Research On LVRT Control Strategy Based On PV Grid-connected Inverter

Given the fact that Photovoltaic inverters’ off-grid,or the trip repeatedly will lead toinsufficient power output of the PV array, the low efficiency, engineers conduct in-depthresearch on inverters working modes under the grid failures. The basic idea is to ensure theinverters’ current less than rated current and being not off-grid.Meanwhile, the inverterstransport active, reactive power, or even complete reactive power to the grid. The thesisstudies on inverters’ fault ride-through capability in the respect of LVRT control strategy.Comparing active current and the rated current under symmetrical grid fault, when thedepth of the voltage drop is less than50%and the active current is less than the ratedcurrent,give out active current reference and MPPT is normal. When the active current is notless than the rated current, disconnect the voltage outside loop,stop MPPT, the active currentequals to the rated current, and limit active power output of the PV array. When the depth ofthe voltage drop is more than50%, disconnect the voltage outside loop,stop MPPT,andinverters inject complete reactive current to the grid. When inverters ride through symmetricalfault successfully, restore MPPT,current inside loop and voltage outside loop control.Inverters inject asymmetric positive, negative sequence current into the grid using P-Rcontroller under grid asymmetrical faults circumstance,achieving output without staticerror,reducing the grid-conected power shock,avoiding complex feedforwardpositive,negative sequence component of the decoupling dual synchronous current controllerwith static error. Using positive, negative sequence elastic control determines currentcontroller’s reference current, realizing active current, reactive current dynamicallyadjustable. Use decoupling capacitors and bidirectional DC/DC converter to decouple DCpower, AC power and to absorb the second harmonics power.Introduce a novel SVPWM modulation to simplify the distribution of sectors and theformula of fundamental space voltage vector working time, reduce the storage space, improveDSP2808efficiency of code execution.Small vectors are used as starting vectors in eachsampling period to reduce the switching frequency, improve midpoint voltage’s fluctuation.Given the small vector having the greatest impact on the midpoint voltage,this topicintroduces equilibrium factor method to assign the working time of the small vector properly, decreasing midpoint voltage fluctuation amplitude, power tube switching losses,grid-connected current harmonics and improving the overall efficiency.