Electric Power Systems Research 80 (2010) 1074–1081
Contents lists available at ScienceDirect
Electric Power Systems Research
journal homepage: www.elsevier.com/locate/epsr
Ground potential rise of multi-grounded neutral and shield wires in joint systems
Janak Acharya 1 , Wilsun Xu ∗,2
Department of Electrical and Computer Engineering, 2nd Floor ECERF, University of Alberta, Edmonton, Canada T6G 2V4
a r t i c l e
i n f o
Article history:
Received 2 November 2009
Received in revised form 20 January 2010
Accepted 22 January 2010
Available online 20 February 2010
Keywords:
Ground potential rise
Grounding
Joint system
a b s t r a c t
Power line faults create the ground potential rise (GPR) on both the neutral and shield conductors when
the transmission lines (TL) and distribution lines (DL) are built on the same structures. The durations
and magnitudes of resulting GPRs are unique for DL faults and TL faults because the corresponding fault
currents are significantly different in terms of their magnitudes and durations. This paper analyzes and
compares the safety impacts of TL faults and DL faults in the joint structures. Approximate formulas are
established to describe the GPR characteristics. Computer simulation results are provided to illustrate
the effects of different parameters on GPRs in various configurations.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The overhead distribution lines (DL) are often built under the
transmission lines (TL) on the same towers. The safety impact of
such configurations under short-circuit conditions are difficult to
understand because the DL-created GPR and TL-created GPR have
unique characteristics. The shield wire of TL establishes the direct
contact with the conductive towers which serve as grounding of
the shield wire. On the other hand, the neutral wire on the same
tower is provided with a separate dedicated grounding assembly,
or is bonded with the shield wire. The conductors used for the
neutral wire and shield wire are not the same. Generally steel is
preferred for the shield wire and Aluminum Conductor Steel Reinforced (ACSR) for the neutral wire. Also the physical positions of
these conductors on the same structure lead to varying degree of
electromagnetic coupling with phase conductors under fault. Consequently, the resulting GPRs are affected even for the same amount
of fault currents in TL and DL.
In the past, a lot of studies have been done for single circuit
multi-grounded configurations and a great deal of literature is
available [1–10], but limited work has studied the composite systems comprised of multiple multi-grounded conductors. Mostly
computer-based methods were preferred for the studies of multigrounded systems. Major shortcomings of such methods include
inability to provide intuitive understanding on the interaction
∗ Corresponding author. Tel.: +1 780 492 5965; fax: +1 780 492 1811.
E-mail address: wxu@ualberta.ca (W. Xu).
1
Student Member, IEEE.
2
Fellow, IEEE.
0378-7796/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsr.2010.01.014
and effects of various factors. Alternatively, analytical methods
can be developed to compensate such shortcomings. Computer
models were used to estimate the GPR of the multi-grounded neutral (MGN) in [2–4] and power flow studies were performed in
[5–7]. Analytical approaches for the GPR analysis were proposed
in [8–10]. The GPR assessment of multi-grounded communication
cable bonded with power line’s neutral wire was performed in [9].
The principles of [9] can be applied to the joint transmission and distribution system with multi-grounded conductors. In our previous
work, analytical methods were proposed to provide understandings of the characteristics of MGN lines [11]. The work done in this
paper complements the previous studies as it is extended to the
joint T&D systems.
It is generally agreed that the TL fault currents are larger, but are
cleared faster than the DL faults. As a result, field engineers assume
that the TL caused GPRs are less severe than the DL caused GPRs.
As will be shown later, this assumption is wrong. Also effectiveness
of bonding the neutral wire with shield wire for lowering the GPR
level is not fully understood yet. This paper reveals a number of
factors that play a crucial role in determining the GPR.
