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Guide to sheath bonding design, in distribution and transmission lines with
HV underground cables
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B1-105
CIGRE 2012
Guide to Sheath Bonding Design, in Distribution and Transmission Lines with
HV Underground Cables
F. GARNACHO1, A. KHAMLICHI1, P. SIMON1, A. GONZÁLEZ2
1
LCOE, 2Gas Natural Fenosa
Spain
SUMMARY
Power electrical networks using HV underground cables are continuously increasing in the big cities. In
order to avoid or reduce transmission losses in cable sheaths provoked by solid-bonding connections
special sheath bonding techniques, such as cross-bonding (CB) or single-bonding (SB) connections, are
commonly used for the HV cable systems. Nevertheless experience has proven that clear design rules are
required to achieve safe and reliable sheath bonding and earth connection systems.
Maintenance and operation experiences in the 220 kV network have showed that failures in the cable
system are not negligible when a short-circuit occurs in the power network. Continuous growing of the
short-circuit power in the high voltage network and progressive increasing of elementary cable section
length between two consecutive accessories require to apply more efficient bonding design criteria.
Sheath overvoltages depend on different factors: place where an eventual short-circuit appears (inside
cable grid, in a substation or in an overhead line), earth resistance value at each earthing point and
architecture used to link elementary cable sections (SB, continuous CB and sectionalised CB). The design
of the bonding system must take into account not only to select sheath voltage limiters but also to
determine the insulation level of the oversheath, joints, terminations and link boxes.
This paper presents the application guide to be applied to sheath bonding design of high voltage power
cable systems in the range between 45 kV and 220 kV. This guide shows temporary over-voltages in
sheaths when different kinds of short-circuits occurs for different sheath bonding configurations. The
results obtained by the guide allow a reliable selection of sheath voltage limiters and insulation level in
order to assure a suitable protection level against eventual short-circuit overvoltages. Different specific
methods have been used to determine overvoltages and they have been compared for a wide range of
cases in order to have an easy and generic numerical tool GSBD adapted to cable systems. The software
package developed allows to define any arbitrary architecture to link elementary cable sections (SB,
continuous CB and sectionalised CB) in order to determine continuous over-voltages in accessory sheaths
and in overvoltages limiters. The programme is used when the architecture applied is not close to the
cases studied in the Guide.
KEYWORDS
Cables, sheath, overvoltages, link boxes, cross bonding, single bonding, ATP.
fernandog@lcoe.etsii.upm.es
1 INTRODUCTION
When a single phase short-circuit occurs in a high voltage network significant overvoltages appear on
power cable sheaths, especially in the cable terminations that are not connected to the earth of SB
configurations and in the sheaths of cross zones of cross-bonding configurations. In the first instants a
transient damped overvoltage of several teens of kilohertz’s of up to several teens of kilovolts is
superimposed to a temporary overvoltage of power frequency that disappears when the short-circuit is
removed by switchgear (see figure 1). Both overvoltages, transient and temporary, provoke significant
stress to be considered in insulation coordination of cables, link boxes and overvoltage limiters.
30
[kV]
20
10
0
-10
-20
-30
0
10
20
(f ile Transitorio50ns80ms.pl4; x-v ar t) v :E1IA
30
40
v :E1IB
50
60
70
[ms]
80
v :E1IC
Figure 1. Voltage evolution on a cable sheaths due to single short circuit.
The rated voltage of overvoltage limiters must be chosen taking into account temporary overvoltage and
its residual voltage is selected in order to get an appropriate protection level, according to transient
withstand voltage of insulation media involved. Consequently, temporary overvoltages must be
determined for a correct selection of overvoltage limiters. In particular, it is very important to determine
the absolute overvoltages that appear in cable outer sheat and the local temporary overvoltages that
appear on overvoltage limiters.
Although ATP software can be used to determine temporary overvoltages, it does not have a user
interface simple enough for project engineers dedicated to high voltage cable projects. In practice, data
tables or alternative flexible numeric tools are required to analyze different influence parameters, (e.g.
earth connection values, length of cable sections, cable arrangements, etc.) on temporary overvoltages for
specific sheath architectures.
2 DIFFERENT KINDS OF SHORT CIRCUITS
In high voltage grids different kinds of short circuits can appear. A special attention must be paid to single
phase short circuits in comparison to three phase short circuits, because the induced overvoltage on
sheaths is not balanced by currents of others phases. However, the relative position between short circuit
point and the voltage supply allows to establish different short circuit scenarios: a) substation-substation
short circuit (figure 2) where the main short circuit current returns through a conductor (sheaths or earth
continuity conductor “ecc”), b) Siphon short circuit (figure 3) where the main short circuit current returns
through earth, c) far away short circuit (figure 4) where the short circuit current returns through both earth
and conductors (sheaths or ecc).
