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The 19th International Symposium on High Voltage Engineering, Pilsen, Czech Republic, August, 23 – 28, 2015
GUIDE TO EHV/HV CABLE SHEATH BONDING APPLICATION SOFTWARE
1
1
1
1*
F. Garnacho , A. Khamlichi and A. Valero
High Voltage Technological Centre (LCOE-F2I2), Eric Kandel, 1 - 28906 Madrid, Spain
*[email protected]
Abstract: This paper introduces a software tool for the sheath bonding design of high
voltage power cable systems. The calculation tools for the design of the bonding system
are required for supplying the maximum value of the temporary overvoltages obtained for
the worst short circuit that can appear in the grid. Sheath overvoltages depend on
different factors: on the place where a short circuit appears, on the earth resistance value
at each grounding connection and on the designed sheath bonding configuration. The
software package uses a general method of circuit analysis adapted to cable systems to
determine temporary overvoltages in complex sheath bonding configurations. The
developed software allows defining any arbitrary architecture of linked elemental cable
sections in order to simulate the real sheath bonding configuration. Temporary
overvoltages in cable joints, terminations and link boxes are determined in this paper.
These values should be used for the correct selection of the sheath voltage limiters used
for protection of the outer sheath, joints, terminations and link boxes insulation. An
example of two cable systems is analysed to demonstrate that small modifications on the
sheath bonding configuration can reduce significantly the temporary overvoltages and
permits the viable selection of the surge arresters used for protection.
1
INTRODUCTION
Experience has proven that guidelines are needed
whereby engineering companies can select the
best sheath bonding configuration and the correct
surge arrester (SVL) to protect cable sheaths for
each particular cable system. The IEEE Standards
Association has recently approved a guide [1] for
helping the development of sheath bonding
configuration in cable system projects.
In spite of the fundamentals of calculating sheath
voltages and currents defined for many years,
many failures in the cable systems occur due to
overvoltages. The continuous increasing of short
circuit capabilities of modern power systems has
accentuated these problems, requiring the
application of more efficient bonding design criteria
and numerical tools for the design of sheath
bonding configurations. The numerical tools are
required for supplying the maximum value of the
overvoltage obtained for the worst fault conditions.
Figure 1: Overvoltages due to short circuit
The maximum absolute and local temporary
voltages shown in figure 2 must be determined. In
particular, the maximum value of the local
temporary overvoltage that can appear on the link
boxes where the surge arresters are placed must
be determined to avoid the failure of these SVLs.
Jc
Ulocal
When a single-phase short circuit occurs in a HV
network significant overvoltages appear on power
cable sheaths, especially in the cable terminations
that are not connected to the earth and in the
sheaths of cross bonding configurations. In the first
instants a transient damped overvoltage of several
tens of kilohertz and up to several tens of kilovolts
is superimposed to a temporary overvoltage of
power frequency that disappears when the short
circuit is removed when the current is extinguished
(see figure 1). Both transient and temporary
overvoltages provoke significant stress to be
considered in insulation coordination of cable
sheaths, cable accessories, link boxes and SVLs.
Je
m
Umn
R
em
Uabsolute
n
R
en
Figure 2: Absolute and local temporary voltages
Although ATP software can be used to determine
temporary overvoltages, in practice, the user
interface is not intuitive enough for the most
engineering companies. The developed software
permits determining temporary overvoltages in a
user-friendly manner.
2
BASIC THEORY
For each single-point bonding (SP) configuration
the following equation can be written:
The application software uses the general theory of
circuit analysis by the mesh current method. The
developed application software permits linking
different elemental bonding connections, named
“major sections”, with overhead lines to simulate
the real sheath bonding configuration.
For each major section of a cable system a set of
equations can be written with a specific boundary
condition. For instance, each sectionalized
cross bonding (SCB) configuration as the shown in
figure 3, satisfies the following system of three
equations:
[Umn] = [Zc( abc)s(αβγ) ]⋅ [Jc( abc) ] +[Zss(αβγ) ]⋅ [Js(αβγ) ]
(1)
[
][
]
U mn = Z ec ( abc) ⋅ J c ( abc) + Z ee ⋅ J e
(3)
where:
[Zec(abc)] = coupling impedances between
the GCC and each phase conductor.
Zee = self-impedance of the GCC.
Je = current through the GCC.
which unknown variables are the current Je
through the GCC, and the voltage Umn. In addition,
the following boundary condition must be also
satisfied:
U mn = Rem · J 'm − Ren · J 'n −( Rem + Ren )· J e
where:
(4)
Ja
[Zc(abc)s(αβγ)] = coupling impedances between
each phase conductor and each phase
sheath, that take into account sheath
transpositions.
