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

dissertation for obtaining a master's degree in electrical engineering

power orientation

abstract

Overvoltages are one of the most important destructive factors and threats to the insulation of power grid equipment and often cause interruptions in service and reduce power quality and system reliability. Overvoltages Transients caused by lightning are much more common and dangerous in the power system and are more important in the insulation coordination of the system.   Therefore, in this thesis, the evaluation of various problems related to lightning overvoltages in a 400 kV system with overhead lines and gas-insulated lines (Gil) using 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

1- Familiarity with lightning and atmospheric discharge phenomenon

The most important task of a power supply system It is required by subscribers with a minimum price and an acceptable level of reliability. 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 cause an increase in the body voltage and the generation of back sparks, 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 are caused by lightning strikes and similar investigations in the country England, which was also carried out in a 14-year period, shows that out of about 5000 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 line insulation, disrupting the expected isolation of the lines and causing the network to be completely blacked out. According to daily observations and experiences, the occurrence of local storms in suitable weather conditions causes the ionization of water vapor molecules and cloud-forming molecules, resulting in positive and negative electric charges. It divides and condenses them.

Droplets carrying positive charges accumulate on one side and negative charges on the other side of the clouds, they create a mass of clouds with electric charges, the appearance of clouds is separated by electric charges and creates two positive and negative poles, so to speak, polarizes 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, atmospheric discharge or natural discharge is also common. rtl;">Abstract: Overvoltages are one of the main threatening and destructive factors in power systems insulations which often cause service trips and consequently reduction in power quality and system reliability. Lightning transient overvoltages are the most dangerous and prevalent ones which possess a great significance in system insulation coordination. Thus, in this paper using EMTP RV software several issues considering lightning overvoltages are evaluated in a 400 kV system with overhead lines and Gas Insulated Line (GIL). A comprehensive study has been conducted in this paper to investigate various issues affecting lightning overvoltages including: First stroke and subsequent stroke of a lightning, overhead line and GIL performance under lightning strokes, Lightning overvoltages caused by the lightning strokes to the towers or to the ground wire and direct strokes to the phase conductors, tower design and dimensions in overvoltages, grounding system for towers and different modeling techniques and soil characteristics and finally the effect of installing surge arresters. The contributing parameters in overvoltages are determined and remedial strategies are introduced.