**4. The Proposed Crowbar Control Strategy**

The crowbar activation and deactivation processes are important issues, there are many used techniques for crowbar activation and deactivation processes. In Ref. [29], an Adaptive Neuro Fuzzy Inference System (ANFIS) is used to produce the crowbar control signal which lead to complicating the crowbar system.

This work aimed to propose a simple crowbar control strategy. As shown in Figure 2, the used crowbar technique is the outer crowbar. The main components of the proposed crowbar control system are voltage measuring units for all phases, a control program and an automatic switch. The main aim of the crowbar control program is producing the control signal which would control the automatic switch. The main idea of the proposed strategy is based on continuously monitoring and measuring the per unit rms terminal voltage for each phase individually. According to the flowchart shown in Figure 3, if the measured rms voltage value, of each phase, is within the predefined voltage constraint limits (more than or equals to 0.7 p.u.), the control scheme would behave in this case as a normal steady-state operating condition, then the control program would set the output control signal to (1). Otherwise, if any one of the measured rms voltage values is lower than the minimum predefined voltage constrain limit "0.7 p.u.", the control scheme would behave with this case as a faulty condition, then the control program would set the output control signal to (0). In the case "Output control signal = 1"; the automatic switch would be switched on and bypassing the crowbar "the crowbar is deactivated", while in the case "Output control signal = 0", the automatic switch would be switched off, activating the crowbar. So, the operation technique of the proposed method is very simple in comparison with the methods that were used in both Ref. [20] and Ref. [29] to achieve the same goal, whereas Ref. [20] used a complex fuzzy control system and improved its performance by adding a PI to the used fuzzy control system. Ref. [29] used a complex ANFIS system. *Energies* **2022**, *15*, x FOR PEER REVIEW 5 of 29 minimum predefined voltage constrain limit "0.7 p.u.", the control scheme would behave with this case as a faulty condition, then the control program would set the output control signal to (0). In the case "Output control signal = 1"; the automatic switch would be switched on and bypassing the crowbar "the crowbar is deactivated", while in the case "Output control signal = 0", the automatic switch would be switched off, activating the crowbar. So, the operation technique of the proposed method is very simple in comparison with the methods that were used in both Ref. [20] and Ref. [29] to achieve the same goal, whereas Ref. [20] used a complex fuzzy control system and improved its performance by adding a PI to the used fuzzy control system. Ref. [29] used a complex ANFIS system.

**Figure 2.** Block diagram of the proposed crowbar technique. **Figure 2.** Block diagram of the proposed crowbar technique.

**Figure 3.** Flow chart of the proposed control for the crowbar.

**Figure 2.** Block diagram of the proposed crowbar technique.

**Figure 3. Figure 3.**  Flow chart of the proposed control for the crowbar. Flow chart of the proposed control for the crowbar. line fault and double line to ground fault. The disturbances occur at the beginning of the transmission line next to the coupling busbar (11 kV Busbar), applied at the instant of

#### **5. Simulation Results** "time = 1 s'' for 150 ms duration. According to the used methodology in Ref. [30], the

system.

The studied system is shown in Figure 4, where the BDFRG (supported by the proposed crowbar) is tied to the network by a transmission line after the coupling transformer. The main data of the simulated BDFRG and wind turbine are described in Tables A1 and A2 (Appendix A) [4]. adequate crowbar resistance value for the studied system was 10 times the secondary "control" winding resistance value. To ensure monitoring the total actual performance of the BDFRG wind turbine under the studied faults, all the protection system devices were deactivated. The simulation works implemented by MATLAB/SIMULINK (2013 b).

minimum predefined voltage constrain limit "0.7 p.u.", the control scheme would behave with this case as a faulty condition, then the control program would set the output control signal to (0). In the case "Output control signal = 1"; the automatic switch would be switched on and bypassing the crowbar "the crowbar is deactivated", while in the case "Output control signal = 0", the automatic switch would be switched off, activating the crowbar. So, the operation technique of the proposed method is very simple in comparison with the methods that were used in both Ref. [20] and Ref. [29] to achieve the same goal, whereas Ref. [20] used a complex fuzzy control system and improved its performance by adding a PI to the used fuzzy control system. Ref. [29] used a complex ANFIS

**Figure 4.** Studied system. **Figure 4.** Studied system.

symmetrical fault occurrence.