1
B
IF
A
Conductor equipotencial
R2
R1
R1
 IF
R2
IF
 IF
IF
C
-
+
A
 IF
Figure 2. Substation-substation short circuit: a) Solid-bonding or Cross-bonding configuration, b) Single-bonding configuration.
IF
IF
B
R1
A
IF
R2
Conductor equipotencial
Iecc
R1
C
-
+
A
IF
IF
R2
Figure 3. Siphon short circuit: a) Solid-bonding or Cross-bonding configuration, b) Single-bonding configuration.
B
R1
R1
R2
IF
´IF
C
-
+
Conductor equipotencial
IF
IF
R2
´) IF
Figure 4. Far away short circuit: a) Solid-bonding or Cross-bonding configuration, b) Single-bonding configuration.
In this paper the same electrical scheme is used for solid-bonding and for cross-bonding configuration,
because in a CB there is a sheath circuit for short circuit current circulation with both ends of the circuit
earthed, in a similar way as in solid bonging configurations.
3 CALCULATION METHOD TO LINK DIFFERENT SHEATHS CONFIGURATIONS
It is impossible to establish simple formulas to determine local and absolute sheath overvoltages in an
arbitrary interconnection architecture of sheaths configurations when a single phase short circuit occurs.
In these cases, it is necessary to use numeric calculation tools. In the following paragraphs a general
method of circuit analysis (GMCA) and a circuit analysis by symmetrical components (CASC) are
presented. Differences of both methods in comparison with results obtained by means of ATP software
are negligible.
3.1 Circuit analysis by symmetrical components
For each section of bonding connection (solid-bonding, single-bonding, cross bonding sectionalized, etc.)
the zero sequence circuit of each section is derived in order to interconnect them in the correct way as the
real arrangement used in the system cable. In the following paragraphs the zero sequence formulas of a
single-bonding configuration with earth continuity conductor are developed.
Overvoltage on the earth cable Umn is given by the superposition of the induced voltages provoked by the
currents of the three phase conductors [Jc(abc)] and the current through the earth continuity conductor Jt:



U mn  Z tc( abc ) J c( abc )  Z tt J t
(1)
2
where:
-
Z
-
conductor calculated by the Carson’s formulas.
Z tt self impedance of the ecc calculated by the Carson’s formulas.
tc( abc )

coupling impedances between the earth continuity conductor (ecc) t and each phase
Taking into account the electric scheme shown in figure 5 and assuming J’tn = 0 the following expression
can be written:
U mn  Rtm ( J 't  J t )  Rtn J t
(2)
where
- Rtm earth resistance of the ecc on the left side.
- Rtn earth resistance of the ecc on the right side.
- Jt’= J’tm current through ecc of the previous section on the left side.
Ja
Jb
Jc
Jt
J’m
m
J’n
Umn
n
Rtn
Rtm
Figure 5. Electric scheme associated to single-bonding configuration with an earth conductor.
Replacing expression (1) in expression (2) the following equation can be written:



 Z tc( abc ) J c( abc )  ( Rtm  Rtn  Z tt ) J t  Rtm J t'
(3)
Applying symmetric components analysis:
 3Z tc0 J c0  ( 3Rtm  3Rtn  3Z tt ) J t 0  3Rtm J t' 0
(4)
where:
-
Z tc0 sequence zero coupling impedance between the three phase conductors and the ecc.
-
J t  3 J t 0 , J t'  3 J t' 0 , J F  3 J c0
JF single phase short circuit current.
The equivalent electric circuit associated to equation (4) is shown in figure 6:
3
-3·Ztc0 ·J c0
3·Ztt
J’t0
+
Jt0
m
J’t0
Jt0
3·Rtm
3·Rtn
Figure 6. Sequence zero circuit of the earth continuity conductor.
If the short circuit current JF , the resistance earth, Rtm y Rtn, and current through phase conductors [Jc(abc)]
are known, the current of the ecc J t 0 can be determined by means of the equation (4), later Umn and
sheath voltages Upa, Upb y Upc, can be determined applying (1) and the following equations:
U
p( abc )
  Z
pc( abc )
J
c( abc )
  Z
p ( abc ) t
J
(5)
t
where:
-
Z
Z
pc( abc )
p( abc ) t
 coupling impedances between the phase conductors and each phase sheath.
 coupling impedances between the phase sheaths and the ecc.
