1 Utilization of System Generated LG Fault on Ground Grid Mesh and Analysis of existing GGS using IEEE 80-2000 and Finite Element Method Akash Patel Electrical Engineering California State University Northridge Northridge, USA [email protected]Professor Bruno Osorno Electrical Engineering California State University Northridge Northridge, USA [email protected]Abstract—the 750kv substation is chosen for given particular type of soil to design square ground grid mesh. By using line to ground fault current generated on 750kv substation by ETAP short circuit analysis, ground grid parameters are determined using simple hand calculations and results are verified by modeling same ground grid using ETAP ground grid system. After getting general idea about effect on system generated fault on GGS, system data from existing 50kv Rawat substation is collected to verify all-important parameters are within the limit or not. To evaluate important GGS parameters, mesh analysis is carried out using both IEEE 80-2000 and Finite Element Methods by using ETAP-12 GGS Module. Shortcoming in existing mesh are highlighted by ETAP warning and alert dialog box and remedial actions are suggested to rectify the problems. Increase in fault current by expansion of substation is consider with possible solution and optimization of ground grid mesh is presented based on modern technology. Finally, comparison is done between IEEE 80-2000 and FEM method to design GGS and effect of ground grid area, number of conductors/rods and spacing between conductors is explained by help of several case study results. Keywords— Ground Potential Rise, Step Voltage, Touch Voltage, Ground Mesh, Electrical Transient Analysis Program, Ground Grid System Module I. INTRODUCTION Under normal and faulty condition, grounding can carry electrical currents into earth with maintaining operating and equipment limits and protect person from danger of electrical shock. After analyzing effects of electrical current on human body, researchers have developed different IEEE standards for earth resistance measurement, surface gradients, grid conductors continuity, equipment grounding, cable routing, cable sheath grounding, indoor installations, lighting protection, current division, generating station grounding practices, testing and different connections of grounding system. Use of single electrode for grounding is not sufficient for safe grounding. Rather than using single electrode if we use grid, we can represent excellent grounding system. To protect human from surface gradients when high current dissipated into the earth, installation of grid with law resistant is required. Horizontal ground grid can reduce danger of high step and touch voltages on the earth surface by using only, when grid is installed in narrow depth. For many installations where soil resistivity change due to freezing or drying effect and resistivity of lower soil remains constant, ground electrodes are useful to stabilize the performance. So, in case of multilevel soil, rods which are penetrating in low resistivity soil more effectively dissipate fault currents. Installation of rods along grid perimeter in uniform or high-to-low soil conditions can moderate steep rise of surface gradient near peripheral meshes. Sometimes size of substation is large or soil has high resistivity. Rather than using large grounding system, we have to use fraction of the land area for grounding purpose and then all areas can be connected with each other by connecting rods. Computer algorithms are based on modeling the individual components comprising the grounding system, developing set of equations which describe the interconnection of these equipments, determine ground fault current flowing from each component into the earth, calculating the potential at any desired surface point due to all individual components-TAP ground grid module is very useful software for design and implementation of ground grid system and it utilizes Finite Element Method,IEEE 80-1986, IEEE 80-2000, IEEE 665- 1995 methods of computation. Existing 500 kv Rawat substation is design and analyze by ETAP-12 using IEEE 80- 2000 and Finite Element method and new 750 kv substation is designed by using data from IEEE safety guide for substation grounding and short circuit analysis of grid. II. SHORT CIRCUIT ANALYSIS AND GROUND GRID SYSTEM There are 9 buses,3 generators,3 transformers,6 transmission lines and 3 loads are connected as shown in figure 1. After creating line to ground fault on bus 5, E-TAP short circuit analysis generate fault currents and voltages as shown in circuit below.Design of square ground grid represented by green area is possible for 750kv substation at bus 5 by updating fault current value at bus 5 and we can easily verify the results with ETAP software. System data for bus,transmission lines,transformers and generators are presented in Appendix A.
