably low footing impedances. Even at locations, where this is not the case, a decision for applying line arresters is not as simple as for distribution lines due to technical and/or economical constraints. Today, all over the world (Fig. 1), line surge arresters are installed on overhead lines of transmission systems with voltages up to 800kV. However, comparing with distribution systems and despite their many advantages, the installation of surge arresters in transmission systems represents a phenomena with a low speed evolution. Abstract: Main external cause for overhead lines outage that may bring negative consequences from technical and economical point of view is represented by lightning strokes. Researches upon this phenomenon has been focused upon various aspects, starting with the lightning strike itself and its consequences and following with finding solutions for better protection of overhead lines. Many countermeasures for improving the lightning performance of the lines have been investigated. Regarding the lightning behavior of overhead lines, the influence of surge arresters, used in various configurations, on the insulation stress is of much interest. The aim of this paper is to study the behavior of an 110kV single-circuit overhead line when surge arresters in different configuration are used. The analysis underlines the effect of using different values of tower foot resistance and lightning current amplitude. Keywords: surge arresters, lightning current, overhead lines 1. INTRODUCTION In order to prevent flashover effects, metal oxide surge arresters, installed in parallel with overhead line insulators, were used in distribution systems since 1975 (first time in Japan, in a 33 kV system). In the ‘80, again in Japan were installed for the first time overhead line surge arresters in 66 kV, 77 kV and 138 kV systems. Many other utility companies – not only the Japanese ones – have followed this example, therefore today in power distribution systems are installed and operate several tens of million of arresters. Apart from general well-known advantages of gapless surge arresters, by comparison with the old SiC arresters and protective gaps, this tendency was sustained by developing in the 1980`s of composite housing for distribution systems surge arresters. The benefits of these technologies like less weight, easy of Figure 2 Arresters installed on 77 kV overhead lines 2. FIELD EXPERIENCE Several studies were made so far, in order to analyze the efficiency of arrester installation methods, for example only on the upper phase, on all three phases or other combinations. In Japan are frequently used three main equipping modes, depending on the number of circuits of the line (Table 1): # 1 – all three phases of one circuit on a two circuits line; # 2 – all three phases of both circuits of the line; # 3 – one or two phases on a circuit of two circuits line [1] . Another important aspect is the connecting mode of the arrester: directly at the ends of protected insulator string or in series with an external gap. Figure 1 Arresters used on overhead lines all over the globe 64 ISSN 1843-6188 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14) From Japan experience gained in this matter by the year 1998, some conclusions could be drawn, regarding the insulation coordination. Firstly, in order to obtain an efficient operation of the arrester, it is essential a correct grading of spaces between gap and lower end of the insulator string. Secondly, in order the arrester to efficiently interrupt the impulse current in a very short time even in polluted environment and bad weather, it is necessary to determine correctly the gap distances and to use an arrester housing specially created for these conditions. From the switching voltages and tripping point of view, it was noticed that arrester performances rise directly proportional with gap length. But, from insulation coordination point of view, it was noticed that performances are better when the gap distance are shorter. In conclusion, it can be said that before using arresters on overhead lines, detailed studies on efficiency of different methods of installing and Figure 3 Compact line model using line arresters In 2008, CIGRE Conference in Croaţia, focused the gained experience in line arresters utilisation field from different parts of the world. One of the many cases of line arresters application presented at this Conference is one of the 123 kV Ston – Komolac line, from southern Croatia, an one-circuit line 44 km length, protected with ground wire and that crosses an area with intense lightning activity and high keraunic coefficient up to 70 days/year. Furthermore, due to very high soil resistivity in that area, the tower foot resistance is very difficult to be maintained below an accepted value. Therefore this line was considered to have very low performances regarding lightning behavior. In the summer of 2007, 110 arresters were installed hanging below the phase conductor (as in Fig.4), on some of the 144 towers of Ston – Kolomac line, in order to improve line performances