Low-Voltage Ride-Through Control Strategy For Photovoltaic Grid-Connected Systems

Nowadays,the penetration level of photovoltaic(PV)grid-connected systems grows rapidly.It impacts the stability of the power systems more and more serious.Under a deep grid voltage dip,the disconnection of PV grid-connected systems will bring adverse effect to the power grid stability.In order to prevent this event from happening,many countries have issued new grid codes,which require the PV power plants should have low voltage ride-through(LVRT)ability.Baesd on a two-stage photovoltaic grid-connected inverter,composed by a front-end boost converter and a downstream three-phase T-type three-level inverter,the LVRT control strategy for PV grid-connected systems will be studied,the key research work is as follows:Firstly,the basic principle of the two-stage PV grid-connected inverter under the normal grid condition is presented.On the basis of the small-signal model,the controller of the front-end boost converter is designed.Then,the small-signal model of the downstream three-phase inverter is established in the synchronous rotating frame.The dual-loop controller including the dc-link voltage outer loop and the inner inductor current loop is designed.The manchnism of neutral-point potential imbalance is revealed,and a solution with zero-sequence voltage injection is given.Simulation results verify the theory analysis.Secondly,based on the small-signal model of the two-stage PV grid-connected inverter,this paper studies the effect of symmetric and asymmetric grid voltage dip on the PV grid-connected system with conventional control strategy.It is concluded that the main incentives that triggers LVRT failure are overvoltage or overcurrent.Besides,the asymmetric voltage dip will lead the even order harmonics get rise to the dc-link voltage and the odd order harmonics arises in the inductor current.Thirdly,based on the analysis of the incentives that triggers LVRT failure,the control strategy for the two-stage PV grid-connected system under grid voltage dip is improved.When grid volage dip occurs,the maximum power point tracking control algorithm of the front-end boost converter is disabled.As a result,the delivered power balance can be achieved,and the amplitude of the dc-link voltage is limited.Further,the voltage at the point of common coupling(PCC)is fed-forward,which can not only limit the overshoot of the inductor current,but also eliminate its negative-sequence components under an asymmetric voltage dip.Based on these improvements,the dc-link overvoltage or the inductor overcurrent of the PV grid-connected system will no longer be triggered,and LVRT failure may be avoided.Simulation results are given to validate the theory analysis.Finally,an 8.2 kW prototype for the two-stage photovoltaic grid-connected inverter is built and tested.The effectiveness of the neutral-point potenial balance scheme is verified.