To effectively mitigate the commutation failure (CF) of the line-commutated converter high-voltage direct current (LCC-HVDC) transmission system, a dynamic series voltage compensator (DSVC) scheme is proposed. The DSVC consists of three integrated gate-commutated thyristor (IGCT)-based full-bridge submodule (FBSM) chains to match the bulk power transmission of the LCC-HVDC. Inserted between the transformer and the AC port of the inverter, the DSVC assists the commutation process by superposing the transient voltage to the AC line voltage and increasing the commutation margin of the inverter valve. Operation and control strategies, which are two critical techniques for the DSVC, are designed based on the working states of the FBSMs, assisting the commutation process and balancing the DSVC capacitor dynamic voltages. The CF detection method is another critical technique of the DSVC control and is improved considering the reverse time of the LCC-HVDC thyristors. Compared with the topologies proposed in existing literature, the DSVC has better performances in safe and stable operation due to the urgent control strategy considering situations beyond its ability. A detailed EMT simulation shows that the DSVC can respond quickly to mitigate CFs and validate the effectiveness of the overall design and key techniques. Furthermore, the operation and control strategy can limit the current stress once it is beyond the capability of the DSVC, which guarantees its feasibility and practicability.

This paper proposes a modular hybrid-full-bridge dc transformer with full-process matching switching strategy (MHDCT) for MVDC power distribution application. The proposed MHDCT merges the advantages of dc transformers based on dual-active-bridge and modular multilevel converter, it not only has good reliability, but also has good modularity and flexibility. Especially, full-process matching switching strategy has good voltage regulation ability to ensure the voltage match of high-frequency-link under various states, then circulating current can be decreased and efficiency can be improved. The universal operation and modulation, voltage, current, power and switching characterizations of MHDCT are analyzed and the control strategy is proposed in the paper. At last, a MHDCT prototype is built and the experimental results verify the correctness and effectively of the proposed solution. The MHDCT will be a practical scheme for MVDC distribution application.

Flexible traction power supply systems (FTPSSs) can solve problems in traditional systems and envision a promising future for rail transit power supply. The traction network voltage is determined by static power converters (SPCs), so the voltage control of SPCs is very important for the safe and stable operation of the system. The uncertainty of the train load, the power redistribution in the case of SPC disconnections from the network because of SPC internal faults, and the real-time cooperative control of multiple SPCs bring challenges to the voltage control of SPCs. Compared to classical error feedback control, active disturbance rejection control (ADRC) has better transient performance and immunity. An ADRC-based voltage-current dual-loop control strategy for SPCs is proposed in this paper. The equivalent block diagram and transfer function are deduced. The stability and immunity of the system are analyzed. Finally, the effectiveness and superiority of the proposed control strategy under different operating conditions are verified by simulation and experiment.

To effectively capture dynamics and interactions of large-scale AC/DC systems integrating (LCC) and modular (MMC) based (MTDC) grids, a numerically accurate and efficient simulation method is extremely desirable. To achieve this objective, a co-simulation method is proposed, where the target system is decoupled into the shifted frequency phasor (SFP) subsystem, the (DP) subsystem, and (EMT) subsystem, respectively. The MMCs are included in the SFP subsystem and implemented on massively paralleled (GPUs). Thus, the simulation efficiency is greatly improved by adopting a much larger time-step, model order reduction technique, and GPU acceleration. The LCCs are represented by dynamic phasors and are included in the DP subsystem. Further, the interactions between SFP and EMT subsystems are reflected by the proposed multi-domain transmission line model (MD-TLM), which can produce instantaneous and phasor values simultaneously. The interface model between DP and EMT subsystems is modeled as a special controlled voltage and current circuit. Finally, the overall co-simulation method is realized by the respective SFP/DP and EMT models, among which their interactions are reflected by the interface models and time sequences of simulations are governed by the overall procedure. The performance of the proposed method has been fully validated on a practical large-scale AC/DC system.

This paper proposes a quasi-optimal power flow (OPF) algorithm for flexible DC traction power systems (TPSs). Near-optimal OPF solutions can be solved with high computational efficiency by quasi-OPF. Unlike conventional OPF algorithms, quasi-OPF does not utilize mathematical optimization algorithms but adopts a new methodology. First, we adopt a new modeling method and successfully reveal the physical meaning of OPF solutions in flexible DC TPSs. Then, by converting the physical meaning of OPF solutions into mathematical expressions, a simple mapping from the power flow solution to the near-optimal OPF solution is obtained, and the quasi-OPF algorithm is designed based on power flow and this mapping. Since calculating power flow is computationally cheap and the mapping is based on simple arithmetic, the quasi-OPF algorithm can solve OPF with much less execution time, achieving subsecond level calculation and a speed-up of 57 times compared to the primal-dual interior point method. The effectiveness is verified by mathematical proofs and a case study with Beijing Metro Line 13. This study provides insight into the physical meaning of OPF solutions and is a powerful tool for flexible DC TPSs to analyze the effects of coordinated control, design real-time coordinated control strategies, and solve operational problems in planning.

In this paper, a novel hybrid-arm modular multilevel converters (HA-MMCs) topology with DC low voltage operation and fault ride-through capability is proposed for unidirectional HVDC bulk power transmission. The HA-MMCs topology is a combination of two types of converter arms: the half-bridge sub-module based arm (HBSM-arm) and the diagonal bridge sub-module based arm (DBSM-arm). Replacing the traditional HBSM-MMC, the HA-MMCs are operated in current and reactive power control modes during normal conditions and can work as the rectifier or the inverter. Since the DBSM can provide bipolar voltages with unipolar currents, the HA-MMCs need not change its control strategy under DC low voltage conditions caused by the high-impedance faults. Under DC pole-to-pole or pole-to-ground faults, the HA-MMCs can both clear the DC fault current and work as the static synchronous compensator (STATCOM) to provide reactive power to support the AC grid. Compared with other MMC topologies with DC fault ride-through capability, the HA-MMCs topology is more economical for significantly decreasing the number of power devices. The operation states, control strategy and DC low voltage operation capability of the HA-MMCs are presented and simulation results of the LCC-HA-MMC hybrid system using PSCAD/EMTDC validate the characteristics and advantages of the HA-MMCs.