with 50 – 60%. Between the installation moment and when that article was presented, 8 months have passed so that, as the authors point out, the period was too short to generalize the resulted conclusions. But in this period, only 4 outages occurred, equivalent to 6 outages per year rate, and that can be seen as an improvement of line behavior by 52%. If this outage rate maintains itself at a low value, it will be considered to extend this pilot project to other important transport lines. [3] Along with the utilization of arresters on above mentioned line, 61 of the arresters installed on the most exposed towers, were equipped with Excount-II monitoring sensors, (Fig 5) with main goal to determine arresters behavior on line in moments when surges occur. This real time scanning intelligent system allow remote control and wireless data acquisition, as Table 1 Application methods on OHL circuits coordination of all component elements are needed. [1] In Nordic states, appeared, starting with 1990, an increasing demand of 420kV compact overhead lines, especially in the residential areas, in order to reduce electromagnetic field intensity and visual impact. Because this area is characterized by a relatively low keraunic coefficient (< 20 days/year), the absence of grounding wires and application of line arresters was considered as a possible solution. In this matter, a feasibility study, that considered the application of line arresters on upper phase at each tower in the compacted line area (Fig 3) in order to protect the line to lightning strikes was developed. In this way, the upper phase behaves like a grounding wire, assuring a 30º shielding angle for lower phases. Compacting the line and implicit reducing the magnetic field were more important than reducing the number of outages due to lightning strikes, so the results were considered satisfying when a reduction with 50%, of magnetic field and a reduction of tower pattern was obtained as well. ELECTRA Magazine published in October 1999 number, an article with the main scope to classify the requirements that arresters must accomplish in order to operate properly, a description of functioning mechanism, an arresters classification and the standard tests that they must pass.[2] Figure 5 Line arresters with monitoring sensors incorporated 65 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14) ISSN 1843-6188 after installation of 21 arresters. In the next years, the outages rate maintained at a very low level. In Brazil, Companhia Energética de Minas Gerais (CEMIG), the main utility company in Minas Gerais area manages 30 substations and approximately 5000 km of overhead lines, outspreaded in south-east Brazil, on an area equivalent to France. [5] The main problems of CEMIG regarding overhead line service are generated by two factors: the keraunic coefficient of the area and soil resistivity of some regions. This environmental conditions determined CEMIG to adopt some measures to avoid energy losses, financial penalties and clients complains. Figure 4. Line arresters hanging on phases of an OHL well as monitoring the arresters activity revealing the number and location of surge levels and current flowing. [4] China is one of the countries with most widely applications of line surge arresters. Since ‘90, there were installed several thousand arresters on lines operating at voltages between 35 kV and 220 kV, encountering no problems in operation. As in many other countries, also in China lightning strike are the main cause for 40 – 70% of outages number, mostly in areas with high lightning frequency, high soil resistivity and rough terrain. For example, in near 20 years of operation, 19 of 33 outages registered on 9 of the most important transmissions lines operating at 500kV in Hubei area, resulted from lightning strikes. Among all protection methods used on international level as: installing of shield wires, reducing the protection angle, reducing tower foot resistance, increase the insulation level, the most efficient was installing line arresters in parall with line insulator strings (Fig 6). Using line arresters, the outage number due to lightning strikes decreased significantly. For example, on 110 kV Xi-bai from Guangdong area, the outage rate decreased from 15,5 outages per 100 km per year Figure 7 Line arrester mounted on a 400kV sustaining tower One of the lines with low performances due to lightning strikes is OHL 230kV Guilman Amorim – Ipatinga 1, which in period 2000-2001 registered 6, respective 5 outages. First actions in analyzing and improving the performances were related to measuring of tower foot resistance, which proved to be at very high levels. In these conditions, the solution was the installation of line arresters on overhead lines. So, through analysis and modeling, it was obtained the optimum method to equip the towers, according to the tower foot resistance values. All these actions had satisfactory results that reflected upon energy quality and clients satisfaction. [5] In Romania, national company SC Transelectrica SA who manages the transmission system