Gaoyuan, LIANG
Year 2010
Grounding Design of Electric Power Plants and Transformer
Substations in High Soil Resistivity Area
Gaoyuan Liang
Inner Mongolia Electric Power Research Institute Network Centre
Abstract: This paper discusses current grounding
standards still maintain 2000V earth potential
design issues of present electric power plants,
regulation value unchanged even though the system
transformer substations in high soil resistivity area,
short circuit capacity is growing, therefore, it is much
by incorporating the actual engineering data of a
strict on the permissible value of grounding grid
220KV substation, analyzes the impact degree of
contact potential and step potential. According to the
grounding design parameters to earth potential,
recent situation analysis of engineering design, it is
contact potential, and step potential. It also can be
very difficult to drop ground resistance below
used as a reference point for peers to discuss in
DL/T621-1997 industrial standards. The following
engineering design.
article will emphasis on exploring current grounding
design issues and solutions of present electric power
Key Words:High Resistivity, Electric Power
plants, transformer substation in high soil resistivity
Plants, Transformer Substation, Grounding,
area, by incorporating the actual engineering data of a
Design
220KV substation.
The main object of grounding design for electric
1.
Overview power plants and transformer substation is that
Along with the rapid demand growth of urban
designing a desirable grounding grid to meet power
residential electric load, there are increasing number
system operational requirements, ensure the safety of
of 220KV transformer substation are seat in city as
personnel and devices. A low resistance grounding
distribution substations in order to meet the electrical
grid obviously benefits the safe running of the
load distribution requirements. The major features of
equipment and personnel against harmful shock.
those transformer substations are: small-scale sites,
However, there are many factors contribute to the
using GIS on 220KV and 110KV high-voltage
safety grounding of electric power plants and
electrical equipment, occupying less area.
transformer substations. Considering the expanding of
current electrical network capacity, there is no safety
The increasing power system capacity has cased high
guarantee of equipment and personnel even though
earth fault currents in those substations. Therefore, the
the grounding grid can be maintained at a low
outstanding
resistance level. There is no direct connection between
grounding
issues
design
in
are:
transformer
small
substation
grounding
area
the resistance of overall grounding system and a
(5,000~10,000 m ), high soil resistivity (typically
harmful shock to persons. Electric power plants and
500~1200Ω·m or higher), lack of condition to lay the
transformer substations with relatively low grounding
external grounding. Although electric power plants
resistance can be very dangerous. On the contrast, by
occupy larger area than transformer substations, it is
careful designing the grounding grid, those with
still a tough challenge in designing ground grid when
higher grounding resistance will become more secure
the soil resistivity is higher and earth current is
(1). The following paragraph will discuss the factors
escalating. In addition, DL/T621-1997 industrial
and the solutions when designing safety grounding for
2
Grounding Design of Electric Power Plants and Transformer Substations in High Soil Resistivity Area
1 Gaoyuan, LIANG
Year 2010
Electric Power Plants, Transformer Substation in high
Step potential difference is the ground surface voltage
soil resistivity areas.
when the distance between the feet of a person is 0.8m.
Because the voltage decreased in soil when earth fault
currents or lighting currents flow into ground, where
2.
The safety grounding design factors the soil resistivity contributes voltage difference the
and solutions most. In the uniform soil, the step potential difference
The aim of sound safety grounding grids design is that
is small due to the lower soil resistivity. Likewise, the
to provide a lower enough secure access point on
step potential difference increased due to the high soil
earth potential, contact potential, and step potential for
resistivity. However, it is far more complicated in the
electric power plants and transformer substations.
non-uniform soil condition. The soil resistivity is still
the main drive of step potential difference on human
Contact potential is formed when earth-fault current
elements, see formula below (3),
flow through equipments and generates distribution
Step potential difference (Allowable Value) Us(V)
potential on the ground surface, the voltage exists
where the distance from ground surface to equipments
is 0.8m horizontal and 1.8m vertically. As a matter of
Us(V)= Ut(v)=(174+0.17ρf)/
Maximum
Grounding
Step
(3)
potential
difference
fact, the existing mesh of grounding grid generates
voltage between every point on the mesh to grounding.
The maximum contact potential difference is the
Usmax(V)
Usmax(V)= Ksmax Ug
(4)
voltage (maximum value) from the centre of mesh to
the earth electrode of grounding grid. When operators
working with charged equipment shell have grounding
Usmax - Maximum Step Potential Difference;
Ksmax
Coefficient of Maximum Step
-
access, it is crucial to avoid potential electric shock
hazard by limiting contact potential difference within
Potential Difference
a security level.
