Tower Grounding Device
A transmission line tower footing is usually made from reinforced concrete, and the tower footing itself is effective as a tower grounding device. Since, the size of the high-voltage steel tower essentially requires a very large tower footing base, the grounding resistance that is required for the tower can be easily satisfied in an area of low soil resistivity.
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However in a high soil resistivity area, the grounding resistance of the tower footing cannot satisfy the required resistance, and additional ground electrodes is therefore added to the tower footings.
When designing a tower grounding device, the following factors should be considered:
Use of concrete-encased tower footing as a natural grounding electrode
An electrode encased with concrete has a good hygroscopic property and the grounding resistance can be lowered effectively. This can satisfy the required grounding resistance in areas with low soil resistivity.
Use of additional grounding electrodes in the tower grounding device
When the natural tower footing cannot reach the required value of grounding resistance, additional ground electrodes should therefore be added. The arrangement and size of such additional ground electrodes are determined by the soil resistivity of the area and tower structure.
Use of deeply buried ground rods in areas with high soil resistivity
When the transmission line passes through an area of high soil resistivity and it is possible to bury the ground electrode in a good conductive soil layer that is not affected by underground water, then deeply buried ground rods may be effective.
The structure of the tower grounding device usually has a relation with the shape of the tower footing. In areas with high soil resistivity, round steel, and profiled bar ground electrodes are buried at the bottom of each footing before installing the concrete-encased base. Ground electrodes are buried together with the tower footings. In order to reach the required value of the grounding resistance, horizontally an extended ground electrode or a deeply buried ground electrode should be added.
Usually, a grounding conductor has a diameter of 10–16mm. As for the burial depth of the electrode, it should not be smaller than 0.5m for a horizontal ground electrode and should not be smaller than 1.0m in farmland. When the ground electrode is buried in rocky soil if the thickness of the surface soil layer is more than 0.1 m, a horizontal ground electrode is allowed to be buried above the rock.
Basic Structures of Tower Grounding Device
The tower grounding devices of transmission line normally use the following structures:
Ring-shaped horizontal ground electrode:
When the resistivity of the soil exceeds 100 Ωm, it becomes impossible to meet the desired grounding resistance by the use of natural ground electrodes, and, therefore, additional ground electrodes should be added.
In this case, we should consider using a rectangular or square horizontal ground electrode which is suitable for the size of footing ditch. It must be noted that in the calculation of grounding resistance, vertical lead wires are seen as dependent parts of the tower grounding device.
Generally, ring-shaped ground electrodes use round steel for the reason that a conductor with a round cross-section has a very low corrosion rate in soil. The ring-shaped ground electrode is placed at the bottom of the footing ditch before fixing the tower base. Since soil excavation is not needed, the expenditure on this kind of ground electrode is very low. For expanding the application range of ring-shaped ground electrodes, we use vertical grounding rods driven into the bottom of the footing ditch.
Horizontal strip-type ground electrode:
The horizontal ground electrodes are buried in a radial pattern neighboring the tower footing, with a physical connection. Generally, the length and number of radial ground electrodes are determined by the resistivity of the soil.
According to field experiences, where soil resistivity is above 4000Ωm, it is effective to use a horizontal ground electrode along the transmission line, connecting with each tower’s ground electrode. This arrangement not only ensures that the transmission lines can substantially withstand lightning but also guarantees that the relay protection can act precisely in the occurrence of a single-phase earth fault.
Deeply buried ground electrode:
When the resistivity of the soil under a footing ditch is much lower than that of the surface soil layer, deeply buried ground electrodes shall be used. The first kind of structure is suitable for soil with good conductivity, and short tubular grounding rods are arranged and connected with the tower footings.
The second kind requires the construction of a single long vertical ground rod by mechanical execution, which shall make contact with deep soil with good conductivity. The length of the vertical ground rod can reach about 15–20 m.
Extending the vertical ground rod can reduce the grounding resistance in clay or sandy clay, the resistivity of which is heterogeneous. Notably, the vertical ground rod is better in sandy clay. For example, in a project in South Africa, the vertical ground rod with a diameter of 20 mm in uniform soil reaches a length of 6 m, and the grounding resistance drops to 47.8% of that of a rod with a length of 2.5 m. Moreover, when the ground electrode reaches a length of 12m, the grounding resistance decreases to 26.4% of that of a rod with a length of 2.5 m.
Major tower grounding devices used in transmission line
The major types of tower grounding devices for transmission lines are shown in the Figure below
Horizontally buried ground electrodes and multiple ground plates are shallowly buried at a depth of 30–80 cm, and it is better to use ground plates in areas with high soil resistivity. The ground electrode with thorns focuses on decreasing the impulse grounding resistance, the length of the ground electrode with thorns is generally 1.5m and the spacing between thorns is 0.1m.
Various tower grounding devices have their own advantages and disadvantages. The lead connecting the ground electrode and tower footing shall be wrapped by insulating tape or covered by asphalt to protect against corrosion.
