Design of Ground-mat for 11kV Sub-station using AUTO GRID PRO

Jawahar M Assistant Professor, Dept. of EEE

Mepco Schlenk Engineering College Sivakasi, Tamilnadu, India

jawahar.m@mepcoeng.ac.in

Alagammal S Assistant Professor, Dept. of EEE

Mepco Schlenk Engineering College Sivakasi, Tamilnadu, India

mshanthilogesh@mepcoeng.ac.in

Jayasankar V Sr. Professor & Head, Dept. of EEE Mepco Schlenk Engineering College

Sivakasi, Tamilnadu, India vjaya@mepcoeng.ac.in

Abstract—Personnel safety cum consistency is the important consideration in the functioning of the power systems. These concerns aid the design of earth-mat. This paper is more focused on the design of earth-mat for a 11kV substation. Earth-mat is basically a solid metallic plate or conductors buried in shallow depths to provide safety measures by reducing the severe touch and step voltages in important areas that are frequently operated by personnel. It also concentrates on minimizing the number of conductors that are used for building the ground-mat using a software toolkit called AUTO GRID PRO, as a result, accuracy of number of conductors, their size and spacing is determined which subsequently reduces the cost of the mat design.

Keywords— Grounding, Earth mat, Safety, AUTO GRID PRO

I. INTRODUCTION

According to IEEE 80 - 10617, grounding is a current carrying connection. Hence, accidentally or intentionally by which a circuit or device is connected to the earth. It is used to establish and maintain the potential of earth. The function of a earthing is to ensure the personnel protection and the community, to minimize the hazards from transferred voltages, to ensure a path for lightening strokes, which in turn guarantee equipment safety, and provide a low resistive path to the earth. Good earthing system reduces the ground potential rise has been presented in [1].

For a safer earthing design, it needs to ensure a way to carry the current into the earth under abnormal and normal conditions. Also, it must provide guarantee that a local personnel should not be dying out. Hence, a relationship between earthing system resistance and the maximum fault current personnel can be ensured is by means of thorough analysis in regards to the location of the ground plates, soil nature and characteristics, etc. In [3], Public assumes that earthing safely done. A low substation ground resistance assures safety. The relation between the ground resistance and the maximum fault current that a person may be experienced.

A. Types of Grounding

Device Grounding: Human protection is ensured by connecting all the current carrying less conductors to the ground. This method of interconnection is said as device grounding. It ensures [1]

• No variation of potential hence no shock current risk between non-current carrying conductors.

• No potential difference between ground and device earthing systems.

• No stationery electron build-up.

Process Grounding: An act of purposely connecting anyone current passing rod of distribution system to the earth.

The grounding system in a premises is momentous.

Everything including the safety of personnel and the device is must. This is the most favorable functioning of the system. It comprises the following:

• Equipotential relationship of the current carrying entities to the grounding system avoids the perilous potentials between conductive entity and the ground.

• The grounding ensures a low resistive current path for grounding problems in the plant, which protects both the public and the device.

• For ground problems in offshore generation systems, a low resistive grounding grid is required to avoid the perilous rise in touch and step voltages.

• Grounding systems ensures low resistive path for lightening strokes.

• The grounding system helps to avoid the electromagnetic interference problems in electronic circuitry.

The manipulation is principally on the procedures given by the IEEE Std 80 (2000), "Guide for safety in AC substation earthing". The earthing manipulation helps in the better design of the grounding system. With the inference achieved from this calculation, we can:

• Estimate the adequate size main earth grid rods. • Ensures the better design to avoid the touch and step

voltages.

These manipulations should be enhanced when the grounding is designed. Post the completion of the groundwork, the need for improvement / redesign can be considered. Preferably, test results of the soil resistivity of the region under study shall be made available for the touch and step voltage manipulations.

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In [4], distinct formula has been proposed to influence the earth resistance of several configurations of rods, a short horizontal wire, a horizontal strip, a buried horizontal plate and a ring of wire.

Recent studies of the horizontal earth electrode using the

transmission line model were carried out. Their results showed that the voltage drop along the horizontal plate was important due to inductance, but only a certain length, the “effective length”, contributed to current dissipation. The effective length was shown to raise with increasing soil resistivity [5].

In [2], a methodical model of a horizontal earth plate based on an equivalent transmission line was used and it confirmed that no major benefit could be achieved by extending the plate length beyond the effective length.

