insulator

1. Insulators
and
HV overhead lines
Condition monitoring

2. Insulators

3. Three main forms
• Gaseous – vacuum, nitrogen, argon, SF6
• Liquid – mineral and vegetable oils
• Solid – glass, ceramic, paper, mica

4. Gaseous insulation
• Purpose – to absorb energy generated under highly stress
• Gases absorb radiation and electrons at specific energies
• Reasons for ionisation of gas
• Electric field acceleration causing collision of charged atoms or
electrons and neutral atoms
• Photo-ionisation
• Interaction of metastable atoms and neutral species
• Thermal ionisation
• Recombination processes
• Cathodic photoelectric emission
• Thermionic emission
• Field emission
• More specific information available in Kuffel

5. Gaseous insulation (2)
• Electron affinity of an
element is dependent
upon its electron shell
construction
• H » H- -72 kJ/mole
• O » O- -135 kJ/mole
• F » F- -330 kJ/mole

6. Gaseous insulation (3)
• Townsend and Paschen developed
characteristic information on breakdown of
gases under high electric fields

7. Charge distribution in gas
breakdown

8. Liquid insulation
• Electronic breakdown
• Stress enhancement from suspended
particulates
• Cavity breakdown
• Electro-convection and electro-
hydrodynamic models
• Static electrification

9. Liquid insulation (2)
• Electronic breakdown – field emission from
cathodic electrode surfaces inject electrons into
the liquid, ionisation from collision with electrical
field accelerated electrons cause breakdown
• Stress enhancement from suspended
particulates – spherical or fibrous particulates
affect field intensities in their vicinity and reduce
the breakdown strength from pure material
values

10. Liquid insulation (3)
• Cavity breakdown – cavities are gas bubbles
generated by localised thermal excitation or
electro-thermo-chemical reactions, these are
less able to withstand the electric field
• Electro-convection and electro-hydrodynamic
models – as dielectrics are generally non-polar
liquids they have the ability to store any charge
injected from electrodes, the movement of these
charged species under the convection and
hydrodynamic movement of the liquid will
determine stress enhancement

11. Liquid insulation (4)
• Static electrification – insulating oils in
transformers become charged as they
pass through filters, pumps, etc. In contact
with paper and pressboard, the oil stays
positively charged with negative charge
transferring to the solid insulator. New oil
and oil with additives has been found to be
less prone to this problem.

12. Solid insulation
• Intrinsic breakdown
• Streamer breakdown
• Electromechanical breakdown
• Edge breakdown and treeing
• Thermal breakdown
• Erosion breakdown
• Tracking

13. Solid insulation (2)
• Intrinsic breakdown – for homogeneous
materials with no faults/inclusions intrinsic
strength measured by slowly increasing
voltage until breakdown occurs,
determined by properties of material and
temperature. Assumed to arise from
electron dissociation from structure,
crossing from valence to conduction band.

14. Solid insulation (3)
• Streamer breakdown – Occurs in
embedded electrode systems, where a
cathodic electron has sufficiently long
pathlength to traverse the insulation from
cathode to anode (c.f. streamer theory in
gaseous systems)

15. Solid insulation (4)
• Electromechanical breakdown – Occurs
where charge trapped within the insulation
cause an attractive force greater than the
material can withstand.

16. Solid insulation (5)
• Edge breakdown and
treeing – Occurs at
regions where dissimilar
materials cause a
breakdown in the weaker
material. Treeing is a gas
filled channel formed in
solid material – structure
varies with material and
field strength

17. Solid insulation (5)
• Thermal breakdown –
Occurs where power
losses cause heating
of the insulation
system, affecting
material properties.
Thermal stability is
field dependent, and
is higher under dc
than ac voltage
• Thermal voltage MV/cm
• Material ac
• Muscovite mica 7 - 18
• HV steatite 9.8
• High-grade porcelain 2.8
• Capacitor paper 3.4 – 4
• Polyethylene 3.5
• Acrylic resins 0.3 - 1

18. Solid insulation (6)
• Erosion breakdown – Occurs from cavities
in insulation materials or at material
boundaries. Cavity filled with gas or liquid,
which breaks down more easily, surface of
cavity reacts to electro-chemical events
and erosion of the system results. Surface
roughening exacerbates field stresses in
the void.

19. Solid insulation (7)
• Tracking – due to formation of conductive paths on the surfaces
of insulation components. Could arise from carbonisation of
insulator, contamination of external surfaces or metallic
deposition from moving parts.

