Yeraltı kablolarının akım taşıma kapasitesi
The Current carrying capacity of underground cables
- Tez No: 14347
- Danışmanlar: PROF.DR. NESRİN TARKAN
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1991
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 144
Özet
ÖZET Elektrik enerjisi iletimi hava hatları veya yeraltı kablolarıyla gerçekleştirilir. Her iki iletim seklinde de iletken kesitinin tayininde birçok faktörün etkisi bulunmakla birlikte bunların en önemlilerinden biri de akım tasıma kapasitesidir. Tez kapsamında bu kavram incelenmeye çalışılmıştır. Giriş bölümünde, yapılan çalışmanın temel konusu açıklanmış ve özellikle kablo sistemlerinin yapısı ve sınıflandırması hakkında genel bilgi verilmiştir. ikinci bölümde iletken direnci konu edilmiştir. İletkenin doğru ve alternatif akım dirençleri açıklandıktan sonra bu dirençlerin artısına neden olan faktörler belirtilerek bağıntıları verilmiştir. Tez konusunun ağırlığını oluşturan üçüncü bölümde akım taşıma kapasitesi kavramı incelenmeye başlanmıştır, îlk olarak, hava hatlarında kullanılan çelik -alüminyum iletkenlerin ısıl denklemi elde edilmek suretiyle akım taşıma kapasitelerine hangi faktörlerin etkide bulunduğu açıklanmış, bu faktörlerin ele alınan bir iletken örneği üzerinde nasıl bir etki yarattıkları sayısal olarak ifade edilmiştir. Daha sonra ise kablo sistemlerinin ısıl incelemesi yapılarak, akım taşıma kapasitesini verecek denklem elde edilmiştir. Burada da kabloların yeraltı veya havada bulunma durumlarına göre hangi faktörlerin etki yaptığı açıklanmış, konuyla ilgili sayısal örnekler verilmiştir. Tezin sonuç bölümünde ise ele alınan konunun bir değerlendirmesi yapılmış, çıplak il etkeni i hava hatlarıyla kablo sistemleri karşılaştırılmış ve kablolarda akım taşıma kapasitesini arttırıcı yöntemler ve önerilerden bahsedilmiştir. xı
Özet (Çeviri)
SUMMARY THE CURRENT CARRYING CAPACITY OF UNDERGROUND CABLES Nowadays, electrical power transmission is mostly realised by overhead lines or underground cables. The transmission of large powers to long distances can be obtained by the application of high and extra high-voltage overhead lines. This is a necessity because of technical difficulties and economical conditions; such conctructions turn out to be of long duration, laborious and costly. Besides, insulation problems for the cables are likely to occur in high voltage transmission. Other than this, overhead lines in the transmission and distribution of energy cannot be used everywhere. Particularly in the city centers, it is more appropriate for the energy to be carried by underground cables, for reasons of esthetics. In addition, the power transmission in such districts forms a larger security, and the accidental percentage is reduced to a high extent. Further, cables are the only means for the connections between high voltage machines and the transmissions to overhead lines in switching stations. For low and medium voltage indoor installation, power cables in air are to be used. The domain of cable system usages indispensably consists of transitions like seas, lakes, etc. connections to islands as underwater cables; ships and covered places like tunnels, channels and mines. In the transmission of electrical energy by overhead lines or underground cables, the cross -sections of conductors are determined by the power.and voltage to be transmitted. Beside many factors as voltage drop, energy loss, stability and economical limitations, it is highly important for keeping the conductors in permissible temperature limitations, to take into consideration the maximum current valuesC current carrying capacities!) which the conductors can carry. Specifically in cable systems, because it is necessary to keep in view the temperature limitation of the cable insulation other than the conductors, this concept signifies a greater importance. The steady state current carrying capacity of long overhead transmission lines may be determined by xixconsideration of system stability» voltage regulation or economic power losses. On the other hand, the capacity of shorter lines may be determined by the maximum permissible temperature of the conductor, which determines the maximum conductor sag, and by the time distribution of the conductor temperature, which determines the rate of annealing and the total loss of tensile strength of the conductor. The current corresponding to the maximum permissible temperature of the conductor under specified atmospheric conditions is known as the thermal rating. The temperature of the conductor depends on the load current, the electrical and the thermal characteristics of the conductor and the atmospheric conditions, such as wind and sun. The relationship between these parameters is known as the heat equation. The overhead line conductors are heated up by Joule heating PT occurring in the conductors, f erro-magnetic heating P of the steel core in ACSR conductors, solar heating P^ and i oni zati onC cor onaD heating P.. To prevent an extra current in conductors, these heat losses have to be transferred to the surrounding air, and this must be realised by convection, radiation and evaporation. If these losses are shown as P, P and P respectively, the heat equation for the steady state. P.+P + P + kP. = P +P + P CID J m s k c r w can be written. It should be noted that only a small fraction of the corona heat diffuses to the conductor to raise its temperature; that is, k J w By substituting this equation into C1D, and by deducing I, the current carrying capacity of an overhead line conductor can be found as: I = P +P +P -P -P -kP, c r w m s k_ iAJ C3D R^ w All of the P losses are in CW/m3. The current carrying capacity of a cable is determined by the maximum permissible conductor temperature and the ambient conditions as far as they are Xlilrelevant for the dissipation of heat. Inadmissibly high conductor temperatures and excessive temperature differences will speed up the ageing process. The cable is heated up by the ohmic losses occurring in the conductors and _ if the cable is operated with a. c. _ in the coverings. There will also be the dielectric losses in the insulation. But the dielectric losses are negligible with PVC cables up to U /U = 3. 