Research On Neutral Point Clamped Three-level Three-phase Photovoltaic Grid-connected Inverters

With the energy crisis and the environment becoming more and more serious, therapid development of photovoltaic grid-connected technology has become theinexorable trend, and the scholars all over the world pay more attention to the study ofphotovoltaic grid-connected inverters. Due to the advantages of better power quality,lower harmonic components and higher voltage-capability, the Neutral Point Clamped(NPC) three-level inverters are widely used in the high-voltage and high-power andrenewable energy generation fields. The topology and controlling strategies areanalyzed and studied deeply in this paper.On the basis of the topology of NPC three-level inverter, this paper introduces theworking principle of three-level three-phase inverter, and then analyzes the SpaceVoltage Vector Pulse-Width Modulation (SVPWM) technology specifically. Thejudgment among sections and the time for working of every space vector are all given inthe paper. For the problem of neutral point voltage imbalance, this paper summarizesthe causes leading to the imbalance and proposes an accurate neutral point voltagecontrolling method: balanceable factor. The method controls the neutral point voltage ina reasonable range through adjusting the working time of positive small vector andnegative small vector, and the feasibility is demonstrated through the simulation andexperiment.This paper establishes a dynamic math model for the NPC three-level three-phaseinverter and all the variables of the inverter are transformed from ABC three-phaseimmobile coordinate system to dq synchronous rotating coordinate system throughthe Park and Clark transformation. Then this paper studies the grid-connectedcontrolling strategy: the indirect Voltage Oriented Control (VOC). The strategy adopts acontrolling scheme that the outer DC voltage control loop and internal current loopwork in coordination. With the strategy, the NPC three-level inverter could work reliably and the active power and the reactive power can also be modulated effectively.This paper designs a10KW NPC three-level three-phase inverter, and the hardwareand the software of the inverter are introduced respectively. The hardware includes thecontrol unit, driving protection unit and the signal sampling unit of the system and thesoftware includes the main loop program, interrupt service program and the SVPWMsubprogram. Through a lot of experiments and the analysis of waves, the availabilityand accuracy of the all strategies proposed in this paper are demonstrated effectively.