The main objective of this paper is to address the abovementioned concerns by using the analytical formulas and computer
simulation. An EMPT-based computer tool was used for the simulation. Since the power industry has accepted the associated
techniques and models, they are not described here. Detailed models can be found in [12]. Sensitivity studies are performed to
illustrate the effect of a number of parameters on the GPR. The
reminder of the paper is organized as follows. Section 2 presents
the problem and the system under investigation. Mechanism of GPR
generation is presented in Section 3. Results are shown in Section
4 and the conclusions in Section 5.
J. Acharya, W. Xu / Electric Power Systems Research 80 (2010) 1074–1081
1075
Table 3
Base case data and their variation range.
Parameter
Typical value
Base case and sensitivity
Grounding
resistance, RG
5–25 Base case: 15 Sensitivity: 7–25 Substation
grounding res.
–
TL-sub: 0.15 DL-sub: 0.15 Grounding interval
Neutral: 40–75 m
Shield: 60–100 m
Base case: 75 m
Sensitivity: 75–600 m.
Parallel exposure
200–3000 m
Base case: 4 km
Sensitivity: 1–5 km
Line length
Shield wire
–
5/16 steel
∼7.5 km
Rdc = 3.5067 /km
The following six basic configurations are identified and studied so
that comparisons can be made in terms of their safety benefits:
(1)
(2)
(3)
(4)
(5)
(6)
D-line with neutral wire (MGN)
T-line with shield wire (MGS)
T&D lines with shield, but without neutral wire
T&D lines with neutral, but without shield wire
T&D lines with neutral and shield isolated
T&D lines with neutral and shield bonded
Fig. 1. (a) Physical layout of TL and DL conductors, (b) Schematic diagram.
Table 1
Conductor positions on the tower.
Conductor
Horizontal
position (m)
Vertical
height (m)
Mid-span
height (m)
T-Line Ph#A
T-Line Ph#B
T-Line Ph#C
D-Line Ph#A
D-Line Ph#B
D-Line Ph#C
D-Line Neutral
T-Line Shield
−2.15
2.15
−2.15
−0.58
0.64
1.85
−0.25
0.15
16.01
16.01
12.91
9.97
9.97
9.97
7.89
19.41
15.06
15.06
11.96
8.61
8.61
8.61
7.08
19.17
2. Study system
Fig. 1 shows a general layout of the study system where the
transmission conductors are positioned above the distribution conductors on the same tower. The TL and DL run in parallel for a certain
distance only. The DL’s neutral wire is insulated from the tower and
grounded with dedicated ground rods (not shown in figure). The
TL’s shield wire, however, is not insulated from tower’s body. So
the tower itself serves as a ground rod. Table 1 provides the horizontal and vertical positions of the conductors, Table 2 shows the
line impedances calculated using the EMTP models [12], and Table 3
provides the system data for base case and sensitivity studies.
The system information was provided by the electrical utility in
Canada. Based on the available data, the line length was chosen to be
7.5 km so that the effect of varying parallel length can be examined.
Table 2
Line impedance data (/km).
Mutual impedances between
D-line phase wire and neutral wire
D-line phase wire and shield wire
T-line phase wire and neutral wire
T-line phase wire and shield wire
Neutral wire and shield wire
zDN = 0.0583 + j0.4734
zDS = 0.0576 + j0.3409
zTN = 0.0579 + j0.3567
zTS = 0.0573 + j0.4030
zNS = 0.0577 + j0.3292
The self-impedances of
Neutral wire
Shield wire
zNN = 0.3966 + j0.9119
zSS = 3.5638 + j0.9518
The GPRs are presented in the form of volts per kA of fault current. This offers two advantages. First, it establishes the basis for
comparison of different configurations irrespective of the fault current magnitudes. Second, the numbers can be indicative to any fault
currents (as they are uncertain). Caution should be exercised while
comparing TL caused GPR/kA and DL caused GPR/kA values because
the fault currents of TL and DL can be quite different. In such cases,
the actual magnitudes of GPR should be considered. The acronyms
NGPR and SGPR are used to denote the GPRs developed in neutral
wire and in shield wire, respectively.