Earth potential rise is cause by a grounding fault, the
voltage rise in ground grid when earth current flows
The formulas to calculate allowable contact potential
through electrode. The shift of potential caused by
difference value (Ut(v))and grounding grid maximum
increasing earth potential is not only threat the
contact potential difference (Utmax(v)) are showing as
equipments and operators, but also creates potential
below:
counterattack voltage hazards to local equipments.
Contact Potential Difference (Allowable Value) Ut(v)
The index of earth current and grounding resistivity
are earth potential.
Ut(v)=(174+0.17ρf)/
Grounding
Grid
Maximum
(2)
Earth potential Ug(v)
Contact
Potential
Difference Utmax(v)
I - Ground short circuit current, A
Utmax=KmaxUg
Utmax
-
Ug (v)=IR
Maximum
R – Earth resistance of grounding devices
Contact
Potential
There are many factors contribute to above three
elements, somehow, the most influential ones are
Difference;
Ktmax
earth fault current (I), duration time (t), shunt
-
Coefficient of Maximum Contact
coefficient of overhead ground wire (K), soil
resistivity
Potential Difference
(ρ),
ground
surface
resistivity
(ρf),
uniformity of soil resistivity etc.
Grounding Design of Electric Power Plants and Transformer Substations in High Soil Resistivity Area
2 Gaoyuan, LIANG
Year 2010
The following paragraphs will also analyze the
considering double-layer soil
requirements, influential factors and solutions to
new contact and step potentials are 773V, 2483V. As
above three security elements in grounding grid
the ground resistance is 2.5Ω, the earth potential value
design. An existing GW substation design data will be
is as high as 28170V, far more great than standard
used to demonstrate influential factors and degrees in
2000V. In order to reach the requirements, the ground
grounding grid design elements.
resistance should below 0.1Ω and it is 4% of the
The original parameters of a substation grounding
original value, which also suggested minimizing soil
design are listed as below:
(ρf =2500Ω·m), the
resistivity down to 34.8Ω·m.
Duration time of ground short circuit (fault)
The argument is focused on how to maintain the
safety of GW substation grounding grid while the
current, t:0.6s
ground resistance cannot be reduced.
Maximum current of earth fault (maximum
shunt coefficient), I:1.5kA
to grounding design parameters
Uniform soil resistivity value (including the
seasonal coefficient),
2.1 The impact of earth fault duration time
As shown on Chart 1, the variation curves of contact
potential and step potential move with earth fault
duration time. It is quite obvious that the fault
：500Ω·m
duration time has a great impact on safety issues.
Available grounding grid area (109*92m), S:
operating much safer. For instance, when fault cleared
10028m2
in 0.1s, contact and step potentials are 819V, 1657V.
Surface resistivity of cushion macadam (moist),
When it is in 0.6s, the contact and step potentials are
334V and 676V, the allowable voltage are all
ρf: 2500Ω·m
According
Therefore, fast fault clearing will guard substation
decreased to 40% of its value in 0.1s. The fault
to
DL/T621-1997,
devices
duration time has a greater influence to contact
grounding at Electric Power Plants, Transformer
potentials and step potentials, especially between
Substation “should dominate horizontal manual
0.1~0.9s. In reality, the switch breaking time and
grounding grids”. GW substation proposed to adopt
protection operating time of circuit breaker units that
compound
are mostly relied on the type of equipments and
grounding
grids
electric
which
combined
horizontal and vertical grounding electrodes.
By
manufacture which normally cannot be changed. Also
engineering calculation, the allowable contact and
it would be too expensive and unrealistic if the
step potentials are 334V, 676V respectively. If
changes have to be made.
Grounding Design of Electric Power Plants and Transformer Substations in High Soil Resistivity Area
3 Gaoyuan, LIANG
2.2 The impact of ground surface resistivity
to grounding design parameters
Year 2010
surface. For instance, GW substation sets pebble
isolation layer with 2500 Ω · m, the number flat steel
required for horizontal ground are 600m (weight
Chart 2 demonstrates the impact of ground surface
2.226t), and it meets step potential requirement by
resistivity to contact potentials and step potentials. It
setting up a 20m mesh distance. Otherwise, the
shows that ground surface resistivity has a greater
number flat steel required for horizontal ground would
impact to the allowable vale of step potentials than
be 20000m (weight 75.4t), and the mesh spacing is
contact potentials. Therefore, it is more effective to
1m which is more than 33 times more than setting
adopt quarantine measures when improving step
pebble isolation layer.
potential hazard. In engineering design, contact
Hence, adopting isolation layer method is more
potentials can be limited by using high soil resistivity
economical wise for substations occupy small area.
material on operating platform.
