# A PMSG Wind Energy System Featuring Low-Voltage Ride-through via Mode-Shift Control
## 2 Low-Voltage Ride-through Capability  
As the capacity of grid-tied wind energy systems increased, such integration has encouraged power utility operators to revise grid codes and to include technical requirements of interconnected wind power plants to ensure grid stability and reliability. These rules vary from country to country, but they share some common requirements, such as LVRT capability, voltage and frequency variation limits, active power and frequency control, reactive power control, voltage regulation and power factor control [24]. As indicated previously, LVRT indicates the ability of a wind energy system to stay connected to the utility network for a short time during grid faults and at voltage dip instances to minimize any power generation losses. Different grid codes are tailored accordingly and vary in terms of voltage dip severity and the allowable operation time for wind turbines to remain connected to the network. A typical LVRT curve is shown in Figure 1, with a special focus on the (E-ON) code, as set by the German Distribution and Distribution Utility operators [25]. In the presence of a grid dip, the wind turbine generators must remain connected to the utility network if the line voltage remains over the limit line in Figure 1a. Besides satisfying the active power requirement, the WECS is required to inject reactive current as specified in Figure 1b without exceeding the current limit of the converter, in order to assist the utility in holding the grid voltage. The amount of reactive power to be injected depends on the amount of grid voltage dip, the amount of reactive current present before the dip occurrence and the rated grid current [16]. According to Figure 1b, LVRT capability should start when a voltage sag is detected below 90% of its nominal value. For voltage sags ranging between 50 and 90%, 2% reactive current needs to be provided for every 1% voltage dip. The WECS must be able to provide 100% reactive current if the grid voltage falls below 50% [26]. As mentioned earlier, LVRT enhancement techniques for PMSG WECS are typically divided into types that rely on the control scheme modification and others that require auxiliary hardware modification. External hardware-based methods include chopper-controlled braking resistance [27], energy storage systems [28], fault current limiters [29] and using FACT devices at the point of common coupling (PCC) [30, 31]. On the other hand, modified controller-based schemes rely on enhancing the applied control scheme to include pitch angle control [32, 33], generator de-loading control [34], or reactive power control [7, 13, 35]. Hardware solutions usually feature less control complexity, yet with more size and cost compared to the alternative software control modifications. Thus, the latter is considered in this paper. Table 1 presents the most recent modified control-based techniques introduced to address the LVRT problem facing grid-tied PMSG WECS [15, 29, 30, 31, 32, 33, 34, 35]. It can be concluded that all presented techniques terminate the MPPT process during sag conditions to limit grid current rises and maintain its value within the safe limit as per grid standards. Additionally, during voltage sag, all techniques inject reactive power (VAR) into the grid as required by grid regulations except [17], which shows deficiency regarding VAR support. Coordinated control between both MSC and GSC for DC-link voltage control is achieved in [18]. Additional auxiliary hardware devices are required in [17, 19, 20, 21] to store the excess power during sag conditions, which adds to system complexity and cost. Finally, [15, 22, 23] prevent any active power injection during faults, thus limiting WECS efficiency due to the loss of active power transferred to the grid during the sag. Figure 1: LVRT requirement: (**a**) voltage limit curves; (**b**) reactive current injection during fault.