1. Pharos University EE-385 Electrical Power & Machines
“Electrical Engineering Dept”
Prepared By:
Dr. Sahar Abd El Moneim Moussa
Dr. Sahar Abd El Moneim Moussa

2. OHTL OVERHEAD TRANSMISSION LINE (Part 1)
Dr. Sahar Abd El Moneim Moussa

3. T.L Main components An OHTL consists of main components: Conductors
Insulators
Support structures
Shield wires.
Dr. Sahar Abd El Moneim Moussa

4. Three-phase conductors, which carry the electric current;
Insulators, which support and electrically isolate the conductors;
Tower, which holds the insulators and conductors;
Foundation and grounding; and
Optional shield conductors, which protect against lightning
Dr. Sahar Abd El Moneim Moussa

5. Types of Transmission lines (according to the voltage)
Extra-high-voltage lines
Voltage: 345 kV, 500 kV, 765 kV
Interconnection between systems
High-voltage lines
Voltage: 115 kV, 230 kV
Interconnection between substations, power plants
Dr. Sahar Abd El Moneim Moussa

6. Sub-transmission lines Voltage: 46 kV, 69 kV
Interconnection between substations and large industrial customers
Distribution lines
Voltage: 2.4 kV to 46 kV, with 15 kV being the most commonly used
Supplies residential and commercial customers
High-voltage DC lines
Voltage: ±120 kV to ±600 kV
Interconnection between regions (e.g., Oregon-California)
Dr. Sahar Abd El Moneim Moussa

7. 500 kV Overhead T.L
Dr. Sahar Abd El Moneim Moussa

8. 275 kV double-circuit steel tower
Dr. Sahar Abd El Moneim Moussa

9. 11 kV Concrete line post Two-units of Suspension type Line Insulator
11-kV Line Conductor
Post-type Distribution Transformer
Concrete Line Post
Dr. Sahar Abd El Moneim Moussa

10. O.H.T.L. Conductors
Overhead transmission lines are composed of aluminium conductors that,even in modest capacities,are stranded in spiral fashion for flexibility.
These are primarily classified as:
AAC: all-aluminum conductors
AAAC: all-aluminum alloy conductors
ACSR: aluminum conductors, steel reinforced
ACAR: aluminum conductors, alloy reinforced
For purposes of heat dissipation, O.HT.L. conductors are bare(no insulating cover)
Dr. Sahar Abd El Moneim Moussa

11. ACSR Conductors of O.H.T.L
Steel reinforcing
strands
For purposes of heat dissipation, O.HT.L. conductors are bare (no insulating cover)
Dr. Sahar Abd El Moneim Moussa

12. O.H.T.L. Insulators PIN-TYPE LINE INSULATORS
Dr. Sahar Abd El Moneim Moussa

13. SUSPENSION-TYPE LINE INSULATORS
Cap of Ceramics
Ball of Malleable Iron
Dr. Sahar Abd El Moneim Moussa

14. Made of porcelain, glass or polymeric material
Insulate and support
Made of porcelain, glass or polymeric material
Higher voltages need more insulator units
Dr. Sahar Abd El Moneim Moussa

15. POST-TYPE LINE INSULATORS
Dr. Sahar Abd El Moneim Moussa

16. OHTL PARAMETERS Transmission line parameter Resistance /m
Inductance H/m
Capacitance F/m
Dr. Sahar Abd El Moneim Moussa

17. Line Resistance Rdc = ρl/A, …………(1) where
R : is the resistance in ohms,
ρ : is the resistivity in ohm meter
L : is the length of conductor in m
A : is the cross sectional area of the conductor in m2
Dr. Sahar Abd El Moneim Moussa

18. Factors affecting the conductor resistance:
Conductor resistance depends on the following factors:
Spiraling:
For standard conductors, alternate layers of strands are spiraled in opposite directions to hold the strands together. This makes the strands or 2% longer, as a result, the dc resistance of a stranded conductor is 1 or2% larger than that calculated from eq.1.
Dr. Sahar Abd El Moneim Moussa

19. ρT1 and ρT2 are resistivity's at T1 and T2 °C respectively.
Temperature :
Resistivity of conductor metals varies linearly over normal operating temperatures according to:
ρT2 = ρT1 (T2+T/T1+T)
where
ρT1 and ρT2 are resistivity's at T1 and T2 °C respectively.
T is a temperature constant that depends on the conductor
material
Dr. Sahar Abd El Moneim Moussa

