Analysis of Output Filter Inductor Current Ripple in Grid-Tie Inverters

Incisive selection of the LCL filter parameters for a grid-tie inverter (GTI) is crucial to meet the grid interconnection standards and at the same time achieve a reduced hardware footprint. Various design methods are available in the literature for selecting the LCL filter parameters. The first step towards the selection of the LCL filter parameters is the estimation of the inverter side inductor (Li), which is usually calculated based on the allowable inverter peak ripple current in order to ensure a minimal losses due to the ripple component. The value or size of i that ensures a certain allowable inverter peak ripple current depends on the inverter configuration, switching technique and the application. This paper presents a comprehensive analysis, comparing the size of i for different configurations and applications of 3-phase GTIs. 3-phase 3-wire and 4-wire inverter configurations employing sinusoidal pulse width modulation (SPWM) are considered in this paper. Active front-end converter (AFEC), distributed generation applications (DGAs) and static synchronous compensator (STATCOM) are the target applications. A step-by-step analysis is carried out by sketching the voltage and current waveforms of i to estimate the inverter peak current ripple at every switching instant for a complete fundamental cycle. Closed-form expressions relating i to the allowable inverter peak ripple current, dc-bus voltage and the switching frequency are developed and compared for the aforementioned GTIs. The proposed analysis helps choose between a 3-wire and 4-wire configuration w.r.t the size of i. Also, from the obtained closed-form expressions, the minimum value of i that ensures the allowable peak ripple current for a chosen GTI can be calculated.