II. EARTHMAT DESIGN

According to IEEE std 80 [1], earth mat can be defined as rigid metallic rod or a set of narrowly gaped plates which are in relation to and frequently held in the ground surface, in order to receive an proactive shield to avoid the severity of touch and step voltages in a critically functioning area. Earth rods are held on the soil surface or the mesh.

A. Purpose Of Earth-Mat Design

• Over-Voltage Protection: Lightening surges can

cause perilous high voltage or electrical disturbance which can be removed by grounding.

• Voltage Stabilization: There are many sources of electricity like transformer, generators etc. , until there is any reference point grounding it is difficult to stabilize the voltage.

• It helps in providing the safety to humans and equipment by providing a current conductive path.

• Grounding can also help in reducing the electromagnetic interference and radio frequency interference received from nearby pole and bad transformer by diverting the stray radio frequency.

In order to ensure the safety of the personnel and the

public, the grounding systems must be designed such that to avoid the very high touch and step potentials both during the abnormal and normal conditions. It provides the continuity of service with the horizontally placed rods along with the vertical ground rods in connection to the ground grid. From the analysis of two scenarios, the first quoted in an AIEE application guide in 1954, and the second published in 1980 (Dawalibi, Bauchard, and Mukhedkar), this process says the most preferably said grounding practices followed at the United States, Europe etc. The importance of using the vertical and horizontal rods is as follows:

In the subsystems of electrical utilities, a single rod providing the safe grounding technique is found to be insufficient. When large number of ground rods is connected in the systems with neutrals of the same system, the importance of the grounding grid system becomes a failure irrespective of the primary purpose of this arrangement. For to develop a good grounding system, the linking rods must be buried in the soil which is holding the effective conductivity. Partially certain utilizes depends on the grid only.

When the current magnitude fritter away into the ground,

it is impossible to place a low resistive grid in order to ensure the ground potential rise within limits for a safer operation by the personnel’s. Hence, the local potential can be made avoided by simple control techniques.

This method of joining the vertical rods with the

horizontal plates would results in the following advantages. • The most effective method to reduce the severity of

step and touch potentials due to the horizontal plates are as follows resulting in the placement of the grids under soil surfaces, sufficiently with long earth rods will steady the performance of both the systems. For numerous placements, this is vital as the dried soil surfaces could vary the resistivity of the soil, yet the lower soil layers would remain no change.

• [2] Fault currents are eliminated in low resistive soils as in the multilayer soil, the resistivity in the upper layers are higher than the lower layers. So, for the numerous Geographic Information Systems and other space-bounded placements, this procedure remains to be the important aspect to be considered.

If the conductors are placed on the outermost perimeter of the grid in higher to lower soil surface fashion, there will be a steep rise in the shallow gradients in the outer meshes. This method helps to evaluate the determination of the potential gradients at the ground surface.

III. AUTO GRID PRO EARTHMAT DESIGN ALOGRITHM

Below is the sequence of procedure to design the ground grid [1]: Step 1: The geographic nature provided in the means of map gives the needed estimates of the areas that need to be grounded. Soil profile and its resistivity are determined by the available three point method or four pin methods. Step 2: Size of the conductor is determined. Fault current across the grid must be the higher limit of the fault current that will be conducted by any current in the earthing prototype. Whereas the time (tc) must be the maximum probable timeto clear the fault. Step 3: The permissible limits of the step and touch potentials are dogged. Hence, the value of the time, (ts) depends on the acceptance of the engineer in work.

Step 4: The primary analysis and design must involve grounding area bordering loop with the adequate crossing rods to ensure ease access for device grounding systems. The preliminary estimates of conductor gaped and the conductor placed is based on the location of grounding and the grid current. Step 5: Primary resistance of the normal uniform soil structure in the grounding system is estimated. Hence, for the final analysis and design, highly accurate resistances are estimated. Process grounding analysis is depending on the modeling of the components in detail which can calculate the resistance of the soil by choosing the soil modes correctly. Step 6: The current flowing through the grid has been determined. In order to avoid the complicated design of the earthing system, total fault current following grid to ground alone must be taken for analysis and design. The current flowing through the grid current must shows the fault current and location of the fault, the decrement factor, and any expansion in the future. Step 7: If the primary design GPR is lesser than the permissible limits of the fault current, no secondary analysis is required. However extra conductors provided for earthing must be required for the analysis. Step 8: The calculation of the touch and step grid potentials as required can be accomplished by the needed analysis for normal soil conditions or by some highly precise analytical techniques. Step 9: However, if the computed mesh potential is lower than the permissible step potential limits, the required design is completed. (move to Step 10). If the calculated mesh potential is above the permissible touch potentials, the preliminary design must be changed (move to Step 11). Step 10: If the step and touch potentials are lesser than the acceptable limits, then the required refinements of the device ground are done. Else, the preliminary structure needs to be changed (move to Step 11). Step 11: When the potentials are above the permissible limits, then the design of the grounding grid has to be revised. Hence, new grid design must include small rod gaps and extra earth conductors. Step 12: Post analysis of the different potentials, extra rods and grid design is required. The extra grid rods may be required for the spaces available near the device that needs to be grounded. The final setup must be equipped with provisions to avoid the perilous effects in special areas with respect to the step and touch potentials.

