OVERHEAD LINE INSULATORS

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OVERHEAD LINE INSULATORS
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• ?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.

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Cross arm
Insulator
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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.

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Insulator Materials

• 1- Toughened Glass?????? ???????
• ? 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.

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Single unit Glass Insulator
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• 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

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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)

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Types of Insulators

• 1- Pin Type Insulators ( (??????? ?????????
• ? 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.

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Double- unit pin insulator 33 kV
Single- unit pin insulator 11 kV
Metal pin
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• 2- Suspension Insulators ( ????? ???????)
• ? 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

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• 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

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Insulator disc (unit)
Insulator string
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• 3- Strain Insulators ( ????? ???????)
• ?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.

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• 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.

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Arcing horn
Strain Insulator with arcing horn
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Voltage Distribution over Insulator
Ground or Tower
Conductor
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• 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 (mlt1).
• V1, V2, V3 the voltage across each unit starting
  from the cross arm towards the power conductor.
• V V1 V2 V3 Line voltage

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• At point A
• I2 I1 i1
• ?C.V2 ?C.V1 ?mC.V1
• V2 (1m).V1
• At Point B
• I3 I2 i2
• ?C.V3 ?mC.(V1V2) ?C.V2
• V3m.V1 (1m).V2 (m (1m)2).V1
• V3 (13m m2).V1

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• For m lt 1 V3 gt V2 gt V1
• Insulator Efficiency ? (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).

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• In general, voltage across the units is given by
• Vn1 Vn.(1m) (V1V2 Vn-1).m
• n 1 V2 (1m). V1
• n2 V3 V2.(1m) V1.m
• n3 V4 V3.(1m) (V1 V2).m
• n4 V5 V4.(1m) (V1 V2 V3).m

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• The voltage across the units cam also given by
• n 1 V2 (1 m). V1
• n2 V3 (1 3 m m2). V1
• n3 V4 V3.(1 6 m 5 m2 m3). V1
• n4 V5 V4.(1 10 m 15 m2 7m3 m4 ). V1

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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.

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• V2 (1m). V1 1.1 V1
• V3 V2.(1m) V1.m 1.31 V1
• V4 V3.(1m) (V1 V2).m 1.651 V1
• V1 V2 V3 V4 38.1 kV
• V1 (11.1 1.311.651)38.1
• V17.53 kV, V2 8.28 kV, V39.86 kV, V4 12.43 kV
• String efficiency (38.1/4x12.43)x10076.6

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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.

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• V3 17 kV, m20 0.2
• V2 (1m).V1 1.2 V1
• V3 V1.m V2.(1m) 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)x10078.03

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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.

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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.

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• 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.

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Improvement of String Efficiency

• 3- Static Shielding (Guard Ring) ( ???? ???????)
• 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.

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• Guard ring serves two purposes
• Equalizing the voltage drop across each insulator
  unit.
• protects the insulator against flash over.

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Guard ring
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• At point A
• I1i1
• 3V.?C1 ?mC.V C1mC/3
• At point B
• I2i2
• 2V ?C22V. ?mC C2mC
• At point D
• I3 i3
• ?C3.V3V. ?mC C3 3mC

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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.

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x
y
z
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• At point A
• I1 Iy Ix i1
• 0.1?C.(V2V3) ?C.V2 ?C.V1 0.25 ?C.V1
• 1.25 V1 -1.1 V2 -0.1 V3 0.0
• At point B
• I2 Iz i2 Iy
• 0.1?C.V3 ?C.V3 0.25?C.(V1V2) ?C.V2
• 0.25 V1 1.25 V2 -1.1V3 0.0
• Also V1 V2 V3 V

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• Solve, to get
• V1 0.295V, V2 0.2985V, V3 0.406V
• ? V/(3x0.406V)x100 82.1
• Find the voltage distribution and insulator
  efficiency without a guard ring.

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More Images for illustration
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