Research On Zero Voltage Ride Through Control Of Grid-connected Photovoltaic Inverter

As the typical representative of clean energy,photovoltaic power generation technology has made great progress and development in recent years.However,with the continuous improvement of photovoltaic power generation,the impact of photovoltaic power station for the safe and stable operation of power grid is gradually emerging.If the large capacity photovoltaic power plant takes off grid automatically during the fault period,it will cause the power grid frequency and voltage collapse,and a huge loss of life and production.In order to minimize the adverse effect of power photovoltaic power plants for the grid during the failure period,and accelerate the self-healing speed of the power grid,China has formulated the relevant grid-connected standard,including zero voltage ride through.Therefore,it is of great significance to carry out zero voltage ride through research to ensure the safe and stable operation of photovoltaic power plants and power grids.Based on mastering the principle and output characteristics of the solar photovoltaic power generation,the thesis analyzes the maximum power point tracking algorithms and topologies of grid-connected inverter.The single-stage grid connected inverter is taken as the research object.Through establishing the mathematical model of the inverter,the thesis determines the grid-connected control strategy that is voltage and current double closed-loop control,and makes the simulation researches.The simulation results show that the perturbation observation method can quickly and accurately search for the maximum power point that in different external environments,the double closed-loop control strategy ensure that the power form photovoltaic cells is delivered to the grid timely,and achieve grid connected control.Aiming at the response characteristics of grid connected inverter during the grid symmetrical and unsymmetrical fault,the thesis discusses separately the changes of inverter DC side voltage,output current and output power,then analyses the cause that photovoltaic power station takes off grid during grid faults is the grid current large.Finally,through simulation experiments to verify the correctness of the analysis.According to the requirements of the zero voltage ride through,taking grid current symmetry,no overcurrent and injecting reactive current in accordance with the connected-grid requirement as control targets,the thesis puts forward to the direct current control,finite control set model predictive control and model predictive control modulation function control method successively.The direct current control method introduces the double closed-loop control to achieve the decoupling of active and reactive component of positive and negative sequence current and the fast tracking of the instruction value.Finite control set model predictive control selects the final switching state through establishing discrete inverter mathematical model during fault period and introducing value function to evaluate all possible switching states.Model predictive modulation function control method is improved on the basis of the model predictive control method.This method gets SPWM signal that based on fixed switching frequency through calculating minimum current error modulation function by taking derivative of variance evaluation function.Finally,the simulation results show that three methods can all achieve zero voltage ride through during grid fault period and meet the requirements.But comparing to the other two methods,model predictive modulation function control method has quick response,smaller overshoot and total harmonic distortion,and better control effect.