Coupling phenomenon of torsional modes

The coupling of the torsional modes of a steam turbine-generator (T-G) set is investigated. The impact of generator load, series capacitor compensation level, power system stabilizer, and auxiliary controllers of a static VAr compensator on the torsional coupling phenomenon is investigated. The system eigenstructure is used to develop an analytical formulation for detecting the coupling and determining its strength. The analytical results are verified by detailed digital computer simulation studies, using the BPA´s Electromagnetic Transients Program. The studies indicate that, depending on the network configuration and parameters, two or more of the torsional modes of a T-G set can be coupled. The coupling level can be significantly affected by the controller parameters and feedback signals of the torsional countermeasure devices. As a result of the coupling, a torsional oscillatory mode can excite and even destabilize its corresponding coupling mode(s)