3.2 General circuit analysis method
A software application that uses the general theory of analysis of circuits has been developed. The
software application allows linking many sections with different bonding connections (SB, CB) with
overhead lines (with or without skywires). For each single-bonding configuration equation (1) can be
written, which unknown parameters are the current through the ecc Jt and the voltage through the ecc,
Umn. For each cross-bonding configuration a system of three equations can be written, which unknown
variables are sheaths currents J  , J  y J  , and voltage, Umn across a major CB section. In addition, for
each bonding configuration the following equation must be satisfied:
U mn  Rtm· J'm  Rtn· J'n ( Rtm  Rtn )·( J  J   J  )
(6)
The resolution of the linear equation system allows determine the unknown variables.
The electrical bonding circuit composed by two CB sections in series with two SB sections shown in
figure 7 with cable arrangement of figure 8 is analyzed by means of the GCA.
Figure 7. Electric circuit of bonding connection.
4
a) Single-bonding formation section.
b) Power cable transversal section.
c) Ecc section.
Figure 8. Cable arrangement.
Figures 9 and 10 show sheaths overvoltages corresponding to a single phase short circuit in substationsubstation and far away short circuit scenarios respectively.
Figure 9. Sheath overvoltages due to a single phase short circuit of 1 kA in a substation-substation short circuit scenario.
Figure 10. Sheath overvoltages due to a single phase short circuit of 1 kA in a far short circuit scenario.
Table I shows a comparison between the GCA method and ATP method for far short circuit scenario. No
relevant differences were obtained using both methods.
Table I. Comparison between GCA and ATP method for figure 7 short circuit.
Voltage (V/kA)
GCA (V/kA)
ATP (V/kA)
Difference (V/kA)
CB1_1
119.8
119.5
0.3
CB1_2
157.2
157.0
0.2
CB2_1
252.9
252.2
0.7
CB2_2
302.9
302.2
0.7
SP1
328.8
328.5
0.3
SP2
347.5
347.1
0.4
5
4 APPLICATION GUIDE
4.1 Overvoltage tables in different sheaths configurations in different laying types
In order to have a fast magnitude order of sheath overvoltages the guide includes many different result
tables with sheath overvoltage values for different sheath configurations (SB, sectionalised CB, continous
CB), for three different short-circuit scenarios: substation-substation, siphon and far away short-circuit,
for the cables arrangements (flat formation -A-, trefoil formation -B- with an ecc laid in the geometric
center of the equilateral triangle, trefoil formation -C- with ecc transposed in the middle, or trefoil
formation -D- with 2 ecc transposed in the middle) used by Gas Natural Fenosa (GNF), and using
different section cable lengths (500 m y 1000 m) and for different earth resistance values on cable ends.
An example is shown in table II, corresponding to local and absolute overvoltages for a single-bonding
configuration in a short circuit scenario substation-substation. Induced overvoltages per unit increase
when the sum R1 + R2 of increases and when the ratio S/d increases also. However, overvoltages decrease
for trefoil formation if the earth conductor is on the geometrical center in comparison with the results
obtained for other ecc positions. In addition, in general, local voltages are bigger than absolute
overvoltages for a substation- substation short-circuit scenario.
0,75S
S
d
s
s
S
S
-B-
-A-
-C-
-D-
Figure 11. Different cables arrangements used by GNF: A: flat formation, B) trefoil formation with an ecc laid in the geometric
center of the equilateral triangle, C) trefoil formation with ecc transposed in the middle, D) trefoil formation with 2 ecc
transposed in the middle).
Table II. Temporary overvoltages (V/kA·km) for a single-bonding section of 500 m, during a single phase
short-circuit in a cable of 220kV-2000mm2 Cu. Considering a substation-substation short-circuit scenario.
