ISSN 1843-6188 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
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,
Technical University Iasi ,
1
ISSN 1843-6188 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
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 likeless weight, easy of 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 surgearresters 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.
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 equippingmodes, 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 modeof the arrester: directly at the ends of protected insulator string or in series with an external gap.
From Japanexperience 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 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 lowkeraunic 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 articlewith 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]
In 2008, CIGRE Conference inCroaţia, focused the gained experience in line arresters utilisation field from different parts of the world.
One of the many cases of line arresters applicationpresented 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 thatcrosses 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 well as monitoring the arresters activity revealing the number and location of surge levels and current flowing.[4]
Chinais 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 35kV and 220kV, encountering no problems in operation.
As in many other countries, also in China lightning strike are the main cause for40 – 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 500kVin 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 110kV Xi-bai fromGuangdong area, the outage rate decreased from 15,5 outages per 100 km per year in 1999, to 4 outages per 100km peryear in 2000, 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 substationsand 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.
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 stringon 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 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 suspensionshackle 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 weresolved, 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 beingused EMTP.
French researchers, in collaboration with Hydro-Quebec 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]
Figure 9. Flashover rate on circuit 2, when both external phases of circuit 1 are protected by arresters
gaplessandgapped
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 approachesto the protection level obtained with ground 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 metal-oxide 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 technical-economical 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.
4. THEORETICAL BASIS
When lightning strikes the tower top, the impulse current flows through the tower and its foot resistance 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 aRLC circuit, the front time of the lightning current wave influences mainly the voltage drop oninductance, 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.
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 wavescircuit. 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 penetrationso that the lightning might strike a phase conductor. The tower is considered as a circuit with uniform distributed parameters, without losses, having three segments: