# Fault ride-Through Control Strategy for Variable Speed Pumped Storage Unit with Full-Size Converter
## 2 Structure and Control Strategy of FSC-VSPSU
### Power Generation and Pumping Control Strategies  
There are two control modes under power generation conditions. Namely, the fast power mode and the fast speed mode. The EESG adopts a constant voltage frequency ratio control strategy to control the rotor side voltage [7]. The converter and its typical \(dq\) decoupled control strategy are shown in Figure 3. \(i_{\text{dref}}\) and \(i_{\text{qref}}\) are the reference values of \(d\)-axis and \(q\)-axis current, respectively. \(\theta_{\text{p}ll}\) is the output angle of PLL. \(k_{\text{R}R}\) and \(T_{\text{R}R}\) are the gain of PWM and modulation wave period, respectively. \(k_{\text{pp}}\) and \(k_{\text{ip}}\) are the proportion and integral gains of the d-axis outer loop. \(k_{\text{p1}}\) and \(k_{\text{i1}}\) are the proportion and integral gains of the d-axis inner loop. \(k_{\text{pq}}\) and \(k_{\text{iq}}\) are the proportional and integral gains of the q-axis outer loop. \(k_{\text{p2}}\) and \(k_{\text{i2}}\) are the proportional and integral gains of the q-axis inner loop. Regardless of the operation mode, the GSC always controls the DC voltage. In the fast power mode, power is controlled by the RSC, and speed is controlled by the governor. Conversely, in the fast speed mode, power is controlled by the governor, and speed is controlled by the RSC. Under pumping conditions, the unit does not control the power directly. The guide vane opening is controlled by the governor, and the speed is controlled by the RSC in the pumping mode. Figure 3: Control model of the converter. Figure 2: The principle of the optimal efficiency control strategy. The governor control diagram of the generation conditions is shown in Figure 4. \(k_{\text{w}}\) is speed measurement gain. \(b_{p}\) is the guide vane opening deviation adjustment coefficient. \(k_{\text{p}}\), \(k_{i}\), and \(k_{d}\) are speed control proportional, integral, and differential gains. \(k_{\text{pm}}\), \(k_{\text{im}}\), and \(k_{\text{dm}}\) are the proportional, integral, and differential gains of the servo mechanism. \(Y_{\text{pid}}\) is the output of the speed of power control. PID, \(T_{\text{o}}\), and \(T_{\text{C}}\) are the water gate opening and closing times constant. \(T_{2}\) is the power delay time. \(k_{\text{pg}}\) and \(k_{\text{ig}}\) are the proportional and integral gains of power control. \(H_{0}\) and \(P_{\text{m0}}\) are the initial water head and mechanical power. \(P_{\text{m}}\) is the mechanical power. \(T_{\text{w}}\) is the rated water hammer efficiency time constant. Figure 4a is the generator speed control block diagram in fast power mode. Figure 4b is the power control block diagram in fast speed mode. The biggest difference between the two modes is that, in the fast speed mode, the speed control loop is removed and replaced by the active power controller. Figure 4c shows the governor servo mechanism. As shown in Figure 4d, a turbine model considering the initial load and water head is used [6]. The governor system and the equivalent simulation model of the pump in the pumping conditions are shown in Figure 5. Under the pumping state, the pump turbine only accepts the guide vane opening command. In Figure 5, \(H_{s}\) is the initial water head, \(H_{\text{loss}}\) is the water head loss, \(H_{d}\) and \(Q_{d}\) are the dynamic water head and flow, \(\eta\) is the pumping efficiency, \(\omega_{g}\) is the unit speed per unit value, and \(f_{e}\) is the total loss coefficient. \(f_{e}\) is shown in Equation (1),  
\[f_{e}=(y_{\text{max}}-y)/y \tag{1}\]  
Figure 4: Governor system diagram in the generation conditions. (**a**) Fast power mode. (**b**) Fast speed mode. (**c**) Servo mechanism. (**d**) Turbine model. where, \(y_{\rm max}\) is the maximum guide vane opening.