Palm Kernel Oil Cake as an alternative to earth resistance-reducing agent.
Eduful, George ; Cole, Joseph Ekow ; Tetteh, F.M. 等
Introduction
The term earthing is defined as the connection of a conductor or
frame of a device to the general mass of earth. Earthing plays an
important role in generation, transmission and distribution for safe and
proper operation of any electric installation. In many of the
applications of earthing, low earth resistance is essential to meet
electrical safety standards. However, in certain soils, it has been
extremely difficult to obtain and maintain a satisfactory earth
resistance values. As a means of reducing the earth resistance, a
chemical treatment and other methods are used. In most developing
countries, materials used as earth resistance-reducing agents are often
imported. As a result, the cost of backfills for lowering earth
resistance value is very high. In a study to determine the most
effective method of installing low resistive earth electrode, majority
of the standard methods were rejected for practicality or cost reasons
[1]. To reduce cost and save scarce foreign exchange for other projects,
there is the need to find cheaper local materials as close substitutes
to the imported materials. Palm kernel oil cake (PKOC) is an organic
material which provides a low resistance when used to backfill around
earth rods. Where chemicals are used, electrolyte such as sodium
chloride, magnesium sulfate, copper sulfate, magnesium chloride, calcium
chloride, ammonium chloride, or the like, is injected into the soil
surrounding the earth electrode to reduce the earth resistance. When
this method is applied, the soil shows an extremely favorable earth
resistance for some period after the treatment. However, the chemicals
are carried away by surface run-off water and subsoil water during
rainfall, and the effect of chemicals lasts only for a period ranging
from several months in an extreme case to 3 years at the longest, the
mean effective period being about 2 years [2]. Accordingly, the chemical
treatment must be repeated after a certain period. However, such
repeated treatments, maintenance, and inspection are very difficult in
remote and deserted places [2]. Also questions regarding environmental
impact of these chemical remained unanswered. Others have raised
concerns about ground water contamination from the chemicals [1].
In most soils, organic matter accounts for up to 10% of the soil
mixture [3]. Organic material plays an important role in soil structure.
It acts as a cementing agent to bind soil particles together. Thus, the
organic nature of the PKOC makes it a promising candidate in the earth
resistance-reducing agents. The PKOC does not only appear to possess the
necessary electrical properties, but is environmentally and economically
friendly material.
A natural method of reducing earth resistance will be replacing all
soil in the effective resistance area with the PKOC. However this method
may not in general make sense economically. For this to be realistic, it
will be necessary to limit the backfill to the area very close to the
earth rod and judged to be accounting for a substantial percentage of
the total earth resistance. The area is referred to as critical
resistance area and its radius as critical resistance radius. In this
paper, the characteristic of PKOC is investigated and its effectiveness
as a backfill in critical resistance area to lower earth resistance is
demonstrated by field test data. It is therefore the principal objective
of this paper to provide an improved and cost- effective method of
reducing earth resistance by use of PKOC.
Methodology
Chemical properties of the PKOC were tested in a Soil Research
Institute. The chemical properties were compared with other earth
resistance-reducing agents. Parameters determined were total nitrogen,
carbon, phosphorus, potassium, calcium, magnesium, and pH (acidity).
Organic carbon was determined using the modified Walkley and Black
method [4] Total nitrogen was determined by the modified Kjeldahl
digestion and distillation method.
Sodium and Potassium were determined by taken 5ml of 1:1 nitric
acid concentrate + perchloric acid, 5ml of concentrated sulphuric acid
and 1g sample PKOC mixed in a digestion flask and placed on
electro-thermal heater for an hour. When sample became clear, it was
left to cool then 50mls of distill water was added to the sample volume;
a concentrate of Sodium and Potassium was read on a flame photometer
after calibrating the instrument with known standards. The concentrate
of the unknown is extrapolated from the standard curve
Moisture content of the PKOC was also examined. Moisture content
was assessed by taking a PKOC sample at a depth of about 0.3 meter and
putting it in a plastic bag immediately. The sample was then weighed and
dried to constant weight in the oven at 70[degrees]C. The weight of the
dried sample was taken and the difference was expressed as a percentage,
with the result being the PKOC's percent moisture by weight [5]
A water- holding-capacity test was carried out by equilibrating a
known weight of the cake (20 g) with a known volume of water at various
time duration.
