**4. Wind turbines and operational grid requirements**

The fast growth and penetration of wind generation through the installation of large number of wind turbines has led to primary concern about the effect of wind power on the transient and frequency stability of the electric utility grid. In several countries, institutional support on renewable energy sources has led to implementation of a large number of wind farms [27]. As the amount of wind power is drastically increasing in years to come, maintaining power system, voltage and frequency stability during a short circuit fault or grid disturbance will be more paramount in order to ensure power supply safety and other important tasks. It would

**Figure 4.** Fault ride through (low voltage ride through) requirement for wind farms as set by E.ON NETZ GmbH.

be imperative to perform new studies to evaluate the behavior of the wind farms during and after severe faults, in order to improve the design of the wind farms in an efficient and economy way. Hence, the most demanding requisite for wind farm is the Fault Ride Through

**Figure 5.** The rule of voltage support during grid fault as set by E.ON NETZ GmbH.

**4. Wind turbines and operational grid requirements**

**Table 1.** Comparative study of fixed and variable speed wind generators.

**Fixed speed wind generator Variable speed wind generator**

3. This wind turbine has superior characteristics such as brushless and rugged construction, low cost, maintenance free and operational simplicity.

8 Stability Control and Reliable Performance of Wind Turbines

4. The technology of this wind turbine requires large reactive power to recover the air gap flux when a short circuit fault or grid disturbance occurs

5. This wind turbine requires the installation of expensive external reactive power compensation devices to provide and support reactive power, thus, increasing the overall cost of the system.

in the power system.

The fast growth and penetration of wind generation through the installation of large number of wind turbines has led to primary concern about the effect of wind power on the transient and frequency stability of the electric utility grid. In several countries, institutional support on renewable energy sources has led to implementation of a large number of wind farms [27]. As the amount of wind power is drastically increasing in years to come, maintaining power system, voltage and frequency stability during a short circuit fault or grid disturbance will be more paramount in order to ensure power supply safety and other important tasks. It would

3. This wind turbine technology has reduction of mechanical

4. The technology of this wind turbine improves the power quality of the grid connected system without expensive external

5. This wind turbine has the required capacity of the power converters for secondary excitation, and this can be less than half for the case of a DFIG system, thus the total cost decreases.

stresses and acoustic noise.

reactive power compensation devices.

**Figure 4.** Fault ride through (low voltage ride through) requirement for wind farms as set by E.ON NETZ GmbH.

**Figure 6.** Grid frequency requirement of wind farms as set by E.ON NETZ GmbH.

(FRT) or Low Voltage Ride Through (LVRT) capability. Wind farms connected to high voltage transmission system must stay connected when a voltage dip or frequency disturbance occurs in the grid, otherwise, the sudden disconnection of great amount of wind power may contribute to the voltage dip and drop of frequency, with terrible consequences in the utility grid. Therefore, the transient and dynamic analysis of wind generators in wind farms are necessary. Several solutions could be used in the stability analysis and improvement of wind turbines during grid disturbances, so that they can contribute to voltage and frequency control. Some of these solutions are the use of power electronic devices and reactive power compensation units like static synchronous compensator (STATCOM), superconducting magnetic energy storage (SMES), energy capacitor system (ECS), crowbar, static series compensator (SSC), a dynamic voltage restorer (DVR), series dynamic braking resistor (SDBR), superconducting fault current limiter (SFCL), passive resistance network, series antiparellel thyristors and among others discussed in the literature.

#### **4.1. Operational grid requirements**

The big challenges that wind farms must face is voltage and frequency dip in the grid during grid disturbances [28]. The magnitude of the voltage is controlled by the reactive power exchange, while the frequency is controlled by the active power. **Figure 4** displays the typical requirement for fault ride through grid code regarding terminal voltage of the wind farm. The wind farm must remain connected to the grid if the voltage drop is within the defined r.m.s. value and its duration is also within the defined period as shown in the curve. **Figure 5** shows the required reactive current support from the generating plants during voltage dip, while **Figure 6** shows the permissible grid frequency requirement [29].
