Stability region of hydropower plant with surge tank at HVDC sending terminal.

This paper aims to study the stability region of hydropower plants (HPPs) at high‐voltage direct current (HVDC) sending terminal, to theoretically predict the frequency stability of isolated HPPs under small disturbances. The overall transfer function of the system is derived, and the Hurwitz stability criterion is applied to deduce the stability region. The theoretical analysis of the stability region and time‐domain simulation results of the system show good consistency, and this proves the accuracy of the model. The focus of this paper is mainly on the influence of the hydraulic‐mechanical aspects. Improperly tuned governor parameters and unfavourable design of the surge tank could lead to the frequency instability of HPPs, which appears as divergent oscillations of the frequency of units. A key suggestion of this paper is that the control strategy should consider the factors from not only the electrical side but also hydraulic‐mechanical aspects, for enhancing the stability of the HVDC sending terminal. Besides, the influence mechanism and parameter tuning of frequency limitation controller (FLC) are investigated and the results demonstrate its capability of quick compensation for power disturbances at the HVDC receiving terminal. [ABSTRACT FROM AUTHOR]