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Advances in Electrical Engineering Systems (AEES) Vol. 1, No. 3, 2012, ISSN 2167-633X Copyright © World Science Publisher, United States www.worldsciencepublisher.org
Practical and Theoretical Investigation of Current Carryi Capacity (Ampacity) of Underground Cables 1
2
2
2
3
Adel El-Faraskoury, Sherif Ghoneim, Ali Kasem Alaboudy, Ragab Salem, Sayed A. 1
Extra High Voltage Research Center, Center, Egyptian Electricity Holding Company, Egypt, Egypt, a.elfaraskoury@yahoo. 2 Faculty of Industrial Education, Suez University, Suez Campus, Suez, Egypt,
[email protected];
[email protected] 3 Faculty of Engineering, Benha University, Shoubra, Cairo, Egypt,
[email protected]
Abstract – In urban areas, underground cables are commonly commonly used for bulk power transmission. The util electricity in factories, domestic premises and other locations is typically performed by cables as they present practical means of conveying electrical power to equipment, tools and other different applications. Estimatio current carrying capacity (ampacity) gains higher potential in recent times due to the continuous increase utilization in modern electric power systems. This paper presents a theoretical study based on relevant IEC sta calculate the ampacity of underground cables under steady state conditions. The ampacity formula stated in IEC are coded using Matlab software. Further, an untraditional experimental ampacity test of a 38/66 kV- XLPE/CU 2 mm cable sample is performed in the extra high voltage research center. This paper proposes a new approach tha complementary laboratory measurements in cable ampacity data preparation. The modified approach gives more estimation of cable parameters. The level of improvement is assessed through comparisons with the traditional calculation techniques. Main factors that affect cable ampacity, such as the insulation condition, soil thermal r bonding type, and depth of laying are examined. examined. Based on paper results, cable ampacity is greatly affected by the conditions and material properties. Keywords – Underground cable; Cable ampacity; Soil thermal resistivity; Bo nding type; Depth of laying
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with $4.99/month. transient temperature rises caused by Special offer forCompared students: Only
sudden application of bulk loads, the calculation of the
Free Foron 30 Days Sign up to this title from the ca well as Read the rate ofvote heat dissipation surroundings. In the case of underground cable sys Not useful Useful Cancel anytime. convenient to utilize an effective thermal resistan earth portion of the thermal circuit which includes of the loading cycle and the mutual heating eff
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Adel El-Faraskoury, et al., AEES, Vol. 1, No. 3, pp. 163-169, 2012
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The maximum temperature that the cable insulation can be endured for long term determines its ampacity. The long term and short term allowable maximum temperatures ensure that the cable can operate safely, reliably and economically. If the operating temperature exceeds certain limit, the insulation aging becomes faster and thus shortens the cable’s life span. In addition, the electrical and mechanical properties, and thermal behavior must be considered in choosing cable to ensure that the heat is not exceeding the limited value while the transmission capability is satisfied. The thermal behaviour of the
cables in underground lines during regimes of normal load or under emergency not only depends on the previous knowledge of the constructive characteristics of the cable and the load curve that submitted, but also of the way conditions where it is installed. Thus other factors will have to be known as: amount of loaded conductors, geometric configuration between the cable, type of grounding of the metallic shields of
the cable, thermal characteristics of the m around of the cable (soil, ducts, concrete, "b etc), effect of the typical variation of the env (humidity and temperature in the land) a interferences caused for external sources of h
maximum temperature that XLPE insulation endu ºC, so when the cable core come to this temper current in the cable core is considered cable am 60287 support a method for calculation the cable of 100% load current, which is a common metho all over the world. To find the ampacity, we first the potential of every node in the circuit analo temperature of the regions between the layers. potential difference between the terminals of and the innermost current source represents the te rise of the core of the cable with respect to the temperature. Therefore the temperature of the cabl the ambient temperature plus Δ t ; Figure 1 show electrical equivalent.
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Master your semester Scribd t as follows: From Figure (1) we can compute Δwith & The New 1 Times York t = W d T 1 (W c + W d W c + Only Special offer∆ for students: 2 $4.99/month.
