im
Journal of	JET v°iume 12 (2°19) p.p. 19-29
Issue 3, November 2019 Type of article 1.04
Technology	www.fe.um.si/en/jet.html
AN ALTERNATIVE METHOD OF INCREASING THE TRANSMISSION PERFORMANCE OF A CONVENTIONAL 110 kV CABLE LINE
ALTERNATIVNA METODA ZA POVEČANJE PRENOSNIH ZMOGLJIVOSTI KONVENCIONALNE 110 kV LINIJE
Dardan Klimenta1R, Dragan Tasic2, Miroljub Jevtic1
Keywords: Ampacity, finite-element method (FEM), hydronic asphalt pavement (HAP), power cable, thermal analysis
Abstract
The purpose of this paper is to show that a significant increase in the ampacity of a 110 kV underground cable line is achievable, if a hydronic asphalt pavement system is applied along the entire line, and if the cable trench is completely filled with high thermal conductivity bedding in order to improve the conduction of heat between the line and the surface of the earth. In such a way, it would be possible to simultaneously collect and then store heat from the sun and cable line. The mutual thermal effects between the 110 kV cable line and the hydronic asphalt pavement, in the presence of solar radiation, wind-driven convection and heat emission along the earth surface, are simulated using FEM-based models for the most unfavourable summer conditions and the most common winter conditions. An adequate experimental background is also provided based on the
R Corresponding author: Professor, Dardan Klimenta, Tel.: +381 65 40 666 40, Mailing address: Kneza Miloša St. 7, RS-38220 Kosovska Mitrovica, Serbia, E-mail address: dardan.klimenta@pr.ac.rs
1	University of Priština in Kosovska Mitrovica, Faculty of Technical Sciences, Kneza Miloša St. 7, RS-38220 Kosovska Mitrovica, Serbia
2	University of Niš, Faculty of Electronic Engineering, Aleksandra Medvedeva St. 14, RS-18000 Niš, Serbia
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existing measurements relevant to the thermal analysis performed. It was found that, compared to the associated base cases, the cable ampacity can be increased up to 92.3% for the most unfavourable summer conditions, and up to 60.3% for the most common winter conditions.
Povzetek
Namen prispevka je prikazati možnost doseganja znatnega povečanja zmogljivosti 110 kV podzemnega vodnika, kadar je vzdolž celotne linije uporabljen hidronični asfalt in v kolikor je kabelski jarek popolnoma zapolnjen z visoko toplotno prevodnim ležiščem, ki izboljša prevodnost toplote med vodnikom in zemeljsko površino. Na tak način bi bilo možno hkrati zbirati in shranjevati toploto sonca in vodnika. Medsebojni toplotni učinki med 110 kV vodnikom in hidroničnim asfaltom ob prisotnosti sončnega sevanja, vetrne konvekcije in oddajanja toplote vzdolž zemeljske površine so v prispevku simulirani z uporabo modelov baziranih na MKE. Za vremenske pogoje so izbrane najboljneugodne poletne razmere in najpogostejše zimske razmere. Zagotovljeno je ustrezno eksperimentalno ozadje na podlagi obstoječih meritev, ki se nanašajo na opravljeno toplotno analizo. Ugotovljeno je bilo, da je možno na tak način v primerjavi s trenutno izvedbo zmogljivost vodnikov za najbolj neugodne poletne razmere povečati do 92,3 %, za najpogostejše zimske razmere pa do 60,3 %.
1 INTRODUCTION
A hydronic asphalt pavement (HAP) system represents an emerging renewable energy technology, i.e., an innovative method for harvesting energy of the sun, [1]. It consists of four main parts, [2]: (i) a heat exchanger, i.e. a pavement-solar heat collecting system, (ii) a heat storage system, (iii) a hydronic circulating pump, and (iv) an automatic monitoring system. Fig. 1 provides a detailed depiction of the operating principle of the HAP system, which includes the cooling of pavement in summer, storing the heat in the ground, and the heating of pavement in winter. Since the earth's surface above power cables usually does not freeze during winter, pavement heating using the HAP system is not relevant for that period. Therefore, it is assumed that all collected heat is used for various applications in buildings and process plants or for thermoelectric conversion.
Together with solar energy, the HAP system from Fig. 1 may simultaneously collect some waste heat from power cables if they are installed below the heat exchanger. Collecting both can decrease the surface temperature of the pavement above power cables by approximately 10 °C when compared with the surface temperature of conventional pavement, [1]. This experimental observation refers to dynamic thermal analysis. This means that the difference corresponding to the steady-state thermal analysis is significantly higher, which makes it possible to use the HAP system in order to increase the cable ampacity under different environmental conditions.
