Control Strategy for VSC-HVDC under Unbalanced Input Voltages

Voltage source converter (VSC) is sensitive to the AC unbalanced input voltages under traditional control strategy, and leads to 2(nd) order DC voltage fluctuation which will be spread from one station to the other through the DC cables. As a result, it appears to be 2(nd) order power fluctuation and power quality problems, or even leads to system instability. So it is necessary to carry out the research on the power characters of VSC station so as to get the effective control method under unbalanced conditions. On this basis, control of the system side power is adopted which take the power consumed by interfacing inductors and loss resistances into account. Delayed signal cancellation (DSC) method is also adopted in this paper to improve the performance of software phase locked loop (SPLL) under unbalanced input voltages. Simulated model is set up by PSCAD/EMTDC software, and the correctness of the proposed strategy is verified by simulated results.