20. Frequency(skin effect):
The ac resistance or effective resistance of a conductor is Rac = Ploss/ I2 Ω.
For dc the current distribution is uniform throughout the conductor cross section . For ac, the current distribution is non uniform . As frequency increases , the current in a solid cylindrical conductor tends to crowd towards the conductor surface with smaller current density at the conductor center . This phenomenon is called skin effect. At power frequency, the ac resistance is at most a few percent higher than the dc resistance.
Dr. Sahar Abd El Moneim Moussa

21. 1- Increases by stranding 2- Increases by temperature
The resistance ,
1- Increases by stranding
2- Increases by temperature
3- Increases by skin effect
Dr. Sahar Abd El Moneim Moussa

22. Inductance of single phase Two Wire line
2- Line Inductance
Inductance of single phase Two Wire line
The total circuit inductance :
Ltotal =4 X 10-7 ln [D / Ds] H/m
Where,
Ds : GMR: Geometric mean radius = r r r r D
Dr. Sahar Abd El Moneim Moussa

23. - Inductance of 3-Phase 3-WireLine with Equal Spacing C
The inductance per phase
L per phase =2 X 10-7 ln [D / Ds] H/m C DD
Lab= Lbc= Lca D D a b D
Dr. Sahar Abd El Moneim Moussa

24. Lab Lbc Lca -Inductance of 3-Phase 3-WireLine with Unequal Spacing
The average inductance is
Laverage =2 X 10-7 ln [Deq / Ds] H/m
Where, C
Lab Lbc Lca
Dac DD
Dbc a
Dab b
Deq =
Dr. Sahar Abd El Moneim Moussa

25. This will cause unequal voltage drops across line conductors,
In case of unequal spacing, the inductances of the three phase conductors are all different.
This will cause unequal voltage drops across line conductors,
Therefore the voltage phasors at receiving-end substation of the line will be unbalanced.
To overcome such unbalance , line transposition is made.
Dr. Sahar Abd El Moneim Moussa

26. IV- Inductance of Transposed lines
L per phase =2 X 10-7 ln [Deq / Ds] H/m
Where,
Deq : Geometric mean distance=
Dr. Sahar Abd El Moneim Moussa

27. V- Bundled Conductors
Bundling of conductors means the use of more than one conductor per phase. It is a common practice for EHV lines.
Bundling reduces the electric field strength at the conductor surfaces , which in turn reduces corona and its effects.
Dr. Sahar Abd El Moneim Moussa

28. CORONA EFFECT:
Corona is an electrical discharge caused by ionization of the air surrounding the conductor, which occurs when the strength of the electric field at the conductor surface exceeds the dielectric strength of the air.
Air at 79 cm pressure and 25 C breaks at 30 kV/cm. If the electric field strength exceeds this value Corona occurs.
Corona is observed by violet-colored light around the conductor, a hissing noise, and Ozone-smell due to ionization of air.
Dr. Sahar Abd El Moneim Moussa

29. L per phase=2 X 10-7 ln [Deq / DSL] H/m
Dr. Sahar Abd El Moneim Moussa

30. DSL = 𝐷𝑠 𝑑 For two conductor bundle
Where,
For two conductors:
DSL = 𝐷𝑠 𝑑 For two conductor bundle
d: is the bundling spacing d a b C D
Dr. Sahar Abd El Moneim Moussa

31. DSL = 3 𝐷𝑠 𝑑 2 For Three-conductor Bundling
For three conductors:
DSL = 3 𝐷𝑠 𝑑 For Three-conductor Bundling a b C D d
Dr. Sahar Abd El Moneim Moussa

32. DSL = 1.091 4 𝐷𝑠 𝑑 3 For Four -conductor Bundling
For four conductors:
DSL = 𝐷𝑠 𝑑 For Four -conductor Bundling a b C D d
Dr. Sahar Abd El Moneim Moussa

33. Example 3: Find the inductance of a 3-phase overhead transmission line if the conductor diameter is 2 cm and the three conductors are placed at the c0rners of an equilateral triangle of side 4 m Solution: (Equal Spacing) D= 4m ,d=2 cm ,r= 1 cm=10-2 m L per phase =2 X 10-7 ln [D / Ds] H/m L per phase =2 X 10-7 ln [4 / (0.78 x10-2 )] = x 10-6 H/m C DD D D a b D
Dr. Sahar Abd El Moneim Moussa

