# Fault ride-Through Control Strategy for Variable Speed Pumped Storage Unit with Full-Size Converter
## 1 Introduction
Similarly, energy storage devices such as supercapacitors can absorb the unbalanced power during voltage dips and release power after grid voltage is restored, ensuring the unit does not operate off-grid [20; 21]. A coordination control strategy for the rotor side converter (RSC) and grid side converter (GSC) has been proposed in [22; 23; 24], which regulates the DC bus voltage during grid faults and improves the LVRT capacity of permanent magnet wind turbines. The unbalanced power can also be stored by changing the rotor speed to stabilize the DC voltage and keep output power balanced. However, it may cause wind turbine overspeed, which affects the safe and stable operation of units [25]. In [26], a control strategy for GSC is proposed to avoid overmodulation and DC line overvoltage, thereby achieving HVRT. In [27; 28], a super capacitor has been proposed to stabilize DC line voltage and enhance HVRT capability. In [29], a control method is presented that incorporates a parallel GSC alongside the FSC to absorb the capacitive reactive current from the power grid, thus lowering the grid voltage. All the above literature is related to the study of fault ride-through strategies for direct-drive wind turbines. Due to differences in the inherent characteristics of the units, fault ride-through requirements, and control strategies, the above fault ride-through strategies may not be applicable to the FSC-VSPSU. Compared to wind turbines, the working power of the FSC-VSPSU can be generated by grid requirements and does not require constant operation at rated power. Additionally, the water turbine is affected by a water hammer and is a nonlinear minimum phase system. The water turbines have larger power, faster speed, greater rotational energy storage capacity, and a wider range of speed operation. Moreover, the power flow of the FSC-VSPSU is bidirectional, which is different from the unidirectional flow of the grid-connected wind turbine systems. Under pumping conditions, FSC-VSPSU has the ability to quickly generate load regulation, thus requiring a certain degree of fault ride-through capability. The FSC-VSPSU often operates complementary to new energy sources to support the operation of the power grid and new energy sources. Therefore, the unique operational dynamics and requirements of FSC-VSPSUs necessitate fault ride-through strategies that differ from existing methods tailored for wind turbines. To fill the gap, this paper uniquely contributes to the field by proposing comprehensive fault ride-through strategies tailored specifically for the FSC-VSPSU. The key contributions of this research are outlined as follows:  
1. The operating characteristics of the FSC-VSPSU during low and high voltage conditions in power systems are studied. Power imbalance or overmodulation of the converter are dominant factors causing DC overvoltage. 2. The fault ride-through requirements of the FSC-VSPSU are proposed. To maximize support for renewable energy and the power grid, a stronger fault ride-through capability is required compared to that of wind turbines. 3. A combined control strategy for fault ride-through of the FSC-VSPSU is proposed. This strategy includes fast power droop, reactive power compensation, rotor kinetic energy regulation, power reversal, low-power protection, a crowbar circuit, etc. Unique fault ride-through combination strategies are applied under various operating conditions and control modes. The remaining part of this paper is structured as follows: Section 2 presents the structure and control strategy of the FSC-VSPSU. Subsequently, Section 3 analyzes the low voltage and high voltage fault characteristics of the FSC-VSPU under different operating conditions. Section 4 proposes the LVRT and HVRT strategies under different operating conditions. The effectiveness of the fault ride-through strategies is verified through simulation analysis in Section 5. Section 6 draws research conclusions.