Lightning Transient PSC28_Amini

No. E-13-PDS-1427

Lightning Transients Analysis When Using

Composite Cross-Arm in Distribution Networks

MohamadAli Amini, AliReza Sedighi

Power Quality Research Group

Dep. Of ECE, Yazd University

Yazd, Iran

[email protected], [email protected]

Abstract— One of the main phenomena having effect on

various voltage levels in power systems is transient over voltages,

specifically over voltages caused by lightning that strike the line.

This type of overvoltage should be properly controlled, so that

equipment would receive minimum damage.

Currently, in 20 kV distribution networks, surge arrester is

one of the protection equipment using against lightning striking

the line.

Recently, the idea of using composite cross-arms in the

distribution networks has intensified, due to the many

advantages that they own. Since, using composite in the cross-

arm construction increases the insulating standing of the poles,

this affair will bring up changes in the operation of the lightning

arresters and insulators.

This paper has studied this subject matter. The simulations

have been carried out by EMTP-Work software.

The results show that in terms of electrical, using composite

in cross-arm construction increases the damage rate of

distribution system in the lightning event.

Keywords— EMTP-Work, Lightning, Arrester, Composite

Cross-Arm, Distribution Network, Guard Wire.

I. INTRODUCTION

Commonly, having a large range, over voltages are disorders that despite usually affect the network for a short time, they may disrupt the proper operation of equipment and they may even damage them[1].

The severity of the damages to the system, caused by over voltages, depends on their range and on their presence duration in the system[2]. Lightning is a type of dangerous over voltages which have a very large range, but their presence duration in the system is quite short[3]. This matter causes various damages to the system. These damages may appear as defects and equipment failures, sudden line outages with no reason, or thermal damages[2].

To avoid such damages and to decrease them, different efforts are available. One of the equipment used against over voltages to protect the network, is Lightning arrester[4].

Overhead distribution networks attach to cross-arm by insulators. In normal state, cross-arm should have enough endurance to bear mechanical pressure, and also it should be

able to connect the ground system and the insulator[5]. This connection is such that, in the event of over voltage and possible electrical failure of insulators, the current leads to the ground system through cross-arm and pole.

In the distribution systems, nowadays, the subject of using composite cross arms is considered. According to researches carried out in Iran and other countries, using composite in cross-arm construction increases the mechanical strength, reduces weight, increases corrosion resistance, and increases their life span. The other advantage of using composite in the cross-arm construction depends on the state that the electrical conductor is separated from insulator and connects to the cross-arm. In such a situation, in the case of using composite cross-arm, single phase to the ground fault rate will decrease[5].

In this paper we are trying to create an interaction between insulators, lightning arresters and the idea of composite cross-arms in the case of lightning striking the line, for the purpose of better and more accurate control of the overvoltage. In the following the simulated feeder have been studied. The second section presents the simulations carried out. The effect of electrical discharge on the feeder has been studied in the third section, and the results are described in the fourth section.

II. SIMULATIONS CARRIED OUT

A. Simulating a typical feeder

In order to studying the effect of increased electrical resistance of the composite cross-arm in comparison to metallic form, and studying the transient behavior of the feeder in the case of lightning strike, we have attempted to simulate a feeder of 20kV, with real data obtained from[6]and[7]. Each and every pole has been considered in the simulation. Fig.1 shows the single-line diagram of feeder. Fig.2 shows a part of the simulated feeder in the EMTP software.

As observed in the Fig.2, for studying the effect of lightning overvoltage on insulators, lightning arresters and transformers, the simulation has been carried out considering all poles.

mailto:[email protected]

mailto:[email protected]

Lightning Transients Analysis When Using Composite Cross-Arm in Distribution Networks 28th Power System Conference - 2013 Tehran, Iran

Fig. 1. Case study single line diagram

Fig. 2. All pole are considered in simulation.

The average distance of adjacent poles is considered to be 60 meters[8].

B. Pole simulation

As shown in Fig.3 for every pole a sub-circuit has been designed that includes insulator, pole resistance and ground resistance.

Fig. 3. Designed sub-circuit for each pole

To simulate the characteristics of the insulator, we have used flashover switches.

Regarding the standard data, threshold voltage level of insulators are considered to be 95 kV[9].

C. Lightning simulation

In order to generate standard lightning waveform 1.2×50 μs, a circuit has been designed as Fig. 4, which according to the standard data, current peak will inject 31 kA into the network[10].

Fig. 4. Lightning simulator circuit

The simulated lightning waveform is shown in Fig. 4.

