N. S. Omar, A. A. Raj and C. K. Chakrabarty/ IJECCT 2014, Vol. 4 (2)
27
Computation of Ampacity Derating of HV and MV
Multiple Circuit Crossing
Nurul Syaza binti Omar1, Avinash Ashwin Raj2, Chandan Kumar Chakrabarty3
123
Department of Electronics and Communication Engineering, College of Engineering, Universiti Tenaga Nasional,
Km. 7, Jalan Kajang-Puchong, 43009 Kajang, Selangor Darul Ehsan
nurulsyaza.omar@gmail.com
Abstract: Cable crossings may occur in substations and
other installations where cable congestion may be
unavoidable. When a cable carrying a high level of
current crosses another cable carrying high current,
additional heat is generated at the crossing point. As a
result the current carrying capacity or ampacity of both
cables can be reduced. To determine the ampacity,
Multiphysics software from Computer Simulation
Technology (CST) is used to model 2D temperature
profile for the cable system in cross over configuration.
The industry specific software which is Power Cable
Ampacity Calculation (CYMCAP) is used to validate the
CST simulation. The CST model can then be used to
recommend the possible ampacity improvement actions
for the cases under study. The results of this paper
graphically show how the temperature of the cables at the
crossing increase and then decrease as the crossing point
is approached, passed and moved away from. This
temperature increase will, in turn, reduce the ampacity of
both cables. The reduction in ampacity can then be
estimated when the temperatures of the cables are known.
These data will allow the planners and engineers to
optimize the lay-out of the cables to achieve the optimum
configuration for the required current load on the cable.
Keywords –Ampacity; cable crossing; cable laying
arrangement;
computer
simulation
technology;
derating; multiple trefoil three phase cable; power cable
ampacity calculation; single core cable; temperature
profile
I.
INTRODUCTION
The important thing in engineering a power plant and over
its service life is to maximize the current carrying capacity
of the cable system to supply the desired load. The cable
needs to operate at temperatures over its lifetime that do
not cause damage or excessive aging of the cable. The
cable must be able to carry huge amounts of current
without overheating. Cable heating is one of the major
problems associated with underground lines. The rating of
a power cable is dependent on its construction and also the
method of installation [1].
Ampacity of electric power cables has been
extensively discussed in the literature for many decades
and is the subject of several international standards which
are International Electrotechnical Commission (IEC) and
the Institute of Electrical and Electronics Engineering
(IEEE). Ampacity computations need solution of the heat
transfer equations which define a functional relationship
between temperature and conductor current within the
cable and in its surroundings [2].
The cable ampacity calculation becomes crucial
when there are multiple incoming and outgoing cable
circuits at the substations which posed new challenge in
determining the ampacity of the underground cables.
Without proper evaluation of the ampacity of the overall
underground cable system, the load transfer capability of
the substation could not be optimized. However, the
ampacity calculation has been accepted based on the
model proposed by Neher and McGrath since the mid
1900’s. Thus it will be the reference for the cases under
study [3]. The industrial software currently used for
multiple circuit in parallel to determine the best cable
layout is CYMCAP. This software provides temperature
profile, ampacity results and other electrical specifications
quite accurately. However for cable crossing layout, it can
simulate only single circuit crossing. As now the need
arises for determining the temperature in multiple circuit
crossing because of increase in loading, therefore it has
become pertinent to determine other software that can
perform such type of cable layout simulation.
This paper is aimed to provide the temperature
profile and ampacity of the high-voltage (HV) cable lay in
the multiple circuit crossing of the cables at the substation
using CST and CYMCAP. The spacing and depth of the
cables buried underground in various cable laying designs
is the main parameters in this paper to determine the cable
ampacity.
The ampacity results provided in this paper enables
planners and engineers to select the optimum cable laying
arrangement, depth of buried and spacing between cables
that achieves the desired load. The results of ampacity
calculation are divided based on different cable crossing
configurations which consist of Single Core Cable
Crossing and Multiple Trefoil Three Phase Cable
Crossing.