Resistivity was measured by the Frank Wenner's method [6].
Earth resistance values were recorded using the DET5/4R Digital Earth
Tester applying the Fall- of-Potential method or the so-called
"62%" rule [6, 7].
To determine the effectiveness of the earth resistance reduction
technique, a 100% PKOC was applied as a backfill in a critical
resistance radius of earth electrode at selected sites and resistance
behavior at the sites monitored over period of three years.
Result and Discussion
Table-1[(a) and (b)] shows chemical composition of PKOC as compared
to other backfills. Major parameters identified to contribute
significantly to the high electrical conduction of the PKOC are carbon,
moisture content and low acidity level. The chemical analysis and
moisture characteristics show that PKOC consists of relatively large
conductive carbonaceous aggregate 56.22%. This demonstrates the ability
of the PKOC to hold its moisture contents for a considerable period and
absorbs moisture from the surrounding soil even in the degraded state.
According to [8], high electrical conductivity of carbon based materials
is observed above 14% of carbon concentration. Further increase in
carbon concentration beyond the critical concentration region (>24%)
causes marginal change in conductivity.
The moisture content of the palm kernel oil cake (air- dried) was
14.2%. Results indicated that moisture holding content increased with
time from an initial level of 240 to 260%. This means that the kernel
oil cake can absorb moisture about two and a half times its weight. It
is our believed that the charge carriers in PKOC are ions in the
moisture plus the ions provided by the slightly acidity of the PKOC.
Additionally, the PKOC conducts by means of free electrons in the
carbon, the same as a metal does, even when dry.
Moisture content is one of the controlling factors in earth
resistance because electrical conduction in soil is essentially
electrolytic. Electrolytic conductivity increases at a rate of
approximately 1.9% per [degrees]C increase in temperature. The moisture
characteristics of the PKOC eliminate the need to use moisture retentive
materials such as silica gel and resins of a lignin type or a urea type.
It is the most probable that the moisture property contributes
significantly in making the PKOC a good material for earth
resistance--reducing agent.
Acidity level (pH) of the PKOC was found to be 5.13. Soil pH is a
measure of the acidity or alkalinity of a soil. A pH below 7.0 is acidic
and above 7.0 is alkaline.
Electrode corrosion is high at pH levels above 7.0. High pH levels
affect durability of earthing system and could result in safety threat
to both personnel and equipment. The durability of an earth electrode
backfilled with PKOC can be expected to be high due to its relatively
low acid content.
Application Technique of PKOC
Test was conducted at three different sites not far from each
other. A plot of resistance R against distance x for the three rods used
for the study is shown in Fig. 1. The rods are 0.5-, 1- and 1.4-m long
and each has a diameter of 14 mm. The resistance is expressed as a
percentage of the total earth resistance. The distance is also expressed
as a percentage of the length of rod 1.
[FIGURE 1 OMITTED]
It is observed that the resistance curves begin to saturate around
40% distance from the rods. Thus the 40% distance becomes a good
proposition for the critical radius. This radius will be used as
critical resistance radius for the study.
Field Validation
To determine the effectiveness of the earth resistance reduction
technique, the 1-m electrode was driven into a soil of resistivity 300
ohm-m. At this resistivity, earth resistance of 236-ohms was measured.
The soil within the critical resistance radius of the 1-m electrode
(0.4-m) was removed and backfilled with PKOC of resistivity 5.7 ohm-m.
[FIGURE 2 OMITTED]
As a result, the resistance dropped to 62.54 ohms representing a
percentage reduction of over 73%.
