Impact of Characteristic Harmonics on the Small-Signal Stability of LCC-HVDC Station

Small-signal stability analysis for Line Commutated Converter based High Voltage Direct Current transmission (LCC-HVDC) is an important reference for the system design and parameter selection, which, however, could be affect by the characteristic harmonics generated by the converter itself. This paper develops an improved small-signal model considering the 11-order and 13-order harmonics, and validates its accuracy by comparing the time-domain responses from the deduced model and the detailed electromagnetic transient (EMT) model in PSCAD. Then, the newly introduced less-damped modes are revealed, and the influence of short circuit ratio (SCR) and the control parameters (taking the parameter of PLL as an example) on the system small-signal stability with and without considering the major harmonics are compared. From the results, it can be concluded that, i) 6 less-damped modes are newly introduced after considering the 11-order and 13-order harmonics, and they are mainly related to the AC system dynamics, and ii) the newly introduced less-damped modes have little impact on the dominant modes trajectory, when the SCR of the Thevenin equivalent model of AC system and the parameter of PLL change.