# Ensuring Stable Operation of Wind Farms Connected to Distribution Networks
## 4 Results  
Standard disturbances in the distribution network were analyzed for belts 0, 1, and 2 (Figure 4). The standard disturbances that lead to the disconnection of power grid components during the short circuit clearance and cause the disconnection of the wind farm from the distribution network were not considered. Figure 5 shows the results of calculations of \(U_{\text{res}}\) at the output of the wind turbine inverters, which were performed in accordance with Table 1.  
Figure 6 shows the voltage-time LVRT characteristics of wind turbine inverters from the grid codes of different countries in comparison with the graph of changes in \(U_{\text{res}}\) at the output of wind turbine inverters (red solid line) when simulating standard disturbances in the distribution network. Figure 5: Graphs of \(U_{\text{res}}\) at the output of wind turbine inverters under standard disturbances in the distribution network. Figure 6: Voltage–time characteristics for LVRT of wind turbine inverters for various countries (green dotted line indicates the voltage provided that the protections are replaced with high-speed ones – without a time delay). Let us analyze the three zones shown in Figure 6, which are characterized by different times of short circuit clearance by relay protection devices:  
* Zone 1 (0-0.2 s) is characterized by the fact that the parameters for setting the LVRT characteristics according to the requirements of the grid codes of Denmark, Italy, Spain, UK, and China do not provide the prevention of wind turbine shutdowns. This is because the value of \(U_{\text{res}}\) at the output of wind turbine inverters is lower than the settings of the LVRT characteristic. The grid codes in some countries allow wind turbine shutdowns when \(U_{\text{res}}\) drops below 15-25% \(U_{\text{nom}}\) with a duration of 0.1-0.15 s. At the same time, the requirements of the grid codes of Germany, Ireland, and Canada make it possible to ensure the stable operation of wind turbines under standard disturbances in the analyzed time interval. * Zone 2 (0.2-1.0 s) is characterized by the fact that most wind turbine manufacturers ensure that the required \(U_{\text{res}}\) value is maintained at the output of wind turbine inverters under standard disturbances. Therefore, with single-phase short circuits (more than 70% of the total number) eliminated by backup protections (the second stage region) with a time delay of up to 1 s, there will be no wind turbine shutdowns. However, the requirements of the grid codes of Denmark, Italy, Spain, and UK do not allow preventing disconnections of wind turbines. * Zone 3 (more than 1 s) is distinguished in that when a single-phase short circuit lasting more than 1-2 s is cleared by backup protections (the second or third stage region), in the vast majority of cases, wind turbines will be shut down. In some countries, however, in distribution networks with a voltage over 69 kV, wind turbines must continue to operate stably up to 2.5 s.  
The long response times of the second and third stages of back-up protection (remote; ground fault protection) are due to the need to ensure the selective operation of relay protection devices based on electromechanical relays that are in operation in large numbers [52; 53]. An analysis of the transient calculations for the distribution networks with standard disturbances indicates that the settings for the LVRT characteristic, given by some manufacturers of wind turbines, require adjustment. This is justified if \(U_{\text{res}}\) at the output of the wind turbine inverter is below 20% \(U_{\text{nom}}\), and also in the event of a short circuit being eliminated by the backup protections with time delays of more than 1 s, which implement the long-range backup function [54; 55]. Otherwise, the wind turbines will be disconnected. A methodology was developed to check the settings of the LVRT characteristic for compliance with the technical requirements for wind turbines. Figure 7 shows a block diagram of the verification algorithm.