OVERHEAD LINE INSULATORS

●The insulators for overhead lines provide

insulation to the power conductor from

ground.

●The insulators are connected to the cross

arm of the tower and the power conductor

passes through the clamp of the insulator.

Cross arm

Insulator

Characteristics of Solid Insulators

1. High Mechanical strength.

2. High Electric strength.

3. High insulation resistance.

4. Free from impurities and moisture.

5. Air and gas free (decrease the dielectric strength)

6. Withstand the flashover phenomenon.

Insulator Materials1- Toughened Glass المقّس�ى الزجاج ● Glass is cheaper than porcelain ● Electric Strength is 140 kV/cm ● It has lower coefficient of thermal expansion

which reduces the strains due to temperature changes

● Moisture condenses on the surface increases the leakage current

● Glass insulators are used up to 33 kV lines.

Single –unit Glass Insulator

2- Porcelain

● Mechanical strength is higher than glass

● Low leakage current

● Less effected by temperature

● Electric strength is 60 kV/cm

● Used with any number of units to increase the

insulation level

Insulation Failure

• High Mechanical stress on insulator.• Defects in insulator material (air, gases,

impurities)• Flashover of insulator (due to over voltages): The flashover voltage is the voltage which will

cause an arc through the air surrounding the insulator. The arc heat can damage the insulator ( the insulators are fitted with arcing horns to keep the arc away from the insulator).

● Faults (short circuits)

Types of Insulators

1- Pin Type Insulators ( ( المّسمارية العوازل

● Pin type insulator consists of a single or

multiple units.

● They are used only up to 33 kV.

● For higher voltages the pin type insulators

are very heavy and more costly.

Single- unit pin insulator 11 kV

Double- unit pin insulator 33 kV

Metal pin

2- Suspension Insulators ( التعليق :(عوازل ● Suspension insulator consists of porcelain

disc units mounted one above each other. ● Each disc is provided with a metal cap at the

top and a metal pin under. ● a string of any number of units can be built

according to the line operating voltage . ● The conductor is suspended below the point

of support by means of insulator string

• The number of discs in a string depends on the

line voltage and the atmospheric conditions

(degree of pollution).

• The usual number of discs are:

Voltage (kV) : 66 132 220 400

Number of discs: 4-5 9-10 15-16 22-23

Insulator disc (unit) Insulator string

3- Strain Insulators ( األجهاد (عوازل

●These are special mechanical strong suspension insulators.

● They are used to take the tension of the conductors at the line terminals, at angle towers, and at road crossings.

● The strings are placed in horizontal plane.● Two or three strings of insulators in parallel

can be used when the tension in conductors is very high.

Arcing horns are used to protect insulators on high voltage Lines from damage during flashover. Over voltages on transmission lines, due to switching operations, lightning, or electrical faults can cause arcs across insulators (flashovers) that can damage them. The horns makes the flashover to occur between themselves rather than across the insulator surface. Horns are normally paired on either side of the insulator, one connected to the line and the other to ground.

Strain Insulator with arcing horn

Arcing horn

Voltage Distribution over Insulator

Ground or Tower

Conductor

Cs : capacitance of each insulator unit.

Ce = m Cs : capacitance to ground

is the capacitance of metal part of the insulator

unit to the tower (m<1).

V1, V2, V3 the voltage across each unit starting from

the cross arm towards the power conductor.

V = V1 + V2 +V3 Line voltage

At point A: I2 = I1 +i1

ωC.V2 = ωC.V1 + ωmC.V1

V2 = (1+m).V1

At Point B: I3 = I2 + i2

ωC.V3 = ωmC.(V1+V2) + ωC.V2

V3=m.V1 +(1+m).V2 =(m +(1+m)2).V1

V3 = (1+3m +m2).V1

For m < 1 V3 > V2 > V1

Insulator Efficiency: η = (V/n.Vmax) x 100

V: Voltage across the insulator string, (phase Volt)

n: number of insulator units.

Vmax: Voltage across the insulator unit near to the

power line (for n = 3, Vmax = V3).

