# High Voltage Ride through Strategy of Wind Farm Considering Generator Terminal Voltage Distribution
## 4 HVRT Strategy Considering Generator Terminal Voltage Distribution
### HVRT Strategy  
As shown in Figure 12, this article put forward a HVRT coordinated control strategy between the DFIG and the synchronous condenser, based on the difference in reactive power output characteristics. \(I_{\text{ex}}\) is the synchronous condenser reactive current command to maintain \(U_{\text{kp}}\) at 1.1 p.u. ; \(I_{\text{sc}}\) is the reactive current output of the synchronous condenser; \(U_{\text{T}}\) is the generator terminal voltage of DFIG; \(I_{\text{ref}}\) is the reactive current command to maintain \(U_{\text{T}}\) at 1.1 p.u. ; \(Q_{\text{re}}\) is the additional reactive power compensation (if \(I_{\text{sc}}<I_{\text{ref}}\)) command; \(Q_{\text{gmax}}\) and \(Q_{\text{gmin}}\) are the upper and lower limits of reactive power compensation  
Figure 11: Wind farm information transmission system. Figure 10: Division of wind turbines. of the grid-side converter, respectively; \(Q_{\rm g}\) is the reactive power command, issued to the grid-side converter; \(Q_{\rm re2}\) is the additional reactive power compensation (if \(I_{\rm sc}<I_{\rm ref}\) and \(Q_{\rm re}>Q_{\rm gmax}\)) command; \(Q_{\rm smax}\) and \(Q_{\rm smin}\) are the upper and lower limits of reactive power compensation of the stator, respectively; and \(Q_{\rm s}\) is the reactive power command issued to the rotor-side converter. This strategy extracts \(U_{\rm kp}\) as the voltage reference point. During a HVRT period, the excitation current target value is generated through stabilizing the voltage of \(U_{\rm kp}\), and transmitted to the excitation system of the synchronous condenser. At the same time, by detecting the reactive current output of the synchronous condenser, it is judged whether it meets the reactive current output requirement (as shown in Equation (15)). If it cannot meet the requirement, then a further reactive power compensation command will be issued to the DFIG. Since the stability of the busbar voltage is prerequisite to ensure that DFIG avoids trip-off accidents during a voltage swell period, priority is given to the reactive output capacity of the grid-side converter. When the maximum reactive power output of the grid-side converter cannot meet the reactive current compensation requirement, the reactive power compensation command will be issued to the generator-side converter, which controls the reactive current output from the stator of the DFIG. According to the above strategy, a coordinated control flow chart is shown in Figure 13. Whether each reactive power source in the wind farm participates in reactive power compensation depends on the voltage swell amplitude of the generator terminal voltage. While generator terminal voltage of all DFIGs is lower than 1.1 p.u., no additional reactive power compensation is required. While the generator terminal voltage of a few wind turbines is higher than 1.1 p.u. and \(U_{\rm kp}\) is lower than 1.1 p.u., only the downstream DFIGs participate in additional reactive power compensation. While generator terminal voltage of some wind turbines and \(U_{\rm kp}\) are higher than 1.1 p.u., and synchronous condenser reactive current satisfies Equation (15), the synchronous condenser and the downstream DFIGs participate in additional reactive power compensation. While the generator terminal voltage of some wind turbines and \(U_{\rm kp}\) are higher than 1.1 p.u., and synchronous condenser reactive current cannot satisfy Equation (15), the synchronous condenser and all DFIGs participate in additional reactive power compensation. \[I_{\rm SC}\geq 1.5\times(U_{\rm T}-1.1)I_{\rm n} \tag{15}\]  
Figure 12: The coordinated control of the DFIG and synchronous condenser. The proposed HVRT strategy can be implemented based on the existing distributed control devices and centralized control device, without additional installations. When an AC failure, a commutation failure, or a DC blocking failure happens, it will cause a transient voltage swell at the grid-connection point of the wind farm. Therefore, the HVRT strategy can be applied to multiple failure scenarios.