Transcript
1
Utilization of System Generated LG Fault on Ground
Under normal and faulty condition, grounding can carry electrical currents into earth with maintaining operating and equipment limits and protect person from danger of electrical shock. After analyzing effects of electrical current on human body, researchers have developed different IEEE standards for earth resistance measurement, surface gradients, grid conductors continuity, equipment grounding, cable routing, cable sheath grounding, indoor installations, lighting protection, current division, generating station grounding practices, testing and different connections of grounding system. Use of single electrode for grounding is not sufficient for safe grounding. Rather than using single electrode if we use grid, we can represent excellent grounding system. To protect human from surface gradients when high current dissipated into the earth, installation of grid with law resistant is required.
Horizontal ground grid can reduce danger of high step and touch voltages on the earth surface by using only, when grid is installed in narrow depth. For many installations where soil resistivity change due to freezing or drying effect and resistivity of lower soil remains constant, ground electrodes are useful to stabilize the performance. So, in case of multilevel soil, rods which are penetrating in low resistivity soil more effectively dissipate fault currents. Installation of rods along grid perimeter in uniform or high-to-low soil conditions can moderate steep rise of surface gradient near peripheral meshes. Sometimes size of substation is large or soil has high resistivity. Rather than using large grounding system, we have to use fraction of the land area for grounding purpose and then all areas can be connected with each other by connecting rods. Computer algorithms are based on modeling the individual components comprising the grounding system, developing set of equations which describe the interconnection of these equipments, determine ground fault current flowing from each component into the earth, calculating the potential at any desired surface point due to all individual components-TAP ground grid module is very useful software for design and implementation of ground grid system and it utilizes Finite Element Method,IEEE 80-1986, IEEE 80-2000, IEEE 665-1995 methods of computation. Existing 500 kv Rawat substation is design and analyze by ETAP-12 using IEEE 80-2000 and Finite Element method and new 750 kv substation is designed by using data from IEEE safety guide for substation grounding and short circuit analysis of grid.
II. SHORT CIRCUIT ANALYSIS AND GROUND GRID SYSTEM
There are 9 buses,3 generators,3 transformers,6 transmission
lines and 3 loads are connected as shown in figure 1. After
creating line to ground fault on bus 5, E-TAP short circuit
analysis generate fault currents and voltages as shown in
circuit below.Design of square ground grid represented by
green area is possible for 750kv substation at bus 5 by
updating fault current value at bus 5 and we can easily verify
the results with ETAP software. System data for
bus,transmission lines,transformers and generators are
presented in Appendix A.
2
Figure 1 [5] developing grounding grid on faulted bus 5
ETAP give short-circuit result as shown in table 3.5. Contribution Line to Ground Fault
From Bus To
Bus
%Voltage At
From
Bus kA
Symm.
rms
ID ID Va Vb Vc Ia 3
Bus5 Total 0 92.91 91.61 10.106 10.106
Bus4 Bus5 0.11 92.91 90.61 5.137 6.252
Bus7 Bus5 0.37 92.87 90.49 4.993 4.004
Table 1 short circuit result
So, 10.106 kA fault current is contribution of Bus 5 to whole
network.By using this fault current value we can design
square grid for Bus 5 by using simple equations as shown
below.data of square grid is presented in Appendix B.
a) Touch and Step criteria:
K= =-0.72
=
b) Determination of grid resistance:
c) Maximum grid current :
d) Ground Potential rise:
GPR=
e) Correction factors:
=1.225.
f) Spacing factors:
g) Mesh voltage:
h) Step voltage:
Result Summary: Calculated Volts Tolerable Volts
Touch 3184.382 V 840.5836 V
Step 1452.7945 V 2696.2398 V
GPR
Table 2 hand calculation result summary
Here the maximum touch voltage exceeds the tolerable limits.
We can verify above result by ETAP. After updating the value
of and X/R from short circuit analysis,we can design
square ground grid by using ETAP as shown in figure 3.7.
3
Figure 2 top view of ETAP square ground grid
The result of ground grid analysis for square grid is shown in
figure 3.
Figure 3 results of ETAP GGS analysis
The value of touch, step voltages, GPR and are similar for
hand calculation as well as ETAP calculation. Even in both
cases calculated touch voltage is higher than tolerable limit.
