Control of Grid-Tied Inverter with L Filter in Weak Grid Considering Grid Impedance and Harmonics

Inductor filter is widely used in grid-tied inverters. If an inverter with inductor filter operates in weak grid condition, voltage measurement error may exist due to the voltage sensor delay and the voltage drop on grid impedance. This can further lead to errors in the phase lock loop (PLL) and even system instability. In this paper, we analyze the voltage measurement error in weak grid condition in detail. All the essential elements in the measurement chain are considered. Analysis shows large DC offset and harmonic components may exist in the sampled voltage. To improve the PLL performance in weak grid condition, a PLL with extra DC offset and 2nd order harmonic rejection capability is presented. Further, the inverter current controller is designed considering grid impedance and grid harmonics. Experiments are performed to verify the presented method.