[Zss(αβγ)] = coupling impedances between
cable sheaths, that take into account sheath
transpositions.
[Jc(abc)] = conductor currents Ja, Jb and Jc.
[Js(αβγ)] = sheath currents Jα, Jβ and Jγ.
which unknown variables are sheath currents Jα, Jβ
and Jγ and voltage, Umn between the ends of the
SCB. In addition, the following boundary condition
must be also satisfied:
Umn = Rem·J 'm − Ren·J 'n −(Rem + Ren )·(Jα + J β + Jγ )
(2)
where:
Rem = earth resistance of the ground
continuity conductor (GCC) on the left side.
Ren = earth resistance of the GCC on the
right side.
J’m = current through GCC of the previous
section on the left side.
J’n = current through GCC of the following
section on the right side.
Minor section
Jα
Ja
Jb
Jc
Jα+Jβ+Jγ
m
Umn
Ren
Rem
Figure 4: Major section defined by a SP
After Je determination sheath voltages Usa, Usb and
Usc can be determined by means of the following
equations:
[U
s ( abc )
] = [Z
sc( abc )
][J
c ( abc )
] + [Z
J’n
(5)
e
Different major sections can be linked in series to
simulate the real sheath bonding configuration of
the cable system to be designed. For each major
section a set of equations can be entered in the
global array that defines the circuit.
Jc
Major section 2: Single-Point
Jα
JF
Jβ
Jγ
n
JM=Jm1
Rem
]J
[Zsc(abc)] = coupling impedances between the
phase conductors and each phase sheath.
[Zs(abc)e] = coupling impedances between
the phase sheaths and the GCC.
Ja
Jα+Jβ+Jγ
s ( abc ) e
where:
Jb
J’m
n
Umn
Major section 1: Cross-Bonding
Jγ
Jc
J’n
m
Jβ
Jb
Je
J’m
Ren
Re1
Figure 3: Major section defined by a SCB
JN=Jn3
Je
J1
J2
R e2
Re3
Figure 5: Simple circuit composed by a SCB + SP
For a better explanation the analysis method using
mesh equations is presented for a simple sheath
bonding configuration composed by two basic
major sections (see figure 5). The equivalent circuit
of the figure 5 is shown in figure 6.
][· J ]
U = −[Z
U = −[Z
][· J ]
[Z
JM
ss ( αβγ )
]
1
c ( abc) s ( αβγ )
c ( abc)
2
Z tt 2
+
ce ( abc )
c ( abc )
+
JN
JM
- Single-point bonding (SP)
- Solid bonding (SB)
- Sectionalized cross bonding (SCB), with three
minor sections.
- Continuous cross bonding (CBC), with any
number of minor sections.
- Overhead line (OL), with any number of
spans.
JN
Re1
J1
J2
Re2
Re3
Figure 6: Equivalent circuit of analysed example
The mesh equations that define the figure 6 circuit
are the following:
[U ] = [M ] ⋅ [J ]
In SP sections is possible to add a second GCC
and both can be transpose at any point along the
cable run, while in SCB and CBC sections the user
could connect a parallel GCC along the major
section. Meanwhile, OL sections can be with or
without grounded wire.
(6)
where:
Re1 + Re2 + Zαα
R + R + Z
[M ] =  e1 e2 αβ
 Re1 + Re2 + Zαγ

− Re2

Re1 + Re2 + Z βα
Re1 + Re2 + Zγα
Re1 + Re2 + Zββ
Re1 + Re2 + Zγα
Re1 + Re2 + Z βγ
Re1 + Re2 + Zγγ
− Re2
− Re2
− Re2




− Re2

Re2 + Re3 + Zee 
− Re2
Zaα Zbα Zcα 
 Re1 ⋅ J M 
Z
 J a  
Z
Z
R ⋅J 
[U ] =  aβ bβ cβ  ⋅  Jb  +  e1 M 
Zaγ Zbγ Zcγ
 Re1 ⋅ J M 

  J c  

− Re3 ⋅ J N 
Zae Zbe Zce 
Figure 7: Example of inserting a SCB section
In any major section joint or termination, earth
resistance value is easily entered.
The resolution of the following linear equation
system allows determining the unknown variables
[J]:
[J ] = [M ]−1 ⋅ [U ]
(7)
where:
 Jα 
J 
[J ] =  β 
 Jγ 
 
 Je 
J 1 = Jα + J β + J γ
3
SOFTWARE DESCRIPTION
3.1
DATABASES
J 2 = Je
3.3
MINOR SECTIONS
Each buried major section consists of at least one
minor section which can also be divided in
subsections in order to match the design to the real
configuration (types of laying and cable).
