This paper researches the transient stability analysis and improvement of isolated renewable energy bases with VSC-DC transmission. First, the transient stability of the system is decomposed into two sub-issues: the transient stability focusing on the dynamics of grid following (GFL) and grid forming (GFM) generators inside the base, as well as the transient stability concerning the interaction between the base and the VSC-DC station. Second, these two sub-issues are analyzed separately, showing that switching GFM generators between the current source state and the voltage source state may negatively affect the transient stability. Finally, an adaptive voltage booster suitable for GFM generators is proposed, which has been proved through a case study to be beneficial for transient stability in the discussed scenes.

The hybrid cascaded HVDC system employs a line commutated converter (LCC) as the rectifier and an LCC in series with multiple paralleled modular multilevel converters (MMCs) as the inverter. MMC arms are susceptible to overcurrent following a severe AC fault at the receiving end, however, its fundamental mechanism has not been totally revealed. Therefore, this article explores the overcurrent characteristics on MMC arms, in terms of both the DC and AC components. Apart from the DC overcurrent component induced by the commutation failure (CF) of the inverter LCC, the AC overcurrent component is also significant. It dramatically depends on the coupling effects among the AC systems of the inverter side. Further, corresponding suppression strategies are proposed, which are applicable to different receiving-end AC fault scenarios. Eventually, the time-domain simulation results from PSCAD/EMTDC validate the effectiveness of the proposed overcurrent suppression control. It is also demonstrated that the presented methods can not only suppress overcurrent for MMC arms, but also reduce the imbalanced power between two sides, as well as improve the dynamic performances of the entire system.