20. Insulators in Overhead lines
• Source – JST Looms, “Insulators for High
Voltages”, Peter Peregrinus Ltd

21. Function
• Mechanical and electrical purposes
• Ideally non-conductive element
• External surface – contamination,
produces non-linear resistance, hence,
leakage current
• Leakage current – heat and
electrochemical change
• Erosion of surface or flashover

22. Design (1)
• Profile needed to overcome pollution and leakage current
• A variety of convoluted profiles exist for string (cap and pin)
insulators - varying the length of the skirts slightly improves
performance, some designs produce acoustic resonances, desert
discs (with open profiles) do badly in slat fog testing

23. Design (2)
• Longrod or line post insulators
• Poorer creepage than equivalent cap and pin
• Helical sheds seem worse than expected –
washing off of pollution not efficient

24. Materials
• Ceramic / Glass
• Polymeric – grp and silicon rubber
• Glass and ceramic have high mechanical strength and
high resistance to chemical attack but are brittle and
readily wettable
• Polymerics can be prone to chemical and photoelectric
attack, low mechanical strength, degradation produces
conductive tracks
• Failure rate – 0.1% failure per annum
• Failure position dependent – electrical stress

25. Adverse conditions
• Ability to prevent/withstand flashover
results from:
– Profile – “best shape” is site dependent
– Attitude – angular displacement
– Surface properties – hydrophobicity,
roughness

26. Flashover prevention
• Optimise shape and creepage
• Washing of insulators – from bottom to top to
prevent polluted run off water causing flashover
• Surface treatment – grease, controlled viscosity
pertolatum gel, silicone paste
• Hybrid insulators – ceramic core with polymeric
coating, still at investigation stage to determine
long term effects
• Resistive glazes – only used where other
methods not applicable

27. Tower string insulator
• Arcing horns provide
protection to
insulators

28. Model of string
• String insulator model – without and with
guard ring

29. Overhead lines

30. Line materials
• Copper
• Aluminium
• Steel-cored aluminium
• Copper-clad steel
• Cadmium copper
• Phosphor bronze
• Galvanized steel

31. Line supports
• UK 33kV supports
• Wooden poles (winter
felled, red fir, pressure
impregnated with
creosote)
• Single, A frame and H
frame
• Pin type ceramic
insulators (cheaper than
string suspension
insulators up to 50kV)
• Lifetime dependent upon
environmental conditions

32. UK Grid lines over 132kV
• Standard painted, or galvanised, steel
tower design
• Wide base to cope with vertical,
transverse and longitudinal stresses
• Steel-cored aluminium cables held by
glass/ceramic tension/suspension units
• Number of cap and pin units, each 250mm
diameter, dependent upon site conditions
(pollution) and voltage

33. Line route selection
• Determined by environmental and
aesthetic considerations
• Computer programs now used in industry
to take such factors into account
• Overhead preferred by industry on cost
grounds but environmental impact should
also be compared

34. Transmission route parameters
Voltage
(kV)
Loading
(MVA)
Right of
way width
(m)
Tower
height
(m)
Index
value
345 500 45.8 27.4 0.037
500 1200 61.0 36.6 0.050
765 2500 76.2 41.2 0.074
1200 7500 91.5 50.4 0.152
• US figures
• Index = Loading / RoW width x Tower height

35. Twin conductor advantages
• Line inductance and inductive reactance
reduced by 25%
• Corona inception voltage 5-10% higher
• More current carried per unit mass of
conductor
• Amplitude and duration of high frequency
vibration reduced

36. Twin conductor disadvantages
• Increased wind and ice loading
• Suspension more complex
• Tendency to dance increased
• Above 250kV advantages outweigh
disadvantages, hence twin bundles for
275kV and quads for 400kV

37. Catenary line sag-tension
• See H M Ryan,
Appendix 5.3

38. Effect of environment on line sag
• Increased temperature causes expansion
of conductor, increasing sag
• Icing of conductor increases weight,
increasing sag
• Calculation to take account of 80km.hr -1 winds
perpendicular to lines with a coating of 3.75mm ice at a
temperature of -5°C is standard on UK lines

39. Electrical fields
• The 30 metre tower shown is
used to suspend two circuit
275 kV high voltage
transmission lines.
• The equipotential-lines
connect points in space with
the same induced voltage. The
value of the induced voltage is
given in kV rms.
• The equipotential line form
depends on the shape and
configuration of the tower and
on the distribution of the
phases.
• The equipotentials are
calculated by a program used
at NKF,

40. Field effects from overhead cables

41. Electrical discharges
• Radio interference
• Corona losses

42. Monitoring lines
• Thermal imaging is being used to identify
discharge events on string insulators
• Fly-pass 3D imaging used to ensure
clearance from ground and tree growth
• New device suggested to increase tension
in cable to combat sag increase