Investigation of DC-Link Stabilization Through BESS for DFIM-Fed Hydropower Unit Under Grid Faults

Reliable operation of large-rated doubly fed induction machine (DFIM) fed hydropower units during grid faults, including voltage dips, unbalanced voltage, and subsynchronous oscillations, is enforced via updated grid regulations. This article aims to improve the stability of a large-rated hydropower DFIM unit by employing a battery energy storage system (BESS) in the dc-link of the back-to-back power converter (BBPC) unit. The control strategy for the BESS has been developed to stabilize the dc-link voltage of the BBPC unit during grid faults and to ensure continuous operation of the test unit. The size of BESS is analyzed based on grid regulations and compliance with the existing BBPC protection devices (dc-link braking choppers and crowbar protection circuits). Through time domain simulation studies, the effectiveness of the proposed method is investigated in a 250-MW variable speed hydropower unit under various grid fault scenarios. Finally, a practical demonstration of the proposed method is carried out in a scaled-down DFIM test facility.