Vertical-Axis Wind Turbine

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A Comparative Study on Variable-Speed Operations of a Wind Generation System Using Vector Control A.J. Mahdi, W.H. Tang, L. Jiang and Q.H. Wu
Department of Electrical Engineering and Electronics The University of Liverpool, Liverpool, L69 3GJ, U.K. E-mail:

Abstract. This paper presents a comparison study among three control methods based on vector control for maximising the output power and improving the performance of a small-scale wind generation system (WGS). The three control methods are a hysteresis-band current controller (HBCC), a PI current controller (PICC) and an improved PI current controller (IPICC) which is based on particle swarm optimisation (PSO). The WGS investigated in this research consists of a permanent magnet synchronous generator (PMSG) directly driven by a vertical-axis wind turbine (VAWT), a current controlled PWM rectifier, and a stand-alone DC load. The principle of maximum power point tracking (MPPT) is to adjust the rotational speed of a wind turbine at optimal speeds that ensures optimal tip-speed ratios (TSR) and maximum power coefficients over a wide range of wind speeds. Simulations are based on actual parameters which are obtained experimentally from a real wind turbine generator system. The simulation results show the effectiveness of the IPICC method compared with the HBCC and PICC methods due to its satisfactory dynamic responses with fast MPPT under wind speed variations. Key Words Variable-speed wind generation systems, maximum power point tracking, vector control, permanent magnet synchronous generator, controlled PWM rectifiers.

1. Introduction
WGS is efficient for a stand-alone hybrid power generation system and a cost effective solution for street lighting utilities. The optimal operation of a WGS is important due to the high initial cost and the low efficiency of the wind turbine generator systems. There are many factors that contribute to increasing wind turbine efficiencies, including the number of rotor blades, a blade pitch angle, and TSR which is the ratio of circumstantial speed to wind speed. In a small-scale WGS, the only possible control variable for yielding the maximum amount of energy from wind is TSR by adjusting the rotational speed to a

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reference value in order to keep TSR at its optimal value and consequently the power coefficient at its maximum value [1]. Operations of WGS can mainly be classified into two types: (i) a constant-speed wind turbine is based on adjusting the pitch angle of wind turbine blades in order to control the output power of a wind turbine. (ii) a variable-speed wind turbine operated by controlling the rotational speed of a wind turbine according to a reference speed that ensures the optimal TSR and a maximum power coefficient. In general, MPPT techniques can be classified into two main categories. The first one is based on the knowledge of wind turbine characteristics which is the power coefficient versus TSR curve, and the second method is rooted on estimating the optimal reference signals of a control system [2]. Vector control strategies have been widely used for PMSG using a PWM controlled rectifier and PWM inverter. Basically, the aim of a vector control strategy for the generator-side is to control MPPT by changing the speed of the wind turbine. The overall control system consists of two main control loops: (i) a speed controller compares the reference speed (which is calculated from a TSR controller) with a PMSG speed and generates a reference torque, (ii) current controllers compare the reference dq-axis currents with the PMSG dq-axis currents to generate switching signal for a PWM current controller. The aim of the vector control strategy for a grid side PWM inverter is to maintain the DC link voltage constant regardless of the amount of the generator power, while keeping sinusoidal grid currents [3]. This paper is organised as follows. In Section 2,...
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