This paper presents a hybrid Modular Multilevel Converter (MMC), which combines full-bridge sub-modules (FBSM) and half-bridge sub-modules (HBSM). Compared with the FBSM based MMC, the proposed topology has the same dc fault blocking capability but uses fewer power devices hence has lower power losses. To increase power transmission capability of the proposed hybrid MMC, negative voltage states of the FBSMs are adopted to extend the output voltage range. The optimal ratio of FBSMs and HBSMs, and the number of FBSMs generating a negative voltage state are calculated to ensure successful dc fault blocking and capacitor voltage balancing. Equivalent circuits of each arm consisting of two individual voltage sources are proposed and two-stage selecting and sorting algorithms for ensuring capacitor voltage balancing are developed. Comparative studies for different circuit configurations show excellent performance balance for the proposed hybrid MMC, when considering dc fault blocking capability, power losses, and device utilization. Experimental results during normal operation and dc fault conditions demonstrate feasibility and validity the proposed hybrid MMC.

This paper presents a new control strategy for a doubly fed induction generator (DFIG) under unbalanced network voltage conditions. Coordinated control of the grid-and rotorside converters (GSC and RSC, respectively) during voltage unbalance is proposed. Under an unbalanced supply voltage, the RSC is controlled to eliminate the torque pulsation at double supply frequency. The oscillation of the stator output active power is then compensated by the active power output from the GSC, to ensure constant active power output from the overall DFIG generation system. In order to provide precise control of the positive-and negative-sequence currents of the GSC and RSC, a current control scheme consisting of a proportional integral (PI) controller and a resonant (R) compensator is presented. The PI plus R current regulator is implemented in the positive synchronous reference frame without the need to decompose the positiveand negative-sequence components. Simulations on a 1. 5-MW DFIG system and experimental tests on a 1.5-kW prototype validate the proposed strategy. Precise control of both positive-and negative-sequence currents and simultaneous elimination of torque and total active power oscillations have been achieved. Index Terms-Converter, doubly fed induction generators (DFIGs), proportional integral (PI) plus resonant (R; PI-R), voltage unbalance, wind energy. NOMENCLATURE V g and I gGrid-side converter (GSC) output voltage and current vectors. V s and V r Stator and rotor voltage vectors. I s and I r Stator and rotor current vectors. ψ s and ψ r Stator and rotor flux linkage vectors. ω s , ω r , and ω slip Stator, rotor, and slip angular frequencies. P s and Q s Stator output active and reactive powers. P g and Q g GSC output active and reactive powers. P total Total output active from the doubly fed induction generator (DFIG) system.

DC fault protection is one challenge impeding the development of multi-terminal DC grids. The absence of manufacturing and operational standards has led to many pointto-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and DC fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode DC component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a DC to DC transformer where two converters operating in the proposed mode are coupled by an AC transformer for voltage matching and galvanic isolation. The proposed DC transformer is shown to be suitable for high-voltage high-power applications due to the low switching frequency, high efficiency, modularity, and reliability. The DC transformer facilitates DC voltage regulation and near instant isolation of DC faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.

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This paper proposes a new breed of high-voltage dc (HVDC) transmission systems based on a hybrid multilevel voltage source converter (VSC) with ac-side cascaded H-bridge cells. The proposed HVDC system offers the operational flexibility of VSCbased systems in terms of active and reactive power control, blackstart capability, in addition to improved ac fault ride-through capability and the unique feature of current-limiting capability during dc side faults. Additionally, it offers features such as smaller footprint and a larger active and reactive power capability curve than existing VSC-based HVDC systems, including those using modular multilevel converters. To illustrate the feasibility of the proposed HVDC system, this paper assesses its dynamic performance during steady-state and network alterations, including its response to ac and dc side faults.Index Terms-DC fault reverse blocking capability, hybrid multilevel converter with ac side cascaded H-bride cells, modular multilevel converter, voltage-source-converter high-voltage dc (VSC-HVDC) transmission system.

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Conventional dual-active bridge topologies provide galvanic isolation and soft-switching over a reasonable operating range without dedicated resonant circuits. However, scaling the two-level dual-active bridge to higher dc voltage levels is impeded by several challenges among which the high dv/dt stress on the coupling transformer insulation. Gating and thermal characteristics of series switch arrays add to the limitations. To avoid the use of standard bulky modular multilevel bridges, this paper analyzes an alternative modulation technique, where staircase approximated trapezoidal voltage waveforms are produced; thus, alleviating developed dv/dt stresses. Modular design is realized by the utilization of half-bridge chopper cells. This way the analyzed dc-dc transformer employs modular multilevel converters operated in a new mode with minimal common-mode arm currents, as well as reduced capacitor size, hence reduced cell footprint. Suitable switching patterns are developed and various design and operation aspects are studied. Soft-switching characteristics will be shown to be comparable to those of the two-level dual-active bridge. Experimental results from a scaled test rig validate the presented concept. Index Terms-DC fault, dc/dc power conversion, dc transformer, dual-active bridge, modular multilevel converter (MMC). NOMENCLATURE T tVoltage transit time between the two dc rails. N Number of cells per arm (or series IGBTs per valve). N s Number of ac voltage steps. T d Dwell time spent in each voltage level. V dc DC-link voltage. m Modulation index. f s , ω s Fundamental frequency (f s = 1/T s ). t d(off ) IGBT turn-off delay time. t f IGBT fall time.