# Ensuring Stable Operation of Wind Farms Connected to Distribution Networks
## 2 Analysis of the Requirements for Stable Operation of Wind Farms
The E.ON (German multinational electric utility company) grid code sets out the following requirements for the implementation of the reactive current injection function during external faults:  
* Control voltage within no more than 20 ms after fixing the voltage dip by injecting a reactive current of at least 2% of \(I_{\text{rated}}\) for every 1% of the voltage dip;
* Ensure the maximum value of the output reactive current, but no less than 100% of \(I_{\text{rated}}\)[28]. The graph of the relationship between the values of the wind turbine reactive current injection and the voltage dip depth during short circuit in the distribution network is shown in Figure 2.  
The dead zone (dotted lines in Figure 2) can be set from 0 p.u. to the required value. In Figure 2, reactive current injection starts at a voltage dip (\(\Delta U\)) of 0.1 p.u. The reactive current injection increment coefficient (\(K\)) can vary from 0 to 10, according to the E.ON requirements \(K=\Delta I_{\text{react}}/\Delta U=2\). In the automatic control system (ACS) of inverters, one can set the priority of active or reactive power supply (combination in proportion). Moreover, the value of the injected reactive current can be added to that in the pre-accident conditions but should be no more than 1.024 \(I_{\text{rnd}}\)[29]. The following conditions are according to the E.ON requirements:  
Figure 1: Simplified single-line diagram of type IV wind turbines. Figure 2: Graph of the relationship between the value of injection of the wind turbine reactive current and the depth of voltage dip during short circuit. * Time for fixing a voltage dip--\(t_{0}\leq 10\) ms;
* Time for stopping the supply of active power--\(t_{1}\leq 10\) ms;
* Maximum time before the start of reactive current injection--\(t_{2}\leq 10.1\) ms;
* Maximum time until the reactive current reaches 0.9 (0.95) \(I_{\text{rated}}\)--\(t_{3}=50\) (55) ms;
* Time for the reactive current to reach the set steady-state value in the range of \(+20\%/-10\%\)--\(t_{4}=80\) ms;
* Maximum short-term excess of the reactive current over the steady-state value--\(I\leq 0.1\)\(I_{\text{rated}}\). All time intervals are counted from the beginning of the voltage dip at the output of the wind turbine inverter during a short circuit. The \(U_{\text{res}}\) value at the output of wind turbine inverters in the case of standard disturbances in the distribution network also depends on the wind farm connection scheme. As a rule, one of the three schemes shown in Figure 3 is used to connect a wind farm to the 69 kV network. There is a high probability of wind turbine shutdowns in the case of standard disturbances near the busbars of the substation (SS) to which the wind farm is connected, according to the scheme in Figure 3a,c. The probability of wind turbine shutdowns is lower when a wind farm is connected according to the scheme in Figure 3b. This is because a disturbance in the power transmission line with a branch line to the wind farm is not subject to consideration according to the requirements of some grid codes. The paper [30] considers the methods for adjusting the LVRT characteristic for a wind turbine based on a dual-fed induction generator (DFIG-WT). The article delivers the results of an assessment of the consistency of the LVRT characteristics of the DFIG-WT with the requirements of the grid codes of various countries. It also provides recommendations for bringing the LVRT characteristics of the DFIG-WT in line with the requirements. DFIGs are widely used due to the numerous advantages they offer. These are variable speed and constant frequency operation, the ability to control active and reactive power, and reduced ratings of power converters. The authors of [31; 32; 33; 34] propose various approaches to bring the LVRT characteristics of the DFIG-WT in line with the requirements of grid codes through the implementation of additional schemes.