IV. CALCULATIONS

A. Soil Resistivity Measurements

Fig. 1. Four Pin Method

From fig 1, Soil resistivity by four-pin method is measured using the below equation,

Whereas, ‘ρa’ be the apparent resistivity of the soil in ohm-meter (Ωm); ‘R’ is the measured resistance in ohm (Ω); ‘a’ be the distance between adjacent electrodes in meter; ‘b’ is the depth of the electrodes in meter.

Fig. 2. Field Area

Soil resistivity measurement in the following field area (24*24) is as follows

TABLE I. SOIL RESISTIVITY MEASURMENTS

Depth Resistance (Ω)

AB AC AD

2m 1.29 0.9 1.15

4m 1.12 0.4 0.7

6m 0.37 0.2 0.41

8m 0.31 0.1 0.35

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From fig 2, the he soil resistivity is found to be 127.255 Ωm using the equation (ρa) and from field dimensions of table 1. Hence, the type found is moist soil in the specimen region taken.

V. RESULTS AND DISCUSSIONS

Various numbers of iterations for different earthmat conductor size were performed, out of which few have been mentioned below. The magnitude of the fault current was 5773A because, Rated power= 110 MW; Voltage= 11kV. Hence,

From fig 3, the number of conductors considered for the earthmat design involves (20X20) has been simulated using Auto Grid Pro and the results are as follows,

Fig. 3. X-Y Plot

Fig. 4. Touch Potential

Fig. 5. Step Potential

Fig. 6. Grounding Grid Design

From fig 4 and fig 5, it is found that step and touch potential has the important involovement in the groundmat design. So, in order to reduce the hotspots, number of iteration has to get increased which involves in increasing or decreasing the number of conductors which consequently reduces the hotspots and provide a perfect mat for operation. In fig 5 and fig 6, Pink areas which are present have to make as pits for future service which involves determining the state of conductors. If any odd, then the pits must be served by salt and earth protection particles.

VI. CONCLUSION

This paper focus on economic design of earth mat for 11kV substation earthing systems. In this paper, Mild steel is used as an earthing conductor material and the results of the design are obtained using AUTO GRID PRO. The various iterations taken for different earthmat conductor numbers are (20X20), (10X10), (16X16), (12X12) have been simulated and the grounding grid design has been generated accordingly. In general, Human body gets shock from step and touch potentials which are dangerous and when a high or a very high potential substations are to be developed, touch and step voltages must be manipulated and the values must meet the Ground Potential Rise (GPR). Therefore, 12X12 grounding grid design is found to be feasible considering the economic aspects and the type of soil, which is most in the region under study.

Acknowledgment The authors would express their gratefulnesss to Electrical

Services Group, Indira Gandhi Centre for Atomic Re-search, Kalpakkam, Dept. of Atomic Energy, Govt. of India for the technical sustenance provided during the project design phase.

References [1] IEEE 80 – 10617, ANSI/IEEE Std.80, IEEE Guide for safety in AC

Substation Earthing, New York, (2000).

[2] R.J. Heppe, “Computation of potential at surface above an energized grid or other electrode, allowing for non- uniform current distribution”, IEEE T. Power Apparatus and Systems, 98, 1978-1988, (1979).

[3] J.G. Sverak et al., Safe substation earthing, Part I: IEEE T. on Power Apparatus and Systems,100, 4281-4290, (1981); Part II: IEEE T. on Power Apparatus and Systems, 101, 4006-4023, (1982)

[4] F. Navarrina, I. Colominas, M. Casteleiro, Analytical Integration Techniques for Earthing Grid Computation by BEM, Num. Met. in Eng. and Appl. Sci., 1197–1206, CIMNE, Barcelona, (1992).

[5] I. Colominas, F. Navarrina, M. Casteleiro, A boundary element numerical approach for earthing grid computation, Comput. Methods Appl. Mech. Engrg., 174, 73-90, (1999).

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