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Doubly fed induction generator (DFIG) driven wind turbines experience poorly damped stator flux oscillations at frequencies around the fundamental, limited low voltage ride through capability and output power fluctuation. In this study, a proposed controller to mitigate stator flux oscillations of DFIG is proposed during autonomous operation mode. In addition, super-capacitors based energy storage is integrated to manipulate wind speed vagaries and contribute to load demand. A 53rdorder small-signal analytical model and eigenvalues study is carried out to optimally determine control system parameters and pinpoint latent system dynamics and stability margins under various operation conditions. Also, a detailed non-linear system is modelled and simulated in Matlab/Simulink environment to assure small-signal model results. The results obtained confirm the accuracy of the analytical model and explore the significance of the proposed damping controller to mitigate stator flux oscillations even during contingencies.

In spite of the contemporary interest in renewable power plants, thermal power plants are still inevitable. Various electric equipment and apparatus are grounded via a large-scale grounding system in thermal power plants. In this paper, the three-dimensional finite-difference time-domain method has been employed to study the performance of such a large-scale grounding system against a lightning strike to a nearby transmission tower. The study has emphasized how a nearby sea, which is utilized for cooling purposes in thermal power plants, influences the ground potential rise on the large-scale grounding system considering soil ionization. The results show that the distribution of the ground potential rise on the large-scale grounding system is quite dependent on the alignment of sea with the large-scale grounding system. In addition, the extent that soil ionization affects the ground potential rise is dependent on the distance between the struck tower and the large-scale grounding system. Index Terms-Electromagnetic fields, finite-difference timedomain (FDTD) method, grounding systems (GSs), lightning strikes. I. INTRODUCTION T HE GLOBAL interest in renewable energy is currently increasing due to environmental considerations. However, the majority of the contemporaneous electric energy consumed all over the world is actually produced from thermal power plants owing to the intermittent nature of renewable resources and their associated technical challenges [1]. Power system apparatus, equipment, and electric circuits inside a thermal power plant are grounded by a large-scale grounding system (LSGS) to protect them against power system electromagnetic transients such as lightning and switching surges. Therefore, a considerable research has been devoted to study grounding systems Manuscript

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Thermal management is one of the most significant challenges for reliability improvement of modular multilevel converters (MMCs). The thermal stress difference caused by the uneven loss distribution between top and bottom IGBTs/diodes will lead to lifetime difference of the IGBTs/diodes which will decrease the reliability of the MMC. In this paper, a thermal optimization strategy for MMCs is proposed, where the maximum thermal stress in the IGBT/diode of each SM can be effectively reduced through injecting optimum second-order harmonic current into the circulating current of MMCs, and accordingly the lifetime of MMCs can be improved by the proposed strategy. Detailed simulation results with PSCAD software and experimental results with 3kW MMC platform are provided to confirm the validity of the proposed thermal optimization strategy for MMCs. INDEX TERMSCirculating current injection, Modular multilevel converters, lifetime, thermal optimization.

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