Research On Control Of Grid-side LCL-VSR And Its Responses To Voltage Dip For Wind Power Generation

The grid-side converter plays a very important role in the wind power generation.,with the increase of system power rating,LCL filter are increasingly being used in grid-side converter. As for the third-order system, whose control is more complex control, and stability decrease, the control strategy commonly used can not achieve the grid-side current-direct control. In response to these shortcomings,firstly, mathematical model of three-phase voltage source PWM converter with LCL filter (LCL-VSC) is established. The impact of various variables is being studied, which build up an adequate theory of foundation for control strategy proposed.Basing on full studying on the control object, a novel control strategy of LCL-VSC based on notch concept is proposed. In order to achieve a grid-side current-direct control, the feedforward variables are made a reasonable choice. Because this method requires the filter capacitor current value, in order not to increase the number of sensors, this paper introduces the concept of virtual flux,which can estimate the capacitive current.To overcome another problem of gride-side converter is voltage dip. With the proportion increasing of wind power generation in power grid, the power grid demand for increasingly high to wind turbine. Low-voltage through technology (LVRT) is one important indicator. To achieve this control strategy, a very important point is that , under the conditions of the voltage dip, can conrol the grid-side converter very good, to make the DC side voltage is stable within a certain range. In this paper, the AC-DC-AC structure commonly used in wind turbine is introduced, and introduce the concept of power balance control. For the general disturbance, the methed can maintain a constant voltage of DC-side very good, and for the voltage drip,which is not very serious,it can limit DC-side voltage fluctuation. Certainly, the double-fed motor control performance can be ensure, to avoid unnecessary crowbar switching. Base on power balance control, the article introduce the minimum capacitance design to be able to make quantitative analysis for the DC-side capacitors. To consider certain extreme conditions, which can lead to the amplitude of DC-side voltage fluctuation, and combined with design constraints,we can quickly calculate the theoretical minimum value of DC side capacitor. In voltage dip control, combining with the capacity of converters, appropriating to relax the DC side capacitor design margins, we can further improve the grid-side converter control performance under the voltage drip situation. This method also has a good theoretical guidance for practical system design.Finally, a 15kVA AC-DC-AC experimental system is established, including grid-side converter with the LCL filter (LCL-VSC) and load-side inverter. Observations each variable waveforms with the active damping or not, the result is same to theoretical analysis and simulation, which verify the effectiveness of active damping; Related experiments of voltage dip control also been done, which also give the correctness of the theory in the certain degree .