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

Abstract

For high-power industrial applications that require direct current, thyristor rectifiers are widely used to convert alternate current to direct current. However, nonlinear current loads introduce harmonics into the grid. These harmonics may reduce power quality, damage equipment, and lead to difficulty complying with standards such as IEEE 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 rectifier topologies in the 1-50MW load range. Using PLECS, several different passive filter types were dimensioned. Total Demand Distortion, power factor, losses and the practical complexity of these filters were then evaluated. The results show that increasing the pulse number of the rectifiers significantly reduces 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 with smaller and less complex filter banks. The study indicates that overall, higher-pulse rectifier configurations with modest filtering are an adequate and practical approach to achieving IEEE 519 compliance in high-power non-linear load operations. Extensive filtering of lower-pulse systems, while capable of achieving acceptable distortion levels, may be less attractive due to the size, cost and complexity of filtering.