Abstract
The resonance stability of modular multilevel converter (MMC) based high-voltage direct current systems has been extensively researched using the impedance-based stability theory. However, when there are coupled multiphase DC lines (CMDCLs), the DC side stability issues would become more complicated and harder to be analyzed by many well-established stability criteria. To address this challenge, this paper proposes a resonance stability analysis method for MMC-based DC grid with CMDCLs, based on the nodal admittance matrix analysis. First, the DC side impedance models of MMCs with typical control modes are established using the harmonic state-space theory. The parameter matrices of CMDCLs are also introduced and their elements are transformed into the s-domain. Second, the nodal admittance matrix of the DC grid when there are CMDCLs is derived in detail. The resonance mode of the DC grid can then be obtained by solving zeros of the determinant of the s-domain nodal admittance matrix. Finally, the resonance stability characteristics of a bipolar four-terminal MMC-based DC grid are investigated. Simulations in PSCAD/EMTDC verify the effectiveness of the analysis results.
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