Transmission lines

Large amount of power generated in generating plants are to be carried to load centers for distribution. This is done by transmission lines. There are basically two modes by which power can be transmitted. They are: 

 1. Overhead lines 

 2. Underground cables 

1. Overhead lines: 

Most of the transmission lines in our context are overhead lines. They are the cheapest and less problematic as far as insulation is considered. The lines can have the voltage level as high as 1000 kv as per the technology permits now. (Voltage levels of 1200 kv and 1500 kv are in the research phase for overhead transmission lines). The problems with these lines are that they are exposed to natural disasters like storms, lightning, heavy rain and snowfall, etc. which creates faults on the lines and severe damage may occur. However in case of Nepal such calamities in small number and are less disastrous and the development of safety measures and protective schemes have prevented the line from heavy damage and ensured the reliability of the systems. 

2. Underground cables: 

In very densely populated areas where overhead lines will be too risky for the safety of life and property, underground cables are used for the transmission and distribution purpose. Because of the cost and the insulation problems they are used only in places where overhead lines could not be installed, viz; transmission across lake, wide river etc. The advantage of these cables is that the aesthetic view of the area is not spoiled, and because they are not exposed to natural disasters, systems reliability is increased. The main disadvantage of this system is the high cost and the insulation problems. In Nepal the only place where underground cables are used is the Singh Durbar area for the distribution purpose.

For the most economic power transmission the two parameters chosen are: the economic transmission voltage and the most economic size of the conductors. The factors to be considered are the line loss, voltage drop and the cost of the conductors used for transmission. All of these decrease with the increase of line voltage. But increase of voltage also increases the insulation cost and other unwanted effects of high voltage which will again increase the cost of the lines. Therefore there is a compromise, and the voltage called the optimum voltage for transmission of a certain amount of power over a certain distance exists and is calculated by considering above factors.                                  

                                                                                                            And the economic size of the conductors for transmission is given by the Kelvin’s law, which states that the most economic size of the conductors is the one which makes the annual cost of the energy wasted in the line equal to annual cost of interest and depreciation on the line.         

Here is the table showing the optimal transmission voltages in terms of power and distance for overhead lines.

Optimum line voltage	Power in MW, which can be transmitted (5% regulation and 90% power factor)
	Distance in miles
in kv	5	10	15	20	30	40	60	80	120	160	240	320
11	12	6	4	3	-	-	-	-	-	-	-	-
33	-	54	36	27	18	13	9	-	-	-	-	-
66	-	-	-	108	72	54	36	27	-	-	-	-
132	-	-	-	-	-	216	144	108	72	54	-	-
220	-	-	-	-	-	-	405	288	216	144	108	72

In our context, almost everywhere overhead lines are used for the transmission purpose. Underground cables are rarely used. So, here is the discussion on the basic components of the overhead lines only.                     An overhead line comprises basically of following components:                                                           
I) Conductors                                                                                           II) Insulators                                                                                           III) Supports                                                                                                                                 

I) Conductors: 

Conductors are the one which conduct electricity and therefore they should have following characteristics for efficient transmission of power:                   a) High electrical conductivity                                                                     b) High tensile strength                                                                             c) Low density i.e. Lighter                                                                       d) Low cost                                                                                                                 

Generally copper and aluminum are used as conductors for the transmission purpose.

                                            

► Copper: 

It has following properties:                                                                       > Highest conductivity                                                                             > Maximum current density value                                                  
> High density                                                                    
> High cost                                                                                                                

► Aluminum:

It has following properties:                                                                       > Conductivity and Current density less than that of copper.                         > Lighter than the copper.                                                                         > Cheaper                                                                                               > Tensile strength is much lesser compared to copper. And this is increased by using steel as the central core. This form of conductor is called Aluminum conductor steel reinforced (ACSR).

