Modular multilevel converters (MMCs) have been widely used in VSC-HVDC, and the capacity of MMCs has reached 1000 MW. The accurate calculation and theoretical analysis of the loss attract more and more attention, especially in the high power applications. This paper presents a simplified analytical calculation model of average power loss for MMC based on the RMS and absolute values of the arm current. In this model, the analytical expressions of the conduction loss and the switching loss are derived in terms of the whole arm of MMC rather than a single submodule. It avoids the effect of the complexity of its switching pattern on the calculation accuracy of the model. The explicit expressions of the presented model show the relationship between the loss and the voltage balancing methods and the main variables of MMC directly in comparison with the conventional loss evaluation methods. PSCAD/EMTDC simulation and experimental results of the $\pm$350 kV/1000 MW MMC in the Luxi Back-to-Back DC Asynchronous Networking Project verify the correctness and the accuracy of the proposed simplified calculation model.

Full-bridge submodule (FBSM) capacitance of FBSM based modular multilevel converter (FB-MMC) can achieve a significant reduction with boosted ac voltages, which makes it a promising candidate in voltage-source-converter based high- voltage-direct-current (VSC-HVDC) scenarios in terms of cost reduction and dc short circuit faults tolerance. With consideration of third-order harmonic voltage injection (THVI), the characteristics of the FBSM voltage fundamental frequency and second-order harmonic fluctuations are analyzed in detail under steady-state and single-line-to-ground (SLG) faults conditions. Both of the optimal phase and amplitude of the injected third-order harmonic voltage are derived constrained by the same modulation margin, based on which the optimized THVI method is proposed. Under the same fluctuations conditions, the energy storage requirements comparison between the FB-MMC with and without the THVI method are also carried out. The feasibility and validity of the proposed THVI method are verified by simulations on a ±200 kV FB-MMC system and experiments on a laboratory-scale platform. With the same fluctuations as the FB-MMC without THVI, both of the simulation results and the experimental results indicate that the FBSM capacitance of the one applying the proposed THVI method can be reduced about 16%.

A decentralized control method for power module (PM) capacitor voltage and switching frequency balancing in modular multilevel converter (MMC) is presented in this paper. The proposed method is composed of a central controller, six arm controllers and several group controllers. The group controller is only in charge of the PM capacitor voltage balancing within a corresponding group and the arm controller is in charge of the PM capacitor voltage balancing and PM switching frequency balancing among groups within an arm. The key advantages of the proposed method are the reduction in the number of the PM capacitor voltages that need to be transmitted and sorted in each group controller as well as the reduction of the communication burden between the group and arm controllers. The proposed method is also highly effective under PM faults. The voltage ripple factor and the PM switching frequency of the proposed method are almost identical to the conventional centralized methods. The RT-LAB based real-time control hardware in the loop (HIL) test results of a ±350 kV/1000 MW MMC system verify the correctness and effectiveness of the proposed method.

This paper introduces a single-phase five-branch direct ac-ac modular multilevel converter (MMC) with the capability of equal frequency conversion for railway power conditioning applications. By analyzing the branch energy balance mechanism, the basic operating principle of the proposed MMC is clarified and the control strategy is also proposed. Comparisons between the proposed MMC and the traditional modular multilevel matrix converter (MMMC) on branch voltage and current ratings as well as capacitor dimension requirements are also presented. The effectiveness of the proposed topology and the control strategy is validated on a down-scaled prototype. Both the theoretical analyses and the experimental results reveal that for compensating low power factor traction loads in Scott transformer based railway power systems, the proposed MMC has decreased branch current stresses and capacitor voltage fluctuations compared to the traditional MMMC, and the required total submodule numbers of these two MMCs are comparable.

Two-photon laser confocal scanning microscopy (TPLCSM) was first used to visualize the uptake and movement of naphthalene (NAP), phenanthrene (PHE), and pyrene (PYR), from separately contaminated hydroponics solutions into living Kandelia candel (L.) Druce seedlings. With this non-destructive and non-chemical extraction technique, the experimental results revealed that three types of polycyclic aromatic hydrocarbons (PAH) were observed moving into the K. candel and its translocation to radicle, hypocotyl and leaf, and the transmission rates of these PAH in the K. candel were in the order of NAP > PHE > PYR. Data demonstrate the manner in which PAH enter, transport, and distribute within the K. candel, and provided us some valuable information on uptake and translocation mechanism of PAH. These findings may help to optimize the phytoremediation strategies of PAH in mangrove wetlands.