Evaluation of Soil Resistivity and Design of Grounding System for Hydroelectric Generating

Station in a Hilly Terrain – A Case Study

C. Prabhakar Distribution Systems Division

Central Power Research Institute Bangalore 560 080, India

R.A. Deshpande Distribution Systems Division

Central Power Research Institute Bangalore 560 080, India

Abstract – This paper presents the study results of soil resistivity measurement and design of grounding system involved for a hydroelectric generating station located on a hilly terrain which were carried out by CPRI.

This paper focuses on Soil resistivity measurement for different seasons in a year such that the worst value to be considered for the design calculation. The effectiveness of temperature on soil resistivity is discussed.

One of the techniques to bring down the ground resistance for the overall grounding system in hydroelectric generating station on a hilly terrain is also discussed.

Keywords – Soil Resistivity, Grounding system, Ground Resistance, Corrosion, Bentonite.

I. INTRODUCTION

The main purpose of grounding installations are protection of the installation, improvement in quality of service and safety of personnel. The essential requirement of effective grounding systems are low earth resistance, adequate current carrying capacity, uniform ground potential on all metal enclosures and structural metals in the generator building and switch yard

It is well known that most of the hydroelectric generating

stations are located on a hilly terrain where abundant water source is available.

More over space availability within the generating

station is limited as pot-head yard/switchyard, machine hall, butterfly valve house are at different elevations. Hence, earthmat design for a hydro power station is a challenging task when compared to the earthmat design for Thermal power stations.

For any grounding system, Soil Resistivity is one of the

vital and prime parameters which determine the resistance of the grounding system. The resistance of any grounding system is determined by the resistivity of the soil surrounding the grounding system.

Earthmat once buried in ground, it is generally not

inspected over a period of time. Hence the cross section of the ground conductor is determined not only by the ground fault current, but depending on the resistivity of soil, the corrosion allowance is also very important.

Central Power Research Institute, Bangalore had recently undertaken a work on measurement, evaluation and design of grounding system for a Hydroelectric generating station situated on a hilly terrain.

This paper presents the details of Soil resistivity measurement and grounding system design based on the site investigations carried out.

II. SYSTEM DESCRIPTION

A proposed Hydro Electric Generating station having a generating capacity of 6 x 68.67MW which is on the hilly terrain was considered for the study of Soil Resistivity Measurement and Design of Grounding system. A. Proposed Grounding System

The proposed grounding system consists of the following: i. Ground mat at power house area ii. Ground mat at Butterfly Valve House area iii. Ground mat at transformer yard iv. Ground mat at pot head yard.

Since the above areas are at difference elevations, all individual grounding systems are to be inter-connected in order to achieve an overall reduction in resistance of the grounding system.

B. Soil Resistivity

The electrical properties of soil constituents are in themselves of great importance, particularly the specific resistance or resistivity of the soil. Soil or earth resistivity, expressed in -cm or -m is the resistance of a cubic unit of earth measured between opposite surfaces. The resistivity is one of the factors in determining the resistance of any grounding system.

Effect of moisture, dissolved salts and temperature has

great influence on the resistivity value of the soil. Upto a certain extent, the resistivity decreases sharply with increase in moisture content, beyond which has a very little effect. Similarly, a small amount of salt present to the moisture results in sharp decrease in resistivity.

With decrease in temperature below 0°C, the resistivity

increases sharply. Because, the moisture present in the soil

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will be frozen making the soil completely dry. To account for the temperature variations during different seasons, measurement of soil resistivity was made at dry winter and dry summer seasons.

III. MEASUREMENT OF SOIL RESISTIVITY

The most widely used method, in practice of measuring the average resistivity of large volumes of undisturbed earth is the Wenner's four electrode method which is outlined in [1] – [4]. In order to measure the resistivity of the earth, it is necessary to pass current through it. This can be done by driving electrodes into the earth, through which current can be led in and out. Wenner’s method of measurement is as shown in Fig. 1.

Fig. 1. Wenners 4-electrode method The four electrodes are buried in four small holes in the

earth, all at depth ‘b’ and spaced in a straight line at intervals ‘a’. A test current I is passed between the two outer electrodes and the potential E between the two inner electrodes is measured with a multi meter or high-impedance voltmeter. Then E/I give the resistance R in which can be substituted in the equation below. The resistivity ‘ρ’ is given by Equation (1), [1] – [5].

2222 4

21

4

ba

a

ba

a

aR

+−

++

=π

ρ (1)

It is to be noted that this does not apply to ground rods

driven to depth ‘b’, it applies only to small electrodes buried at depth ‘b’ with insulated connecting wires. However, in practice four rods are usually driven into earth in a straight line at intervals ‘a’, to a depth not exceeding 0.1a. Thus we assume b=0 and (1) reduces to

ρ = 2 πa R (2)

where ρ = resistivity of the soil in Ω-m a = equidistant spacing of the electrodes in metres R = Resistance in .

