# Ensuring Stable Operation of Wind Farms Connected to Distribution Networks
## 2 Analysis of the Requirements for Stable Operation of Wind Farms  
One of the mandatory requirements for generating plants of various types is the regulation of their response to standard disturbances in the power system, accompanied by short-term voltage dips [16; 17; 18]. International experience shows that over the past decade, the requirements for the connection and operation of wind farms as part of power systems have been gradually tightened [19; 20]. The main objective was to ensure the reliable operation of energy systems in the face of an increase in the share of wind farms in the structure of generating capacities [21; 22]. Consider the main stages of evolutionary changes in the requirements for choosing wind turbine protection settings, as exemplified by the countries with a large number and total capacity of wind farms (Germany, Denmark, and Belgium):  
1. _Ensure that the wind turbines are turned off in case of voltage dips with their subsequent turning on after a short circuit in the distribution network is eliminated._  
This was necessary to exclude the impact of wind turbines on the operating conditions of the distribution networks and the relay protection devices in them while reducing the residual voltage (\(U_{\text{res}}\)) at the wind turbine terminals to 85% \(U_{\text{nom}}\) and below [23]. With a 15-20% increase in the share of wind farms in the structure of generating capacities, it was necessary to tighten the requirements for protection settings. This is because the shutdown of high-capacity wind farms during the clearance of a short circuit in the distribution network by the correct operation of the relay protection devices aggravates the post-accident conditions [24; 25]. 2. _Prevent wind turbine shutdowns in case of voltage dips of a given depth and duration._  
Some countries have developed requirements to ensure the stable operation of wind farms as part of power systems in accordance with a given voltage-time LVRT characteristic. The LVRT characteristic is calculated in each country individually, given the types of relay protection devices used, their operation algorithms and settings, as well as high-voltage circuit breakers. If the voltage dip is below a given depth and longer in duration, the wind turbines can be disconnected from the grid. This is undesirable but with the correct calculation of the LVRT characteristic, wind turbine outages will occur quite rarely. 3. _Ensure the stable operation of the wind turbine within the limits of the LVRT characteristic with the reactive current injection at a given rate of rise._  
This function is necessary to ensure the reliable operation of relay protection devices in the distribution network, increase the stability of power systems, and boost the speed of voltage recovery after the short circuit clearance. The requirements for wind turbines were made stricter due to the need to prevent system-wide accidents and their development with the disconnection of electricity to consumers [26]. To continue the operation of wind farms after the introduction of new requirements, it was necessary to re-certify all wind turbines. The LVRT characteristic is set by the wind turbine manufacturer in the inverter. A simplified single-line diagram of a type IV wind turbine is shown in Figure 1 explains the principle of implementing the LVRT characteristic. The LVRT characteristic is implemented in the wind turbine as follows:  
* Line (interphase) voltage (\(U_{\text{AB}}\), \(U_{\text{BC}}\), \(U_{\text{CA}}\)) is measured at the inverter output (690 V);
* Inverter protection is triggered when any of the line voltages drops below the specified setting in terms of the voltage dip (it is deeper) and in terms of its duration (it is longer), in accordance with the voltage-time LVRT characteristic;
* When the inverter protection is triggered, the wind turbine circuit breaker is switched off at a voltage of 690 V [27].