Research On The Low Voltage Ride Through Technology Of Grid-connected Photovoltaic Converter

As the increased penetration of photovoltaic (PV) power generations in power system, moden grid codes concerning grid-connected PV converter are developed. As a result, the steady-state operation of grid-connected PV converter under normal grid conditions is well understood and applied. The low voltage ride through (LVRT) operation and control of the grid-connected PV power system under grid voltage fault conditions has been the main subject of much recent research and development worldwide. Therefore, the LVRT operation and control of the grid-connected PV converter under grid voltage symmetrical and asymmetrical fault were deeply studied in this dissertation. Under distorted grid voltage conditions, the control of the grid-connected PV converter was also deeply studied. Some important conclusions and the research results were obtained.The impact of symmetrical voltage drop on the AC side current and DC side voltage of PV converter are analyzed. On this basis, the redistribution of reference value of active current and reactive current by grid voltage feed-forward control links is used to suppress converter output current surge. The converter is running in static reactive compensation (STATCOM) mode that provides reactive power support to the grid. The simulation results verified the effectiveness of the LVRT control strategy.Via symmetric-component method, the mathematical model of converter is established under grid voltage asymmetrical fault. Based on the model, the influence of grid-connected impedance’s power fluctuations on DC bus voltage is analyzed. In view of the negative sequence current of AC side and DC bus voltage fluctuation problem, the control strategy of restraining negative sequence current of AC side and the control strategy of restraining quadratic fluctuations of DC voltage were respectively proposed. Considering converter output current restrictions, the PV converter’s output active power is reduced in proportion to retained grid voltage ensuring low voltage ride through of PV converter in the grid fault process. The simulation and experimental results verified the feasibility of the proposed scheme.The mathematical model of converter is established under distorted grid voltage conditions. Based on the model, the influence of distorted grid voltage on the operation of PV converter and grid-connected impedance’s power fluctuations on DC bus voltage is analyzed. The control strategy of suppressing harmonic current of AC side and the control strategy of eliminating the DC bus voltage fluctuations were respectively proposed. To improve the phase-locked loop, proportional integral-resonant (PI-R) controller was used to design control system. The simulation results verified the effectiveness of the control method.