Modeling And Frt Control Of Grid-connected Photovoltaic Systems

The integration of power generation systems from photovoltaic(PV)system has been increasing worldwide in response to increase of environmental issues related to utilization of fossil fuels and continuous increase of global energy demand.In the last few decades,the cumulative installed capacity of PV has grown at an extraordinarily large rate globally.In line with this,the PV power generation system are expected to account for a significant share of future power generation systems,as the cost of PV modules and the entire PV system declines due to technological advances and government incentives under the provision of clean energy,and green economy.This increase in PV systems integration causes a significant challenge to grid operation in terms of power system stability and reliability.Thus,the successful integration of PV systems into grid requires an effective control strategy that operates well under normal and abnormal grid conditions as well as in strong and weak scenarios,and fulfils the grid code requirements that are aimed at facilitating reliable and safe grid integration of power electronics-based power generation systems.Accordingly,this thesis has developed model and control structure for two-stage grid-connected PV system under normal and abnormal grid conditions.In normal operation,a multi-layered control structure for regulateing two-stage grid-connected PV system has been developed to meet grid requirements and effectively facilitate grid integration of PV system.The developed control scheme performs MPPT function,which is implemented by a boost DC-DC converter and regulates current injected into grid with required power quality.The required power quality is achieved by controlling the grid-connected inverter in the multi-layer control structure,where current control is done in the dq-frame using the PI controller.In addition,a complete modeling of two-stage PV system operating under balanced grid fault was developed to facilitate the integration of PV systems that reliably operate through low voltage while providing reactive current during voltage dips based on grid requirements.The developed model provides proper current reference generation and appropriate controller design to effectively achieve low voltage ride-through(LVRT)to enhance power quality and stability.Further,the developed scheme helps to achieve the provisions of grid code and to guarantee smooth operation for PV system under balanced grid faults with current amplitude limit functions.Moreover,this thesis also proposes a comprehensive control strategy for grid-connected two-level photovoltaic system operation.The proposed control structure provides a comprehensive LVRT strategy for grid-connected photovoltaic systems under unbalanced grid faults.The proposed strategy includes a power reference calculation method to meet the requirements of the grid code;active and reactive power control strategies;and their controllers through flexible power quality control and peak current limiting to effectively implement LVRT.In addition,the strategy includes detailed development of a new current reference generator in the dq frame to achieve flexible power quality control during LVRT and peak current limiting to avoid overcurrent in the inverter.The developed current reference generator enables reliable operation of the photovoltaic system under different grid faults.Furthermore,an FRT scheme that reduces PV power generation has been proposed to prevent an excessive increase in DC voltage during a fault.In this work,PV array power reduction is employed to avoid DC link voltages from increasing with DC link choppers.First,the reduced PV array power is injected into the grid;however,if the DC voltage increases due to a very large drop or long fault duration,the DC link chopper will be activated to prevent the DC link voltage from excessively increasing beyond the allowable limit.In addition,the proposed control strategy includes an improved hybrid grid synchronization and sequence separation method,which in turn is employed for grid voltage amplitude calculation that further used for fault detection.The detailed modeling and control strategies developed were validated through extensive field testing and simulation to assess its efficacy.Finally,this thesis has investigated the impacts PV system integration on weak grids.The studied impacts of PV system include the analysis of influences of different control strategies for PV power plants in weak grids on output of PV system.A simplified dynamic PV power plant model has been developed to conduct the investigation.The comparison of impacts of different control strategies during grid faults have been analyzed under different load modeling and grid strengths along with different reactive power control modes.An improved control strategy with reduced active current proportional to remaining voltage magnitude at the point of common coupling(PCC)that restores to its pre-fault value with pre-set ramp rate has been recommended.MATLAB/SIMULINK simulations validates the effectiveness of the control strategies of PV system connected to utility grid.