III. IEEE 80-2000 METHODS CASE STUDIES
By using the same 9 bus power system,designing of 500kv
substation is possible at bus1 by using ETAP IEEE 80-2000
method as shown in figure 4 [5],
Figure 4[5] sample connection diagram for IEEE 80-2000 method grid
design
Actually ground grid system for 500kv substation is already
laid and functioning in Rawat substation,so existing data is
collected to verify whether all important parameters are with
in the described limits or not.The collected data from actual
substation are represented in Appendix C.
a) IEEE 80-2000 Method Case 1:
Initially after specifing surface material,soil resistivity and
depths for various layers in ETAP soil editor and selecting
weight of person, proper IEEE method and details of short
circuit current and fault duration by using ETAP GRD study
case editor and specifying conductor/ rod parameters for grid
and ground grid dimentions by using in ETAP IEEE group
editor with the data given in table 3 ,we can design and
analyze protection scheme for priliminary gound grid mesh by
finding ground potential rise,ground system resistance,touch
and step potentials using IEEE 80-2000 method. Gri
d
Len
gth
(ft)
Numbe
r of
conduct
ors
Rod
Data
Conductor
Size
AWG/kcmil
Depth
ft
ft
ft
In X Directio
n
In Y Direc
tion
Diameter
inch
Length
ft
No
Of
Rods
4/0 0.9
8
472.
44
32
8.08
14 10 0.472 6.56 50
Table 3[4] input ground grid parameters of IEEE 80-2000 method case 1.
The ground grid system is presented by a top view,a soil
view,a 3-D view for graphical arrangement of the conductors
and rods as shown in figures 5,6 and 7.the soil view is used to
edit the soil properties of the surface, top, and lower layers of
soil. The 3-D view is used for the three-dimensional display of
the ground grid which can be rotate or view by any angle. The
Top View is used to edit the ground conductors/rods of a
ground grid.
Figure 5 soil view of GGS case 1.
Figure 6. 3-D view of GGS case 1.
Figure 7 top view of GGS case 1.
4
The result of ground grid analysis case 1 is shown in figure 8.
Figure 8 results of case 1 analysis
As shown in figure 8, ground grid is not functioning properly
due to higher touch voltage and temperature rise than safer
limits. Moreover, either we should have to increase or
decrease area to design proper ground grid.
b)IEEE 80-2000 Method Case 2:
To overcome issue created in case 1, redesign of ground grid
system is possible by changing number of conductors and
number of rods as shown in table 4[4]. Gr
id
Le
ngt
h
(ft)
Number
of
conduct
ors
Rod Data
Conduc
tor Size
AWG/k
cmil
De
pth ft
ft
ft
In X
Direction
In Y
Directio
n
Diameter
inch
Len
gth ft
N
o Of
R
ods
4/0 0.9
8
47
2.44
3
28.
0
8
20 16 0.598 6.5
6
54
Table 4 [4] input ground grid parameters of IEEE 80-2000 method case
2.
3-D View of grid and the results of ground grid analysis case
2 are shown in figure 9 and 10.
Figure 9. 3-D view of GGS case 2.
Figure 10 results of case 2 analysis
So, temperature rise issue can be solved by changing number
of conductors and rods. But, higher touch voltage and grid size
issues are still exist.
c)IEEE 80-2000 Method Case 3:
There is an expansion in actual substation by replacing old
power transformer with new one which increase fault current
from 40 A to 45 A.To fulfill this new requirement,changes are
required in ground grid mesh as shown in table 5[4]. Gri
d
Len
gth
(ft)
Numbe
r
Of
conduc
tors
Rod
Data
Condu
ctor Size
AWG/
kcmil
D
epth
ft
ft
ft
In X
Direction
In Y
Direction
Diamet
er inch
Length
ft
No
Of Ro
ds
4/0 0.
9
8
472.
44
328.
08
22 19 0.634 6.56 65
Table 5[4] input ground grid parameters of IEEE 80-2000 method case 3.
3-D View and the result of ground grid analysis case 3 is
shown in figure 11 and 12.
Figure 11. 3-D view of GGS case 3.
