Reconsideration of Grid-Friendly Low-Order Filter Enabled by Parallel Converters

High-order filters, such as inductor-capacitor-inductor ( LCL) filter, have been popular in grid-tied power converters. Although featuring small size, LCL filters are not grid friendly due to inherent resonance, especially when a large number of converters are in parallel operation in todays electric grid to attain modularity, reliability, and redundancy advantages. Thus, this article reconsiders the low-order L filter in parallel converters to eliminate the resonance and in turn to simplify the control. It is found that by interleaving a certain number of converters, the L filter will be sufficient to meet the harmonic limit requirements of the standards, while the filter size can be even smaller than the LCL filter. This further contributes to cost reduction as a promising solution for grid-friendly converters.

I. Introduction

Grid-tied power converters employ passive filters to attenuate the high-frequency harmonics within limits set by the standards, e.g., the IEEE std 1547–2018 [1] or the IEEE std 519–2014 [2]. To reduce the size and cost, high-order filters, e.g., the inductor-capacitor-inductor () filter, are popular in many ground-based noncritical applications [3], [4]. To further reduce filter size, more complicated high-order filters, such as filter, have been proposed in the literature [3]. While featuring excellent harmonics filtering performance, high-order filters are not grid friendly due to their inherent filter resonance [5]. With the increasing penetration of power electronics, the utility grid will face severe resonance issues caused by the high-order filters [6], [7].