| The calculation of lightning overvoltages is critical for substation voltage uprating and it is essential to control their effects on substation equipment properly. The most severe stress is caused by a lightning flashover in close proximity to the substation. The various means which can be employed to control lightning overvoltage effects can be roughly divided as: (1) Surge Arresters; (2) Surge Modifying Devices; (3) Transmission Line Design.; Commercially available Metal Oxide station type surge arresters at line entrance and other locations, in addition to the usual at the Transformer location have been applied successfully to control lightning overvoltages in uprated substations. This type of surge arrester can limit the peak overvoltages to arrester's discharge voltage, producing a fast front flat tail wave, only at arrester location. Further away from the arrester, the overvoltage is higher due to the separation effect. It has been experimentally verified that the amplitude and duration of the flat top wave have more effect on system insulation than the initial rate of rise. With measured times to flashover of 8 to 40 {dollar}mu{dollar}s, it is doubtful from the view point of surge energy, that a fast phenomena in a very short interval at the beginning can influence the results. The influence of surge reflection is also negligible. The duration of the flat tail is limited when the surge arrester ceases to conduct significant current.; In the case of uprated substations, the maximum allowable overvoltage is determined by the available clearances. How close the arrester must be, is determined not only by the crest and steepness of the overvoltage but also by the duration, polarity, and frequency of occurrence.; There are many aspects of transmission line design that offer opportunities in reducing the frequency and severity of the lightning surges imposed on the substation. Improved lightning performance of transmission lines is commonly achieved by taking advantage of one or more of the following: Shielding, Grounding, Route Selection, Underbuilt Ground Wires and Guys for certain type of Towers.; A voltage across the tower can be observed only during the front of the stroke; during the tail voltage across the tower reduces considerably, i.e., there is hardly any potential difference across the tower and footing resistance. It is essential to minimize voltage across the tower in this small interval of time. Use of guys and underbuilt ground wires seem to offer a good option to minimize these voltages.; From a substation uprating point of view, measures must be taken to minimize the lightning overvoltages along a short section of transmission line called "The Limiting Distance". The substation is protected by natural distortion and attenuation outside the limiting distance. The limiting distance in turn determines the "Separation Distance".; In this dissertation the use of underbuilt ground wires and guys (only within the limiting distance) has been explored. The lightning performance of that section of line can be improved considerably, the amplitude and duration of the flat top wave can be decreased, and so increase in margins and separation distances in the substation results. This has considerable impact on electrical power system and insulation design. |
