“Understanding Subsynchronous Resonance Using Real Time Digital Simulation and Nyquist Stability Criterion”

Dept: Electrical & Computer Engineering

Chair(s): Dr. Mischa Steurer, Dr. Sastry Pamidi, Dr. Yuan Li

Abstract

Subsynchronous Resonance (SSR) is a unique power systems phenomenon that was relatively unknown until a 1970’s incident at the Mohave Power Station damaged two turbine-generators in the exact same segment of their shafts. This unlikely outcome indicated that the mechanical properties of these shaft segments were somehow related to their failures. Unknown at the time, was that a disturbance in the electrical system created an SSR signal causing rotor torques on the shaft that coincidentally matched its natural oscillation frequency. The result being an increasing energy exchanging loop between the electrical and mechanical sides of the power system ending with shaft failure. This thesis proposes that instability issues like the Mohave case can be identified and corrected early on using an Electromagnetic Transients Program (EMTP) and the Nyquist Stability Criterion. The results prove that the Nyquist plots of the mechanical and electrical gains over the subsynchronous frequency range (0 – 60 Hz) accurately predict the shaft designs at risk of SSR by encircling the point (-1,0) on their graphs. Moreover, simulation outputs confirm that the torque waveforms predicting high-risk shaft segments exceed safe operating limits. The successful demonstration of this predictive process can then be developed to analyze the stability of related phenomena, such as subsynchronous oscillations (SSO), which may impact modern microgrid systems containing power converters.