Small Signal Stability Analysis and Improvement of Hybrid AC/DC System Based on Probability Method

High voltage direct current (HVDC) transmission is widely used in the interconnection of regional Power grid, which makes the traditional AC low frequency oscillation more complex. Most of researches on AC/DC low frequency oscillation only consider single operation mode. Under multi-operating conditions, probability method is used to study and suppress the low frequency oscillation characteristics of hybrid AC/DC system in this paper. The plug-in modeling technology (PMT) is applied to establish the small disturbance model of AC/DC transmission system and DC power modulation controller. In this paper, the probabilistic load flow under multi-operating conditions is calculated, and characteristic root is solved by combing mixing center distance and semi-invariant combination Gram-Charlier series expansion methods. Based on four-machine two-area hybrid AC/DC system, power system stabilizer (PSS) and additional DC power modulator (DCM) are designed by using center frequency method and test signal method respectively. According to control effect of the controllers, the coordination principle of DCM and PSS is presented. The simulation results show that the probability small-signal stability of power system can be improved significantly.