(
)(
)
+ W s )T 2 +
(1)
From expression (3) one can compute the amp Read Free For 30thermal Days up to vote on this title resistances cable bySign calculating the Useful Not useful R of the core of factors λ and the ac resistance Cancel anytime. The loss factors λ take into account eddy losses in circulating currents, while R considers the te
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Adel El-Faraskoury, et al., AEES, Vol. 1, No. 3, pp. 163-169, 2012
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initial temperatures of the core and sheath are also given. Secondly, iterative calculate their temperatures under this current. Thirdly, change the core current continually based on the temperature difference between the core and its allowable maximum temperature, until this difference is smaller than the given error [6]. When the conductor is energized, heat is generated within the cable. This heat is 2 generated due to the I R losses of the conductor, the dielectric losses in the insulation and losses in the metallic component of the cable. The ampacity of the cable is dependent on the way this heat is transmitted to the cable surface and ultimately dissipated to the surrounding. The thermal resistances control heat dissipation from the conductor. Thus the efficiency of heat dissipation is dependent upon the various thermal resistances of the cable material and the external backfill and soil plus the ambient temperature around the cable. If the cable is able to dissipate more heat, the cable can carr y more current. In the Neher-McGrath method [1], the thermal resistances are either computed from basic principles or from heuristics. One can appreciate, from Figure 3, that some of the internal layers of a cable can be considered as tubular geometries. The following expression is used for the computation of the thermal resistance of tubular geometries:
and that work fine for the conditions tested [5]. code by Matlab is provided to calculate the am different cases and the flowchart that explained th is shown in Figure 2.
3. Factors affecting cable ampacity 3.1 Effect of soil resistivity
Dry soils have much higher thermal resist moist soils. With sufficiently high ampacity the away conditions can occur. If cable current is hig it will generate sufficient heat and if it is mainta long enough time, the soil will become unstabl circuit will have to be de-rated or overheate ampacity varies with change of soil resistivity with-conduit and the conduit-less cable. Cable am proportional to soil conductivity; rising soil dissipates more heat, and increase cable ampacity.
3.2 Effect of cable depth
Depth affects the ampacity of cables that bu conduit and conduit-less, in both homogen heterogeneous soil. Soil conductivity is red ρ r 2 1 increasing the cable depth in the soil as well as You're Reading a Preview T = ρ (4) = ln dissipation, less ampacity. The closer the cable A 2π r 1 Unlock full access with a free the trial.rate of cable ampacity changes wil surface, [7]. Equation (4) is applicable for most internal to the cable layers (T 1 , T 2 , T 3 ). For complicated geometries and for the With Free Trial Download 4. Test arrangement layers external to the cable, such as three-core cables, duct banks, etc., heuristics are used. The external to the cable thermal resistivity is commonly computed assuming that the 4.1 Theoretical study for ampacity surface of the earth in the neighborhood of the cable installation is an isothermal. Kennelly made this assumption Free Foron 30this Days Sign up to vote title been propos A Read computer program has in 1893 and it is still being used. This assumption allows for MATLAB to calculate current carrying Useful Notthe useful Cancel anytime. the application of the image method to compute the external (ampcity) for different underground cables. The Special offertoforthe students: Only $4.99/month. cable thermal resistance (T 4 ). The following is presented in Figure (1). The program takes int expression results from the image method: the
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Adel El-Faraskoury, et al., AEES, Vol. 1, No. 3, pp. 163-169, 2012
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shape of the soil particles determines the surfac area between particles which affects the ability of conduct heat. Figure 3 and Figure 4 show the va ampacity for cable with soil resistivity and soil tem respectively. For underground cable system the transfer mechanism is by conduction. Si longitudinal dimension of a cable is always mu than the depth of the installation, the problem is c a two-dimensional heat conduction problem. shows the effect of depth on cable ampacity and shows the variation of ampacity with cable temper
Start
Sheet Music Read input data 1. Cable data 2. Temp. data 3. Soil resistivity
Calculate 1. The outer diameter of the cable, sheath diameter and insulation diameter and different thicknesses 2. DC Resistance and AC resistance 3. Dielectric losses 4. Sheath loss factor 5. Armor loss factor 6. Thermal resistivities
Ampacity Calculation
Flat Formation
E mergency
Comparison of cable ampacity between circuit and double circuits with manufactu
Table 1.
Trifoil Formation
S teady state
Emergency
Steady state
Ampacity Bonded number
Double bond
Single bond
Double bond
Double bond
240 mm2 (Amp.)
400 mm2 (Amp.)
630 mm (Amp.)
Flat
502
648
839
Trefoil
478
616
796
Flat
435
584
702
Trefoil
460
558
672
You're Reading a Preview Double
Flat
484
649
778
Bonded Unlock full access withEmergency a free trial.
Trefoil
511
620
746
Flat
497
640
829
Trefoil
445
550
774
Single bond
Double bond
Write the ampacity of the different cases
Single Bonded
Ampacity Calculation for different cable depth
Flat Formation
Trifoil Formation
Double Bonded
Write the ampacity of the different cases
Ampacity Calculation for different soil temperature
Flat Formation
Trifoil Formation
Write the ampacity of the different cases
Download WithManufacturers Free Trial
Ampacity Calculation for different soil res istivity
Master your semester with Scribd & The New York Times Flat Formation
1000
Trifoil Formation
Write the ampacity of the different cases
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Figure 2. Flowchart of the Matlab program used for ampacity
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800
4.2 Experimental study temperature method)
SoilTemp for trfoil SoilTemp for flat
750
) A ( t n e r r u C
700
650
600
550 10
15
20
25 30 Degrees (C)
35
40
45
Figure 4. Effect of soil temperature on the ampacity of cable
820
Depth for trfoil Depth for flat
800 780 760 ) A 740 ( t n e r r u 720 C
- Ambient temperature - Conductor temperature - Over-sheath temperature - Heating Download With Free Trialcurrent
Unlock full access with a free trial.
