Electric Transmission and Distribution : Concepts -- 2 ( sai saikumar jn)

Electric Transmission and Distribution : Concepts

Transmission line constants :

The transmission lines up to 50 kilometer length and voltages up to 33 kV are considered as short lines. Lines more than 50 kilometers in length and carrying voltages above 33 kV are called long transmission lines. If V_s and V_r be respectively the sending end and receiving end voltages (corresponding currents being 1_s and l_r) then

V_s = AV_r + BI_r and I_s = CV_r + DI_r

The values of constants A, B, C and D primarily depends on the R, L, C parameters of the line.

For short lines,

V_s = V_r + LZI_r
I_s = I_r
A = D =1 ,
B=Z

And

C=0,
AD-BC =1,
A=D

For medium lines (T-network) :

A = D = 1 + (YZ)/2
B = Z + YZ² /4
C= Y,
AD-BC=1,
A=D

( π network ) :

A= 1+ (YZ)/2 B + Z,
C = Y + Y²Z/4 ,
D= 1+ YZ / 2

Long lines :

A = cosh √(YZ) ,
B =√ (Z/Y) sinh √(YZ) ,
C = (Y/Z)^1/2 sinh (YZ)^1/2 ,
D = cosh (YZ)^1/2 ,
AD - BC = 1,
A = D

Transmission Line Voltage Regulation:

Voltage regulation = ( Sending end voltage - Receiving end voltage) / Receiving end voltage

The permissible variation in voltage at the consumer ends ± 10% with reference to nominal voltage. Following methods are used to control the voltage during transmission over long distances
(i) Use of tap changing transformers
(ii) Use of shunt capacitors at the receiving end during heavy loads or low power factors
(iii) Switching in shunt reactors during low loads to neutralize the effect of shunt capacitance of the long transmission lines.
(iv) Use of booster generators.

Skin effect in transmission lines:

When alternating current flows through a conductor it is always linked with flux. The flux linking the central portion of the con doctor is always more than that at the surface. This results in higher current density at the surface of the conductor as compared to that at the core of the conductor. This is known as skin effect. This has following effects.
(i) Skin effect causes increase in dc resistance of the effective dc resistance
(ii) Skin effect is high for conductors of magnetic materials
(iii) Skin effect is more predominant at higher frequencies
(iv) In case of dc, skin effect does not exist.

Proximity effect in transmission lines:

For a conductor carrying alternating current, the current density in the conductor depends on the internal flux distribution. When two ac current carrying conductors non parallel, the internal current distribution in both the conductors gets rearranged in such a way so that ac current density on the side nearer to the other conductor is lesser than that at the other end. The effect. is known as proximity effect. As a result of this effect the effective dc resistance of the conductors increases.

Corona :

Conductors carrying electric power at higher voltage have strong electrostatic field around them. This results in a violet visible discharge, called corona. It is accompanied by hissing sound, vibrations, power loss and radio interference.

The voltage between the conductors at which the surrounding air is ionized is called the break down voltage or disruptive voltage. Break down voltage depends on atmospheric conditions and also surface conditions of the conductor.

The electrical stress at the surface of a conductor is given by

E = V / r₀ log _e (S/ r₀)

where,
V = phase to earth voltage,
r₀ = radius of conductor
S = equivalent equilateral spacing between 3 phase conductors.

Factors affecting corona

1.Atmosphere:

There is more tendency during stormy weather.

2. Conductor size:

Rough and irregular surfaces give rise to more corona because unevenness of the surface decreases the value of breakdown voltage stranded conductors having rough surface have more tendency for corona.

3. Spacing between conductors:

Shorter distance between conductors increases tendency for corona.

4. Line voltage:

Higher voltages have increased chances of corona.

5. Insulators:

A insulator should have high mechanical and electrical strength and should be non-porous free from impurities and cracks and should have high ratio of puncture strength to flash over.

Pin type insulators are used up to 33 kV. Suspension type insulators are used for voltages higher than 33 kV. Strain insulators are installed at the dead end of the line or whenever mere is comer or sharp curve.

Transmission lines > String Efficiency:

A string of suspension insulators consists of a number of porcelain discs connected in series through metallic links. The figure shows 3 strings of suspension insulators. The porcelain portion of each disc is in between two metal links. Shunt capacitance exists between porcelain discs as shown. The string efficiency is defined as the ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor. Thus,

String efficiency = Voltage across the string / n x voltage across disc nearest to conductor

where n is the number of discs in the string.

(i) using longer cross-arms
(ii) grading the insulators
(iii) using a guard ring.

 from ur's -- Bellapuri saikumar
                          ( www.facebook.com/saikumar544)