Effect of Turbulence on Fixed-Speed Wind Generators

The influence of wind energy connection to the grid has increased greatly and turbulence or unreliable characteristics of wind energy are expected to produce frequency and voltage changes in power systems and protection system equipment. To prevent these changes, it is necessary to study the working point change due to turbulence. In other papers, the voltage and transient stability analysis have been studied during and after turbulence [2] and the impact of WTGs (wind turbine generators) on the system frequency, inertia response of dif‐ ferent wind turbine technologies, and comparison between inertia response of single-fed and doubly-fed induction generators have been examined. Moreover study of the frequency change alone was conducted using Dig-SILENT simulator for FSWTs (fast-speed wind tur‐ bines) with one-mass shaft model [2].


Introduction
The influence of wind energy connection to the grid has increased greatly and turbulence or unreliable characteristics of wind energy are expected to produce frequency and voltage changes in power systems and protection system equipment.To prevent these changes, it is necessary to study the working point change due to turbulence.In other papers, the voltage and transient stability analysis have been studied during and after turbulence [2] and the impact of WTGs (wind turbine generators) on the system frequency, inertia response of different wind turbine technologies, and comparison between inertia response of single-fed and doubly-fed induction generators have been examined.Moreover study of the frequency change alone was conducted using Dig-SILENT simulator for FSWTs (fast-speed wind turbines) with one-mass shaft model [2].
In this chapter both frequency and grid voltage sag change are presented with MATLAB analytically and also by SIMULINK simulation in FSWTs with one-and two-mass shaft turbine models to compare both results and a new simulation of induction machine without limiter and switch blocks is presented as a new work.The first part of study is frequency change effect on wind station by SIMULINK that shows opposite direction of torque change in comparison with previous studies with Dig-SILENT.The second part of study is effect of frequency and voltage sag change on wind station torque due to turbulence in new simulation of induction generator that is new idea.

Wind turbine model
The equation of wind turbine power is whereρ is air density, Ais area of turbine, C p is power coefficient and υ w is wind speed.
The C p curve and equation are shown in Fig. 1 and given by equation ( 2) and (3) whereθ pitch is blade pitch angle, λis the tip speed ratio described by equation ( 4).The parameters are given in Table 1.
where R is blade radius.

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The curve of Fig. 1 has positive slope before C p max and it has negative slope after C p max .

One-Mass Shaft Wind Station Model
Where, T m is the mechanical torque, T e is the generator torque, Cis the system drag coeffi- cient and J is the total inertia.
Table 1 shows the parameters of the one-mass shaft turbine model and induction generator.

Generator Wind Turbine
R s = .011Ωc 1 =.44 Table 1.Parameters of one-mass shaft turbine model and generator.

Two-Mass Shaft Induction Machine Model
This model is used to investigate the effect of the drive train or two-mass shaft, i.e., the masses of the machine and the shaft, according to the equation (8) [3], [4].In this equation,J t is wind wheel inertia, J G is gear box inertia and generator's rotor inertia connected through the elastic turbine shaft with a κ as an angular stiffness coefficient and C as an angular damping coefficient.
T G is the torque of the machine, T t is the turbine torque, δ t is the angular turbine shaft angle, δ G is the angular generator shaft angle, νis the inverse of the gear box ratio and J G andJ t are the inertia of the machine shaft and turbine shaft, respectively.
The Parameters, defined above, are given in Table 2.
This model is described as equation ( 8).
Table 2. Parameters of two-mass shaft model.

Induction Machine and Kloss Theory
In a single-fed induction machine, the torque angular speed curve of equation ( 12) [1] is nonlinear, but by using the Kloss equation (13), equations ( 9), (10), and (11), this curve is linearly modified [1], [2] as shown in Fig. 2. Therefore, the effect of frequency changes in wind power stations can be derived precisely by equation ( 12) and approximately using equation (13), as shown in Figs.2-6.Equations ( 11) and ( 12) are given in per unit, but the associated resistances are in ohms.For small changes in the slip according to the Kloss approach in equation ( 13), the torque changes as follows [2]: Then: and Thus, the new angular operation speed [2] is

Simulation of wind generator with frequency change
During turbulence and changes in the grid frequency, the torque speed (slip) curves change in such a way that as the frequency increases, the torque is increased at low wind speeds; 6 and 10 m/s, in contrast to Fig. 6 and decreases at a high speed of 13 m/s [2], as shown in Figs.7-15 show the electrical torque and mechanical speed of the induction machine for the one-and two-mass shaft turbine models at wind speeds of 6, 10, and 13 m/s to validate Table 7.

