A General Approach of Damping Torsional Resonance Modes in Multimegawatt Applications

In large-drive applications, load-commutated inverters (LCIs) are one of the most used technologies mainly because of their excellent reliability records. However, LCIs are known to generate interharmonics. They can interact with the mechanical system at torsional natural frequencies of the rotating train, on both the inverter and rectifier sides in weakly connected power systems, such as offshore oil and gas platforms. These interactions can lead to accelerated shaft fatigue, lifetime reduction, gear damage, and system blackouts. On the other hand, pulsewidth-modulated (PWM) voltage-source inverters (VSIs) are known to produce less torque ripple compared to LCIs. Consequently, VSIs are supposed to be less subject to exciting torsional resonances in mechanical shaft systems. Even with reduced torque ripple, mechanical failures have been consistently reported due to motor air-gap torques supplied by PWM drives. This paper is focused on solving torsional vibration issues. Due to system uncertainties, these issues cannot be excluded in the design phase of LCIs and VSIs for high-power applications. With regard to LCIs, the dc-link inductance is used as an integrated energy-storage unit. Motor and generator interactions are therefore decoupled. Excited eigenmodes on the grid side are damped with the rectifier; the inverter is driving the variable-speed motor. This approach is also successfully applied to damping resonance modes in the motor side. Simulation and selected experimental results on a 30-MW LCI system are provided to validate the proposed design approach. As for VSIs, a more general approach of damping or controlling excited shafts is proposed. This method is successfully applied using the dc-link capacitor as an integrated energy-storage source. This approach is used to optimize the design of new systems; otherwise, it is used to improve the performance of existing systems with minor modifications.