3. Mechanism of GPR generation
Ref. [11] presents the mechanism of GPR generation, but for
the line-to-ground fault only. The basic principles are described
in this section. The faults involving a single ground conductor will
be investigated first and then more complex schemes will be dealt
with. A generic term ‘ground conductor’ refers to either a neutral
wire or a shield wire. The GPR can develop in the ground conductor under two distinct conditions: (1) unfaulted ground conductor
– when the fault does not involve this conductor, and (2) faulted
ground conductor – when the fault involves this conductor.
3.1. GPR of the unfaulted ground conductor
An unfaulted ground conductor experiences the GPR when the
nearby phase conductor is at fault. This occurs when phase conductor falls on the ground without making any contact with the ground
conductor. The fault current flowing in the phase conductor induces
a voltage on the multi-grounded conductor (Fig. 2a). The induced
voltage (EG ) is distributed along the fault-exposure length and can
be determined as
EG = eg l = zmutual l · IF
(1)
where zmutual is the mutual impedance between phase and ground
conductors and l is the length of the exposure with fault current.
The voltage (eg ) of one segment can be modelled as an equivalent
current source connected between the two grounding points based
on the principle of Thevenin to Norton circuit conversion (Fig. 2b).
Note that the downstream portion of the ground wire does not
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Fig. 3. The injection of current sources during the line-to-wire fault: (a) line-to-wire
(or contact fault), (b) equivalent circuit at node 1, and (c) equivalent circuit at node
2.
3.2. GPR of the faulted ground conductor
Fig. 2. Illustration of GPR mechanism in the multi-grounded conductor: (a) fault in
a parallel multi-grounded conductor, (b) conversion of voltage source into current
source, (c) equivalent model with current injection into the ground wire, and (d)
final equivalent circuit of ground wire.
expose to the fault current, so there is no induced voltage or current.
Since current IG is independent of the length of grounding span, all
other segments have the same current.
IG =
eg l
z
= mutual IF
zgg
zgg l
The representative cases of the faulted ground conductor are:
insulation failure of phase conductor causing short circuit with
shied wire via tower, or shield wire falling on the phase wire, or
phase conductor falling on neutral wire, etc. In this category, the
ground conductor contacts the phase conductor. These faults are
categorised as the contact faults in this paper and are different
from ground faults. Fig. 3 illustrates the case of contact fault. The
mechanism of GPR generation is similar to that of unfaulted ground
conductor case, but the ground conductor will carry a portion of
fault current entering from the fault point in addition to the current
produced by induction (Fig. 3c). Thus the GPR at the fault location
will be much different from that of line-to-ground fault. The GPR
of the islanded end (node 1) will be the same.
The GPR at fault location (node 2) is given by
GPR2 = (IF − IG )(Zeq-2u //Zeq-2d )
(6)
The current involved in (4) is IG only, but it is modified in (6) as
(IF − IG ) due to short circuit of phase and ground conductors.
(2)
3.3. Effect of the other parallel ground conductor
where zgg is the self-impedance of the ground conductor.
Fig. 2b can be further transformed into that of Fig. 2c without
affecting the nodal voltages and segment currents. Assuming that
the distance between node 1 and node 2 is large enough so that the
influence of current source (IG ) at one node (e.g. node 1) does not
affect the voltage of the other current-injected node (e.g. node 2),
the neutral network can be modelled as shown in Fig. 2d. The GPRs
of the node 1 and node 2 are given by
GPR1 = IG (Zeq-1 //RG ) ≈ IG Zeq-1
(3)
GPR2 = −IG (Zeq-2u //Zeq-2d //RG ) ≈ −IG (Zeq-2u //Zeq-2d )
(4)
Fig. 4 shows a phase wire, a ground wire and a parallel wire
(also grounded). A fault (line-to-ground or line-to-wire) occurs at
the location of node 2. The fault current will induce currents in
the ground wire and in the parallel wire. Consider one segment
of the ground wire to explain the GPR developed in it (Fig. 4b). In
where Zeq-1 is the equivalent impedance, Zeq-2u and Zeq-2d are the
equivalent impedances seen upstream and downstream from the
node 2, respectively. The minus sign is placed in (4) to signify that
the GPR1 and GPR2 are of opposite polarity due to directions of
associated currents.
