Complexity of Thyristor-based Rectifier Operation and Optimal Harmonic Mitigation Strategies

Sammanfattning

For high-power industrial applications that require direct current, thyristor rectifiersare widely used to convert alternate current to direct current. However, nonlinearcurrent loads introduce harmonics into the grid. These harmonics may reduce powerquality, damage equipment, and lead to difficulty complying with standards such asIEEE 519. This thesis investigates the effect that different rectifier topologies, operation,and filtering solutions have on mitigating harmonics.This thesis sets out to evaluate harmonic mitigation strategies for various rectifiertopologies in the 1-50MW load range. Using PLECS, several different passivefilter types were dimensioned. Total Demand Distortion, power factor, losses andthe practical complexity of these filters were then evaluated.The results show that increasing the pulse number of the rectifiers significantlyreduces the amount of harmonics that occur and the filter requirements. A 6-pulse rectifier required extensive and complex filtering to achieve acceptable results,while 12- and 24-pulse rectifiers achieved significantly lower harmonic distortion withsmaller and less complex filter banks. The study indicates that overall, higher-pulserectifier configurations with modest filtering are an adequate and practical approachto achieving IEEE 519 compliance in high-power non-linear load operations. Extensivefiltering of lower-pulse systems, while capable of achieving acceptable distortionlevels, may be less attractive due to the size, cost and complexity of filtering.