Hello learners. The earthing system design is an important activity in substitution engineering. As we discussed in the self-learning model, the substation earthing is directly associated with the safety of both operating personnel and equipment. In this session, we are going to see how to design the aftermath for an outdoor substation as per the IEEE-80-2013 standard. Let us have a look at the flowchart, which you've got familiarized with through the self-learning model. However, we will revisit the same while solving a case study. Now look at the case study which indicates that 33/11kV outdoor substation is constructed in a plot area of 50 meter by 50 meter. The ground ambient temperature is 40 degree Celsius, and the electrical resistivity of the soil measured at substation plot is 30 Ohm meters. Also the maximum estimated single-phase earth fault current at 33 kV bus is 18 kilo amperes. The short time rating of the various equipment are one second. The substation is provided with numerical production relay at the maximum fault clearing time including backup protection relay operation is less than 0.5 second. The earth grid resistance shall be maintained less than one Ohm. Now the objective is to design your safety grounding system for this substation. Now we will discuss about solution. In Step 1, we'll summarize the basic data available pertaining to the substitution. The details listed here are; plot size, length is 50 meters and the width is 50 meters, resistivity of soil is 30 Ohm meters, single phase to ground fault current is 18 kilo Amperes. The maximum fault clearing including backup relay operating time is 0.5 second. Now let us calculate the conductor size as per Step 2. The objective here is to determine the cross-sectional area. That is to know whether it can carry the required fault current for the specified duration without any damages to the conductor. You have to remember that improperly sized conductor will not be able to carry the required current and thereby it may even thermally damage the conductor due to high temperature rise. The temperature rise is limited by providing adequate cross-section area of the conductor. In our case, material of the earth grid given is MS FLAT. Here, as per the industry practice, the type of joint considered as welding for below ground grid conductors. Refer the table above, IEEE-80-013, which gives the various material constants. You may also note from the table that as the type of material changes, the material constant changes. Look at the equation for finding the cross-sectional area of the conductor. You can note that the cross-sectional area is proportional to the fault current and inversely proportional to the fault current to stand duration. That is T_c. The type of material like steel, aluminum, copper, etc, will have influence on the conductor cross-sectional area as the electrical resistivity, that is Rho, changes with respect to material. The type of joint influences the cross-sectional area as a joint could be made of welding, bracing, bolted, etc. Each type of joint, that is K_m, will have different temperature withstand capability. Substituting the various parameters in the equations appropriately, we can find out the cross-section area of the conductor required. The final parameter is selected here are shown and you can make clear observation on the same. It is important to be noted here is that even though the steel material melting temperature is 1,510 degrees Celsius, the nature of joint determines the final temperature of the conductor. In our case, the mild steel will be welded under recommended temperature as per the National practices CBIP Design of Earthmat for the High Voltage Substitution. The final temperature considered is 620 degree Celsius. Upon substitution of these parameters in the equation, we can find that the minimum size required is 215 square mm. Let us investigate additional factors. The soil generally contains moistures and minerals. Due to the same, the Earthmat material is likely to get corroded. The normal life of the earth grid is around 25 to 30 years, which is almost the same as life of a substation. Therefore, sufficient corrosion allowance must be provided for the earth contractor against this requirement. As per the national standard, IS 3043, code of practices for earthing where you can note that classification of soil with respect to each corrosion aspects. It also recommends use of steel conductor with a large cross-sectional area as a mitigation measure. Further, there is an additional table which indicates the corrosion allowances based on electrical resistivity. In our case, the resistivity is 3o Ohm-m. This comes under the moderately corroded soil and accordingly, the corrosion elements considered is 15 percent. Therefore, the minimum cross-section required considering the corrosion is 247 square mm. The nearest standard size available in the industry is 40 by 8mm MS FLAT, considering minimum thickness of 6mm for main grown mat as per the CBIP manual. Therefore, the selected cross-section is 320 sq mm, which is meeting the requirement. This is the process for selection of the cross-section of the conductor. Having learned the methodology for conductor sizing in Step 2, we will now see the third step for determining the tolerable potential. As we are aware that when a single-phase fault current flows through the station yet to the remote earth, the surface potential is generated. If a person happens to be in the substation at that moment, he may experience a shock due to the potential difference when he walks on the substation surface. This is called the step potential. In case he touches any object, he may experience a shock due to the potential difference when standing and keeping both legs close to each other. This is called touch potential. Now, the body can withstand certain potential difference, and therefore, we need to ensure that the potential experienced by the body of the person due to the surface potential is to be limited within the tolerable level. Gravel spread into substation premises reduces the step and touch potential experienced by the humans. When higher surface potential exists on the substitution plot, the same can be limited by using gravel. That is the reason the substitution is spread with gravel layer. So when the person stands on the top of the gravel layer, some potential is dropped and reduced potential is experienced by the body of the person. The process of assessing the tolerable potential is called as tolerable potential calculation. Now we can find out the tolerable potential using the formula. If you inspect the equation, you have been given two options of body weight. One is 50 Kg, another is 70 Kg. Also, you can observe that as the body weight increases, the tolerable voltage by the person increases. We can consider the body weight as 50 Kg on conservative basis to ensure the higher safety level. The value row yes, that is gravel resistivity, which we can consider it as 3,001 0hm-m. The thickness of the layer can be 80 to 150 mm as per class 12.5 of IEEE-80. Therefore, we have considered the thickness as a 100 mm as per industry practices. Another important aspect is shock duration, t_s. This is function of time record for clearing the fault. Recall the production grading aspects which you learned from over-current protection module, we can calculate the production operating time for clearing the fault. Let us say with the actual relay grading analysis, if we can demonstrate that within 0.5 second the fault is cleared, including the backup relay operating time, then we can consider duration for t_s as 0.5 seconds. In our case, we can consider it as 0.5 seconds as per the details given in the case study. Therefore, by substituting all the parameters, they get the following tolerable touch potential, E_touch, that is 675 volts. The tolerable step potential, E_step is 2210 volts.
