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| Photo
1 |
| In
other words, the second objective of Standard 80 is to
make sure that all metallic non-current-carrying
substation parts are effectively grounded. |
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| Figure
1 |
| A typical substation grounding system designed for
installation in low resistance soil per Standard 80
might look something like the following: a network of
4/0 AWG bare copper conductors buried 12 to 18 inches
below grade in a grid pattern spaced 10 to 20 feet apart
and extending outside the perimeter fence. |
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| Photo
2 |
| An instrument, which can be used to
measure the soil resistance |
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IAEI
News>Issue Listing>November/December 2002 >Other Code—NESC Substation Grounding — Part 2
Other Code
NESC Substation Grounding — Part 2 |
The almost two hundred pages and ninety five equations of IEEE Standard 80-2000 provide a well explained procedure for the design of safe and practical grounding systems for electric substations. |
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The almost two hundred pages
and ninety five equations of IEEE Standard 80-2000
provide a well explained procedure for the design of
safe and practical grounding systems for electric
substations. There are two objectives of a safe
grounding system as detailed in Clause 4.1: "To
provide means to carry electric currents into the earth
under normal and fault conditions without exceeding any
operating and equipment limits or adversely affecting
continuity of service," and "To assure that a
person in the vicinity of grounded facilities is not
exposed to the danger of critical electric shock"
under normal and fault conditions. One might ask,"
What is dangerous about grounded facilities?"
Grounded facilities are only as safe as the quality of
the ground. Facilities are only safe when they are
effectively grounded. Effectively grounded, as defined
by the NESC on page 6, is "Intentionally connected
to earth through a ground connection or connections of
sufficiently low impedance and having sufficient
current-carrying capacity to limit the buildup of
voltages to levels below that which may result in undue
hazard to persons or to connected equipment." In
other words, the second objective of Standard 80 is to
make sure that all metallic non-current-carrying
substation parts are effectively grounded. [Photo
1]
Hazardous Voltages
The voltages that Standard 80 is concerned with are
touch voltage, step voltage, meshvoltage, metal-to-metal
touch voltage and transferred voltage. Touch voltage is
the potential difference between the earth surface on
which a person may stand and the surface of a facility
the person is touching. Step voltage is "The
difference in surface potential experienced by a person
bridging the distance of 1 m with the feet without
contacting any grounded object." Mesh voltage is
"The maximum touch voltage within a mesh of a
ground grid." Metal-to-metal touch voltage is
"The difference in potential between metallic
objects or structures within a substation site that may
be bridged by direct hand-to-hand or hand-to-feet
contact." Transferred voltage is "A special
case of touch voltage where a voltage is transferred
into or out of the substation from or to a remote point
external to the substation site." The people that
Standard 80 is trying to protect are the people who work
within substations and the general public who may
approach a substation. Though we can partially reduce
the hazard to workers within a substation by requiring
them to wear personal protective equipment like
insulated footwear, the hazard to the public, someone
outside the fence, can only be reduced by proper design
of the grounding system.
A Typical Grounding
System
A typical substation grounding system designed for
installation in low resistance soil per Standard 80
might look something like the following: a network of
4/0 AWG bare copper conductors buried 12 to 18 inches
below grade in a grid pattern spaced 10 to 20 feet apart
and extending outside the perimeter fence. [Figure
1]
At crossings, the conductors would be securely bonded.
Ground rods would be securely bonded to the grid at
corners and at junction points along the perimeter. All
aboveground metallic facilities including structures,
transformers, breakers, and fencing would be securely
bonded to the grid with grounding conductors. Additional
ground rods would be securely bonded to the grid at
major facilities and surge arrester locations.
The Design Process
The process of design as described in Standard 80 is one
of acquiring data on the proposed substation and site
characteristics, developing a preliminary design,
calculating the various hazardous voltages at many
locations within and outside the substation to determine
the relative safety of the design, modifying the design
as necessary, and recalculating the hazardous voltages
to insure the design meets the objectives. The process
may have to be worked through many times before the
objectives are achieved.
Input Data
A large part of the design process is the accumulation
of data for input into the equations. The layout of the
facilities to be installed in the substation must be
complete. The available fault-current magnitudes must be
known. In order to determine the maximum fault
durations, the protection scheme must be complete. The
effect of future changes must be considered. For
example, making changes to an existing grounding system
to accommodate increases in available fault current can
be very expensive. The soil resistance characteristics
must be measured at the site of the proposed substation.
Soil Resistance
Soil resistance is often the most difficult information
to obtain. The electrical resistance of soil varies with
soil temperature, soil moisture content, current
magnitude, and soil type. Since soil type often varies
with depth and location, the soil resistance profile at
a proposed substation site may be quite complex. It is
rare to find uniform soil resistance over the entire
area and at various depths. The discussion of soil
characteristics starts in Clause12 of Standard 80.
"To determine the actual soil resistivity, tests
such as those described in IEEE Standard 81-1983 should
be performed" (Clause 12.4).
IEEE Standard 81-1983
IEEE Standard 81 is the IEEE Guide for Measuring Earth
Resistivity, Ground Impedance, and Earth Surface
Potentials of a Ground System. The most accurate method
for measuring soil resistance is what they call the
Wenner four-point method. This method is discussed in
detail in section 7, page 12 of Standard 81. The four
point method consists of driving four short ground rods
into the soil, placed in a straight line, separated by a
distance (a). The rods are only driven into the ground
to a depth (b) = a /10. The test instrument induces a
current (I) into the soil between the two outer rods and
then measures the voltage (V) on the two inner rods. The
resistance is equal to the voltage divided by the
current (V/I). The instrument does the calculation. The
instrument reading then has to be multiplied by 2pa to
get the average resistance.
The resistance we measure with a rod separation of 2
meters is the average resistivity of the soil to the
depth of 2 meters. The resistance we measure with a rod
separation of 4 meters is the average resistivity of the
soil to the depth of 4 meters. If the 2-meter resistance
and the 4-meter resistance are different, then the soil
type, moisture content, and/or temperature of the top
two meters of soil is different from the bottom two
meters of soil. One instrument, which can be used to
measure the soil resistance, is shown in photo
2.
The soil resistance measurement instruments cost from
$1800 to $4500 and it is not uncommon for two people to
spend three days taking measurements at a 100 m by 100 m
site. The goal is to develop a three dimensional
analysis of soil resistance. Many measurements should be
taken on the site including measurements at various
depths. The number of measurements necessary to obtain
an adequate resistance profile for a site is dependent
upon the variability of the soil. It is common to find
several layers with dramatic differences in resistance
below the surface. Changes in lateral resistance are
usually more gradual. The number of measurements should
be increased in areas of great change in resistance and
in areas of high resistance. Interpretation of the soil
resistivity measurements and the development of a soil
model are the most difficult part of soil data
procurement. In Part
3, I will talk about soil model
development and continue with the design process.
If you have general questions
about the NESC®, please call me at 302-454-4910 or
e-mail me at dave.young@conectiv.com .
National Electrical Safety Code® and NESC® are
registered trademarks of the Institute of Electrical and
Electronics Engineers.
Dave Young is a senior engineer with Conectiv Power
Delivery of Wilmington, Delaware, where he has been
working with and teaching the NESC® for over 31 years.
He is a member of the NESC Interpretations Subcommittee
and the NESC Overhead Line Clearances Subcommittee 4.
Dave is also vice-chairman of the Delmarva Division of
the IAEI Chesapeake Chapter.
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