Equation (2) gives approximately the average resistivity of the soil to depth ‘a’.

In the case of the proposed site of Generating station, this method was chosen. In order to account for the seasonal variations, measurements were carried out during Summer and Winter season at each locations within the site.

A. Measurement of Soil Resistivity during Summer & Winter season

Unlike in thermal power stations, area available for laying the earth mat is very limited in power stations located on a hilly area. Hence, earthmats are proposed to be laid at different sections which are located in various elevations which are to be interconnected finally. Measurements were made at each location with different directions such that entire area to be covered during the investigations of soil resistivity. As per Wenners method of measurement, by varying the adjacent electrodes to the required spacings on the surface, the resistivity at the depth which is equal to electrode spacings on the surface can be determined. The results of soil resistivity are as shown in Table 1.

Table 1: Measurement of Soil Resistivity at site.

Location Soil Resistivity during Summer

Soil Resistivity during Winter

Pothead yard 345.24 -m 570.24 -m Butterfly Valve house

392.44 -m 568.16 -m

Referring to Table 1, it is seen that measurement made during winter season was higher than that made during summer season which clearly indicate that resistivity increases with decrease in temperature. Thus temperature has great impact on soil resistivity. This brings out clearly the need for burying earthing system sufficiently below the frost level, especially in this type of regions where very low temperatures are experienced. Probably the explanation for the high increase of soil resistivity is that the freezing of the moisture in the soil is analogous to making it dry.

IV. DESIGN OF GROUNDING SYSTEM The grounding system for the present study is designed to achieve the desired objectives to i. Ensure safety of operating personnel from hazardous

potential gradients ii. Protect the equipment from ground faults. The design mainly involves determination of ground conductor size, layout of conductors, computation of grounding system resistance and potential gradients. The design of grounding system has been carried out using in-house developed software at CPRI. The proposed grounding system consists of the following: i. Ground mat in power house ii. Ground mat in Butterfly valve house iii. Ground mat at transformer yard iv. Ground mat at pothead yard v. Service bay All the above grounding systems are proposed to be inter-connected in order to achieve an overall reduction in grounding system resistance.

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V. DESIGN CONSIDERATIONS A. Fault current System study to compute the maximum ground fault current is essential to determine the ground conductor size. Considering the future system growth, a value of 40 kA was chosen for the grounding system design. B. Fault duration (For step & touch potential) In order to calculate the maximum step & touch potentials for maximum permissible body current, fault duration of 0.5 second is assumed which is fault clearing time of back-up protection relays [7]. C. Fault duration (For conductor design) Ground conductor once placed in the ground is generally not inspected. It is prudent to make it suitable for the maximum possible time for which the fault current may flow in the system. Hence for determining the ground conductor size, a value of 1 second is adopted as fault duration time [3]. D. Ground conductor size Based on the magnitude of fault current, earth conductor cross sectional area is computed such that it is adequate enough to carry maximum ground fault current at the station Calculation of ground conductor size is based on the equation as given in [1]. Mild steel of electrical grade is considered as the grounding conductor material as it is the common practice adopted in India. On soil, steel corrodes faster than copper. The extent of corrosion depends upon properties of soil. Hence depending on the value of soil resistivity, a corrosion factor of about 26% [6] and standard commercially available size of the conductor the cross sectional area of conductor is finalized as 800 sq. mm.

VI. DESIGN OF GROUNDING SYSTEM A. Grounding system for Power house The power house at this location is at different elevations. The natural ground available at all these elevations is proposed to be utilized for laying the ground mat. As all these ground mats at different elevations are inter-connected and therefore, they have been treated as one entity in the design calculations. It is stated in [1], [8] that the gradient problem that exists in outdoor substations should not be present within an indoor facility (building). This is on the assumption that the floor surface is effectively equivalent to a conductive plate or close mesh grid that is always at substation ground potential. In view of this, hazardous potential gradients would not occur within an indoor facility. Therefore, in the design of grounding system for power house area the reduction of the grounding system resistance has been the main design criteria. Each mesh size of the grid is finalized as 1.0m x 1.0m in order to achieve minimum ground resistance.