In order to create the model and find the ampacity of
the multiple circuit crossing configuration, CST
Multiphysics is used and CYMCAP to validate only the
single circuit crossing due to its limitation stated earlier.
II.
SOFTWARE DESCRIPTION
This section covers the details regarding the software
used in the simulation.
N. S. Omar, A. A. Raj and C. K. Chakrabarty/ IJECCT 2014, Vol. 4 (2)
A. Computer Simulation Technology
CST is one of the most precise and effective
computational solution for electromagnetic designs. CST
consists of many modules such as CST Microwave Studio,
CST EM Studio, CST Particle Studio and CST
Multiphysics Studio (MPS). Nevertheless, because the
used of CST in this project is to determine the ampacity of
underground cables for cable crossing, the suitable
modules is CST MPS. CST MPS is a capable and easy-touse tool for mechanical and thermal simulation. The main
function of it is to generate heat transfer and measured the
temperature from the cable model created [4].
B. Power Cable Ampacity Calculation (CYMCAP)
CYMCAP is dedicated to the calculation of ampacity
and temperature rise for power cable installation. It
addresses steady-state and transient thermal cable rating as
per analytical techniques described by Neher-McGrath and
the IEC287© and IEC 853© International Standards. It
was developed jointly by Ontario Hydro (Hydro One),
McMaster University and CYME International, under the
auspices of the Canadian Electricity Association.
CYMCAP also helps increase system reliability and
support the proper utilization of the installed equipment
[5].
III.
Horizontal spacing between cables is 0.09158
meter which is 1 diameter of 132kV 800mm2
cable.
The cables touch.
There are also few variable parameters for multiple
trefoil three phase cable crossing which are:
Depth is varied from 1.5 meter to 6 meter.
Horizontal spacing and the vertical spacing of the
circuit is varied between 1 cable diameter and 2
cable diameter.
The final section of this paper presents the temperature
profile of cable crossing between 132kV 1200mm2 Cu
with 33kV 800mm2 Cu cables and 132kV 1200mm2 Cu
with 33kV 1600mm2 Cu cables at 90o.
IV.
AMPACITY GUIDELINE
In this section, the types of cables used in the
simulation are shown in Figure 1 to Figure 4. In the
ampacity calculation, the cable layer swellable tape is not
included in the modeling since the contribution of this
layer to ampacity is negligible.
SCOPE OF PAPER
This paper only covers cables that are direct buried
underground without pipe. The type of cables included in
this paper are 33kV 630mm2 Copper (Cu), 132kV
800mm2 Copper (Cu), 132kV 1200mm2 Copper (Cu) and
132kV 1600mm2 Copper (Cu). It is assumed that all the
cables installed in a same type of laying configuration will
carry same amount of current continuously. This current
is limited by the cross-linked polyethylene (XLPE)
continuous operating temperature of 90oC core surface
temperature. There are two types of cable crossing
configuration: single core cable crossing and multiple
trefoil three phase cable crossing.
A. Simulation Description
For single core cable crossing, the type of cable used is
both the same which is 132kV 800mm2 Cu at 90o. There
are three arrangements which are single crossing, double
crossing and triple crossing.
For multiple trefoil three phase cable crossing between
132kV 800mm2 Cu and 33kV 800mm2 Cu cables at 90o,
the arrangements is the positioning of 132kV cables below
of 33kV cables.
The calculation of the ampacity is based on these
assumptions:
Ambient Temperature : 30oC
Thermal resistivity: 1.2oC M/W
Bonding configuration: Cross bond with equal
minor section length
There are fix parameters for single core cable crossing
which are:
Length of cable is 1 meter
Depth of buried is 1.5 meter
28
Figure 1. 33kV 630 mm2 XLPE Cu
Figure 2. 132kV 800 mm2 XLPE Cu
N. S. Omar, A. A. Raj and C. K. Chakrabarty/ IJECCT 2014, Vol. 4 (2)
V.
29
CABLES AMPACITY FOR DIFFERENT TYPES OF
CABLE CROSSING CONFIGURATION
The results of the ampacity calculations are divided into
two parts based on the different types of cable crossing
configuration.
