# Ensuring Stable Operation of Wind Farms Connected to Distribution Networks
## 4 Results
The block diagram of the algorithm (Figure 7) shows the sequence of actions to design a wind power evacuation arrangement or check the results of power flow calculations performed by the design organization. Each stage of the algorithm involves making a decision on the need to apply methods to prevent unnecessary shutdowns of wind turbines during typical disturbances in all three operating zones discussed above. The topology and operating conditions in the distribution network may vary, which is why transient calculations are essential for every distribution network repair scheme, whose number is huge. In the event that the alternative methods are technically efficient, it is advisable to choose the cost-effective one. Next, we will consider methods ensuring the stable operation of wind turbine inverters under standard disturbances in the distribution network. 1. _Upgrading relay protection devices and automatic controls in the distribution network adjacent to the wind farm._ Zone 1 (Figure 6) requires reducing the response time of the main protection of power lines by replacing the electromechanical relay protection devices with digital ones, which have a higher sensitivity and a shorter response time. It may also be necessary to replace high-voltage circuit breakers with modern (SF6 or vacuum) ones, with a shorter opening time. However, the reduction in the short circuit clearance time does not prevent wind turbine shutdowns at close short circuits with a deep voltage dip. Zone 2 (Figure 6) requires modernizing the backup protections to reduce the response time of the second and third stages to 0.9-1.0 s. It may also be necessary to reduce the response time of the breaker failure protection by changing the settings or by replacing it with a digital one. Zone 3 (Figure 6) requires placing the second sets of relay protection devices with absolute selectivity as backup protection on power lines and implementing the automatic introduction of backup protection acceleration (remote; ground fault protection). This will increase the value of \(U_{\text{res}}\) at the output of wind turbine inverters, as shown by the green dotted line in Figure 6.  
Figure 8 shows the requirements of the grid codes of various countries for the value of \(U_{\rm res}\), at which the wind turbine should not be disconnected. The calculated values of \(U_{\rm res}\) at the outputs of the wind turbine inverters when the reactive current injection function is activated at the wind turbine during a short circuit in the distribution network are shown by a red solid line, and when deactivated, by a blue one (Figure 8). The analysis of Figure 8 suggests that the activation of the reactive current injection function at the wind turbine produces a significant positive effect in the form of an increase in the \(U_{\rm res}\) value at the output of wind turbine inverters from 0.028 \(U_{\rm nom}\) to 0.222 \(U_{\rm nom}\), i.e., 5-8 times, depending on the distance between the wind farm and the 220 kV substation. The \(U_{\rm res}\) value goes up as the distance from the wind farm to the 220 kV substation increases, which is due to the generation of significant reactive power by extended 110 kV transmission lines with their low active power load. The comparison of \(U_{\rm res}\) at the output of wind turbine inverters with the LVRT characteristics shows that the activation of the reactive current injection function helps to prevent unnecessary shutdowns of wind turbines produced by most manufacturers. In the event that the wind turbine does not have the function of reactive current injection in case of short circuit in the distribution network, a requirement for the wind turbine should be established similar to the requirement of the grid codes of Germany, Ireland, and Canada (\(U_{\rm res}\) = 0 p.u.). This will prevent excessive shutdowns of wind turbines in zone 1 (Figure 6). off by its protection.