**5. Simulation and results**

induction motors which parameters are presented in the Appendix, and all other compo‐

Scenario 1 3,271.0 1,015.0 3,385.0 971.3 Scenario 2 1,660.0 1,050.2 3,388.4 972.3

> **Generator 3 (MW)**

Scenario 1 2,747.0 1,736.0 1,154.0 1,200.0 / -364.0 1,200.0 / 100.0 Scenario 2 1,115.5 1,736.0 1,154.0 1,200.0 / -14.3 1,200.0 / 200.0

The intermittent characteristic of wind generation is considered following the wind regime

0 50 100 150 200 250

Time (sec.)

**Load at bus 8 Load at bus 11 P(MW) Q(Mvar) P(MW) Q(Mvar)**

> **DFIG (PV mode) (MW/Mvar)**

**DFIG (PQ mode) (MW/Mvar)**

nents are modeled as in scenario 1.

**Generator 1 (MW)**

6

**Figure 6.** Wind speed regime.

8

10

12

Wind speed (m/s)

14

16

18

**Generator 2 (MW)**

presented on Fig. 6, with the initial wind speed of 12 m/s.

**Scenarios**

232 Advances in Wind Power

**Table 1.** Load Scenarios

**Table 2.** Generation Scenarios

**Scenarios**

To evaluate the different impacts caused by DFIG control modes on long-term voltage sta‐ bility, two cases are analyzed:


All simulations considered both DFIG control modes alternatively: power factor control mode (0.99 leading) and voltage control mode. The results and analysis are presented next in the following sections. The simulations were conducted using the softwares ANAREDE for load flow calculations and ANATEM for transient stability simulations [18,19].

## **5.1. Case A: Static load model (Scenario 1)**

This case considers the successive increase on system total demand, with increments of 0.1% every second in respect with the initial load from scenario 1 as presented in tables 1 and 2. The load increases up to 200 seconds and the simulation time is 250 seconds.

As load increases, the voltage at bus 11 decreases, causing OLTC to operate in order to maintain voltage close to the reference level for both control modes as shows Fig. 7. Howev‐ er, while OLTC improves voltage level at bus 11, it progressively depresses voltage at bus 8 with each tap-changing operation, mainly when power factor control mode is employed in wind generation as shows Fig. 8. In voltage control mode, DFIG maintain the voltage level at bus 8 with its capacity to supply reactive power support. Fig. 9 presents the reactive pow‐ er injected and absorbed by DFIG. Note that when OLTC starts to operate close to 60 sec‐ onds, the DFIG starts to inject more reactive power in the system until it reaches its maximum limit.

The voltage reductions at load buses 8 and 11 are directly reflected in the field current of generator 3, because with the load increase, the generator AVR (Automatic Voltage Regula‐ tor) would quickly restore terminal voltage by increasing excitation, which results in addi‐ tional reactive power flow through the inductances of transformers and lines, causing increased losses and voltage drops. At this stage, generator G3 tends to reach its field cur‐ rent limit with the load ramp increase as shows Fig. 10. In this scenario 1, generator 3 does not suffer over-excitation (the OEL is not activated), and the long-term power system volt‐ age stability is maintained in both reactive power control modes, at least apparently. How‐ ever, as can be seen in Fig. 10, the DFIG's voltage control mode has a positive effect in the power system voltage stability since it tends to delay the OEL operation because the field current level is smallest from 110 s up to 250 s, providing less risk of protection interven‐ tions and system security degradation.

**Figure 7.** Voltage at bus 11

**Figure 8.** Voltage at bus 8

**Figure 9.** Reactive power injected / absorbed by DFIG.

**Figure 7.** Voltage at bus 11

234 Advances in Wind Power

0.95

**Figure 8.** Voltage at bus 8

1

1.05

Voltage at Bus-8 (p.u.)

1.1

0 50 100 150 200 250

Voltage Control Power Factor Control

Time (sec)

**Figure 10.** Field current at G3.

Fig. 11 shows the OLTC behavior during the load ramp increase. The results show that the OLTC reaches its upper limit when the DFIG wind generators are configured to control the power factor, and in this case the wind turbines cannot provide reactive power to support the voltages in the system. On the other hand, when the voltage control mode is used, the OLTC does not reach the upper tap limit, increasing the long term voltage stability margin.

Fig. 12 shows the system PV curve when DFIG is operating in both power factor and voltage control modes. These curves were obtained by increasing the load and plotting the voltage at bus 8 considering the dynamic aspect of the equipments in the system. This curve indi‐ cates the maximum loadability point, which is the maximum power the system can provide. The results show that when DFIG is operating in voltage control mode it increases signifi‐ cantly the maximum loadability point (nose point), since this equipment can supply reactive power to the system through voltage control. It is important to mention that the PV curves contours are irregular since they represent the discrete actuation of OEL and OLTC.

