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Damping subsynchronous resonance using HVDC supplementary controls.

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ABSTRACT: In power transmission lines, series capacitors are used to increase power transfer capability. However, series capacitors are the primary source of subsynchronous torsional oscillations of nearby turbogenerator shafts. These electronic components in transmission lines expose the turbogenerator system's torsional dynamics to a resonance phenomenon. Typically, this phenomenon appears in the form of supersynchronous and subsynchronous resonance. Subsynchronous resonance is often the most dangerous for its harmful effects, leading to shaft failure and power system instability. It differs from supersynchronous resonance, which does not adversely affect system stability. A turbogenerator set commonly consists of natural modes with frequencies smaller than the power system rated frequency. When the torsional frequencies resulting from the transmission line's series capacitors affect the turbine-generator shaft's operation by coinciding with one of its natural frequencies, they induce the phenomenon known as subsynchronous resonance. Supersynchronous resonance refers to torsional oscillations whose frequencies exceed the nominal frequency of the electrical network. Researchers have used many approaches to solve the subsynchronous resonance phenomenon in power systems. This research focuses on exploiting supplementary controls of a parallel HVDC system to address the subsynchronous resonance problem. However, owing mainly to the converter's current regulators and many other parameters, the HVDC system can also potentially induce subsynchronous oscillation on nearby turbine-generator shafts. This work focuses on a power system comprising a modified IEEE First Benchmark Model to incorporate a parallel HVDC link. This power system is subjected to torsional instability from either the resonant line AC line, HVDC controls, or both. Thus, this study contrasts the effectiveness of single-mode and multimodal damping controllers when the turbogenerator shaft's torsional dynamics undergo instability from the parallel AC-DC system. Furthermore, this thesis utilizes the time domain-based test signal and phase correction methods to design the supplementary subsynchronous damping controllers. These two approaches allow obtaining the optimal parameters of the damping controllers. The controller consists of a Power System Stabilizer to control the inertial mode and Synchronous Damping Controller to control other unstable torsional modes to achieve positive damping. This research uses the FFT analysis to assess the performance of supplementary HVDC damping controllers on the torsional dynamics of the turbogenerator shaft in the parallel AC-DC system.


Masters Degree. University of KwaZulu-Natal, Durban.