5
Figure 12 results of case 3 analysis
ETAP ground grid analyzer show warning signal related to
size of ground grid area and higher touch voltage than
allowable limit due to larger than 10000 sq.m grid area. So,
for safety purpose we have to minimize the grid area.
d)IEEE 80-2000 Method Case 4:
when ground grid mesh of this substation is designed , very
less methods for designing ground grid are available.In this
case study, new modern analysis and optimization techniques
are applied for ground grid design by using the details
presented in table 6 [4]. Gri
d
Le
ngt
h
(ft)
Numbe
r
Of
conduct
ors
Rod
Data
Con
ductor
Size
AWG/kc
mil
De
pth ft
ft
ft
In X
Direction
In Y
Direction
Diam
eter inch
Le
ngth
ft
No
Of Ro
ds
4/0 2 393.7
272.31
25 23 0.63 6.56
42
Table 6[4] input ground grid parameters of IEEE 80-2000 method case 4.
3-D View and the results of ground grid analysis case 4 is
shown in figure 13 and 14.
Figure 13. 3-D view of GGS case 4.
Figure 14 results of case 4 analysis
By decreasing size of ground grid area and proper utilization
of ground conductors and rods,we can design ground grid
mesh which fullfill all required limits of touch potential,step
potential and temperature.
IV. FEM CASE STUDIES
Using the same data from Appendix C ,we can design ground
grid mesh for 500kv substation at bus 3 by using ETAP FEM.
Figure 15 [5] ground grid representation for FEM.
a)FEM Case 1:
By using ETAP ground grid Finite Element module and input
datas given in table 7[4], design and analyze of priliminary
ground grid design is possible.we can modify each individual
conductor/rod parameter by using FEM group editor and rod
editor. Grid
Lengt
h
(ft)
Numb
er
Of
condu
ctors
Rod
Data
Con
duct
or Size
AW
G/kcmil
Dep
th
ft
ft
ft
In X
Direct
ion
In Y
Directio
n
Diam
eter
inch
Len
gth
ft
No
Of
Rods
4/0 0.9
8
472.4
4
328.08 20 15 0.472 6.56 50
Table 7[4] input ground grid parameters of FEM case 1.
6
Different views of ground grid are shown in figure 16, 17, 18.
Figure 16. 3-D view of GGS case 1.
Figure 17 top view of GGS case 1.
Figure 18 soil view of GGS case 1.
The result of ground grid analysis case 1 is shown in figure
19.
Figure 19 results of case 1 analysis
As shown in figure 19, ground grid is not functioning properly
due to higher touch voltage and temperature rise than safer
limits.3-D Plots are used only with the FEM method, and are
available for Absolute/Step/Touch Voltages. As shown in
figure 20, some voltages are quite high on voltage axies
represented by red colour in step potential profile,while in
case of touch potential as shown in figure 21, most of area is
green which represent that touch voltage is very low.
Figure 20 step potential profile of case 1.
Figure 21 touch potential profile of case 1.
Figure 22.absolute potential profile of case 1.
b)FEM Case 2:
To overcome issue created in case 1, redesign of ground grid
system is possible by changing number of conductors and
number of rods is as shown in table 8.[4]. Grid
Lengt
h
(ft)
Num
ber
Of
condu
ctors
Rod
Data
Conductor
Size
AWG/kc
mil
Depth
ft
ft
ft
In X Direct
ion
In Y Directio
n
Diameter
inch
Leng
th
ft
No Of
Rods
4/0 0.98
472.44
328.08
25 20 0.614 6.6
57
Table 8[4] input ground grid parameters of FEM case 2.
3-D View and The result of ground grid analysis case 2 is
shown in figure 23 and 24.
7
Figure 23. 3-D view of GGS case 2.
The result of ground grid analysis is shown in figure 5.13.
Figure 24.results of case 2 analysis
So, by changing number of conductors and rods, we resolve
temperature rise issue. But, still we need to focus on higher
touch voltage.3-D voltage profiles are shown in figure 25, 25
and 27.
Figure 25.step potential profile of case 2
Figure 26.touch potential profile of case 2
Figure 27.absolute potential profile of case 2
c) FEM Case 3:
There is an expansion in actual substation by replacing old
power transformer with new one which increase fault current
from 40 A to 45 A.To fulfill this new requirements ,
modifications are required in grid configuration as shown in
Table 9.[4].
