soil resistivity measurement & effects

7/28/2019 Soil resistivity measurement & effects

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Study and analysis of different parameters affecting substation grounding

Experiments on gravel

Gravel is rock particles composed of unconsolidated rock fragments. Substation

upper layer is covered with gravel. They are used in upper level layer due their high

resistivitys above 50000 , thereby reducing step and touch potential. Gravel also

protects vegetation(i.e. weeds, grass, moss, any type of organic matter on top of a gravel

layer) to grow on the substation grounds which affects soil resistivities adversely causing

decrease in resistance drastically. Gravel is also used to keep the area clean, drain well,

and be non-muddy. Thus the gravel provides an insulating layer and increases the

allowable step- and touch-voltage. The low soil resistivity is needed below the gravel

layer.

Setup for gravel testing:

Since gravel is one of the important factor regarding substation grounding,

various factors affecting gravel resistivity are studied and analysed.

For carrying out experiment we have referred Referance.

Gravel to be tested is taken in a bucket(should be insulator with enough mechanical andbreakdown strength), which is cutted in a bottom and a G.I. plate with holes is

fitted(strainer like structure). G.I. plate is used in order to get a good current conductivity

and mechanical strength. Holes are drilled to give a way to water for seepage in wet and

saltwater testing.

Gravel are filled in a bucket and the upper layer of a gravel is covered with

aluminium foil. Connections are bought from both lower G.I plate and upper aluminium

foil for supplying voltage across it.

Procedure:

Setup is done as explained above. Aluminium foil is kept above gravel and a

weight is kept above it for few minutes in order to ensure foil is touching with gravel at

maximum possible point with uniformity. While performing experiments we have faced


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problems with aluminium foil as it is tattering due to weight and roughness of gravel.

This creates problem of non-uniform flow of current and increase in electrode resistance.

To overcome this foil is used with G.I. plate which gives mechanical strength and good

conductivity for electrode.

After improvements in the setup we have performed to check resistivity of gravel

under different conditions. That is for dry gravel, wet gravel, with salt water. For wet and

salt water readings are taken at periodic time slots which gives different readings due to

seepage of water through strainer at the bottom of the bucket.

Salt water readings:

For salt water different salt solutions are prepared with different percentage

of salt content in water. This salt solutions are prepared with the help of electronic

balance and PH meter. PH meter is used in order to ensure that water using for

experiments should not be acidic or basic in nature and is within prescribed limit given in

reference.

Different samples used in testing are given below:

1. Dry Gravel2. Wet gravel with waterPH level3. Salt water with

Basic calculations for volume measurement for bucket:


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Readings:

Calculations:

Conclusion table:

Graph:


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Conclusions drawn:

Soil resistivity measurements:

Why soil resistivity

For Soil resistivity measurement another test cell is prepared. For test cell a

non conductive rectangular box with lid having enough strength bear a pressure applied

on soil during tests. It should not get deformed which will introduce errors in

measurements. Box chosen should be sharp at corners in order to avoid miss calculationsin volume measurement. A simple plastic or PVC box will do a work. A small hole is

drilled in a lid enough for a passage of a thermometer for recording temperature while

tests on frozen soil.

At the both end faces of the box along the length, metal plates are fitted with BIT

terminals, in order to provide connections for applying test voltage and measurement of

current through soil. Plates used are G.I. type, in order to get good conductivity and

uniform flow of current.

Test procedure:

Experiments are performed for detailed analysis on effect of moisture, dryness and

temperature on soil resistivity. These analyses are very important regarding effective


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grounding. (Remember here soil so taken for measurement is black soil. Resistivitys

different for different soils, due to minerals and soil composition.)

Again for soil resistivity testing different samples are prepared with different

combination to get good simulation of atmospheric conditions. Different samples

prepared are as follows,

1. Dry soil2. Wet soil with water percentage3. Frozen soil at -40C.

Dry soil is prepared by drying in an oven for 3 hours.

After samples are prepared each samples is filled in a box with enough pressure soas to get uniformity and avoid air bubbles and voids which may disturb resistivity

measurements. 5-6 readings are taken for a factor of safety for each sample, and an

average is taken as a resistivity of that soil sample.

For testing with frozen soil sample readings are taken at different time

intervals with taking reference to temperature. So we will get resistivity

measurements at wide temperature ranges.

Basic Calculations for volume of soil:

Readings:


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Resistivity calculations:

Conclusion table:

Graph:

Conclusion from graph:


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soil resistivity measurement & effects

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