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This paper is on a wind energy conversion system simulation of a transient analysis due to a blade pitch control malfunction. The aim of the transient analysis is the study of the behavior of a back-to-back multiple point clamped five-level full-power converter implemented in a wind energy conversion system equipped with a permanent magnet synchronous generator. An alternate current link connects the system to the grid. The drive train is modeled by a three-mass model in order to simulate the dynamic effect of the wind on the tower. The control strategy is based on fractional-order control. Unbalance voltages in the DC-link capacitors are lessen due to the control strategy, balancing the capacitor banks voltages by a selection of the output voltage vectors. Simulation studies are carried out to evaluate not only the system behavior, but also the quality of the energy injected into the electric grid. © 2015 Elsevier Ltd. All rights reserved.Keywords: Wind energy; five-level power converter; drive train; fractional-order control; simulation. IntroductionEnvironmental problems such as global warming and habitat preservation have to be faced by the society in nowadays in order to achieve a sustainable development. One of the causes for the global warming is said to be the increase on the level of CO 2 in the atmosphere due to fossil fuel burning [1]. However, nowadays fossil fuel burning is needed for supplying the majority of the energy demand [2]. Research and development is on the way to achieve lower pollutants emissions on conversion of energy to convenient forms of usage in order to achieve a sustainable development. Conversion of energy from renewable energy sources are in nowadays a political attractive option [3] and a wind energy conversion system (WECS) is an economically viable exploitation [4], experiencing a significant expansion [5,6]. Although, onshore WECS deployments are cheaper than offshore ones, new suitable available onshore sites for deployments are becoming scarce, particularly in Europe [7]. So, the offshore WECS is becoming a further attractive option.

This paper deals with a computing simulation for an offshore wind energy system taking into account the influence of the marine waves action throughout the floating platform. The wind energy system has a variable-speed turbine equipped with a permanent magnet synchronous generator and a full-power fivelevel converter, injecting energy into the electric grid through a high voltage alternate current link. A reduction on the unbalance of the voltage in the DC-link capacitors of the five-level converter is proposed by a strategic selection of the output voltage vectors. The model for the drive train of the wind energy system is a two mass model, including the dynamics of the floating platform. A case study is presented and the assessment of the quality of the energy injected into the electric grid is discussed.

a b s t r a c tThis paper presents an integrated model for an offshore wind turbine taking into consideration a contribution for the marine wave and wind speed with perturbations influences on the power quality of current injected into the electric grid. The paper deals with the simulation of one floating offshore wind turbine equipped with a permanent magnet synchronous generator, and a two-level converter connected to an onshore electric grid. The use of discrete mass modeling is accessed in order to reveal by computing the total harmonic distortion on how the perturbations of the captured energy are attenuated at the electric grid injection point. Two torque actions are considered for the three-mass modeling, the aerodynamic on the flexible part and on the rigid part of the blades. Also, a torque due to the influence of marine waves in deep water is considered. Proportional integral fractional-order control supports the control strategy. A comparison between the drive train models is presented.Ó 2016 Elsevier Ltd. All rights reserved.

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