34. ( This example will be solved in the lecture)
The three conductors of a 3 phase transmission line are arranged in a horizontal plane and are 4 m apart. The diameter of each conductor is 2 cm. calculate the inductance per phase per m of the line
( This example will be solved in the lecture)
Dr. Sahar Abd El Moneim Moussa

35. The line-to-neutral capacitance:
2- Line Capacitance
Capacitance of single phase Two Wire line
The line-to-neutral capacitance:
Cn =2 ᴨ Ɛo / ln [D /r] F/m
Where,
Ɛo : is the permittivity of air = 8.85 x F/m r r r D
Dr. Sahar Abd El Moneim Moussa

36. The line-to-neutral capacitance:
- Capacitance of 3-Phase 3-WireLine with Equal Spacing
The line-to-neutral capacitance:
Cn =2 ᴨ Ɛo / ln [D /r] F/m C DD D D a b D
Dr. Sahar Abd El Moneim Moussa

37. The line-to-neutral capacitance:
- Capacitance of 3-Phase 3-WireLine with Unequal Spacing
The line-to-neutral capacitance:
Cn =2 ᴨ Ɛo / ln [Deq /r] F/m
Where, C
Dac DD
Dbc a
Dab b
Deq =
Dr. Sahar Abd El Moneim Moussa

38. V- Bundled Conductors Deq = Dsc = 𝑟 𝑑 for a two- conductor bundle
The line-to-neutral caacitance:
Cn =2 ᴨ Ɛo / ln [Deq /Dsc] F/m
Deq =
Dsc = 𝑟 𝑑 for a two- conductor bundle
Dsc = 3 𝑟 𝑑 2 for a three- conductor bundle
Dsc = 𝑟 𝑑 3 for a four-conductor bundle
Dr. Sahar Abd El Moneim Moussa

39. Example 5: A 200 km, 3-phase transmission line has its conductors placed at the corners of an equilateral triangle of 2.5 m side. The radius of each conductor is 1 cm. calculate the line to neutral capacitance of the line. Solution: (Equal Spacing) D= 2.5 m ,L=200 km ,r= 1 cm=10-2 m Cn =2 ᴨ Ɛo / ln [D /r] F/m Cn =2 ᴨ Ɛo / ln [D /r]= 2 𝜋 𝑋 8.85 𝑋 10 −12 ln −2 = x F/m  C = x x 200 x 1000 = 2 x F C DD D D a b D
Dr. Sahar Abd El Moneim Moussa

40. ( This example will be solved in the lecture)
The three conductors of a 3 phase transmission line are arranged in a horizontal plane and are 4 m apart. The diameter of each conductor is 2 cm. calculate the line to neutral capacitance per m of the line
( This example will be solved in the lecture)
Dr. Sahar Abd El Moneim Moussa

41. Example 7: The 3 phases of a TL are placed at the corners of an equilateral triangle of side 12.5 m. Each phase consists of 2 bundled conductors, the conductor diameter is 4.6 cm and the bundle spacing is 45 cm. Find L and C of the TL. Solution: Dab = 12.5 m D bc = 12.5 m D ca = 12.5 m D conductor=4.6 cm d= 45 cm C d a b D
Dr. Sahar Abd El Moneim Moussa

42. L per phase=2 X 10-7 ln [Deq / DSL] H/m
Deq = DabDbcDca = 12.5 m
Ds= (4.6/2) x 10-2 x0.7788= cm
DSL = √Ds d = cm
L per phase=2 X 10-7 ln [Deq / DSL] H/m
= 2 X ln = 9.87 x H/m
Dsc = √rd =10.7 cm
Cn =2 ᴨ Ɛo / ln [Deq /Dsc] F/m
= 2 𝑋 𝜋 𝑋 8.85 𝑋 10 −12 ln = x F/m
Dr. Sahar Abd El Moneim Moussa

43. ( This example will be solved in the lecture)
The three conductors of a 3 phase transmission line are arranged in a horizontal plane and are 4 m apart. The diameter of each conductor is 2 cm. calculate the line to neutral capacitance per m of the line
( This example will be solved in the lecture)
Dr. Sahar Abd El Moneim Moussa