In a DC distribution network, the modular multilevel high-frequency DC converter (MDCC) can achieve the electrical isolation, voltage conversion and power transmission between the low- and medium-voltage DC buses. In this paper, the quasi-square-wave (QSW) modulation, which avoids the dv/dt stress problem and maintains a high DC voltage utilization, is applied in MDCC, and the mathematical model of QSW modulation with a dual-phase-shift (DPS) scheme is established. Based on the model, the influences of staircase rising and falling process in QSW modulation on MDCC performance, including power characteristic, current stress, switching characteristic and efficiency characteristic are analyzed. The influences of QSW modulation under DPS scheme on MDCC performance are also investigated. Moreover, to achieve a comprehensive optimization of MDCC performance, a multi-objective optimized control strategy based on QSW modulation under DPS scheme is proposed. Finally, a MDCC prototype is set up and the experimental results verify the correctness and effectiveness of the analysis and proposed strategy.

With the fast development of advanced switches and magnetic cores, high-frequency oscillation (HFO) caused by high dv/dt of fast switching devices and stray capacitances of transformer in dual active bridge (DAB) presents a significantly challenge. In this article, a comprehensive analysis, design, and optimal methodology of HFO for DAB with wideband gap (WBG) switching devices and nanocrystalline transformer cores is proposed. The stray parameters equivalent circuit of DAB is built and the key influencing factors to mitigated the HFO are identified by establishing a time-domain analytical model. Based on the theoretical analysis, this article points out for the first time that the traditional method of decreasing the stray capacitance of the transformer to eliminate HFO is not suitable for high dv/dt occasions when using WBG switching devices. Correspondingly, a novel elimination method of HFO by equating the voltage changing time of dv/dt to the oscillation cycle is proposed, through which the minimum voltage spike amplitude (VSA) of HFO can be achieved mathematically. A 6.6-kW DAB prototype with three different transformers is developed and an approximately 95% reduction of the VSA can be achieved when the proposed HFO elimination method is adopted, which verifies the effectiveness of the proposed method.

Medium-voltage dc (MVdc) power distribution grid is a key link to build dc power networks. In this article, a novel input-series-output-parallel modular dc transformer (DCT) with the high-frequency-link (HFL) voltage buck-boost matching strategy is proposed for MVdc grids application. The proposed DCT is composed of a current-fed switched capacitor structure and corresponding dual-active-bridge converters, and they need to operate collaboratively to achieve the great property. Comparing with the traditional switched capacitor-based DCT (SCDCT), the most salient merits of the proposed DCT include the HFL voltage buck-boost adaptive regulation, the low conducting and switching current in switches and the capacitor voltages self-balance characteristic. These merits are all beneficial for the improvement of efficiency, operating reliability, switching performance, and cost of DCT systems. Meanwhile, the proposed DCT reserves the advantages of SCDCT including redundant submodule design and dc fault isolating capability. The topology, buck-boost modulation principle, capacitor voltage self-balance analysis, switching behaviors, and comprehensive comparison with the traditional SCDCT are completed in detail. Finally, the experimental results validate the effectiveness and correctness of the proposed solution.

The ripple effect phenomenon wherein the capacitor ripple voltage generates the fundamental-frequency output ac voltage of a cascaded H-bridge-based static synchronous compensator (CHB-StatCom) is revealed and investigated in this work. First, an observation method of the ripple effect for the CHB-StatCom is presented. Then, the reason why CHB-StatCom has a ripple effect is explained. Subsequently, the accurate mathematical model of the CHB-StatCom considering ripple effect is reformulated. Based on the remodeled mathematical model, the cognition inaccuracy of the conventional design of a CHB-StatCom is analyzed, and a new cost-majorization-oriented (CMO) design principle is proposed to optimize the cost of CHB-StatCom. The proposed CMO design needs a lower dc-link voltage to generate the same maximum fundamental-frequency output ac voltage compared with the conventional design, thereby reducing the cost of CHB-StatCom. The simulation and experimental results validate the correctness and effectiveness of the analysis.

The linear modulation problem of modular multilevel converter (MMC) is considerably complicated by capacitor voltage ripples (CVRs) and circulating current suppression control (CCSC). This article examines the linear modulation range of MMC and its influence on parameter design and operating characteristics, taking into account the CVR effect and CCSC. The deviation between the actual voltage output and the reference is revealed and analyzed, and the analytical expression of this deviation is determined. The effects of CCSC on linear modulation are also revealed. Based on the article, a set of equations are proposed to determine the exact reference wave while accounting for the CVR effect and CCSC and precisely assess linear modulation. Subsequently, the linear modulation range of MMC, which is defined as the maximum selectable valve-side voltage that can satisfy whole operating region linear modulation in this article, can be exactly determined. Furthermore, how the CVR effect and CCSC influence the linear modulation range, parameter design, and operating characteristics are analyzed using case studies. The proposed theory and method are also verified through simulation and experimental results.

A new grid-forming (GFM) high-voltage direct current (HVdc) rectifier, named flexible line-commutated converter (Flex-LCC), is proposed in this article to collect the power generated by large-scale islanded or weak-grid-supported renewable energy systems. The Flex-LCC uses a series-connected hybrid valve based on the LCC and full-bridge modular multilevel converter (FBMMC), and adopts a new control. As a result, the Flex-LCC works as a GFM rectifier and retains the same dc voltage and power carrying capacity, dc fault clearing, and power reversal functions as an LCC. Particularly, the Flex-LCC has three salient features: the control of the Flex-LCC is decoupled from the LCC part's firing angle, the internal reactive power allocation is flexible, and the valve-side ac voltage of the FBMMC part has a wide operation range due to the decoupling of dc-port voltage and capacitor voltage. These bring reduced interharmonics, enhanced stability, and a smaller capacity boundary to the FBMMC part for the Flex-LCC. Furthermore, utilizing the reasonable design for the actual capacity and valve-side ac voltage of the FBMMC, the Flex-LCC can effectively balance cost, footprint, and weight, and match the current-carrying capacity between the series-connected LCC and FBMMC. The topology, control, characteristics, and design of the Flex-LCC are analyzed in detail in this article. Simulations and experiments validate the theory.