Investigating the effect of lightning on the reliability of power transmission lines

Dissertation for Master's Degree in Electrical Engineering

Power Trend

Abstract

Overvoltages are one of the most destructive and threatening factors to the insulation of power network equipment and often cause interruptions in service and reduce power quality and system reliability. Transient overvoltages caused by lightning are more common and dangerous in the power system and are more important in coordinating the insulation of the system. are   Therefore, in this thesis, the evaluation of various issues related to lightning overvoltages in a 400 kV system with overhead lines and gas-insulated lines (Gil) using the EMTP RV software has been done. Lightning caused by lightning striking the tower or ground wire and direct lightning striking the phase wires, the influence of the structure and dimensions of the tower in creating these overvoltages, the design of the grounding system of the towers and its modeling techniques, the characteristics of the soil and the effect of installing lightning arresters in different places, finally, by introducing and determining the effective parameters in creating overvoltages, helpful strategies have been introduced. 

Key words: EMTP RV, lightning overvoltage, reliability, flashback, lightning arrester, overhead lines and line with gas insulation. Therefore, reliability studies in power systems are one of the most important tasks defined in planning the development of these systems. Reliability, especially in transmission and super distribution systems, is one of the topics that has been given much attention by regional power companies in recent years. Lightning is of great importance as the most important external source causing transient overvoltages in the power network, the importance of transient overvoltages caused by lightning can be found in its destructive role in the power network. In this thesis, while introducing the types of overvoltages created in the power system, with the introduction and modeling of lightning and flashback phenomenon and the effective factors in the occurrence of this phenomenon, we analyzed the overvoltages created as a result of the main and return wave of lightning using the EMTP RV [1] software. There are conventional methods to reduce the return spark and improve the performance of transmission lines and increase reliability, the third of which is reducing the resistance of the mast and installing lightning arresters.  Therefore, in this report, by modeling the high frequency of the ground system rods, investigating the impact of the structure and dimensions of the mast, the impact of the ground impedance on the occurrence of flashback and the installation of lightning arresters, and an effort to reduce overvoltages caused by lightning strikes. 

Energy transmission lines and high voltage stations in national energy transmission networks are the most vulnerable and sensitive parts of the network to natural phenomena such as lightning or electrical atmospheric discharge. When lightning occurs, it is possible to hit transmission line conductors that contain mostly high voltages. Many transmission networks pass through mountainous areas with high altitude. Many of these areas face the phenomenon of lightning, especially in spring and autumn. If the impedance of the tower is high, lightning can increase the voltage of the body and produce a return spark, resulting in outages in the power system.

Investigations conducted in the United States and Canada during the 14 years of operation of about 25,000 miles of power transmission lines show that 26% of power outages in 345 kV transmission lines were caused by lightning strikes, and similar investigations in England, which were also in the same period A 14-year study shows that out of about 5,000 outages in power distribution lines up to 33 kV, 47% of them were caused by lightning. Therefore, it is clear that one of the most important tasks that must be studied and developed in order to increase the reliability of power transmission and distribution lines is how to eliminate the effect of lightning. The need to investigate lightning is due to its major importance in causing disruptions in electricity supply and nationwide interruption of the network and energy needed by consumers.Any atmospheric discharge appears on any point of the energy transmission lines in the form of an electric arc in the air gaps of the lines' insulation, disrupting the expected isolation of the lines and causing the network to face a complete blackout. According to daily observations and experiences, the occurrence of local storms in suitable weather conditions causes the ionization of water vapor molecules and the molecules that make up the clouds, and divides the resulting positive and negative electric charges and

Figure (1-1): Division of positive and negative electric charges

Drops carrying positive charges are accumulated on one side and negative charges on the other side of the clouds, they create the mass of clouds with electric charges, the appearance of clouds is separated by electric charges and creates two positive and negative poles, so-called polarizing it. The creation of two negative and positive poles creates the intensity of the electric field between them, the accumulation of more and more charges increases the intensity of the electric field continuously. If the electric field exceeds the intensity of the air resistance field, an arc and electric discharge occurs between the two positive and negative poles in the cloud. With the density of charges in the cloud and their polarization, similar electric charges are induced on the surface of the earth, the intensity of the electric field appears at the boundary between the cloud and the earth. So that arcing and atmospheric discharge happens inside the clouds, between the clouds, between the clouds and the ground. The arc appears as a suitable electric path due to the approach of non-identical electric charges and their attraction to each other in the distance between two poles. With the occurrence of an arc, the positive and negative electric charges accumulated on both sides of the cloud or the cloud and the earth flow towards each other, gather together, neutralize each other, and create complete molecules, this is the practice of discharging electric charges. With the discharge of charges, the intensity of the electric field is reduced, the arc is suffocated and the discharge of charges is stopped. The path of discharge of charges and the place where the arc occurs is ionized for a few milliseconds, after the arc is suffocated, the above path is ionized again and has full dielectric properties. Occurrence of discharge arc is accompanied by intense light and sound, for this reason, the above phenomenon is called lightning. Because the occurrence of arc or lightning occurs as an atmospheric phenomenon in order to transfer electrical charges from the cloud to the ground and vice versa, therefore it is also known as natural discharge or natural discharge. This action happens especially in thunder clouds, these clouds are called cumulonimbus. The discharge between the clouds or the clouds with the ground starts following the increase in the intensity of the electric field and the ionization of the narrow and uncertain path of the air in the distance between the two electric poles.

The occurrence of multiple and consecutive arcs in a very short time interval of a very variable number of 1-10 According to the amount of cloud load, their set is called Restrike arcs. Each of the arcs is called Stroke, due to the very small time interval of the arcs. Thus, a lightning or lightning may be limited to one arc or include consecutive arcs.

In the statistical analysis of an arc sample consisting of 6 consecutive discharges, the current range of the first arc has increased to 80 kA in a time interval of 10 to 15 microseconds. While in the next successive arcs, the current range increases to a maximum of 20 kiloamperes within 1 to 2 microseconds.

One of the main characteristics of shock waves and atmospheric discharge waves is the speed of their current increase in the time interval of the wave front. This speed of increase is expressed in kiloamperes per microsecond. This is how the speed of the surge of atmospheric discharge voltage is defined. This speed is shown by dividing the maximum wave voltage by the time of its front in terms of kilovolts per microsecond. The lower the speed of the surge, the more reliable is its discharge by arresters. Waves with a wavefront of less than 1 µs are called atmospheric discharge waves.