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This paper proposes a multifunctional control strategy and associated control algorithms for distributed wind-turbine (WT) based nano-grids connected to a distorted utility-grid. The contribution is on a new strategy with innovative control algorithms to coordinate multiple converters for a multitasking operation of the nano-grids. The novelty is on a unique control design with feasibilities: maximizing the generated power from WT, maintaining power quality in both ac- and dc-sides under critical conditions of the power grid, and improving power quality against distortion from local nonlinear loads under a reduced switching frequency. A robust fast-dynamic predictive control method is developed for current controllers to fulfil the multifunction. Unconstrained deadbeat control inputs are derived in twofold targets: ensuring fast dynamic response and significantly reducing both the computation and switching frequency for finite predictive control. The control system is applied on a permanent-magnet synchronous generator (PMSG) WT-based nano-grid connected to a distorted utility-grid. An OPAL-RT-based real-time platform is used for comparative studies among the proportional integration (PI) control, finite predictive control (FS-MPC), and proposed control method. The performance verification exhibits the power quality improvement in both the nano- and utility-grids under critical conditions via high-performed regulation of currents, voltages, reactive power, and rotor-speed of the PMSG-WT. [ABSTRACT FROM AUTHOR]

This article proposes a new control structure for grid-interfaced converters to enhance ancillary services from emerging electric vehicle (EV) charging stations (CS) under unbalanced and distorted grid conditions. Compared to existing methods, the proposed control structure is relatively simple yet effective for both regulating powers and minimizing harmonics under unbalanced and distorted grid conditions. A simple direct predictive control methodology is proposed where the positive fundamental component of currents is controlled for power regulation while other negative sequence or harmonic current components are set to zeros. Consequently, the charging station operation is enhanced with efficient ancillary services to the electric grid. The novelty originates from a simple predictive control structure in an abc-natural frame without adding any decoupled or separated current-component controllers. Comparative studies on both Matlab/Simulink platform and OPAL-RT-based real-time system among proportional integral (PI) control, finite predictive control, and the proposed method are conducted to verify the efficacy of the proposed structure. The comparative results show that the control structure improves the control performance and significantly enhances the support for the local loads under unbalanced, harmonic distorted grid conditions. [ABSTRACT FROM AUTHOR]

A high-frequency isolated multilevel cascaded-type bipolar direct PWM ac–ac converter is proposed. By cascading N-unit cells, the converter can produce symmetric bipolar voltage gain of ±ND. It does not require a separate ac source for each unit cell, and can provide higher ac output voltages with multiple levels using low-voltage rating devices, decreasing output voltage harmonics and filtering requirements. Moreover, phase-shift PWM switching is employed, increasing the effective frequency of the output filter, further reducing its requirements. The converter can be utilized as a dynamic voltage restorer (DVR) for grid voltage regulation, with the following features: No need for an external dc source, separate ac sources, and a line-frequency voltage injection transformer. Injection of higher voltage magnitudes than the line voltage, mitigating deep voltage sags. Injection of a negative series voltage to provide compensation for voltage swells. The detailed circuit operation is explained together with component design guidelines and application as DVR. Finally, experimental results are provided using an 800-W scale-down laboratory prototype. [ABSTRACT FROM AUTHOR]

Single-phase matrix converters (MCs) are undergoing rapid developments due to their bipolar voltage gain (as required in dynamic voltage restorers) and step-changed frequency operation (used in high step-up ac–dc rectifiers, and to provide medium frequency isolation in traction and wind turbine converters, etc.). However, existing buck–boost MCs require high-voltage/current rating devices, and suffer from large component voltage/current stresses and ripples, significantly degrading their efficiency. This article proposes a highly efficient single-phase buck–boost MC; consisting of eight IGBTs (implemented using two full-bridge IGBT modules), second-order input and output filters, and a small value film capacitor. Efficient discrete inverting and noninverting buck and boost operations are proposed (same as that of sixteen-switch cascaded buck–boost MC) with significantly lower component voltage/current stresses and ripples. In addition, flexible inverting and noninverting buck–boost operations are proposed with independent control of buck and boost duty ratios. All of the proposed operations are free of commutation issues, compatible with reactive loads, and provide smooth input and output currents. Furthermore, all the switches have same voltage stresses, and all four switches in a full-bridge module experience the same current stresses. A comprehensive description of circuit operation is presented based on a number of proposed switch modulation strategies with a nonunity power factor load, followed by design guidelines, and comparative evaluations with existing topologies. Finally, experimental verification results are presented, using a 400-VA laboratory prototype converter. [ABSTRACT FROM AUTHOR]

