Impact of Background Voltage Harmonic Mitigation Techniques on Coupled Frequencies in VSC-Based EV Fast Charging

In active front end-based EV fast charging stations, background voltage harmonics produce current not only at the frequencies of the distorted voltage, but also at other coupled frequencies. Various mitigation techniques, such as increasing inner control loop gain, grid voltage feedforward, and selective harmonic compensation, have been adopted in industry to reduce the emissions originating from distorted background voltage. However, although these techniques are effective in suppressing the current at the harmonic orders present in the background voltage, they deteriorate the emission at coupled frequencies. This paper provides the theoretical explanation of this phenomenon, which is verified by simulation of a two-level active front end in PSCAD/EMTDC. This paper also discusses the proper treatment of current emission due to background voltage harmonics.