Mathematical Analysis on Harmonics Mitigation Using a Phase-Shifted Transformer based Twelve-Pulse Variable Frequency Drive

Harmonics are among the serious problems that affect the power quality of an electrical system. However, harmonics nowadays are inevitable due to the ever-increasing number of Variable Frequency Drives (VFDs), Uninterruptible Power Supply (UPS) and other non-linear loads. In this regard, IEEE519 Standard specifies the allowable Total Harmonic Distortion (THD) and Total Demand Distortion (TDD) at a given point of common coupling (PCC). In this practical research, we have performed an extensive mathematical analysis to investigate the efficiency of a harmonic filter based on two transformers with 30 electrical degrees phase difference in reducing THD and TDD and maintain these parameters within the acceptable range defined by the standard. Analysis was carried out using actual data collected from the King Fahad Medical City Faculty of Medicine (KFMCFOM) Building Riyadh, Saudi Arabia. The results demonstrated that the phase-shifted transformer-based method removes some of the harmonic components caused by six-pulse VFDs and acts as a twelve-pulse VFD when connected with phase-shifted transformers in parallel. Accordingly, implementing the proposed method resulted in an over 50% reduction of THD and TDD values of the test system.