BONDING SYSTEM FOR SG MAAW–SG MERAH 132KV UNDERGROUND
CABLE PROJECT AND ACCESSORIES FOR HT CABLE SHEATH BONDING
Prepared by Mohammad Shahril Ardianshah, Power Cables Malaysia Sdn Bhd
INTRODUCTION
Referring to the requirement stated in SG
MAAW-SG MERAH 132KV UNDERGROUND
CABLE PROJECT, the power cables to be
supplied shall be suitable for operation at a
system voltage of 132kV. Such high voltage
cables, the metallic sheath shall be
designed sufficiently to carry fault current
which served by the combination of
extruded metallic lead alloy, copper wire
and copper equalizing tape.
and transfer electrical energy. Metallic
sheath is part of the long network thus for
safety purpose it must be grounded to
avoid hazard when touched and to prevent
rapid degradation of the outer sheath
jacket.
Eliminate sheath losses- Special bonding
arrangement will be able to significantly
reduce the sheath losses in single-core
cables. Sheath losses occur through
circulating currents in the system and eddy
currents in the cable sheath.
PURPOSE OF METALLIC SCREEN
Other than providing effective return path
for fault current, the purpose of metallic
sheath is also used to control the stress in
cable insulation and provides shielding for
electromagnetic radiation.
PURPOSE OF BONDING SYSTEM
1)
2)
3)
Safety purpose
Eliminate sheath losses
Eliminate induced sheath voltage
Safety purpose- Power cable represent by a
long capacitor network which used to store
From above ampacity equation, minimizing
the ratio of sheath losses λ1, current load
capacity I under certain circumstances may
substantially increase.
Eliminate induced sheath voltage- Sheath
currents caused by the induced sheath
voltage in unbalanced earthing systems.
This current is proportional to the sheath
losses thus the induced sheath voltage may
cause deration on the current load capacity.
Since the induced sheath voltage is a
function of the rated conductor current I
and cable length L, long run length
application is limited.
To meet the standing voltage limit, both
cable ends have to be bonded sufficiently to
the earthing system
bonded to the earth at both ends via link
boxes.
Advantages- This arrangement does not
require SVL as the sheath induced voltage
has been eliminated. It’s also eliminates the
need for the parallel continuity conductor.
Disadvantages- This system suffered
deration on the current capacity as a result
of significant increases on the circulating
current. Circulation current is proportional
to the current load and cable length. Thus,
it normally only suitable to be used for LV
and MV application with short length route
of more than 500 meters up to 1000 meters
2) CONTINOUS CROSS-BONDING
TYPE OF SHEATH BONDING
1)
2)
3)
Solid bonding
Continuous cross bonding
Single point bonding
Note: Single point bonding will not be elaborated in
this paper
1) SOLID BONDING
Basic circuit arrangement- Solid bonding
arrangement is the most simple and
common method where the cable screens is
Basic circuit arrangement- The sheaths is
cross bonded at the end of each identical
minor section throughout the whole cable
length. It is again transposed so that each
conductor occupies all three positions for
one third of total route length.
The purpose of sectionalizing the sheath
into minor sections and cross connecting
them is to approximately neutralize the
total induced sheath voltage in three
consecutive sections.
Advantages- The principal of its advantages
is that while induced sheath current are
inhabited during normal balanced load
operation, the sheath will form a
continuous path from end to end of the
cable circuit and are grounded at both ends.
Sheath current can therefore flow during
ground fault, and the necessity for the
parallel continuity conductor is removed.
In addition, by the elimination of ground
continuity conductor, the cable sheaths
function more effective as screening
conductor during ground fault comparing to
parallel continuity conductor. Hence, the
voltage induced in parallel cable are less
during ground fault in a cross bonded
system than for a similar single point
bonding.
Disadvantages- Cross bonded system is
technically complicated which require
expertise for installation. To achieve an
exact balance of induced sheath voltage;
the arrangement of the three minor
sections must be identical to form a major
section. The number of matched minor
section should be preferably being exactly
divisible by three.
Due to its complexity, it is obvious that this
system is the most expensive compared to
other bonding type. Numbers of link boxes
and Sheath Voltage Limiter SVL require to
be installed throughout the total route
length.
BONDING METHOD COMPARISON
Earthing
Method
Sheath induced
voltage
Circulating
current
Sheath Voltage
Limiter
Installation cost
Ampacity
Application
Solid Bonding
No
High
Not require
Cross Bonding
Occur only at cross
bonding points
Low and almost
negligible
Require
Cheap
Expensive
Derated
Improved
Short distance
Allow for long distance
installation up to 1 Km installation
Only suitable to be
used for LV and MV
application
Suitable to be used for
HV installation
CONCLUSION
There are so many disagreements as to
whether the cable metallic sheath should
be solidly bonded or continuous crossbonded
Solidly bonded system is the cheapest and
commonly used method but there are few
restrictions for its implementation in SG
MAAW-SG MERAH 132KV UNDERGROUND
CABLE PROJECT.
Solidly bonded system is hardly be used in
high voltage application due to the
existence of high circulating current which
will de-rate the cable current capacity. The
total project distance is also a barrier to
implement this type of bonding system.
Cross bonding system is quite an expensive
method and requires expertise for its
installation but in other hand, since the
circulating current and induce sheath
voltage is eliminated; this system is suitable
to be used for high voltage application and
for long distance installation.
