Thyristor branch loop (TBL)-based hybrid MMC (HMMC) is one of the promising technical solutions to handle the DC-line short-circuit faults in high voltage direct current (HVDC) systems. However, the submodule (SM) capacitor precharging characteristics of TBL-HMMC is quite different from conventional MMCs, due to the special configuration. During SM precharging and system startup process, the TBL-HMMC may suffer from surge current, SM capacitor under-voltage, unbalanced capacitor voltages between unipolar full-bridge (UFB)-SMs and half-bridge (HB)-SMs, and unbalanced capacitor voltages between upper and lower arms. A SM grouping-based capacitor precharging strategy is proposed in this paper, which could precharge both UFB-SMs and HB-SMs in upper and lower arms to their nominal voltage, and therefore achieving a smooth startup for the TBL-HMMC based HVDC system. Electromagnetic transient simulation studies using PSCAD/EMTDC tool are conducted to confirm the effectiveness of the proposed strategy.

In the bipolar HVDC system based on TBL-HMMC, a voltage sag can occur on the AC side during DC short-circuit fault clearing. In this article, the DC short-circuit fault clearing process for TBL-HMMC and its fault equivalent circuit are presented. This paper analyzes the underlying reasons for voltage sag resulting from thyristor triggering and submodule blocking and proposes a voltage optimization control strategy to suppress the transient voltage sag. In addition, compared with TBL-HMMC without proposed voltage optimization control, the proposed approach can effectively suppress AC side voltage sags in order to improve power quality, without adversely affecting the fault clearing process. Simulation studies using PSCAD are conducted to investigate the operation of the bipolar TBL-HMMC under DC short-circuit fault clearing and to verify the effectiveness of the proposed voltage optimization control method.

Low frequency transmission technologies can significantly enhance power transfer capacity and distance of alternating current transmission by reducing transmission frequency. This paper proposes a dual bipolar modular AC-AC converter (DBMAC) topology for the frequency conversion between low-frequency (15~20 Hz) and grid-frequency transmission (50 Hz). The operation principle is elucidated, showing the principal of the frequency decoupling. The mathematical models of DBMAC are developed and the control strategies are given for both input and output sides. The simulation is conducted and its results verify the effectiveness of the proposed DBMAC.

Continuous-control-set model predictive control (CCS-MPC) has been extensively adopted in dual active bridge (DAB) converters to achieve rapid and precise output voltage regulation. However, the voltage control performance of conventional CCS-MPC exhibits significant sensitivity to parameter variations in DAB system modeling. Discrepancies between model and actual parameters may degrade voltage tracking accuracy. To address this limitation, this paper proposes a continuous-control-set model-free predictive control (CCS-MPC), where an ultra-local data-model for the DAB converter is established and the real-time updates of this data-model is implemented through the Runge-Kutta method, thereby eliminating parameter dependency in output voltage regulation and enhancing system parametric robustness. Experimental results validate the effectiveness of the proposed approach through comprehensive verification.