Small Signal Stability Analysis of Damping Controller for SSO Mitigation in a Large Rated Asynchronous Hydro Unit

In a recent large-rated doubly fed induction machine (DFIM)-based asynchronous hydro unit (AHU), speed control through the rotor-side power converters and active power control through the hydro governor unit are preferred by the plant operators in view of the water hammering effect and protective equipment. In addition, when linked to the series-capacitor compensated transmission line, the DFIM-fed asynchronous hydro unit is experiencing increasing sub-synchronous oscillation (SSO)-based power system stability difficulties. To investigate the sub-synchronous oscillations of this machine, a DFIM coupled to a series capacitor compensated transmission line performed a small-signal frequency stability analysis, which would be presented in this paper. Initially, the models of transfer function for the hydro governor unit, DFIM, and rotor side power converters are constructed. Based on the models obtained, small-signal stability analysis and the eigen value analysis are then performed. Afterward, an enhanced frequency system response model for the DFIM based AHU integrated with the series capacitor compensated transmission line is presented. Furthermore, the regulation of the converter controller optimization/damping controller to mitigate the sub-synchronous oscillations using a genetic algorithm is proposed. The stability analysis of the power system that incorporates a 250 MW DFIM-fed asynchronous hydro unit is examined using the combined model. The analysis of small signal stability is validated using 250 MW time domain simulation results and a 2.2 kW experimental test setup.