PKOC has been applied at seven selected sites with different
types/texture of soil. Average soil resistivity values and moisture
content at the sites are presented in Table. Figure-3 shows test results
at these sites over a period of 3 years. The results show an average
earth resistance improvement of about 50%. It was noted; see Fig-3, that
the PKOC was very effective in high resistive soil with low moisture
content. Percentage change in earth resistance value over 3years with
PKOC application in high resistive soil was over 80%. This significant
change is attributed to indirectly extending the cross sectional area of
the earth electrode by the PKOC, ability of the PKOC to absorb moisture
and the presence of charge carriers in the form of ions in the slightly
acidity of the PKOC.
[FIGURE 3 OMITTED]
Conclusion
The study has so far shown that the PKOC provides and maintain an
excellent earth resistance-reducing effect for a long period of time.
The PKOC is quite stable with resistance to acids and alkalis, and are
able to maintain permanently earth resistance-reducing effect
substantially without being lost by rainfall. As a result, fluctuation
of earth resistance due to seasonal changes of climate and rainfall is
substantially negligible.
Reference
[1] Martin D. Conroy and Paul G. Richard, 1993: Deep Earth
Grounding Vs Shallow Earth Grounding. The Power Quality '93
Conference.
[2] Higashimura, Einosuke (Tokyo, JA) et al: Earth
Resistance-Reducing Agent and Method of Reducing Earth Resistance by use
of Same. http://www.freepatentsonline.com (Date assessed: 17th August,
2007).
[3] http://www.dpiw.tas.gov.au/inter.nsf/WebPages/TPRY-5YW6YZ?open.
(Date assessed: 27th August, 2008).
[4] Brian A. Schumacher, 2002: Methods For The Determination Of
Total Organic Carbon (Toc) In Soils And Sediments.
http://www.epa.gov/nerlesd1/cmb/research/papers/bs116.pdf. (Date
assessed: 13th February, 2008.
[5] Roy B. Carpenter, Jr. and Joseph A. Lanzoni Designing For A Low
Resistance Earth Interface (Grounding). An LEC Publication Revised 2007
[6] Getting Down to Earth (A practical guide to Earth Resistance
Testing).- Megger Publication.
[7] Burger, H.R., 1992. Exploration Geophysics of the Shallow
Subsurface. Prentice Hall PTR, Upper Saddle River, NJ.
[8] Zhang, Wei et al, 2007: Effect of carbon black concentration on
electrical conductivity of epoxy resin-carbon black-silica
nanocomposites. Journal of Materials Science, Volume 42, Number 18,
September 2007, pp. 7861-7865(5)
(1) George Eduful and Joseph Ekow Cole
(1) Electricity Company of Ghana P.O. Box 1980 Kumasi-Ghana E-mail:
georgeeduful@yahoo.com
F.M Tetteh
Soil Research Institute--Center for Scientific and Industrial
Research, Kwadaso-Kumasi Ghana
Table 1: Chemical composition of PKOC as compared to other backfills.
% Composition
LEC
Sample PKOC Crystal LEC Coke Marconite
Parameter activator GAF
Carbon 56.22 3 4.45 3.9 0.51
Nitrogen 46.86 0.04 0.14 0.68 0.05
Crude
Protein 30.25 0.25 0.88 4.25 0.31
Calcium 0.32 0.16 0.84 0.16 0.08
Magnesium 4.48 0.12 1.92 0.32 0.07
Phosphorous 0.42 Trace 0.2 0.003 0.081
Potassium .57 1.43 0.42 0.02 Trace
Moisture 240.2 11.53 191.72 56.69 62.5
Sodium 0.002 40.50 1.43 -- --
(a)
LEC
Sample PKOC Crystal LEC Coke Marconite
Parameter activator GAF
pH 1:1 5.34 -- 7.76 5.62 5.55
Ph 1:2.5 4.76 9.53 7.65 -- --
(b)
Table 2: Soil Resistivities and Moisture contents at 7 sites of
investigation.
Substation Top-timbers G. Gate Chief's P
B15
Average
Resistivity 6653 5200 570 5499
([OMEGA].m)
Moisture
Content(%) 1.2 1.9 2.1 1.2
Tano-Odum Berebe-T Unity Oil
Average
Resistivity 5125 550 4500
([OMEGA].m)
Moisture
Content(%) 1.5 2.4 2