with voltages above 220kV, also confronted with the necessity of improving both the reliability of transmission lines and power quality offered to the customer. So, the company focused on 220kV and 400kV overhead lines, especially on the segments that cross areas with high keraunic coefficient and high soil resistivity, where shielding wires are missing. In a first stage, installing the line arresters parallel with every insulator string on OHL 400kV Braşov–Gutinaş sector 130-145 was decided. (Fig. 7) [6] Results show a decisive decrease of flashover rate on this OHL after installing the line arresters, meaning that this solution is justified both economical and technical. There are still some operating problems due Figure 6 Line arrester mounted on a sustaining tower in 1999, to 4 outages per 100km per year in 2000, 66 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14) ISSN 1843-6188 The comparative results between the cases in which two types of arresters were used, presented in Fig. 9, shows that the arrester type has no significant influence upon the outages rate on the unprotected circuit 2, when protected circuit 1 is equipped on the external phases. [7] As mentioned before, the arresters application on overhead lines was studied also as a solution of reducing the visual impact of the lines. Two researchers from Denmark have made an analysis on a 400 kV line with the configuration presented in Fig. 10, in order to find if ground wires can be eliminated and arresters can be used to assure line protection instead. So, using PSCAD/EMTDC they have modeled a line with frequency depending distributed parameters. To the obtained model (which includes also shield wires) were applied lightning current impulses of 100kA – 1.2/50µs, considering that this value can be applied to the system without producing the insulation flashover at a certain tower. The conclusions show that in a this kind of configuration of the 400kV OHL, from the protection level point of view, only the protection with arresters on upper phase at all towers approaches to Figure 10 the protection level 400kV line configuration obtained with ground in Denmark wires, considering that in this study only lightning strikes on upper phase were taken into analysis. So, in order to renounce to the shield wires in favor of line arresters application on the upper phase, this analysis must be extended and must be overviewed the lightning strikes on the other two phases. Another conclusion shows that renouncing to the shield wires, installing of line arresters might demand a resizing of the geometrical distances in tower configuration, in the direction of increasing them. In this case, must be reconsidered the visual impact created by these modifications, and compared both from the technical-economical point of view and safety in operation. As it can be seen in those presented above, line metaloxide arresters application on overhead lines in order to protect them, remains a modern solution that can prove to be both reliable and viable from the technicaleconomical point of view. But, the complexity of the phenomena that occur when lightning surges appear and arrester function leave enough space for questions and opportunity studies. Figure 8 Line arrester separated by phase conductor by connection assembly breakage to some errors in assessment of area environmental stresses. These leaded to wrong dimensioning of the arrester assemble and appearance of a weak point. Due to extremely strong wind, the connection device broke and three of the arresters were disconnected from the phase conductor. Because of the same strong wind, the arresters turned and the suspension shackle was unscrewed from the arresters upper fixture, leading to arresters falling down and be damaged. It is shown in Fig 8 a part of the connecting wire hanging (red) and another on the phase wire (yellow). So, these problems were solved, in order to eliminate the damages related to mechanical behavior. But, using the line arresters on OHL is still both a modern one and viable one, in order to improve the stability of power system. 3. RESEARCH RESULTS From the theoretical point of view, the operation of line arresters was modeled for different types of lines and different installing methods, in different programming environments, most frequent being used EMTP. French researchers, in collaboration with HydroQuebec Canada, have modeled in a reviewed version of EMTP (EMTP-RV), a 400 kV double-circuit line and the results were presented in 2008 in Croatia. The line is protected with two ground wires, but a significant number of towers are situated in mountain areas where an optimal value for the tower foot resistance is hard to obtain. Therefore it was suggested to use line arresters. Firstly it was decided to make a detailed study of the technology to use was carried out, concerning the type of arrester best fitted (gapped or gapless), the arresters class, the towers where the arresters will be installed and the phases that will be protected. [7] 4. THEORETICAL BASIS Figure 9. Flashover rate on circuit 2, when