**-0.5**

**0**

**0.5**

**Va (p.u)**

**1**

**1.5**

*5.1. Symmetrical Fault (Three Line to Ground Fault)*  The per unit rms terminal voltage of the BDFRG, as shown in Figure 5, at the instant of fault occurrence, the terminal voltage dropped to zero p.u. for 150 ms due to the occurrence of the studied three phase to ground fault; then, after fault clearance, the terminal voltage returns to its original value (1 p.u.). To examine the efficacy of the proposed crowbar control technique, under the occurrence of different heavy disturbances, this work shows and analyses the performance of the studied system without and with using the proposed crowbar control strategy. The studied disturbances are: three-line to ground fault, single line to ground fault, double line fault and double line to ground fault. The disturbances occur at the beginning of the transmission line next to the coupling busbar (11 kV Busbar), applied at the instant of "time = 1 s" for 150 ms duration. According to the used methodology in Ref. [30], the adequate crowbar resistance value for the studied system was 10 times the secondary "control" winding resistance value. To ensure monitoring the total actual performance of the BDFRG wind

**Figure 5.** Per unit rms terminal voltage (Va) of the studied wind farm main coupling point under

**0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4**

**T i m e (s)**

turbine under the studied faults, all the protection system devices were deactivated. The simulation works implemented by MATLAB/SIMULINK (2013 b). **Figure 4.** Studied system.

The studied system is shown in Figure 4, where the BDFRG (supported by the proposed crowbar) is tied to the network by a transmission line after the coupling transformer. The main data of the simulated BDFRG and wind turbine are described in Tables

To examine the efficacy of the proposed crowbar control technique, under the occurrence of different heavy disturbances, this work shows and analyses the performance of the studied system without and with using the proposed crowbar control strategy. The studied disturbances are: three-line to ground fault, single line to ground fault, double line fault and double line to ground fault. The disturbances occur at the beginning of the transmission line next to the coupling busbar (11 kV Busbar), applied at the instant of "time = 1 s'' for 150 ms duration. According to the used methodology in Ref. [30], the adequate crowbar resistance value for the studied system was 10 times the secondary "control" winding resistance value. To ensure monitoring the total actual performance of the BDFRG wind turbine under the studied faults, all the protection system devices were deactivated. The simulation works implemented by MATLAB/SIMULINK (2013 b).

#### *5.1. Symmetrical Fault (Three Line to Ground Fault) 5.1. Symmetrical Fault (Three Line to Ground Fault)*

*Energies* **2022**, *15*, x FOR PEER REVIEW 6 of 29

**5. Simulation Results**

A1 and A2 (Appendix A) [4].

The per unit rms terminal voltage of the BDFRG, as shown in Figure 5, at the instant of fault occurrence, the terminal voltage dropped to zero p.u. for 150 ms due to the occurrence of the studied three phase to ground fault; then, after fault clearance, the terminal voltage returns to its original value (1 p.u.). The per unit rms terminal voltage of the BDFRG, as shown in Figure 5, at the instant of fault occurrence, the terminal voltage dropped to zero p.u. for 150 ms due to the occurrence of the studied three phase to ground fault; then, after fault clearance, the terminal voltage returns to its original value (1 p.u.).

