A DC Bus Voltage Control Strategy for Grid-connected Photovoltaic Inverters under Variable Active and Reactive Power Set-Points

The integration of new and advanced functionalities to grid-tied photovoltaic inverters looks forward to improving the power quality, reliability, and stability of the distribution grid. In that case, the injection of controlled active and reactive power is essential to achieve the integration of those capabilities. Firstly, this paper presents an analysis of the current waveform quality in a grid-tied photovoltaic inverter under different power factor scenarios. The operation of a three-phase inverter is analyzed by simulations in MATLAB/Simulink for different apparent power set points and a sweep in the power factor. For each case, the quality of the injected currents was analyzed by using as evaluation criteria the Total Harmonic Distortion (THD) and the RMS value of the tracking error. The results show that the photovoltaic inverter reaches a better performance when it operates under leading or unit power factors. Furthermore, it is shown that the operation under lagging power factors can result in high tracking errors increasing the THD of the injected current. To overcome this drawback without increasing the high-frequency harmonics in the output currents under low modulation ratios, a reference voltage control strategy for the DC bus is proposed, which achieves a correct tracking of the reference current for any power factor value under nominal operating conditions. Simulation results of the proposed strategy are presented, which verifies the good performance of the power inverter and the improvement of the THD values.