I. Introduction
Grid-connected inverter designers are making continuous efforts to reduce the size, weight, cost, power loss, and failure rate of the grid interface filters [1]–[4]. A grid interface filter, or line filter, is the filter connected at the AC side of a voltage-source converter (VSC) to suppress the switching frequency current ripple. It is often costly and consumes a large portion of power loss, weight, and size in a grid connected inverter, especially in medium to high power applications. A commonly recognized method to reduce the size and weight of a grid interface filter is to increase the order of a filter, so that less inductance is required. However, these higher-order power filters, including LCL filters, contain one or multiple resonant peaks which require passive or active damping. Passive damping leads to extra power loss and reduces filter attenuation [5], [6]. Active damping requires extra sensors and/or extra computation power, and their effects are often limited by digital control delay and parameter uncertainty [7]–[9].