# Fault ride-Through Control Strategy for Variable Speed Pumped Storage Unit with Full-Size Converter
## 4 Fault Ride-Through Control Strategies
### LVRT Strategy in Generation Condition  
Under power generation conditions in fast power mode, the RSC rapidly adjusts real power, often within hundreds of milliseconds, for swift power reduction, as shown in Figure 7. The DC voltage deviation undergoes processing through a dead zone and limiting link, adjusted by the PI controller to output the necessary power regulation, which then integrates into the power reference to form a new regulation command. Moreover, the GSC provides reactive power during grid voltage drops [24]. To protect against DC overvoltage in engineering practice, a crowbar circuit is often necessary [22]. Figure 6: The technical requirements for fault ride-through of wind turbines and FSC-VSPSU. (**a**) LVRT. (**b**) HVRT. Figure 7: The fast power reduction strategy. However, in fast speed mode, the aforementioned strategies become inapplicable for the FSC-VSPSU. The speed of power regulation is slower in this mode as it is governed by the turbine governor, not allowing for rapid power reduction. Here, the unit speed is managed by the RSC and can be adjusted within milliseconds. Given the greater inertia of FSC-VSPSU compared to wind turbines, employing rotor energy storage is particularly effective at absorbing excess turbine-side power, thus mitigating DC overvoltage. When the generator speed increases from \(n_{0}\) to \(n_{1}\), the rotor stores part of the kinetic energy, which can be expressed as:  
\[\Delta E_{\text{k}}=E_{\text{k}0}-E_{\text{k}1}=E_{\text{k}0}\left[1-\left( \frac{n_{1}}{n_{0}}\right)^{2}\right] \tag{14}\]  
where \(E_{\text{k}0}=1/2T_{\text{j}}{n_{0}}^{2}\). \(E_{\text{k}0}\) is the initial kinetic energy, and \(E_{\text{k}1}\) is the kinetic energy corresponding to the rotational speed \(n_{1}\). \(\Delta E_{\text{k}}\) is the decreased kinetic energy. \(T_{\text{j}}\) is the rotational inertia. Figure 8 shows the rotor energy storage control strategy. The DC voltage deviation passes through a dead zone and limiting link, and after being adjusted and limited by the PI controller, the corresponding speed adjustment amount for rotor kinetic energy adjustment is output. This is superimposed on the optimal speed reference to form a new speed adjustment instruction. Due to the limited rotational inertia and the finite capability to increase speed, the effectiveness of reducing input power through rotor energy storage is constrained. Directly reducing the output power of the prime mover would be more effective; however, water has a significant inertia with a time constant typically between 2-5 s. The mechanical governor also adjusts slowly, fully opening the guide vanes in about 12-15 s, making rapid power reduction challenging. However, the water hammer effect offers a reverse regulation characteristic of power, which can be leveraged to moderately reduce mechanical power. This approach is termed the power anti-regulation strategy (PARS). Figure 9 illustrates the PARS, which is implemented by integrating an additional guide vane opening adjustment into the existing power regulation PID in Figure 4b. Initially, a step signal is applied at the onset of the fault ride-through, instantly increasing the guide vane opening. This increment, due to the water hammer effect, reduces the mechanical power, aiding in mitigating DC overvoltage. After peaking in power reversal, the guide vane instruction is progressively decreased to zero, utilizing the water hammer effect for LVRT control. However, frequent activation of this strategy may cause accelerated mechanical wear of the guide vanes. Figure 8: The rotor energy storage control strategy. Figure 9: The power anti-regulation control strategy. Simultaneously, during the LVRT process, the GSC can inject reactive current to support grid voltage. Consequently, activating reactive current injection control is essential.