Fault-tolerant Control of a Wind Turbine with a Squirrel-cage Induction Generator and Stator Inter-turn Faults Vinko Leˇ si´ c 1) , Mario Vaˇ sak 1) , Nedjeljko Peri´ c 1) , Gojko Joksimovi´ c 2) and Thomas M. Wolbank 3) 1) Faculty of Electrical Engineering and Computing, University of Zagreb, Zagreb, Croatia 2) Faculty of Electrical Engineering, University of Montenegro, Podgorica, Montenegro 3) Faculty of Electrical Engineering and Information Technology, Vienna University of Technology, Vienna, Austria vinko.lesic@fer.hr, mario.vasak@fer.hr, nedjeljko.peric@fer.hr, joxo@ac.me, thomas.wolbank@tuwien.ac.at Abstract—Faults of wind turbine generator electromechanical parts are common and very expensive. This paper introduces a fault-tolerant control scheme for variable-speed variable-pitch wind turbines that can be applied to any type of generator. We focus on generator stator isolation inter-turn fault that can be characterized before triggering the safety device. A simple extension of the conventional control structure is proposed that prevents the fault propagation while power delivery under fault is deteriorated as less as possible compared to healthy machine conditions. Presented fault-tolerant control strategy is developed taking into account its modular implementation and installation in available control systems of existing wind turbines to extend their life cycle and energy production. Simulation results for the case of a 700 kW wind turbine are presented. I. I NTRODUCTION Increasing interest in renewable energy sources and their growing impact on today’s energy production motivated dif- ferent branches of science to make contributions in price re- duction, energy quality and market competence of renewables. After a huge breakthrough and an average growth rate of 27% in last 5 years, wind energy today is a well-tested technology with total world installed capacity of about 197 GW in 2010 [1]. Because low-turbulent and strong winds are favorable, re- mote locations are best suitable for wind turbines. This intro- duces difficult and expensive maintenance procedures and rises availability concerns. Different fault-tolerant control algorithms have been introduced in [2] and they mostly propose different kinds of redundancies for sensors and electronic components. Focus here is on generator electromechanical faults, which are besides gearbox and power converters faults the most common in wind turbine systems [3]. Due to the lowest failure rate among all kinds of machines [3], many already installed wind turbines have a squirrel-cage induction generator (SCIG). In our recent papers [4] and [5] we proposed a general idea of fault-tolerant control algorithm for generator electromechanical faults and its application in suppressing the rotor-bar defect in SCIGs. Algorithm is based on proper extension of widely adopted control strategies used in wind turbines, mainly on torque control loop with field-oriented control (FOC). Focus of this paper is to research and develop a fault- tolerant control strategy for stator isolation faults that cause about 20% of machine faults [6]. We introduce a modulation of the generator flux using already available control system in order to suppress the fault propagation and to keep the electrical energy production possible under emergency circumstances. There are two main issues which hinder the proposed fault- tolerant control algorithm application on suppressing stator isolation faults: • very slow dynamics of machine rotor-flux transients, which are dependent on the rotor time constant, • whole procedure needs to be performed with the high frequency of voltage supplied to the machine stator. Main advantage, as well as the motivation, is that proposed algorithm is not only restricted to an SCIG but can be applied to any type of generators used in wind turbines (doubly-fed induction machine, synchronous generator with wound rotor, synchronous generator with permanent magnets). This paper is organized as follows. The stator inter-turn isolation fault is briefly described in Section II. Section III presents the mathematical model of wind turbine, as well as most-widely-adopted wind turbine control strategy. In Section IV a mathematical model of an SCIG is described explaining the theoretical basis used to form a control system extension. A fault-tolerant approach and control algorithm that enables wind turbine operation under stator isolation fault is proposed and described in Section V. Section VI provides simulation results. II. STATOR ISOLATION FAULT Some of the most common causes of stator isolation faults are moisture in the isolation, winding overheating, or vibrations (especially due to fallen stator slot wedge). Modern voltage- source inverters also introduce additional voltage stress on the inter-turn isolation caused by the steep-fronted voltage surge [7], [8]. There are two main kinds of stator faults (Fig. 1). One is the isolation fault and short circuit between two different machine phases. The time elapsed between fault occurrence and triggered safety device is about one third of a second. If there is a short circuit between turns of the same phase the time elapsed between incipient fault and triggered safety device The 12th IEEE International Workshop on Advanced Motion Control March 25-27, 2012, Sarajevo, Bosnia and Herzegovina 978-1-4577-1073-5/12/$26.00 ©2012 IEEE