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 VSC-HVDC 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. T Disconnector Auxiliary DC Breaker Hybrid DC Breaker Main DC Breaker Current Limiting Reactor Residual DC Current Breaker Fig. 1. ABB Proactive VSC-HVDC Circuit Breaker [1].0885-8977 (c)
This paper presents a comprehensive analysis of the limitations and the key dynamics of closed-loop active power control systems for VSC-HVDC, regarding stability, performance and robustness. Detailed dynamic models are derived and the controllability and robustness issues for VSC active power control are identified. Limitations imposed by ac system strength, converter operating point and current control design on the stability and performance of the two leading active power control principles are addressed, using frequency response analysis and time domain simulations. The dynamic interactions between the active power control design and the dc voltage droop control are examined. The simulations are performed using average-value VSC models and a high-fidelity modular multilevel converter model. Impacts of the active power control design on dynamic behaviors of multi-terminal dc (MTDC) systems are investigated using a four-terminal model. This paper provides a systematic study on the key stability and performance issues associated with the active power control. Furthermore the methodology offers a framework for the analysis of more complex active power and dc voltage droop controllers for future MTDC systems.
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