Solutions for Interharmonics in Variable Speed Drives for LNG Plants

Abstract

Variable speed drives can cause components of any frequency known as interharmonics. Large variable speed drives in the order of tens of megawatts can replace Gas Turbines in driving cooling compressors in a Liquefied Natural Gas Plant. The drives used in this application are among the largest drives that are built today. As a consequence the effect of the interharmonics is also much larger than in other applications. These interharmonics can cause torques on the resonance frequencies of the Gas Turbine Generators used to produce the electric power. Without mitigation this can cause fatigue or even catastrophic failure of the shaft. Reliability is of utmost importance as every day a plant is down can cost tens of millions of Euros due to lost revenue. First the basic principles including the creation and the effect of interharmonics are thoroughly researched. Hereafter different solutions, both new and existing, are researched. The most promising solution is first tested on small scale to find the most effective way to limit the torque caused by resonance. A model of a full scale plant is built to verify its effectiveness.