680 660
0.6
0.8
1
1.2 Depth (b)
1.4
1.6
1.8
Master your semester with Scribd & The New York Times Figure 5. Effect of depth on ampacity
900
Special offer for students: Emerg Onlyflat$4.99/month. 800
The calibration should be carried out in a dr situation at a temperature of 20 ± 5 ºC. Te recorders should be used to measure the conduc sheath and ambient temperature simultaneou calibration should be performed on a minimum ca 10 m, taken from the same cable under test. IEC cable system test approach and requires a minimu of the cable. The length should be such that the lo heat transfer to the cable ends does not a temperature in the center 2 m of the cable by mo C. During calibration and during the test of the should be calculated according with either IEC 60853[9], based on the measured external temp the oversheath (TC S ). The measurement should with a thermocouple at the hottest spot, attached t the external surface. The hottest current should b to obtain the required value of the calculated temperature, based on the measured external temp the over-sheath [9]. The cable that used for c should be identical to that used for the test, and (path) of heat should be identical. After stabiliz been reached the following should be noted the curve as in Figure 7.
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700
640 0.4
(Calibration
Emerg trfoil
2
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equilibrium has been established. it must develop a consistent heating cycle to maintain the conductor temperature adjustment generally cannot be made in sufficient time during testing due to the large thermal time
constants of high voltage cables. In this test, t sample 38 /66 kV – CU/XLPE/LEAD/HDPE – 2 mm with 15m length as shown in Table II and Fig
Table2. Heating cycle for xlpe cables xlpe – cu- 38/66kv- 1x 630 mm2 No. of heating cycle
Required Heating steady current at conductor stable temp. condition
ºC
Amp.
20 95-100
ooling per cycle
Heating per cycle
Total duration hr
1600
8
Stable temp. hr
hr
Voltage Per cycle
hr
2U 0
2
16
2 4
72
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Download With Free Trial Figure 8. Heating cycle for cable sample 38/66 kV – 1x 630 mm 2
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Niv Hai-qing, Shi Yin- Xia, Wang XaaoZhang Yao “Calculation of ampacity of si cables with sheath circulating current based o The theoretical and practical study for cable ampacity method,” Guangzhou, 510640, China. estimation under steady state conditions shows that the [3] IEC Standard: Electric Cables – Calcu underground cable ampacity depends on the cable geometry Current Rating – Part 1: Current rating installation, its depth as well as on the soil thermal (100% load factor) and calculation of losses, resistivity. Cable ampacity is proportional to soil General. Publication IEC-60287-1-1, 1994+A conductivity; when soil conductivity increases, cable [4] IEC Standard: Electric Cables – Calcu ampacity will be increased. The results show that the cable Current Rating – Part 2: Thermal resistance ampacity decreases with the increase of cable depth 1: Calculation of thermal resistance. Publica installation under soil surface. By using MATLAB with the 60287-2-1, 1994+A2:2001. steady state conditions based on IEC standards and comparing [5] Francis Codeleon “Calculation of undergro with manufacturers, it gives good results. In facts that stand ampacity,” CYME International T& D, 2005. out the importance of interaction with the manufacturers, [6] T. IVO, Domingues, Oliverira, et al. ’Devel designer and installers of the line for attainment of coherent one specialist system to determine the dynam data with the reality. The maximum operating temperature capacity of underground transmission lines w of a cable is typically limited by its insulation material but cables,’ B1-202- CIGRE 2006. can also be limited by the maximum temperature which the [7] Amin Mahmoud, Solmaz Kahourzade, R.K surrounding environment can be withstood without Computation of Cable Ampacity by Finite degradation. Method Under Voluntary Conditions” Journal of Basic and Applied Sciences, 5(5): Acknowledgment 2011 [8] IEC Publ. 60840, 3rd ed., “Power Cables with The authors would like to express his great thanks to the Insulation an their Accessories for Rated Volt team work of the Extra High Voltage Research Centre for 30 kV (Um =36 kV) up to 150 kV (Um=170 k providing their facilities during this work. You're Reading a Preview Methods and requirements”, 2004-4. [9] IEC Standard: Electric Cables – Calculati Unlock full access with acyclic free trial. References and emergency current rating of cables Cyclic rating of cables greater than 18/30 (3 [1] J. H. Neher, McGRATH, ’The calculation of the emergency ratings for cables of all. Publica Download With Free Trial temperature rise and load capability of cable system", 853-2, (1989-2007). AIEE Transaction, vol.76, part 3, Octoper 1957, pp.752-772.
5. Conclusions
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