# Ensuring Stable Operation of Wind Farms Connected to Distribution Networks
## 4 Results
With remote short circuits, when \(U_{\text{res}}\) at the STATCOM terminals is higher than 80% \(U_{\text{nom}}\), the voltage at the output of wind turbine inverters increases from 80.6 % \(U_{\text{nom}}\) to 85.7% \(U_{\text{nom}}\) (i.e., by 6.3%) [56]. The cost of STATCOM (10 MVA), excluding the costs of design, installation, and commissioning work, is approximately $1.8 million at current prices. In zone 3 (Figure 6), however, wind turbines are unnecessarily turned off only when the backup protection response time in the distribution network is more than 1 s. Thus, the performance of STATCOM is extremely low, given its significant cost.  
The main technical characteristics of CLR are as follows:  
* Nominal voltage: 35 kV;
* Nominal current: 1000 A;
* Inductive reactance (\(X_{\text{CLR}}\)): from 1 to 10 Ohm.  
This research study also addressed the option of series connection of the CLR in the circuit of a 35/110 kV step-up transformer of the wind farm on the 35 kV side. Multivariate calculations of transients were performed during a three-phase short circuit at a 110 kV power transmission line in the case of its shutdown (near the buses of the 220/110 kV substation) for various \(X_{\text{CLR}}\) values. The results of the calculations of the \(U_{\text{res}}\) value at the output of the wind turbine inverters show that the greatest effect is achieved at \(X_{\text{CLR}}\) = 7.5 Ohm (Figure 9).  
The use of CLR made it possible to increase \(U_{\text{res}}\) at the output of wind turbine inverters with a short circuit clearance time of up to 0.9-1 s as follows:  
* From 8% \(U_{\text{nom}}\) to 25% \(U_{\text{nom}}\) (increase by 213%) for three-phase short circuits;
* From 40% \(U_{\text{nom}}\) to 60% \(U_{\text{nom}}\) (increase by 50%) with two-phase short circuits;
* From 68% \(U_{\text{nom}}\) to 76% \(U_{\text{nom}}\) (increase by 12%) with single-phase short circuits.  
The obtained LVRT characteristic when connecting the CLR (red dotted line in Figure 6) ensures the stable operation of wind turbines manufactured in accordance with the requirements of the grid codes of the UK, China, Spain, Denmark, Ireland, Germany, Canada, and Italy. For each case of the wind farm connection to the distribution network, \(X_{\text{CLR}}\) should be determined based on the results of transient calculations.  
The cost of a CLR with \(X_{\text{CLR}}\) = 7.5 Ohm, excluding the costs of design, installation, and adjustment work, is about $300,000 at current prices. The CLR performance is quite high, although its cost is six times less than that of STATCOM. For the economic analysis to be correct, one should take into account the cost of electricity losses in the CLR and operating costs throughout its life cycle. 4. _Making changes to the hardware of wind turbine inverters._  
Figure 9: Graph of \(U_{\text{res}}\) at the output of wind turbine inverters versus \(X_{\text{CLR}}\). An individual technical solution is to make changes to the hardware of wind turbine inverters to adjust the LVRT characteristic. The stable operation of the inverter can be provided by maintaining the balance of power transmission from the wind generator through the rectifier to the DC link and to the input of the inverter. In each link of the inverter controller, the wind turbine ensures that the operating parameters are maintained within the range of feasible values. A specific feature of the inverter is that with the help of the phase-locked loop (PLL) system, the shape, frequency, and phase of the alternating current are formed at its output in accordance with the shape, frequency, and phase of the voltage in the wind farm collection network. The stable operation of all units of the wind turbine inverter controller is possible at a voltage of at least 20% \(U_{\text{nom}}\) in the wind farm collection network.