Harmonic analysis of grid-tied active front end inverters for the frequency range of 0-9 khz in distribution networks: Addressing future regulations

The existing international standards define grid harmonic limitations only up to the 40th or 50th harmonic. Increasing trend of grid-tied inverters in renewable energy systems causes grid distortions beyond the existing harmonic standardization range leading to low power quality, consumer load damage and excess power loss in the power network. Hence, this paper investigates the influencing factors towards these harmonic emissions of grid-tied inverter systems. Controllers for the inverters are designed for rated operation condition assuming the DC-link voltage and grid voltage is ideal, even though these systems mostly operate at partial powers and distorted DC-link and grid voltage conditions in practice. Hence, this paper presents an analysis of grid current distortion when the inverter operates with fluctuating DC-link voltage, different power levels and controller gains of the proportional resonant controller. The results show high gains of the controller, partial power operation and low frequency fluctuations of DC-link voltage cause increase in harmonics up to 2 kHz while there is no significant effect for 2-9 kHz harmonics. Further, the study demonstrates the significance of grid current distortion when grid voltage is distorted at the frequency where output impedance of inverter together with grid impedance is minimum. This analysis is presented for both stiff and weak grid conditions since the grid distortion varies with different grids.