The equivalent impedance of the multi-grounded ladder [11] is
approximated as
Zeq =
s · zgg · RG
(5)
where s is the grounding interval and RG is the grounding resistance.
The impedances Zeq-1 , Zeq-2u and Zeq-2d are equal due to symmetry
of the ladder. The ratio GPR1 to GPR2 is approximately 2. Therefore,
the maximum GPR is located at node 1.
Fig. 4. The effect of parallel ground conductor: (a) a faulted phase conductor and two
parallel ground conductors, and (b) induced voltages and their equivalent currents.
J. Acharya, W. Xu / Electric Power Systems Research 80 (2010) 1074–1081
1077
addition to the fault-current-induced voltage (eg ), the current of
parallel conductor will induce another voltage (ep ) in the ground
wire, but with opposite polarity. As a result, the GPR at node 2 will
be affected. Considering the line-to-ground fault, the GPR at node
2 is given as
GPR2 = (IG − IGp )(Zeq-2u //Zeq-2d )
where
zgp
IGp =
IP
zgg
and
IP =
zmutual
IF
zpp
(7)
(8)
where IGp is the current induced in the ground conductor due to
current of another parallel conductor (IP ) and zpp is the impedance
of the parallel conductor. Comparing (7) and (4), the first term is
modified by IGp . If the phase conductor contacts the ground conductor directly or indirectly during the fault, the resulting GPR at
fault location (node 2) will be
GPR2 = [IF − (IG − IGp )](Zeq-2u //Zeq-2d )
(9)
Again, comparing (9) with (6), the current IG is modified by
subtracting IGp .
The above principle can be applied to examine the effect TL’s
shield wire on the GPR of DL’s neutral wire and vice versa.
3.3.1. Effect of shield wire on GPR of the neutral wire
In this case, IG represents the current induced by the fault current on neutral wire and IGp represents the current induced by the
shied wire’s current. Then
IGp =
zNS
zDS
×
IFD = 0.031IFD
zNN
zSS
(10)
where zNS is the neutral-to-shield mutual impedance, zDS is the
mutual impedance between the distribution phase wire and shield
wire and IFD is the fault current on distribution line. The current
induced by the shield current (IGp ) is relatively small compared to
IG (=0.48IFD ), i.e. 6.5% of IG . Therefore the presence of the shield wire
does not significantly affect the GPR of the neutral wire. However,
it should be emphasized that if there were faults on transmission
line, the fault current will be significantly higher than IFD and the
resulting neutral GPR will increase accordingly.
3.3.2. Effect of neutral wire on GPR of the shield wire
In this case, IG represents the current induced by the fault current on shield wire and IGp represents the current induced by the
neutral wire’s current. Then
IGp =
zNS
zTN
×
IFT = 0.033IFT
zSS
zNN
Fig. 5. The effect of the sources on both ends of T-line: (a) T-line with two sources
and parallel D-line, (b) current sources in the MGS during the T-fault, and (c) current
sources in the MGN during the T-fault.
(11)
where zNS is the neutral-to-shield mutual impedance and IFT is
the fault current on transmission line. The current induced by the
neutral current (IGp ) is 30% of IG (where IG = 0.11IFT ), which is considerable. Therefore the presence of the neutral wire actually helps
to lower the GPR of the shield wire. If the faults occur on distribution line, the GPR would not be higher than those originally caused
by the transmission faults.
Fig. 6. Bonding of neutral and shield wire in the parallel section.
After the injected currents and their locations are identified, the
mathematical procedure described earlier can be directly applied
with appropriate values. The results are provided in Section 4.
3.5. Bonding of D-line neutral and T-line shield
The bonding of neutral and shield wires essentially combines
the grounded node of neutral and grounded node of shield into
a single node as shown in Fig. 6. As a result, self-impedances of
these wires will be in parallel, so are their grounding resistances.
