LCL type supply filter for active power filter - Comparison of an active and a passive method for resonance damping

First-order L type supply filters do not always offer sufficient attenuation of switching ripple currents in voltage-source active power filter applications. By using third-order LCL filters the level of the switching frequency current components can be reduced more effectively. However, because of the resonance phenomenon of the supply filter and the active filter wide operating bandwidth, the use of the LCL filter in the active filter applications is challenging. This paper compares an active and a passive method for LCL filter resonance damping, assesses their suitability for the active power filter application and presents their benefits and drawbacks. The passively damped LCL filter utilizes a control system designed for an active filter with a first-order L type filter, while the active damping method examined is based on the cascaded control of the mains side inductor and the capacitor currents. First, the frequency responses of the transfer function models are examined and second, the topologies are studied through experimental tests. The results presented show that both topologies filter the harmonics effectively and the switching ripple is attenuated. The active method requires more current sensors, while additional damping resistors are needed in the passively damped system. However, there are only slight differences both in the filtering results and the power losses.