**4. Simulation and discussion**

#### **4.1 Steady-state simulation**

The system under study is shown in **Figure 10**. The 2 MW, 1500 rpm, 50 Hz, 690 V, 1760 A, and 12,732 Nm torque DFIG model was used to model and simulate the systems. The simulation was used to analyze the challenges with power system stability of integrating the WTG into the grid, considering intermittent wind characteristics and the problem of slip convergence. This task was executed by creating a steady-state Matlab function, to calculate the steady-state operation points and reveals how the rotor speed of the modeled DFIG involves the power flow of the studied system. Speed array and torque array were considered as inputs into the stimulated threeblade wind turbine connected with DFIG. Two different work frames of generation strategies *Qs* ¼ 0 and *Idr* ¼ 0 were considered here.

#### *4.1.1 Steady-state simulation results and analysis*

Simulations were carried out for variable wind speeds ranging from 5 m/s (cut-in speed) to 25 m/s (cut-off speed) in progressive steps of 2 m/s, with reactive power *Qs* ¼ 0, (red plot) and *Idr* ¼ 0 (green plot) as a control strategy separately. The results for the DFIG voltage, torque, generated real power, efficiency, and consumed reactive power in both the methods are plotted in **Figures 11**–**18**. The influence of two different generation strategies does not make big differences for variables such as *Tem*, *Pt*, *Ps* and *Pr*, however, some other variables such as *Is*, *Ir*, *Qs*, and *Qr*, (**Figures 14**–**17**) was found to have some big differences in amplitudes, concerning rotor speed.

**Figure 11** shows the DFIG's torque vs. speed characteristics, which stimulate the three-blade wind turbine with a minimum speed of 900 rpm and a maximum speed of 1800 rpm. The DFIG can perform above and under the synchronization speed for power generation. The generation model of DFIG matching negative torque values

**Figure 10.** *Steady-state simulation program block model.*

*Simulation Analysis of DFIG Integrated Wind Turbine Control System DOI: http://dx.doi.org/10.5772/intechopen.103721*

**Figure 11.** *The graph of torque (Tem) vs. rotor speed (n).*

**Figure 12.** *DFIG's active power Pt (W) vs. speed n (rpm).*

**Figure 13.** *DFIG stator and rotor active power Ps & Pr (W) vs. speed n (rpm).*

**Figure 14.** *DFIG Is (A) vs. n (rpm), red plot: Qs = 0, green plot: Idr = 0.*

covers from the negative slip to the positive slip state. Therefore, the turbine target power and electromagnetic torque features of variable speed DFIGs are unlike the customized constant-speed induction machine. **Figure 12** shows the plotting for the total mechanical power of the turbine shaft, which is the product of torque and speed, from sub synchronous to super synchronous speed, with a maximum power value of

**Figure 15.** *DFIG Ir (A) vs. n (rpm), red plot: Qs = 0, green plot: Idr = 0.*

**Figure 16.** *DFIG Qs (VAR) vs. n (rpm), red plot: Qs = 0, green plot: Idr = 0.*

**Figure 17.** *DFIG Qr (VAR) vs. n (rpm), red plot: Qs = 0, green plot: Idr = 0.*

**Figure 18.** *DFIG Vr & vs vs. speed n (rpm), red plot: Qs = 0, green plot: Idr = 0.*

�2.54 MW at 1800 rpm. **Figure 12** shows, the rotor's active power *Pr* is absorbed by the induction machine at below synchronous speed, and the active power is supplied above the synchronous speed from the induction generator to the grid. **Figure 13**, shows, with *Qs* ¼ 0 (red plot) as an adopted control strategy the stator current value *Is* is on the lower side. **Figures 14** and **15** shows during *Idr* ¼ 0 (green plot) as a control

*Simulation Analysis of DFIG Integrated Wind Turbine Control System DOI: http://dx.doi.org/10.5772/intechopen.103721*


#### **Table 1**

*WT simulated parameters at defined DFIG speeds.* strategy the rotor current *Ir* is on the lower side and *Qs* is on the higher side. **Figure 16** shows the rotor reactive power *Qr* ¼ 0 at synchronous speed 1500 rpm, with both control strategy *Qs* ¼ 0 and *Idr* ¼ 0, indicating the reactive power varies according to the wind turbine speed. **Figure 17** shows, a constant stator voltage *Vs* amplitude throughout the variable speed range, while the variable rotor voltage *Vr* amplitude is very low at synchronous speed 1500 rpm, with two peak voltage amplitudes at a minimum and maximum rotor speeds.

