Multi-tuned Narrowband Damping for Suppressing MMC High-Frequency Oscillations

Modular multilevel converters have been widely reported to experience high-frequency oscillations with the power system to which they are connected in HVdc applications, and the root cause is that the system resonance frequencies fall into the delay-induced non-passive region of the MMCs. This paper presents a multi-tuned narrowband damping control scheme to selectively and narrowly enhance the passivity of an MMC around multiple resonance frequencies, and thereby damp the associated high-frequencies oscillations. First, this paper presents a detailed process for simplifying the complete matrix-based impedance model of MMC into a high-frequency impedance model suitable for damping design. Second, using the simplified model, the effect and comprehensive design of the proposed multi-tuned damping controller are described from a virtual admittance perspective, making it possible to simultaneously damp the MMC impedance around multiple resonance frequencies in the high-frequency range. The proposed design method carefully takes into account the coupling effect among multiple damping controllers targeting oscillations at different frequencies, in order to avoid potential unintended interferences. The performance of the proposed narrowband damping method is verified in EMT simulations by implementing it to suppress high-frequency oscillations in three different MMC-based applications.