The paper deals with the possibility of replacing a mechanical planetary gear system by a magnetic planetary gearbox. The magnetic planetary gearbox has many advantages such as contact-free, no gear lubrication, high-speed-reduction ratio and high durability. Firstly, the principle of operation, design equations and Willis relation for the magnetic and the mechanical planetary gears are given. A comparative study between the two systems in terms of torque transmission capabilities is presented.Index Terms-Gearbox, high torque density, magnetic planetary gears, mechanical planetary gears, Willis equation. NOMENCLATURERelated to sun, planet, ring, and planet carrier gears respectively (mechanical gear).Related to inner rotor, steel pole pieces, outer rotor, and carrier steel pole pieces respectively (magnetic gear). Rotational velocity of the inner rotor and the sun gear.Rotational velocity of the planet gear.Rotational velocity of the outer rotor and the ring gear.Rotational velocity of the carrier steel pole pieces and the planet carrier.Basic train gear ratio.Number of teeth of the sun, the planet and the ring.Diameter of the sun, the planet and the ring.Outer diameter of the ring gear.Pitch circle diameter, base circle diameter.Input torque at the sun gear or at the inner rotor.Output torque at the ring gear or at the outer rotor.Torque at gear one (the sun gear).Gear ratio.Tangential force.Tangential force exerted by the planet on the sun teeth.

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This paper addresses the problem of overcurrent relays (OCRs) coordination in the presence of DGs. OCRs are optimally set to work in a coordinated manner to isolate faults with minimal impacts on customers. The penetration of DGs into the power system changes the fault current levels seen by the OCRs. This can deteriorate the coordinated operation of OCRs. Operation time difference between backup and main relays can be below the standard limit or even the backup OCR can incorrectly work before the main OCR. Though resetting of OCRs is tedious especially in large systems, it cannot alone restore the original coordinated operation in the presence of DGs. The paper investigates the optimal utilization of fault current limiters (FCLs) to maintain the directional OCRs coordinated operation without any need to OCRs resetting irrespective of DGs status. It is required to maintain the OCRs coordination at minimum cost of prospective FCLs. Hence, the FCLs location and sizing problem is formulated as a constrained multi-objective optimization problem. Multi-objective particle swarm optimization is adopted for solving the optimization problem to determine the optimal locations and sizes of FCLs. The proposed algorithm is applied to meshed and radial power systems at different DGs arrangements using different types of FCLs. Moreover, the OCRs coordination problem is studied when the system includes both directional and non-directional OCRs. Comparative analysis of results is provided. ª 2015 Faculty of Engineering, Alexandria University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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