Download Earth Potential Rise (EPR) Computation for a Fault on

National Electrical Engineering Consultancy
Design
Management
4 – 54/60 Links RD
St Marys, NSW, 2760
ACN: 132586675
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ABN: 86132586675
NATIONAL ELECTRICAL ENGINEERING
CONSULTANCY
Earth Potential Rise (EPR) Computation for a
Fault on Transmission Mains Pole
Prepared by:
M. Nassereddine
Earth Potential Rise (EPR) Computation for a Fault on Transmission Mains Pole
National Electrical Engineering Consultancy
Design
Management
4 – 54/60 Links RD
St Marys, NSW, 2760
ACN: 132586675
Construction
www.NeecGroup.com
[email protected]
ABN: 86132586675
Abstract—The prologue of new High Voltage (HV) transmission mains into the community necessitates earthing design to
ensure safety compliance of the system. Conductive structure such as steel or concrete poles is widely used within HV
transmission mains; the earth potential rise generated by a fault on these structures could direct to an unsafe condition. This paper
endeavor to provide information on the input impedance of the OHEW system for finite and infinite transmission mains, the
definition of finite and infinite system was discussed, maximum EPR due to pole fault were discussed, and simplified equations
for EPR assessments were introduced and discussed for the finite and infinite conditions.
I. INTRODUCTION
T
he benefits of electricity are numerous but mishandling it can cause damages to properties and may inflict
injuries and fatalities. High voltage transmission deploys conductive poles such as steel and concrete structure.
The assumption that any grounded object can be safely touched is not always correct. A serious hazard may result
during a ground fault. Fault on transmission mains structure create an earth potential rise (EPR) which could lead to
unsafe touch condition on these structure or unsafe transfer voltage to nearby conductive infrastructure. High voltage
substations are fed by transmission mains. The route of transmission mains could be in close proximity to residence
and community infrastructures, many people believe touching transmission mains are safe, this statement is not
always correct, this statement only stand if the earthing system at the base of the pole is capable on absorbing the
fault energy generated by the high voltage fault.
The EPR generated on the structure could lead to unsafe voltage transfer to near by conductive infrastructures such
as metal fence, water pipe line and telecommunication circuit.
Transmission mains poles are exposed to two types of fault conditions:
Fault at the high voltage substation due to the split factor concept
Fault at the transmission poles
Also transmission mains are divided in two categories depending on its length:
Infinite transmission mains
Finite transmission mains
This paper discusses the transmission mains EPR under pole fault for the two length conditions, it examine the
EPR on the faulted pole and its maximum possible magnitude.
Also this paper introduces an estimated methodology for quick assessment on the maximum EPR under pole fault.
II. THEORETICAL STUDY
A. Design Factors
The earth potential rise on the transmission pole under fault will depend on the following factors; these factors are
considered the most important elements when it comes to pole EPR assessment:
Fault Location
Fault Current Magnitude on the structure
Infinite or finite transmission mains
Substation earth grid resistance
Pole earth grid resistance as seen from the OHEW connection point
Conductor arrangement on the transmission poles
Type of the OHEW
Numbers of the OHEW (in this paper, single OHEW were used)
Soil Resistivity Structure
Pole surrounding infrastructure
In this paper, transmission mains pole fault is assumed to have the same magnitude as substation fault.
B. Line Impedance
Figure 1 represents an overhead transmission line that connect two substations,
Earth Potential Rise (EPR) Computation for a Fault on Transmission Mains Pole
National Electrical Engineering Consultancy
Design
Management
4 – 54/60 Links RD
St Marys, NSW, 2760
ACN: 132586675
Construction
www.NeecGroup.com
[email protected]
ABN: 86132586675
Figure1: OH transmission mains layout with OHEW
C. Infinite System
For an infinite transmission line, equation 1 can be used to determine the OHEW input impedance
Ze =
Zs
+ ZsZ p
2
(1)
Where
Z s is the OHEW self impedance for the average span in ohms
Z p is the pole earth grid resistance in ohms
Z e is the line impedance as seen from the fault
Figure 2 represents the line impedance
Z e under a fault at the substation,
Figure 2: Line impedance for a fault at the substation
Figure 3 represents a pole fault located in the middle of the transmission mains, if half the transmission main line
is considered to be infinite, equation 1 can be used to determine
Z e1 and Z e 2
Figure 3: Fault at the transmission pole
Earth Potential Rise (EPR) Computation for a Fault on Transmission Mains Pole
National Electrical Engineering Consultancy
Design
Management
4 – 54/60 Links RD
St Marys, NSW, 2760
ACN: 132586675
Construction
www.NeecGroup.com
[email protected]
ABN: 86132586675
Under the infinite condition, the maximum pole earth potential rise can be found using equation 2:
PoleEPR = (1 − ς )I f ZT
(2)
Where
ς is the coupling factor
I f is the fault current
ZT is the total impedance as seen from the fault location
The worse case scenario is represented by a fault on the phase located at greater distance d
Where
d is the distance between the Phase conductor under fault and the OHEW
Figure 4 shows the phase conductor arrangements in relation to the OHEW, in this figure, fault on phase C
represents the worse case scenario when assessing the EPR for a fault on this pole.
