Abstract
The small-signal stability of the LCC-HVDC system connected to the weak AC grid is of great concern. This paper derives the SISO model of the LCC inverter station. On this basis, the influence mechanism of AC system strength on system stability is studied via the Bode stability criterion, and the coupling interaction between different control loops is quantitatively evaluated. Results reveal that, as the AC grid becomes weaker, the system stability margin is diminished characterized by the larger phase lag in the phase-frequency response. Also, the interaction between DC voltage controller (DVC) and phase locked loop (PLL) control loops is intensified. Then, a derivative-feedback based compensation (DFC) control is presented to improve the LCC system stability under the weak AC grid condition. Theoretical analysis and simulation results indicate that the presented DFC control can effectively improve the stability and dynamic performances of the LCC system, which allows the system to operate stably even connected to the extremely weak AC grid.
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