On the design of LCL filter with passive damping in three-phase shunt active power filters

This paper is focused on the LCL filter with damping resistance used to interface the shunt active power filter of an active DC-traction substation with the power supply. Two main requirements to be met by the interface filter are emphasized, in terms of significant attenuation of the current switching harmonics and ensuring the unalterated flow of the current to be compensated by active fitering. The behaviour of the LCL filter is analysed on the basis of frequency response associated to the currents-based transfer functions. First, the influence of the passive filter's parameters on the magnitude response is pointed out, resulting in useful aspects for a proper design. Then, the analysis of the damping resistance on the losses shows the existance of a maximal value of losses, when attenuation at switching frequency and maximal frequency to be compensated by active fitering are both imposed. Thus, the imposion of ratio between losses and maximal value of losses provides an additional condition leading to a concrete set of filter parameters. It is shown that there are practical limitations and a compromise must be made between the two requirements. The results of the Matlab/Simulink-based simulation of the active filtering regime of an active DC-traction substation with 12-pulse series diode rectifier and LCL coupling filter designed as proposed illustrate good performance of the system in terms of the LCL filter behavior.