The currents associated with the shield wire and those associated
with neutral wire will combine in the bonded section. The GPR is
function of equivalent impedance and amount of injected current.
Therefore, the effect of bonding can be examined through equivalent impedance of the bonded network and the injected currents at
specific nodes.
3.4. Transmission line supplied from both ends
Fig. 5 shows the double circuit T&D lines where T-line is supplied
from two opposite ends. During a D-line fault, there is no effect
on NGPR whether T-line is supplied from one end or both ends.
However, for the T-line faults, the presence of downstream supply
(right source) causes injection of additional current sources in the
downstream section of the neutral wire corresponding to the fault
location. The procedure describe earlier is applicable to this as well.
Fig. 7. Bonding schemes, (a) separately grounded, (b) grounded together.
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J. Acharya, W. Xu / Electric Power Systems Research 80 (2010) 1074–1081
For node N4S4, the current sources are
IN4S4 = ISN + INS − INF − ISF + (0 or IF )
The last term will be zero if the fault is line-to-ground and IF if
the fault is line-to-wire. Thus the GPR of the nodes that are within
the parallel section will be affected. The current injected in the node
outside the parallel zone is not affected, (e.g. Node S1). The impact
of modified current injection in these will affect the GPR of the
other nodes as well. It is very difficult to establish simple, but accurate, formula for this situation. More accurate results are shown in
Section 4, here the focus is to show the peak GPR are affected.
Fig. 8. The currents injected in the bonded neutral and shield network.
3.5.1. Effect on equivalent impedance
Fig. 7 shows two schemes of bonding for a grounding span. In
Fig. 7a, the neutral wire and shield wire having their own grounding assembly are bonded at each grounded nodes, so there are 2
grounding resistances RGN and RGS at each location. In Fig. 7b, the
neutral wire and shield wire have the common grounding resistance RG .
The equivalent impedance of the bonded circuits for scheme of
Fig. 7a is
Zeq-2g ≈
s · (zNN //zSS ) × (RGN //RGS )
(12)
and for scheme of Fig. 7b is
Zeq-1g ≈
s · (zNN //zSS ) × RG
(13)
The equivalent impedances were computed using (12) and (13)
and were compared with the equivalent impedances of MGN and
MGS individually. The equivalent impedance given by (12) was 33%
of the MGS alone and 65% of the MGN alone (given by (1)). On the
other hand, the equivalent impedance given by (13) was 47% of
MGS alone and 90% of MGN alone. It implies that larger reduction
in equivalent impedances can be achieved for MGS than for MGN
as a result of bonding. This is true because the neutral conductor
has smaller impedance and adding a more resistive shield conductor in parallel will have relatively smaller impact on the equivalent
impedance. However, the neutral wire helps to reduce the equivalent impedance from the shield wire’s perspective. As the GPR
is directly proportional to the equivalent impedance, the GPR will
decrease compared to before bonding.
3.5.2. Effect on current injection
Consider the circuit of Fig. 5 again. For simplicity, ignore the
source on the right side of the transmission line. Therefore, only
the left hand side source will contribute to the fault current and
the induced currents are on the left side of the fault location only.
Due to the bonding, the Fig. 5b and c can be combined together,
resulting in a circuit shown in Fig. 8. So the subscripts L and R in the
currents referencing the left source and right source, respectively,
are dropped. Bonding of the neutral wire and shield wire also affects
the amount of current due to following currents:
• The fault current at the fault location
• The induced neutral current and the induced shield current
• The current induced in the shield wire by neutral current and the
current induced in the shield wire by neutral current
From Fig. 8, it can be seen that the parallel sections of neutral
and shield wires (N2–N4 or S2–S4) are combined. Nodes N2 and
S2 become a single node (N2S2) and nodes N4 and S4 into a single
node (N4S4) and are of great interest.
