Ion-Flow Environment of HVDC and Hybrid AC/DC Overhead Lines

For various applications, HVDC overhead lines (OHL) offer technical and economic advantages over conventional HVAC systems. Boosted by increased demand for transmission capacity, the number of HVDC systems has thus steadily risen over the past decades. Corona effects - processes related to partial electric discharges caused by high electric field stresses at conductor surfaces - are an important factor in the overall environmental, technical and economic performance of HVDC OHLs. They include corona losses, audible noise and the drift of ions towards ground or to other conductors. This latter aspect plays an important role in so-called hybrid AC/DC towers, in which HVAC and HVDC systems run in parallel on the same tower. This thesis investigates the fundamental effects that lead to the generation of corona discharges on DC conductors. The influence of conductor surface properties on the corona behavior during wet weather is given particular attention. Furthermore, the corona current coupling between different bundles in bipolar DC and hybrid AC/DC systems is studied in detail. A series of indoor laboratory experiments (monopolar DC, bipolar DC and hybrid AC/DC) form the basis of the presented investigations. Using simulated rainfall and novel imaging methods, the relationship between wetting behavior and corona properties is established. In bipolar DC and hybrid AC/DC measurements, the selective placement of corona sources is used to differentiate between coupling effects caused by Laplace (space-charge-free) fields and mechanisms driven by Poisson (space-charge-enhanced) fields. A key finding of this thesis is that the conductor surface type strongly influences wet weather HVDC corona losses and the drying rate after a rain shower. The latter is important since audible noise is increased during the drying phase. The presented results and methods can be used in the selection or further improvement of OHL conductors for HVDC applications. It was also determined that the local concentration of water drops on the lower side of the conductor influences the corona current coupling in bipolar HVDC systems. Neglecting this directional bias in ion flow simulations was shown to lead to significant prediction errors, in particular for narrow pole separations. Finally, for hybrid AC/DC systems it was shown that DC corona currents in AC and DC conductors are strongly affected by corona emissions on AC conductors. The fundamental reason for this was determined to be the existence of a net DC ion drift from AC conductors to DC conductors. Thus, there exists a bipolar space charge environment between a coronating AC and DC conductor. Neglecting AC corona in ion flow simulations was shown to lead to large under-predictions of the DC corona current in AC conductors. This thesis presents a simple empirical method to account for AC corona in hybrid AC/DC ion flow simulations