**Figure 5.** Per unit rms terminal voltage (Va) of the studied wind farm main coupling point under symmetrical fault occurrence. **Figure 5.** Per unit rms terminal voltage (Va) of the studied wind farm main coupling point under symmetrical fault occurrence. The active power of the BDFRG (with and without using the proposed crowbar) is

The active power of the BDFRG (with and without using the proposed crowbar) is shown in Figure 6. As obvious in the case of "without using the proposed crowbar", during the fault, the active power totally dropped to zero kW for 150 ms (fault duration time). In the case of using the proposed crowbar, during the fault, the active power was effectively improved and quickly returned to its pre-fault value. shown in Figure 6. As obvious in the case of "without using the proposed crowbar", during the fault, the active power totally dropped to zero kW for 150 ms (fault duration time). In the case of using the proposed crowbar, during the fault, the active power was effectively improved and quickly returned to its pre-fault value.

**Figure 6.** Active power of BDFRG with and without the proposed crowbar under symmetrical fault occurrence. **Figure 6.** Active power of BDFRG with and without the proposed crowbar under symmetrical fault occurrence.

reached about 5.04 kvar. While in the case of using the proposed crowbar, after fault clearance, the absorbed reactive power was reduced to 2.165 kvar only and quickly improved

**Figure 7.** Reactive power of BDFRG with and without the proposed crowbar under symmetrical

The reactive power of the BDFRG (with and without the proposed crowbar) is shown in Figure 7. As shown, the reactive power was adjusted at zero value (unity power factor) before the fault occurrence. Following the clearance of the fault, in the case of "without

**0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4**

**T i m e (s)**

until reaching its pre-fault value.

fault occurrence.

**Crowbar resistance=10 × secondary resistance**

**Without the proposed crowbar**

**-6**

**-5**

**-4**

**-3**

**-2**

**R e a c t i v e P o w e r ( k v a r )**

**-1**

**0**

**1**

**2**

**-2**

**0**

**2**

**4**

**A c t i v e P o w e r ( k W )**

**6**

**8**

**10**

occurrence.

**Without the proposed crowbar**

**Crowbar resistance=10 × secondary resistance**

The reactive power of the BDFRG (with and without the proposed crowbar) is shown in Figure 7. As shown, the reactive power was adjusted at zero value (unity power factor) before the fault occurrence. Following the clearance of the fault, in the case of "without using the proposed crowbar", the absorbed reactive power, by the BDFRG from the grid, reached about 5.04 kvar. While in the case of using the proposed crowbar, after fault clearance, the absorbed reactive power was reduced to 2.165 kvar only and quickly improved until reaching its pre-fault value. The reactive power of the BDFRG (with and without the proposed crowbar) is shown in Figure 7. As shown, the reactive power was adjusted at zero value (unity power factor) before the fault occurrence. Following the clearance of the fault, in the case of "without using the proposed crowbar", the absorbed reactive power, by the BDFRG from the grid, reached about 5.04 kvar. While in the case of using the proposed crowbar, after fault clearance, the absorbed reactive power was reduced to 2.165 kvar only and quickly improved until reaching its pre-fault value.

**Figure 6.** Active power of BDFRG with and without the proposed crowbar under symmetrical fault

The active power of the BDFRG (with and without using the proposed crowbar) is shown in Figure 6. As obvious in the case of "without using the proposed crowbar", during the fault, the active power totally dropped to zero kW for 150 ms (fault duration time). In the case of using the proposed crowbar, during the fault, the active power was effec-

*Energies* **2022**, *15*, x FOR PEER REVIEW 7 of 29

tively improved and quickly returned to its pre-fault value.

**0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4**

**T i m e (s)**

**Figure 7.** Reactive power of BDFRG with and without the proposed crowbar under symmetrical fault occurrence. **Figure 7.** Reactive power of BDFRG with and without the proposed crowbar under symmetrical fault occurrence.

