How Lightning Arresters Save Power Systems ? | Detailed 3D Animation

When a direct lightning strike occurs on a live substation conductor, a huge amount of electrical energy is injected into the system within just a few microseconds. The voltage rises to several times the normal operating level of the substation. As a result, the insulation of equipment such as transformers [music] experiences severe dielectric stress, far beyond what it is designed to withstand. At this point, the arrester intervenes. The surge current [music] is diverted through the arrester and safely discharged to earth in most cases, thereby preventing excessive voltage rise and protecting other equipment. There are many types of lightning arresters. In this video, we will focus on the gapless metal oxide arrester, which is the most widely used type in modern power systems due to its superior efficiency, >> [music] >> fast response time, and high reliability. In a gapless arrester, there is no open-air gap between the live terminal and the earth terminal, unlike earlier gapped silicon carbide arresters. >> [music] >> A metal oxide arrester is built with a single effective active element, the column of metal oxide varistors. >> [music] >> The metal oxide varistor column, along with its supporting structure, forms the only active part of the arrester. The varistor column consists of individual metal oxide resistor discs [music] stacked on top of each other. Each metal oxide disc is primarily made of zinc oxide, >> [music] >> along with small amounts of bismuth oxide and antimony oxide, and is further doped with other metal oxides such as [music] tin, cobalt, and nickel. A very smooth surface finish is maintained on the top and bottom electrodes to maximize the contact area between adjacent discs in the stack. The diameter of the discs plays a crucial role in determining the current handling capacity during a lightning surge. Typically, the diameter of a metal oxide varistor disc ranges from 30 to 100 mm. The height of a single metal oxide disc generally lies between 20 to 45 mm, >> [music] >> and it is closely related to its voltage handling capability. However, a single disc of large height and diameter is not manufactured, since the greater the height and diameter, >> [music] >> the harder is to achieve sufficient homogeneity of the resistor material during manufacturing. For mechanical support, several supporting rods made out of fiberglass reinforced plastic material encircle the metal oxide varistor column like a cage. Holding plate, also [music] made of fiberglass plastic, additionally provided at regular intervals to prevent the supporting rods from being bent apart, [music] and also limit the sagging of the whole construction towards the housing walls. The key reason for using metal oxide material is its highly non-linear voltage-current behavior. In ordinary ohmic materials, such as a resistance wire, voltage and current share a linear relationship at a given temperature. This means that as the voltage increases, the current through the wire also increases proportionally in accordance with Ohm's law. That is why such materials are called ohmic. A metal oxide varistor is a non-ohmic material, where voltage and current do not follow a linear relationship. Its resistance changes with the applied voltage, allowing it to behave very differently under normal and surge conditions. The height of the metal oxide varistor column depends on the voltage handling capability of the arrester. The voltage handling capability of a single metal oxide disc is typically expressed in volts per millimeter, and generally lies between 300 to 400 volts per millimeter. To achieve, for example, a lightning impulse protective level of 800 kilovolts with each disc having a height of around 50 millimeters and a voltage handling capability of 300 volts per millimeter, a stack of approximately 54 discs is required. This results in a combined height of about 2.7 [music] meters. For practical and mechanical reasons, such a long active column is not housed within a single housing. Instead, [music] the arrester is divided into two sections or, in extra high voltage systems, [music] sometimes three sections to facilitate easier transportation, handling, [music] and improved mechanical strength of the overall structure. The housing of a lightning arrester is typically made of quartz or alumina porcelain. For medium voltage applications, silicone rubber housings are more commonly used. We will take a closer look at silicone rubber arresters later in this video. However, for most high voltage applications, porcelain housings are still preferred. The strength of the porcelain housing isn't just about wall thickness. The geometry of the porcelain, especially its diameter, [music] plays a crucial role as well. In addition to protecting the active components from environmental influences such as pollution, dust, [music] and moisture, the arrester housing must also provide adequate creepage distance [music] to prevent surface leakage currents. For this reason, it is equipped with