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An alternative method of increasing ihe transmission performance of a conventional 110 kV cable line
This paper considers the possibility of increasing the ampacity of the 110 kV underground cable line by an application of the HAP system, under the most unfavourable summer conditions and the most common winter conditions. It is assumed that the HAP system is applied along the entire cable line and that the cable trench is completely filled with one of four different bedding materials with high thermal conductivities. FEM-based models in which the cable beddings have a standard size of 1.2 m x 0.65 m are used as the base cases for the summer and winter periods. Finally, the steady-state thermal analysis reached a number of interesting conclusions.
2 FEM-BASED STEADY-STATE THERMAL MODELS
Fig. 2 illustrates a cross-sectional view of the experimental apparatus that was used in [1] for testing HAP system performance. The experiment is relevant for the validation of the model proposed herein. According to [1], the experiment was conducted with the following environmental conditions and material properties:
(i)	temperature of the air contacting the pavement surface Ta=32.93 °C;
(ii)	heat transfer coefficient due to wind-driven convection h=6 W/(m2K);
(iii)	solar irradiance incident on the pavement surface <2s,*=800 W/m2;
(iv)	solar absorptivity and thermal emissivity of the pavement surface «=0.90625 and s=0.9, respectively;
(v)	water temperature at inlets of the heat collecting pipes Twi=23.2 °C;
(vi)	water temperature at outlets of the heat collecting pipes Two=28.2 °C;
(vii)	water velocity in the heat collecting pipes vw=0.06 m/s or vw=1 l/min;
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(viii)	thermal conductivity of the water flowing through the pipe indicated by a white circle in Fig. 2 ¿=0.61331 W/(mK) - at TW=(TW,,+Tw,0)/2=25.7 °C;
(ix)	thermal conductivity of the still water in the pipes indicated by grey circles in Fig. 2 ¿=0.64399 W/(mK) - at 50 °C;
(x)	thermal conductivity of the copper pipe ¿=385 W/(mK), and
(xi)	thermal conductivity of the limestone asphalt mixture/pavement ¿=1.583 W/jm-K).
The inner and outer diameters of the copper pipes (i.e., heat collecting pipes) are equal to dp, ,=0.019 m and dp, O=0.0222 m, respectively.
1-6 - thermocouples
Bj, B,. B3 udiubsitic boundaries
Figure 2: A cross-sectional view of the experimental apparatus used in [1] for testing HAP system performance (not to scale). Note: In this experiment, water was not flowing through the two pipes indicated by grey circles, while water was flowing through the pipe indicated by a white
circle
Fig. 3a shows the computational domain used for the two base cases, while Fig. 3b shows the computational domain used for the proposed alternative method with a HAP system consisting of seven copper pipes. The first base case relates to the following environmental conditions and material properties, [3,4]: (i) Ta=40 °C; (ii) wind velocity v„=0.22 m/s; (iii) Qs,s=1000 W/m2; (iv) temperature of referent soil of Trs=20 °C; and (v) thermal conductivities of the cable bedding, backfill, native soil and conventional asphalt pavement amounting, respectively, to 1 W/(mK), 0.4 W/(mK), 0.4 W/(mK) and 1.2 W/(mK). These conditions represent the most unfavourable summer conditions. The second base case relates to the most common winter conditions, namely [4]: T„=5 °C, v„=0.22 m/s, Qs,s=500 W/m2, T„=10 °C, and thermal conductivities of the cable bedding, backfill, native soil and conventional asphalt pavement amounting, respectively, to 1 W/(mK), 0.4 W/(mK), 0.4 W/(mK) and 1.2 W/(mK).
The cables in Fig. 3 are of the type AXLJ 1x1000/190 mm2 64/110 kV. All the construction elements of these cables are described in [3]. For the purposes of simulations using the COMSOL software, these cables are modelled with an equivalent construction composed of the aluminium conductor, cross-linked polyethylene (XLPE) insulation, copper screen, and outer polyethylene (PE) sheath with outer radii 19.2 mm, 35.65 mm, 38.35 mm and 42.15 mm, respectively, as well as with thermal conductivities 239 W/(mK), 0.286 W/(mK), 385 W/(mK) and 0.286 W/(mK). In addition, it is assumed that dimensions and thermal conductivities of the blocks representing the HAP system in Fig. 3b correspond to the ones from Fig. 2.
