Abstract
The increased use of computers and other nonlinear loads is beginning to effect the power quality of 208/120V three phase four wire distribution systems. Office buildings, medical facilities and factories that employ large numbers of these nonlinear loads are experiencing excessive triplen harmonic currents flowing in the distribution systems neutrals. Primarily third harmonic, the presence of these triplen harmonic currents is tied to wiring failure, transformer overheating, and an overall decrease in distribution system efficiency. In order to determine how prolific the problem of excessive triplen harmonics is a survey of Texas A&M office buildings is undertaken. The results of this survey along with some typical current and voltage waveforms is presented. This thesis then proposes two new techniques to help deal with extensive triplen harmonic currents in neutral conductors. The first approach is a simple passive filtering technique which directs any excessive triplen harmonic currents away from the distribution transformer and back to the load. This technique employs an E-core shell type transformer as the primary filtering device. The efficiency of this technique is observed under balanced and unbalanced loading conditions and simulations as well as experimental results are completed in order to validate the design. The second approach is an active technique that employs the same E-core transformer as the passive approach. In addition to the transformer, the active approach also employs a two switch PWM controlled active filter. The closed loop control of the active filter guarantees cancellation of neutral current harmonics and drastically improves the overvall system performance by eliminating the possibility of transformer overheating due harmonics. Simulations and experimental results of the filter operation is also presented and discussed. Filter operation under balanced and unbalanced loading conditions is examined and appropriate design examples are provided.
Packebush, Paul Norman (1994). New techniques to deal with the problem of excessive neutral currents in buildings containing large numbers of computers and other nonlinear loads. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1994 -THESIS -P119.