PDF《wind energy conversion system》

1 power supply devices used in communication systems in

1997 [3]. Vienna recti?er has important features which

2 makes it preferable for using as the generator-side converter in a WECS. The harmonic distortion in the AC

3 side of this converter is relatively lower and also the power factor is close to unity. The switching scheme is

4 much simpler and the converter has lower cost and higher e?ciency because there are just three active switches

5 in the structure and the voltage stress imposed on those switches is small [4]. Moreover, it does not have dead

6 time and is capable of high frequency operation. The power ?ow in this converter is unidirectional and therefore

7 it is not suitable for many applications in which the power ?ow is required in both directions. However, the

8 power ?ow in wind energy conversion is unidirectional from AC side to the load and therefore, Vienna topology

9 is an advantageous choice for this application.

10 Model predictive control (MPC) is one of the most advantageous methods for controlling power electronic

11 converters. The basic principle of this method is to utilize an appropriate model of the system to predict the

12 future behavior of a control variable until a prede?ned horizon in time. This information is used by the controller

13 to determine the next optimal actuation based on minimizing a pre-de?ned cost function. This method has

14 some notable advantages: The concepts are simple and easy to understand and implement. The multivariable

15 case can be easily considered and the nonlinearities can be included into the model. Moreover, the dead times

16 can be compensated and the constraints can be easily treated [5].

17 The most important reason to control the machine-side converter is to maximize the captured energy

18 from the wind. The main conventional maximum power point tracking (MPPT) methods employed in wind

19 energy conversion are tip speed ratio (TSR), optimal torque control (OTC), power signal feedback (PSF) and

20 perturbation and observation (P&

O), also known as hill-climbing search (HSC) [6, 7]. In the TSR algorithm, the

21 rotational speed reference is determined by keeping the tip speed ratio at its optimal value [8, 9]. Although this

22 method is simple and fast, but it is not accurate enough in tracking the maximum power point. Moreover, this

23 method requires measurement of the e?ective wind speed, which is costly, inaccurate and di?cult to accomplish

24 [7]. The OTC method adjusts the generator torque to its optimal value obtained from the optimum torque-speed

25 curve of the system [1, 8]. The PSF method has a similar principle to the OTC algorithm. In this method,

26 the optimal power-speed curve is used as a look-up table or a mathematical expression in order to determine

27 the optimal power reference required for tracking the maximum power point [10]. The PSF and OTC methods,

28 although do not need wind speed measurement, but require knowledge of system parameters and air density,

29 and are highly dependent on these parametric values and will be a?ected by the changes in these parameters.

30 Optimal characteristic curves are also required which will change as the system ages [6, 11].

31 P&

O search algorithm is widely utilized in order to implement MPPT in WECSs [12, 13]. This algorithm

32 is simple since prior knowledge of the parametric values or wind speed measurement are not required [1, 7].

33 However, it has one main drawback, which lies in the challenge of choosing the right perturbation step size. P&