Requirement for a Tower Grounding Device
The grounding devices for the transmission tower and distribution tower are used to direct any lightning current and power-frequency short-circuit currents into the earth. They are designed to maintain suitable lightning protection levels for transmission and distribution lines and ensure the safety of persons around the towers.
Table of Contents
The impulse impedance of a grounding device for a transmission or distribution line has a direct relationship with the trip-out rate of the transmission or distribution line, which has a considerable influence on the effectiveness of the applied lightning protection. When designing the lightning protection of a transmission line, besides constructing the overhead ground wire to reduce the possibility of lightning strikes by grounding overhead earth wire, the potential at the top of the tower should be suppressed to reduce the potential difference between the tower top and the phase conductor in order to reduce back-flashover.
When lightning strikes the transmission tower, lightning current i flows into the earth through the tower and the tower grounding device, and the rise in tower top potential is mainly determined by the overall performance of the tower grounding device
where RI is the impulse grounding resistance of the tower grounding device, L is the inductance of the tower body which is roughly proportional to the height of the tower and b is the division factor of the lightning current on the ground wire.
When the potential difference between the tower top cross-arm and phase line caused by induction and coupling exceeds the flashover voltage of the insulator, a back-flashover will occur along the insulator string of the tower which can have a negative influence on the power system operation and even lead to a power transmission failure when the disc ruptures. It can be seen from the above equation that the tower top potential is directly determined by the impulse grounding resistance of the tower grounding device. Therefore, a proper earthing design for the grounding device can considerably improve the operating reliability of a transmission or distribution line.
From the above equation, it is seen that the lower the impulse grounding resistance is, the smaller is the voltage across the insulator and the smaller is the probability of back-flashover. Therefore, at the design phase of a transmission line, the impulse grounding resistance of the tower grounding device is an important parameter to be considered, and it is usually smaller than the power-frequency grounding resistance.
The impulse impedance depends on many factors, such as the characteristics of the soil, the peak and waveform of the impulse current, and the geometry of the grounding device. So in the practical design of an impulse grounding device, the power-frequency grounding resistance is usually used as the reference, and the impulse coefficient is also considered. If the power-frequency grounding resistance is between 10 and 15Ω, the design is considered to be good. In an extra-high voltage network, a grounding resistance of 10Ω is always required.
Components of the tower grounding device
Grounding conductor: The metallic path that will conduct the fault current or lightning current from the tower to the ground. These conductors are usually of copper or galvanized steel.
Ground Rods: These are long rods that are driven in the earth to establish a low resistance path for the fault current.
Earthing grid: It is a network of grounding conductors interconnected and buried in the soil around the tower base for efficient distribution of the fault current into the earth.
Bonding: Bonding ensures a good electrical connection of all metallic parts of the tower to the grounding system. Bonding results in an equipotential zone in the tower and reduces the chances of shock during a fault.
