Tainan Science-Based Industrial Park (NSIP) is an important Hi-Tech park of high-valued productions. The supplied power quality will influence profits of these manufacturers directly. Owing to the large amount of underground power cables used, this thesis introduces the transmission structure of NSIP, including the substation and nearby transmission lines. The line model and the voltage variation caused by the reactive power are investigated. The Matlab and ASPEN-Oneliner packages are used to simulate the influence of reactive power under different line parameters, reactor, load variation on voltage variation and line loss.