Seeking the closest voltage instability point of AC/Hybrid MTDC system

For analyzing the static voltage stability of the hybrid-MTDC system which is combined by current source converter (CSC) and voltage source converter (VSC), a method to find the closest voltage instability point is proposed. Based on the algorithm of improved continuation power flow, this paper considers the transformation of the converter turns ratio and the conversion of the converter's control modes in the calculation procedure. Trace the changing direction from the initial point to the breakdown point and use the self-adaptive step control strategy, gradually approach to the most dangerous load growth direction. Calculation comes to an end until get the closest voltage instability point, meanwhile the minimum load margin is obtained. AC/Hybrid three-terminal DC system based on IEEE14 is analyzed by this method. Simulation results show that the proposed method is effective and feasible. The closest voltage instability point can accurately reflect the voltage instability.