Design of transmission line

STEP 1              
Selection of appropriate voltage level:
 The guiding factor for selection of voltage level of a transmission line are:
(a) Economical factors
(b) Technical requirements
(c) standard voltage levels
(d) Existing scenario. 


a) Economical factor:
For three phase Nc ckt transmission line: Pl =3*Nc*Il^2*Rc and P= sqrt(3)*Vll**Ill*cos(x)*Nc 
Rc=pl/A
therefore Pl/P=P/(Nc*V^2*cos^2(x))*pl/A=percentage power loss

For same percentage power loss: A=K* P.l/(V^2*Nc) =>this shows that the percentage power loss decreases by increasing no of circuit or by increasing the system voltage or by increasing the conductor cross section area. Conductor volume=3*Nc*A*l = K1*P*l^2 / V^2 => The conductor cost is independent of the no of ckt for same percentage power loss.

Veconomical=5.5[Lt/1.6+(P*1000)/Nc*cos(x)*150]^.5

The standard voltage level nearest of the Veconomical is chosen. Then it is checked for technical requirement fulfillment.

b)Technical requirement:

Plimit =sqrt(3)*V*Imax*cosx
Plimit=K*V^2/(x.l)
where V=system voltage
Imax=maximum current that the line can carry, depends upon thermal capability.
x = inductance per unit length
l = length of transmission line
If the power to be transmitted (P)< Plimit then the transmission line is said to satisfy the technical requirement.

It is checked by the following method:
surge impedance loading (SIL)= V^2 /Zc
Plimit/SIL = (KV^2/l)/(K1* V^2) = K2/l = m-factor
Zc=400 ohm for normal line, for double circuit Zc=400/2

length (km)                 m-factor limit
80                                   2.75
160                                 2.25
240                                 1.75
320                                 1.35
480                                 1.0
640                                 0.75
To calculate the Plimit, first we calculate SIL and then m-factor for the given length of the transmission line from the above table. If required value not found on the table use interpolation. Then Plimit= SIL*m-factor
If Plimit>P (power to be transmitte) => satisfied.. otherwise repeat with another voltage level or double ckt

c) Standard voltage is chosen because all the measurement instruments and the switchgear are available at standard voltage level.

d) Existing scenario also influence the selection of voltage level. If there is one line with 66 KV then if we need to connect this line with other station, we chose 66 kv and check for single or double ckt.

STEP 2
AIR CLEARANCE DESIGN:

It includes air clearance design and design of physical insulator at towers.  In air clearance design: insulation must withstand normal system voltage continuously and overvoltages for short times. The overvoltages for short times are:

i.              Temporary overvoltage – due to unsymmetrical fault, Ferranti effect (10% of system voltage up to 220 KV and 5% for voltage greater than 220 KV) and Ferro resonance.

ii.             Switching overvoltage

iii.            Lightning overvoltage

The factors affecting the air clearance between charged conductor and earth metallic structure of the tower:

# Voltage level – phase voltage, peak voltage

# maximum system voltage – highest system voltage at any point along the lines of all condition

Line configurations:

i.              Single circuit

ii.             Double circuit

iii.            Horizontal

iv.           Vertical

v.            Triangular

a= 1cm per KV and 20 cm margin=(Vll*√2*1.1/√3 )+ 20   cm

Cl = a(1+tan x )       

l=asec x

b=1.5 a

y= (l+a)/sqrt(1-(x/y)^2*(l+a)^2/Cl^2)

d=√3 *(Cl +b/2)

d=√3 *(Cl) for double circuit line

STEP3

INSULATOR DESIGN

 No of disc for different voltage level: bit complex because the voltage distribution for different discs along the string are not equal. Dielectric strength of disc varies from different voltage level. It should withstand for sudden abnormal voltage: to minimize the cost of insulator, minimization of overvoltage at lower level is required.

1.       External overvoltage -  lightning overvoltage

2.       Internal overvoltage – short term  and long term

i.                     Unsymmetrical fault: The phase voltage is given by Vph= K*Vll    the value of k varies between (1/√3) to 1. The higher the value of k => cost of insulator is high.  Lower the value of k => grounding cost is high. K=0.8 is taken for Nepal.

ii.                   Ferranti effect:  1.1*Vll up to 220 KV and 1.05* Vll for greater than 220 KV

iii.                  Vxc= Ix*Xc

Ix=V/(2Xtr-Xc)

iv.                 Switching overvoltage Vs/w=K1*E , E is the instantaneous voltage at the instant of switching. If line is assumed lossless and there is no trap charge K=2. If line is assumed lossless and there is –Vpeak trap charge, K=3. Since line has certain resistance, value of k decreases slightly. In practical case K=2.8 is chosen.

v.                   Lightning overvoltage: Lightning current=natural phenomena =>10KA from probability distribution of lightning current. Lightning voltage= depends upon system voltage and tower footing resistance.

Vlightning= I*Rt+Vll*√2*1.1/√3

                                         The standard voltage which an insulator must withstand known as basic insulation level (BIL) of that insulator.  Low voltage lines are very much prone to lightning whereas high voltage lines are dominated by switching over voltages. These facts are greatly considered for designing a transmission line.

Protective ratio (α) = BIL of L.A/ Temporary O/V≈3

Therefore for 11KV, BIL=3*.8*11*1.1

Switching surge ratio (SSR) = Maximum overvoltage/ system peak voltage

Therefore Vs/w(max)=SSR*Vpeak

                                                    KV                                SSR

                                                     220                                2.8

                                                     400                                2.4

                                                           765                                1.8

The insulation voltage is in increasing order for Lightning arrestor, transformer and Line insulator.