Impact of DC Breaker Systems on Multiterminal VSC-HVDC Stability

The use of VSC-HVDC grids for offshore wind farm integration will require the use of dc breaker systems and they, at present, will require dc reactors to limit the rate of rise of fault current. The introduction of large dc reactors throughout a VSCHVDC system can have significant impact on its stable operation and will require additional control. This paper analyses this problem and proposes a PSS-like control (DCPSS) to aid dc grid stability and cope with this effect. A generalized analytical model for studies on dc voltage control is presented. Key stability and transient performance issues caused by the use of the dc reactors in a multi-terminal system, are investigated by analyzing poles, zeros and frequency responses of both open-loop and closed-loop models. Design and location identification methods for the DCPSS are provided. Excellent damping enhancement is achieved by this controller. The analytical studies and time-domain simulations in this paper are performed based on two VSC-HVDC models.