Corona Characteristics and Audible Noise of Hybrid AC/DC Transmission Lines

Abstract

The concept of hybrid overhead lines with both alternating current (AC) and direct current (DC) bundles suspended from the same tower combines the benefits of both, AC and DC transmission. While traditional AC transmission allows a densely meshed grid and easy transformation, DC offers bulk transmission with higher efficiency over long distances and additional features of power control. If existing AC infrastructure is converted to such a hybrid tower structure, the electric power transmission capacity can be significantly increased. In times of low acceptance for new overhead line (OHL) projects, this permits to upgrade existing corridors and possibly avoids the construction of new towers. As the conversion is expected to cause only a negligible change to the existing visual impact, the acceptance is assumed to be higher and therefore, construction delays can be reduced to cope with the rising demand for transmission capacity. However, with close proximity of AC and DC bundles, coupling between both systems is unavoidable, causing a possible increase in the corona effects such as audible noise (AN), radio interference (RI), ion current density and electric field at ground. As these phenomena can be perceived as a nuisance and are subject to regulatory limits, they have to be reduced to an acceptable limit. This thesis investigates the AC/DC corona performance with a focus on the audible noise and the influence of coupling phenomena on hybrid tower geometries. A simulation tool is developed to calculate the electric field and predict the audible noise based on empirical formulas. Several series of indoor measurements are conducted using a rain simulator to increase the understanding of the transient corona characteristics during weather changes. It is shown that while the audible noise tends to increase with rain under AC energization, it decreases with rain for DC. This behavior as well as the transition time to steady state is found to be dependent on rain intensity and the applied electric field. The effect of coupling is studied in both in- and outdoor tests. These tests reveal, that the DC bias and AC ripple due to the coupling tend to clearly affect the corona characteristics and the audible noise for both AC and DC corona. While the AC ripple and positive bias generally increase the audible noise level, a negative bias increases the audible noise in most cases, but also causes a reduction in some rare cases, depending on the state of corona and the relative degree of coupling. While an existing prediction model agrees quite well with the case of AC ripple, larger deviations are observed between the experimental data and the simulated curves, especially regarding a negative bias. Therefore a different prediction approach is proposed and further measurements are advised. Finally, different strategies to improve the corona performance are reviewed. Using different bundle arrangements and geometries on a tower, an optimized design is proposed for a typical tower geometry which allows to reduce both magnetic field and audible noise below the initial levels. It is demonstrated that the corona performance and audible noise can be improved through the use of different conductor surfaces. For the case of a hybrid tower conversion, the audible noise can also be significantly reduced using hydrophilic conductors under mixed AC/DC field stresses.