Abstract
With respect to sensitivity, selectivity and speed of operation, the current differential scheme is a better way to protect transmission lines than overcurrent and distance-based schemes. However, the protection scheme can be severely influenced by the Line Charging Capacitive Current (LCCC) with increased voltage level and Current Transformer (CT) saturation under external close-in faults. This paper presents a new UHV/EHV current-based protection scheme using the ratio of phasor summation of the two-end currents to the local end current, instead of summation of the two-end currents, to discriminate the internal faults. The accuracy and effectiveness of the proposed protection technique are tested on the 110 kV Western System Coordinating Council (WSCC) 9-bus system using PSCAD/MATLAB. The simulation results confirm the reliable operation of the proposed scheme during internal/external faults and its independence from fault location, fault resistance, type of fault, and variations in source impedance. Finally, the effectiveness of the proposed scheme is also verified with faults during power swing and in series compensated lines.
Highlights
1 Introduction To accommodate increased load demand and the need for transmitting large amounts of power over long distances, high volta ge long transmission lines play a large role in the operation of a power system
5.5 Performance on series compensated lines The nonlinear behavior of a MOV (Metal Oxide Varistor) connected across the series compensated capacitors to protect them from overvoltage creates asymmetry in phase impedance [29]
The simulation results show that the proposed scheme is highly reliable for internal faults, having good sensitivity for high resistance faults and security from Current Transformer (CT) saturation during close-in external faults
Summary
To accommodate increased load demand and the need for transmitting large amounts of power over long distances, high volta ge long transmission lines play a large role in the operation of a power system. In view of selectivity, sensitivity, and speed of operation, a pilot wire differential protection scheme is preferred to distance-based and overcurrent approaches because of its high sensitivity with crisp demarcation of the protective zone [2] This method improves the speed of operation and can provide favorable performance during power swings and external faults [3]. In [22], both sequence and phase currents are used to revamp the sensitivity of the Alpha (α)-Plane scheme to provide better reliability under various power system operating conditions This scheme incorporates a total of 5 characteristics to provide complete protection of lines under all types of shunt faults, including 3 phase elements (87LA, 87LB, 87LC), and 2 sequence elements (87LG and 87 L2) of the zero and negative sequences [21,22,23]. F1 and F2 Represents internal and external faults of protecting zone (line M-N)
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