This paper proposes a novel wide-area control strategy to enhance the transient stability of a large inter-connected power grid. The proposed control strategy achieves maximal damping of the inter-area oscillations by calculating the optimal number of cluster centers based on a novel online coherency grouping. To achieve this objective, the critical electro-mechanical mode is selected using localness index method during transient conditions. In addition, the online identification of the full model is performed to estimate the states to be controlled during the grid disturbances. Thereafter, a discrete linear quadratic regulator (DLQR) is directly applied to the estimated states, and the corresponding signals are channelized through the loop of most controllable and observable machines of each coherent group. The efficacy of the proposed method is tested on IEEE 39 bus and IEEE 68 bus standard benchmark model. The results demonstrate faster damping with minimal computation burden compared to existing solutions such as a classical slow coherency and a multi-input multi-output (MIMO) based wide area damping controllers. [ABSTRACT FROM AUTHOR]

DC short-circuit fault ride through is one of the most important characteristics for the modular-multilevel converters (MMC) employed in high-voltage direct-current (HVdc) transmission systems. During the faults, for providing a fault-tolerant control with reactive power compensation to the grid, the MMC normally changes its structure, which requires a remarkable modification and burden computation for implementing the modulation technique in the MMC. This paper proposes an alternative submodule configuration of the MMC based on a carrier-phase-shift PWM scheme, which is easily implemented for switching the operation modes of the MMC from normal condition to fault-tolerant control. The arms of the proposed MMC are configured by typical half-bridge submodules (HBSM) and suggested series-connected triple SMs (SCTSM) in interleaving series, where the SCTSM is composed of three HBSMs connected in series through an additional IGBT and a clamp diode. With the additional IGBTs and diodes, the SCTSMs can produce bipolar output voltages and the MMC can be restructured to operate as three-phase cascaded multilevel converter during pole-to-pole short circuits to control the converter currents. In addition, the cost and power loss of the proposed MMC are lower than those of the existing MMCs based on full-bridge SMs (FBSM), a hybrid of HBSMs and FBSMs, and clamp-double SMs. PSIM simulation results for the 300 MW–300 kV HVdc system with the proposed MMCs are shown to verify the effectiveness of the scheme. [ABSTRACT FROM AUTHOR]

Fault-current handling capability of the modular multilevel converters (MMCs) under dc-cable short-circuit conditions is a major concern for the MMC applications on the high-voltage direct-current (HVDC) transmission systems, where the MMCs based on half-bridge submodules (SMs) cannot block the fault currents to protect the converter devices. In this paper, a comprehensive review for the fault-ride-through capability of the HVDC transmission systems based on the MMCs adopting different SM schemes is presented, where the MMCs can block the fault currents and compensate the reactive currents to the electric grid during the dc faults. An analysis of the dc short-circuit faults in the MMC is introduced and then the operation principle of different SM circuits building the MMC for blocking the fault currents is highlighted. The fault-tolerant operation of these MMC schemes as static synchronous compensator to enhance the ac grid stability during the dc faults is also investigated. A comparison in terms of investment cost, loss, volume, and controllability for various MMC topologies is performed. Comprehensive simulation results for the most promising topologies of the MMC with a capability of fault-ride through under dc-fault conditions are finally presented. [ABSTRACT FROM AUTHOR]

This paper presents a new control strategy for voltage source converter based Multi-Terminal High Voltage Direct Current (MTDC) systems. The proposed control approach ensures accurate power sharing between the droop-controlled converter stations. By communicating the power-sharing index between neighboring converters, the proposed approach achieves exact droop control operation independent of the DC system topology and line parameters. The pilot voltage droop based controller, which is an alternative communication-based approach for achieving precise power sharing, was used as a base case for comparison. Modal analysis is carried out to reveal the sensitivity of the system's eigenvalues to the changes in control parameters (e.g., power droop gain, proportional integral gains of the proposed controller) and the latencies in the communication. It is demonstrated that the proposed strategy remains in the stable operation even when excessive latencies are encountered in the communication. Nonlinear simulations are conducted in the MATLAB/Simulink environment in four- and five-terminal MTDC grids, validating the capability of the proposed controller to achieve the desirable performance under various operational conditions. [ABSTRACT FROM AUTHOR]