Several types of high voltage direct current (HVDC) breakers have been introduced and commercialized. Each of them has advantages and disadvantages. Among them, the hybrid HVDC breaker is highly successful. One of the most important concerns that the hybrid HVDC breaker has faced is high power loss throughout its fault current breaking process. The hybrid HVDC breaker comprises a high voltage bidirectional main HVDC breaker. A significant number of electronic switches need to be connected in a series where anti-parallel diodes are essentially embraced. During fault inception, a number of series solid-state switches and a number of series diodes dramatically increase the power loss of the main breaker. This study, firstly, studies the power loss of the hybrid HVDC breaker and later develops a structure of a full-bridge hybrid breaker (FBHB) to reduce the losses of the current structure both in the normal and fault protection states. In this paper simulations are done based on PSCAD. In addition to the analytical study and simulations, we show that the developed structure substantially decreases the amount of power lost during the normal operation and fault current breaking stage.

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In this paper the ability of fuzzy logic to design power system stabilizer for generating proper control signal has been demonstrated. The selected TCSC is mounted at the center of transmission line and fuzzy power system stabilizer uses the variation of electric power and its derivative as input signal. Also using triangular membership function, the complementary signal is obtained. The study system is simulated using MATLAB Simulink 6.5. Then, applying a symmetrical three phase short circuit fault , oscillations are created in power system. The major area of this paper is damping this oscillation by using fuzzy power system and TCSC. Finally, in this study a comparision between controllers of PSS and PSS+TCSC and also FPSS+TCSC to damp this oscillation is done.