EEPB463 High Voltage Technology Chapter 5

EEPB463 HIGH VOLTAGE

TECHNOLOGY

Dr Azrul Mohd Ariffin BN-3-017

[email protected]

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BREAKDOWN IN SOLID DIELECTRICS

• Solid dielectric materials are primarily used to insulate conductors from one another, in addition to provide mechanical support

• Solid dielectrics have higher dielectric breakdown strength compared to liquids and gases

• A good dielectric should have low dielectric loss, high mechanical strength, should be free from gaseous inclusions and moisture, and be resistant to thermal and chemical deterioration

• In gases, conduction is limited to positive and negative charge carriers, and its rapid growth is due to formation of electron avalanches

• In solids however, conduction is not only due to charge carriers but includes currents due to polarization processes; thus the mechanism is much more complex

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• Nevertheless several distinct mechanisms have been put forward to explain quantitatively the breakdown processes in solids

• Mechanism of failure and breakdown strength changes with the time of voltage application

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Breakdown strength

Time

intrinsic, electromechanical th

erm

al

erosion, electrochemical

102 10-8


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Breakdown in Solid

Dielectrics

Intrinsic

Electro-mechanical

Thermal

Treeing Tracking

Internal discharges

Electro-chemical


Intrinsic Breakdown

• When voltages are applied for a very short period of time, the electric strength of a solid dielectric increases up to an upper limit which is called the intrinsic electric strength

• However the material under test needs to be pure and homogeneous, temperature and environmental conditions are carefully controlled and sample is stressed that there are no other external discharges; thus very rare to accomplish experimentally

• The stress required for intrinsic breakdown is very high; in the order of several MV/cm

• Intrinsic breakdown is obtained in times of the order 10-8 s and therefore has to be electronic in nature

• This type of breakdown depends upon the presence of free electrons capable of migration through the lattice of the dielectric

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• In pure dielectric materials, conduction and valence bands are separated by large energy gap

• At room temperature electrons cannot acquire sufficient thermal energy to cross this gap

• In reality, materials contain imperfections and impurities that may act as traps for free electrons

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VALENCE BAND

Band gap

CONDUCTION BAND

Density states, N(E)

Energy, E


• When an electric field is applied, these ‘trapped’ electrons gain sufficient energy to move into the conduction band

• As the applied field increases, more and more trapped electrons are freed and therefore conduction increases

• Moving electrons will collide with solid molecules to cause ionization to some extent

• As more and more moving electrons are produced, electron avalanche occurs (similar to gases) and this will eventually lead to complete breakdown

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VALENCE BAND

Band gap

CONDUCTION BAND

Density states, N(E)

Energy, E


Electromechanical Breakdown

• When solid dielectrics are subjected to high electric fields, failure can occur when the electrostatic compressive forces exceed its mechanical compressive strength

• Compression forces arise from electrostatic attraction between charges that appears when voltage is applied

• Pressure exerted when the field reaches about MV/cm may be several kN/m2

• If d0 is the initial thickness of specimen material of Young’s modulus Y, and is compressed to a thickness d under applied voltage V, then the electrically developed compressive stress is in equilibrium with the mechanical compressive strength if

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electrical compression

mechanical compression


• Re-arranging the equation gives

• Mechanical instability occurs when d/d0 = 0.6

• Thus the highest apparent strength, Emax prior to breakdown is

• The above equation however, ignores the possibility of plastic deformation when the material is subjected to high electrical stresses

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can you work out on how to get the last equation?


Thermal Breakdown

• Conduction current due to the application of high field across a solid dielectric can cause heat to be continuously generated

• Equilibrium is reached when the heat generated becomes equal to the heat dissipated

• Breakdown occurs when generated heat exceeds dissipated heat

• Consider the following figure

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Heat generated / dissipated

Temperature

E2

E3

E1

T1 T2

heat generated

heat dissipated


• Heat dissipated is represented as straight line and heat generated at different fields are shown by curves

• For field E1, thermal equilibrium is achieved at T1. Below this value, heat generated exceeds heat dissipated thus breakdown may occur. Beyond T1, heat loss is greater than heat generated; thus breakdown will not occur

• For field E2, thermal equilibrium is achieved at T2. Breakdown may occur both below and beyond this value as heat generated is higher than heat dissipated

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Temperature

E2

E1

T1 T2

heat generated

heat dissipated


• For field E3, thermal equilibrium is not achieved for any temperature values so this level of field will almost likely cause breakdown due to thermal

• By identifying the thermal breakdown stress for a particular material, a temperature limit can be set for each operating stress level

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Temperature

E3

heat generated

heat dissipated

explain how temperature limit is set to prevent thermal breakdown?


Breakdown due to Treeing

• In practical insulation systems, solid material is stressed together with one or more other materials

• Thus breakdown voltage will be influenced more by the weak medium than the solid


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d2

d1 V1

V

ε1

ε2

dielectric

dielectric

electrode

electrode


• Relationship between the fields across the dielectrics, E1 and E2 is given by

• Say the applied voltage at the electrodes is denoted as V

• Substituting the equations will give

• Thus the voltage across each dielectric can be written as

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• Since 2 > 1, field becomes very significant across d1; sparking can occur in the gap and charge accumulation takes place at the surface of solid insulation

• As time passes, breakdown channels spread through the solid insulation in an irregular ‘tree’ like fashion leading to formation of conducting channels

• Eventually the conducting path will bridge the electrodes and cause total failure of insulation

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Breakdown due to Tracking

• Tracking is a formation of a permanent conducting path (usually carbon) across the surface of an insulation

• Leakage current passes through the solid insulation surface and this leads to formation of spark

• Heat resulting from the small sparks causes carbonization and form permanent ‘carbon track’ on the surface

• This further increases the electric stress over the rest of the insulating region

• Insulation failure occurs when carbonized tracks bridge the distance between electrodes

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can you relate the design of insulation strings and bushings with tracking?


