Widespread adoption of electric vehicles will require AC power distribution systems to accommodate high penetrations of power electronic loads, placing increasing demands on the power quality of grid-connected converters. Recent developments in devices and circuit topologies have potential to improve the intrinsic power quality of these grid interface inverters, reducing the need for passive filters and associated reactive power consumption. Wide bandgap devices, such as SiC, have recently gained much attention due to their low switching losses facilitating raised PWM frequencies. However the high cost of SiC together with electromagnetic interference (EMI) resulting from very rapid switching transitions necessary to realize low switching losses cause concern. Previous research in Si MOSFET modular multilevel converters (MMC) suggests a high efficiency alternative with potential for lower EMI. Si MOSFET MMC benefits are enhanced with parallel-connected devices and slowed switching, made possible by low effective switching frequency. This paper uses experimental results to explore the impact of parallel connection and slowed switching on Si MOSFET MMC losses, and presents improved Si MOSFET switching loss models to resolve inaccuracies observed with conventional Si MOSFET models. EMI is then compared between SiC and Si MMC using carefully controlled relative measurements of radiated EMI.

Adoption of distributed submodule (SM) capacitors in a modular multilevel converter (MMC) necessitates complex controllers to ensure the stability of its internal dynamics. This paper presents comprehensive analysis and assessment of different proportional resonant (PR)-based control schemes proposed to stabilize the internal dynamics and ensure ac and dc sides power quality of the MMC within a dc transmission system. With the consideration of passive component tolerances, different energy and voltage based control schemes under various conditions are analyzed. It has been established that without vertical voltage balance control, unequal passive component values in the upper and lower arms of the same phase-leg may cause: unbalanced fundamental currents in the arms, unequal dc voltage across the arms, and fundamental oscillations in the common-mode currents that lead to fundamental frequency ripple in the dc link current. The theoretical analysis that explains this mechanism is presented, and is used to show that vertical voltage balancing is necessary for the nullification of arm voltage difference and suppression of odd oscillations caused by capacitive/inductive asymmetry between arms of the same phase-leg. Simulations support the theoretical analysis and the effectiveness of voltage balancing in ensuring correct operation, independent of tolerances of the MMC passive elements and operating conditions. A new direct method for elimination of fundamental oscillations in the common-mode and dc link current is proposed. Experimental results from a singlephase MMC prototype validate the presented theoretical discussions and simulations.

Distributed energy resources are often connected to low-voltage distribution networks where the grid voltages may be unbalanced. This leads to an unwanted ripple in the output active power at twice the fundamental grid frequency. In this article, a new model predictive current controller is proposed for unbalanced grids. The variable switching frequency of existing model predictive controllers is fixed and the power quality is improved. A Kalman filter estimator is used to extract the positive and negative sequence components. A new calculation time compensation technique is proposed, which offers superior accuracy to existing approaches. A grid voltage discretization compensation strategy is outlined, and its effectiveness is demonstrated. Finally, the system stability is verified theoretically. Simulation and laboratory results are included to prove the robustness of the proposed controller and support the theoretical analysis.

This paper describes the operating principles and theoretical relationships that underpin the modelling and control system of the enhanced modular multilevel converter (EMMC). A full-scale model of a point-to-point HVdc link that employs EMMCs is used to examine its performance during normal operation in all four quadrants, and resiliency to symmetrical and asymmetrical ac and dc faults. Results of exhaustive simulation studies reveal that the improved ac and dc power qualities, which are achieved by incorporation of a few full-bridge cells into the arms of conventional half-bridge modular multilevel converter (HB-MMC) with medium-voltage cells to create the EMMC do not affect its ac and dc fault ride-through capability nor its dynamics during normal operation as active and reactive power set-points being varied. In addition, a variant of the EMMC is proposed, in which the number of full-bridge cells to be added into the arms of HB-MMC could be increased to offer bespoke features beyond that explicitly defined in original vision of the EMMC, such as reduced dc voltage operation during pole-toground dc fault, and potential extension of fault clearance times in multi-terminal HVdc grids. Moreover, the validity of the new variant has been confirmed using results obtained from highfidelity HVdc link models developed in EMPT-RV platform, in which the EMMCs are replaced by the proposed variant.

This paper presents an alternative implementation of a modular multilevel converter (MMC) that generates a large number of voltage levels per phase with high resolution voltage steps from a reduced number of cells per arm. The presented MMC employs a half-bridge chain-link of medium-voltage cells and a full-bridge chain-link of low-voltage cells in each of its arms. The total blocking voltage of the full-bridge chain-link is equivalent to half that of the medium-voltage half-bridge cell. The use of half and full-bridge cells with two distinct rated voltages in each arm permits full exploitation of the full-bridge cells to generate high resolution multilevel voltage waveforms with fine stepped transitions between major voltage steps of the medium-voltage half-bridge cells. In this manner, errors in the synthesis of the common-mode voltages of the three phase legs due to switching of the cell capacitors in and out the power path are reduced. The nested multilevel operation of the proposed MMC results in a number of voltage levels which is related to the product, rather than the sum, of the numbers of half and full-bridge cells. Detailed comparisons with existing MMC implementations show that the proposed MMC implementation offers the best design trade-offs (superior AC and DC waveforms with reduced control and power circuit complexity). The validity of the proposed MMC implementation is confirmed using simulations and experimentally.

This paper presents a small-signal analysis of different grid side controllers for full power converter wind turbines with inertia response capability. In real wind turbines, the DC link controller, the drivetrain damping controller and the inertial response might present contradictory control actions in a close bandwidth range. This situation might lead to reduced control performance, increased component stress and non-compliance of connection agreements. The paper presents an analysis of the internal wind turbine dynamics by considering different grid-side converter control topologies: standard current control used in the wind industry, standard current control with inertia emulation capabilities and virtual synchronous machines. Comments are made on the similarities between each topology and the negative effects and limits, and possible remedies are discussed. Finally, the conclusion poses that the inclusion of a DC link voltage controller reduces the ability of a converter to respond to external frequency events without energy storage. The degradation increases with the DC link voltage control speed.