II) Insulators: 

The live conductors are insulated from line supports i.e. poles or towers by insulators. Basically the insulators are of following forms:                             a) Shackle insulators                                                                                 b) Pin insulators                                                                                       c) Suspension or Disc insulators                                                                                           

a) Shackle insulators:                                                                              > Also known as spool insulator.                                                               > Used for low voltage distribution lines of voltage range 400/230 V.             > Fixed on the poles by means of nuts and bolts.                                                                                                                                                

b) Pin insulators:                                                                                
> Used for the voltage range of 11 kv to 33 kv.

> Consists of steel pin screwed in at the centre of two or three shreds.           > Shreds are of porcelain for insulation of conductors.

> Numbers of shreds increases with the voltage.                                                                                                                                                     c) Suspension insulators:                                                                        > Pin insulators become complicated and heavy for high voltages.                   > Hence, Suspension type insulators are used for voltage exceeding 33 kv.   
> Consists of string of interlinking number of disc made of glass or porcelain.   > Number of disc depends upon the line voltage. e.g. For 66 kv the number of discs is 5.                                  

III) Supports: The line supports are either poles or towers. The overhead transmission lines are held to the insulators which are themselves supported by the poles or towers. The various types of line supports in practice are:                                                                                                         

a) Wooden Poles:                                                                                  > Used for low voltage distribution lines (400/230V).                                   > And span range for these poles are between 40 – 50 meters.                                           

b) Steel tubular poles:                                                                                                              > Used for higher voltages up to 33kv.                                                     
> Usual span for these poles are 50 – 80 meters.                                         > Must be earthed for safety purpose.                                                                                                 

c) R.C.C Poles:                                                                                        > Mechanically very strong and have longer life.                                           > Used for higher voltages up to 33 kv.                                                     > Span range used is between 80 – 100 meters.                                                                     

d) Lattice steel towers:                                                                          > Span range used is between 100 – 200 meters.                                         > Used for higher voltages of 33 kv.                                                                                       

e) Steel towers:                                                                                      > Used for long transmission lines and high voltages (66 kv and above).         > Usual span used for these towers are between 200 – 500 meters.               > Mechanically very strong and useful in crossing large valleys, rivers, mountains, etc.

S.no	Transmission lines	Length (km)	Circuit type	S.no	Transmission lines	Length (km)	Circuit type
i.        132 kv Transmission lines
1	Anarmani-Duhabi	85	Single	11	Hetauda-Gandak P/S	154	Single
2	Kusha-Katiya (India)	19	Single	12	Bharatpur – Pokhara	97	Single
3	Duhabi- Hetauda	282	Double	13	Bardaghat-Butwal	43	Double
4	Hetauda – KL2 P/S	8	Single	14	Butwal –KGA P/S	58	Double
5	Bharatpur-Marsyangdi P/S	25	Single	15	KGA P/S –Lekhnath	48	Single
6	Marsyangdi P/S –Suichatar	84	Single	16	Pokhara-Modikhola P/S	37	Single
7	Suichatar -KL2 P/S	34	Single	17	Butwal- Tanakpur P/S	407	Single
8	NewBhaktapur–  Lamosangu	48	Double	18	Pathalaiya-       NewParwanipur	17	Double
9	Suichatar –New Bhaktapur	26.9	Single	19	Marsyangdi-M. Marsyangdi	44	Single
10	Lamosangu – Khimti P/S	46	Single		Total	1562.9
ii.	66 kv Transmission line
1	Chilime P/S –Devighat P/S	43.56	Single	10	Teku – K3 (Underground )	3.5	Single
2	Trisuli P/S – Balaju	29	Double	11	Suichatar –K3	6.9	Single
3	Debighat P/S – Balaju	30	Single	12	New Patan –New Baneshwor	2.8	Single
4	Debighat P/S – New Chabel	33	Single	13	Bhaktapur – New Chabel	12	Single
5	Balaju-Laincahur	2.3	Single	14	New Baneshwor – Sunkoshi P/S	61	Single
6	Balaju –KL1 P/S	36	Double	15	Debighat Trisuli	4.56	Single
7	KL 1 P/S Birgunj	72	Double	16	Indrawati-Panchkhal	10	Single
8	Suichatar –Teku	4.1	Single
9	Suichatar –New Patan	4	Double		Total	354.72