Nonlinear dynamic characteristics analysis and adaptive avoid vortex-coordinated optimal control of hydropower units under grid connection

Abstract

Hydropower units operate as the first option for renewable energy supply. The coupling characteristics of hydro-turbine governing system (HTGS), misaligned shafting (MSH), and power grid (PG) (HTGS-MSH-PG) have always been omitted. Meanwhile, the safety hazard from the draft tube vortex zone (DTVZ) and the difficulty of coordinating the ultra-low frequency oscillation (ULFO) and primary frequency regulation (PFR) performance of hydropower unit grid-connected operation have not been resolved. In this study, we aim to investigate the interaction mechanism and nonlinear dynamic characteristics of the HTGS-MSH-PG coupling system and determine the optimal control strategy for considering the DTVZ, ULFO, and PFR of hydropower units. First, a novel coupling model of HTGS-MSH-PG is established. Thereafter, the stability of the HTGS-PG coupling system is examined by employing the Hopf bifurcation theory, and the damping characteristics are obtained using the hydraulic damping model. The nonlinear dynamic behavior of misaligned distance, nonlinear vibration characteristics of MSH, and influence of MSH vibrations on HTGS and PG are revealed based on this model. Finally, an adaptive avoid vortex-coordinated optimize control strategy which considers the means to prevent a strong DTVZ and coordinate the ULFO and PFR performance of HTGS, MSH, and PG is proposed. NSGA-II is introduced to obtain the Pareto optimal solution set, and the optimal scheme is determined by the fuzzy satisfaction method. Moreover, the nonlinear dynamic characteristics of hydropower units under optimal control are revealed. The results demonstrate that the nonlinear dynamic characteristics of the HTGS-MSH-PG coupling system mainly include the quasi-periodic multi-frequency vibration of MSH and multi-frequency dynamic performance of HTGS and PG. The adaptive avoid vortex-coordinated optimal control strategy can effectively prevent a strong DTVZ, coordinate the ULFO and PFR performance of HTGS, MSH, and PG, and increase the satisfaction rate by 72.2%. These results lay a theoretical and technical foundation for the safe, stable, and optimal operation of hydropower units.