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An alternative method of increasing ihe transmission performance of a conventional 110 kV cable line
Figure 3: Presentation of the computational domains for (a) base cases, and (b) proposed method with a HAP system consisting of the same seven copper pipes (not to scale)
The governing equation for 2D FEM-based modelling of steady-state heat transfer has the following form [3,4]:
-q * ^ Ui-i * ^ i+a=o
dx ^ dx) dy ^ dy
(2.1)
where T is the unknown temperature in K; x, y are the Cartesian spatial coordinates in m; and Qv is the volume power of heat sources in W/m3. The thermal conductivities of all bedding materials appearing in the FEM-based models are listed in Table 1. The governing equation is non-linear due to the existence of the radiation boundary conditions.
All the required material and surface properties are constant and selected so that the obtained results are optimistic from the engineering point of view. Accordingly, the values of the thermal and electrical conductivities which correspond respectively to the associated dried-out states
(of the cable bedding and the native soil] °C) are taken into account.
and the continuously permissible temperature (Tcp=90
Table 1: Thermal conductivity of all used bedding materials in the dried-out state
Cable bedding material	k W/(m-K)
Fine aggregate, Serbian standard [5]	1
Fine aggregate, US standard [6]	1.47
Limestone fine aggregate [7]	2.15
Quartzite fine aggregate [7]	5.38
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The volume power of heat sources Qv in each 110 kV conductor with a radius of 19.2 mm and a geometric cross-section area of 5'c=1158.117-10"6 m2 is
Rac (Tcp ) 2
Qv = -2^ ■ 12	(2.2)
where Roc(TCp)=40.91094-10-6 Q/m is the effective conductor resistance to the flow of alternating current per unit length of each 110 kV cable at Tcp=90 °C, and I is the ampacity or load current in A. The a.c. resistance takes into account the skin and proximity effects, as well as losses in the metal screen. According to the facts given in [3,4], the volume powers of heat sources in the cable insulation and metal screens of the 110 kV cables are equal to zero.
The adiabatic boundary condition:
k ■ — = 0	(2.3)
dn
is used to model boundaries surrounding the left-hand, bottom and right-hand sides of the two domains in Fig. 3, where the constant temperature boundary condition:
T = To( x, y) = Trs + 273.15	(2.4)
also needs to be satisfied simultaneously [3,4]. In the Eqn. (2.3) and Eqn. (2.4), n is the length of
the normal vector n in m, while T and T0 are the unknown and specified temperatures of the corresponding boundaries in K, respectively.
The heat transfer along the earth and pavement surfaces is represented by a combination of the convection boundary condition:
kdJ_ = ^ (T - Ta)	(2.5)
on
and the radiation boundary condition:
OT
k■ —— = s ■ aSB ■T 4-a-Qs,s.	(2.6)
on
where T is the unknown surface temperature of the earth or pavement in K, a=0.6 and s=0.94 are the solar absorptivity and thermal emissivity for a dry grassy surface, a=0.87 and s=0.93 are the solar absorptivity and thermal emissivity for the asphalt surfaces in Fig. 3, h=12.654 W/(m2 ■ K) is the heat transfer coefficient due to convection for a dry grassy surface when va=0.22 m/s, h=8 W/(m2 ■ K) is the heat transfer coefficient due to convection for the asphalt surfaces in Fig. 3 when va=0.22 m/s, and e%B=5.67 ■ 10-8 W/(m2 X4) is the Stefan-Boltzmann constant. More details relating to the boundary conditions (3-6) can be found in [3,4].
The heat transfer between the inner surfaces of the heat collecting pipes and the water flowing through them is represented by the following convection boundary condition:
k= h^ (T - Tw)	(2.7)
on
where T is the unknown temperature of the corresponding inner boundary in K, which is also higher than Tw=(Tw>,+Two)/2. Accordingly, the associated Reynolds number Re and heat transfer coefficient h are
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An alternative method of increasing ihe transmission performance of a conventional 110 kV cable line
v • d
Re = —ZL < Recr = 2300	(2.8) v
, 3.66 • k
h = —--(2.9)
dPi
where k is the thermal conductivity of water at Tw in W/(mK), vis the kinematic viscosity of water at Tw in m2/s, and Recr is the critical Reynolds number for smooth pipes. Therefore, the wall temperature of each copper pipe is assumed to be constant. The values of k, v, Re and h respectively amount to 0.61331 W/(mK), 8.5776 107 m2/s, 1329.04 and 118.14 W/(m2K) for Tw=25.7 °C.
3 RESULTS AND OBSERVATIONS
The results obtained by simulating the temperature distribution over the transverse cross-section of the experimental apparatus (Fig. 2) for the known environmental and operating conditions are shown in Figs. 4a and 4b. It is found that the temperature of the surface above the pipes can be decreased by about 12.9 °C with the flowing of water. In comparison with the corresponding experimental result of the dynamic thermal analysis, [1], this temperature is higher by about 2.9 °C and represents the maximum possible value that can be reached for Tw,i=23.2 °C. According to [8], for Tw,i=13 °C, the temperature of the surface above the pipes can be decreased by about 18 °C.