For node N2S2, the current sources are
IN2S2 = INF − ISN − INS
3.6. Comparison of TL-fault and DL-fault caused GPRs
The magnitudes of the GPR caused by TL faults are higher than
those caused by the DL faults because the fault current of DL is generally much lower that of TL. The safety impact of GPR on human is
determined by the magnitude and duration of the GPR. According to
the theory of electrocution established by Dalziel [13], the threshold current leading to electrocution is inversely proportional to the
square root of the current duration. Further considering the fact the
touch and step voltages are in proportion to the GPR, we can thus
compare the safety impact associated with the GPR created by the
TL faults and DL faults. A TL-fault poses higher risk if
GPRT-fault
>
GPRD-fault
tD
=
tT
(14)
where tD and tT are the fault clearing times of the DL and TL, respectively, and is defined as the breaker trip factor (BTF). The breakers
used in distribution systems are much slower than the breakers
used in transmission systems. For example, DL and TL have 30-cycle
breakers and 5-cycle breakers, respectively. Then
=
30
= 2.45
5
Thus the TL-faults will be more risky if GPRT-fault is 2.45 times
greater than the GPRD-fault . Using the equations derived in Section
3 and the system data given in Section 2, the highest GPR produced
in the neutral wire by the faults on D-line and T-line are:
NGPRD-fault = 0.50IFD (line to ground fault, islanded end)
NGPRT-fault = 0.30IFT (line to shield fault, fault location)
where the IFD and IFT are the fault currents of D-line and T-line,
respectively. Then
0.3IFT
NGPRT-fault
=
> 2.45
NGPRD-fault
0.5IFD
or
IFT
>4
IFD
(15)
i.e. the TL-fault poses a higher risk if IFT > 4IFD . The highest shield
GPR produced by the D-line and T-line faults are:
SGPRD-fault = 0.26IFD for a line-to-neutral fault and bonded
SGPRT-fault = 0.98IFT for a line-to-shield fault, unbonded
This results that a TL fault poses a higher risk if
IFT
> 0.65
IFD
(16)
Although the breaker trip factor () depends on the selection of
breakers, most TL faults will have a fault current IFT that satisfies
(16), if not both (15) and (16). Thus we can conclude that the TL fault
is more severe than the DL fault even though a TL fault is cleared
faster.
J. Acharya, W. Xu / Electric Power Systems Research 80 (2010) 1074–1081
1079
Table 4
GPRs for line-to-ground fault and line-to-wire fault.
Ground potential rise
NGPR (D-line fault) (V/kA)
SGPR (T-line fault) (V/kA)
Line-to-ground fault
Line-to-wire fault
Fault
location
Islanded
end
Fault
location
Islanded
end
250
110
500
220
300
980
500
220
Table 5
Summary of NGPR results obtained analytically.
Configurations
D-line only
D-line + T-line but no shield
D-line + T-line with MGS
D-line + T-line and
MGN–MGS bonded
a
D-line fault
(assume, IFD = 1 kA)
T-line fault
(assume, IFT = 5 kA)
V/kA
V
V/kA
V
500
500
500a
500a
500
500
500
500
–
190
180
300
–
950
900
1500
Fig. 9. NGPR profiles for the T-line-to-shield fault (except for D-line only).
No change because max NGPR occurs outside the parallel zone.
4. Results
Table 4 shows the GPR results for line-to-ground fault and lineto-wire fault. For the distribution line, the NGPR at the fault location
for the line-to-neutral fault is marginally higher that that for the
line-to-ground fault. This is because a large part of the fault current
(IFD ) flows into the neutral wire, leading to less current dissipating
into the earth through (RG ). Consequently the NGPR is less although
the neutral wire comes in contact with phase wire. For the transmission line, however, the SGPR for the line-to-shield fault is by
far larger as the shield wire does not carry a significant part of the
fault current. In summary, the faults involving the multi-grounded
wire (neutral or shield) are worse. It is important to note that the
line-to-wire fault has virtually no impact on the GPR of the islanded
end (node 1) of the multi-grounded wire (Fig. 3 and Table 4).