But rain is a critical element where using pebble
Considering the surface soil resistivity has greater
isolation layer, the resistivity sharply drops when the
impact on the allowable value of contact and step
ground surface is wet or rain. Therefore, it is quite
potentials, ground electrode material usage can be
important to wear insulating shoes when entering
reduced by paving high resistivity material on the
operating areas in raining days or wet seasons.
In the uniform soil environment, there is a negative
resistivity. It has a much obviously result to decrease
linear correlation between earth potential and soil
contact and step potentials by reducing soil resistivity
Grounding Design of Electric Power Plants and Transformer Substations in High Soil Resistivity Area
4 Gaoyuan, LIANG
Year 2010
rather than earth potential. And the grounding grids
According to the calculate formula, there is a positive
become secure when resistivity drops down to certain
linear correlation between earth potential and earth
lower value. In applied engineering, it is difficult and
current. It is an effective method to maintain low earth
expensive to reduce resistivity partially, not to
potential, maximum contact and step potentials of
mention dropping down to certain lower value. From
ground grid by decreasing system earth fault current,
above analysis, in order to meet contact potentials, it
increasing ground current distribution, and cutting
is un-realistic and un-achievable to reduce earth
down earth current. In practice, system capacity,
resistance from 2.5 Ω to 0.1 Ω, even though by
impedance, and operating methods are determining
investing great amount of labour and material.
earth fault current. Also fault current in two-phase and
For example, a substation in Shenzhen city, the
single-phase grounding can be limited to certain point
measured earth resistivity was 3.08Ω before applying
by
any resistance reducing measurements, even though
recommended
adopted the methods of connecting
restricting system operation. With the system capacity
earth to trench,
system
adjustment.
to
earth
However,
decrease
fault
earth
current
it
is
not
potential
will
by
replacing soil, adding resistance reducing agent, and
expending,
increasing
burring earth deep, earth resistivity only dropped
accordingly. To decrease earth current, the ideal
down to 2.6Ω, however, the investment was increased
method is increasing the diversion effect of overhead
7,000,000. Even with the recommended “explosion
ground wire, but should considering the affect from
grounding technology” which could reduce earth
wiring tower potential.
resistivity by 20%~50%, only reached to 0.62~1.5Ω.
It still does not meet owner required 0.5Ω. When
decrease contact potential to 1243.5 ~ 3108.8V, still
2.4 The impact of grounding grid size to
grounding design parameters
does not meet standard specification of 444.1V. It is
For the grounding grid has sufficient area, there is a
quite challenge to maintain earth potential at 2000V as
negative linear correlation between earth resistance
well as resistivity reached to 0.1Ω.
and square root of ground grid area. With larger
All above, soil resistivity contributes the most to earth
ground grid area, earth resistance is relatively smaller.
potential, while ground surface resistivity affects
Hence, it is an effective way to decrease earth
contact and step potentials the most. In conclusion, the
resistance by increasing ground grid area in the design.
most effective way to solve contact potential and step
As shown in chart 3, when doubling the ground grid
potential issues is that paving high resistivity isolation
area, earth potential and earth resistance are all
layers on the ground.
decreased by 30%. If grounding the lowest resistance
part of soil, the result will be more remarkable. But
2.3 The impact of earth current (or shunt
coefficient
parameters
)
to
grounding
design
from the practical perspective, on few electric power
plants and substations at the seaside could achieve it,
the rest simply lack of land to adopt this method.
Grounding Design of Electric Power Plants and Transformer Substations in High Soil Resistivity Area
5 Gaoyuan, LIANG
Year 2010
2.5 The impact of grounding material quantity to grounding design parameters
To certain grounding grid, increasing the amount of
potential difference from centre of mesh to grounding
grounding material can effectively decline grounding
electrode, and step potential. For instance, there are
contact potential, especially to step potential. As show
cases, in foreign countries where the operation
in chart 4, it demonstrates the reflection of contact and
platform is made from 0.6m * 0.6m (1) metal mesh.
step potential to horizontal grounding material
However, it is not economic wise to solve the
quantity. Mainly because of the additional grounding
excessive contact and step potential issues by
material,
horizontal
adopting excessive grounding material. Reasonable
grounding electrode, not only reduced mesh spacing
use of grounding material and control project cost are
of ground grid, the ground surface potential
must be considered in design.
especially
by
increasing
distribution became more even, but also solved the
is more than 6~7 times (4). Take GW substation for
In summary, by measuring grounding grid of existing
instance, its earth potential is 28.7kV, which is 14.3
electric power plants and substation, also including
times of standard 2kV.
new designed grounding grid, the actual measurement
plants and substations located in high soil resistivity
results of earth, contact and step potentials from
area, it is also not cost effective and realistic by
majority substations are beyond standard. In some
decreasing earth potential to standard. It is a huge
cases, there are few substations are above standard
challenge in ground designing to achieve a reasonable
earth potential limit by 10 times, and contact potential
safe ground grid when the high potential exists.
