Synthetic Inertia Control To Support Microgrids Frequency Regulation

In recent years,the ever-increasing energy demand around the world with the desire for reducing carbon footprint has led to an expanded integration of renewable energy sources(RESs)into the power grid.Since RES are normally coupled to power grids through power electronic converters,conventional power systems gradually evolve into small-scale systems named microgrids(MGs).In which,the concept of MG provides an appealing infrastructure for integrating the growing penetration of RESs.However,the high level of RESs penetration may result in numerous technical and operational issues to such grids.Furthermore,replacing conventional generation units with RESs reduces the system inertia,causing high-frequency fluctuations,and this issue may be exacerbated when MGs operated in islanded mode.Hence,this thesis focuses on proposing appropriate control strategies to support the frequency regulation of low-inertia MGs.One of the effective solutions towards regulating the frequency of low-inertia MGs is to emulate the behavior of conventional synchronous generators(SGs)virtually into MGs,thus improving system stability and resiliency.This concept is defined as synthetic inertia control(SIC),where a proper inertia control technique can be used in combination with energy storage systems(ESSs)to provide supplementary inertia power to the MG.However,in the face of various grid uncertainties,the low system inertia still poses a threat to the frequency stability of MGs.Fortunately,the robust control approaches have available tools to deal with RES/parameters uncertainties and disturbances.Taking advantage of that,a new robust SIC scheme is proposed in this thesis to support the frequency stability of islanded MGs that are critically influenced by high RESs fluctuations,as well as load perturbations and system uncertainties.The robust synthetic inertia controller is designed based on the coefficient diagram method(CDM)which has a simple structure and flexible design procedure.The proposed strategy assured better dynamic performance in terms of frequency excursion,and a better restoration process.Furthermore,when applying the SIC scheme,frequency measurement platforms such as phase-locked loop(PLL)are required to obtain the estimation of the system frequency data.However,the use of PLL can lead to greater system frequency fluctuation and instability.Therefore,a new SIC scheme based on an optimal robust controller is proposed to support frequency regulation in a two-area interconnected MG considering frequency measurements effects,system uncertainties and nonlinearities,and high RESs penetration.The proposed optimal controller is utilized for well-tuning the key parameters of the proposed robust controller,which eliminates the drawbacks of trial and error methods commonly used for estimating robust controller parameters.Moreover,the constraints of the robust controller based on CDM have been estimated by a novel metaheuristic optimization algorithm named chaotic crow search algorithm(CCSA),to reduce the computational time and enable the search process to be carried out at higher speeds.The simulations results proved that the proposed robust control strategy assured better dynamic performance and high robustness in face of high RESs penetration,loads/RESs disturbances,and parametric uncertainties.On the other side,another effective approach for emulating real SGs characteristics is to enable grid-connected power converters to behave like virtual synchronous generators(VSGs).Modular multilevel converters(MMCs)have proved their superiority over traditional voltage source converters,particularly in medium or high voltage applications.In this regard,a comprehensive VSG design based on MMC as an interface between RESs and the MG is proposed in this thesis to take over the responsibility of MG frequency regulation that was earlier provided by SGs.In which,a power/frequency control strategy devoted to the outer loop controller of the MMC converter is adopted to generate the required phase angle and frequency for controlling the MMC to behave as a VSG.In the same context,an enhanced SIC scheme,which includes emulating both inertial and damping properties,is proposed in this thesis and applied to MMC integrated with battery ESS.The MMC in this case is employed as an interlinking converter(IC)between hybrid AC/DC MG.The proposed control scheme is provided to support frequency regulation in the AC sub-grid as well as DC voltage restoration in the DC sub-grid,and ensure a reliable power balancing for the hybrid AC/DC MG.The MMC capacitors are developed to cope with short-term power fluctuations introduced by inertia emulation while the batteries are developed to compensate for the slow-varying power fluctuations resulting from damping emulation.These improvements and strategies have been tested and analyzed via MATLAB-Simulink environment to reach simple,secured,and applicable control methods.