Research On Control Method Of Three-phase Grid-connected Converter With LCL-filter

With the development of global economy, the increasing social demand for electricity has caused shortage of fossil energy together with the problem of environmental pollution, which has attached more and more attention all over the world. With excellent prospects, the grid-connected distributed generation technology based on renewable energy has got into a hot spot for research. Since grid-connected converter is the key part in renewable energy distributed grid-connected system, then study of its structure and control strategy is of great importance for application and popularization of this technology. Based on three-phase LCL grid-connected converter system, this paper has studied the following three aspects: optimization of filter parameters, improvement of control strategy and design of grid-voltage sensorless.Firstly, the relationship between parameters of LCL filter was analyzed. Aimed at minimizing damping resistance loss, the inductance value ratio was determined, also other LCL filter parameters were designed. Secondly, considering the traditional grid voltage orientation vector control strategy has problems such as large amount of calculation and complex structure, Mathematical models of LCL three-phase grid-connected converter were respectively established in three-phase static coordinate, two-phase static coordinate and two-phase rotating coordinate. Double closed-loop control strategy was proposed using instantaneous power outer loop and inverter-side current inner loop for the control of LCL grid-connected converter. PI controller was designed to realize constant frequency quasi direct power control.Finally, sliding mode variable structure theory was applied in the LCL grid-connected converter direct instantaneous power control. The active power and reactive power were used to construct sliding mode variable, and exponential reaching law with the Slid function was utilized to weaken the chattering. Then LCL grid-connected converter realizes unity power factor operation and the higher harmonics of the output current are filtered out effectively. To further improve system stability, virtual flux variables were introduced in direct instantaneous power control algorithm. By using a modified first-order low-pass filter in virtual flux observer, the grid side voltage sensors can be canceled, which realizes control of system without grid-side voltage sensor.Simulation results show that the grid-voltage sensorless sliding mode variable structure direct instantaneous power control strategy has good stabilit y and dynamic response.