The subject of the doctoral dissertation extends to the field of transient overvoltages with steep fronts and compactions of 400 kV overhead lines for the Slovenian area, where, as of now, this is an unused tool. Compacting reduces the tower head (or increases it, depending on the purpose) of the overhead line tower and consequently brings the phase conductors closer. This introduces changes, especially at the highest volage levels, that can directly impact the number of insulator flashovers on the overhead line, caused by transient overvoltages. These are categorized into those caused by switching overvoltages and atmospheric discharges, which are crucial in addressing the 400 kV insulation level. Based on minimal clearances and interphase distances, it is necessary, from the standpoint of withstand voltages, to verify the distances and geometry of the flashover of the double-circuit compacted overhead line with supporting insulators.
Regarding transient overvoltages with steep fronts, which can lead to insulation breakdowns and flashovers, special attention was given to lightning strikes as the central factor. For each area where the overhead line is incorporated, it is essential to verify key parameters such as lightning current amplitude, frequency of occurrence, and current waveform. To analyse these parameters, advanced systems for the localization of atmospheric discharges were used, such as the SCALAR system in Slovenia. This provided insight into the classification of atmospheric discharges and the ability to determine the duration of the phenomenon itself, with key data on lightning characteristics needed for insulation coordination analyses.
Lightning strikes on the overhead line can conclude in three ways, but only two are prominent and must be considered: in the case of a direct strike on a phase conductor or on a shield wire. Therefore, various approaches to concluding lightning strikes on objects were analysed, as they define the object's exposure to direct strikes.
The behaviour of the overhead line under the influence of external factors is based on electrogeometric models, analytical tools used to explore lightning strike possibilities. These models rely on the concept of a striking distance. The leader advancing from the cloud towards the ground must approach an object on the ground to a critical distance (striking distance) necessary for the final breakdown and return stroke. Due to lightning strikes and overvoltages, the potential changes both on the conductor side and on the overhead line tower, which is normally unenergized but grounded. Thus, the insulator must withstand overvoltages resulting from potential differences between them.
When the withstand voltage of the insulation is exceeded, a flashover occurs. Composite insulators used in compacted lines easily withstand voltage stresses within the material. However, a flashover occurs due to surface flashover on the outer side, along the surface or above the insulator. In moist conditions, especially on a contaminated insulator surface, an arc can form along the surface. This is not relevant for transient overvoltages but power frequency overvoltages that are not problematic for this voltage level and are therefore excluded from consideration. This is due to the fact that the grounding factor for the neutral point of transformers is the lowest among all voltage levels in our transmission systems. To mitigate overvoltages and arc movement away from the insulator (thermal loads), protective rings with horns are usually installed on the insulators. Since the spark gap distance between the horns is the smallest, the electric field is the strongest, and this is where a flashover occurs. Therefore, when analyzing insulation overvoltages, appropriate sparkover modelling is required. Different models describing such sparkovers were evaluated, including the key model based on the U-t characteristic of flashover, integral methods, and physical models. The latter received significant attention as it considers all three stages of discharge development, from corona inception, streamer development, to final leader formation and flashover.
An important parameter that needed to be addressed is the wave impedance of the conductors and the coupling factor. The latter affects the induced voltage magnitude in the conductors when lightning strikes a shielding wire and is related to the geometry of distances between conductors. The impedance of the overhead line defines how the overvoltage wave will be reshaped, limited, or amplified. In this context, the significant role of soil resistivity and the grounding resistance of the tower is not forgotten, as they are inherently linked and ensure the dissipation of electric charges into the ground.
There are multiple approaches to performing overvoltage analyses, but it is meaningful to have one that is most suitable for Slovenian conditions. To this end, a simplified procedure was developed that provides insight into the reliability of the overhead line or exposure to transient overvoltages with steep fronts. Simulations and the use of advanced methods provide a good insight into the reliability of apparent insulation. However, tests are what confirm and demonstrate real conditions on the line. For this purpose, a prototype of a compacted insulator set was constructed, and both standard and non-standard testing approaches were used. The goal was to approximate the shape of the lightning impulse waveform, which was analysed through direct observations of lightning strikes (SCALAR). Attention was also given to corona discharge, which represents the ionization process of air surrounding metallic fitings of insulators and conductors at high voltage. In general, corona phenomena in AC voltage can be divided into positive and negative half-periods, as ionization events occur in the nanosecond range. This phenomenon has several consequences, one of the main ones being radio interference disturbances, which are measured (RIV - radio interference voltage), and noise on the line. The wideband sound component of corona arises due to micro discharges on the surface of metallic parts when the electric field value at a given location is less than the critical value. Besides RIV, partial discharges or current "pulses" occurring in the air are also measured, and they must be limited.
Optimizing the insulator set is crucial and can be done in several ways. One of the most useful approaches for our case is optimizing spark gap by changing geometry. This directly affects the apparent extension of the insulator chain and consequently raises the critical field strength, resulting in an optimal shape for the 400 kV compacted insulator set.
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