Reactive power sensitivity-based novel overvoltage suppression strategy for parallel VSC-and-LCC-HVDC transmission system

The rapid deployment of both line-commutated converter-based high-voltage direct current (LCC-HVDC) and voltage source converter-based high-voltage direct current (VSC-HVDC) does not only prolificate AC/DC grid coupling but also form new DC transmission pattern with VSC and LCC converter stations interconnected by parallel AC grids (namely Parallel VSC-and-LCC-HVDC Transmission System, PVL-HVDC). Such a new pattern provides technical potential in addressing LCC commutation failures (CF) and subsequent AC bus overvoltage resulting from LCC reactive power surplus by using VSC reactive power capability. This paper proposes a communication-free novel overvoltage suppression strategy (NOSS) for PVL-HVDC. The proposed NOSS firstly obtains the sensitivity of voltage at VSC side and reactive power at LCC side using the improved Jacobian matrix. In addition, a delay compensation module is incorporated to enhance the response speed of NOSS control. Based on the NOSS, it precisely coordinates VSC reactive power to mitigate AC bus overvoltage at LCC side via their interconnected AC grids. Simulation studies are carried out using a detailed PVL-HVDC model established in PSCAD/EMTDC platform, proving that the proposed NOSS can effectively suppress LCC-HVDC AC-side overvoltage via VSC-HVDC reactive power coordination.