In the minor section menu the user can choose the
power cable and GCC from the database, and the
run length and the soil resistivity value are entered
directly by the user. In the case of OL sections, the
conductor, the bundled configuration, the grounded
wire and the type of tower are eligible from the
database, and the operating temperature, the line
length and the mean span length are entered
directly.
Initially, the EHV/HV Cable Sheath Bonding
Application Software requires the user to fill in a
cables and overhead lines database. An Excel
library should be completed with the information
provided by cable and wire data sheet. Likewise,
parallel ground continuity conductor (GCC) and
overhead grounded wire specifications should be
included in this library database.
Figure 8: Example of editing a cable laying
3.2
MAJOR SECTIONS
The software allows designing any power circuit
with multiple bonded cable system and overhead
lines by inserting in series all the desired major
sections from the following:
Any type of cable laying or OL laying to be used
must be included in an Excel database, although
they can be later modified directly on the layout
viewer in order to fit the design to the real laying
configuration.
3.4
VOLTAGE SOURCES
3.7
Two types of voltage sources can be considered:
substation or power grid where cable is connected
(see Figure 9). Source data can be entered either
by their short circuit currents (three-phase and
single-phase) or by their short circuit impedances
(direct and homopolar).
SIMULATION OUTPUTS
When the case study is completely designed and
the analysis scenario selected, the user can
proceed with a full simulation which will be
completed in a few seconds. The software allows
evaluating the influence of the earth resistance
values by reproducing the same case study with
three different values in the same processing. The
following results are obtained from the simulation:
3.7.1 Absolute sheath voltages
COUPLING OF DOUBLE-CIRCUIT CABLE
Since sheath standing voltages are modified by the
presence of a closely second circuit, the software
permits designing double-circuit geometries. Two
independent circuits can be analysed with a
common or different voltage source. Furthermore,
any cable subsection can be coupled to another
subsection or minor section of a second circuit
system in order to simulated parallel double-circuit
cable with any desired laying configuration.
3.7.2 Local sheath voltages
The same outputs as for absolute voltages turn out
for the maximum values of local overvoltage that
can appear on the link boxes.
12
9
100.0% R t
150.0% R t
100.0% R t
200.0% R t
150.0% R t
200.0% R t
8
10
7
b)
a)
8
6
5
U (k V )
3.5
U (k V )
Figure 9: Types of voltage sources: a) substation
source; b) power grid source
The maximum absolute overvoltage values that
can appear in the cable outer sheath of every joint
or termination are shown directly over the circuit
diagram, plotted on a line graph and exported
numerically to Excel, in order to analyse the outer
sheath insulator.
4
6
4
3
2
2
1
0
EI0
EI1_I
EI1_D
EI2
EI3
EI4
EI5
EI6
EI7
0
EI0
EI1_I
EI1_D
EI2
EI3
EI4
Empalme/Terminal
EI5
EI6
EI7
Figure 11: Example of: a) absolute sheath voltage
graph; b) local sheath voltage graph
3.7.3 Ground voltages
Figure 10: Example of coupling a double circuit
SIMULATION SCENARIOS
3.7.4 Induced sheath currents
Two kinds of analysis can be performed:
3.6.1 Temporary analysis
Sheath temporary overvoltages are
caused by the following fault conditions:
typically
-
Three-phase fault
Single-phase fault to substation earth
Single-phase fault to local earth
Fault due to wire breakage in OL, from left or
right side
- Fault due to backflashover in OL
- Internal cable fault
In addition, induced current values are also shown
over the circuit diagram by arrows indicating the
flow direction.
9
100.0% R t
Another possible scenario consists on inserting a
three-phase load for steady-state simulation.
200.0% R t
b)
7
6
a)
5
4
3
2
1
0
EI0
3.6.2 Steady-state analysis
150.0% R t
8
U (k V )
3.6
The software also provides the user maximum
voltage values on the points connected directly to
ground.
EI1_D
EI4
EI7
Figure 12: Example of: a) ground voltage graph;
b) induced sheath current presentation
4
CASE STUDY
- Fault due to wire breakage in OL from left
side (WB-L), located in each span.
- Fault due to wire breakage in OL from right
side (WB-R), located in each span.
- Fault due to backflashover in OL (BFO),
located in each span.
- Internal cable fault (IN), in two cases: located
between Sub.1 and L1 at 450 m distance, and
between Sub.3 and T1 at 50 m distance.
In order to demonstrate the technical capabilities of
the EHV/HV Cable Sheath Bonding Application
Software, the local sheath voltages of a doublecircuit are analysed below for all possible fault
conditions.
