# High Voltage Ride through Strategy of Wind Farm Considering Generator Terminal Voltage Distribution
## 4 HVRT Strategy Considering Generator Terminal Voltage Distribution  
### Extraction of Key Voltage Node  
According to Section 2, the generator terminal voltage of each wind turbine in the wind farm must be considered during the HVRT period to avoid the trip-off accident happening at the end of the collection line. Therefore, the grid-connection node, the node with the lowest or highest generator terminal voltage in the wind farm cannot be selected as the reference point for reactive power compensation. Consequently, this article put forward a method for extracting key nodes.  
According to the analysis result in Table 1, the voltage distribution of all wind turbines can be depicted as shown in Figure 9. The lowest voltage node in the wind farm is \(U_{1}\), and the highest voltage node is \(U_{10}\). According to the nonlinear least-squares approximation, the voltage distribution curve can be fit with  
\[\left\{\begin{array}{c}U(L)=-0.000175L^{3}+0.00007143L^{2}+0.009739L+0.9975\\ U_{1}(L)=0.007075L+0.9975\end{array}\right. \tag{14}\]  
According to Equation (14), when \(L\) varies in range (0-4), there is a unique tangent point between \(U(L)\) and \(U_{1}(L)\). The unique tangent point is the key voltage node, \(U_{\text{kp}}\), that we take. The key voltage node, \(U_{\text{kp}}\), is not an actual wind turbine node, but a virtual node.  
By extracting \(U_{\text{kp}}\), the entire wind farm's wind turbines can be divided into two regions. One is the set of wind turbines with a node voltage lower than \(U_{\text{kp}}\), and the other is the set of wind turbines with node voltage higher than \(U_{\text{kp}}\). According to Figure 9, by limiting the voltage of \(U_{\text{kp}}\) to within 1.1 p.u. during a HVRT period, the safe and stable operation of wind turbines with node voltages lower than \(U_{\text{kp}}\) can be realized. As shown in Figure 10, the key voltage node, \(U_{\text{kp}}\), divides all wind turbines into two parts: the upstream wind turbines and the downstream wind turbines. The upstream wind turbines refer to the wind turbine set with node voltage lower than \(U_{\text{kp}}\), and the downstream wind turbines refer to the wind turbine set with node voltage higher than \(U_{\text{kp}}\). The number of downstream wind turbines is small, and further inductive reactive power output can be performed through the wind turbine's own reactive power compensation capability.  
Figure 9: The voltage distribution of all wind turbines.  
It should be noticed that the generator terminal voltage detection and the extraction of key voltage point are updated in real time according to the variation of the wind farm. According to China's national technical rule for connecting wind farms to power systems, promulgated in 2011, wind farms are required to be equipped with certain voltage/var management devices before being put into operation [23]. These voltage/var management devices have the characteristics of quick adjustment speed, high precision, simple installation, and being maintenance-free. As shown in Figure 11, each wind turbine is equipped with an independent distributed control device, which is able to detect the wind turbine information (including generator terminal voltage) and transmit to a centralized control device, receive control instructions from the centralized control device, and transmit to the wind turbine. The HVRT strategy proposed in this article is based on real-time voltage transmission. The centralized control device generates the voltage distribution curve of the wind farm, and calculates the reactive power compensation reference point (the key voltage node). During a HVRT period, the centralized control device generates the corresponding reactive power compensation instructions, and transmits to the synchronous condenser and wind turbines [24].