A Variable Inductor Based LCL Filter for Large-Scale Microgrid Application

Three-phase LCL filters, compared to L or LC filters, are preferred in high-power converters in microgrid applications because of their better capability of harmonic attenuation. Fixed value inductors are mostly adopted in traditional designs to achieve low current harmonics at rated power. However, due to the intermittent nature of renewable energies, microgrid converters have to operate under light-load conditions, which sometimes result in worse harmonics performance. This paper proposes a variable filter inductor, which has three-time higher inductance at low currents compared to its inductance value at rated current. It not only reduces current harmonics under all operating conditions but also helps to stabilize the microgrid by inducing less LCL resonant propagation. The inductor's magnetic design is illustrated and verified by finite-element simulation. MATLAB/PLECS circuit simulation shows a reduction of current harmonics when using the variable inductor under light-load conditions. Experimental results from a 1-MVA prototype confirm the theoretical expectations.