Analysis of Sub-Synchronous Oscillations in Grid-Connected Wind Farms and Hybrid PV/Battery Systems

This paper presents methods to evaluate and analyze sub-synchronous oscillations (SSO) in wind energy systems utilizing doubly-fed induction generators (DFIG) and hybrid photovoltaic (PV)/battery energy storage systems (BESS) in series-compensated electrical networks. SSOs are evaluated on four test systems depicting the Induction Generator Effect(IGE), Sub-synchronous Torsional Interactions (SSTI), and Sub-Synchronous Resonance (SSR). Simulations conducted using PSCAD/EMTDC reveal how variations in series compensation, inertia constants, stator resistance, and Short Circuit Ratio (SCR) can cause SSO, impacting grid stability. Specifically, IGE is examined under different stator resistance values (0.01 pu and 0.015 pu), and torsional interactions are evaluated by adjusting inertia constants. Additionally, the study examines the role of integrated PV and BESS system controls in mitigating SSOs. The voltage harmonic injection method analyzes inverter harmonics across different dispatch levels, SCR levels, and power factors. A comparative analysis between traditional series capacitors and Thyristor-Controlled Series Reactors (TCSR) indicates that TCSR provides superior damping, reducing oscillatory amplitude in active and reactive power by up to 30 percentage points. With the increase in compensation levels, the resonant point shifts to the sub-synchronous region (20-40Hz). Moreover, inaccurately modeled active and reactive power controls can induce undamped oscillations in the sub-synchronous region depicted by a negative resistance in this region, affecting radially connected, series-compensated lines.