Mechanism analysis of sub-synchronous frequency impedance characteristic of TCSC

A theory and numeric approach is adopted to directly calculate time domain curves of sub-synchronous frequency voltage, by using small signal method, adding a tiny sub-synchronous frequency component to the fundamental current source. The variety rule of sub-synchronous frequency voltage at beginning and end of thyristor's switching-on is analyzed, and the sub-synchronous frequency equivalent impedance of thyristor controlled series compensation (TCSC) is formulated. The result shows sub-synchronous frequency impedance characteristic curves by formulation is closed to numeric approach. Furthermore, by researching the change process of modulation effect along an increase in conduction angle, the mechanism explanation of sub-synchronous frequency impedance characteristic form is given from angle of period modulation.