Simulation of wind station with one-mass and two-mass shaft turbine models
The results of simulations of a simple grid, fixed-speed induction machine, and one-mass and two-mass shaft turbines are given in Tables 8 -10 and Figs.16-42.For an induction wind generator using the induction block in SIMULINK with high voltage sag i.e. 50% with frequencies 50 and 52 and equal to 13, C p becomes negative, and the results are unrealistic.
Then results of 50% voltage sag are realistic in new simulation of induction machine in Tables 8 -10.Where P is poles number, λ ds and λ qs are flux linkages and leakages, respectively, and i qs and i ds are stator currents in q and d circuits of dqo system, respectively.
Then i matrix produced by the λ matrix is given by equation (20).
( ) To create the torque in equation ( 19), it is necessary to determine the currents in equations (30)-(33) from the stator and rotor currents by using current meters.

Conclusion
As frequency changes and voltage sag occurs because of turbulence in wind stations in ridethrough faults, the system's set point changes.The theoretical and simulation results results are similar for one mass shaft and two mass shaft turbine models.At lower wind speeds; 6 and 10 m/s, the directions of the changes in the new working point are the same as those of the frequency changes.At a higher wind speed; 13 m/s, the directions of these changes are opposite to the direction of the frequency changes.Simulation results of high grid voltage sag with SIMULINK induction block has error and new simulation of wind induction generator in synchronous reference frame is presented without error and in 50% voltage sag, new simulation of wind generator model has higher precision than that in 10% and 20% voltage sags; however, this model can simulate wind generator turbulence with voltage sags higher than 50%.Although results of new simulation of induction machine with wind turbine for 50% voltage sag and frequencies 50 and 52 have been presented in this chapter.

Figure 1 .
Figure 1.Curve of C p for different tip speed ratios λ .

Figure 3 .
Figure 3. Mechanical and linear electrical torque versus slip.

Figure 4 .
Figure 4. Mechanical and electrical torque versus frequency curves per unit with V sag = 10% .

Figure 6 .
Figure 6.Mechanical and electrical torque versus frequency per unit with V sag = 50%.

Figs. 3
Figs.3, 4, 5, and 6 illustrate that for lower wind speeds of 6 and 10 m/s, as the synchronous frequency f s and V sag change, the T e and T m values of the rotor change in the same direction as the frequency of the network, as shown in Tables III, IV, V, and VI.These figures and tables give the results for V sag = 0% (i.e., only the frequency changes), 10%, 20%, and 50%.However, for a higher wind speed of 13 m/s, as f s and V sag change, the T e and T m values of the rotor change in the opposite direction to the changes in the frequency of the network.

Figure 43 .
Figure 43.Induction machine Model in dqo systemThe new simulation of induction machine is in dqo system and synchronous reference frame simulation on the stator side; n (Transfer coefficient) is assumed to be 1. Circuit theory is used in this simulation, and it does not have saturation and switch blocks, unlike the MAT-LAB-SIMULINK Induction block.In Fig.43, L M is the magnetic mutual inductance, and r andL are the ohm resistance and self-inductance of the dqo circuits, respectively.The ma- chine torque is given by equation (19).In this equation, i d ,qs andλ d ,qs , the current and flux pa- rameters, respectively, are derived from linear equations (20)-(23); they are sinusoidal because the voltage sources are sinusoidal.

Table 3 .
Analytical MATLAB results for different frequencies.

Table 7 .
Simulink simulation results for one-and two-mass shaft models Effect of Turbulence on Fixed-Speed Wind Generators http://dx.doi.org/10.5772/51407

Table 8 .
Simulation results by SIMULINK for one and two mass shaft model for V sag = 10%

Table 9 .
Simulation results by SIMULINK for one and two mass shaft model for V sag = 20%

Table 10 .
Simulation results by SIMULINK for one and two mass shaft model for V sag = 50%