basics of transmission lines

1. Electrical Transmission
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2. Presentation Topic:
Mechanical Design of Overhead
Transmission Lines
Darya Khan Bhutto
M.E (Energy systems)
( 291-17-0010 )
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3. Introduction To OVER HEAD T/M Line
• Universally Adopted .
• Subjected by Stormy weather conditions & external
interferences .
• The successful operation of Overhead T/M line
depends to great extent on its mechanical design .
• In general overhead line should have great
Mechanical Strength . 13-EL-42

4. An Overhead T/M Line has Following Components :
• Conductors
( Carry Power from Sending end station to
receiving end station )
• Supports
( Provide Suport to Conductors & keep those
@ suitable level above the ground )
• Insulators
( Provide Electrical Isolation to Poles )
• Cross-Arms
( These provide Support to the Insulators )
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5. CONDUCTORS
( Types of Conductors )
(1) COPPER Conductor : ( 97.4 % )
• High conductivity + Greater tensile strength .
• Also called Ideal Conductor for T/M .
• Reason to it’s high conductivity .
• Costly & Non-availability for T/M .
(2) STEEL CORE COPPER CONDUCTOR (SCCC) :
• Steel is used in the center surrounded
by the copper conductors .
• Tensile strength + Flexibility increases .
• High cost , Low Availability .
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6. (3) CADMIUM COPPER CONDUCTOR ( CCC ) :
 ALLOY OF CD + CU .
 TENSILE STRENGTH INCREASES 50% THAN CU ALONE .
 BUT IT’S CONDUCTIVITY REDUCES 15% .
 BUT COSTLY , THAT’S Y RARELY USED .
4) ALL ALUMINUM CONDUCTOR ( AAC ) : ( 61% )
 ONLY ALUMINUM IS USED .
 HAS LOW CONDUCTIVITY AS COMPARED TO “CU” .
 LOW TENSILE STRENGTH BECAUSE OF LIGHT IN WEIGHT .
 CHEAP
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7. 5) Steel core Aluminum Conductor ( SCAC )
OR
Aluminum Conductor Steel Reignforced (ACSR )
 To increase tensile strength , Steel is reignforced in Aluminum conductors .
 Diameter increases & Corona loss is reduced as compare to Cu .
 Weight = 75% of Cu .
 Suitable for Transmission Lines .
6) All Aluminum Alloy
Conductor ( AAAC ) :
 Mg + Si with Al Conductor .
Provides better tensile strength
 but less conductivity .
 Suitable for populated urban areas
and populated industrial areas or central
areas .
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8. 02) Line Supports
• Supporting Structure of T/M Lines consists of Poles
& Towers .
• Function of Line Supports are :
i) Maintaining proper spacing between conductors .
ii) To provide min: ground clearance of conductor &
earth .
• Line Supports should have these properties .
i) High mechanical strength .
ii) Cheap in cost
iii) Longer Life
iv) Easy availability . 13-EL-42

9. SOME TYPES OF LINE SUPPORTS :
1) WOODEN POLES : ( SUITABLE FOR 11KV )
 CHEAPEST AMONG ALL LINE SUPPORTS .
 CAN BE USED FOR SHORT DISTANCE UPTO 50M T/M OR DISTRIBUTION .
 HIGHLY INSULATED BECAUSE OF WOOD . THAT’S WHY MOSTLY USED FOR
DISTRIBUTION .
 BUT HAVE SHORTER LIFE TIME & REQUIRES PERIODICAL INSPECTION .
 TWO TYPES OF WOODEN POLES ARE USED , SHOWN BELOW :
I) SINGLE POLE SHAPED II) DOUBLE POLES SHAPED
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10. 2) RCC Poles :
 Recently have become more popular
because of High Mechanical Strength .
 Have longer lives due to little deterioration .
 Require little maintenance
 Also have insulating property .
 But they are suitable for 11KV .
 High cost of transportation due to heavy weight .
. 3) Steel Poles :
 Have good mechanical srength .
 Possess longer life and can be used for longer
distance than wooden poles .
 Suitable usage is for High Voltage & Extra
High Voltage .
 Double Circuited Steel Poles provide
Continuity of Supply during
breakdown situation .
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11. 03) Overhead Line Insulators
• Overhead Insulators are used to isolate electrically
the conductors from towers .
• Also prevent leakage current from conductor to
cross arms .
• An Insulator should have properties :
i) High Electrical Resistance .
ii) Non-porous .
• Electrical Failure of Insulators occurs due to :
i) Puncture
ii) Flash over
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12. Types Of Line Insulators :
01) Pin Type Insulators :
• Earliest developed overhead insulator .
• used in power network up to 33KV system.
• In 11 KV system we generally use one part type insulator .
• Beyond 33 KV , Pin Type Insulators become bulky
& uneconomical .
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13. 02) Suspension type Insulators :
• Consists of number of Porcelain discs , connected
in series to form a string .
• Used for Voltage > 33 KV .
• Each disc is designed for low voltage ,
say 11KV .
• The number of discs used depend
upon working voltage .
Suppose for 66 KV working voltage ,
6 discs in series will be provided on string .
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14. 03) Strain Type Insulators :
• Consists of assembly of suspension type insulators .
• Used When there is a dead end or there is a sharp corner in
transmission line .
• Used for higher transmission .
• Used in parallel arrangement .
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15. 04) Shackle Type Insulators :
• Similar to strain type insulators , but used for low
voltage .
• 1st these type of insulators were used as Strain
type. Later on these were only used for distribution.
• These are single Porcelain material , mounted
horizontally or vertically .
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16. 04) Cross-Arms
• Cross arms of transmission tower hold the
transmission conductor.
• The dimension of cross arm depends on the level of
transmission voltage, configuration and minimum
forming angle for stress distribution.
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17. Types of Cross-Arms:
01) Line Arms :
• Straight line form .
• Mounted equally weight distributed to poles .
• Used for Short Transmission but Mostly used for
Long Transmission ( Double Circuited Transmission System ) .
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18. 02) Side Arms :
• Also called Braced Horizontal arms .
• Three single arms are used on poles ,
fitted alternatively .
• Used for Single Circuit Transmission .
• Mostly used for Short Transmission .13-EL-42

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