Suppression of Sub-Synchronous Resonance and Torsional Oscillations in Doubly Fed Induction Generator Based Wind Turbines

Abstract

This thesis proposes novel approaches for mitigating Sub-Synchronous Resonance (SSR) and torsional vibrations in Doubly Fed Induction Generator (DFIG) based wind turbines. The control approach proposed for SSR mitigation is based on the mechanism of SSR which is identified as being due to the unstable electrical dynamics in the DFIG rotor circuit. It is then reasoned that this could be rectified if the problematic dynamics were eliminated. This is achieved using Sliding Mode Control (SMC) which collapses the rotor dynamics into algebraic equations and constrains them into following predefined reference currents, alleviating the SSR issue. The stability of the remaining dynamics are analysed and evaluated to be stable. This method is also extended to cover cases where parameter variations may be present due to variations in operating conditions or machine loading conditions. The proposed torsional vibration mitigation method uses the generator and turbine speeds to calculate the twisting speed and twist angle between the generator mass and the turbine mass of the drivetrain. This information is then used to design a sliding surface which when reached converges the twist angle to a prescribed value. The relationship between the drivetrain twist angle, drivetrain stiffness and torque is used to compute a prescribed drivetrain twist angle for a given torque value. This relationship is used to express a Maximum Power Point Tracking (MPPT) torque reference as a twist angle reference. The controller then enforces and maintains this twist angle on the drivetrain. Due to this operation, both torsional vibration suppression and torque control for MPPT can be achieved with a single controller, in contrast to previous methods which required separate controllers for MPPT and torsional vibration suppression. The proposed solutions for SSR mitigation and torsional vibration suppression are tested by implementing the control algorithms on the Rotor Side Controller of the detailed DFIG model provided in the RTDS platform. Simulation results under varying operation conditions validate the efficacy of the proposed methods.