R1/R2
U
local /
Phase conductor and ecc
0
absolute
arrangement
y
0
local
0,25
0,25
y
y 0,25
0,5
y 0,25
0,5
0,5
0,25
0,5
10
10
10
10
20
y
y
y
y
y
y
y
y
y
0,5
10
10
0,25
0,5
10
20
10
20
20
T-A
S/d=1,12
159
175
182
182
186
196
196
196
196
197
197
197
197
T-A
S/d=2,29
290
335
350
350
358
379
379
379
379
380
380
380
380
T-C
S/d=1,12
225
253
263
263
269
285
285
285
285
285
285
285
286
T-C
S/d=2,29
290
335
350
350
358
379
379
379
379
380
380
380
380
T-D
S/d=2,93
214
226
229
229
231
236
236
236
236
237
237
237
237
T-A
S/d=1,12
159
125
99
138
113
28
28
192
188
107
76
137
107
T-A
S/d=2,29
220
187
158
204
176
70
75
272
267
169
133
206
169
T-C
S/d=1,12
225
192
162
210
181
74
79
279
274
174
138
211
174
T-C
S/d=2,29
290
261
226
285
252
125
130
373
366
247
203
291
247
T-D
S/d=2,93
214
189
169
198
181
119
121
233
231
175
155
195
175
absolute
6
4.2 Overvoltages examples for different sheath connection architectures.
Elemental different bonding sections (SB, CB) are linked in order to create different interconnection
architectures. Temporary overvoltages on cable sheaths depend on the architecture created. Although, the
correct way to know temporary overvoltages is using the software tools described in section 3, it is very
important to study some examples in order to have general design criteria.
Table III shows local overvoltages per unit (V/ kA) in the different sheaths crosses of three cross-bonding
linked for a substation-substation short circuit scenario when a far away short occurs, considering length
cable section of 500 m and 1,000 m for each CB section. The resistance earth value has been changed
between 0,5  to 20 . Greater overvoltages are obtained for far away short-circuits than for substationsubstation short circuits. In addition, overvoltages are proportional to section lengths when the short
circuit appears between substations, however, the length does not affect significantly on sheaths
overvoltages (less than 10%) for far away short circuit scenarios. Must be headline that overvoltages
increases signigicantly when the earth value on the cable end increase to 5  or 10 ).
Table III. Local overvoltages per unit (V/ kA) on crosses of 3 CB linked.
Length
Substation-substation short-circuit
Far away short-circuit
R1
R1
R2
R3
R2
R3
Tensiones de pantalla absolutas. CBS + CBS + CBS.
Falta monofásica fase C. Pasante lejana
R4
Tensiones de pantalla absolutas. CBS + CBS + CBS. Falta monofásica fase C. S/E-S/E
46
2000
1
2
R1
()
0.5
0.5
R2
()
5
10
R3
()
5
10
R4
()
0.5
0.5
3
4
5
6
0.5
0.5
0.5
0.5
5
10
10
1
10
5
10
1
0.5
0.5
0.5
0.5
40
1
2
3
4
38
36
1
2
R1
()
0.5
0.5
R2
()
5
10
R3
()
5
10
R4
()
0.5
0.5
3
4
0.5
0.5
5
10
10
5
0.5
0.5
Case
1600
1400
1200
V/kA
Caso
Caso
Caso
Caso
42
V/kA
500 - 500 - 500 (m)
Case
1800
44
Caso
Caso
Caso
Caso
Caso
Caso
1000
800
34
1
2
3
4
5
6
600
32
400
30
200
28
CB1_1
CB1_2
CB2_1
CB2_2
CB3_1
CB3_2
0
CB1_1
CB1_2
Tensiones de pantalla absolutas. CBS + CBS + CBS. Falta monofásica fase C. S/E-S/E
80
V/kA
70
1
2
3
4
65
1
2
R1
()
0.5
0.5
R2
()
5
10
R3
()
5
10
R4
()
0.5
0.5
3
4
5
6
0.5
0.5
0.5
0.5
5
10
10
1
10
5
10
1
0.5
0.5
0.5
0.5
Case
2500
1
2
R1
()
0.5
0.5
R2
()
5
10
R3
()
5
10
R4
()
0.5
0.5
2000
3
4
5
6
0.5
0.5
0.5
0.5
5
10
10
1
10
5
10
1
0.5
0.5
0.5
0.5
V/kA
Caso
Caso
Caso
Caso
75
3000
1000
55
500
CB1_2
CB2_1
CB2_2
CB3_1
CB3_2
CB2_2
CB3_1
Case
Caso
Caso
Caso
Caso
Caso
Caso
1500
60
50
CB1_1
CB2_1
0
CB1_1
CB3_2
Tensiones de pantalla absolutas. CBS + CBS + CBS.
Falta monofásica fase C. Pasante lejana
3500
1,000 -1,000 - 1,000 (m)
R4
CB1_2
CB2_1
CB2_2
CB3_1
1
2
3
4
5
6
CB3_2
5 SELECTION CRITERIA FOR OVERVOLTAGE LIMITERS AND OUTER SHEATH
PROTECTION
For a specific laying arrangement (trefoil formation, flat formation, etc.) and for a specific bonding
connection (SB, CB), sheath overvoltage limiters (SVL) used for outer sheath protection, should
withstand temporary overvoltages, Ut that appears between sheaths and earth. This overvoltage depends
on induced local voltage ulocal (V/kA) and on the short circuit current value that is foreseen in the grid for
a specific short circuit scenario (substation-substation short circuit, siphon short circuit or far away short
circuit).
