EE 301: Experiment No: 3

SUSPENSION INSULATOR STRING

Instructed by: Mr. W. D. Prasad

Name : R. T. LiyanageIndex No : 040218KGroup : EE-1Course : B.Sc. EngineeringField : Electrical EngineeringPerformed on : 22/05/2007Submitted on : 05/06/2007

EXPREIMENT: Suspension Insulator String

OBJECTIVES: To study the voltage distribution of suspension insulation by constructing models.(a) With identical insulator units(b) With Graded Units(c) With identical units & grading ring

APPARATUS: 1. capacitors2. Variac3. Digital Voltmeter

THEORY:

Potential Distribution over a String of suspension Insulators:

(a) String with identical insulator units:

Fig 1

Let,

m = Capacitance per Insulator = C′ Capacitance per Ground C

Let V be the operating voltage (line to ground) V= V1 + V2 + V3 + V4

From the Fig 1 I2 = I1 + Ic1

V 2 = V 1 + V 1

(1/ωmC) (1/ωmC) (1/ωC)

ωmCV2 = ωmCV1 + ωCV1 (ω angular frequency)

V2 = V1 (1 + 1/ m)

V

V1

V2

V3

V4

C

C

C

C′

C′

C′

C′

I1

I2

I3

Ic1

Ic2

Ic3

I4

Also, I3 = I2 + Ic2

ωmCV3 = ωmCV2 + ωC (V1+V2)V3 = V2 (1+1/m) + V1 / m

Substituting V2 by V1 and mV3 = V1 (1 + 1/ m) 2 + V1 / mV3 = V1 (1 + 3/ m + 1/m2)

Similarly V4 can be expressed as as,V4 = V1 (1 + 6/m + 5/m2 + 1/m3)

Normally m>1

V1<V2<V3<V4

Thus the lowermost unit is full stressed or utilized. As m increases the division of voltage becomes more equalized. “String Efficiency” is a measure of utilization of material in the string and is defined as,

String Efficiency = Voltage across string n * Voltage across unit adjacent to line

Where, n is the number of insulator units in the stringWhen n=4:

String Efficiency = (V1+V2+V3+V4)4V4

(b) String with Graded Units:Since the voltage for a given current is inversely proportional to the capacitance, the unit

nearest to the cross arm should have the maximum capacitance in order to reduce the maximum voltage across it. Further as we move to the power conductor the unit capacitance should be progressively increased. By correct grading of capacitances, completely equality of the voltages can be achieved and this is called “Capacitive Grading”.

100V

V

V

V

V

C

C

C

C1

C2

C3

C4

I1

I2

I3

Ic1

Ic2

Ic3

I4

Fig 2

From Fig 2, I2 = I1 + Ic1 = ωV(C1 + C) = ωVC2

Therefore, C2 = C1 + CVoltage producing Ic2 is 2V. Ic2 = 2 ωCVNow I3 = I2 + Ic2

ωVC3 = ωV(C1 + C) + 2ωC VC3 = 3C + C1 = C1+ (1+2) C

Thus we can write capacitance for the nth unit as;Cn = C1+ (1+2 + 3…………….. + [n-1]) C

(C) String with identical units and Graded ring:The voltage distribution is equalized in this method by providing grading, or guard ring

in the form of a large metal ring surrounding the bottom unit and connected to the metal work at the bottom of this unit, and therefore to the line. The arrangement is shown below.

Denoting the capacitances to the shield by x, y, z….. We can write

I2 = I1 + Ic1 – Ix

I3 = I2 + Ic2 – Iy etc.

But if the voltage is V across each unit and all n units are identical, the currents I1, I2, I3…. etc must be equal.Hence,

Ix = Ic1, Iy = Ic2, Iz = Ic3 etc.