both external phases of circuit 1 are protected by arresters gapless When lightning strikes the tower top, the impulse current flows through the tower and its foot resistance and gapped 67 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14) to earth. If the ground wires are missing, the voltage applied to the insulators strings can be considered equal to tower potential at the console level. When ground wires are present, a contribution is added by the coupling with the phase conductors. The stress of line insulation appears only at the stroked tower, and when ground wire are present, to the next towers too. If a flashover appears on an insulation string, a part of the lightning current will be taken over by the phase conductor. The size of this component depends on ration between surge impedances of conductor and tower. Therefore, current waves propagate in both directions apart from stroken tower creating insulation stresses at the adjacent towers. Insulation stresses are mainly influenced by three factors: lightning current amplitude, lightning current front duration and tower foot resistance. Another parameter, tower surge impedance can be considered constant because it is identical for all towers of a line. A problem that can be solved by assisted simulation is the rate of voltage drop along the tower, which influences the stress on insulation of different phases and complicates in this way the determination of critical current level. Because the tower model is a RLC circuit, the front time of the lightning current wave influences mainly the voltage drop on inductance, so it is expected that while the front of current impulse gets shorter, this voltage drop become more important. When grounding wires are missing, lightning can strike the phase conductor and the lightning current propagates in both directions, inducing the appearance of a potential wave equal to ItZca/2. If an insulation flashover does not appear, the voltage wave propagates and attenuates due to losses on conductor resistance and corona discharge. On the un-stroked phases, lower voltages are induced by the coupling with stroked phase. When the lightning strikes the ground wire between the towers, the lightning current divides in two equal components which flow in opposite directions and arriving to the towers, they will draw a part of it, so the amplitude of current flowing forward on the ground wire decreases gradually. ISSN 1843-6188 penetration so that the lightning might strike a phase conductor. The tower is considered as a circuit with uniform distributed parameters, without losses, having three segments: the first, between tower top and R phase console, the second, between R phase console and S/T phases console the last, between S/T phases console and footing resistance. In this way, the voltage distribution along the tower can be observed. The tower surge impedance can be computed with rel. (1) 2h ZT 60 ln 2 1 r (1) Figure 11 Lightning strike in an OHL with ground wire in absence/presence of arrester where r is the radius of the tower considered as cylindrical.[8] The behavior of the overhead line was studied regarding the insulation stress and current amplitude through arresters, in some cases: with and without grounding wire and with and without arresters simultaneously. For all above mentioned cases a parametrical study was accomplished, considering credible ranges of variation for main factors: the front time of the current impulse and tower foot resistance, keeping the same amplitude of lightning current. In the first step they were used both ramp and standard double-exponential shaped current impulse, in order to verify the results obtained analytically and to confirm the accuracy of simulations. A first observation is that in case of lightning strike to the top of tower, homologue values of voltages on phase insulation strings and tower foot resistance are considerably higher when standard shaped current impulse was used. The explanation is that standard shape impulse has variable front slope and the maximum one is higher than that in case of ramp shape impulse with the same front time. In case of lightning strike in the span, the simulations conducted to similar results for both shapes of the lightning current impulse. So, in order to be closer to the reality, in the foregoing simulations only standard double-exponential shaped current impulse was used. Also in the foregoing 5. CASE STUDY It will be considered a one circuit 110 kV overhead line, build on concrete towers. The ATP model of line is made by 10 spans of 200 m ended by two segments of 25 km with length, in order to avoid the influence of the reflected waves from the opposite end. The line is modeled as a traveling waves circuit. In order to observe easier the phenomena that will appear, the symmetry of ATP model will be preserved relating to the striking point. If the lightning current strikes at the span middle, the symmetry of the model will be maintained by adding a new span, equally divided by the impact point. When grounding wire is present it is considered implicitly