Laboratory Experiment 08

SUSPENSION INSULATOR STRING

INSRUCTED BY:-Miss. Priyanka Liyanarachchi

NAME: -V.I.P. Dasanayake

INDEX NO: -090075M

FIELD: -Electrical engineering

GROUP: - 3

Calculations

V

V1

V2

V3

V4

C

C

C

C′

C′

C′

C′

I1

I2

I3

Ic1

Ic2

Ic3

I4

(a) String with identical insulator units:

Let,

m=Capacitance per InsulatorCapacitance perGround

=C 'C

Let V be the operating voltage (line to ground)

∴ V = V1 + V2 + V3 + V4

Voltage Distributions

m=C'

C=6

1=6

V2 = V1 (1 + 1/ m) = V1 (1 + 1/ 6) = 1.17 V1

V3 = V1 (1 + 3/ m + 1/m2) = V1 (1 + 3/ 6 + 1/62) = 1.53 V1

V4 = V1 (1 + 6/m + 5/m2 + 1/m3) = V1 (1 + 6/6 + 5/62 + 1/63) = 2.14 V1

V= V1 + V2 + V3 + V4

100 = V1 + 1.17 V1 + 1.53 V1+ 2.14 V1

5.84 V1 = 100

V1 = 17.12 V

Voltage distribution

V1 V2 V3 V4

Theoreticalvalues

17.12 20.03 26.19 36.64

Practical values

17.29 19.71 25.9 37.15

Figure 1

100V

V

V

V

V

C

C

C

C1

C2

C3

C4

I1

I2

I3

Ic1

Ic2

Ic3

I4

Theoretical String Efficiency (V ¿¿1+V 2+V 3+V 4)

4V 4

¿

4V4

= 17.12+20.03+26.19+36.64 ¿ ¿4∗36.64

= 0.68

Practical String Efficiency = (V ¿¿1+V 2+V 3+V 4)

4V 4

¿

=(17.29+19.71+25.9+37.15)

4∗37.15

= 0.67

(b) String with Graded Units:

Figure 2

Given that C1 = 6 μF and C = 1 μFSo,C2 = C1 + C = 6 + 1= 7 μF

C3 = 3C + C1 =3*1 + 6= 9 μF

C4 = 6C + C1 =6*1 + 6= 12 μF

Theoretical voltage distribution across V1, V2, V3 &V4 capacitors =100

4=25 V

Voltage distribution

V1 V2 V3 V4

Theoreticalvalues

25.6 24.82 24.93 24.48

Practical values

25 25 25 25

∴Theoretical String Efficiency = (V ¿¿1+V 2+V 3+V 4)

4V 4

¿

¿25+25+25+25¿ ¿4∗25

= 1.00

100V

z

V

V

V

V

C

C

C

C′

C′

I1

I2

I3

Ic1

Ic2

Ic3

(n-1)V

(n-2)V

(n-3)V

xC′

C′I4

y

Ix

Iy

IZ

Practical String Efficiency = (V ¿¿1+V 2+V 3+V 4)

4V 4

¿

= (25.6+24.82+24.93+24.48)

4∗24.48

= 1.02

(C) String with identical units and Graded ring:

Figure 3

Given that C1 = 6 μF, C = 1 μF and n= 4

∴ x = C

(n−1) = 63 = 2

y = 2∗C

(n−2) = 2∗6

2 = 6

z = 3∗C

(n−3) = 3∗6

1 = 18

Theoretical voltage distribution across V1, V2, V3 &V4 capacitors =100

4=25 V

Voltage distribution V1 V2 V3 V4

Theoreticalvalues

24.78 24.00 24.99 26.40

Practical values

25 25 25 25

∴Theoretical String Efficiency = (V ¿¿1+V 2+V 3+V 4)

4V 4

¿

¿25+25+25+25¿ ¿4∗25

= 1.00

Practical String Efficiency = (V ¿¿1+V 2+V 3+V 4)

4V 4

¿=(24.78+24.00+24.99+26.40)4∗26.4

=0.95

DISCUSSION

(a) INSULATORS AND INSULATOR MATERIALS

Meaning of the terms electrical insulation and dielectric are very similar to each other that is a material or object which does not contain any free electrons to permit as a current. In other words when a voltage is placed across an insulator no current flows. Therefore we can put an insulator to separate electrical conductors by each other without passing any current. These objects are called insulators. There are several types of insulator such as pin type, suspension type, strain type and shackle type, Post type insulators, Spool insulators

. Some of the well known insulator materials are silicon dioxide and Teflon. Although some of the insulators like rubber-like polymers and most plastics are good enough to insulation purposes. Normally these two types are used to electrical wirings and cables. Because those have lower bulk resistively.

