In spite of a range of remedial measures, lightning remains one of the leading causes of line tripping disturbances. Therefore, solutions to lessen its impact remain a priority for line designers and power system operators across the globe.
Installation of transmission line arresters along the route is regarded as the one of the most economical and also one of the simplest strategies to deal with lightning disturbances. Alternatives such as reducing tower footing resistance are typically far more expensive, especially in mountainous areas. So is the option of replacing an overhead line with underground cable.
This article, contributed by arrester experts, Bengt Johnnerfelt and Jorge Luiz de Franco of TE Connectivity, discusses essential system parameters for successful installation of line surge arresters. The end goal of the process is optimizing selection of a non-gapped line arrester and coordinating it with the disconnector so as to mitigate problems of line tripping due to lightning.
In order to be in a position to optimize installation of line surge arresters, certain important data need to be collected beforehand. These include:
1. Information on Towers
• height & configuration of cross-arms;
• location & length of phase insulators;
• maximum phase conductor movements during maximum wind speed design;
• location of mechanical dampers;
• tower footing impulse resistance along line;
• location of shield wire(s), if any.
2. Information on Line Insulators
3. System Parameters
• reclosing times & scheme;
• short-circuit current;
• data on substation arresters;
• decisive TOV requirements;
• targeted reduction in trippings/yr for the system;
• maximum switching voltage along the line for switching protection.
4. Environmental Data
• ground flash density and/or number of thunder days/yr.
With all this information assembled, selection of a line surge arrester (LSA) can start with a system study to identify possible solutions using available programs such as EMTP, Sigma, etc.
Applications for LSAs
There are basically four main applications of an LSA. The most common is reducing tripping either from lightning hitting the tower or shield wire and causing back flashover or from direct lightning strike to a phase conductor causing a flashover from line insulator to tower. This application will also prevent flashovers due to induced overvoltages from lightning hitting tall nearby structures such as trees or wind turbines.
For this application both non-gapped line arresters (NGLAs) and externally gapped line arresters (EGLAs) can be used. If the target were to have no flashovers, no matter how these might originate, an LSA would have to be installed on every tower and on all phases of a line. This, however, is generally not costeffective except in cases where the consequences of line tripping are especially harmful to the system or its customers. One such example might be double EHV circuits coming from a generation plant where any double circuit tripping could result in a major blackout. Rather, in most cases the number of LSAs as well as on how many phases these are installed will depend on tower footing resistance as well as the outage reduction target. Moreover, charge requirements will depend greatly on whether shield wires are used or not since risk of high direct strike charges increases in the latter case.
A second main area of application of NGLAs is using them in place of reclosing resistors to control switching voltages along an EHV line. This application requires an NGLA be installed on all three phases but only on a few selected towers – typically in the middle for shorter lines or at points one-third and two-thirds along the overall length for longer lines. Here, the NGLA can be used either separately or in combination with other systems such as controlled switching. The goal is always no tripping due to switching. Typically, the energy/charge requirement for such NGLAs is one class lower than for substation arresters since they cover only half or a third of the line.