**Figure 11.** Tap position.

**Figure 12.** PV curve at bus 8.

Fig. 11 shows the OLTC behavior during the load ramp increase. The results show that the OLTC reaches its upper limit when the DFIG wind generators are configured to control the power factor, and in this case the wind turbines cannot provide reactive power to support the voltages in the system. On the other hand, when the voltage control mode is used, the OLTC does not reach the upper tap limit, increasing the long term voltage stability margin.

Fig. 12 shows the system PV curve when DFIG is operating in both power factor and voltage control modes. These curves were obtained by increasing the load and plotting the voltage at bus 8 considering the dynamic aspect of the equipments in the system. This curve indi‐ cates the maximum loadability point, which is the maximum power the system can provide. The results show that when DFIG is operating in voltage control mode it increases signifi‐ cantly the maximum loadability point (nose point), since this equipment can supply reactive power to the system through voltage control. It is important to mention that the PV curves

0 50 100 150 200 250

Voltage Control Power Factor Control

Time (sec)

0.9

**Figure 11.** Tap position.

0.95

1

1.05

Tap (p.u.)

1.1

1.15

236 Advances in Wind Power

contours are irregular since they represent the discrete actuation of OEL and OLTC.

#### **5.2. Case B: Static and dynamic load models (Scenario 2)**

This case considers the successive increase on system demand from bus 11, with increments of 0.1% every second in respect to the initial load from scenario 2 as presented in tables 1 and 2. The load increases up to 200 seconds and the simulation time is 250 seconds.

As seen in Figs. 13 and 14, the power factor control mode results in a heavy reactive power demand from the power system, leading to a very low voltage profile at load buses 11 and 8. In this case, constant power factor strategy decreases the long-term voltage stability margin, resulting in the voltage deterioration at load buses caused by relevant effect of the OEL com‐ bined with the OLTC action. In voltage control mode, the DFIG maintain the voltage level at bus 8 with its capacity to supply reactive power to the grid, as shows Fig. 15.

**Figure 13.** Voltage at Bus 11.

**Figure 14.** Voltage at Bus 8.

**Figure 15.** Reactive power injected / absorbed by DFIG.

**Figure 13.** Voltage at Bus 11.

238 Advances in Wind Power

**Figure 14.** Voltage at Bus 8.

Fig. 16 shows the field current behavior from generator 3. The DFIG's power factor control mode increases the field current demand and the OEL begins to operate at 225 s reducing the current, and as a consequence, the reactive power injected by G3 decreases.

On the other hand, when the DFIG is operating with voltage control mode the OEL is not activated, increasing the voltage stability margin. The DFIG's voltage control mode dem‐ onstrates that can be utilized in order to improve the long-term voltage stability in a sys‐ tem with a high wind penetration. From these results, one can conclude that the DFIG's voltage control mode has a beneficial effect in the voltage stability when the power sys‐ tem is submitted to load increase, considering the dynamic aspects of the OEL and OLTC combined with the load characteristics adopted. It is important to highlight that load char‐ acteristics and power system voltage control devices are among key factors influencing voltage stability.

**Figure 16.** Field Current from G3.

Fig. 17 shows the OLTC behavior during the load ramp increase. It is observed that the min‐ imum and maximum tap positions are reached when the DFIG wind turbines are config‐ ured to control the power factor, in order to support the voltage at bus 11. This is a great disadvantage of power factor control mode. The reactive power system reserves are insuffi‐ cient and the OLTC tap changing is detrimental to voltage profile, increasing the risk of long-term voltage instability.

On the other hand, the voltage control strategy provides a delay on the OLTC actuation. Be‐ sides, the OLTC does not even reach the upper tap limit. When the OLTC is not changing its tap position, the reactive power absorbed decreases as well as the transmission line losses, causing a smallest drop in voltages. In this case, the power system is much more prone to maintain the voltage stability.

#### **Figure 17.** Tap position.

**Figure 16.** Field Current from G3.

240 Advances in Wind Power

long-term voltage instability.

maintain the voltage stability.

Fig. 17 shows the OLTC behavior during the load ramp increase. It is observed that the min‐ imum and maximum tap positions are reached when the DFIG wind turbines are config‐ ured to control the power factor, in order to support the voltage at bus 11. This is a great disadvantage of power factor control mode. The reactive power system reserves are insuffi‐ cient and the OLTC tap changing is detrimental to voltage profile, increasing the risk of

On the other hand, the voltage control strategy provides a delay on the OLTC actuation. Be‐ sides, the OLTC does not even reach the upper tap limit. When the OLTC is not changing its tap position, the reactive power absorbed decreases as well as the transmission line losses, causing a smallest drop in voltages. In this case, the power system is much more prone to

The DFIG's terminal voltage control mode based on the rotor excitation current allows the maintenance of reactive power consumed by the motor as shows Fig. 18. In this case, there are no extra static or dynamic reactive compensation demands for maintaining the power system long-term voltage stability. On the other hand, the DFIG's power factor control mode causes an increase in the reactive power drawn by the motor, which is necessary to maintain the power system reactive power balance. In this case, the motor is subject to a sudden stall that can cause a voltage collapse manifested as a slow decay of voltage in a sig‐ nificant part of the power system.

