Distance Protection Essay

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Chapter 1 Introduction At the beginning of the 20th century, e ective protection coordination using over- current or directional over-current relays was no longer sucient due to high clearing times and operation with spur lines and rings circuits. Therefore distance protection became the most important protection technology. Distance relays determine the apparent impedance based on voltage and current and operate when the apparent impedance falls within operating zones for an appropriate amount of time. But one of the major problems of distance relay in practice is that the apparent impedance may enter a protective zone due to some reasons but not faults. Our project is to propose a new solution for this problem which is appropriate to Sri Lankan power system. Figure 1.1: Distance Relay 1.1 Background Transmission lines operate close to their operating limits due to factors like dereg- ulated market environment, economics, right of way clearance, and environmental requirements. Therefore transmission line protection is very important for electrical system. Distance protection is the most widely used method to protect transmission 1 CHAPTER 1. INTRODUCTION 2 lines due to its numerous characteristics and merits. Distance relays use power trans- former (PT) and a current transformer (CT) to measure voltage and current in order to calculate impedance of the power line as shown in the Figure 1.2. Distance relay use the impedance per unit distance of the transmission line to identify faults. The impedance per unit distance is fairly constant so these relays respond to the distance between the relay location and the fault location. Figure 1.2: Distance Relay PT and CT Connections In usual practice, distance relays are constructed to give three zones protection in forward direction and one zone in reverse each zone has di erent time delaying.

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