In general, voltage across the units is given by:

Vn+1 = Vn.(1+m) + (V1+V2+……+ Vn-1).m

n= 1 V2 = (1+m). V1

n=2 V3 = V2.(1+m) + V1.m

n=3 V4 = V3.(1+m) + (V1 +V2).m

n=4 V5= V4.(1+m) + (V1 + V2 +V3).m

The voltage across the units cam also given by:

n= 1 V2 = (1+ m). V1

n=2 V3 = (1+ 3 m +m2). V1

n=3 V4 = V3.(1+ 6 m +5 m2 + m3). V1

n=4 V5= V4.(1+ 10 m + 15 m2 + 7m3 + m4 ). V1

Example

An insulator string for 66 kV line has 4 units.

The capacitance to ground is 10% of the

capacitance of each insulator unit. Find the

voltage across each insulator unit and string

efficiency.

V2 = (1+m). V1 = 1.1 V1

V3 = V2.(1+m) + V1.m = 1.31 V1

V4 = V3.(1+m) + (V1 +V2).m =1.651 V1

V1 +V2 +V3 +V4 = = 38.1 kV

V1 (1+1.1 +1.31+1.651)=38.1

V1=7.53 kV, V2= 8.28 kV, V3=9.86 kV, V4= 12.43 kV

String efficiency = (38.1/4x12.43)x100=76.6%

3

66

Example

Find the voltage distribution of an insulator of

3 units, if the maximum voltage of each unit

is 17 kV, and the capacitance to ground is

20% of unit capacitance, also find the

insulator efficiency.

V3 = 17 kV, m=20% = 0.2

V2 = (1+m).V1 = 1.2 V1

V3 = V1.m+ V2.(1+m) =1.64 V1

V1 = 17/1.64 = 10.36 kV

V2 = 1.2x10.36 = 12.43 kV

V= V1 + V2 + V3 = 39.8 kV

Insulator efficiency = (39.8/3x17)x100=78.03%

Improvement of String EfficiencyMethods of Equalizing Potential (p.386)

1- Reducing the ground capacitance relative to the

capacitance of insulator unit (reduce m where m = ce/cs):

This can be done by increasing the length of cross arm and

hence taller supporting tower which uneconomical.

Improvement of String Efficiency

2- Grading of insulator units: It can be seen that the unequal

distribution of voltage is due to the leakage current from the

insulator pin to the tower structure. The solution is to use

insulator units with different capacitances.

This requires that unit nearest the cross arm should have

minimum capacitance (maximum Xc) and the capacitance

should increase as we go towards the power line.

This means that in order to carry out unit grading,

units of different types are required. This requires

large stocks of different units which is uneconomical

and impractical.

3- Static Shielding (Guard Ring): ( الحماية (حلقة

This method uses a large metal ring surrounding the bottom

insulator unit and connected to the line. This ring is called a

grading or guard ring which gives a capacitance which will

cancel the charging current of ground capacitance.

Improvement of String Efficiency

Guard ring serves two purposes:- Equalizing the voltage drop across each insulator

unit.- protects the insulator against flash over.

Guard ring

L

At point A:I1=i1

3V.ωC1= ωmC.V C1=mC/3

At point B:I2=i2

2V ωC2=2V. ωmC C2=mC

At point D:I3 =i3

ωC3.V=3V. ωmC C3= 3mC

Example

A 3-unit insulator string with guard ring. The

capacitance to ground and to guard ring are

25 % and 10 % of the capacitance of each

unit. Determine the voltage distribution and

string efficiency.

x

z

y

At point A:I1 + Iy = Ix + i1

0.1ωC.(V2+V3) + ωC.V2= ωC.V1 + 0.25 ωC.V1

1.25 V1 -1.1 V2 -0.1 V3 =0.0

At point B:I2 + Iz = i2 +Iy

0.1ωC.V3 + ωC.V3= 0.25ωC.(V1+V2) + ωC.V2

0.25 V1 + 1.25 V2 -1.1V3 =0.0

Also: V1 + V2 + V3 = V

Solve, to get:

V1= 0.295V, V2 = 0.2985V, V3 = 0.406V

η = V/(3x0.406V)x100= 82.1 %

Find the voltage distribution and insulator

efficiency without a guard ring.

More Images for illustration