Fig. 19 shows the system PV curve for both DFIG control modes. In this case, the maximum loadability points for voltage and power factor control modes don't show much difference between each other as the previous case (static load). This occurs since in this case the load is incremented only at bus 11, which is controlled by the OLTC. Then, the active power absor‐ bed at bus 8 shows only a small drop, which is reflected to the PV curve. The load at bus 8 (motors) was not incremented because the motor would consume too much reactive power from the system.

**Figure 18.** Reactive power absorbed by the motors.

**Figure 19.** PV curve at bus 8.

#### **6. Conclusion**

Fig. 19 shows the system PV curve for both DFIG control modes. In this case, the maximum loadability points for voltage and power factor control modes don't show much difference between each other as the previous case (static load). This occurs since in this case the load is incremented only at bus 11, which is controlled by the OLTC. Then, the active power absor‐ bed at bus 8 shows only a small drop, which is reflected to the PV curve. The load at bus 8 (motors) was not incremented because the motor would consume too much reactive power

0 50 100 150 200 250

Time (sec)

from the system.

242 Advances in Wind Power

1020

**Figure 18.** Reactive power absorbed by the motors.

1040

1060

1080

1100

1120

Reactive Power Absorbed by Motors (Mvar)

1140

1160

1180

Voltage Control Power Factor Control

1200

This paper presented studies analyzing the impacts of different control strategies from DFIG wind turbines on power system long-term voltage stability by time domain simulations. The study considered the dynamic models of generator OEL and OLTC transformer combined with static and dynamic load representations. The simulation results confirm the expecta‐ tion that when DFIG operates in power factor control mode the voltage stability margin is poor, mainly when the motor model is used to represent a part of the load. The results clear‐ ly show that DFIG's voltage control mode enhances voltage stability margin. The voltage control is more robust than power factor control when the power system is subjected to a slow load increase, a process that involves the OEL and OLTC actions and interactions. The use of DFIG in voltage control mode also increased the maximum loading point as well as delayed the OEL and OLTC actuation, helping to avoid problems of voltage collapse in the power system. As part of the ancillary services, the voltage control mode may have an in‐ creasing market ahead. This is an important feature that must be considered in the choice of the control strategy to be used on DFIG wind turbines.

#### **Appendix**

```
Generators 1, 2 and 3 parameters (p.u. on base of machine rating):
            Ra = 0.0046Xd = 2.07X'
                                   d = 0.28X"
                                             d = 0.215
             Xq = 1.99X'
                         q = 0.49X"
                                   q = 0.215Xl
                                              = 0.155
                     T'
                       d0 = 4.10 sT'
                                   q0 = 0.56 s
                   T"d0 = 0.033sT"
                                  q0 = 0.0062 s
                  G2: H = 2.09s, Sb = 2200 MVA
                  G3: H = 2.33s, Sb = 1400 MVA
     SCIG and DFIG parameters (on base of machine rating):
                rs = 0.85%Xs = 5.776%Xm = 505.9%
                 rr = 0.712%Xr = 8.094%H = 3.5 s
                    Poles = 2Power = 1140 HP
                  DFIG wind turbine parameters:
             Rotor diameter = 58 mGear ratio = 74.5
                        DFIG power curve:
                        OLTC parameters:
           Time delay for the first tap movement = 30 s
         Time delay for subsequent tap movement = 5 s
                  Dead band = ±1% bus voltage
                      Tap range = ±16 steps
                  Step size = 5/8% (0.00625 pu)
  Induction Motor parameters (p.u. on base of machine rating):
                    Xm = 3.3 Rs = 0.01Xs = 0.145
                       Rr = 0.008Xr = 0.145
                       H = 0.6s, 4,826.0 HP
    Generic network parameters (p.u. on base Sb = 100MVA):
                Line 5-6:R = 0.0 X = 0.0040B = 0.0
             Line 6-7:R = 0.0015 X = 0.0288B = 1.173
              Line 9-10: R = 0.0010X = 0.0030B = 0.0
               T1:R = 0.0X = 0.0020Ratio = 0.8857
               T2: R = 0.0X = 0.0045Ratio = 0.8857
               T3:R = 0.0 X = 0.0125Ratio = 0.9024
               T4:R = 0.0 X = 0.0030Ratio = 1.0664
               T5:R = 0.0 X = 0.0026Ratio = 1.0800
```