Accurate and General Small-Signal Impedance Model of LCC-HVDC in Sequence Frame

To reveal the interaction between AC and DC voltage and current of line commutated converter (LCC) based HVDC, a detailed model of LCC in abc frame is developed in this article to capture the nonlinear characteristics using switching functions. Using Fourier transforming, the switching functions are then decomposed into different harmonic components, which significantly reduces the difficulties associated with LCC model linearization. An accurate LCC small-signal impedance model is thus obtained through linearizing the model and analyzing the dynamics of the overlap angle. The impedance model developed is general and applicable to inverter mode, rectifier mode, AC/DC side impedance calculation and different controller designs. The developed impedance model is validated by comparing to the measured impedance from time-domain simulation. The interactions among the impedances of the AC and DC terminals, and the connected network condition of the remote converter terminal are further analyzed. It reveals that the grid strength at inverter side has significant impact on the small-signal rectifier AC impedance, while the grid strength of the rectifier has relatively minor impact on the inverter AC impedance. This indicates the coupling of the small signal behaviors of the two AC networks integrated by LCC-HVDC system. To assess the interactions between the inverter and rectifier, stability analysis of a simplified AC network containing a MMC-HVDC system and a LCC-HVDC system, is conducted based on the developed model. The analytical results are verified by comparing with the simulation results.