Master of Engineering (Electrical), MEE - UniMAP Portalportal.unimap.edu.my/portal/page/portal30/Lecturer Notes... · Master of Engineering (Electrical), MEE ... CONDUCTION AND BREAKDOWN

Dr. Mohd Afendi Mohd Piah 1

Faculty of Electrical Engineering, Faculty of Electrical Engineering, UniversitiUniversiti TeknologiTeknologi MalaysiaMalaysia

Master of Engineering (Electrical), MEEMaster of Engineering (Electrical), MEE

HIGH VOLTAGE INSULATION AND ELECTRICAL DISCHARGEHIGH VOLTAGE INSULATION AND ELECTRICAL DISCHARGEMEP 1632MEP 1632

By,By,

Dr. Mohamed Dr. Mohamed AfendiAfendi b. Mohamed b. Mohamed PiahPiahInstitute of High Voltage & High CurrentInstitute of High Voltage & High Current

Faculty of Electrical EngineeringFaculty of Electrical Engineering


HIGH VOLTAGE INSULATION AND ELECTRICAL DISCHARGEHIGH VOLTAGE INSULATION AND ELECTRICAL DISCHARGE

What is high voltage (HV) insulation?

High voltage (HV) apparatus needs careful design of its insulation and the electrostatic field profiles.

Media of insulation used:GasesVacuumSolidComposite

Breakdown strength depends on pressure, temperature, humidity, field configurations, nature of applied voltage, imperfections in dielectric materials, material & surface conditions of electrodes, etc.


HIGH VOLTAGE INSULATION AND ELECTRICAL DISCHARGEHIGH VOLTAGE INSULATION AND ELECTRICAL DISCHARGE

The developed electric field stress and dielectric strength of the insulating materials are the important factors in HV systems.

Electric field distribution is developed due to the space chargedensity caused by the application of HV stress across the insulating materials.

Most commonly methods for determining potential distribution are

Electrolytic tank methodNumerical method

Finite difference method (FDM)Finite element method (FEM)Charge simulation method (CSM)Surface charge simulation method (SSM)


CONDUCTION AND BREAKDOWN IN GASESCONDUCTION AND BREAKDOWN IN GASES

The build-up of high currents in a breakdown is due to the ionization process.

The ionization process is governed by the various physical conditions of gases.

Ionization ProcessesIonization by collision – a free electron collides with a neutral gas molecule and generates new electron and positive ion.



Ionization by radiation (photo-ionization) – excitation of the atom to a higher energy state, dissociation of diatomic molecule.

Secondary ionization process – secondary electrons are produced.

Electron emission due to positive ion impact.Electron emission due to photons.Electron due to metastable and neutral atoms.



Townsend’s current growth equation

Number of electrons reaching the anodend = no exp(αd)

Average current = the number of electrons travelling/secI = Io exp(αd)

α : average number of ionizing collisions made by an electron per cm travel in the direction of the field.(is called Townsend’s first ionization coefficient)

no : electrons emitted from the cathode.Io = initial current at the cathode



Current growth in the presence of secondary process

Total number of electrons n reaching the anode becomes

n = no” exp(αd) = (no + no’) exp(αd)

n = n exp ( d )1 - [exp( d ) - 1]

o αγ α

no’ : num of secondary electrons frm secondary process.

no” : total num of electrons leaving the cathode

γ : Townsend’s secondary ionization coeff.

I = I exp ( d )1 - [exp( d ) - 1]

o αγ α

Townsend’s breakdown criterion

γ [ exp(αd) - 1 ] = 1



Current growth in a Townsend discharge

In the region T1 and T2, the current increases steadily due to the Townsend mechanism



Breakdown in electronegative gases

Give high breakdown strength. Free electrons get attached to neutral atoms or molecules to form negative ions.

Electron attachment represents an effective ways of removing electrons – Direct and dissociate attachment.

Gases that attachment process occurred, SF6, O2, freon, CO2and fluorocarbon.

An attachment coefficient (η) is defined in the Townsend breakdown criterion.



Streamer theory of breakdown in gases

Why the streamer theory is proposed?

Because the Townsend mechanism was found to have certain drawbacks in explaining the breakdown at atmppressure, ie.

In practice, bd voltage depends on the gas pressure and the geometry of the gap.

In ionization process, predict time lags = 10-5 s, but in actual practice, bd was observed in very short time (10-8 s)

Townsend mechanism predicts a very diffused form of discharge, but in actual practice, discharges are filamentary and irregular.



Proposed by Raether, Meek & Loeb (1940).

Predict the development of a spark discharge directly from a single avalanche – transform the avalanche into a plasma streamer.

Field distortion in a gap due to space charge

V/cm

px

x)exp( 10 x 5.27 = E 2/17-

r

⎟⎠⎞⎜

⎝⎛

αα



Paschen’s Law

1 = 1 - pdV f pdexp

pdV f 12 ⎥

⎦

⎤⎢⎣

⎡

⎭⎬⎫

⎩⎨⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛ V = f (pd) : Paschen’s law

For the effect of temperature and pressure, experimental results of bd potential in air:

2/1

T 760pd 293 6.08 +

T 760pd 293 24.22 = V ⎥⎦

⎤⎢⎣⎡

⎥⎦⎤

⎢⎣⎡



Vacuum Insulation

When the gas is liberated in the vacuum gap, the breakdown can occur by the Townsend process.

Categories of the breakdown mechanism in vacuum.

Particle exchange mechanism

A charge particle emits from one electrode –impinges on the other electrode – liberates oppositely charged particles.Cumulative process.



Field emmision theory

Anode heating mechanism

Electrons produced at small-projections of the cathode bombard the anode – causing local rise in temperature – release gases vapours.

