Assessment of Torsional Resonance in Turbine-Generator Shaft Due to Multiple EAF Operation

Abstract

The oscillating torques between different mass sections of turbine-generator shaft are amplified at resonance, which indicates that the oscillating torques absorb more energy from the system. This may cause violent swing motion creating torsional stress and even catastrophic failure in the mechanical system. The torsional stressing may arise as a result of torsional resonance induced by pulsating components in the electrical torque of the generator. Hence, the interaction between the electrical system and the turbine-generator has to be analysed in advance to assess the severity of torsional stress to the shaft system to ensure undisturbed operation.

The frequencies of torsional oscillations are in the subsynchronous range. There are many sources of subsynchronous frequencies in the electrical network such as capacitor compensated transmission line, EAF loads, load-commutated inverters, PWM voltage-source inverters etc. These equipments induce pulsating components in the air-gap torque of synchronous machine and through which the torsional oscillating torques are amplified during torsional resonance.

The EAF load causes power quality problems such as flicker, voltage imbalances, harmonics and interharmonics. If the subsynchronous frequencies created by EAF leads to resonance, the respective matched torsional modes of the turbine-generator system are excited causing torsional oscillating torques amplification. The magnitude of torsional resonance depends on the proximity of stimulating frequency and natural frequency, on the magnitude and duration of the excitation and on the damping of the excited natural mode.

Torsional oscillating torques amplification due to multiple EAF operations at the same point of common coupling (PCC) or at close proximity has not been studied yet. Investigation of torsional excitation due to multiple EAF operations is the main focus of this thesis.

In this work, the three-phase EAFs are modeled using MATLAB/Simulink from mathematical model of electric arc, where the resistances of the EAFs varied deterministically in the frquency range of 1.2-45.5 Hz to meet the actual characteristics of EAF.