Hvt 4 Breakdown in Liquids Chalmers

8/9/2019 Hvt 4 Breakdown in Liquids Chalmers

1/16

Department of Electric Power Engineering

Electric breakdown

in liquids and

solids


2/16


Insulating liquids

• mineral oil• esters and synthetic hydrocarbons

• silicones and fluorinated oils

• chlorinated and phosphate fluids

• electronegative fluids (refrigerants)

• liquified elemental gases


3/16


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.


4/16Department of Electric Power Engineering

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.


5/16



• 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

•


6/16



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.


7/16



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).


8/16


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.


9/16


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).


10/16


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


11/16


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


12/16


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


13/16


Thermal ageingend-point principle


14/16


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.


15/16


Electric ageing

( ) ( ) BE A E L −=

exp( )

nkE E L

−=


16/16


Multi-factor ageing

( ) ( )( )( )

N E E T B L E T L

−∆−=

00 //1exp,

Documents

Hvt 4 Breakdown in Liquids Chalmers