A. Single Core Cable Crossing
The purpose of this simulation is to validate the CST
result with CYMCAP result. This is due to the limitation
of CYMCAP software that can only simulate a single
crossing. Based on the small percentage error between
both the results, the CST result will be used for multiple
single and trefoil three phase cable crossing. Figure 5 until
Figure 7 below shows the three configurations of single
core cable crossing arrangements. ‘d’ is the horizontal
spacing between cables.
Figure 3. 132kV 1200 mm2 XLPE Cu
Figure 4. 132kV 1600 mm2 XLPE Cu
Figure 5. Single Crossing
A. Cables Current Carrying Capacity
The cables ampacity is limited by the cable insulation
material and conductor size. All the cable in this paper is
XLPE insulated cables. The ampacity is limited by 90oC
core surface temperature condition in continuous
operation. Table 1 below shows the transformer rating and
the expected current loading for the respective cables sizes.
Equation (1) is used to find the expected current loading.
S = 3 Io V
(1)
d
S presents the transformer rating of the cable, Io is the
current loading of the cable and V is the voltage of the
cable.
d
Figure 6. Double Crossing
Table 1. Cables Expected Current Loading
No.
1
2
3
4
Type of Cable
33kV 630mm2
XLPE Cu
132kV 800mm2
XLPE Cu
132kV 1200mm2
XLPE Cu
132kV 1600mm2
XLPE Cu
Transformer
Rating
(MVA)
Expected
Current
Loading
Io(A)
90
525
150
656
150
656
d
d
240
1050
d
d
Figure 7. Triple Crossing
N. S. Omar, A. A. Raj and C. K. Chakrabarty/ IJECCT 2014, Vol. 4 (2)
Table 2 below shows the result of all cable crossing
configuration. Each of the cable behaves as a heat source
for neighboring one. The percentage error in the last
column is the deviation of CST from CYMCAP.
Table 3. Result for Top Cable 33kV 630mm2 Cu and Bottom Cable
132kV 800mm2 Cu
No
Depth
Y(m)
1
1.5
3.0
6.0
Table 2. Result of Single Core Cable Crossing
CYMCAP
Ampacity(A)
Type of
Configuration
Single Cable
Crossing
Double Cable
Crossing
Triple Cable
Crossing
CST
Ampacity (A) error(%)
784.0
770.9
-1.7%
n/a
670.3
n/a
n/a
502.8
n/a
CST result for the single cable crossing is hence validated
by the small percentage error between both software
simulations as shown in Table 2. Thus, another two
simulations are done using the single crossing CST result
which is 770.9A as the reference.
B. Multiple Trefoil Three Phase Cable Crossing
Ampacity of cables crossing are calculated to obtain
the optimum horizontal spacing between each circuit,
vertical spacing between top and bottom circuits and depth
of buried. Figure 8 below show the cable crossing
arrangements with 90° crossing which is the top three
circuits are 33kV cable, bottom three circuits are 132kV
cable configuration. There are two types of cable used in
this section which is 33kV 630mm2 Cu and 132kV
800mm2 Cu. The horizontal spacing between each circuit,
X; vertical spacing between top and bottom circuits, Z; and
the depth of buried, Y; are varied for the cable crossing
arrangements. X is using the cable diameter of the cable
itself. For example, horizontal spacing between 33kV
circuit is the cable diameter of 33kV itself. Same goes to
132kV. For Y, it is fixed with the used of 132kV cable
diameter.
Y
33kV
X
Z
132kV
X
Figure 8. Multiple Crossing Trefoil configuration: Top three
circuits are 33kV cable, bottom three circuits are 132kV cable
configuration
The results of the ampacity calculations are tabulated in
Table 3 below. The depth of buried and spacing between
cables are varied for different cable arrangements of
multiple trefoil three phase cable crossing configuration.
Each of the cable behaves as a heat source for one another.
