Earth Fault Currents in Three Phase systems

University essay from Blekinge Tekniska Högskola/Institutionen för tillämpad signalbehandling

Author: Mohammad Kamrul Hasan; Faham Hossain; [2015]

Keywords: Earth; Fault;

Abstract: This paper focuses on the grounding methods for distribution systems and the characteristics and behavior of earth fault currents. At First different existing grounding methods such as isolated neutral, solidly grounded, Resonant grounding, low and high impedance groundings are introduced. Secondly, focus is placed on further describing these methods and the viability of each of these methods in different scenarios. Then these methods are further analyzed by using equivalent circuit designs. Using the equivalent circuit helps to derive the formulations and equations. It is shown that the designing of these systems in fact follows basic electrical properties such as voltage dividing and current dividing principles. The derived equations are further tested experimentally to prove the characteristics and behavior of these methods. Finally, the report is concluded by testing some of these methods (isolated neutral and grounding via resistor methods) in the laboratory and the results obtained are analyzed with the theoretical results to determine the characteristics and behavior of these methods. In this manner it is shown experimentally that in case of isolated neutral systems the summation of currents through all the phases (current flow through the line model) is zero when there is no earth fault. In case of an earth fault occurring across one of the phases the neutral-to-ground voltage becomes equal to the voltage across the faulted phase and also the sum of currents passing through the healthy phases is equal to the current flowing through the faulted phase. This phenomenon is proved in the second experiment. The third experiment proves the relation between the earth fault current and current via the resistor and the currents via the healthy phases whereby it is shown that the fault current is equal to the root square summation of currents of resistor and healthy phases. This is one of the characteristics of grounded via resistor system. The next experimental setup also focuses on the characteristics of grounded via resistor system whereby it is shown that the fault current and the total phase current are independent of any external load. The last experiment proves the phenomenon that phase-to-phase voltage remains intact during times of earth fault and the system continues the operation uninterrupted as it should be in case of isolated neutral grounding system.

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