As you can see in figure 13, there are in the
example two circuits connected on their right side
to the same voltage source reproducing a common
substation (Sub.2). The first circuit (Sub.1-Sub.2a)
is composed by two linked SCB followed by a long
SP with the SVL at the right end. The second
circuit (Sub.3-Sub.2b) is composed by one short
SP followed by an OL of 30 km long. From the
other side of the OL continues a SCB plus two SP
with their not-grounded points coinciding at the
middle joint (L10).
Table 1: Local sheath voltages in the case study
without improvements (kV)
ULOCAL
Furthermore, there are three coupled subsections
reproducing a double-circuit laying configuration:
minor section L5-L6 is coupled with a subsection of
L8-L9 and the minor section L6-Sub.2a is divided
in two equal length subsections, each coupled to
the facing SP minor sections of the second circuit,
L9-L10 and L10-Sub.2b respectively.
Circuit Sub.1 – Sub.2a
Circuit Sub.3 – Sub.2b
Fault
L1
L2
L4
L5
S/2a
T1
L7
L8
3F-S
3,3
3,1
1,6
2,5
4,4
0,2
1,5
1,5
L10L L10R
0,7
0,7
1F-S
4,8
4,5
4,5
5,7
10,7
0,1
4,3
4,0
0,8
0,8
WB-L
1,1
1,1
1,2
1,5
0,1
0,2
1,1
1,2
0,1
0,1
WB-R
2,3
2,4
2,6
3,1
0,7
0,0
5,5
4,0
0,8
0,8
BFO
5,3
5,1
5,6
6,8
7,0
1,6
14,3 12,8
8,6
8,5
IN
6,0
5,4
4,8
5,8
10,8
0,2
2,5
2,0
0,7
0,7
Max.
6,0
5,4
5,6
6,8
10,8
1,6
14,3 12,8
8,6
8,5
Noting the last row of table 1, with the present
sheath bonding design the outer sheath integrity is
compromised and the selection of the SVLs is
complicated in the link boxes of Sub.2a, L7, L8,
L10 (left side) and L10 (right side). The proposals
for improvement are described below in order to
reduce these maximum local sheath voltages
obtained in table 1 and select the SVLs correctly:
The high voltage considered is 220 kV, having
40 kA of short circuit currents (three-phase and
single-phase) in each substation. The same type of
power cable and trefoil laying formation has been
considered along the whole system in order to
annul their influence on the local sheath voltages.
- Improvement num. 1: reduce the earth
resistance value in every transmission tower.
In this example 20 Ω is changed by 10 Ω.
- Improvement num. 2: add a parallel GCC
along the SCB sections. In this case, this
improvement is applied in the major sections
Sub.1-L3, L3-L6 and T86-L9.
- Improvement num. 3: add a second GCC in
the SP sections. Is the case of the major
sections L6-Sub.2a, Sub.3-T1, L9-L10 and
L10-Sub.2b.
- Improvement num. 4: break up facing SP
sections and avoid having SVLs in substation
terminations by switching the grounding
connection points of the SP major sections. In
this case study it is proposed moving the notgrounded point from Sub.2a to L6 and from
L10 (left side) to L9 (right side).
The OL is composed by 85 spans with 20 Ω of
earth resistance value in each tower. The earth
resistance value at substation considered is 1,5 Ω
while at each joint is 5 Ω. The length of each minor
section or subsection is shown in figure 13.
The following fault conditions were analysed
separately obtaining the maximum local sheath
voltages shown in table 1:
- Three-phase fault (3F-S), located in each
substation.
- Single-phase fault to each substation earth
(1F-S).
Sub.1: 220 kV
Sub.1
L1
900m
L2
300m
L3
600m
1.5 Ω
L4
200m
L5
800m
L6
400m
5Ω
Sub.2a
450m
450m
5Ω
Sub.2: 220 kV
T1
AP
AP
T86
1.5 Ω
Sub.3: 220 kV
Sub.3
L7
30.00 km. 85 Spans
100m
1.5 Ω
700m
10 Ω
10 Ω
20 Ω
Figure 13: Case study without improvements
L8
700m
L9
300m
400m
L10
450m
5Ω
Sub.2b
450m
5Ω
Sub.1: 220 kV
Sub.1
L1
900m
L2
300m
L3
600m
1.5 Ω
L4
200m
L5
L6
800m
400m
5Ω
Sub.2a
450m
450m
5Ω
Sub.2: 220 kV
T1
AP
AP
T86
1.5 Ω
Sub.3: 220 kV
Sub.3
L7
30.00 km. 85 Spans
100m
1.5 Ω
700m
10 Ω
L8
L9
700m
300m
L10
400m
10 Ω
450m
Sub.2b
450m
5Ω
5Ω
10 Ω
Figure 14: Case study with improvements
These proposals for improvement, represented in
figure 14, are justified by countless real studies
carried out by the authors of this article. The
maximum local sheath voltages obtained for the
case study with each proposed improvement are
shown in table 2. In the last row the percentage
difference between with and without improvements
cases is calculated.
