Research On The Inverter And Low Voltage Ride-Through Control Strategy Of Grid-Connected PV System

With the further economic globalization, the world energy crisis has been increasing and the irrational use of traditional fossil energy causes a series of environmental problem. For solar energy resources is unlimited,universally,clean and economic make the solar power technology has been developed rapidly. PV installed capacity in the proportion of newly installed capacity all over the world is increasing. As the core component of the grid-connected PV systems, the design performance of the inverter’s control system directly affects the performance of the whole system. Meanwhile, with the proportion of the PV power station increasing, the low voltage ride-through ability is directly related with the security of power systems, attracts widespread attention and becomes a hot research.In this paper, the control strategy under normal and fault operation state of the power grid of three-phase single-stage grid-connected inverter is studied in depth. Its low-voltage ride through control strategy under grid voltage fault state is preliminary explored.Firstly, the establishment of a PV array model and the implementation of the MPPT method are introduced. A method is suggested by combining constant-voltage method and incremental conductance method, which can effectively solve the oscillations exist in the traditional method and shortens the track time. Simulation results show that its optimizing speed increases about5-fold than traditional incremental conductance, and the tracking accuracy is significantly improved.Secondly, the grid voltage vector oriented photovoltaic grid control system in normal operation state of the power grid was simulated based on full knowledge of photovoltaic grid power characteristics. With the utilization of coordinate transformation advantage that it can convert AC capacity to DC capacity, the system controller is designed by converting mathematical model of the PV inverter system to the dq rotating coordinate system. The voltage and current double closed-loop system is designed and a specific parameter calculation method is given. Meanwhile, a detailed study of the space voltage vector modulation method is included, and the SVPWM simulation module is created by programming.Again, the accurate extraction of positive and negative sequence component in fault grid state is crucial. In this paper, a notch designed is added to the traditional software PLL, so that it can pinpoint the phase angle of the positive sequence component in the grid failure to achieve the control system design based on double dq coordinate transformation. There is also a depth study of control strategy targeting with suppress the grid to negative sequence current and DC voltage fluctuation.Finally, the LVRT technical requirements of PV system are described in detail and dynamics change of the inverted system power feature is analyzed in depth. A LVRT control strategy based on reactive current compensation is proposed on the premise of meeting the low voltage ride through technical requirements. This control strategy can take full advantage of the characteristics of the inverted system without adding additional hardware devices, through which the photovoltaic power plants are able to to meet the low voltage ride through requirements and to provide a certain amount of reactive power support to the network-connected voltage.This paper researches in depth of the PV inverted system and designs the control strategy in the condition of normal operation and fault running state of the power system. Simulation studies of PV LVRT control strategy are performed on this simulation platform after a good performance of the control system in simulation results. Simulation results demonstrate that the strategy can make the grid curren of photovoltaic power plants being limited to less than1.1times of the rated current when network voltage drops. Meanwhile, a certain amount of reactive power can be transported to the grid and network voltage increases, so that the photovoltaic power plants remain network-connected operation in grid failure state.