Research On Load Unbalance Control Method For Photovoltaic Microgrid

With the gradual rise of distributed photovoltaic power generation,photovoltaic microgrid has been widely used as a small grid connecting distributed energy.As a new generation system,photovoltaic microgrid can make the power supply in remote mountainous arems be solved,and can solve the energy problem of power system through natural energy.Elewever,due to the influence of sunshine,load and equipment in photovoltaic power generation system,photovoltaic microgrid requires more stringent power quality problems much as voltage and current.When the unbalanced load is connected,the output voltage of photovoltaic microgrid is unbalanced,and the short voltage of users is unbalanced,which has a serious impact on the life of the equipment.If the fault occurs,the output fault current will ecceed the unbalanced current value,and it will burn the equipment easily.The poor power(?)of PCC points has a serious:impact on users or power grids,resulting in fluctuation and instrability of power grids.According to the above situation,combined with the unstable influence of voltage and carrent,a control method combining unbalanced voltage and fault current is proposed in this pager.Voltage control is accomplished by primary control and secondary control.Primary(?)carries out P/Q power calculation and virtual sag control.P/Q power calculation caicukates the voltage and current of PCC points.The power obtained is converted into walltage and frequency through sag control.Sag control can distribute the input and output parwer of photovoltaic inverters.Because of the voltage and frequency in primary control,sag(?)can distribute the input and output power of photovoltaic inverters.The rate is(?).In the secondary control,the deviation of voltage amplitude and frequency is further eliminated by the standard judgement of the secondary control,so that the deviation is within a eartain range of deviation.Fault current control method by adding fault current judgment in(?)control,when the load is unbalanced or when the fault occurs,the unstable current weill far exceed the rated current.If the fault current exceeds twice the rated current,it will be switched to the fault current limiter to control,so as to always ensure that the fault current works within twice the rated current.Finally,three DG simulation models are built on MATLAB/Simulink simulation platform.One DG works under voltage control and the other under current control.The three-phase load imbalance under voltage control and three-phase unbalance under voltage and current conitrol are simulated and compared.The waveforms of voltage and current before and after adding load in three-phase balancing situation prove that adding load affects voltage but current waveform has greater influence,which is within a certain control range.In the three-phase unbalanced situation of voltage control,the waveforms before and after adding compensation are compared to prove the rationality of the proposed control method;in the three-phase unbalanced situation of voltage and current control,the three-phase unbalanced situation is proved.I hen,the fault of three-phase to ground,two-phase to ground and two-phase to ground are verified respectively.The waveform shows that the voltage and current control method not only has a good control effect on voltage but also on current.