Sub-Synchronous Torsional Interaction Study and Mitigation Using a Synchro-Phasors Measurement Unit

In a power plant, sub-synchronous resonance is not encountered very often, but when it occurs, it can cause a very serious problem and severe damage. Many efforts have been investigated to study and hence mitigate a resonance produced between electrical synchronous machines and the electrical grid that may arise for frequencies other than the fundamental one (50 Hz). Natural resonances in the electrical grid incorporating series capacitors can appear for sub-synchronous frequencies and can be both a series and parallel resonance nature. Mitigation techniques are required for a power plant with an extensive turbine-generator string located near a long power transmission line with series capacitors. Due to the severe consequences, power plants that risk sub-synchronous resonance (SSR) may be equipped with appropriate protection. However, if the sub-synchronous resonance frequencies of the network coincide with any of the mechanical frequencies of the turbine-generator shaft, torsional interaction that is called sub-synchronous torsional interaction (SSTI). If the electrical damping for a specific frequency in the network is insufficient or negative in comparison to mechanical damping, it may lead to this sub-synchronous torsional interaction. This phenomenon can be hazardous causing fatigue in the turbine-generator shaft, which results in the failure of the power generation unit. It can also occur due to the interaction between a control system of converters and the turbine generators. In this research paper, a study using different methods of analysis developed by transmission system operators (TSOs) and the manufacturers with one case study is presented. Additionally, different mitigation techniques, such as filtering and damping, are suggested. Furthermore, a demonstration for measuring principles as well as monitoring and protection against SSTI using a synchro-phasors measurement unit has been presented.