that it will capture the lightning strike, excluding the possibility of shielding 68 ISSN 1843-6188 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14) observations, for easy comparing the results, the current impulse front time will be 1.5 s. Considering the lightning strike at the tower top when ground wire is present (Fig. 11) and comparing the line behavior in absence of arresters (Fig. 12), respectively of arrester presence on R phase (Fig. 13), the following conclusions may be drawn: - in case of lightning strike at tower, the presence of the arrester limits the voltage on protected phase insulation to 15% up against the case in which the arrester is missing; - when the arrester is missing, the voltage top value on R phase at the adjacent tower decreases with 50%; - the presence of the arrester on a phase (R in this case) limits the voltage top value only on the protected insulation strings, while the stresses of same phase insulation on adjacent towers is at least equal to that in case the arrester is missing (if not higher); - for the other two unprotected phases (S and T), the top voltage value on insulation strings at hited tower drops with aproximately 30% when arrester is present on R phase. From the current through arresters point of view, considering the case when the lightning hits the tower, and the arrester is present only on R phase, the current through arresters represents 20% of lightning current if the line is not protected with grounding wire and 10% when the grounding wire is present. If the insulation on S phase is also protected by an arrester, the value of the current through this arresters drops at 10%, respectively 7% of the lightning current. The presence of arrester also on the T phase does not change very much the current distribution, unless maybe in a small rate the current through S phase arrester (t predictible aspect). Coming back to the more conservative cases, when the current impulse front time varies beteen 0,5 s and 3 s, in order to see its influence and the influence of tower foot resistance, following conclusions can be drawn, valid in all studied cases: - increasing the time of lightning current slope reduces the voltage levels on phase line insulation. The slightest dependence appears for voltages on tower foot resistance, which is expected to happen because they are modeled only by their dispersion resistance. The tower potential and stresses on insulation strings decrease when the time of current slope increases, because the tower inductance component Ldi/dt, reduces accordingly. - the tendency of voltages decreasing when of lightning current front time increases depends on the tower foot resistance too, becoming smaller while the foot resistances become higher. 1 UR 2 3 4 5 1 US 2 3 4 5 Figure 12. Lightning strike at tower top (GW present) Stresses on hit tower insulation and the following 4 towers 2 3 UR 4 5 1 1 US 2 3 4 5 Figure 13. Lightning strike at tower top (GW present – arrester on R phase) . Stresses on hit tower insulation and the following 4 towers Before install arresters on overhead lines, detailed studies are necessary concerning their behavior, the efficiency of different methods of installation and coordination of component elements. ; The presence of arresters assures the insulation protection of the respective tower, but might increase the stresses on insulation of the adjacent towers; The increase of front time of the lightning current impulse reduces the voltages on line phases insulation; The decrease of stresses when time front of current impulse increases, is influenced by the tower foot resistance, becoming smaller while the foot resistances become higher. 6. CONCLUSIONS In the last years the application of line surge arresters on overhead lines is developing exponentially, transforming this solution in one of higher and higher performance and viable from technical-economical point of view. 69 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14) 7. REFERENCES [1] 33-301 Experience and Effectiveness of Application of Arresters to Overhead Transmission Lines, T. KAWAMURA Shibaura Institute of Technology, a.o. CIGRE SESSION 1998 [2] 33.11.03 Application of Metal Oxide Surge Arresters to Overhead Lines – Working Group 33.11. Task Force 03 A. Schei, Conventor of WG 33.11 – ELECTRA No 186, OCTOBER 1999 [3] Lightning Performance Improvement Of 123 kV Line Ston – Komolac By Use Of Line Surge Arresters M. PUHARI, a.o., CIGRE COLLOQUIM Cavtat 2008 [4] First Experience in Monitoring of Line Surge Arresters Installed on 110 kV Transmission Line Ston – Komolac in Croatia, S. BOJIĆ, a.o. Croatia [5] Application of line surge arrester on a 230 kV transmission line CEMIG’s Experience, A.C.O.ROCHA, a.o. BRAZIL [6] Romanian Field Experience in Mounting and Exploitation of Line Arresters On High Voltage Overhead Electric Lines, Stelian Alexandru Gal – CN Transelectrica SA, a.o. Romania [7] Reduction of the double-circuit flashovers on a 400 kV overhead line, A. Xemard, a.o. Canada [8] Hileman A., Insulation Coordination for Power Systems, CRC Press, 1999 70 ISSN 1843-6188