As mentioned earlier, the maximum GPR will occur at the
current injection nodes. Tables 5 and 6 show the summary of maximum NGPR and SGPR for the T-line and D-line configurations.
The reminder of this section shows the computer simulation
results. Figs. 9 and 10 depict the GPR profiles for different configurations under the line-to-shield fault. The fault occurs on T-line about
3.5 km from the substation. For the single circuits, same distance
was considered for the fault from the source. The T-line fault current
was approx 12.6 kA and the D-line fault current was about 2.5 kA.
Fig. 9 shows that the maximum NGPR occurs on the upstream
end of the parallel section. But the maximum SGPR always occurs
at the fault locations. The T-line faults significantly increase the
NGPR compared to D-faults and the situation becomes worst when
the neutral and shield wire are bonded. On the other hand, the
maximum SGPR of the single T-line remains the same even after
introduction of D-line (unbonded case) as shown in Fig. 10. It
decreases significantly when the neutral and shield are bonded.
These results agree with the analytical results.
Fig. 10. SGPR profiles for the T-line-to-shield fault.
Fig. 11. NGPR profiles for different fault locations (fault on T-line).
The GPR profiles are further illustrated in Figs. 11 and 12 for
various fault points along the T-line. The GPR/kA index is used in
these figures since only the T-line faults are compared. As seen in
Fig. 11, the NGPR has two equal peaks at the ends of the parallel
section when the fault lies downstream of the parallel zone. On
the other hand, the SGPR profile in Fig. 12 exhibits extremely high
peaks at the fault locations. The main reason is magnitude of fault
current.
Table 6
Summary of SGPR results obtained analytically.
Configurations
T-line only
T-line + D-line but no neutral
T-line + D-line with MGN
T-line + D-line and MGN–MGS
bonded
D-line fault
(assume, IFD = 1 kA)
T-line fault
(assume, IFT = 5 kA)
V/kA
V
V/kA
V
–
95
54
266
–
95
54
266
980
980
983
300
4900
4900
4915
1500
Fig. 12. SGPR profiles for different fault locations (fault on T-line).
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J. Acharya, W. Xu / Electric Power Systems Research 80 (2010) 1074–1081
Fig. 16. Comparison of NGPR for D-line faults and T-line faults.
Fig. 13. NGPR profiles for different fault locations (fault on T-line, N–S bonded).
Fig. 17. Comparison of SGPR for D-line and T-line faults.
Fig. 14. SGPR profiles for different fault locations (fault on T-line, N–S bonded).
For the bonded circuits, the NGPR and SGPR profiles are shown
in Figs. 13 and 14, which are different from Figs. 11 and 12 for
unbonded cases. Fig. 13 shows that the NGPR becomes the highest
at the fault location provided that the fault occurs in the parallel
zone. If the fault moves outside this zone (downstream), the maximum NGPR occurs at the upstream end of the parallel section. In
Fig. 14, the SGPR is the highest for the fault outside the parallel
zone and is similar to that of Fig. 12 (unbonded). However, if the
fault occurs in the parallel zone the maximum SGPR will decrease
drastically (compare Figs. 12 and 14).
4.1. The maximum GPR magnitudes
This case involves T-line fault (to shield) in the middle of the
parallel section. It is clear from Fig. 15 that the NGPR increases
significantly when the fault occurs on T-line.
The results (Fig. 15) also reveal that the bonding of neutral and
shield increases the NGPR. However, the maximum SGPR decreases
to the level of NGPR. This is for the following reason. The NGPR
is produced by the induced current when unbonded. But after
bonding, the T-line fault current is directly involved in the NGPR.
Although the equivalent impedance is reduced due to bonding, this
effect is dominated by the amount of T-fault current. On the other
hand, the decrease in SGPR is the reflection of reduction in equiva-
Fig. 15. Max GPR magnitudes for different configurations (fault on T-line).
lent impedance. The T-fault current involved in SGPR is almost the
same before and after bonding.