Table 2: Local sheath voltages in the case study
with improvements (kV)
ULOCAL
Circuit Sub.1 – Sub.2a
Selected SVL residual voltage (Ures) for each link
box must assure an appropriate protection margin
taking into account the insulation level for transient
overvoltages, and considering the effect of
distance between the SVLs and the insulation to
be protected. This topic is explained in detail in
reference [2], therefore, in the improved case study
of this paper, in particular, a correct selection of
the SVLs would be as shown in the table 3:
Table 3: Selection of SVLs in the final case study
Circuit Sub.3 – Sub.2b
SVL
Circuit Sub.1 – Sub.2a
Without
Improv.
6,0
5,4
5,6
6,8
10,8
1,6
Improv.
L1
L2
L4
L5
S/2a
T1
ULOCAL
2,0
2,2
num.1
6,0
5,4
5,7
7,0
10,8
1,6
13,2 12,0
8,5
8,5
Ur ≥
2,0
2,2
num.2
2,0
2,2
0,7
1,7
10,7
1,5
2,5
4,0
8,6
8,5
Ures ≤
6,0
6,6
2,1
num.3
4,8
5,3
5,0
6,1
9,3
0,9
5,8
6,9
3,4
3,8
L1
L2
L4
L5
L6
T1
L7
L8
L9R L10R
6,0
5,4
5,8
7,1
10,8
0,1
13,4 12,0
8,5
8,5
2,0
2,2
0,7
1,7
9,3
0,9
2,5
3,4
3,8
num.4
With all
Improv.
%
14,3 12,8
L7
L8
4,0
8,6
8,5
L10L L10R
-67% -59% -88% -75% -14% -44% -83% -69% -60% -55%
Interpreting the results from table 2, it could be
said that the improvements number 2 and 3
significantly reduce the local sheath overvoltages.
In this case, the improvement number 1 can be not
considered. Meanwhile, it is known that by
removing facing SP sections (improvement num. 4)
transient opposite polarities disappear, avoiding
double overvoltage which would have endangered
joint insulators. Additionally, removing SVLs from
gas-insulated substation (GIS) terminations is
proposed because the coordination between the
cable SVL rated voltage and the GIS enclosure
bypass SVL rated voltage is usually not easy.
However, despite all the improvements the local
sheath voltage obtained in the long SP section L6Sub.2a remains dangerous. The only possible
solution would be to divide the major section in two
separated SP sections just like the configuration in
sections L9-L10 and L10-Sub.2b. Finally, selection
criteria for surge voltage limiters should be
considered for outer sheath protection. In this
sense,
SVL
should
withstand
temporary
overvoltages in order to be in the safety side. The
SVL rated voltage (Ur) is chosen greater or equal
to the local temporary sheath overvoltage.
With all
Improv.
6
L1
L2
L4
Circuit Sub.3 – Sub.2b
L5
L6
T1
L7
L8
L9R L10R
0,7
1,7
9,3
0,9
2,5
4,0
3,4
3,8
0,7
1,7
9,3
0,9
2,5
4,0
3,4
3,8
5,1
27,9
2,7
7,5
12,0 10,2 11,4
CONCLUSION
Significant temporary overvoltages can appear in
cable sheaths when short circuits occur in high
voltage power cable systems. The developed
EHV/HV Cable Sheath Bonding Application
Software
allows
designing
any
arbitrary
architecture of linked elemental cable sections in
order to simulate the real sheath bonding
configuration and to determine the temporary
overvoltages. The simulation outputs permits the
viable selection of the surge arresters used for
protection. This software is strongly recommended
to cable project designers, cable manufacturers
and transmission system operators.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial
support of Red Eléctrica de España (REE) and
Unión Fenosa Distribución (UFD).
REFERENCES
[1] IEEE Std 575™-2014 “IEEE Guide for Bonding
Shields and Sheaths of Single-Conductor
Power Cables Rated 5 kV through 500 kV”.
[2] F. Garnacho, A. Khamlichi, P. Simon and
A. Gonzalez: “Guide to Sheath Bonding
Design, in Distribution and Transmission Lines
with HV Underground Cables”, CIGRE, B1105, 2012.