Date of Award
Doctor of Philosophy (PhD)
Collins, Edward R.
The elevated neutral-to-earth voltage (NEV), and the related phenomenon called stray voltage, is analyzed in multigrounded distribution systems. Elevated NEV is typically caused by fundamental frequency currents returning to the source via the neutral conductor and earth. However, harmonic distortion is also found to contribute to elevated NEV. A multiphase harmonic load flow algorithm is developed to examine the effects of various factors on the NEV, including unsymmetrical system configuration, load unbalance and harmonic injection. To fulfill this objective, the system modeling is adapted to include the neutral conductor into the component equivalent circuit. The overhead transmission line is remodeled in detail based on the Carson's line theory. The neutral and earth return paths are represented explicitly in the model. Additionally, the harmonic analysis, embedded in the load flow algorithm, is demonstrated using a single-phase uncontrolled capacitor-filtered rectifier model.
The algorithm and the associated models are tested on an IEEE example system. The load flows are performed under different system and load conditions, including both linear and non-linear loads. The accuracy of the developed algorithm is verified by comparing the model predictions with field measurements on real multigrounded distribution feeders.
Unbalanced loading and system asymmetry are observed to be the important source of the elevated NEV. The magnitude is shown to be a function of the earth resistivity, residual return current, feeder length and the neutral conductor size. Additionally, the harmonic injection from nonlinear loads tends to deteriorate the NEV by injecting additive triplen harmonic current into the return path. Three-phasing of single-phase laterals, a common distribution system upgrade method, is examined for its effectiveness to mitigate elevated NEV when the system has harmonic loads. As expected, it is found that three-phasing is effective when the system has low distortion. However, three-phasing is less effective for alleviating NEV when the feeder is loaded with an appreciable amount of single-phase non-linear devices.
Jiang, Jian, "Elevated Neutral-to-Earth Voltage in Distribution Systems Including Harmonics" (2006). All Dissertations. 42.