International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056 Volume: 05 Issue: 08 | Aug 2018 www.irjet.net p-ISSN: 2395-0072 © 2018, IRJET | Impact Factor value: 7.211 | ISO 9001:2008 Certified Journal | Page 250 Mathematical Flashover Model of Polluted Insulators based on the Arc Root Voltage Gradient Criterion Akshata A Tadkod M.Tech, Department of Power System Engineering ---------------------------------------------------------------------***--------------------------------------------------------------------- Abstract – In this report the flashover mechanism of polluted insulators under AC voltage, a new arc propagation criterion which is based on an arc root voltage gradient is proposed. The arc channel is considered as an equivalent distributed parameter circuit model instead of using the arc voltage- gradient equation. The parameters of the arc model are obtained from the electromagnetic field distribution of the arc and the gas discharge theories. The arc root is considered as parallel paths including the polluted layer. The variation of the voltage on the arc root is related to the capability of arc propagation. This model takes the microscopic mechanism of arc root ionization into consideration, which can improve the accuracy of the flashover model. Key Words: Arc root voltage gradient; Flashover mechanism; Circuit model of insulator; AC arc 1. INTRODUCTION Now a day, the electric power requirement has increased significantly. To full fill the requirement of power, electrical companies need to increase the transmission lines efficiency. Each individual consumer has liberty in electrical markets to choose their own supplier companies for improved and reliable service. The companies has to increase and maintain efficiency which is based on continuity of the supply to the consumers, and avoiding the faults, causes financial losses for both consumers and companies. one of the major problem to maintain the reliability is the consequence formed by contaminated insulators of transmission lines. The fundamental reason for flashover is the deposition of pollution on the insulators. When existed contaminants on surface of insulators combined with humidity of the fog or dew, rain the insulator may initiate to fail. Combination of humidity and contaminants, create a layer that turn into a conductor. This allows circulating the currents which will build up the condition of short circuit, because of a decrement in the resistance of surface of insulator. Unless there is a natural cleaning or an adequate maintenance, the electrical activity will be affected by a possible flashover in the insulator. The flashover of polluted insulators is one of the fundamental factors undermining the protected operation of the power grid, which can prompt extraordinary monetary losses to the entire power system; hence it is of incredible incentive to concentrate the contamination flashover characteristics and mechanisms from both the engineering and academic viewpoints. AC flashover is most complex than the pollution flashover underneath DC, due to arc reignition and arc extinction during the course of the AC flashover process, which is went with the distortion and pulses of the leakage current. Pollution flashover of dynamic and Static models were progressed and applied for insulation coordination, in view of the empirical formulae of arc reignition from the arc tests. From the current research, the criteria for arc propagation and re-ignition and arc extinction are considered as the key points of the flashover model. Prior criteria for arc propagation depends on, as power (P) varies the arc length propagation (x) also varies i.e. dP/dx > 0, this is not a sufficient condition but necessary condition. 2. BASIC AC MATHEMATICAL FLASHOVER MODEL OF POLLUTED INSULATORS Most of the models used to predict the flashover voltage of contaminated insulators are concluded from the Obenaus contamination flashover model, which is appeared in Figure 1. Figure 1 represents a perfect model going for foreseeing the proliferation procedure of the curve on contaminated protectors. With this model, the basic voltage to keep up the curve can be derived from U = AxI −n + R p I .....(1) where U (V) is the peak value of the voltage applied , x (cm) is the arc length, I (A) is the peak value of the arc current, R p ȋΩȌ is the resistance of the remaining contamination layer, and A, n are the arc characteristic constants. Figure 1. Circuit flashover model. The Obenaus model is limited in explaining the AC arc extinction and reignition. Claverie and Porcheron proposed that AC pollution flashover should meet the arc reignition condition,