Analysis of Turbine Generator Shaft Torsional Vibration Caused by Self-commutated Converters

Abstract

SUMMARY This paper presents a simple analysis tool for torsional vibration of the rotor shaft of a turbine generator which is interconnected with a power system with self-commutated converters. The evaluation is based on analysis of measurement data in the field and instantaneous value-based power system simulation. A number of power converters with large capacity are installed in industrial power systems. Especially, interharmonics originating from self-commutated converters cause torsional vibration at the rotor shaft of rotating machines. The interaction between rotating machines and converters is known as sub-synchronous torsional interaction. The resonance between interharmonics from converters and the natural frequency of the rotor shaft causes large torsional vibrations, leading to shaft damage. In this paper, the influence of interharmonics on turbine generators is investigated. If there were air gap torque fluctuations with a frequency near the natural frequency caused by interharmonics, which have frequencies below 100 Hz of the sideband wave caused by asynchronous pulse width modulation, the rotor shaft may be damaged by the torsional vibration. In this paper, the emerging condition of the torsional vibration and the impact on the generator are investigated.