Black Start Capability of Synchronous and Asynchronous Hydro Generating Units

Power system restoration using synchronous machine (SM)-based hydro generating units (HGU) is well-established in hydropower plants (HPP) worldwide. In contrast, the adoption of doubly fed induction machine (DFIM)-based units, commonly used in wind power, is limited in hydropower applications. Although there is some research on black start restoration with DFIM-based wind power units, comprehensive hydropower studies are lacking. This study addresses the challenges of black start restoration in HPPs following the upgrade of the Tehri HPP transmission line (TL) from 400 kV to 765 kV, significantly increasing the reactive power requirements for charging the extra high voltage (EHV) TL. Due to this upgrade, a single HGU at Tehri HPP cannot meet these demands, necessitating new strategies. The Tehri HPP is used as a case study. This research involves mathematical modelling and simulation to compare black start restoration capabilities using large-rated 250 MW, SM and DFIM-based HGUs. The results show that the 765 kV EHV TL requires significantly higher charging MVAr than the 400 kV EHV TL. DFIM-based HGUs provide faster voltage build-up and better control over voltage and speed, while SM-based HGUs require less backup power for restoration. Real-time validation using an OPAL-RT OP4200 eFPGA-based Hardware-In-the-Loop (HIL) setup demonstrates that the total black start time for an SM-based HGU is around 4000 s, consistent with actual hydropower operations. The study proposes solutions such as parallel HGU operation, additional backup power, adding shunt reactors and intermediate substation charging to optimize black start strategies in upgraded high-voltage systems.