This paper analyzes the characteristics of the submodule (SM) voltage fundamental frequency and second-order harmonic ripples with the consideration of negative voltage state of the full-bridge SM. The optimal operation point, which minimizes the fundamental frequency ripples, is derived. Both the design principle and analytical expressions for SM capacitance dimensioning are presented, and the comparison between energy storage requirements with and without negative voltage states in the arm reference voltages are also carried out under the same constraints on the transmission power, modulation index, and SM voltage fluctuations. However, the SM voltage fundamental frequency ripples increase significantly with reactive power increasing. Therefore, a circulating current injection (CCI) method is proposed in order to suppress the ripples caused by reactive power. Comparative simulations on a ±200 kV hybrid modular multilevel converter (MMC) and experimental results on a laboratory prototype verified the correctness of the presented design principle and the effectiveness of the proposed CCI method. It is shown that the energy storage requirements of the hybrid MMC with negative voltage in arm reference voltages can be reduced about 15%, and it achieves a significant reduction about 45% with the proposed CCI method.

This article presents an in-depth study on the circuit dynamics and control strategy of the recently proposed full-bridge five-branch modular multilevel converter (FB5B-MMC) to qualify its application potential for comprehensive power quality management of cophase railway power system (RPS) with detailed consideration of nonideal operating conditions. In particular, the circuit dynamics coupling induced by branch inductor parameter discrepancy, and the effects on branch voltages, branch currents, capacitor voltage stability, and fluctuations brought by compensating nonlinear traction loads are thoroughly investigated. With the presence of nonlinear traction loads and branch inductance discrepancy, current controllers based on the repetitive control theory are proposed to achieve high-performance suppression of the grid-side rich-content harmonic currents along with other power quality management goals. In addition, branch energy controllers are well designed to ensure capacitor voltage stability against disturbances. Therefore, the power quality management performance and converter operating stability can be effectively enhanced. The effectiveness of the FB5B-MMC and the proposed control strategy for cophase RPS power quality management are verified by means of simulations and experiments.

Series connection of low voltage (LV) power cells have been considered as an alternative microgrid configuration to supply rated voltage power to PCC loads. However, the major focus in the power regulation of system in the previous studies. Similar to the conventional shunt DG units based islanded microgrid, the PCC voltage harmonic problem can be serious without any active control. In order to provide enhanced power quality in this system, a flexible PCC harmonic voltage compensation approach is developed via the hierarchical control of series-connected power cells. Firstly, the selected low order PCC harmonic voltage components are mitigated by using closed-loop equivalent impedance shaping method, via cooperation among the PCC central and power cell local controllers. Secondly, the medium range lumped output LC filter resonance is suppressed with the help of internal virtual impedance, which is implemented only at the power cell local controllers. Finally, the high frequency switching ripples of PCC voltage are mitigated by phase-shifting PWM in distributed power cells, through exploring the output current signal of power cells as an inherent synchronizer for carrier shifting control. It has been experimentally validated that with the proposed approaches, the PCC voltage quality can be significantly improved in broad frequency range without any interferences.

The DC voltage dynamics of droop-controlled voltage source converter based multi-terminal HVDC system (VSC-MTDC) are more complicated due to their transient overshoot and steady-state deviation. Firstly, to solve the DC voltage dynamic problems of the droop-controlled VSC-MTDC system with strong adjacent AC system, a practical reduced-order model of a VSC-MTDC system is established in DC voltage control timescale. Besides, the droop-based outer controller is considered as a droop admittance with lag-lead characteristic, which provides a physical explanation of DC voltage overshoot caused by droop controller coefficients. Moreover, the maximum DC voltage deviation that may occur under any power disturbance can be easily estimated by applying the frequency response analysis to the reduced-order model. Secondly, the damping of droop-controlled VSC can be easily enhanced by adding a simple compensator at the input of the DC voltage feedback loop. In this way, the droop admittance can present like a pure conductance for limiting DC voltage deviation under power disturbances. Finally, electromagnetic transient simulations are conducted to validate the accuracy of the reduced-order model of VSC-MTDC systems and the effectiveness of the proposed damping compensation method for DC voltage overshoot.

In this paper, a bypass pulse injection based PWM approach is proposed for mitigating line current harmonics of grid-tied current source converters (CSCs) operating at low switching frequency. Firstly, the relationship between CSC output harmonic current and the format of bypass pulse is analyzed. Then, through online adjustment of both bypass pulse location and width, the 7th harmonic component of output current is actively regulated to mitigate the corresponding 7th line harmonic current. Meanwhile, this action may be associated with additional amplification of 5th output current. To deal with this issue, the unsymmetrical selective harmonic elimination (SHE) modulation with online adjustable switching angles is applied to further mitigate the 5th harmonic line current. It has been experimentally validated that with the application of both the bypass pulse injection and the unsymmetrical SHE based modulation approaches, both the 5th and the 7th harmonic line currents can be simultaneously suppressed with ultra-low switching frequency below 1kHz.

Featuring high modularity, scalability and transformerless structure, cascaded H-bridge (CHB) based power quality compensator (PQC) is a promising power quality solution for railway power system (RPS). Firstly, branch rating requirements of the conventional CHB based PQCs for co-phase RPS are analyzed under two typical traction transformer schemes, i.e., the V-v scheme and the Scott scheme. Then, a novel CHB based PQC with a special wiring connection style of the Scott transformer is proposed, which achieves reduced total submodule quantity and total branch capacity compared with the conventional solution in V-v and Scott schemes. In addition, a branch current reference generation method is introduced for both branch energy balancing and power quality regulation, which avoids the complicated zero-sequence circulating current injection issue and thus facilitates controller implementation in practice. The effectiveness of the proposed CHB based PQC and the corresponding control strategy is verified by both simulations and experiments.