The effect of bonding system to equivalent
circuit capacity illustrated as below table
Direct buried
Lay in duct
Crossbond
133.1 MVA
134.0 MVA
Solid Bond
111.6 MVA
98.9 MVA
Note: Trefoil, double circuit, ambient temp 30oC, spacing c-c
circuit 350mm, soil TR 1.2k.m/W, buried depth 6500mm
The cross-bonding link boxes are used to
restrain over voltage rise of sheath at
insulation joint, to minimize induce sheath
voltage, to reduce and dispel annular
electric current of sheath to improve the
cable capacity and prevent the outer sheath
from puncturing so as to run safely and
reliably.
ACCESSORIES FOR HT CABLE SHEATH BONDING
1) Cross bonding LINK BOXES
2) Sheath Voltage Limiter SVL
1) Cross bonding LINK BOXES
2) Sheath Voltage Limiter SVL
The main purpose of the sheath voltage limiter
(SVL) is to limit the voltage stress across the
cable jacket during transient events.
Link boxes are one of the common
accessories for above ground cable
bonding. Cross bonding link boxes allow
metallic sheath of cable to be transposed at
insulation joint with surge voltage
suppression and reduction of circulating
currents
At transposition/cross bonding locations, it is
important to install the SVL at these locations to
eliminate any possibility of insulation
breakdown of the cable jacket or link box
insulation.
THERMAL RESISTANCE OF CABLE
All non-conducting materials such (XLPE, PE
and PVC) in the cable will impede heat flow
away from the cables which make the
conductor temperature to rise rapidly. Heat
gives the greatest influence on the cable
current capacity.
Heat flow is inversely proportional to the
magnitude of thermal resistance. Therefore,
higher thermal resistance will cause lower
current capacity. Metallic parts in the cable
usually have a very low thermal resistance.
Its contribution to cable current capacity is
almost zero and can be neglected from
rating computation.
T4’’: Thermal resistance of the duct
itself
3) T4’’’: External thermal resistance of
duct
2)
Note:
U, V, Y, can be obtain from IEC60287-2 Table 4
De is external diameter of cable
Θm is steady state air temperature inside duct
Current rating of a cable is a function of the
following thermal resistance:
T1: Thermal resistance between
conductor and sheath
2) T2: Thermal resistance between
sheath and armor
3) T3: Thermal resistance of external
covering
4) T4: External thermal resistance
1)
From above list, T4 give the greatest
influence on the current rating
computation.
External thermal resistance for in duct
installation is mainly depending on:
DUCT EXTERNAL THERMAL RESISTANCE
1)
For cable laid in duct, the external
resistance of a cable consists of three parts:
2)
1)
T4’: Thermal resistance of air or
liquid between cable surface and
duct internal surface
3)
4)
5)
Soil or backfill thermal resistivity
k.m/W
Duct thermal resistivity k.m/W
Depth of buried mm
Diameter of duct and cable mm
Mutual heating of neighboring cable
EFFECT OF SOIL THERMAL RESISTIVITY TO
CABLE CURRENT RATING
Thermal resistivity depends on the type of
soil. Normally, soils that impede heat flow
away from the cable have higher thermal
resistivity for example sand. Clay has lower
thermal resistivity due to its ability to
contain moisture and ability to disperse
heat.
EFFECT OF CABLE DEPTH
As the cable buried deeper, the thermal
resistance of soil surrounding the cable for
both homogeneous and heterogeneous soil
increase. Heat dissipation becoming less
causing the soil conductivity reduced.
The ampacity depends upon the rate of
heat generation within the cable as well as
the rate of heat dissipation from the cable
to the surroundings.
Burried depth, mm
L
3000 6500 8000
Equivalent circuit capacity
MVA 146.5 134.0 131.1
Note: Trefoil, double circuit, ambient temp 30oC, spacing c-c
circuit 350mm, soil TR 1.2k.m/W
From above table, current carrying capacity
decrease as the cable is buried deeper in
the soil.
Soil thermal resistivity is proportional to
T4’’’: External thermal resistance of duct.
Therefore, selection of low soil TR may lead
to a significant increase on the cable
current capacity.
Soil thermal resistivity k.m/W
External TR of duct
Equivalent circuit capacity
TR
T4''''
MVA
1
2.13
144.7
1.2
2.56
134.0
2
4.26
106.6
Note: Trefoil, double circuit, ambient temp 30oC, spacing c-c
circuit 350mm, buried depth 6500mm
Soil thermal resistivity gave great influence
to the equivalent circuit capacity thus it is
important to consider the location of
installation. Thermal resistivity can be
improved by using a special grade of
backfill.
GROUPS OF CABLES
Almost all cases, cables are installed in a
group. Even in a single circuit of a 3 phases
system, mutual heating mechanism can
happen between its adjacent phases.
In general principle, hottest cable will carry
lesser current carrying capacity when
compared to the same cable installed in
isolation. This decrease is attributed to the
mutual heating mechanism.
The influences of grouping on the current
carrying capacities are based on the ratio of
the cable diameter, D as well as the spacing
center to center between circuits, S.
In case of SG MAAW-SG MERAH 132KV
UNDERGROUND CABLE PROJECT, a group of
2 circuits installed, parallel to each other
and buried at maximum of 8000mm
Spacing center to center, mm
Equivalent circuit capacity
S
MVA
350
134.0
500
136.5
1000
142.0
Note: Trefoil, double circuit, ambient temp 30oC, buried depth
6500mm, soil TR 1.2k.m/W
From above simulation, reduction in
ampacity as the circuit get nearer to each
other is the result of an increase in the
external thermal resistance of a cable or
circuit in a group.