5/6 kV, paper -insulated cables up to U /U = °8/30 kV and XLPE cables up to U /U = 64/110 kV operating voltage. Under steady-state conditions, the heat dissipated is equal to the sum of all the losses in the cable. The heat flows by conduction to the surface of the cable and, if this is arranged in free air, is transferred to the surrounding air by convection and radiation. With cables buried in the underground, the heat generated by losses flows from the surface of the cable by thermal conduction via the ground into the atmosphere, whilst the cable itself is heated up. The difference between conductor temperature and ambient temperature is nearly proportional to the total losses. The law of heat flow is analogous to Ohm's law, with the total heat flow P corresponding to electric current I, the difference in temperature AT corresponding to voltage U and the total thermal resistance ZS corresponding to electrical resistance R. Thus, AT = P. ES is analogous to U = I.R. C43 The total losses P is the sum of. the Joule losses P and the dielectric losses P.. The heat flow is the sum of all losses generated in the cable. To reach the surrounding air from its point of origin, the heat flow must overcome the thermal resistance S, of the cable and the thermal resistance of the air S or of the ground S caused by the transfer of heat from the surface of the cable to the surrounding media. In accordance with the analogy existing between the heat flow and the electric current Cequation 43, an equivalent circuit diagram C Figure 13 may be drawn for the heat losses flowing from the cable and the temperature rise they produce. The thermal resistance S, of the cable for n number of current carrying conductors is given by S. S, = 1S + S. + S. C5D k n ex ca The component resistances are those of the insulation S, and of the inner and outer protective i s coverings S, and S. CThe thermal resistance of the c i_ d ât. metallic components can be assumed to be negligible.!) XIVConductor temperature ?- Conductor losses j Thermal resistance of insulation Sisyn Conductor temperature“SÇ-, «. Dielectric losses P, a ?*”Sheath losses Thermal resistance of inner layers S. 4 - Armour losses > Thermal resistance of outer sheaths S“ ca Thermal resistance corresponding to convection and radiation S_ Conductor losses Thermal resistance of insulation S,.,/ Sheath losses Thermal resistance of inner layers S. Armour losses Thermal resistance of outer sheaths S Thermal resistance, of the ground S”j Ambient temperature j ^^ losseg Pj+p | **i«t temperature. ^^ losses v^ Figure 1. Equivalent Circuit Diagrams for the Heat Flow in a Cable. If the cable is operated with a.c., the fictitious thermal resistance is n + C1+\.)S. 1 ci Sk = + S 1 + \ + X2 ca C6D of the cable will be used. The effective resistance Ca.c. resistance} of a cable, its d. c. resistance being P., is arrived at as follows: R = R + AR w cn/m] C7D xvwhere AR shows the extra resistance and can be given as : AR = Cy + y + X“ + X_D R. C8D s -'p 1 2 Here y, y, X. and X_ are the resistance increment ^^ ?. ^ s P 1 2 coefficients. y = constant due to skin effect, y = constant due to proximity effect, XJT = constant due to sheath losses, X^ = constant due to armor losses. Then the effective resistance can be obtained as : R =Cl+y +y + X, + X”D R. C9D w.'s Jp 1 2 From Figure 1, the fictitious thermal resistance due to dielectric losses can be obtained as : S, S* = - İ5-. + S. + S C1CD kd 2n ci ca Then the law of heat flow, at = p.cs; + S_D + P,CS' + S_D CUD c J k E d kd E 2 can be written. By substituting P. = nR I and by deducing I, the current carrying capacity of an underground cable for operation with a. c. can be found as : I = AT - P.CS', + S“D c d kd E nR CS,' + S”D w k E CI 2D With the cable installed in free air, the thermal resistance of the air S instead of S will be taken into account. The current carrying capacity is determined by : - conductor resistance and losses due to the current C Joule losses}, - dielectric losses in the insulation, - conductor temperature and ambient temperature C temperature difference!), xvi- thermal resistance of the cable, - installation conditions Cin the ground or in free air D Further» economical limitations have to be taken into consideration for the current calculations. The current carrying capacity of high-power cables depends considerably to the losses occurring in conductors and insulation. There are three possibilities to reduce the losses of the conductor if the material of the conductor and the current I are fixed. These possibilities are: - Decreasing the dc resistance by enlarging the cross section. - Reducing the skin-effect factor y and the proximity-effect factor y by special constructions of the conductor. - Lowering of the conductor temperature T by for ced cool i ng. The first possibility is limited since with increasing the cross section also the skin and proximity effect factor but also the dielectric losses will increase. According to the second possibility there are a lot of proposals to reduce the skin and the proximity effect factor. So conductors were constructed of several conductors with a segmental shape and these segments are isolated to each other CMilliken conductor D. By these means reached reduction of the skin and proximity effect factors is remarkable. On the other side this construction causes considerable costs. In order to reduce these manufacturing costs and to limit at the same time the skin and proximity effect factors, a round conductor with a great hollow duct has been chosen. This conductor has got lower losses than a normal round conductor of the same cross section. If the diameter of the hollow duct becomes very great the losses could fall beneath the values of the losses of a Mi 11 iken conductor. On the other hand the demand of material will increase if the diameter of the hollow duct is growing greater and greater. An additional disadvantage is the increase of the dielectric losses if the conductor diameter becomes large. In this hollow duct could be brought a stainless steel pipe and through this pipe cooling water could be pumped..Then the mean temperature of the conductor will be lowered and therefore, the conductor losses will be decreased. Of course it must be made a total balance of xvi iall losses since there is at least an additional demand of power to pump the cooling water around. Therefore, the motor -cable should be regarded. This cable pumps the water around without any help from outside. So it can be tested whether the reduction of the conductor losses could be greater than the additional pump losses or not. XVI 11
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