Research On Proportional-resonant Controller For Grid-connected Converter With LCL-filter

With the overusing of the coal, nature gas and other fossil fuel, the human hadto search for more durable, more environmental new energy to meet the increasingneeds of people. It has been being widely studied that how to make full use of thegeneration power of solar energy, wind energy, tidal energy and other new energypower, so the grid-connected converter plays an important role in the transformationof electrical energy. The transformed power is either transmitted to electricity grid,or used to drive different loads independently. In this paper, the control policies ofgrid-connected converter under the symmetric grid and asymmetric grid are studied.First of all, the mathematical model of the grid-connected converter withLCL-filter is analyzed, after which the article builds respectively mathematicalmodels of the static coordinate system and the rotating coordinate system under thesymmetric and asymmetric power grid. At the same time, the positive sequence andnegative sequence decomposition problem of each electric is considered under theasymmetric power grid, which adopts a modified instantaneous symmetricalcomponent method to make an effective positive sequence and negative sequencedecomposition of the grid voltage and current. The results are verified by thetheoretical analysis and the simulation. Secondly, the LCL-filter parameters,including converter side inductance, power grid side inductance and the filtercapacity, are designed in detail.Secondly, based on the control strategy research of grid-connected converter,this paper firstly analyzes and researches the traditional grid voltage oriented vectorcontrol strategy. Under the symmetric grid, this control strategy can be very good tomeet grid standards, the access current becomes sine wave, the harmonic distortionrate is very small, as well as the grid power factor is closed to1. Under theasymmetric grid, because there are the positive and negative sequence componentsin power grid voltage and access current, the dc side voltage, the active and reactivepower and the dq axis component of the access current contain second harmonicfrequency component during steady state operation, which seriously affects thequality of grid. Based on this, a kind of commonly used control method is doubledq-PI current control strategy based on the positive and negative sequence component theory, which can complete the decoupling control of the dq axial currentcomponent in the positive and negative sequence respectively. Through thesimulation analysis the control effect is verified to be good, and it will be able torealize the control of various electric amount according to the different controltarget.At last, the double dq-PI current control strategy based on the positive andnegative sequence component theory has some shortages, such as the large amountof calculation, complex parameter adjustment of four needed PI controllers, thepositive and negative sequence decomposition problem of the access current andreal-time performance degradation. In the light of these, this paper puts forward acurrent control strategy of based on PR (proportional-resonant) controller. Above all,the paper analyzes the grid-connected control strategy and the parameters of the PRcontroller are designed in detail. Then the simulation model is built in theMATLAB/Simulink and tested in the symmetric grid and asymmetric grid. Theresult is similar to the result of the double dq-PI current control strategy, however itcan make the calculation of the proposed control strategy reduce by half and doesn’tneed decoupling control.