If we consider about the number of insulator plates and amount of voltage of the supply. There is a relationship between above two factors. The number of plates is equal to the [amount of voltage in kilo volts /11] or [(amount of voltage in kilo volts /11)-1]. It depends on the several factors. The insulators are designed as plate shape due to the various technical reasons. A single disc-shaped piece of porcelain, grooved on the under-surface to increase the surface leakage path, a metal cap at the top and to a metal pin underneath. The cap is recessed so as to take the pin of another unit, and in this way a string of any required number of units can be built up. The cap is secured to the insulator by means of cement. And also it is designed to hang the above plate. Those plates are made from glass, porcelain, or composite polymer materials. Porcelain insulators are made from clay, quartz or alumina and feldspar, and are covered with a smooth glaze to shed dirt. Insulators made from porcelain rich in alumina are used where high mechanical strength is a criterion. Porcelain is the most frequently used material for insulators because it works as a good insulator and it has strength to bare the wire tension. Porcelain insulators has smooth and shiny surface which help in shedding the rain water. The cost for the porcelain is high so sometime glass is also used because it is cheaper than the porcelain but glass insulator cannot use for voltages higher than 11kV

Types of insulators

Pin-Type Insulators: -The pin-type insulator is secured to a cross-arm on the transmission pole and attached to a steel bolt or pin. Two typical porcelain pin-type insulators are illustrated in the following picture. The usual working voltage refers to insulators for use in an industrial atmosphere in this country. Smaller insulators are used in clean conditions. These differ in construction in that they consist of two or three pieces of porcelain cemented together. These pieces form what are called petticoats. They are designed to shed rain and sleet easily.

Strain Insulators: -The strain insulator is a variation of the suspension insulator and is designed to sustain extraordinary pulls. On a transmission line, this strain insulator often consists of an assembly of suspension insulators. Because of its peculiarly important job, a strain insulator must have considerable strength as well as the necessary electrical properties. Although strain insulators come in many different sizes, they all share the same principle; that is, they are constructed so that the cable will compress the porcelain

Post type insulators: - These are similar to pin type insulators and they are generally used for higher voltage applications with the height and number of petticoats being greater for the higher voltages. This type of insulators can be mounted horizontally or vertically. The insulator is made of one piece of porcelain and its mounting bolt or bracket is an integral part of the insulator.

Spool insulators: - these are usually used for low voltage distribution. The spool insulator may be mounted on a secondary rack or in a service clamp. Use in mounting house service wires.

Suspension type insulators: - This type of insulators are normally use in high voltage transmission. The insulators are hanged in the cross arm and the transmission conductors are mounted lower end of the insulator. The entire unit of suspension insulators is called a string. It is important to note that in a string of suspension insulators one or more insulators can be replaced without replacing the whole string. Each insulator is a large disc shape piece of porcelain grooved on the undersurface to increase the surface leakage path between the metal cap at the top and the metal pin at the bottom of the insulator. The cap at the top is increased so that it can take the pin of another unit and in this way a string of any required number of insulators can be built. There are several types of suspension insulators a one of the type is shown on the following picture

(b) Comparing suspension insulator with a others

Suspension Insulators

The cost of pin-type insulator increases very rapidly when the working voltage is increased. So these types of insulators are uneconomical. And also the replacements are expensive. Due to the above mentioned reasons suspension insulators are used for high voltage lines. There are several types of suspension insulators such as I-shape suspension insulators and V-shape suspension insulators

ADVANTAGES:-

If there any failure of an insulator then no need to replace whole string. Only particular unit has to be replaced. .

Because the line is suspended flexibly, the mechanical stresses are reduced. But we use pin-type insulators, the rigid nature of the attachment results in fatigue and ultimate brittleness of the wire. On the other hand the string is free to swing.

In the situations like an increase in the operating voltage of the line. In suspension type it can be met by adding the requisite number of units to each string without replacing all insulators with pin type.

Each insulator is designed for a comparatively low working voltage about 11kV. Then the insulation for any required line voltage can be obtained with using a string of a suitable number of suspension insulators.

DISADVANTAGES

The suspension insulator suspends conductors below the cross-arm. So the suspension type required a higher tower to get the same conductor height above the ground than the towers which use pin or post types insulators. So cost for build the tower for suspension insulator string would be high.

Because of the free suspension, the amplitude of swing of the conductors may be large compared with that on a pin-type insulated line so the spacing should be increased.

Another disadvantage is that this construction associated with high electrostatic stresses in the porcelain immediately between the links, so that the liability to puncture is greater than with other types

Suspension insulator strings are more expensive when compared to pin and post types.

(c) Advantages of The Insulator Grading Methods

For very high voltage lines the insulator grading method is very economical. This is very easy and very simple method which uses in very high voltage transmission systems.

String with identical unit and graded ring method is very simple method. It can be easily taken the same voltage distribution throughout the string accurately than string with graded units method. Since there is an equal voltage distribution throughout the insulators in the string, no insulator will be over stressed.

(d) Reasons for differences between practical and theoretical values

1) The capacitors which we used aren’t ideal so they may not be represented the actual values.2) The components which we used for this practical can be worked with errors.

3) Human errors could be occurred when taking the readings4) Errors could be occurred when reading from the meters with naked eyes.5) Connecting Wire resistance wasn’t included in calculations6) Errors caused due to internal resistance of the meters.7) Internal capacitive components of used equipments may have affected to the readings.