Research On Ultra Low Frequency Oscillation Restriction Strategy In The Hydro-power Dominant System

Depending on the geography,the forms of the available primary energy source,and the level of technological development,power networks across the world have their particular fundamental features for energy generation.Many nations have vast hydraulic resources in hilly areas that are perfect for power generation.As a result,as these nations expand and prosper,their hydro-resources have become a key element of their energy supply,aiding to minimize the region’s dependency on fossil fuel-based electricity production.Modelling electric power systems allows us to reproduce the dynamic behaviour of the interacting components in electric power systems.Although computer simulation has been a developed research trend for decades in electrical engineering,some phenomena and observations are still not easy to capture.In the case of hydro-dominant power systems,among other phenomena,low-frequency and very low-frequency oscillations have been observed and suspected to be the cause of major incidents.Accurate representation of hydro-electric power systems deals with the effects of the water inertia(water transients,water-hammer),nonlinearities in the controls and actuators,hydraulic coupling of turbines,control strategies.The proposed research aims to focus on the accurate representation of these hydro-electric power systems and further bring improvements to their integration to large-scale power systems regarding frequency stability.The level of accurate modelling helps reproduce and understand better oscillatory observations by computer simulation.New control strategies are also proposed to damp the oscillatory modes while considering the effect of the dynamic behaviour of coupled systems and nonlinearities in their controls.Finally,the drawbacks of large-scale integration of these hydro-power plants are depicted and investigated in an attempt to bring improvements.The research content of this present document is divided into 7 chapters.The first chapter explores the trend in hydropower generation and hydro-turbine technology,then investigates the major developments in terms of control and optimization.Existing control techniques,available in the industry and research environment,are cited and benchmarked to optimize the operation and integration of the hydropower plant.The second chapter provides the foundation to design the hydropower plant simulation model for frequency dynamics.It puts on view the theory and method of hydropower plant modelling which the design of governor controller,hydro-governor and hydro-turbine models are based-on in MATLAB and other main available commercial softwares.Associated equations and physical coefficients are given and discussed.Chapter 3 discusses the mechanism of ultra-low frequency oscillations occurring in the hydro-dominant power grids.The model of the prime-mover,which is constituted by the hydro-turbine,the governor and its integrated controller,is mathematically re-established.The established model is then studied in the frequency-domain when integrated with conventional control loops and controller tuning methods.For a better consideration of real-life operation aspects,the representation of the non-linearities,which are inherent to the mechanical system,is also investigated.This chapter provides detailed research for designing and accurately simulating the occurrence of ultra-low frequency oscillations which is mandatory before investigating with advanced control techniques as new solutions.This thesis also explores the benefit of integrating virtual inertia into hydro-dominant power grids.Chapter 4 goes in depth with vehicle to grid(V2G)schemes serving to emulate inertia in such a way to appropriately damp the ultra-low frequency oscillations.The V2G scheme and the control methodology are presented as long as the results of successful implementation.Chapter 5,in the same path,investigates the integration of wind turbines into the hydro-dominant grids.The ability of doubly-fed induction generators(DFIG)to provide virtual inertia,is made use of.Proper inertia emulation and frequency control technique is presented.The subsequent dynamicresponse shows effective results in providing enough damping to the grid which gains resilience to the occurrence of ultra-low frequency oscillations.Finally,Chapter 6 draws the conclusion and discusses the innovations and future prospects.