U t (V)  u local ·I cc kA
(7)
7
In order to have a security margin the rated voltage of the overvoltage limiter Ur (withstand power
frequency voltage for 10 s) is chosen greater or equal to the temporary overvoltage (e.g. overvoltage
during short circuit).
U r  u local ·I cc kA
(8)
Local voltages per unit, ulocal , must be determined for each project, either by means of table data
included in the Guide either by means of a software application developed for this purpose. On the other
hand the maximum short circuit currents must be evaluated taking into account the specific performances
of the grid.
SVL selected for each voltage level of the system must assure an appropriate protection margin taking
into account the insulation level for transient overvoltages, and considering the effect of distance between
the SVL´s and the insulation to be protected.
U

MP %    pt  1  100
 U res

(9)
where:
Up-t:
Ures:
withstand voltage sheath-earth, for lightning impulses 1,2/50 µs
Residual voltage of the SVL´s.
Table IV shows the characteristic values of the SVL´s used by GNF and the protection margin
considering the withstand voltage required for each voltage level in order to compensate the distance
effect between limiter and the insulation protected.
An additional energy analysis performed for the GNF grids allowed to recommend overvoltage limiter of
class 2, except for overvoltage limiters of rated voltages of 3,3 kV, that is recommended a class 3.
Table IV. Protection margin.
Uo/U
(kV)
Up-t 1,2/50
(kV)
26/45
30
36/66
30
76/132
127/220
37,5
47,5
Limiter characteristics
Protection level
MP (%) =100∙(Upt/Ures-1)
Ur (kV)
Uc
(kV)
Ures
(kV)
3,3
2,7
10
200
5
4,0
14
114
3,3
2,7
10
200
5
4,0
14
114
3,3
2,7
10
275
5
4,0
14
168
6
4,8
18
108
3,3
2,7
10
375
5
4,0
14
239
6
4,8
18
164
6
5
20,6
131
9
8
24,6
93
8
On the other hand, the outer sheath should withstand temporary overvoltages that appears between
sheaths and reference earth. This overvoltage depends on induced absolute voltage uabsolute (V/kA) and on
the short circuit current value that is foreseen in the grid for a specific short circuit scenario (substationsubstation short circuit, siphon short circuit or far away short circuit):
U outer sheath
50Hz
 u absolute·I cc ( kA)
(10)
As a reference, a typical value for the power frequency (50 Hz/1 min) insulation level for the outer sheath
is 10 kV.
CONCLUSION
Significant overvoltages can appear in cable sheaths when short circuits occur in high power grids.
Taking into account that temporary and transient overvoltages on cable sheaths depend on the sheath
connection architectures, specific short circuit studies must be performed for each project of a new cable
system. Although ATP package is a good numerical tool to determine transient and temporary
overvoltages, it is not usually used by the project designers of underground cable lines.
Specific software packages have been developed to determine sheath overvoltages of cable systems, one
of them applying circuit analysis by symmetrical components (CASC) and other by a general method of
circuit analysis (GMCA). Both software packages allow to define any arbitrary architecture to link
elementary cable sections (single-bonding, continuous cross-bonding and sectionalised cross-bonding) in
order to determine continuous over-voltages in accessory sheaths and in overvoltages limiters for the
different short-circuit scenarios (substation-substation, Siphon and far away fault).
In addition application guide has been elaborated with overvoltage tables for different sheaths
configurations used by GNF utility for different laying types. Application guide and software packages
have allowed improve bonding configurations and the SLV selection of existing underground cables of
GNF utility and new underground lines.
9
BIBLIOGRAPHY
[1]
“Estudio de sobretensiones en las pantallas de los cables de alta tensión de los circuitos
Mazarredo-Mediodía y Cerro de la Plata-Mediodía en caso de cortocircuito monofásico en la
subestación de Mediodía”. Report nº 200612300541. LCOE-FFII.
[2]
The design of specially bonded cable systems. Working Group 07, of study Committee nº 21
(HV insulated cables). ELECTRA nº 28. 1973.
[3]
The design of specially bonded cable systems. Part II. Working Group 07, of study Committee
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[5]
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[9]
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