A modular multilevel resonant dc/dc converter (MMRDC) possesses high power and high voltage rating, which is adopted for high step-ratio interconnection in dc grid applications. However, the current stress and reactive power of MMRDC are high with the conventional quasi-square-wave modulation scheme when in a wide voltage range and light load conditions. Hence, this article proposes an optimized asymmetric trapezoidal wave (ATW) modulation scheme that reduces current stress over wide load and voltage ranges by separately modulating the duty cycles of string output voltage rising and falling processes. The time-domain mathematical model of MMRDC with the ATW modulation scheme is established. Then, based on the soft switching characteristic and reactive current analysis, the optimized operating point of MMRDC can be derived while considering low current stress and zero voltage switching constraint of low voltage side full-bridge switches. On this basis, the numerical solutions of optimized control variables are obtained, and a control block diagram is presented for the practical implementation of the optimized ATW modulation scheme. Finally, a 4 kW MMRDC prototype is constructed, and the experimental results verify the correctness and effectiveness of the analysis and the optimized ATW modulation scheme.

From flexible interconnection among feeders to hybrid alternating current (AC) and direct current (DC) distribution structures of future smart distribution systems, medium-voltage DC distribution centers with flexibly interlinked multiple microgrids (MGs) will have wide applications on the demand side. A generic coordinated control framework based on a distributed cooperation scheme is proposed for such DC centers, as opposed to centralized control structures. A novel unified control only using local measurements is proposed for these interlinking converters. During normal power disturbances, automatic coordinated power control and mutual support among sub-systems can be realized, thereby improving DC voltage and AC frequency stability to enable multiple MGs to be treated as a real unified cluster. Moreover, with this method, interlinking converters can realize seamless transition in power dispatching mode, common DC bus voltage control mode, and MG support mode without communication and control system switching. A simplified dynamic model has been developed to verify the proposed control strategy. This work is expected to provide a new solution for flexible interconnection and operational control of large-scale MG clusters.

Modular Multilevel Converter (MMC) is widely used in high power motor drive, energy storage and flexible DC power transmission & distribution system, and the parameter identification for sub-module is essentially important for reliable operation of MMC. This paper proposes a digital twins (DT) based method for model construction and parameter identification of the MMC half-bridge sub-module (HBSM). Firstly, the DT model of the HBSM is built, which consists of a simplified MMC sub-module (SM) equivalent model and an arm current simulator based mission profile emulator (MPE). Secondly, a modified particle swarm optimization (MPSO) algorithm is proposed to achieve the online identification of internal parameters in the HBSM, further achieving the real-virtual interaction feedback for real-time sensing of the operation state of the HBSM. Finally, a non-intrusive DT MPE for the HBSM is constructed in MATLAB. The simulation and experimental result shows that the average errors of the actual and model-calculated capacitor voltage values are within 0.1%, and the parameter identification results combined error is less than 1%. The digital twin model established by the proposed method can effectively identify the parameters of MMC sub-modules under different operating conditions.

DC grid is essential to efficiently integrating renewable energy and flexible access to DC loads. DC transformer is the key equipment to interconnect DC systems with different voltage levels, establishing DC grids. This paper proposes a resonant modular multilevel DC transformer with half-bridge submodule series connected at the medium-voltage side and full bridges parallel connected at the low-voltage side, which significantly reduces the voltage and current stress along with the number of power devices and transformers. Quasi-square-wave modulation and phase-shifted control increase the AC link frequency and achieve fixed frequency output voltage control. The transmission power characteristics are analyzed based on the fundamental harmonic approximation analysis method. And the key parameter design procedure and output voltage control strategy are presented on this basis. Finally, the feasibility of the proposed topology and theoretical analysis is validated by a 4kW/600V/100V prototype.