DC terminal impedance modeling of LCC-based HVDC converters

Stability of the dc bus voltage in high-voltage dc (HVDC) transmission systems is a critical problem, and depends on the control of each HVDC terminal, as well as on the dynamics of the dc network. The HVDC converter at each terminal can be represented by an impedance together with a voltage or current source. Such impedance models can then be combined with the impedance of the dc network to provide a system model for small-signal stability analysis. This paper presents small-signal impedance modeling of line-commutated converters (LCC) for such HVDC system stability analysis. The impedance models are developed by using the harmonic linearization method and taking into account the ac grid impedance as well as control of the converter. Rectifiers and inverters are modeled separately by assuming different control modes. The developed models are provided in analytical forms and validated by detailed circuit simulation.