Research On Control Strategies Of Fault Ride-through For Single-phase Grid-connected Photovoltaic Inverters

As the "bridge" between the photovoltaic(PV)distributed power generation systems(DPGs)and the power grid,the power quality and the stability of the grid connected PV inverters should be guaranteed by using the reliable control strategies.If a mountain of power electronics converters are off-grid when a grid fault occurs,the shortage of the power supply for the load demand would exacerbate the grid voltage fluctuations,and the grid fault scales would be expanded.Many countries require grid-connected power inverters with the low-voltage ridethrough(LVRT)capability in their updated grid code.Moreover,in extreme cases,i.e.,the grid voltage dips to zero,the zero-voltage ride-through(ZVRT)capability is also required.At present,many attempts have been made in the literature for the three-phase power system.And by using few grid information to realize single-phase grid-connected PV generation fault ride-through(FRT)operations with high-efficiency and high-reliability in the single-phase lowvoltage DPGs,that has become an urgent problem to be solved.In this thesis,taking singlephase grid-connected transformerless PV inverters as the research object,aim at designing,evaluating and verifying reliable control strategies,to achieve high precision grid fault monitoring and steady FRT operation.In the single-phase grid-connected inverters,the accuracy and dynamics of the grid synchronization unit have a direct impact on the performance of FRT operations in terms of stability.Thus,the most commonly-used and recently-developed PLL synchronization methods(T/4 delay PLL,EPLL,SOGI-PLL)for single-phase grid-connected inverters have been explored,and the comprehensive design guidelines to fine-tune their dynamic performance parameters are given.The simulation and experimental results are presented to assess the performances of the prior-art PLL methods in response to grid faults in terms of detection precision and dynamic response.However,the estimated frequency outputs have steady-state errors for ZVRT operation.In this case,a control strategy by modifying the single-phase grid system PLL scheme is then introduced,to improve the estimation accuracy of grid voltage frequency for the stability of the FRT operation.The transfer delay-based PLL(TD-PLL),i.e.the T/4 delay PLL,is still the favorites due to the simplicity,and easy implementation.However,it suffers from phase-offset errors and doublefrequency oscillatory errors,when the grid frequency drifts away from its nominal,or under distorted conditions.To tackle these issues,a transfer delay-based adaptive frequency locked loop(TD-AFLL)is proposed,A mathematic proof indicates that the proposed TD-AFLL can reject both phase offset errors and double-frequency oscillatory errors.Moreover,fast dynamics and relatively high accuracy of the TD-AFLL are achieved due to the adaptive frequency estimation based on the transfer delay structure.Notably,the harmonic immunity of the TDAFLL is poor,and it can be enhanced by adding a high-order adaptive observer-based filter,thus consolidating the applicability of the TD-AFLL in the DPGs.Furthermore,it becomes essential to combine a fast and stable current control mechanism with a dedicated control strategy for the FRT operations.In the conventional FCS-MPC for singlephase grid-connected inverters,with limited switching states and variable switching frequency,while these lead to current ripples,tracking errors,and scattered harmonic spectrums.Thus,a constant switching frequency FCS-MPC(CSF-FCS-MPC)method specifically suitable for single-phase grid-connected inverters is proposed.The CSF-FCS-MPC synthesizes the switching states in one sampling period.In addition,and the time allocation method is presented in detail.The performance of the proposed CSF-FCS-MPC is experimentally compared with the conventional FCS-MPC.The benchmark verifies the effectiveness of the proposal in terms of constant switching frequency MPC,and thus resulting in an almost ripple-free output current with high quality(i.e.,low harmonics).The power control loop is accurately and rapidly to regulate the output active and reactive powers of the grid-connected inverters in case of the FRT operations.According to the current active grid requirements,the inverters should provide reactive current injection to support the grid recovery.In this thesis,based on the single-phase instantaneous power theory,two flexible power control methods(i.e.,the ??-stationary reference frame power control and dq-rotating reference frame power control)are explored,and a power phase-angle shifting power control method is proposed.All the three power control methods are evaluated for grid-connected single-phase PV systems in the case of LVRT and ZVRT operations.Simulation and experimental results are presented,which verifying that three methods can help the PV systems to temporarily ride-through the grid low-voltage faults.However,the ??-stationary reference frame power control method is not suitable for use in ZVRT operations.The power phase-angle shifting control method has a better dynamic response.