A new control strategy for SSSC to improve low-frequency oscillations damping

Abstract

When power systems are expanded and connected together with weak tie lines, the low-frequency oscillations are increased and the stability margin of the power system decreases. Therefore, when designing the transmission system to be used, it is necessary to maintain the dynamic stability of the power system, and to make sure to have the most possible stability margin. SSSC is a FACTS device connected in series with power transmission lines. SSSC can control power flow very effectively and can inject series inductive or capacitive voltages into the line. In this paper, a new control strategy based on voltage measurement for the SSSC is proposed. The commonly used strategy for SSSC is based on line current measurement; consequently, the injected voltage is in quadrature with the line current. But in the proposed strategy, the injected voltage is in quadrature with the ac voltage at the point of common coupling (PCC) of SSSC. With modifications done in the proposed strategy, compared with the conventional control method, it can be adjusted easily to control the power flow of the transmission line. The simulation is performed in single-machine and multi-machine power systems. The analysis of the results under different disturbances show that the proposed strategy has an appropriate response with respect to the changes in the system states and it is relatively faster in damping power system oscillations, compared with the conventional strategy.