30
2
1.5
3.0
6.0
3
1.5
3.0
6.0
4
1.5
3.0
6.0
Horizontal
Spacing
X(m)
1 cable
diameter
2 cable
diameter
Vertical
Spacing
Z(m)
1 cable
diameter
132kV
cable
2 cable
diameter
132kV
cable
1 cable
diameter
132kV
cable
2 cable
diameter
132kV
cable
Current
33kV
630mm2
In(A)
324.7
320.1
Current
132kV
800mm2
In(A)
453.0
442.5
315.5
432.1
336.2
320.1
484.2
473.8
313.2
458.2
333.9
330.4
489.4
484.2
324.7
473.8
331.6
329.3
317.8
502.4
492.0
486.8
The optimum spacing and depth of cables buried
underground shall be selected to obtain the required
current carrying capacity of the cables. Based on Table 3
above, the optimum depth of buried chosen is 1.5 meter.
This is because, the cable is laid progressively deeper
under the ground the heat dissipation becomes
correspondingly more difficult. Hence, as the depth of
laying increases, the current carrying capacity also
decreases. In order to find the optimum spacing, the
percent of derating is calculated by using (2).
Derating %
Io
I n / Io
100%
(2)
From the calculation done, the lowest percent of
derating is 27.3% for 33kV cable and 12.3% for 132kV
cable which the optimum horizontal spacing and optimum
vertical spacing is 2 cable diameters. Reference [6] showed
this percent of derating which the author claims the
derating factor can be greater than 10% even if the heat
source is perpendicular to the rated cable.
The temperature influence on the external heat source
on the rated cable will depend on both the crossing angle
and the distance between the circuits. The vertical distance
between the circuits plays a smaller role as far as the
variation in the temperature is concerned. The influence
temperature can be high at the point of intersection, even
for a perpendicular crossing [7][8]. However, this paper
only considered the angle of 90o only.
VI.
TEMPERATURE PROFILE
Electric cables transmitting capacity were mainly
restricted to its heat insulation level. To ensure the safe
operation of the cable, mastering the temperature of the
cable ontology was needed. Therefore, this section shows
two results of the computed core temperature for cable
crossing. Figure 9 shows the arrangement of cable crossing
for 33kV 630mm2 cable with 132kV cable. The computed
temperature for 33kV 630mm2 cable with 132kV
N. S. Omar, A. A. Raj and C. K. Chakrabarty/ IJECCT 2014, Vol. 4 (2)
31
1200mm2 cable and 33kV 630mm2 cable with 132kV
1600mm2 cable are listed in Table 5 and Table 6. Figure
10 and Figure 11 show the profiles of temperature at the
point of crossing. The temperature is from the point of
crossing up to +/- 20 centimeters. This trend of results can
be extended to other cables. The ampacity calculation is
according to the following parameters as shown in Table 4.
Table 4. Parameters of Ampacity Calculation
No.
1
2
3
4
Parameters
Type of Buried
Bonding Arrangement
Ambient Temperature
Thermal Resistivity
Conditions
Direct Buried
Cross Bonded
30oC
o
1.2 C M/W
Table 6. The Result for Computed Core Temperature between 33kV
630mm2 and 132kV 1600mm2
33kV
132kV
Figure 9. Top circuit is 33kV cable, bottom circuit is 132kV cable
configuration
Table 5. Result for Computed Core Temperature between 33kV
630mm2 and 132kV 1200mm2
Distance
along
132 kV
1200mm2
cables
(cm)
20
18
16
14
12
10
8
6
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
Core
Temperature
(oC)
118.4
119.1
119.4
119.8
120.4
121.2
121.8
122.5
122.9
123.2
123.8
123.9
123.4
123.1
122.3
122.1
121.7
121.4
120.7
120.1
119.6
Distance
along
33kV
630mm2
cables
(cm)
20
18
16
14
12
10
8
6
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
Figure 10. Graph of Core Temperature versus Distance of Core
Along the 33kV 630mm2 cable and 132kV 1200mm2 cable
Core
Temperature
(oC)
116.8
117.5
117.9
117.9
118.2
118.4
118.6
118.9
119.2
119.7
