Grounding conductors selection and connecting leads selection should be done considering the design life of the substation and should meet the following requirements
Good conductivity. There should be no major local potential difference on the grounding conductor caused by its self resistance.
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In the worst scenario of the high amplitude and duration of a fault current, the grounding conductor should not be fused and mechanically deteriorated.
Reliable mechanical strength and strong robustness. We must give attention to the exposed parts, which are vulnerable to natural damage.
Even when exposed to corrosion or physical abuse, the grounding conductor should be able to maintain its normal function.
While choosing conductor materials, the thermal stability, corrosion in soil, conductive performance and the cost should be taken into account. In order to design a ground device thoroughly, only use of a large-sized conductor should be avoided. Hence, each aspect should be analyzed carefully.
Thermal Stability
In an effectively grounded system, the short-circuit current flowing into the grounding conductor and grid, usually in the range of kA to dozens of kA, which disperses into the earth from the grounding grid electrode and generates heat in the conductors. However, the duration of the short-circuit current is very short, depending on the action time of the protection device for the circuit breaker which is nearest to the short-circuit point, and the operating time of the circuit breaker.
Generally, this is only some hundred milliseconds. In this short time duration, the heat generated cannot be transferred to the surrounding soil fully, so almost all the heat generated by the fault current results in raising the temperature of the conductor.
where E is the energy generated by the short-circuit current, V is the volume of the grounding conductor, r is the conductor resistivity, ΔT is the temperature rise in a conductor absorbing an energy of E and the conductor’s specific heat under constant pressure is CP.
When the temperature of the conductor exceeds a certain value and is naturally cooled in the soil itself, the mechanical properties of the conductor sharply reduces, particularly in the connections or joints of the conductor.
Thus, when a conductor encounters a high short-time electrodynamic force, it will be damaged. When the short-circuit current is very high and also the conductor temperature is high that it reaches melting point, then the conductor will be melted. These two reasons can break the grounding conductor and result in a disintegrated grounding grid, which therefore, greatly reduces the reliability of the ground device.
Different metals have different short-term maximum allowable temperatures. When the temperature of the conductor exceeds this, its performance will be degraded. Similarly, each material has its own melting point. The higher the short-term maximum allowable temperature or the melting point temperature of the conductor, the better will be the its thermal stability.
The short-term maximum allowable temperature of copper is 300 ° C, with a melting point of 1083 ° C. That of steel is 400 ° C, with the melting point of 1550 ° C. Therefore, steel has a better thermal stability than copper.
Corrosion Rate of Grounding Conductors in Soil
Metals buried in soil will be corroded and this kind of corrosion is due to electro-chemical process. Water with dissolved minerals present in the soil acts as an electrolytic solution. However, soil corrosion is more complicated than simple electrolyte corrosion.
The degree of soil corrosion on a metal conductor that is burried can be expressed by a corrosion rate. The average rate of corrosion can be expressed by the weight loss per unit time per unit area or the corrosion depth of the metal surface. The rate of corrosion of common steel in the soil is about four to five times that of copper. The corrosion rate of galvanized steel inside soil is about one to two times that of copper. Thus, copper is more corrosion resistant.
Conductivity of the Conductor
In a large grounding grid, where a large short-circuit current disperses into the soil through the grounding conductors, the potential difference at different locations on the grounding grid are not the same due to the existence of conductor resistance and inductance.
According to relevant results, larger the size of grounding grid, the lower is the grounding resistance. Higher resistance of conductor, results in greater potential difference between different places on the grounding grid.
Therefore, the control of the difference in potential should be considered during design phase in order to prevent accidents. In addition, the resistivity of steel is about as eight times that of copper so, under the same short-circuit current, the steel will generate much more heat, leading its temperature rise to be much higher, which is more adverse to the thermal stability. Therefore, the conductivity of copper is better.
