Power system operation and control required models of generators, lines and loads to be accurately estimated, this is to enable operators make a reliable decision on the system.  Generators and lines models are so far considered accurate, while load models are considered perplexing due to invention of new types of loads, distribution system are transforming from passive to active. Future distribution systems are desired to be smart and for a network to be smart the system as to be fully and accurately represented. Penetration of renewable energy and application of power electronic devices as well as participation of active customers in distribution systems make traditional methods of load modeling absolute. Accurate load modeling is required to address the new challenges evolving in the task of power system operation, control and stability studies. It is also an interest of power system researchers globally to realize Smart Networks (SNs), in which accurate load models are required. This work described a review of techniques and approaches for load modeling from traditional methods to the state of art in the area. In addition, gaps in the literature as well as research directions are also pointed out.

Renewable energy sources (RES) are being integrated to electrical grid to complement the conventional sources to meet up with global electrical energy demand. Among other RES, Wind Energy Conversion Systems (WECS) with Doubly Fed Induction Generator (DFIG) have gained global electricity market competitiveness because of the flexible regulation of active and reactive power, higher power quality, variable speed operation, four quadrant converter operation and better dynamic performance. Grid connected DFIG-based WECS are prone to disturbances in the network because of direct connection of stator windings to grid. The ability of the Wind Turbine (WT) to remain connected during grid faults is termed the Fault Ride-Through (FRT) capability. The grid code requirement for integrating the DFIG-based WTs to power networks specified that they must remain connected and support the grid stability during grid disturbances of up to 1500 ms. The use of compensation devices offers the best FRT compliance thereby protecting the DFIG and the converters from voltage fluctuations and over currents during the grid fault. The paper presents a review of techniques employed in ensuring FRT compliance. The article also proposes the state-of-the-art techniques for compensating voltage sag/swell and limiting the fault short-circuit current. Keywords: Renewable energy sources, DFIG, wind turbine system, fault ride-through, grid codes, dual-functional DVR

The RFEH design challenges can be broadly classified into overall radio frequency direct current (RF-to-DC) power conversion efficiency (PCE), form factor, operational bandwidth (BW), and compactness. A detailed overview of the essential components of an RFEH system is presented in this paper. Various design approaches have been proposed for the realization of compact RFEH circuits that contribute immensely to mm-wave rectenna design. Effective mechanisms for configuring the rectenna modules based on the recommended spectrums for the RFEH system were also outlined. This study featured a conceptual viewpoint on design tradeoffs, which were accompanied by profound EH solutions perspectives for wireless power communications. The work covers some challenges attributed to 5G EH in mm-wave rectenna: from a controlled source of communication signals to distributed ambient EH and system level design. Conversely, the primary targets of this work are to: (I) examine a wide range of ambient RF sources and their performance with various antennae and RF-rectifier layouts; (II) propose unique rectenna design techniques suitable for current trends in wireless technology; (III) explore numerous approaches for enhancing the rectenna or RF-rectifier efficiency in a low-power ambient environment; and (IV) present the findings of a comprehensive review of the exemplary research that has been investigated. These are aimed toward addressing the autonomous system’s energy challenges. Therefore, with the careful management of the reported designs, the rectenna systems described in this study would influence the upcoming advancement of the low-power RFEH module.

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