DC Impedance Modeling of Push-Pull DC Auto-Transformer for MMC and LCC HVDC Interconnections

The push-pull DC autotransformer (PPDAT) is a potential power equipment to interconnect the MMC and LCC HVDC systems. However, the harmonic stability of the PPDAT remains an unsolved challenge. In this article, considering the dual closed-loop control and bidirectional DC voltage operation of the PPDAT, its DC impedance models are proposed using the harmonic transfer function (HTF) method. Both the DC impedance models at the high/low voltage side considering the harmonic coupling and internal dynamics of the hybrid MMC/LCC HVDC system are deduced. The accuracy of the detailed DC impedance models is validated in PSCAD/EMTDC simulations. The impedance analysis under bidirectional DC voltage operation scenarios reveals that the DC voltage polarity affects the low frequency region. Besides, the main impact of PPDAT's parameters on the wideband impedance characteristics is disclosed. It shows that unreasonable parameter design will bring negative damping characteristics near the resonance point (RP). Moreover, stability analysis considering the interaction of PPDAT and LCC-HVDC at the high-voltage side is conducted. The results indicate that the potential resonances can be readily predicted through the Nyquist diagrams and the negative damping of the PPDAT may cause instability at RP.