A Magnetic Integrated LLCL Filter for Grid-Connected Voltage-Source Converters

High-order passive filters, such as the well-known LCL filters, are normally employed in the grid-connected power conversion systems to effectively attenuate the switching frequency harmonic introduced by the modulation of power converters. Although much more compact than the conventional single-inductor L filters, such passive filters are still bulky and expensive when compared with their active counterparts, e.g., the semiconductor switches. In order to improve the system power density and reduce its cost, the magnetic integration technique has been widely adopted so that the discrete inductors of passive filters are replaced by the integrated inductor, resulting in smaller magnetic cores and, therefore, the decreased volumes of passive filters. For conventional magnetic integrated LCL filters, the converter-side inductor and the grid-side inductor are integrated together and their coupling coefficient is intentionally minimized. In this letter, this coupling coefficient is fully utilized and properly designed, and the resulting coupling effect is equivalent to inserting an additional inductor into the filter capacitor branch loop. The integrated inductor and the filter capacitor can form an integrated LLCL filter, which exhibits the advantages of both the LLCL filter and magnetic integration, e.g., enhanced harmonic attenuation, reduced filter inductances, and system volume without adding the extra trap inductor. Finally, experimental results obtained from a single-phase grid-connected voltage-source converter interfaced by the proposed integrated filter are presented to validate its effectiveness.