Current control optimization for grid-tied inverters with grid impedance estimation

The control of a PWM inverter is affected by variable inductance present in the power line of the grid. This paper presents a new implementation of grid-tied inverter with weighted-average-current feedback control (WAC) and an embedded online technique to determine the grid line impedance. By adding an inter-harmonic voltage periodically in the modulation of the PWM and measuring the current response, it is possible to estimate the load impedance connected to the inverter, and the grid impedance can be obtained by subtracting the low-pass filter from the load impedance. Consequently, current control optimization can be carried out such as rearrangement of poles or zeroes and adjustment of the current loop gains to obtain the desired dynamic response even under changing line conditions. The presented technique can be implemented with the existing sensors and the digital processor of the inverter, provides a low cost and adaptive approach for current control of grid-tied inverters. Simulation on a 5 kW grid-tied inverter demonstrates the control method and the impedance measurement.