There are three reasons for which we use substation earthing.
- To earth the neutral point of the system.
- To earth, the overvoltages created due to lightning and switching impulses.
- From the safety point of view, to make all the noncurrent carrying parts of the substation connected with earth.
We should connect the neutral point of all transformers either directly or through an impedance to the earth. For proper functioning of relays direct earthing is more preferable than through an impedance. As per IE rule, or Indian Electricity rules, we should connect the neutral point of any power transformer, through two individual earth electrodes. In other words, we must connect the neutral point of a power transformer with two separate treated earth pits. For star-connected winding, the star-point (or neutral point) is directly connected to the earth or to the earth pits but in the case of delta winding, we need an additional earthing transformer to obtain the neutral point. We then connect the neutral point to the earth-electrodes or pits. As per rules, we should connect each lighting arrester to an individual earth pit. In that case, the minimum distance is maintained between the earth pit and the earthing point of the lightning arrester. Also, the connection is maintained as straight as possible. We should not use any iron or GI pipe for covering the earthing connection. The metal part of each and every piece of equipment which does not carry any current should be connected to the earth. The earth resistance of a substation depends on the operating voltage, size of the substation, size, and numbers of transformers in the substation.
The value of earth resistance and the number of electrodes should be such that in any faulty condition there must not be any dangerous voltage created anywhere in the system that means the fault current should go to the earth through the maximum possible wider path. The earth path should be so designed that the fault current can easily pass through it without creating any overvoltage in any part of the earthing system. The earthing resistance of the system should be such that there should not be any voltage rising beyond the limit between neutral and earthing points even during the surge. There should not be any beyond the limit rise of the voltage even during lightning conditions.
Depending upon the nature of the soil and the size of the substation the number of GI pipes or roads. If the number of earth pit or electrodes are increased the earthing resistance decreases because in that case, the fault current will get more path to pass to the earth. In that case, there will be no risk of rising step voltage beyond the limit. Each electrode is so designed that it can carry at least 250 to 300 ampere. The diameter of the pipe used for the electrode is 40 mm where its length should be 3 meters.
The lower the resistance of the sub-station, the better. This only applies to small and medium-size substations. Larger stations have lower earthing resistance as well as some additional measures.
The earth resistance of a substation should be as low as possible. For small or medium-sized substations only a low value of earth resistance is more than enough for the purpose. But for a big substation, only a low value of earth resistance is not a sufficient criterion. This is because here the fault current level of a big substation is quite high. When this high current will flow through the earth there may be a high step potential developed under the ground. This is not desirable. So there should be some arrangement by which the potential difference between any two points on the ground would be under limit under any high faulty condition. In other words, the step potential and touch potential should be under the safe limit. In that case, there will be a number of earth spikes borrowed at a regular interval of 4 to 5 meters around the substation.
Suppose we have connected one piece of equipment with the earth through an earth pit. During a faulty condition, there may be a huge current passing through the earth connection to the ground. Due to this huge current, there may be a high potential difference being developed between two points on the ground. If any human being standing on the ground at the same time, he or she may feel an electric shock for that. The voltage difference between the steps of the human being is known as step potential. To reduce this step potential it is always required to maintain an equipotential zone below the ground of a substation. This is done by placing an earth grid under the ground for the entire switchyard of the substation. In that case, there are a number of earth pits in the switchyard area. All the earth pits are connected together with the help of a conductor grid buried in the ground. This grid is called the earth grid or sometimes the earth mat of the substation. In that case, when any electrical fault occurs in any equipment, the fault current gets a number of parallel paths to flow to the earth. So the fault current gets divided and thereby current through any of the paths remains in limit. Therefore there will be no chance of generating high step voltage in any situation. Moreover, the grid provides an equipotential level of the ground. The earth grid also increases the reliability of the earthing system. As there are many earth electrodes (pits) connected together by the earth grid, in the case of failure of any of the electrodes there will be no problem for passing the earth fault current to the ground. Suppose a situation, where a single earth electrode is connected to equipment. In the failure of that electrode, the fault current does not get the path to flow to the ground.
What is earthing?
Electrical earthing means using the huge ground body as a conductor. Practically the potential of the earth is zero. Earthing of any equipment or system means, connecting the equipment or system to the earth. Suppose there is a lamp connected to the positive battery terminal. Obviously, the negative terminal of the battery should be connected to the other point of the lamp for glowing the lamp. If the negative terminal is connected to the earth or ground and in the same way the other terminal of the lamp is connected to the earth or ground, the lamp starts glowing. This is because in that case, the earth behaves as the return conductor of that simple circuit. But the question may arise in the mind that the resistivity of the earth soil is much higher than that of a copper or aluminum conductor, how it behaves like a good conductor.