# Fault ride-Through Control Strategy for Variable Speed Pumped Storage Unit with Full-Size Converter
## 6 Conclusions  
This paper analyzes the characteristics of the FSC-VSPSU under low voltage and high voltage fault conditions. Based on this analysis, the fault ride-through requirements and control strategies are proposed. The dominant factor causing DC overvoltage during low-voltage events is power imbalance. The high-voltage process is mainly affected by the overmodulation of FSC and the power imbalance. In order to provide better support for the renewable energy or the power grid, it is recommended that the fault ride-through ability of the FSC-VSPSU is stronger than that of wind turbines. In terms of LVRT, the critical issues include power imbalance and DC overvoltage or undervoltage. Under power generation conditions, adopting a combination strategy of power droop, reactive power compensation, and crowbar circuits is advocated. In fast-speed mode, a strategy integrating rotor kinetic energy regulation, power reversal, reactive power compensation, and crowbar circuits is suggested. Under pumping conditions, it is prudent to implement low power protection strategies. The application of the proposed LVRT strategy enables the FSC-VSPSU to handle low voltage faults down to a minimum of 0.15 pu, actively bolstering the grid voltage and reducing potential peak DC voltage to a stable 1.1 pu. In terms of HVRT, the primary issues involve the DC overvoltage caused by the overmodulation of the converter and the power imbalance. The strategies for power generation conditions are still applicable. Under pumping conditions, fast speed regulation can effectively solve the problems of power imbalance and DC overvoltage during fault processes. It is recommended to adopt a combination strategy of fast speed regulation, reactive power compensation, and crowbar circuit. Implementing the proposed HVRT strategy ensures that the FSC-VSPSU can endure high voltage faults up to 1.35 pu, actively supporting the grid voltage while preventing overmodulation, and maintaining DC voltage within a safe range of 1.1 pu. Figure 22: Simulation results of the HVRT (in pumping state). (**a**) Rotor side active power. (**b**) Rotor side reactive power. (**c**) Grid side active power. (**d**) Grid side reactive power. (**e**) Unit speed. (**f**) DC voltage. (**g**) Water head. (**h**) Water flow rate. **Author Contributions:** Conceptualization, H.S. and L.D. ; methodology, Y.W. ; software, X.Z. and P.P. ; validation, H.S., G.C. and Y.L. ; formal analysis, H.S. ; investigation, X.Z. and J.L. ; resources, H.S. ; data curation, H.S. ; writing--original draft preparation, H.S. and J.L. ; writing--review and editing, L.D. and J.L. ; visualization, H.S. ; supervision, H.S. ; project administration, L.D. ; funding acquisition, L.D. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was supported by the Science and Technology Project of State Grid Sichuan Electric Power Company of China (521997230004). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data presented in this study are available in article. **Conflicts of Interest:** The authors declare no conflicts of interest. The funders had no role in the design of this study, in the collection, analysis, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.