120.4
119.8
119.5
119.1
118.9
118.6
118.2
117.7
117.5
117.2
117.0
Distance
along
132 kV
1600mm2
cables
(cm)
20
18
16
14
12
10
8
6
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
Core
Temperature
(oC)
122.2
122.8
123.5
123.7
123.9
124.1
124.2
124.4
124.5
124.8
124.9
124.9
124.7
124.3
124.1
123.5
123.0
122.6
122.3
122.1
121.7
Distance
along
33kV
630mm2
cables
(cm)
20
18
16
14
12
10
8
6
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
Core
Temperature
(oC)
121.2
121.6
122.1
122.2
122.5
122.8
123.2
123.4
123.6
123.9
124.2
123.8
123.7
123.3
123.1
122.8
122.5
122.2
122.0
121.7
121.4
Figure 11. Graph of Core Temperature versus Distance of Core
along the 33kV 630mm2 cable and 132kV 1600mm2 cable
N. S. Omar, A. A. Raj and C. K. Chakrabarty/ IJECCT 2014, Vol. 4 (2)
Both cable loadings were not derated and the loading
used was taken based on independent simulation of single
circuit’s buried 1.5m to achieve a 90°C temperature at
core.
At 0cm, both cables obtain its highest temperature at
core because that is the touching point of both crossings.
The 132kV 1600mm2 cable temperature at this point is
124.9°C and the 33kV 630mm2 cable temperature is
124.2°C. The temperature decreases linearly as the length
increases away from the point of intersection. From the
results, the extent of cable crossing is an important factor
that is going to determine the amount of derating during
continuous operation and as such cable installers can now
find ways to mitigate this by using different materials
between the cables layer at only point of intersection [9].
VII. CONCLUSIONS
The objective of this paper which is to use CST in
finding the ampacity is achieved. CST Multiphysics is
able to simulate and determine the ampacity and derating
of single core multiple crossing and multiple trefoil three
phase cable crossing. The simulations have shown that
when a cable crosses each other, derating of its current
carrying capacity or ampacity is required in determining
the optimum cable laying arrangement. Furthermore, the
angle of crossing might be one of the main parameters in
determining the cable ampacity; which may dictate the
bottleneck of the whole cable system. In addition, Figure
10 and Figure 11 prove that cable has its highest
32
temperature at 0cm which is the touching point of both
crossing.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
George J. Anders, “Rating of Electric Power Cables: Ampacity
Computations for Transmission, Distribution, and Industrial
Applications,” New York, McGraw-Hill, 1997.
George J. Anders, “Rating of Electric Power Cables In
Unfavourable Thermal Environment,” Canada, McGraw-Hill,
2005.
Pascal Vaucheret, R. A. Hartlein, Senior Member, IEEE, & W. Z.
Black, Fellow, IEEE, “Ampacity Derating Factors for Cables
Buried in Short Segments of Conduit,” IEEE Trans. Power
Delivery, vol. 20, no. 2, pp. 560-565, Apr. 2005.
CST – Computer Simulation Technology available at:
http://www.cst.com, retrieved on 10 August 2013.
CYME – Power Engineering Software and Solutions available at:
http://www.cyme.com, retrieved on 10 August 2013.
George J. Anders, Fellow, IEEE, and Eric Dorison, “Derating
Factor for Cable Crossings With Consideration of Longitudinal
Heat Flow in Cable Screen,” IEEE Trans. Power Delivery, vol. 19,
no. 3, pp. 926-932, Jul. 2004.
George J. Anders and Heinrich Brakelmann, “Cable Crossings Derating Considerations Part I - Derivation of Derating Equations,”
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Derating Curves,” IEEE Trans. Power Delivery, vol. 14, no. 3, pp.
715-720, Jul. 1999.
George J. Anders and Heinrich Brakelmann, “Ampacity Reduction
Factors for Cables Crossing Thermally Unfavorable Regions,”
IEEE Trans. Power Delivery, vol. 16, no. 4, pp. 444-448, Oct.
2001.