III. STUDY THE EFFECT OF ELECTRICAL DISCHARGE ON

THE LINE

A. Insulator failure and current discharge through the pole to

the ground system

As shown in the lightning waveform (Fig. 5), increasing rate in the initial steps is very high. Once the lightning strike the line, the discharged current increases the line voltage and will damage the insulator. Due to the Overvoltage applied to the line, when the overvoltage is more than the insulator‘s withstand, the insulator loses its insulating standing and works as a conducting route leading the lightning current to the ground.

As observed in Fig.3, In order to simulate this subject, whole pole and ground resistance is considered as an


3

equivalent resistance. The current passing through this damaged insulator, transfers to the ground over this resistance.

Fig. 5. Simulated lightning wave form

Then, if the line voltage is examined according to Fig.6, at the location of lightning strike, then it would observed that in the early moments of lightning strike (part A), suddenly, the voltage increases resulting from wave advance up to 95 kV, leads to damage the nearest insulator, and a route including the damaged insulator, total resistance of the ground and pole will be created to discharge the current.

The wave length current of lightning, passing from created route, increases the line voltage highly. This overvoltage is higher than the rated voltage several times, and is shown in Fig.6; Part B.

Produced high voltage waveform starts to move from the location of lightning strike on the line, towards the both sides, and may cause electrical discharge in the adjacent insulators.

Fig. 6. Line voltage due to lightning strike

B. Studying the increased electrical resistance of lightning

dischargeroute

The equivalent resistance inserted in the pole modeling, is a function of ground resistance, pole resistance and cross-arm resistance. By changing cross-arm material from metal to composite, certainly the equivalent resistance will increase. In this section we have studied the effect of this resistance growth on the line voltage waveform in the lightning transient conditions. As stated in the previous section, the current injected to the line by surge, would transfer to the ground through this resistance.

Fig.7 represents the overvoltage peak changes as a function of the equivalent resistance of the pole and the ground, after lightning current discharge.

More increasing in cross-arm resistance, the more increasing in peak of the generated transient waves. It is worth noting that the first part of Fig.7’s curve, represents the behavior of the metal cross-arms that have a resistance of 5 ohms. In such a condition, after damaging the first insulator, voltage peak generated on the line, will be about 10pu. While the distal part of the curve shows the behavior of cross-arms with high resistance (like composite cross-arms) that after damaging the first insulator, a voltage peak of about 50 pu on the line will be generated. Transient wave moving with a higher peak, in the first stage will damage adjacent insulators more seriously and then in the next stages will increase the entrance voltage of transformers and the other installed elements, and this incident may make the damage to the network and equipment more acute.

C. Investigating the use of lightning arresters in the network

with composite cross-arm

Considering the explanations presented in the previous section, using composite cross-arms would exacerbate the damages caused by the lightning strike. In this section the idea of using additional lightning arresters has been surveyed in order to control the generated transient wave. Therefore, we have tried to control the generated overvoltage, by increasing the number of lightning arresters in the system. For this purpose, with the assumption of using composite cross-arms on all poles, the typical network, has been investigated in two forms.

Fig. 7. Increasing line peak voltage due to increasing equivalent resistant


4

Fig. 8. Increasing entrance voltage of transformer due to increasing pole

resistant

Entrance voltage waveform in the selected distribution transformers with a proper distance from the lightning location, has been considered as a criterion for investigating the idea. The location of lightning strike and the Transformer has been shown in Fig.1.

First of all we assume a condition in which lightning arresters of the system are just installed in the place of distribution transformers. In such a condition, input voltage peak variation, has been plotted in curve i of Fig.8 as a function of equivalent resistance of the pole and cross-arm.

As observed in the figure, by cross-arm resistance growth, the input voltage of transformer, due to lightning strike will be several times higher than the rated voltage. So, it will be difficult to control the overvoltage by using zinc oxide lightning arresters and input gap lightning arresters of transformers. To overcome this problem, it is proposed to increase the number of installed lightning rods. Curve ii, in Fig.8 shows the input voltage of the transformer, as a function of the pole resistance and the ground resistivity, in a condition that the number of system lightning arresters has been increased. Comparing to the previous form, the voltage level has improved in the transformer entrance. As it could be realized from this graph, although this affair will be useful in terms of stability, and will be able to prevent lightning strike the transformer, it should be noted that the installed equipment increase, would entail the repair and maintenance cost growth. Therefore, one of the subjects that can be considered later in this work is determining the number and proper location of additional installed arresters. In addition to the above solution, it is possible to prevent the lightning damages in power energy distribution networks by using guard wires, which will be discussed later.

