Small-signal dynamics of hybrid LCC-VSC HVDC systems

This paper (i) develops a comprehensive small-signal dynamic model of a hybrid LCC-VSC HVDC link and (ii) based on eigen-analysis of the model, investigates the HVDC link dynamic modes and the impact of parameters on system dampings. Validity and accuracy of the small-signal (linear) dynamic model is verified based comparing the results with those from time-domain simulation in PSCAD/EMTDC platform. The eigen-analysis results show that (i) parameters of DC-current controller of LCC, DC-voltage controller of VSC, DC-side capacitor of VSC and smoothing reactor of LCC can highly affect the dampings of the oscillatory modes and even cause instability, and (ii) larger proportional and integral gains of VSC DC voltage control and smaller proportional and integral gains of LCC DC current controller can enhance dampings of dominant modes. The study also shows that larger DC-side capacitor and smoothing reactor of VSC, and smaller smoothing reactor of LCC may lead to oscillations/instability.