# Fault ride-Through Control Strategy for Variable Speed Pumped Storage Unit with Full-Size Converter
## 1 Introduction  
As the integration of new energy sources into power systems grows, challenges concerning electricity equilibrium, grid management, and safety protocols intensify. Advancements in energy storage technologies offer fresh approaches to mitigating the variability inherent in renewable energy outputs. Among these, pumped storage stands out as the most established, cost-effective, and environmentally friendly solution [1]. It offers capabilities including peak trimming, frequency adjustment, and emergency support, serving a crucial role in power systems. Variable-speed pumped storage units (VSPSUs) are extensively researched for their efficiency, operational flexibility, and swift responsiveness [2; 3; 4]. The VSPSU is categorized into two types: doubly fed and full-sized. Relative to doubly fed units, the full-size converter variable speed pumped storage unit (FSC-VSPSU) provides superior speed control and enhanced capability to handle fault disturbances [5]. With the reduction of full power device costs, FSC-VSPSU will have broader development prospects in small and medium-sized capacity applications [6; 7]. Current research on the FSC-VSPSU has extensively focused on simulation modeling, control strategies, and applications to enhance grid stability and efficiency. Significant work includes developing simulation models for these units to study their behavior under various operational conditions [8; 9; 10]. Researchers have also explored advanced control strategies such as virtual inertia and oscillation suppression to improve dynamic responses and stability in power systems connected with FSC-VSPSUs [11]. Furthermore, methods have been proposed to utilize FSC-VSPSU for smoothing power fluctuations [12]. However,these studies predominantly address the performance of pumped storage under non-fault conditions. The characteristics and control strategies under power grid fault conditions are not covered. The research on the low voltage ride-through (LVRT) strategy of FSC-VSPSU only focuses on the power, voltage, and current changes caused by voltage drop without proposing specific solutions [13]. There have been no reports of the high-voltage fault ride-through (HVRT) strategy of the FSC-VSPSU. Therefore, there is a notable gap in addressing the performance of FSC-VSPSU under fault conditions, particularly the strategies required for effective fault ride-through--a critical aspect for operational resilience. Similarly, many scholars have explored fault ride-through strategies for high-voltage DC systems affected by AC faults. These solutions also fall into two categories: enhancing control strategies and incorporating additional power electronic equipment [14]. Enhancing control strategies often involves adjusting converter control parameters or switching control methods to modulate power output during faults to ensure the high-voltage DC system's stability [15; 16]. Moreover, additional power electronic devices such as flywheel energy storage systems, supercapacitor energy storage devices, or fault current limiters can be used to absorb the unbalanced power during the fault and thus achieve fault ride-through [17; 18; 19]. The FSC-VSPSU also incorporates a back-to-back converter similar to those used in HVDC systems. However, transferring these strategies to FSC-VSPSU systems is not advisable. This is primarily because the distinct application contexts lead to different fault characteristics, and such strategies do not take into account the specific prime mover characteristics. Nowadays, there is a large amount of literature on fault ride-through strategies for direct-drive wind turbines with similar structures. These fault ride-through strategies are divided into two main categories, one by adding additional equipment and the other by improving the control strategy. A dynamic braking resistor can be connected in parallel on the DC side to dissipate unbalanced power and achieve LVRT.