However, there are parts of the asphalt pavement surface between the pipes whose temperature is decreased by only 3.7 °C. One way of overcoming this is to increase the number of pipes filled with flowing water. Fig. 4c shows temperature distribution over a domain having seven copper pipes filled with flowing water. The domain and pipes have the same dimensions as those in Fig. 2. In this case, the temperature of the entire surface above the seven copper pipes can be decreased by 14-16 °C. This observation is significant, because it indicates that it is possible to reduce the amount of heat radiated back from the pavement surface.
How the HAP system affects the conductor temperature and load current of the 110 kV cables can best be illustrated by concrete examples of the temperature field distribution over the two computational domains in Fig. 3. Accordingly, Figs. 5a and 5b show the temperature field distributions over the domains in Figs. 3a and 3b, respectively. The temperature distribution in Fig. 5a is obtained for the environmental conditions and material properties corresponding to the first base case (i.e., for the most unfavourable summer conditions). In addition, the result in Fig. 5b is obtained for the same environmental conditions and thermal conductivities of the cable bedding, native soil and limestone asphalt mixture/pavement amounting, respectively, to 5.38 W/(mK), 0.4 W/(mK) and 1.583 W/(mK). In both simulations, the volume powers of heat sources and load currents were Qv=10250 W/m3 and I=538.7 A, respectively.
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Figure 4: Temperature distribution over the domain in Fig. 2 for cases when (a) water flows through only one of the three pipes, and (b) water flows through each of the three pipes; (c) temperature distribution over a domain having seven copper pipes filled with flowing water
Surface: Temperature [°C] Contour: Temperature [°C] Subdomain marker: Temperature [°C]
Max: 89.991 Max: 70,0
		
	& 89.!	31168 \ 1
(a)l	\ /	;7o i
-2.4 -2 -1.6
-0.6 0 0.4
Surface: Temperature [°C] Contour: Temperature [°C] Subdomain marker: Temperature [°C]
Min: 44.52a Max: 60,359
Min: 50,0 Max: 50.0
< i_ ut_.1 t		
		in\ | max:| 6Q. 367182
	•n v---- ----"	50
		
(b)		
-1,4 -1.2 -1 -0.8 -0,6 -0.4 -0.2 0 0,2 0,4 0.6 0.8 1 1,2 1,4 1.6
Min: 31,428 Min: 40.0
Figure 5: Temperature distributions over the domains shown in (a) Fig. 3a, and (b) Fig. 3b
According to Fig. 5, the HAP system in combination with the cable trench that is completely filled with fine quartzite aggregate affects the load current of the 110 kV cable line to a considerable extent. In particular, with the installation of the HAP system, the maximum
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An alternative method of increasing ihe transmission performance of a conventional 110 kV cable line
system above the 110 kV cable line can be used simultaneously to control the ampacity and to collect heat from the sun and cables.
For the purpose of determining the ampacity of the 110 kV cable l ine Icp, a sequence of simulations over the two computational domains (in Figs. 3a and 3b) is performed with the thermal conductivities of the selected cable bedding materials (Table 1), for the most unfavourable summer conditions and the most common winter conditions. The values for Qv are gradually increased from an arbitrary prescribed initial value (e.g., 10 kW/m3) to its continuously permissible value (corresponding to the temperature Tcp=90 °C). The volume powers of heat sources Qv obtained in this manner are listed in Table 2. Then, these volume powers of heat sources and the Eqn. (2.2) are used to calculate the corresponding cable ampacities (for I=Icp). The ampacities Icp of the considered 110 kV cable line are also listed in Table 2. In order to simplify comparisons between the results, the inlet water temperature is assumed to be constant (at 23.2 °C), regardless of the environmental conditions considered.
Table 2: Volume powers of heat sources in cable conductors and corresponding ampacities estimated for the computational domains in Figs. 3a and 3b, different cable bedding materials and different environmental conditions
Cable bedding material
Computational domain
Results obtained for the most unfavourable summer conditions
Results obtained
for the most common winter conditions
k			Qv	I=Icp	Qv	I=Icp
W/(m-K)			W/m3	A	W/m3	A
1	F	g. 3a	10250	538.7	21185	774.4
1	F	g. 3b	14680	644.6	23735	819.7
1.47	F	g. 3b	18520	724.1	28985	905.8
2.15	F	g. 3b	23195	810.3	35225	998.6
5.38	F	g. 3b	37910	1035.9	54465	1241.7
4 CONCLUSION
The most important conclusions that can be drawn from the presented results are:
•	If the conventional asphalt pavement above the 110 kV cable line is changed with the HAP system, combined with the cable trench that is completely filled with the high thermal conductivity bedding, then the corresponding ampacity can be increased up to 497.2 A for the most unfavourable summer conditions and up to 467.3 A for the most common winter conditions.
