Small Signal Analysis Of LCC-HVDC System With STATCOM At The Inverter Busbar

Static synchronous compensator(STATCOM)has been installed to improve the dynamic behaviors of line commutated converter based high voltage direct current(LCC-HVDC)system and thus to suppress the commutation failure.However,the close coupling between STATCOM and LCC-HVDC system could deteriorate the dynamic characteristics of the control systems and even result in system instability especially under weak AC grid conditions,which has not been addressed in the existing literatures.To make deeper study on this problem,this paper developed a linearized small-signal model of a LCC-HVDC system with STATCOM at the inverter side.The accuracy of the small signal model was verified by comparing its dynamic responses with the simulation results from PSCAD/EMTDC under the same small disturbances.By the eigen-analysis based on the small-signal model,this paper investigated the impact of STATCOM on the small-signal stability margin of LCC-HVDC system comparing the feasible region of the control parameters of LCC-HVDC system with or without STATCOM.The results show that the close interaction between the AC voltage controller of STATCOM and PLL(Phase Locked Loop)of LCC station,deteriorates the system stability margin,which makes LCC-HVDC system with STATCOM more prone to experience oscillation/instability under weak AC grid conditions.Then,this paper proposed a novel supplementary coordinated damping-control(SCDC),for LCC-HVDC system with STATCOM under weak AC grid conditions.Based on the tracked frequency from PLL of LCC station,the SCDC approach introduces a damping coefficient and generates an additional component to the input of AC voltage controller of STATCOM.The SCDC approach can effectively mitigate the oscillations/instability induced by the controller interaction of STATCOM and LCC-HVDC system and thus effectively improve the stability margin of the system.The results based on the electromagnetic transient(EMT)simulations indicate that the proposed SCDC approach can not only enhance the system stability,but also slightly improve the transient performances of the overall system.