Maurice Apap

Electrical insulation systems (EISs) are the principal reliability bottleneck of modern electrical machines (EMs). Among the many stresses acting on insulation, thermal stress is the most pervasive because it accelerates chemical reactions that progressively erode dielectric and mechanical integrity, ultimately dictating service life. As EMs migrate into compact, high-power-density platforms—automotive, aerospace, and industrial drives—designers need lifetime models that are not merely explanatory but actionable, linking operating temperatures and missions to quantified ageing and risk. This review article traces the evolution of thermal-ageing modelling from fundamental chemistry to a practical design tool. The historical empirical lineage of Arrhenius equation, Arrhenius–Dakin model, and Montsinger model is first revisited, clarifying their assumptions, parameter definitions, and the construction of thermal endurance curves. A discussion then follows on extensions that address deviations from first-order kinetics and demonstrate how variable temperature histories can be incorporated through cumulative damage formulations suitable for duty-cycle analysis. Since models are required to be anchored in data, accelerated thermal ageing (ATA) practices on representative specimens are outlined, alongside a description of the Weibull post-processing for deriving percentile lifetimes aligned with design targets. Building upon these foundations, the Physics-of-Failure (PoF) approach is introduced as a reliability-oriented design (ROD) methodology, in which validated lifetime models guide material selection and geometry optimisation while supporting prognostics and health management during operation. The emerging trend towards a hybrid PoF–AI approach is also discussed, which integrates artificial intelligence to identify nonlinear degradation patterns and drifting parameter relationships beyond the reach of empirical models, with physical constraints ensuring that predictions remain consistent with known ageing mechanisms. Such integration enables the learning process to adapt to operational variability and coupled stress effects, thereby improving both the accuracy and physical interpretability of lifetime estimation. The review aims to provide a concise view of models, tests, and workflows that convert thermal-ageing knowledge into robust, design-time decisions. By linking empirical and physics-based insights with modern data-driven learning, these developments support proactive maintenance, sustainable asset management, and extended operational lifetimes for next-generation EMs.

This paper presents the controller design of paralleled Buck converters using Droop Control to obtain common load sharing for DC Microgrid operation. Proportional Integral (PI) controllers are used to provide nested current and voltage control of the Buck converters. Droop control is applied to obtain load sharing between the paralleled converters. Then, a voltage restoration loop is applied utilizing another PI controller to restore the desired voltage in the dc microgrid, correcting any voltage deviations caused by the droop loop. The operation of the controllers is tested by simulating two paralleled Buck converters operated in the continuous current mode while sharing a common resistive load.

Droop control has limitations with respect to current sharing since the output current delivered by the inverters depends on their output impedance ratios. In addition, harmonic voltage drops due to the flow of harmonic currents induce voltage distortion at the point of common coupling (PCC). Virtual impedance loops were proposed in literature to improve the current sharing between the inverters by normalizing the output impedance of the inverters. However, virtual impedance loops have constraints in this respect since the improvement in the current sharing occurs through redistribution of the current harmonics which can add to the voltage distortion at the PCC. This paper compares the performance of resistive, inductive, inductive-resistive and resistive-capacitive virtual impedance loops with respect to current sharing and voltage harmonic distortion at the PCC. Simulation results are given for a single phase microgrid setup to achieve a fair performance comparison of the different virtual impedance techniques.

Optimization of the islanded and grid-connected operation of microgrids is important to achieve a high degree of reliability. In this paper, the authors consider the effect of current harmonics in single phase microgrids during both modes of operation. A detailed analysis of the effect of the output impedance of the considered primary control loops on the harmonic output of the considered voltage source inverters is initially carried out. A virtual admittance loop is proposed to attenuate the current harmonic output in grid-connected operation that is generated due to the grid voltage distortion present at the point of common coupling (PCC) and due to local non-linear loads. This paper also considers the harmonic current sharing and resulting voltage harmonics at the PCC during islanded operation of the microgrid. A capacitive virtual impedance loop was implemented to improve the harmonic current sharing and attenuate the voltage harmonics at the PCC. Experimental results are given to validate the operation of the proposed algorithms.

Microgrids are an effective way to increase the penetration of distributed generation into the grid. They are capable of operating either in grid-connected or in islanded mode, thereby increasing the supply reliability for the end user. This paper focuses on achieving seamless transitions from islanded to grid-connected and vice versa for a single phase microgrid made up from voltage controlled voltage source inverters (VC-VSIs) and current controlled voltage source inverters (CC-VSIs) working together in both modes of operation. The primary control structures for the VC-VSIs and CC-VSIs is considered together with the secondary control loops that are used to synchronize the microgrid as a single unit to the grid. Simulation results are given that show the seamless transitions between the two modes without any disconnection times for the CC-VSIs and VC-VSIs connected to the microgrid.

When paralleling multiple inverters that are capable of operating as an island, the inverters typically employ the droop control scheme. Traditional droop control enables the decentralized regulation of the local voltage and frequency of the microgrid by the inverters. The droop method also enables the inverters to share the real and reactive power required by the loads. This paper focuses on some of the limitations of parallel islanded single phase inverters using droop control. Algorithms with the aim to address the following limitations in islanded operation were proposed: reactive power sharing and reduction of the voltage harmonic distortion at the point of common coupling (PCC). Experimental results were then presented to show the suitability of the proposed algorithms in achieving reactive power sharing and in improving the voltage harmonic distortion at the PCC.

Parallel inverters having LCL output filters cause voltage distortions at the point of common coupling (PCC) in islanded microgrids when non-linear loads are present. A capacitive virtual impedance loop could be used to provide selective harmonic compensation in islanded microgrids, instead of introducing additional active or passive filters into the system that could compromise the stability of the microgrid. However, the performance of these compensation loops becomes degraded when a virtual resistance is introduced with the aim to improve the overall stability of the parallel inverters. With the capacitive virtual impedance, there is effectively a compromise between the additional stability provided by the virtual resistance and the harmonic compensation due to the virtual capacitance. This paper focuses on overcoming this limitation of the capacitive virtual impedance with additional virtual resistance for selective harmonic compensation in islanded microgrids. Simulation results were given to show the suitability of the proposed algorithms in reducing the voltage harmonics at the PCC.

This paper presents a Model Reference Adaptive System (MRAS) applied to the sensorless control of a DFIG system. The performance of the MRAS is enhanced by introducing a voltage model bandpass filter approximation to compensate for drift. Control is performed by a rotor-side matrix converter and is limited to the most common speed range in wind systems. Moreover a detailed analysis of the effects of current harmonics on the MRAS is carried out and a means of mitigation is proposed.

This paper describes the electronics and sensorless control algorithm implemented in an experimental setup to operate a 3 phase, high pole number, brushless DC motor through a single stage inverter. The use of a high PWM switching frequency imposed on the motor phases for torque and speed control generates noise on the sensing and drive circuitry. Thus the first part of this paper focuses on designing noise immune feedback and control circuitry containing the required galvanic isolation. The second part of this paper focuses on the development of a sensorless algorithm based on back-emf voltage sensing to commutate between phases. The outcome of this algorithm is useful when the motor is operated from medium to high speed. This sensorless algorithm uses synchronous sampling of direct terminal voltages measurement proposed in [1]. Then through an innovative digital estimation technique, the zero crossing of the back-emf of the concurrently unenergized phase is calculated. This technique avoids the use of passive filters on terminal voltage sensing thus eliminating undesired phase shifts that cause erratic phase commutation. Validity of this technique during steady state and transient operation is shown through simulation and practical results obtained with the experimental setup. Moreover the limitations encountered on employing the designed algorithm on the experimental setup are also discussed.