# High Voltage Ride through Strategy of Wind Farm Considering Generator Terminal Voltage Distribution
## 3 Reactive Power Output Characteristics of Wind Farm Transmission Systems
### The Reactive Power Output Characteristics of Synchronous Condensers  
Compared with traditional synchronous condensers, the state-of-art synchronous condenser is optimized in terms of transient response and overload capability. Compared with SVC and STATCOM, the synchronous condenser has the following advantages [16; 17]:  
1. When the power grid fault happens, the response speed of the synchronous condenser to output reactive current based on its own physical characteristics will be faster.
2. When the power grid voltage sags, the synchronous condenser can output more reactive power.
3. The lifetime of the synchronous condenser is longer.  
When the grid voltage swells or sags, the reactive power response of the synchronous condenser is mainly divided into two parts. The first part is that the reactive power response comes from its physical characteristics. This reactive power is naturally generated when the power grid voltage changes and decays with time. The second part is the reactive power based on excitation control, and this reactive power is caused by controlling the excitation current, and requires a certain response time. The reactive current of the first part is shown as [17]:  
\[i_{\rm d}=\frac{F_{\rm g0}-U_{\rm b+i}}{X_{\rm d}}+\left(\frac{F_{\rm g0}^{ \prime}-U_{\rm b+i}}{X_{\rm d}}-\frac{F_{\rm g0}^{\prime}-U_{\rm b+i}}{X_{\rm d }^{\prime\prime}}-\frac{F_{\rm g0}^{\prime}-U_{\rm b+i}}{X_{\rm d}^{\prime \prime}}\right)e^{-\frac{i}{\
u_{\rm d}}}+\left(\frac{F_{\rm g0}^{\prime}-U_{ \rm b+i}}{X_{\rm d}^{\prime\prime}}-\frac{F_{\rm g0}^{\prime}-U_{\rm b+i}}{X_{ \rm d}^{\prime}}\right)e^{-\frac{i}{\
u_{\rm d}}}-\frac{U_{\rm b-i}-U_{\rm b+i}} {X_{\rm d}^{\prime\prime}}e^{-\frac{i}{\
u_{\rm d}}}\cos(\omega t+\delta_{0}) \tag{13}\]  
where \(i_{\rm d}\) is the d-axis current; \(X_{\rm d}\) is the d-axis reactance; \(X^{\prime}{}_{\rm d}\) is the d-axis reactance; \(X^{\prime}{}_{\rm d}\) is the d-axis transient reactance; \(E^{\prime}{}_{\rm q0}\) is the transient potential; \(E{}_{\rm q0}\) is the no-load potential; \(U_{\rm 0}\). is the generator terminal voltage before the fault occurs; \(U_{\rm 0+i}\) is the generator terminal voltage after the fault occurs; \(T^{\prime}{}_{\rm d}\) is the d-axis transient short circuit time constant; \(T^{\prime\prime}{}_{\rm d}\) is the d-axis sub-transient short circuit time constant; \(T_{\rm a}\) is the stator winding time constant; \(\omega\) is the angular velocity; and \(\delta_{0}\) is the initial phase before the fault occurs.  
It can be seen from Equation (13) that the instantaneous reactive power output after the fault occurs is mainly determined by the voltage change amplitude and the sub-transient reactance. The response time of reactive current output based on its physical characteristics is generally 30-40 ms, and the response time of reactive current output by excitation control is generally several hundred milliseconds. Therefore, the reactive power output characteristics of the two parts should be comprehensively considered when adopting the state-of-art synchronous condenser as a reactive power compensation device.