A Comprehensive Approach to Flexible LVRT Strategies for Inverter-Based PPMs Enhancing Voltage-Support, Overcurrent Protection, and DC-Link Voltage Quality

This paper focuses on enhancing the resilience of Power Park Modules (PPMs), connected to the grid via power electronic units, under asymmetrical voltage sag conditions. We particularly address three interconnected constraints crucial for Low-Voltage Ride-Through (LVRT) capability of PPMs: grid voltage support, overcurrent protection, and the DC-link voltage quality. Existing literature often overlooks the holistic consideration of these factors, prompting this study to introduce and compare three flexible LVRT strategies compliant with recent German grid codes. The proposed strategies use distinct approaches to reinforce the LVRT capability of the PPMs. Strategy A employs static gain factors to control positive- and negative-sequence reactive current (PSRC and NSRC) and the negative sequence of the active current (NSAC). Strategy B aims to enhance the DC-link voltage quality by eliminating real power oscillations through the flexible choice of NSAC and gain factors controlling PSRC and NSRC. Strategy C enhances DC-bus voltage quality by eliminating NSAC and employing flexible equal gain factors for PSRC and NSRC, mitigating both real and imaginary power oscillations. Testing on an inverter-based PPM, involving 400 kW PV module and 200 A, storage system, reveals the superior performance of strategy C. Additional tests evaluate Strategy C's performance with var/voltage regulation equipment (On-Load-Tap-Changing transformer), demonstrating stable and promising performance. Through the introduced approaches, the authors believe that this research work is useful for developing future grid codes and improving the safety and reliability of grid-connected power systems.