Gri
d
Len
gth
(ft)
Num
ber
Of
cond
uctor
s
Rod
Dat
a
Co
ndu
ctor
Siz
e
A
W
G/k
cmi
l
De
pth
ft
ft
ft
In X
Direc
tion
In Y
Directio
n
Dia
met
er
inch
Len
gth
ft
N
o
O
f
R
o
ds
4/0 0.9
8
472.
44
328.
08
29 26 0.64
2
6.8
9
6
0 Table 9.[4] input ground grid parameters of FEM case 3.
3-D View and the result of ground grid analysis case 3 is
shown in figure 28 and 29.
Figure 28. 3-D view of GGS case 3.
8
Figure 29.results of case 3 analysis When I analyze the new designed grid after expanding
warning signal related to higher touch voltage than allowable
limit.3-D voltage profiles are shown in figure 30, 31 and 32.
Figure 30.step potential profile of case 3.
Figure 31.touch potential profile of case 3.
Figure 32.absolute potential profile of case 3.
d)FEM Case 4:
when ground grid mesh of this substation is designed , very
less methods for designing ground grid are available.In this
case study,new modern analysis and optimization techniques
are applied for ground grid design by using the details
presented in Table 5.4. [4]. Gri
d
Len
gth
(ft)
Num
ber
Of
cond
uctor
s
Rod
Dat
a
Con
duct
or Size
AW
G/kcmil
Dep
th
ft
ft
ft
In X
Direc
tion
In Y
Directi
on
Dia
mete
r inch
Leng
th
ft
N
o
Of Ro
ds
4/0 1.6
4
393.
7
272.31 26 22 0.47
2
6.56 50
Table 10.[4] input ground grid parameters of FEM case 4.
3-D View and ETAP ground analysis is presented in figure 33
and 34.
Figure 33. 3-D view of GGS case 4.
Figure 34.results of case 4 analysis
3-D view of different potential profiles are shown in figure 35,
36 and 37.
9
Figure 35.step potential profile of case 4.
Figure 36.touch potential profile of case 4.
Figure 37.absolute potential profile of case 4.
By decreasing size of ground grid area and proper utilization
of ground conductors and rods,I design ground grid mesh
which fullfill all required limits of touch potential,step
potential and temperature.
V. CONCLUSION
From the result of ETAP simulation on several case studies, we can derive below aspects, magnitude of touch potential, step potential, absolute potential, and ground resistance decrease with increase in grid area. So, it’s better to use larger grid in the area where land is cheap. Effect of spacing on touch and step potential is opposite. If we reduce the spacing between conductors, mesh potential decrease, while step potential may be increase and effect of mesh potential is severe than step potential. The effect of increasing conductors can be varied for different cases, but mainly installation of horizontal conductors decreases the touch potential. Normally if we install more number of vertical rods, temperature rise, mesh potential,
ground impedance decrease. Simulated results are nearer to actual value by using IEEE 80-2000 and FEM method; in addition results are more accurate than hand calculation ETAP simulate ground grid system within a second, while for FEM method E-TAP took around 5-6 minutes.so, we can do faster calculation by using IEEE 80-2000 method compare to FEM method. Ground grid design using FEM is costlier than IEEE 80-2000 method because greater number of conductors and rods are required in FEM compare to IEEE 80-2000 For the same size grid. Mesh designed by FEM method will be more durable, long lasting and have ability to withstand the excessive fault currents more effectively. In FEM method we can modify the data for individual conductor or rod by conductor/rod editor, while in IEEE 80-2000 method we can’t change data for individual conductor/rod.
VI. BIBLIOGRAPHY
[1] Design analysis and optimization of ground grid mesh of extra high
voltage substation using an intelligent software.
[2] IEEE guide for safety in AC substation grounding.
[3] IEEE guide for measuring earth resistivity, ground impedance and earth
surface potentials of a grounding system.
[4] A comparison of ground grid mesh design and optimization for 500kv
substation using IEEE 80-2000 and finite element methods.
[5] Transient stability analysis of IEEE 9 bus electric power system.
