Breakdown refers to the destructive discharge inside the insulator under the action of electric field, the insulation resistance decreases, the current increases, and the phenomenon of damage and perforation occurs.Important causes of accidents in electrical equipment.Voltage scale when it occurs“breakdown voltage ”Its value is related to the type, thickness and use environment of materials.[1]
Chinese name
breakdown
Foreign name
breakdown
Interpretation
Insulation material loses dielectric function
Classification
Electric breakdown, thermal breakdown, light breakdown
dielectricLoss of dielectric function in a short time (10~10s) under the action of high enough electric field strength.yesDielectric breakdownOne of the forms.Under the action of electric field, a small amount offree electronkinetic energyIncrease whenelectric field intensityWhen it is large enough, free electrons continuously hit theIonsAnd transfer energy to ions to ionize them, thus generating newSecondary electronThese secondary electrons gain energy in the electric field to accelerate their movement, and then hit and ionize more ions to produce more secondary electronschain reactionLike an avalanche, it generates "electron tide", which rapidly increases the current through the medium, leading to breakdown.[2]
Thermal breakdown
Under the action of electric field, the solid dielectric changes from insulation state to good conduction state due to the accumulation of internal heat and high temperature.[3]
In the actual insulation structure, in addition to the heat generated by the current in the conductor will be transferred to the solid dielectric, under the action of the electric field, the solid dielectric itself will alsopolarizationAnd fever.At the same time, solid dielectrics also radiate heat around. If the heat exceeds the heat dissipation, the temperature of solid dielectrics will rise.As the loss of solid dielectrics increases with temperature, so does the heat generation.Accordingly, the heat dissipation also increases with the increase of the temperature difference between the solid dielectric and the surrounding environment.If the heat generation and heat dissipation are equal at the temperature that the solid dielectric can withstand, theheat balance, solid dielectric works normally;If the heat generation is always greater than the heat dissipation, the temperature of the solid dielectric will keep rising, and eventually the solid dielectric will carbonize, melt or crack, lose its insulation performance, and thermal breakdown will occur[3]。
Light breakdown
Various damages occur in transparent media due to the effect of strong laser field. Optical breakdown refers to the damage caused by breakdown due to avalanche ionization caused by strong laser, which is different from the damage caused by direct laser heating, which is called thermal breakdown.
The physical mechanism of light breakdown in solids is that there are a small number of quasi free electrons in solids (electrons thermally excited to the conduction band, or electrons generated by multiphoton photoionization). These electrons gain energy in the laser field and make the latter collide with atomsionization； Repeated this process for many times, electrons multiply rapidly, and plasma is formed due to avalanche ionization;Plasma heating and expansion formationshock waveCauses local damage to solids, which is shown asfunnelShape damage track (if it is caused by inclusion, it is spherical).[4]
breakdown voltage
Announce
edit
sendDielectric breakdownUnder the action of a strong enough electric field, the dielectric will lose its dielectric properties and become a conductor, which is called dielectric breakdown, and the corresponding voltage is called breakdown voltage.The electric field strength during dielectric breakdown is called breakdown field strength.Different dielectrics have different breakdown field strength at the same temperature.When the distance d between the capacitor medium and the two electrode plates is fixed, it is known from U1-U2=Ed that the breakdown field strength determines the breakdown voltage.The breakdown field strength is usually called the dielectric strength of the dielectric.The dielectric strength plays a key role in improving the withstand voltage capability of capacitors.Attached table is the relative dielectric constant of various dielectricsεR and dielectric strength.[5]
dielectric
Relative permittivity
Breakdown field strength, 1e6/(V/m)
atmosphere
one point zero zero zero five
three
water
seventy-eight
—
mica
3.7～7.5
80~200
Glass
5~10
10~25
porcelain
5.7～6.8
6~20
paper
three point five
fourteen
Bakelite
seven point six
10~20
polyethylene
two point three
fifty
titania
one hundred
six
Tantalum oxide
eleven point six
fifteen
barium titanate
～103～104
three
The main factors affecting the breakdown voltage are:
(1) Electrode shapeRadius of curvatureThe smaller the breakdown voltage, the easier the discharge.
(2) Polarity of electrode: When the rod electrode discharges to the flat electrode, when the rod electrode is negatively charged, the breakdown is more difficult, and the breakdown voltage is high. When the rod electrode is positively charged, the breakdown voltage is low, and the breakdown is easy to occur.
(3) When the pressure of the gas decreases or the temperature increases, the breakdown voltage decreases due to the increase of the kinetic energy of the electron, which is prone to spark discharge.
(4) The breakdown voltage decreases with the increase of humidity.
(5) The action time of the voltage is very short, and the breakdown voltage is high.The lower the breakdown voltage, the easier the spark discharge will occur and the easier it will become the ignition source of combustibles.[6]
