This paper reviews the current state of the art of condition monitoring and fault diagnosis techniques for permanent magnet (PM) machines. It also takes into account the past research on this subject as appropriate. The discussion in the paper includes issues related to common abnormalities and specific faults in permanent magnet machines and drives, such as magnet damage and demagnetization, rotor eccentricity, unbalanced magnetic pull, open and short-circuit windings, and switch faults. In this paper, a detailed review is present on the sources of these faults, their analytical model and fault detection tools. Finally, an integrated methodology for condition monitoring of PM machines is discussed.

As the number of inverters increases in the power grid, the stability of grid-tied inverters becomes an important concern for the power industry. In particular, a weak grid can lead to voltage fluctuations at the inverter terminals and consequently cause inverter instability. In this paper, impacts of circuit and control parameters on the stability of voltage source inverters are studied using a small-signal state-space model in the synchronously rotating dq-frame of reference. The full-order state-space model developed in this paper is directly extracted from the pulsewidth modulation switching pattern and enables the stability analysis of concurrent variations in the three-phase circuit and control parameters. This paper demonstrates that the full-order model of a grid-tied active (P) and reactive (Q) power (PQ)-controlled voltage source inverter (VSI) can be significantly reduced to a second-order model, preserving the overall system stability in the case of grid impedance variations. This paper also shows that a decrease in the grid inductance does not necessarily improve the stability of grid-tied VSIs. The system stability is a function of both the grid R/X ratio and grid inductance. Despite the grid-side inductor of the LCL filter is in series with the grid impedance, they have different impacts on the stability of a grid-tied PQ-controlled VSI, i.e., an increase in the filter inductance may improve the system stability in a weak grid. These findings are verified through simulated and experimentally obtained data.

Many Electric Traction Drive Systems (ETDS) are driven by electric motors primarily composed of high-energy Rare-Earth (RE) Permanent Magnets (PMs) such as Neodymium-Iron-Boron (NdFeB) and Samarium-cobalt (SmCo). Given the high cost of RE materials, there is a great interest among various industry sectors in searching for breakthrough technologies that reduce the consumption of high-energy RE magnets in their electro-mechanical energy conversion systems. The reduction of the magnetic volume is typically realized either by replacing a portion of the high-energy RE magnet with the low-energy Ferrite magnet (knows as hybrid structure) or by modifying the magnetic arrangement of the machine itself, where a portion of the physical RE magnets is removed and replaced by induced magnets (known as consequent-magnetic pole structure). For this purpose, this paper presents a review on the current state-of-the-art of motor typologies with Less-Rear-Earth-PMs (Less-RE PM) and their key design challenges, followed up by a series of recommended guidelines to overcome related design challenges.

Traditionally, a grid-interactive inverter providing ancillary services is called a smart inverter. However, broader features will be required for the next generation of inverters that can be categorized as self-governing, self-adapting, self-security, and self-healing. For grid-interactive inverters, the self-governing feature can be identified as the capability of inverters to operate in grid-following and grid-forming control modes, where the self-adapting is referred to as more flexibility realized by adaptive controllers for stable dynamics of inverters under various grid conditions. Moreover, for supervisory control and economic dispatch in a grid with high-penetration of inverter-based power generators, a minimum communication might be necessary, but it can place grid-interactive inverters in danger of being hacked when self-security becomes essential to identify malicious setpoints. Furthermore, the self-healing is defined as fault-tolerance and stress reduction under abnormal conditions. It suggests that after realizing these features, an inverter is called a smart inverter. In this paper, the advancements toward achieving these features for grid-interactive inverters are reviewed.

This article compares two strategies for seamless (re)connection of grid-forming inverters to a microgrid powered by droop-controlled inverters. While an incoming inverter must be synced to the microgrid, seamless syncing and power-sharing are technical challenges for grid-forming inverters. In the first strategy, called the output-sync method, an incoming inverter is synced to the microgrid, and then the circuit breaker is closed for power-sharing. In the second strategy, called the controller-sync method, the inverter initially contributes at zero power following the microgrid frequency, and then the controller is transferred to the mode of power-sharing. Remarkably, the circuit breaker can be kept closed during the syncing process using the controller-sync method as the inverter starts from zero power contribution. In this novel strategy, two controller sets run in parallel, i.e., two parallel control paths are in place for obtaining the magnitude and the phase angle of the PWM reference signal. These parallel paths are synced for seamless transitions when only one of them is engaged to generate the PWM reference signal. The efficacy of these control strategies has been tested in a hardware setup of a microgrid fed by two 5kVA 208V droop-controlled inverters, and the results are presented in this article. This article is also accompanied by a video clip demonstrating the performance of the output-sync and controller-sync methods in the laboratory.

In this paper, a feedforward virtual inductance control strategy is developed for the stability of voltage source inverters in weak grids. A weak grid is characterized by a low short-circuit ratio (SCR) and a low inertia constant, in which, voltage source inverters (VSIs) become susceptible to voltage distortions and instability. The proposed feedforward virtual inductance term is integrated to the current control loops of grid-tied VSIs. In this paper, the effectiveness of the virtual inductance control strategy in weak grids is first proved through closed-loop transfer function analysis, and then, the system's root locus analysis to determine the stability region of the system versus the grid impedance. The developed virtual inductance controller is also verified through laboratory experiments for different weak grid scenarios.