Research On The Control Strategy Of NPC Three-level PV Grid-connected Inverter

Multi-level inverters have been widely used in high voltage and high power applicationfields because of their numerous advantages as following, lower withstand voltage level forpower semiconductors, lower switching losses, lower harmonic distortion, as well as betterquality of output waveform. Multi-level inverter is based on three-level inverter, So analysisand research on the three-level inverter is important. The main advantages of Space VectorModulation (SVPWM) are following, larger range of modulation, higher DC voltageutilization and better suited implementation on a digital computer. So the grid-connectioncontrol strategy of the whole system based on a SVPWM simplified algorithm was designed,the concrete steps as following.It is kowned that diode clamped three-level is the earliest, most applications structure byanalyzing the current research status on three-level inverter at homeland and abroad. So thediode clamped (NPC) three-level inverter was choosed as the research object, and thetopological structure and working principle was discussed in detail.Traditional SVPWM has disadvantages of intensive calculation operations and lowprecision. In this paper, a SVPWM simplified algorithm was applied, each big sector of thespace vector plane was divided into two right-angled triangle areas, the space voltage vectoris synthesized by several switch vectors corresponding to the right-angled triangle whichcontains the terminal of the reference vector. As a result, the calculation is greatly reduced, thecontrol to midpoint voltage of NPC three-level inverter, and the difficulty of the overallcontrol algorithm is reduced.A kind of three-phase phase-locked loop based on dq coordinate transformation principlewas proposed in view of the problem that traditional phase-locked loop could not obtainaccurate phase under the condition of voltage distortion. Using T/4(T is the three-phasevoltage cycle) delay calculation method in order to separate positive and negative sequencecomponent, the negative sequence component influence on phase was effectively curbed.Thesimulation and experiment results show that the phase-locked loop has dynamic response speed, good stability, and a strong inhibitory effect on the voltage distortion.NPC three-level inverter grid control strategy based on the SVPWM simplifiedalgorithm was desigened. MATLAB simulation was down and the results show that very goodgrid current waveform is obtained, the current harmonic content is less than1%.Test platform of10kw NPC three-level photovoltaic (PV) grid inverter was Establishedwith DSP2808and CPLD as the control core. Grid-connection control strategy based onSVPWM simplified algorithm was verified. The test results show that grid current harmonicis less than3%, the power factor is above0.99, the machine efficiency is up to98%, and it hasperfect protection function under the rated output power.