Propagation of AC Background Harmonics in MMC HVdc Multiterminal Systems Due to Resonances and Mitigation Measures

Modular multilevel converter (MMC)-based multiterminal HVdc (MTdc) transmission is a favorable option for asynchronously interconnecting several weak ac systems. This paper investigates the potential resonance of such an MMC-MTdc system excited by background harmonics in any of the MMC's ac systems and the undesirable transfer of these harmonics to the remote ac systems. First, the harmonic transfer rule across one MMC is established using phase leg voltage analysis and it is then used to develop an equivalent circuit model of the MMC-MTdc system. This model is used to investigate the mechanism of resonance excited by ac-side background harmonics. Finally, a control strategy is proposed to mitigate this resonance. Analytically calculated results from the derived model and the proposed control strategy are validated via electromagnetic transients simulation of a five-terminal MMC-MTdc network.