Abstract
The high-frequency resonances (HFRs) around 700 Hz and 1.8 kHz have occurred in Yu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">′</sup> E project. Mechanism studies indicate that HFRs are caused by the interactions between the negative damping inductance characteristics of the MMC and the capacitive characteristics of the AC system. In this paper, two optimized methods regarding the current control loop are presented to suppress the HFRs. The first one is to use a round(x) function inserted into voltage feed-forward loop instead of low pass filter (LPF). The proposed method has less negative impact on transient characteristics of an MMC. The second one is to employ a second-order damping controller, including the parameters design, to reshape the MMC impedance around the multiple risky regions. The core of HFRs suppression is that the real part of the MMC impedance near the multiple intersection frequencies should be positive according to impedance stability theory. Combined with the proposed control methods and core of HFRs suppression, the simplified model of MMC is adopted to design the parameters using the analytical expressions. Moreover, the general steps of parameters design are presented. Finally, the optimized methods to suppress HFRs are verified by time-domain simulations.
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