For those electric power
Grounding Design of Electric Power Plants and Transformer Substations in High Soil Resistivity Area
6 Gaoyuan, LIANG
3.
Complete
Year 2010
ground
grid
design
measurement
insulated frames can be applied in real scenarios. If
the ground grid is located within the fences, it is
necessary to set isolation area out side of the fences
By applying essential measurements to reduce
and displaying “electric danger” sign to minimize
resistance, the earth potential still cannot be reduced
potential electric hazard to staff. If the ground grid is
to the standard requirement. Grounding grid design
beyond fences, the step potential must satisfy the
should be focused on dealing with voltage equalizer
safety standard.
and isolation, especially on the particular area on
ground grid.
3.1 Voltage equavalent issue on ground grid
edge
Theoretically, electric power plants and substations
are designed on the basis of uniformed soil resistivity.
Actually, soil resistivity is not equivalent either
horizontally or vertically. According to DL/T621-1997
standard, it is difficult for the non-equalizing treated
ground grid to meet contact and step potential
requirement even if the earth potential is below 2000V.
3.3 Control cable grounding issue
It is important to strengthen the control of cable
grounding to prevent counterattack damage to the
cable and secondary equipment. If the cable is not
very long, grounding one end of cable jacket can
reduce current outside of the single-conductor cable.
For a long cable, both sides and the skin of the joint
all should be grounded. To eliminate the EMF, both
ends of shielding control cables should be grounded.
The control cable jacket spacing may be too large at
each grounding point, which will cause large current
flows through cable jacket when earth fault potential
happened. To avoid current damage, the best solution
is installing a separate ground cable on cable jacket
where between grounding point and parallel to the
control cable.
In the case of GW substation, step potential meets
requirement when soil resistivity dropped down to
3.4 Telecommunication cable grounding issue
34.8Ω, however, the contact potential still needs to be
It is important to isolate communication cable to avoid
the shift potential damage.
Communication lines need to take into measurement
in design to ensure the safety of staff and
communication terminal devices. In order for the
substation communication terminal and remote
terminal isolation, consider using fiber optic
communications cable, eliminating high potential of
transfer.
worked on by potential equalizing treatment. Since the
edge effect of earth current, there is a huge potential
gradient on the ground grid edge (1). The design
should be focused on dealing with the voltage
equalize on grid edge.
To prevent potential difference over the standard, the
main measurements are:
a)
Increasing the density and depth along
vertical electrode on ground grid edge
b)
Burying two or more horizontal electrode
around the edge. And the inbuilt electrodes
should go deeper with the distance far away
from substation, which is called the “hat
brim” voltage potential area.
c)
Changing the distance between horizontal
electrodes on grid. The density of horizontal
electrodes should be higher at where close
to the grid edge.
3.2 Metal frame grounding issue
To prevent the possible earth potential damage caused
by metal frame around electric power plants and
substations, connecting frame to ground grid or using
3.5 Low voltage power supply line grounding
issue
It is critical to isolate low voltage neutral lines to
avoid the shift potential damage caused to residence
from high voltage substations.
In substations or
nearby, neutral lines should be treated as "charged"
conductor, and isolate neutral lines with substation
grounding system by using insulator has a high
potential resistance. Meantime, the neutral wire
should be placed in a secure location to prevent staff
exposed to risks, isolate when necessary.
3.6 Metal pipes grounding issue
All kinds of metal pipes should be isolated to prevent
shifting potentials.
Pipes and metallic conductors should be connected to
grounding systems to avoid internal hazard with
substations. To prevent the high potential spread out
along metal pipes, isolation zone should be set around
Grounding Design of Electric Power Plants and Transformer Substations in High Soil Resistivity Area
7 Gaoyuan, LIANG
Year 2010
fences near power plants and substations. Most
importantly, the isolation zone has to be in great
distance to avoid nearby soil bypass.
Isolation area must be able to withstand the potential
difference between a substation and nearby ground earth potential.
among metals, therefore personnel hazard
still obvious even the potential is small on
GIS shell. It is the best solution to reduce
touch potential and step potential hazard by
grounding
4.
multi-point
of
GIS
shell.