sheds whose design can vary considerably depending on the application. Both ends of the housing are fitted with aluminum flanges, which also serve as directional pressure relief devices to release any rapid buildup of internal pressure during an arrester overload. >> [music] >> These flanges are secured to the porcelain body using sulfur cement. >> [music] >> Two very strong compression springs, supported by rings inside the flanges, are installed [music] at both ends of the metal oxide varistor column to firmly brace the active part within the housing. A sealing system [music] consisting of a sealing ring and a pressure relief diaphragm made of high-grade steel or nickel [music] is used to completely seal the active part within the housing and prevent the ingress of moisture throughout the service life of the arrester. >> [music] >> A clamping ring is used to tightly secure all the internal components within the flange housing. Now that we have discussed the construction of a metal oxide arrester, let us take a look at some of its technical parameters. The maximum continuous operating voltage, MCOV, of a metal oxide arrester is one of its key parameters. It represents the voltage level at which the arrester is designed to operate continuously. The MCOV must be at [music] least 5% higher than the highest continuous phase-to-earth voltage of the system in which it is installed. During normal operation, when the system voltage is at or below the MCOV, the metal oxide blocks offer extremely high resistance. As a result, [music] only a very small current, on the order of a few microamperes, flows through the arrester. This small current is known as leakage current. Because of this, [music] the arrester does not create a phase-to-ground fault and has no impact on the system during normal operation. In the event of a lightning strike, the system voltage can rise to anywhere between three to 10 times its normal level. At such high voltages, the arrester offers a very low resistance to the incoming surge due to the non-linear voltage-current characteristics [music] of the metal oxide discs. Even a moderate increase in voltage causes the current to rise dramatically by several orders of magnitude. This enables the arrester to rapidly divert the surge to earth, limiting the voltage at its terminals to a safe [music] level, and thereby protecting critical equipment from damage. During both normal operation and surge conditions, >> [music] >> the current flows from the live terminal to the earth terminal through the metal oxide varistor stack, [music] with the flanges providing a conductive path. A device known as a surge counter is connected in series between the arrester's earth terminal [music] and ground. It is used to record the number of surge events and [music] to monitor leakage current. The energy handling capability of an arrester is one of its most important parameters, as it is directly related to its thermal stability. [music] During normal operation, the leakage current is very small. However, even this small current generates some heat [music] within the metal oxide column. Under steady conditions, this heat is safely dissipated to the surroundings. Metal oxide varistors [music] exhibit temperature-dependent behavior. If the heat generated exceeds the rate at which it can be dissipated, the temperature of the discs begins to rise. This increase in temperature leads to higher leakage current, which in turn produces even more heat. If this cycle continues unchecked, the arrester becomes thermally unstable and may heat up to the point of self-destruction. This condition is known as thermal runaway. [music] During a lightning surge, the current can reach extremely high values, sometimes up to 200 kiloamperes, depending on the severity of the strike. Even though this current flows for only a few microseconds, >> [music] >> it generates a significant amount of heat within the metal oxide discs. As a result, [music] the temperature of the discs rises sharply. After the surge is discharged [music] and the system voltage returns to normal, the arrester must quickly dissipate this heat. If it fails [music] to do so, the elevated temperature can increase the leakage current, potentially pushing the arrester into thermal runaway. The construction of silicone rubber arresters is relatively simpler compared [music] to porcelain housed arresters. Although the core principle remains the same, both use metal oxide discs [music] as the active element. Their behavior under fault conditions differs. In the event of thermal runaway, >> [music] >> the internal pressure build-up can cause the silicone rubber housing to rupture safely without damaging nearby equipment. In contrast, porcelain [music] housings are brittle and require a dedicated pressure relief system to safely release internal pressure. If not properly relieved, the porcelain housing can shatter and potentially damage surrounding equipment. I hope you found this video helpful. If you did, please give it [music] a like and consider subscribing for more videos like this.