B. Grounding system for pothead yard The grounding system for pothead yard is designed according to [1] where in the dangerous potential gradients within the yard is to be controlled besides achieving a low value of the ground resistance. For this purpose, grounding system consist of a combination of horizontal (grid) conductors & vertical ground rods placed at the periphery of the grid. C. Application of Bentonite to reduce the earth resistance As the site was located in a rocky area, soil resistivity being very high and with available area earmarked for the pothead yard the required earth mat safe design was difficult to achieve by conventional means. Application of bentonite was considered as one of the option to overcome the difficulties in the design. As per earlier studies carried out at CPRI also, Bentonite is an attractive alternative in difficult soil conditions. Application of Bentonite over the flats of the grid reduces the soil resistivity drastically due to the very low resistivity of Bentonite, which is typically of the order of 3-20 -m. As per CPRI studies on commercially available Bentonite, a reduction of soil resistivity to the tune of 60-70% of the original value is reported. In view of the above, a reduction of soil resistivity by about 50% is assumed for further calculations. D. Application of Gravel Since the soil resistivity at pothead yard is high, the step and touch voltages within the pothead yard during ground faults are also expected to be high. Under the circumstances, it is necessary to spread a layer of gravel/crushed rock covering the surface of the substation to artificially increase the contact resistance and reduce shock currents. There are different varieties of gravel available for this purpose. Resistivities of these varieties varies from an average value of 3000 -m to 25000 -m. In this case, it is clear that gravel with a resistivity of 14000 -m with a thickness of 0.2 inch was adopted for calculation. Otherwise, the gravel layer may not be effective in reducing the shock currents through the human body. E. Grounding system for Butter fly valve house The butterfly valve house is at an Elevation of 878.0m. The effective area available for laying the earth mat is 69.0m x 10.50m. The design of ground mat like in powerhouse is proposed at Butterfly valve House and same computer program is utilized to arrive at the design parameters. The proposed grid is to be laid 0.3m below the finished level. Each mesh of the grid is recommended as 1.75mx1.75m size as there is only a marginal decrease in the grounding system resistance for closer mesh.

106 2014 International Conference on Advances in Energy Conversion Technologies (ICAECT)

F. Design of Auxiliary grounding system For designing an auxiliary grounding system, the objective is only to obtain a low value of ground resistance so that this auxiliary mat when interconnected with the main ground mat will offer a reduced value of ground resistance. Accordingly, a suitable design is carried out. The dimension of the auxiliary grid proposed have been optimized in view of the space limitation and found to be of 10m x 8m size. An area of 10m x 8m preferably close to the main pothead area has to be excavated to a depth of 2 metres. This is to be completely filled with Bentonite clay mixed with water in the ratio of 1:4. A depth of 2 meters is chosen only to ensure that the assumption of uniform resistivity in the vertical direction is valid. Resistivity of Bentonite clay varies from 3 to 20 -m and it has to be ensured that the resistivity of Bentonite to be used at this location is also of this order. Nonetheless as a pessimistic value, 20 -m is used as Bentonite resistivity in the design of auxiliary grounding system. G. Computation of grounding resistance Analytical expressions as given in [1] were employed to compute the ground resistance of individual grounding systems based on the configuration of the electrodes proposed at different locations. The values of ground resistances of individual grounding system are presented in Table 2. From this, the overall grounding system resistance is computed as 0.59 .

Table – 2 : Ground Resistance of Individual locations

Sl No

Description Ground Resistance ( )

1 Power House 3.976 2 Service Bay #1 8.612 3 Service Bay #2 9.84 4 Transformer yard 11.455 5 Pothead yard (Eastern) 11.703 6 Pothead yard (Western) 9.053 7 Butterfly valve house 15.954 8 Auxiliary Grounding system 1.14 Overall grounding system 0.59

VII. CONCLUSION

This paper discusses about the seasonal variation of soil resistivity for a generating station situated on a hilly terrain. It is a challenging task to design a safe grounding system design for Powerhouse, Transformer yard, Butterfly valve house and Pothead yard where the soil resistivity is very high due to hilly terrain. The need for grounding system at different areas within the location so as to reduce the overall grounding system is also discussed. Interconnected grounding system resistance at this location is 0.59 which is the acceptable value as per [1] and [3].

VIII. ACKNOWLEDGMENT The authors wish to thank the authorities of CPRI for according permission to present this paper. The authors also wish to thank Dr. R S Shivakumara Aradhya, Additional Director, CPRI for his encouragement during this study.

IX. REFERENCES

[1] IEEE Std.80-2000, “IEEE Guide For Safety in AC Substation Grounding.”.

[2] ANSI/IEEE Std.81-1983, “IEEE Guide For

Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System”.

[3] “Indian Standard Code of Practice for Earthing”,,

IS:3043/1987, Bureau of Indian Standards, New Delhi.

[4] F.C. Wenner, “A method of Measuring Earth

Resistivity”, U.S. Bureau of Standards, Scientific Paper 258, pp. 469-478, 1915.

[5] G.F. Tagg, “Earth Resistances”, George Newnes

Limited, London, 1964. [6] “Manual on Earthing system”, Central Board of

Irrigation and Power, New Delhi, 2006.

[7] Central Board of Irrigation and Power, Technical report No. 49-1989: ‘Earthing system Parameters for HV, EHV and UHV substations’

[8] IEEE Std 665-1995 “IEEE Guide for Generating

Station Grounding”.

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