D. Using guard wire as a protective equipment to prevent

damages resulting from lightning

In 1985, Rusk introduced the following equation to calculate the inductive voltage in lines resulting from indirect collision with lightning:

= 36.5 kV (1)

Where is the maximum of the lightning current at the

moment of collision in kA, h is the height of the line from the ground in m, and y is the distance of the collision point from the line in m. According to this equation, if the lightning current is 31 kA, the height of the line from the ground is 12 m and the place of collision is within a 1 m distance, then the additional inductive voltage resulting from the indirect collision of lightning will be 93 kV that is below the insulation stability level of the distribution transformer. According to Eq. (1), as the distance between the collision point and the line increase, the inductive voltage will decrease. In order to appraise the performance of the guard wire, it has been added to the under-investigation system. The guard wire can be fixed in different distances from the line conductor; according to [2], this wire has been fixed within a 20 cm distance. Having imposed the lightning on the guard wire at the location that shown in Fig.1, as it can be seen in Fig.9,voltage peak at transformer entrance that shown in Fig.1 will be about 3 pu.

In the presented condition that composite cross arms have been installed and also guard wire has been used, the overvoltage results from inducting voltage to line could be simply controlled by the arresters at the entrance of the transformer regarding to the amplitude of the inductive voltage and its presence time in the network. It must be mentioned that in the case of using this method to optimize the voltage level at the entrance of the transformer in lightning conditions, guard wires must be properly grounded in any poles. Here, according to [2], the resistant used for the guard wire grounding is considered to be 5 ohm. Studies have been shown increasing the resistant will increase the inducted voltage amplitude in each phases which can affect the arresters operation.

I. CONCLUSION

One of the phenomenon having effect on various levels of voltages of power systems is transient over voltages, specifically over voltages caused by lightning. Using composites has recently boomed in designing and constructing cross-arm. But changing cross-arm material from metal to composite, the resistance of the lightning current discharge route is being increased. Resistance growth increases the input peak voltage of transformer. Two proposed methods for controlling the additional voltage are increasing the number of the arresters fixed at the network, and use of the guard wires. Using the above methods can optimize the voltage level at the entrance of the distribution transformer.

Fig. 9. Lightning caused overvoltage after striking the guard wire.


5

REFERENCES

[1] N. Balijepalli, S. Venkata, W. Richter and D. Christie, “Distribution System Reliability Assessment Due to Lightning Storms” IEEE Trans. Power Del., vol. 20, no. 3, pp. 2153–2159, July 2005.

[2] D. Ahmadi, M. Oskuei and A. Parhizgar, “Capability Analysis on Distribution Transformer Arrester Deleting in Urban Region” , PSC 24th Conf., Iran, 2009.(In Persian)

[3] R. Zoro, R. Mefiardhi, R. Aritonang and H. Suhana,” Observation on Improved 20 kV's Overhead Distribution Lines Against Lightning” , IEEE Conf. Properties and applications of Dielectric Materials, Indonesia, pp. 979 – 983, June 2006.

[4] V. Podporkin, Y. Enkin, S. Kalakutsky, E. Pilshikov, and D. Sivaev, “Overhead Lines Lightning Protection by Multi-Chamber Arresters and Insulator-Arresters” IEEE Trans. Power Del., vol. 26, no. 1, pp. 214–221, Jan. 2011.

[5] M. Khalili and M. kheiri, “Practical Analysis of Composite Cross-arms in Distribution Networks”, PSC 24th Conf., Iran, 2009. (In Persian).

[6] A. Sedighi, M. Haghifam, O. Malik and M. Ghassemian, “High Impedance Fault Detection Based on Wavelet Transform and Statical Pattern Recognition’ IEEE Trans. Power Del., vol 20, no. 4, Oct. 2005.

[7] M. Haghifam, A. Sedighi and O. Malik, “Development of a fuzzy inference system based on genetic algorithm for high-impedance fault detection” , IEE Proceedings on Generation, Transmission and Distribution, vol. 153,issue 3, pp. 359-367, May 2006.

[8] M. Arabani, P. Islamzadeh, “Engineering Standpoints on Energy Transmission Line Designing”, Electrical Engineering Services Co. , Moshanir, 1998(in Persian)

[9] International Electrotechnical Commission, "Artificial Pollution Tests on High Voltage Insulators to be Used on AC Systems", International Standard IEC 507, April 1991.

[10] H. Mohseni, K. Loux, “An Investigation of Over-voltages Caused by Lightning in Transformer Winding Based on the Monte Carlo Method” University College of Engineering, University of Tehran, Vol. 55, 1994(in persian).

[11] International Standard IEC 99-4,” Surge Arresters. Part 4: Metal-oxide Surge Arresters without Gaps for A.C. Systems”.

Documents

Lightning Transient PSC28_Amini