Electrons ionize atom of the gas.

Produce secondary electrons.



Cathode heating mechanism

Existence of the pre-breakdown current at the sharp point of the cathode surface.

Current causes resistive heating at the tip of sharp point – tip melts and explodes – initiates vacuum discharge.



Clump mechanism

Experimental factors that influence the vacuum bdmechanism: gap length, geometry and material of the electrode, surface uniformity, presence of extraneous particles and residual gas pressure


CONDUCTION AND BREAKDOWN IN LIQUIDCONDUCTION AND BREAKDOWN IN LIQUID

Liquid dielectrics act as heat transfer agents and arc quenchingmedia.

Most factor that affects the electrical strength, is the presence of water droplets suspended in the liquid.

In practice, choice of liquid dielectric is based on chemical stability properties.

Types of liquid dielectrics;

Mineral oilSynthetic hyrocarbonsChlorinated hydrocarbonsSilicone oilsesters



Conduction and breakdown in pure liquids

At low fields – the current is due to dissociation of ions.

At high fields, the current generated is due to the field –aided electron emission by a Townsend mechanisms.

Breakdown voltage depends on the field, gap separation, cathode work-function, temperature, liquid viscosity, density and molecular structure of liquid.



Conduction and breakdown in commercial liquids

Not chemically pure and have impurities, e.g. gas bubbles, suspended particles, etc. – Reduce the breakdown strength.

Breakdown mechanism depends on the nature and condition of the electrodes, liquid physical properties, impurities and gases.

Classification of bd mechanisms;

Suspended particle mechanismParticle experience a force and drives the particles towards the max stress area – becomes aligned and bridging the electrode gap.

Give rise to local field enhancement depending on the shape – local bd will occur – results in the formation of gas bubbles.



Cavitation and Bubble Theory

The processes for the vapour bubbles formation.Gas pockets at the electrode surface.Electrostatic repulsive force between space charges –overcome surface tension.Gaseous products – dissociation of liquid molecules by electron collisions.Vapourization of the liquid by corona discharge.

Thermal mechanism

High density current pulses give rise to localised heating – lead to the formation of vapour bubbles.Elongation to a critical size or completely bridges the gap fromthe formed bubbles.Breakdown strength depends on pressure and molecular structure.



Breakdown strength of liquid;

Vb = Adn

d : gap lengthA : constantn : constant (always < 1)

Bd also depends on the voltage nature, applied voltage mode and the time application.

Actual mechanism of bd in oil is not a simple phenomenon. Bdvoltages are determined by experimental investigations only.


CONDUCTION AND BREAKDOWN IN SOLIDCONDUCTION AND BREAKDOWN IN SOLID

Good dielectric – low dielectric loss, high mechanical strength, free from gaseous inclusion and moisture, resistant to thermal and chemical deterioration.

Higher bd strength compared to liquids and gases.

When bd occurs, solid get permanently damaged.

Breakdown mechanism:

Intrinsic/Ionic bdElectromechanical bdThermal bdElectrochemical bdTreeing and trackingInternal discharges



Intrinsic Breakdown

Occurs at a very short duration of HV applied.

Depends on the presence of free electron, which capable of migration thru the lattice of the dielectric

Electronic breakdown

Assumed to be electronic in nature.Initial conduction electrons density assumed to be large –electrons collisions occur.Electrons gain energy and cross to conduction band. The process is repeated and more electrons are developed in conduction band.



Avalanche or Streamer Breakdown

Cummulative ionization – similar to bd in gases.

Conduction electrons gain energy –cause liberation of electrons from the lattice atom by collisions.

Energy gained exceeds the lattice ionization potential – additional electrons will be liberated –formation of an electron avalanche.

In practice – bd occurs from many avalanche formed – extending step by step through the entire thickness of the materials.



Electromechanical Breakdown

Due to electrostatic compressive forces that exceed mechanical compressive strength.

The highest apparent electric stress before breakdown, if the thickness of specimen do is compressed to d under applied HV;

E = Vd

= 0.6 Ymax

o o r

1/2

ε ε⎡

⎣⎢

⎤

⎦⎥

Mechanical instability occurs when d/do = 0.6



Thermal Breakdown

Conduction current flows – heats up the specimen and the temperature rises.

Heat generated transfers to the surrounding medium by conduction and radiation.

Bd occurs when heat generated > heat dissipated.

Thermal instability in solid dielectrics



Heat generated is proportional to the frequency –thermal breakdown is more serious at high frequency.

Thermal bd stresses are lower under a.c. condition then d.c.



Breakdown of solid dielectric in practice

Chemical and electrochemical deterioration and breakdown –presence of air and other gases – Chemical reactions occur.

Oxidation – surface cracksHydrolysis – lose electrical and mechanical propertiesChemical action – chemical instability – oxidation, cracking and hydrolysis

Chemical and electrochemical deterioration increases very rapidly with temperature.



Breakdown due to Treeing and Tracking

Occurs over a long time of electrical stress; presence of conducting path. Leakage current phenomena – formation of spark and carbon track.

Treeing occurs due to the erosion of the material at the tips ofthe spark. Breakdown channels spread through the insulation – formation of conducting channels.

Surface Tracking – caused by dry-band arcing. Formation of carbon track on the insulation surface.

Tracking occurs even at low voltages, whereas treeing requires high voltage.



Breakdown due to Internal Discharges

Occurs when solid material contains voids or cavities.

Electric field strength in the voids is higher than across the dielectric.

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Master of Engineering (Electrical), MEE - UniMAP Portalportal.unimap.edu.my/portal/page/portal30/Lecturer Notes... · Master of Engineering (Electrical), MEE ... CONDUCTION AND BREAKDOWN