This paper presents a hybrid modular dc solid state transformer (HMDCSST) composed of a series resonant-dual active bridge (SR-DAB) and a phase shift-dual active bridge (PS-DAB), aiming at improving the transfer efficiency as well as maintaining the control flexibility. The key problems in terms of modeling and control strategy are discussed in the paper. The generalized average and small signal models of the HMDCSST are derived and confirmed by simulation, in which two LC filters are also considered to reduce the ripple components of the input and output currents. Based on the models, the control strategies to achieve flexible control of the output voltage and power are designed. Moreover, as a key problem for HMDCSST, the design principle of the number for each type of DAB is discussed to meet the demands of the grid. Finally, a three-module prototype of HMDCSST consisting of one PS-DAB and two SR-DAB modules was built-up and tested, and the results proved that HMDCSST has higher efficiency than the traditional DCSST based on PS-DABs purely, without sacrificing the output voltage and power regulating capability in the meanwhile.

This study investigates the analytical approach for determining the spectral components of a hybrid pulse width modulation (HPWM) strategy for cascaded H‐bridge (CHB) multilevel inverters. By decomposing the HPWM voltage into a staircase waveform and a non‐sinusoidal pulse width modulation waveform and applying the one‐dimensional Fourier series as well as double Fourier series methods, the closed‐form solution for arbitrary level HPWM voltage is derived. Based on the derived solution, analysis of HPWM output with a changed modulation index is also performed, with good stability and control dynamic demonstrated. The spectral analytical expression is verified extensively by comparison of theoretical calculation results with those of the simulation and laboratory experiment, which coincide with each other very well. Furthermore, a brief switch function rotation balancing method for HPWM‐based CHB inverter is utilised, and is also validated by experimental results.

This article addresses the decoupling control design for the cascaded power electronic transformer (PET) in input-series–output-parallel connection. Due to the coupling and interaction between the submodules and between substages, in essence it is a complex nonlinear system. A nonlinear control strategy based on feedback exact linearization (FEL) is developed to decouple all the control objectives so as to realize the dc-link and output voltages fast stabilizing and the submodule balancing without coupling and interactions. Based on the separation of the ac current time-scale and dc voltage time-scale, the FEL control law is derived, and the specific tuning process of control parameters is given for desired control bandwidths. Taken the dynamic influences of the current-loop, filter, and delay into account, further analyses are made and compared with a dual active bridge (DAB) balancing control-based strategy to show the improvement on decoupling effect and dynamic performance of the proposed control. Finally, the simulation and experimental results verify the theoretical analysis, which exhibit better dynamic features and minimal coupling effect under the condition of bidirectional power changes and parameter inconsistency and uncertainty.

Sparse modular multilevel converters (SMMCs) are a new type of lightweight high-voltage large-power AC/AC converter that significantly reduces the number of components compared to modular multilevel matrix converters (M3Cs). This study proposes a fault ride through a control strategy for SMMC to address the issues of arm energy imbalances and valve-side overvoltage, which occur during asymmetric faults on the low-frequency side. First, we establish models of the energy deviation of the arms under asymmetric short-circuit faults on the low-frequency side of SMMC. We also study the influence mechanism of the control strategies on the arm energy imbalance during faults. On this basis, an arm energy balancing strategy based on zero-sequence voltage injections combined with AC voltage control is proposed; this can achieve arm energy balance and suppress the negative sequence current and overvoltage of the SMMC. Finally, we construct a simulation model of an offshore wind power low-frequency transmission system based on the SMMC. The simulation results show that the proposed energy balance strategy can realize the stable operation of the low-frequency transmission system (LFTS) under asymmetric faults on the low-frequency side, that the maximum capacitor voltage deviation during the fault does not exceed 10% and that capacitor voltage returns to normal 0.25 s after the fault occurs.