Surge protection of power lines
One of the conditions for the safe and proper operation of the electricity network is adequate surge protection. Protecting the network requires the use of the right tools and measures and the correct selection and operation of equipment to protect the power installations. How are overhead lines protected against surges?
Surge protection of the electricity network is a complex issue that is part of securing the proper operation of the network. At first, it should be noted that we are talking about surge protection in the electricity network, not in buildings. This is because these are of a secondary nature and are intended to protect equipment from surges transmitted from the grid as well as those generated within the electrical installation of the building. However, the power grid and its individual components, including the substation, power poles, line conductors, etc. are also subject to surge protection. It is one of the conditions for the safe and correct functioning of the network. Before discussing the methods and tools used for this purpose, it is first worth identifying the risks involved.
Impact of atmospheric factors on the electricity network
The basic threat against which surge protection is used in the power grid is atmospheric factors, and especially lightning discharges. The first category of threats is direct discharge, i.e. a situation in which lightning strikes directly into the overhead line or near it. The threats in this situation are:
- surge wave generated by a discharge in the line wires
- arcing on insulators
- arcing between line wires
As observations and practice show, medium voltage lines or low voltage overhead lines are most often exposed to direct lightning strikes on the wires. Cases of hitting power poles or pole crossbars are so sporadic that they are often omitted in studies and analyses of the network safety profile.
When lightning strikes overhead lines, a high-peak voltage wave is created in the line, accompanied by a surge. The waves spread out in both directions from the point of impact toward the poles.
The second category of threats are induced overvoltages, caused by lightning strikes not directly on the line or network elements and their immediate surroundings, but in its vicinity, e.g. a tree or the ground. We talk about induced overvoltages when the impact occurs at a distance of at least four times the height of the suspension of the line wires. If the lightning channel is located closer, we are dealing with a direct lightning strike.
The task of surge protection in the event of direct or induced discharge is to influence the energy of the surge wave by dividing it and weakening it on its flow path.
Surge protection on overhead lines
In the case of medium voltage lines, the dilemma that needs to be considered in the case of surge protection is whether it is better to use insulators with a higher flashover voltage, which limit the surge wave behind the insulator to a greater extent but may be associated with more frequent line disruptions, or insulators with a lower flashover voltage, which limit the risk of line disruptions but limit the surge wave behind the insulator to a lesser extent.
Surge arresters are also an important part in network protection. Their task is to mitigate the steepness of the surge wave. In the case of medium voltage lines (and also high voltage lines), e.g. blow-out arresters, valve arresters or gapless surge arresters with oxide varistors are used. The latter surge arresters are currently gaining popularity and are gradually replacing other solutions.
In the case of low voltage overhead lines, so-called SPD (Surge Protective Device) surge arresters are used. These are arresters with one connection, with one branch of protection, type 2. For installation on wires, they should have a line terminal appropriate for the cross-section and type of wires on which they are mounted (bare or insulated wires). They should also be equipped with special disconnectors that allow the device to be disconnected, e.g. in the event of a failure, to prevent a permanent short circuit in the network. Since they are exposed to various negative atmospheric factors, surge arresters should be placed in a cover made of a material resistant to UV radiation, corrosion or erosion.
An important issue concerning SPD arresters is the method of their installation. They should be placed between each phase conductor and the protective-neutral PEN conductor. They should also be used in LV network supply substations, line attachments, along the line, at line divisions – on both sides of the pole, at the end of the line, or at connections directly supplying installations in various types of buildings.