20-07-2013, 01:11 PM
Maximum Power Extraction Schemes & Power Control in Wind Energy Conversion System
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Abstract
Nowadays research focus is towards the Variable speed power generation in instead fixed speed power generation in wind energy conversion system. With variable speed, there will be 20-30% increase in the energy capture compared to the fixed-speed operation. For Variable speed wind energy conversion system the maximum power point tracking (MPPT) is a very important requirement in order to maximize the efficiency. Every year a number of publications appear in various journals and conferences/seminars claiming to offer better and faster MPPT techniques for wind energy conversion system (WECS). On the other hand, wind turbines are designed to give maximum output at wind speeds around 15 m/s (30 knots or 33 mph). For stronger winds it is necessary to dissipate part of the excess energy of the wind to avoid damaging the wind turbine. So all wind turbines are designed with some power control mechanism. This paper provides the brief idea about the power control techniques on modern wind turbines and a comprehensive review and critical analysis of MPPT techniques, which is very helpful for present researcher and students working in this area.
INTRODUCTION
Variable-speed wind turbines are designed to operate at an optimal rotation speed as a function of the wind speed. The power electronic converter may control the turbine rotation speed to get the maximum possible power by means of a maximum power point tracking (MPPT) strategy. It is also possible to avoid exceeding the nominal power if the wind speed increases by means of Power Control mechanism of Wind Turbines Therefore all wind turbines are designed with some power control mechanism. The power generated by a wind turbine can be expressed as; P=0.5ρπR2V3CP (λ, β); where ρ = air density ; Kg/m3, R= turbine rotor radius, V= wind speed, & CP=turbine power coefficient that represents the power conversion efficiency of a wind turbine. The physi-cal meaning of the CP curve is the ratio of the actual power delivered by the turbine and the theoretical power available in the wind. CP is a function of the tip speed ratio (TSR), λ, as well as the blade pitch angle (β) in a pitch controlled wind turbine.
Pitch Controlled Wind Turbines
In a pitch controlled wind turbine the electronic controller of turbine checks the power output of the turbine several times per second. When the power output cross a threshold limit, it sends an actuating signal to the blade pitch mechanism which quickly turns the rotor blades slightly out of the wind. On the other hand, the blades are turned back into the wind whenev-er the wind goes down again. Thus the rotor blades have to be able to twist around their longitudinal axis (to pitch). The pitch mechanism is usually operated using hydraulics. Varia-ble speed pitch-regulated wind turbines have two methods for affecting the turbine operation, namely speed changes and blade pitch changes. Power optimization strategy, employed when the speed is below the rated wind speed, to optimize the energy capture by maintaining the optimum tip speed ratio. Power control strategy, used above the rated wind speed of the turbine is for limiting the output power by changing the blade pitch to reduce the aerodynamic efficiency.
POWER CONTROL TECHNIQUES IN WIND TURBINES
Wind turbines are designed to give maximum output at wind speeds around 15 m/s (30 knots or 33 mph). A good wind tur-bine can start spinning in 5 mph (in between 2 to 3 m/s) winds. For stronger winds it is necessary to dissipate part of the excess energy of the wind to avoid damaging the wind turbine. Therefore all wind turbines are designed with some power control mechanism.
The Neural Network-Fuzzy Logic Based Techniques
Control systems for variable-speed wind turbines are pro-gressively evolving toward effective and innovative control systems based on soft-computing methodologies, such as fuzzy systems and artificial neural networks. Actually, Fuzzy control algorithms offer many advantages over traditional controls since they give fast convergence, they are parameter insensitive, and accept noisy and inaccurate signals. [16]-[22]. Artificial neural networks (ANNs) are particularly useful to implement nonlinear time-varying input–output mapping. In the past, ANN has been applied for various control, identifica-tion, and estimation schemes in power electronics and drives. For example, feedforward ANNs were selected for implemen-tation of pulse width-modulation (PWM) techniques [23], [24]. The ANN-based PWM has advantages of fast parallel compu-tation, learning capability, and fault tolerance, which are not possible by standard PWM implementation methods.
CONCLUSION
Nowadays, the most common wind turbine configurations are based on the variable-speed pitch-control and the fixed-speed stall-control concepts. The variable-speed pitch-control con-cept is the currently preferred option mainly because of its good power control performance, low mechanical stresses and emergency-stop power reduction features.A systematic review and analysis of the techniques available for MPPT in WECS has been given in this research paper. This review paper spans all the possible publications till date. The analysis concludes the two best techniques to be [34] and [41]. Both the algo-rithms have adaptive tracking with self-tuning capability. But [34] needs both wind speed and generator speed sensors. While [40] has a clear edge over [34] and all the rest for being absolutely mechanical sensorless. Furthermore, in contrast to [34], the algorithm of [40] does not construct a lookup table and therefore is easy on the memory requirements.