The maximum NGPR for D-line and T-line faults are shown in
Fig. 16. The NGPRs caused by the T-line faults are much higher than
that caused by the D-line faults. For T-line faults, the bonding configuration gives the worst NGPR. The maximum NGPR for the D-line
faults is not affected whether the shield is present or not. There is
no effect of bonding on the maximum NGPR because the maximum
occurs outside the parallel zone of T&D lines. The maximum SGPR
for D-line and T-line faults are shown in Fig. 17. The SGPRs caused by
the D-line faults are negligible compared to those caused by T-line
faults. Unlike the NGPR, the SGPR will be reduced due to bonding
for the T-line faults.
The following conclusions can be drawn from the above figures:
• The maximum NGPR will increase significantly as a result of fault
on T-line.
• The shield wire on top of the T-line does not have any noticeable
effects on NGPR. However, if the shield and neutral are bonded,
the NGPR will increase considerably. Note that the effect of bonding applies in the parallel zone only.
• For the T-line faults, the maximum SGPR does not increase by the
D-line configurations (Fig. 15). Similarly, for the D-line faults, the
maximum NGPR will not be increased by the T-line configurations.
4.2. Effect of T&D parallel exposure length
The parallel length of T-line and D-line was varied from 1 to
5 km. Fig. 18 shows that the NGPR increases initially with the length
of exposure when the fault occurs on T-line (middle of parallel section). However, it does not increase further when the parallel length
is more than 3 km. Fig. 19 shows that the SGPR is independent of
the exposure length when the fault occurs on T-line.
Fig. 18. Max NGPR for parallel exposure lengths (T-line fault in the parallel section).
J. Acharya, W. Xu / Electric Power Systems Research 80 (2010) 1074–1081
1081
• The line-to-wire faults are generally more severe than the lineto-ground faults. The effects of these faults are identical on the
parallel line.
• Bonding of neutral and shield does not improve GPR. It will create
higher level of GPR in the parallel circuit.
• The GPR caused by T-faults can still be dangerous than that caused
by D-faults even if T-faults clear faster.
• The GPR will increase for a certain range of parallel exposure
length.
Fig. 19. Max SGPR for parallel lengths (T-line fault in the parallel section).
Fig. 20. Max NGPR for parallel lengths (fault on T-line d/s of the parallel section).
Fig. 21. Max SGPR for parallel lengths (fault on T-line d/s of the parallel section).
It can be seen from Fig. 18 that the NGPR for the bonded configuration is much higher than that for the unbonded configuration.
This is particularly true if the fault location is inside the parallel
zone (where bonding exists). But if the fault occurs outside, the
NGPR will be less for bonded case compared to the unbonded case
(Fig. 20). Similarly the effect of bonding on the maximum SGPR is
relevant when the fault is inside the parallel zone (Fig. 19) and the
maximum SGPR does not change due to bonding if the fault location
is outside the parallel zone where bonding does not exist (Fig. 21).
5. Conclusions
The GPR characteristics of multi-grounded neutral (MGN) and
shield (MGS) in the joint systems are illustrated. Main conclusions
of this study are summarized as follows:
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Janak Acharya (S’05) received the B.Sc. degree in electrical engineering from Tribhuvan University, Nepal, in 2002. He obtained the M.Sc. degree in electrical engineering
from the University of Saskatchewan, Saskatoon, Canada, in 2005. Currently, he is
working toward the Ph.D. degree at the University of Alberta, Edmonton, Canada.
His research interests are power quality and reliability.
Wilsun Xu (F’05) received the Ph.D. degree from the University of British Columbia,
Vancouver, Canada, in 1989. He was an engineer with BC Hydro, BC, Canada, from
1990 to 1996. Dr. Xu is presently a NSERC Industrial Research Chair and a Professor
with the University of Alberta, Edmonton, Canada. His main research interests are
power quality and harmonics.