#### *4.1.2 Evaluation of simulation modeling at defined speeds*

**Table 1** shows the specified wind turbine DFIG speeds that are compared and used to evaluate the simulation model parameters with the steady-state model parameters as obtained from **Figures 11**–**18**. The simulation graphs shown in **Figures 19** and **20** represent the torque vs. time and rotor current vs. time characteristics at 1356 rotor rpm and steady-state simulation period of 1.5 sec and 1691 rotor rpm at a steady-state simulation period of 2.0 sec for the entire modeling period of 3.0 sec. Simulated torque values �6050 Nm and � 9450 Nm, rotor current values 1200 amps and 1790 amps, and stator current values 1325 amps and 1850 amps at pre-defined speeds are close to steady-state parameter values as shown in **Figures 11**–**18**.

#### *4.1.3 Simulation model of DFIG using wind turbine MPPT block*

In this section, a 2 MW stator power DFIG model and a three-blade wind turbine model with gear ratio *n* ¼ 100, blade radius 42 m, *Cp* ¼ 0*:*42, and *λopt* ¼ 7*:*2 were used for the wind turbine maximum power point tracking simulation control as shown in **Figure 21**. **Figures 22** and **23** show the wind turbine MPPT simulation model characteristics at 8 m/sec and 10 m/sec of wind speed. Observed in **Figure 11**, the torque response for the wind speed and *iq* current indicates that more oscillations occur at the low torque due to the fact reduced mechanical inertia. The more the mechanical inertia, the more the torque oscillations. On achieving the steady-state condition at

**Figure 19.**

*Torque vs. time graph @ 1356 rotor rpm and @ 1691 rotor rpm.*

**Figure 20.**

*Ir vs. time graph @ 1356 rotor rpm and @ 1691 rotor rpm.*

*Simulation Analysis of DFIG Integrated Wind Turbine Control System DOI: http://dx.doi.org/10.5772/intechopen.103721*

**Figure 21.** *WT MPPT control model.*

**Figure 22.** *The dynamic state WT MPPT graph of speed vs. time (sec).*

**Figure 23.**

*WT MPPT torque vs. time characteristic curve.*

2 sec of modeling time, wind turbine speed and correspondence torque values were tabled for angular speed of 140 rad/sec, at a torque of 5500 Nm, and a mechanical wind turbine power output approximately equal to 770 kW was obtained. Further, the wind speed was increased from 8 m/sec to 10 m/sec and the steady-state simulation at 6 sec of modeling time was observed. On achieving the steady-state at 6 sec of modeling time, wind turbine speed and correspondence torque values were tabulated for 170 rad/sec, at 8800 Nm respectively and a mathematical wind turbine power

outcome equivalent to 1.49 MW was recorded. These two simulated outcomes are very close to the steady-state characteristics graph numerical values as shown in **Figures 11** and **12**.

## **5. Conclusions**

This study was to focus on investigating the influences of the integration of wind power generators into the power grid systems. The rotor side converter control unit is utilized for, real and reactive power control by regulating the rotor current and the speed of the DFIG. With the computed stator voltage, stator current, rotor current, and the rotor location by encoder response signal the active PI measured and controlled procedure results in a considerable enhancement in control system sturdiness and advances its indemnity to produced system noise. The engaged PI control unit attests to the grid side converter control by sustaining the stable generated power frequency and voltage with the grid frequency and voltage. The controller scheme and the simulation mode controller employed for the study assure the wind generator supplying into the grid at varying wind speeds behaves like a synchronous generator, at a zero Hz rotor frequency.

### **Acknowledgements**

I wish to thank the almighty God for giving me life and enabling me to reach the heights that I have reached.

I wish to thank my parents and my siblings, for their tireless and relentless love, continuous support, and the countless sacrifices they have made on my behalf. To my family, for being great inspirations and believing in us even when we have stopped believing in ourselves. This would not have been possible without the family's help.

Finally, I wish to everyone not mentioned above but directly or indirectly contributed to our work, your input is much acknowledged.
