This review paper discusses power quality considerations for direct current (DC) electric power distribution systems, particularly DC microgrids. First, four selected sample DC architectures are discussed to provide motivation for the consideration of power quality in DC systems. Second, a brief overview of power quality challenges in conventional alternating current (AC) distribution systems is given to establish the field of power quality. Finally, a survey of literature addressing power quality issues in DC systems is presented, and necessary power quality considerations in DC distribution system design and operation are discussed.

Polyphase permanent magnet synchronous motors exhibit outstanding advantages over the traditional three-phase traction motors in electrified transportation applications. This manuscript introduces a novel direct-torque model predictive control technique aiming at developing a highly reliable, loss-minimizing and energy efficient five-phase permanent magnet synchronous motor drive. Leveraging the underlying constrained optimization process, the direct torque model predictive control approach optimizes the torque delivery and speed regulation quality, improves the flux and torque ripples, reduces higher-order current harmonics, minimizes motor drive losses, and boosts power efficiency of the electric powertrain. dSPACE DS1104 hardware-in-the-loop and computer simulation studies are jointly utilized to verify the effectiveness and robustness of the proposed direct torque model predictive control approach in polyphase permanent magnet synchronous motor drive applications.

This paper addresses the protection of a high voltage direct current (HVDC) transmission system, utilizing the modular multilevel converter (MMC) topology in addition to incorporating a hybrid transmission corridor (transmission line including overhead line and cable sections). A solution is proposed for identifying the section within which a dc fault is located for the purpose of maintaining power delivery. A detailed model of the MMC-HVDC system is simulated using PSCAD and an in depth fault analysis is performed. A characteristic signal is discovered and then implemented into a novel solution. The end result is a fault section identification protection algorithm, implementing protective relay coordination to protect the system from false circuit breaker reclose as well as enabling fast system restart for nonpermanent faults. This restart protection algorithm is implemented without the use of a communications channel between converter stations, introducing novelty and quick restart response.

Gallium nitride (GaN) devices show particular promise for space-rated power conversion applications that rely on MOSFET technology whose performance is severely limited by the radiation hardening processes. Although the number of radiation-hardened (rad-hard) GaN devices is low, the current space-rated selection pool can still yield significant efficiency and power density improvements. However, the context of GaN research is often future-oriented such that the application of GaN to common, proven, space-rated converter designs is rare. The presented work quantifies the performance benefits of market available, space-rated GaN HEMTs over rad-hard MOSFETs for a synchronous forward converter, which remains an extremely popular topology for isolated, medium-power, dc–dc conversion on various satellite systems. Two 75-W, space-rated forward converters were designed, implemented, and benchmarked, with the power switch technology being the single variable of change. By forming Pareto-optimal fronts of the key device metrics, optimal rad-hard MOSFETs were chosen so that the baseline converter performance was considered the best case. The frequency limitations of common, available, rad-hard PWM controllers limited the power density in the GaN and Si converters alike; however, efficiency gains proved sizeable. The GaN-based converter saw a peak efficiency of 86%, which was a 4.54% improvement over the Si baseline. Detailed efficiency and loss differential plots are presented, which show the GaN converter’s reduced sensitivity to the input voltage.

Three-phase inverters for grid-connected applications typically require some form of grid voltage phase detection in order to properly synchronize to the grid and control real and reactive power. This phase detection is usually based upon some type of grid voltage sensing. However, in the present work a method is proposed whereby the phase angle of the grid can be accurately identified solely via the grid current feedback. In the proposed current control scheme, the only measurement is the output inverter current and no phase locked loop (PLL) exists within the scheme. This phase observer is incorporated into a current controller which can manage the injected power to the grid. The design of this combined observer/controller system is motivated and validated via a Lyapunov stability analysis. The performance of the proposed control scheme has been compared with the conventional PI PLL-based control scheme which needs both a grid voltage and current sensor. Experimental results utilizing a Controller+ Hardware-in-the-Loop test bed have been presented in order to validate the proposed observer/controller scheme under normal operation, weak grid, distorted grid and faulty grid conditions. To summarize, this scheme archives current control for a grid connected inverter using only current feedback. In comparison, traditional methods require voltage and current feedback along with a PLL to synchronize with the grid creating a cascaded closed loop control system.

Traditional protection methods such as over-current or under-voltage methods are unreliable in inverter-based microgrid applications. This is primarily due to low fault current levels because of power electronic interfaces to the distributed energy resources (DER), and IEEE1547 low-voltage-ride-through (LVRT) requirements for renewables in microgrids. However, when faults occur in a microgrid feeder, system changes occur which manipulate the internal circuit structure altering the system dynamic relationships. This observation establishes the basis for a proposed, novel, model-based, communication-free fault detection technique for inverter-based microgrids. The method can detect faults regardless of the fault current level and the microgrid mode of operation. The approach utilizes fewer measurements to avoid the use of a communication system. Protecting the microgrid without communication channels could lead to blinding (circuit breakers not tripping for faults) or nuisance tripping (tripping incorrectly). However, these events can be avoided with proper system design, specifically with appropriately sized system impedance. Thus, a major contribution of this article is the development of a mathematical framework to analyze and avoid blinding and nuisance tripping scenarios by quantifying the bounds of the proposed fault detection technique. As part of this analysis, the impedance based constraints for microgrid system feeders are included. The performance of the proposed technique is demonstrated in the MATLAB/SIMULINK (MathWorks, Natick, MA, USA) simulation environment on a representative microgrid architecture showing that the proposed technique can detect faults for a wide range of load impedances and fault impedances.