Hava taşıtlarında kablolarda elektriksel ve ısıl zorlanmaların incelenmesi
Investigation of electrical and thermal stresses in aircraft cables
Yazar: BURHAN MERT YILDIZ
Danışman: PROF. DR. ÖZCAN KALENDERLİ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektrik Mühendisliği Ana Bilim Dalı / Elektrik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Havacılık sektöründe ihtiyaçlar ve görev gereksinimlerin zaman geçtikçe artması bu
alanda yeniliklerin doğmasına neden olmuştur. İşletme maliyetlerini ve yakıt
tüketimini azaltmak ve bu doğrultuda uçakların ağırlığını düşürmek amacı ile daha
yüksek gerilimli uçak kavramı ortaya çıkmıştır. Artan gerilim seviyeleri ile birlikte
kablolama sisteminde ağırlık ve hacimsel olarak azalma sağlanmaktadır. Buna
karşılık artan gerilim seviyesi ile birlikte fazladan yalıtım çalışmalarına gereksinim
duyulmaktadır.
Bu çalışmada uçaklardaki kablolama sistemine yönelik fiziksel zorlanma olayları ele
alınmıştır. Havacılıkta kullanılan kablolama sistemlerin elektriksel ve ısıl analizleri
üzerine çalışılmıştır.
Kablolarda elektrik alan incelemeleri için Comsol'da elektrostatik bir model
oluşturulmuştur. Kablo demetlerinde oluşan elektrik alan hesaplamaları sonlu
elemanlar yöntemi ile yapılmıştır. Yalıtkan malzemenin içindeki hava boşluğunun
elektrik alan üzerine etkisi incelenmiştir. Ayrıca yalıtkandaki hava boşluğu için farklı
şekiller incelenmiş olup farklı şekillerdeki hava boşluklarının kabloda oluşturduğu
elektrik alan şiddetleri incelenmiştir. Kablo yalıtkanındaki boşluklarda elektrik alan
şiddetinin yüksek olduğu gözlemlenmiş ve elektrik alan şiddetinin yüksek olduğu
bölgelerde hava boşluğu olduğunda bu değerin daha da arttığı görülmüştür.
Kabloların içerisinden akım geçtikçe kabloda bir ısınma meydana gelmektedir. Artan
sıcaklık ile de kablonun akım taşıma kapasitesinde azalma meydana gelmektedir. Bu
tezde kablodan geçen akımın sıcaklık ile ilişkisi incelenmiştir. Bu doğrultuda
Comsol'da ısı aktarımı aracı kullanılarak model oluşturulmuştur. Ayrıca kablolar
arasındaki uzaklığın akım taşıma kapasitesine etkisini gözlemlemek adına analizler
yapılmıştır. Kablolar birbirlerine yaklaştıkça sıcaklığın arttığı ve kablonun akım
taşıma kapasitesinin azaldığı görülmüştür.
Kablolarda kısmi boşalma başlangıç gerilimleri yüksek olmasına rağmen havacılıkta
irtifanın artması ile basıncın düşmesi ve kullanılan gerilim değerinin de artması
sonucu kısmi boşalma başlangıç geriliminin düşmesine neden olmaktadır. Bu yüzden
kablolarda oluşan üç elektriksel boşalma türü de matematiksel olarak hesaplanmış bu
değerlerin en küçük olanı kısmi boşalma başlangıç gerilimi olarak seçilmiştir. Aynı
zamanda bu gerilim değerine güvenli çalışma gerilimi olarak da ifade edilebilir.
Farklı kablo boyutları ve aynı hava boşluğu mesafeleri için gerilim değerleri
hesaplanmış ve güvenli çalışma gerilimleri bulunmuştur. Aynı zamanda kablo
yalıtkanını da değiştirerek kablo yalıtkanının güvenli çalışma gerilimi üzerine etkisi
incelenmiştir.

Abstract:
The increasing needs and mission requirements in the aerospace industry have led to
innovations in this field. The concept of higher voltage aircraft has emerged in order
to reduce operational costs, fuel consumption and reduce the weight of aircraft
accordingly. Weight and volume decreasing of wiring system are provided with
increased voltage level. Whereas extra insulation work is required with increased
voltage level.
In this study, stress events related to the wiring system in aircraft are discussed.
Electrical and thermal analysis of wiring systems that used in aerospace industry
have been studied. In addition, safe operating voltages for different cable sizes and
different insulation materials have calculated and the results have compared.
Electrostatic model is created in Comsol in order to electrical field investigation of
cables. Electrical field calculations in cables have been made with finite element
method. The air gap in the insulation material on the effect of electric field has
investigated. It has been observed that the electric field value is high in the air gaps
in the cable insulation and this value increases even more when there is an air gap in
the regions where the electric field value is high.
