Model Predictive Control For T-type Three-level Photovoltaic Grid-connected Inverter

The photovoltaic grid-connected inverter is the key equipment that convert the initial DC power generated by the solar panels into the AC power used directly for the user. The requirements for performance of grid-connected inverter is high. This thesis uses the model predictive control methods to research T-type three-level photovoltaic grid-connected inverter deeply focusing on the two questions that neutral-point potential fluctuating and generating common-mode voltage.Firstly, based on consulting a number of domestic and foreign literatures, this thesis reviews the status of domestic and foreign development on photovoltaic inverters, and describes the neutral-point potential balancing and common-mode voltage of T-type three-level inverter.Secondly, in view of the neutral-point potential fluctuating of T-type three-level grid-connected inverter, a model predictive control method is used to study. Model predictive control optimizes the cost function g, cost function g is consist of the square error between the load predictive current and the reference current and the voltage divider between the two capacitor voltage. This method preferentially selects the switching states minimizing neutral-point potential as the gate drive signals applied to the inverter, enabling the inverter neutral-point potential balancing. Using MATLAB/Simulink to simulate and analysis, the results show that the deviation range of neutral-point potential is narrow, and the THD and the tracking error of grid current are both small, three-level output voltage waveform is closer to the sine wave.Finally, since the T-type three-level grid-connected inverter generates great common-mode voltage, which is harmful to the system and personal safety, the improved model predictive control is used for the common-mode voltage suppression. On the basis of ensuring the neutral-point potential balancing for the three-level inverter, the cost function g is improved, the common-mode voltage suppression target is added to the function g, so that the switching states generating smaller common-mode voltage are selceted and applied to the inverter. Meanwhile, in order to solve the time delay when using model predictive control in the third and fourth chapters, a two-step prediction is used for delay compensation. Using MATLAB/Simulink to simulate and analysis, the results show that the common-mode voltage of T-type three-level inverter is effectively suppressed to a lesser extent, and the grid current ripple is greatly reduced, waveform quality has markedly improved, the quality of the three-level voltage output waveform is high.