Key factors of designing ground grid Working together with GIS manufacture to
in high soil resistivity area find out the suitable grounding material size,
a)
Metallic fences grounding
connecting
In the event of earth fault grounding, there
requirement, also the emphasis should be
is a higher potential gradient difference near
focused on DC component, switch actions
fences around power plants and substations
and maximum potential when earth fault
caused by edge effect of earth current.
grounding happened and connect equal
Designers should be alerted that electric
potential properly.
shock hazard still exists regardless of
b)
d)
points
and
shell
potential
System capacity development
whether the fences connected to grounding
System capacity should be designed by the
grid or not, particularly in high soil
life cycle of power plants and substation (1)
resistivity area, it may be worse.
rather
Focusing on voltage equivalent on main
development
building, auxiliary workshop, and coal
DL/T621-1997. By studying cases, it has
transmission system in addition to booster
been found that the follow-up supplement
station of power plants.
and optimize are difficult and expensive.
Above areas are normally considered as
e)
than
standard
plan
5~10
years
recommended
by
Note the side effect caused by un-even soil
crucial parts in the design, it has created
resistivity
touch and step potential hazard due to large
Grounding grid design is based on the
mesh grounding. For example, a LNG
condition of equivalent uniformed soil
power plant in Guangdong province, its
resistivity. Due to the actual effect, the
earth potential value is 20.8kV, if decreasing
designed ground grid is not safe, especially
it down to 20kV, the soil resistivity needs to
in high soil resistivity area. Therefore,
drop from 791Ω · m to 76 m Ω. Under this
followed by grounding grid construction,
circumstance, step potential could still meet
measuring
safety standard even not laying horizontal
distribution curve and eliminate unsafe
electrodes to equivalent potentials. However,
elements to complete setting of grounding
if not paving high soil resistivity isolation
grid.
actual
ground
potential
layer, the required touch potential cannot be
meet even expending mesh distance up to
1m. When the earth potential is 20.8kV, it is
Conclusion a)
With increasing capacity of the power
required to set a 37m mesh distance on
system and decreasing floor area of power
grounding grid to meet step potential
plants and transformer substation network, it
requirement.
isolation
is a great challenge to design grounding
measurements should to take into account to
grids in high resistivity area and it requires
prevent grounding when staff operation
designers to be more careful.
The lower earth resistance does not mean a
safe ground grid, and ground grid resistance
is not the direct unsafe cause to personnel
and device. Theoretically, no matter how
much ground grid resistance is, a careful
Therefore,
devices.
c)
5.
GIS grounding
As GIS could easily cause touch potential
b)
Grounding Design of Electric Power Plants and Transformer Substations in High Soil Resistivity Area
8 Gaoyuan, LIANG
c)
Year 2010
designed ground grid will be very safe. On
the other hand, although the ground network
resistance is low, potential hazard elements
are not considered in design, it is very much
insecure with a grounding grid without
eliminate excessively high touch potential,
step potential, transfer potential.
To stop 2~10kv system valve type arrester
actions, methods can be adopted by
4.
Yang Ziyi, 2000, The Comprehensive
Analysis Of Current Substation Grounding
Design
5.
IEEE
Guide
for
Generating
Station
Grounding – IEEE Std665-1995.
decreasing resistance, distribute current and
earth level.
d)
Besides
decreasing
grounding
grid
resistance to lower the earth level, it is much
easier to increase shunt coefficient of
overhead
arrester
wire
to
decrease
grounding current.
e)
To decrease step potential, there are few
methods can be adopted such as increasing
quantity
of
horizontal
electrodes
or
designing a suitable mesh size to meet
requirements.
Distributing
the
unequal
layout of mesh can greatly reduce the cost
of ground grid design.
f)
In order to limit step potential and not using
excessive
horizontal
alternative
solutions
electrode,
are
laying
the
wood
floorboard on operation platform, paving
asphalt and gravel mixture to isolate
platform or using metal ground platform.
g)
In power plant ground grids design, it is
important to take full advantage of existing
building foundations, underground water
pipes to connect equal potential and
eliminate dangerous potentials in entire
power plant ground grid.
Bibliography 1.
IEEE Guide for Safety in AC Substation
Grounding – IEEE Std80-2000
2.
DL/T
621-1997
AC
Electric
Devices
Engineering
Design
Manual
Grounding
3.
Electric
(Electrical Transmission Circuit), BeiJing,
1989, Water Resources and Electric Power
Press.
Grounding Design of Electric Power Plants and Transformer Substations in High Soil Resistivity Area
9