VCω = (n-1) Vxω

100V

z

V

V

V

V

C

C

C

C′

C′

I1

I2

I3

Ic1

Ic

2

Ic

3

(n-1)V

(n-2)V

(n-3)V

xC′

C′I4

y

Ix

Iy

IZ

Fig 3

2VCω = (n-2) Vyω 3VCω = (n-3) Vzω

Which yieldx = C / (n-1)y = 2C / (n-2)z = 3C / (n-3)

In general, capacitance from the shield to the pth link from the top,Cp= pC / (n-p)

CALCULATIONS:(a) Voltage Distributions

m = C′ = 6 = 6 C 1

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)V3 = 1.53 V1

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

V= V1 + V2 + V3 + V4

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

5.84 V1 = 100

Theoretical Practical V1 = 17.12 V 17.15 V V2 = 20.03 V 19.74 V V3 = 26.19 V 25.97 V V4 = 36.64 V 36.48 V

Theoretical String Efficiency = (V1+V2+V3+V4) 4V4

= (17.12 + 20.03 + 26.19 + 36.64) 4 * 36.64

= 0.68

Practical String Efficiency = (V1+V2+V3+V4) 4V4

= (17.15 + 19.74 + 25.97 + 36.48) 4 * 36.48

= 0.68

(b) C1 = 6 μF C = 1 μF

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 =100 / 4 =25 V

Theoretical String Efficiency = (V1+V2+V3+V4) 4V4

= (25 + 25+ 25+ 25) 4 * 25

= 1.00

(c) C1 = 6 μF C = 1 μF n= 4x = C / (n-1) = 6 / 3 = 2y = 2C / (n-2) = 2*6 / 2 = 6z = 3C / (n-3) = 3*6 / 1 = 18

Theoretical voltage distribution =100 / 4 = 25 V

Theoretical Practical V1 = 25.00 V 28.00 V V2 = 25.00 V 23.40 V V3 = 25.00 V 24.10 V V4 = 25.00 V 24.10 V

Theoretical String Efficiency = (V1+V2+V3+V4) 4V4

= (25 + 25+ 25+ 25) 4 * 25

= 1.00

Practical String Efficiency = (V1+V2+V3+V4) 4V4

= (28.00 + 23.40 + 24.10 + 24.10) 4 * 24.10

= 1.03

DISCUSSION:

Electrical insulation is a material or object which contains no free electrons to permit the flow of electricity. When a voltage is placed across an insulator, no charge or current flows. An object intended to support or separate electrical conductors without passing current through itself is called an insulator.The term electrical insulation has the same meaning as the term dielectric.

Some materials such as silicon dioxide or teflon are very good electrical insulators. A much larger class of materials, for example rubber-like polymers and most plastics are still "good enough" to insulate electrical wiring and cables even though they may have lower bulk resistivity. These materials can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands, of volts).

High-voltage insulators

- High-voltage insulators used for high-voltage power transmission 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. - Glass insulators were (and in some places still are) used to suspend electrical power lines. Some insulator manufacturers stopped making glass insulators in the late 1960s, switching to various ceramic and, more recently, composite materials.- Recently, some electric utilities have begun converting to polymer composite materials for some types of insulators which consist of a central rod made of fibre reinforced plastic and an outer weathershed made of silicone rubber or EPDM. Composite insulators are less costly, lighter in weight, and have excellent hydrophobic capability. This combination makes them ideal for service in polluted areas. However, these materials do not yet have the long-term proven service life of glass and porcelain.

The insulation of an overhead line is achieved by providing suitable "air gaps" between the live conductors and any item of each potential, and also suitable "air gaps" between the individual phases. Man-made insulators (the insulation strings) are still needed to insulate the conductors from the supports.

Two kinds of insulation are used to insulate the conductors from the supports: suspension isolation and anchoring isolation

Suspension insulation  

Insulators (the vertical string of discs) on a 275 kV suspension pylon

   Between the two anchoring towers of a wholesection, the attachment of conductors to the intermediate suspension towers is achieved by the suspension insulation.

There are generally two types of insulators: 1) I-shape (The left and the right phases on the figure) 2) V-shape (The middle phase on the figure)

Anchoring insulation    The anchoring insulation is designed for the dead span section towers or the anchoring towers of a multi-span section. The connection to the anchoring towers plays a very important role in the movement of overhead lines 

These two types of insulation strings are made of several sub-elements fitted one into each other such as the shown on the next figures. On the right figure, you can see what happens when a short-circuit between one phase and the ground occurs at the location of the insulator string: the current, along the red lines traced on the two left figures, twists the insulator string.

Reasons for the differences between experimental and theoretical results

1) Human errors when taking the readings

2) Errors involved in the measuring equipments

3) Capacitances used are not ideal

4) Resistances of the connection wires