Grid commutated rectifiers are still commonly used in high power DC-applications, due to their superior power density and low losses. To comply with grid standards, additional filters to compensate reactive power and to mitigate current harmonics need to be installed. This paper investigates an active filter configuration using a doubly fed induction machine (DFIM). Sizing of this filter is deduced in a case study of a multi-MW-scale electrolyzer. Results indicate that operating the DFIM at nearly synchronous rotational speeds reduces rotor and inverter current demand at fundamental frequencies, however, high-frequency harmonics impose significant voltage drops over leakage inductances and thus consume the aforementioned advantage.