The rotor speed of the BDFRG (with and without using the proposed crowbar) is

The rotor speed of the BDFRG (with and without using the proposed crowbar) is shown in Figure 8, which has a reference value equal to 1160 rpm. During the fault, in the case of "without using the proposed crowbar", the rotor rapidly accelerated, then after fault clearance, the rotor speed reached about 1464 rpm, which led to a decrease in the power coefficient of the WT from 0.48 to less than 0.3846 as shown in Figure 9. In the case of using the proposed crowbar, the rotor speed increased instantaneously to about 1213 rpm only, then quickly improved. shown in Figure 8, which has a reference value equal to 1160 rpm. During the fault, in the case of "without using the proposed crowbar", the rotor rapidly accelerated, then after fault clearance, the rotor speed reached about 1464 rpm, which led to a decrease in the power coefficient of the WT from 0.48 to less than 0.3846 as shown in Figure 9. In the case of using the proposed crowbar, the rotor speed increased instantaneously to about 1213 rpm only, then quickly improved.

**Figure 8.** Rotor speed of BDFRG with and without the proposed crowbar under symmetrical fault occurrence. **Figure 8.** Rotor speed of BDFRG with and without the proposed crowbar under symmetrical fault occurrence.

**Figure 9.** Power coefficient of wind turbine with and without the proposed crowbar under symmet-

The primary and secondary currents of the BDFRG (without and with using the proposed crowbar) are shown in Figures 10 and 11 in the same order. In the case of "without using the proposed crowbar", during the fault, both currents were increased for a certain

**0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4**

**T i m e (s)**

rical fault occurrence.

**Without the proposed crowbar**

**Crowbar resistance= 10 × secondary resistance**

**0.38**

**0.4**

**0.42**

**0.44**

**P o w e r C o e f f i c i e n t ( C p )**

**0.46**

**0.48**

**0.5**

**1200**

**1250**

**1300**

**1350**

**R o t o r S p e e d ( r p m )**

**1400**

**1450**

**1500**

**Reference**

**Without the proposed crowbar**

**Crowbar resistance= 10 × secondary resistance**

**0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4**

**T i m e (s)**

rpm only, then quickly improved.

occurrence.

**Figure 9.** Power coefficient of wind turbine with and without the proposed crowbar under symmetrical fault occurrence. **Figure 9.** Power coefficient of wind turbine with and without the proposed crowbar under symmetrical fault occurrence.

**Figure 8.** Rotor speed of BDFRG with and without the proposed crowbar under symmetrical fault

The rotor speed of the BDFRG (with and without using the proposed crowbar) is shown in Figure 8, which has a reference value equal to 1160 rpm. During the fault, in the case of "without using the proposed crowbar", the rotor rapidly accelerated, then after fault clearance, the rotor speed reached about 1464 rpm, which led to a decrease in the power coefficient of the WT from 0.48 to less than 0.3846 as shown in Figure 9. In the case of using the proposed crowbar, the rotor speed increased instantaneously to about 1213

The primary and secondary currents of the BDFRG (without and with using the proposed crowbar) are shown in Figures 10 and 11 in the same order. In the case of "without using the proposed crowbar", during the fault, both currents were increased for a certain The primary and secondary currents of the BDFRG (without and with using the proposed crowbar) are shown in Figures 10 and 11 in the same order. In the case of "without using the proposed crowbar", during the fault, both currents were increased for a certain period. After the fault clearance, the primary current was increased to about 212% (19.74 A) of the pre-fault value (9.32 A) and the secondary current was increased to about 216% (28.25 A) of the pre-fault value (13.05 A), while in the case of using the proposed crowbar, as shown in Figures 10b and 11b, after the fault clearance, both the primary and secondary currents were effectively improved. As the primary current increased to (15.25 A), while the secondary current increased to (22.86 A) and then quickly both the primary and secondary currents were effectively improved.

The dc link voltage of the BDFRG (with and without using the proposed crowbar) is shown in Figure 12, which has a reference value equals to 710 V. Under the fault occurrence, in the case of "without using the proposed crowbar", the dc link voltage was decreased to about 499.2 V. In the case of using the proposed crowbar, the dc link voltage decreased instantaneously to about 587.8 V only, then the dc link voltage improved and returned quickly to its pre-fault value.