•	HAP systems can be used to control the thermal environment of underground cable lines in order to increase their ampacities.
•	The proposed innovative method is new, it can be easily implemented within current practices, and it would result in significant financial and engineering benefits.
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Acknowledgements
This paper was based on research conducted within the project TR33046 funded by the Ministry
of Educat ion, Science, a nd Techn ologicalDevelopment of the Republic of Ser^a .
References
[1]	R.B. Mallick, B.-L. Chen, S. Bhowmick: Harvesting heat energy from asphalt pavements: developmknt of and comparison between hemerical models and experiment, International Journalof Sustainable Engineering, Vol. 5, Issue 2, pp. 159-169, 2012
[2]	Z. Zhou, X. Wang, X. Zhang, G. Chen, J. Zuo, S. Pullen: Effectiveness of pavement-solar enhrgy system - An exparimental stndy. Allied Energy, Vol. 138?, pp. 1-10, v015
[3]	D. Klimenta, B. Eerrvic, J. Klimenta, M. Jevtic, M. Milrvanrvic, I. Krstic: Controlling the thermel environmrnt of undeeground cable lines using the pavement surface radiation properties, l°T Generation, Transmissian and Distrtaution, Vol. 12, Issue 12, pp. 29682976, 201 8
[4]	D.O. Klimenta, B.D. Eerrvic, J.Lj. Klimenta, M.M. Jevtic, M.J. Milrvanrvic, I.D. Krstic:
Con trolling the the rmal environment of un de^ro und power cables adjacent to heating pipeline u sing the pavement surface radiation properties, Thermal Science, Vol. he Issue 6, pp. 2625-2640, a018
[5]	D. Klimenta, S. Nikrlajevic, M. Sredrjevic: Controlling the thermal environment in hot spots of baried power eanles, Europem Tra nsartions on □ectricml Power, Vol. 17, Issue 5, pp. 427-449, 2007
[6]	S.Y. King, N.A. Halfter: Underground Power Cables, 1st edition, Longman, London and New York, 1982
[7]	R.B. Mallick, B.-L. Chen, S. Bhrwmick, M.S. Hnlen: Capturing solar energy from asphalt pavements, The Internationa| ISAP Sympos ium on Asphalt Pavements and Environment ,ISAP 20083), Zu rich, Switrerland, August 18-20, 2008
[8]	D. Klimenta, D. Tasic, M. Jevtic: Increasing the ampacity of a 110 kV underground cable line by an applicatioe of a hydnonic asphalt pavement system, The 14th loternatibnal Conference on Applied Electromanneticn - nEC 2019, Niš, Setüa, August 26-28, 2019
Nomenclature
Symbol	Symbol meaning
dp,i	inner diameter of the heat collecting pipe, in m
dp,o	outer diameter of the heat collecting pipe, in m
h	heat transfer coefficient due to convection, in W/(m2K)
I	load current, in A
Icp	ampacity, in A
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An alternative method of increasing ihe transmission performance of a conventional 110 kV cable line
n	length of the normal vector n , in m
n	normal vector
Qs,s	solar irradiance, in W/m2
Qv	volume power of heat sources, in W/m3
Ra
effective conductor resistance to the flow of alternating current per unit length of each 110 kV cable at 7^=90 °C, in Q/m
Re	Reynolds number, dimensionless
Recr	critical Reynolds number, dimensionless
Sc	geometric cross-section area of each 110 kV conductor, in m2
T	unknown temperature, in K
To	specified temperature, in K
Ta	temperature of the air contacting the pavement surface, in K or °C
Tcp	continuously permissible temperature, in °C
Trs	temperature of referent soil, in °C
average temperature of water flowing inside the heat collecting pipe, in K or °C
Tw,i	inlet water temperature, in K or °C
Tw,o
outlet water temperature, in K or °C
Va	wind velocity, in m/s
Vw	water velocity, in m/s or l/min
x, y	Cartesian spatial coordinates, in m
a	solar absorptivity, dimensionless
S	thermal emissivity, dimensionless
v	kinematic viscosity of water, in m2/s
OSB	Stefan-Boltzmann constant, in W/(m2K4)
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