A Frequency-Adaptive Differential Mode Active Filter for Mitigating Supraharmonics in the 10 kHz to 80 kHz Range

This paper presents a frequency-adaptive differential mode (DM) active filter designed to mitigate supraharmonics (SH) generated by switching power electronics on the AC side, operating within the 10 kHz to 80 kHz frequency range. The proposed active filter does not require any prior knowledge of the primary source of pollution on the low-voltage grid. Instead, the filter is able to automatically identify the dominant SH in the covered frequency range and infer from this the dominant switching frequency f_sw. The notch function of the filter is then tuned to f_sw to maximally remove the detected dominant SH band. Only SH within about 2.3 kHz of the tuning frequency are removed, minimizing the impact on narrowband power line communication (NB-PLC) signals on the same lines. It is demonstrated that the filter can be expanded with extra controlled notch functions that additionally remove SH clustered around mf_sw, with m > 1. During operation, the system tracks the detected switching frequency f_sw, and can even cope with abrupt changes thereof. A prototype of the filter concept has been developed and its operation has been verified in a controlled grid test setup with different grid-connected loads. Measurements with a single-tone SH demonstrate full suppression up to the noise floor (40 dB). For in-range sideband SH originating from a chopper load operating with SPWM, 36 dB suppression at the first sideband has been measured. Furthermore, under the variable switching frequency scenario, the filter maintains an average attenuation of approximately 17 dB for a ±500 Hz frequency deviation around a 20 kHz center frequency at a 10 Hz modulation frequency. For a smaller deviation of ±100 Hz, a suppression of 25 dB is achieved.