Figure 4: Conductor Phase arrangement on transmission pole
D. Finite System
If the transmission line is considered to be finite, the input impedance of the OHEW can be computed using
equation 3:
Z e = Z OHEW − In // (Z s − connection + Z g )
(3)
Where
Z OHEW − In is the ling impedance for N number of poles
Z s −connection is the OHEW self impedance between the last pole and the substation
Figure 5 shows a fault on a pole for a finite transmission line,
Z e1 can be found using equation 5, similar
Earth Potential Rise (EPR) Computation for a Fault on Transmission Mains Pole
National Electrical Engineering Consultancy
Design
Management
Construction
4 – 54/60 Links RD
St Marys, NSW, 2760
ACN: 132586675
computation for
www.NeecGroup.com
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ABN: 86132586675
Ze2
Figure 5: Pole fault for finite transmission line
The finite impedance on one side can be found using the following approach
Z OHEW − In = Z s +
1
1
+
ZP Z +
S
1
1
1
1
+
Z P Z + ...... + Z +
S
S
1
1
1
+
ZP ZS + ZP
NEEC established equation 4 to compute the finite system OHEW input resistance
Z OHEW − In − NEEC
AZ s Z P + Z P2
=
BZ s + CZ P
(4)
Where
A, B and C are function of N
N is the number of section in an infinite system
Similar to the infinite line, the separation distance between the OHEW and faulted phase has an impact on the pole
EPR, the worth case scenario is represented with the fault on the phase located far to the OHEW, similar to the
infinite line, the finite line with pole arrangement as shown in figure 4, fault on phase C represents the worse case
scenario.
III. EPR UNDER POLE FAULT
The worse case EPR under a pole fault can be divided into two sections
1. Fault under finite system
2. Fault under infinite system
Figure 6 shows the EPR for a pole fault under transmission mains where half its length is considered to be finite,
which mean the fault on the middle pole has a finite line both its end. The lowest EPR is presented in the middle as
both substation earth grids for a part of the OHEW input impedance due to the finite line, the worse case scenario is
represented by a pole fault near the substation.
Earth Potential Rise (EPR) Computation for a Fault on Transmission Mains Pole
National Electrical Engineering Consultancy
Design
Management
4 – 54/60 Links RD
St Marys, NSW, 2760
ACN: 132586675
Construction
www.NeecGroup.com
[email protected]
ABN: 86132586675
Figure 6: Pole EPR for a finite middle line
Figure 7 shows the EPR for a pole fault under transmission mains where half its length is considered to be infinite,
which mean the fault on the middle pole has an infinite line both its end. The worse case scenario is represented by a
pole fault in the middle of the line
Figure (7) Pole Fault EPR for an infinite system
Earth Potential Rise (EPR) Computation for a Fault on Transmission Mains Pole
National Electrical Engineering Consultancy
Design
Management
4 – 54/60 Links RD
St Marys, NSW, 2760
ACN: 132586675
Construction
www.NeecGroup.com
[email protected]
ABN: 86132586675
IV. CONCLUSION
This paper provides a process on how to assess the maximum EPR under pole fault; it shows that for infinite
transmission mains it is possible to estimate the maximum EPR value with minimum input data.
Also this paper shows how it is possible to estimate the EPR along the feeder route under different pole fault
location, it shows that the worse case scenario is presented with a pole fault near the substation for a finite system
and by a pole fault in the middle of the line for an infinite system.
This approach enhance the process of earting design when it comes to transmission mains, it gives the designer
guidance if further pole earth grid design is required for touch and step voltage compliance.
It shall be noted that the pole earth grid shall comply with the lightning protection requirements.
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[7]
Andrenyi J. “Analysis of Transmission Tower Potentials During Ground Faults” 1967 IEEE Transaction on Power Apparatus and Systems,
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Nassereddine M, Hellany A, Nagrial M, “Analysis of the impact of the OHEW under full load and fault current” 2010, International Jurnal
of Energy and Environment (IJEE), Volume 1, Issue 4, pp. 727-736. [ORS ID: 216605]
Nassereddine M, Hellany A, Nagrial M. Rizk J. “Soil Resistivity Structure and its implication on the Earth Grid of HV substation” 2011
World Academy of Science, engineering and Technology , Vol 60, pp 1322-1326, [ORS ID: 222496]
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Earth Potential Rise (EPR) Computation for a Fault on Transmission Mains Pole