Breakdown due to Internal Discharges (Erosion)

• Solid insulating materials often contain cavities or voids within the dielectric material or on boundaries between the dielectric and electrodes

• These voids are filled with a medium of lower dielectric breakdown strength and permittivity than that of the solid insulation

• Thus electric field strength in the voids is higher than that across the solid dielectric

• The field in the voids may exceed their breakdown value and thus breakdown may occur even under normal working voltages


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• C1 represents the capacitance of the void, C2 is the capacitance of the dielectric in series with the void and C3 is the capacitance of the rest of the dielectric

• The capacitance of the void and the dielectric in series with the void are be given as

• If V is the applied voltage, the voltage across void V1 is

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V1 C1

C2

C3 V d t


• Under applied voltage V, V1 also increases until it reaches the breakdown value Vi of the gap t and discharge occurs

• Vi is called the discharge inception voltage

• V1 decreases due to discharge and starts increasing again until it reaches Vi again

• A new discharge then occurs

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Vi

-Vi

V

V1


• Several discharges may take place during the rising part of applied voltage V

• Similarly, cavity discharges also occur at the decreasing part of the applied voltage

• This group of discharges give rise to positive and negative current pulses on raising and decreasing voltage applied

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Vi

-Vi

i

V

V1


• These internal discharges (also called partial discharges, or PD) can cause gradual erosion of the material, and consequently decreases insulation ‘s lifetime

• Current pulses of PD can be observed and this will be discussed in the next chapter

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Vi

-Vi

i

V

V1


Breakdown due to Electrochemical Deterioration

• Air and other gases may exist in dielectric solids, and thus can lead to chemical changes under continuous application of electrical stresses

• Presence of oxygen in air can cause materials to undergo oxidation

• Presence of moisture or water vapour on the surface of a solid dielectric can cause hydrolysis, which can cause the material to lose its electrical and mechanical properties

• Even in the absence of electric field, chemical reaction can still occur due to the effects of temperature where deterioration can occur rapidly leading to breakdown

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BREAKDOWN IN LIQUID DIELECTRICS

• Liquid dielectrics are mainly used as impregnants in HV cables, and for filling up of transformers, circuit breakers

• In addition to insulation, liquid dielectrics also act as heat transfer agent

• Most commonly used liquid dielectrics are petroleum oils (transformer oils), and for high temperature application, silicone oils and fluorinated hydrocarbons are employed

• Conduction and breakdown mechanisms in liquid dielectrics can be explained based on the type of the material

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pure liquids

• electronic breakdown

commercial liquids

• suspended particles

• cavity breakdown

breakdown in liquid

dielectrics


Electronic Breakdown

• Impurities are not present in pure liquids, so their breakdown mechanism must be electronic in nature

• The process follows Townsend mechanism of current growth where three distinct regions are identified

• At very low fields, electrons are generated from cathode by field emissions, and then achieve saturation due to dissociation of ions

• At high fields, electrons get multiplied and thus current increases rapidly according to Townsend’s primary and secondary ionization, before breakdown occurs

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voltage

current


Breakdown due to Suspended Particles

• In commercial liquid dielectrics, solid impurities may be present either as fibres or as dispersed solid particles

• Permittivity of these particles will be different from the permittivity of the liquid; thus they will become polarized in the presence of a field

• Assuming these impurities to be spherical particles of radius r and the applied field is E, then each particle experiences a force F

where 1 and 2 are the permittivity values of the liquid and the particle respectively

• In the case of 1 < 2 , such as paper, the force is directed towards areas of maximum stress

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• If impurities are of gas bubbles for instance, 1 > 2 and thus the force has an opposite direction

• Under continuous application of voltage, these particles become aligned due to the force exerted and thus form a stable chain bridging the electrode gap

• This will lead to breakdown between the electrodes

• Breakdown strength of liquids containing impurities is much less than that of pure liquids

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electrodes

liquid dielectric

aligned impurities

breakdown path


Breakdown due to Cavities (Gas Bubbles)

• Insulating liquids may contain gaseous inclusions in the form of bubbles

• Bubbles are formed due to

-existence of gas pockets at the surface of the electrodes

-changes in temperature and pressure

-gaseous products due to dissociation of liquid molecules by electron collisions

-liquid vaporization by corona-type discharges from sharp points and irregularities on the electrode surface

• Once a bubble is formed, it will be elongated in the direction of electric field due to electrostatic forces

• Breakdown occurs when the voltage drop along the length of the bubble becomes equal to the field to ionize the gas in the bubble

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• Breakdown field is given as

where E0 is the field in the liquid, is the surface tension of the liquid, 1 and 2 are the permittivity values of the liquid and bubble respectively, r is the initial radius of spherical bubble and Vb is the voltage drop in the bubble

• This expression indicates that the critical field strength required for breakdown of liquid depends upon the initial size of the gas bubble which is affected by hydrostatic pressure and temperature of the liquid

• However, the theory does not take into account the initial amount of bubble produced

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EEPB463 High Voltage Technology Chapter 5