The effect of different geometric shapes of air gap in the cable insulation on the
stress has been examined. Besides the effect of air gaps that have different
geometric shapes, the effects of the location of air gaps on the cable, the dimension
of air gap and number of air gaps have been examined. Electrical fields for as the
variety geometric shape of air gaps that are sphere, cube, cone and cylinder shape air
gaps have calculated on the cable. When looking at the stress values of the electric
field according to the analysis values, it was seen that the cube shape of the air gap in
the cable has the highest value of the stress of the electric field.
In order to examine the effect of the positions of the air gaps in the cable, electric
field analyzes have made for a distance of 6.5 mm, 8.5 mm, 10.5 mm and 12.5 mm
from the conductor of cable. According to the analysis results, the electric field stress
in the air gap closest to the conductor has 50 kV/m, while the electric field strength
in the air gap farthest from the conductor has calculated as 18 kV/m. In this case, it
has been observed that the electric field stress value that occur the air gap in the
insulation decreases as it goes away from the conductor.
In order to observe the effect of the size of the air gaps in the cable, the analysis of
the spherical air gaps that have different radius have been made. Electric fields have
calculated for the sphere radius of 0.25 mm, 0.5 mm, 0.75 mm and 1 mm. It has
been observed that as the radius of the air gaps increases, the electric field in the air
gaps increases.The effect of the number of air gaps in the cable have been investigated. Therefore,
four air gaps have placed randomly on the cable insulation and the results have been
analyzed. Electric field analyzes have made on the cable for three, two and one
number of air gaps. It has been observed that when the number of air gaps is higher,
the electric field values that occur in the air gaps are large, and as the number of air
gaps decreases, the electric field values that occur in the air gaps decrease. When the
obtained results have evaluated, it was understood from the analysis results that the
positions of air gaps in the insulation of cable should also be taken into
consideration.
Heat occurs in the cables as the current flows through the cables. The current
carrying capacity of the cable decreases with the increasing temperature. In this
thesis, the relationship between the current passing through the cable and the
temperature has been investigated. For this reason, model was created using the heat
transfer tool in Comsol. In addition, analyzes have been made to observe the effect of
the distance between the cables on the current carrying capacity. It has been
observed that as the cables get close to each other, the temperature increases and the
current carrying capacity of the cable decreases. The cable selected for analysis can
withstand up to 200°C. Cables have placed vertically, horizontally and triangular
positions, and thermal analyzes have made for three cables that have distance
between 75 mm, 30 mm and adjacent. The current values obtained when the cables
reach the temperature they can withstand have been compared. It has been observed
that the current carrying capacity of the cables decreases as the cables approach each
other in both horizontal and vertical orientation. The lowest current carrying capacity
is calculated in the cable placement in triangle layout.
In addition, by changing the ambient temperature, the effect of different temperature
values on the current carrying capacity of the cables was also examined. Current
carrying capacity has been calculated for each identical cables at 20 ° C, 50 ° C and
70 ° C. According to the analysis results, it has been determined that the current
carrying capacity value of the cables decreases as the ambient temperature increases,
they had reverse ratio.
Although the partial discharge inception voltages (PDIV) of in cables are high, as a
result of decreasing pressure with increasing altitude and the increase the operating
voltage level in aerospace causes a decrease the PDIV.
Therefore, the electrical discharges in the cables have been mathematically
calculated, and the smallest of these values has been chosen as the PDIV. The lowest
PDIV could be used to determine a safe operating voltage (SOV) for a cable system.
PDIVs have calculated for different cable sizes and same air gap distances and safe
operating voltages have found. At the same time, by changing the type of cable
insulation, the effect of cable insulation on safe operating voltage has been
investigated.
Calculations of partial discharge voltages in cables are made for different cable sizes.
In addition, safe operating voltages have been calculated for different types of cable
insulations are PTFE, XLPE and PVC. For this, safe working voltages between
cables and between cable and grounded surface have been calculated. As a result of
the analysis, it was observed that the lowest safe operating voltage occurred between
the cable and ground surface. Safe operating voltage is lower in cables with large
diameter values. Additionally, when the insulation types are evaluated, PVC insulated cables with the highest dielectric coefficient have lower safe operating
voltages. For this reason, it would be more advantageous to use PTFE insulators with
lower dielectric coefficient in cables.
Consequently, while making cable design or cable selection in aircraft, attention
should be paid to environmental conditions as well as the technical characteristics of
the cable. While the ambient temperature of the cable affects the current value that
the cable can carry, the settlement place of the cables gains also important.
The effect of the voltage applied to the cables that causes the electric field stress on
the discharge event on the cable that will occur in the cable has been examined
within the scope of this study. The effects of the type and size of the cable insulation,
the air gaps occur in the cable insulation on the electric field stress in the cable were
examined. Cables to be designed considering these factors will be longevity and
effective against all kinds of electrical and thermal stress that may occur in aircraft
cables.
In aircraft, cables are placed in the air environment via the support of structural parts.
Although the cable selection is made by considering the current carrying capacity of
the cable, as in the literature, there are differences in calculation criteria. SAE
AS50881 standard is taken into consideration when choosing cables in aerospace
industry.