Hvt 4 Breakdown in Liquids Chalmers
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Transcript of Hvt 4 Breakdown in Liquids Chalmers
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8/9/2019 Hvt 4 Breakdown in Liquids Chalmers
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Department of Electric Power Engineering
Electric breakdown
in liquids and
solids
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Department of Electric Power Engineering
Insulating liquids
• mineral oil• esters and synthetic hydrocarbons
• silicones and fluorinated oils
• chlorinated and phosphate fluids
• electronegative fluids (refrigerants)
• liquified elemental gases
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Department of Electric Power Engineering
Liquid properties of significance• Transformers high breakdown strength, low losses, high
electrical resistance, resistance to partial
discharges, chemical and thermal stability,
cooling properties.
• Capacitors resistance to partial discharges, high
inception voltage, low losses, low viscosity,gas absorbing properties.
• Cables electrical and gassing characteristics.
• Switchgear carbon formation and extinguishing.
• Bushings electrical and thermal properties.
• Electronics electrical and thermal properties, fire
resistance.
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Breakdown in liquids
• electron initiated• impurity initiated
• factors influencing
breakdown in technicalliquids- humidity and impurity content
- polarity and duration of voltage- shape and roughness of
electrodes
- temperature and pressure
- liquid volume
Influence of pressure on electric strength of
transformer oil in a weakly non-uniform field.
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Department of Electric Power Engineering
Breakdown in liquids
• At ambient temperature and pressure the following
intrinsic breakdown values have been measured:
hexane 130 kV/mm
benzene 110 kV/mm
mineral oil 200 kV/mm (depends on viscosity)
• Liquified gases also show high intrinsic strength:
liquid oxygen 240 kV/mm
liquid nitrogen 170 kV/mm
•
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Department of Electric Power Engineering
Breakdown in liquids
Influence of water content on breakdown
strength of transformer oil containing
cellulose fibres (1.4 mmg/l) with reference
to a pure and dry oil.Breakdown strength of transformer oil for
different types of voltages.
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Department of Electric Power Engineering
Breakdown in liquids
Influence of temperature on breakdown
strength of dry (1) and humid (2)
transformer oil.
Influence of oil content (size effect) on
breakdown strength of transformer (50 Hz).
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Department of Electric Power Engineering
Breakdown in solids• Different mechanisms
- intrinsic (electronic)- thermal
- partial discharges
- water treeing
- electochemical ageing
• Conduction processes
cannot be neglected
(>105 V/cm)
External and internal sources of conduction electrons: (a) Fowler-Nordheim
emission, (b) Shottky emission, (c) Poole-Frenkel effect.
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Department of Electric Power Engineering
Intrinsic breakdown in solids
Temperature dependence of dc
breakdown strength in polyethylene.Temperature dependence of dc
breakdown strength in quartz crystal (1)
and quartz glass (2).
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Department of Electric Power Engineering
Thermal breakdown in solids
• Heat generation by
conduction
dielectric losses ω
2 E σ
20 E ε ε ′′
ac thermal breakdown voltage
PVC (high loss) - 50 to 100 kVoil/paper (low losses) - about 750 kV
PE (extremely low losses) - up to 5 MVHeat balance in an insulating system
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Department of Electric Power Engineering
Partial discharge breakdown• PDs cause slow deterioration
of solid insulating materials• PDs are the most frequentcause for breakdown in acinsulating systems
• PDs cause discharge-induced physical-chemicalreactions on void walls:- temperature increase- reactions and wall erosion- space charge injection
Possible defects causing partial discharges
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Department of Electric Power Engineering
Ageing of electrical insulationageing processes in insulating materials may have either
physical or chemical naturethermally activated chemical reaction
)/exp( kT W K k
−=α
Montsinger formulated in 1930 so called 10(8) °C rule
[L = Aexp(-BTc)]
Later Dakin assumed that changes in a physical
property P should be related to the concentration C of
an important chemical constituent of the insulation
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Department of Electric Power Engineering
Thermal ageingend-point principle
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Department of Electric Power Engineering
Thermal ageingtemperature index TI
)/exp(0 kT W L L −=
The temperature index TI is thetemperature in °C (derived from
the endurance characteristics) for
which the expected life time
should be equal to 20 000 hours.
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Department of Electric Power Engineering
Electric ageing
( ) ( ) BE A E L −=
exp( )
nkE E L
−=
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Department of Electric Power Engineering
Multi-factor ageing
( ) ( )( )( )
N E E T B L E T L
−∆−=
00 //1exp,