electrical insulation in power systems - …portal.unimap.edu.my/portal/page/portal30/lecturer...

Dr M. Afendi M. Piah, MIEEE, CMCIGRE1

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Electrical Insulation in Power SystemsElectrical Insulation in Power Systems

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INTRODUCTIONThe economic development & social welfare of any modern society depends upon the availability of a cheap and reliable supply of electrical energy.

Major function of power systems ; to generate, transport and distribute electrical energy over large geo-graphical areas in an economical manner while ensuring a high degree of reliability and quality of supply.

Transmission of electrical power over long distances is best accomplished by using high voltage (HV), extra high voltage (EHV) or ultra high voltage (UHV) power lines.

Voltage classification


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INTRODUCTIONHigh voltages are also extensively used for many industrial, scientific and engineering applications such as:

Electrostatic precipitators for the removal of dust from flue gases.Atomization of liquids, paint spraying and pesticide spraying.Ozone generation for water and sewage treatment.X-ray generators and particle accelerators.High power lasers and ion beams.Plasma sources for semiconductor manufactureSuperconducting magnet coils, etc.

In all such applications, the insulation of the HV conductor is of primary importance.

For proper design and safe and reliable operation of the insulation system, knowledge of the physical and chemical phenomena of the insulating material is very important.


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INTRODUCTIONIn addition, the basic processes which lead to degradation and failure of such materials and appropriate diagnostic techniques are of prime importance since any such failure can cause temporary or permanent damage to the system.

PROPERTIES OF DIELECTRICSThere are several properties of a dielectric which are of practical importance for an engineer.

1) DC Conductivity (σ)

EJ

=σJ : current density (A/m2)E : electric stress (V/m)J : current density (A/m2)E : electric stress (V/m)


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PROPERTIES OF DIELECTRICSFor most insulating materials, σ depends upon the material purity, its temperature T and electric stress E.

kTEAeT /)( −=σ k : Boltzman constantA : constantk : Boltzman constantA : constant

In addition due to polarization effects, σ also depends upon time of application of the stress. It influences the power losses in a dielectric and controls the electric stress distribution under direct voltage applications.

ρσ 1= ρ : resistivityρ : resistivity


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PROPERTIES OF DIELECTRICS2) Dielectric Permittivity (εr)

Also called relative permittivity or dielectric constant.

Generally εr is not a fixed parameter but depends upon temperature, frequency and molecular structure of the insulatingmaterial.

or C

C=ε


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PROPERTIES OF DIELECTRICS3) Complex Permittivity, Loss Angle and Dissipation Factor

To determine the response of dielectrics to alternating voltages.

Model the dielectric by a parallel RC network. R represents the lossy part and C is the capacitance of the dielectric.

δδ

δω

tan and small;ry usually ve is angle Loss

tan ;

⋅−=≈∴

⋅−==

cRc

Rc

jIIII

jIICVjI


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( ) *tan current, Total

εωδεεω VCjjVCjIIII

orro

cR

=−=∴

+=

Where ε* = complex relative permittivity having a real part (dielectricconstant, εr) and an imaginary part equal to the loss factor (εr tan δ).

Tan δ commonly known as loss tangent or dissipation factor and usually depends upon frequency, applied electric stress and the temperature.

ro

roVCCV

εωεσδ

δσδεωδω

=

==

tan

as; related are tan and eFurthermortantan LossPower 22


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PROPERTIES OF DIELECTRICS4) Polarization

Most of the electrons in insulating materials are bound and not free to move. Under the influence of an electric field, the resultingelectrostatic forces create some level of polarization forming dipoles.

In polymer materials, permanent molecular dipoles are reorientedin electric field. Interfacial polarization is observed for heterogeneous materials.

In this case, mobile conduction charges are held up at some boundary within the dielectric.


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PROPERTIES OF DIELECTRICS5) Dielectric Strength

The maximum value of applied electric field (V/m) at which a dielectric material, stressed in a homogeneous field electrode system, breakdowns and loses its insulating property.

Depends upon the purity of material, time and method of voltage application, type of applied stress as well as other experimental and environmental parameters.

Under inhomogeneous field, breakdown strength needs to be defined and sometimes referred as the nonuniform field dielectric strength.


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CLASSIFICATION OF INSULATING MATERIALS1) Gases

In normal state, most gases are good insulators. Recently, metalclad switchgear and gas insulated cables filled with compressed sulfur hexaflouride gas (SF6).

In special applications (Van de Graaf accelerators or in measurement capacitors), other gases or mixtures of SF6 with gases such as N2, O2, CO2, air and N2O are also used.

An ideal gaseous insulator should be cheap, chemically and thermally stable, and should not form toxic, corrosive or flammable products under prolonged electrical stress. Good heat transfer and arc quenching properties.


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CLASSIFICATION OF INSULATING MATERIALS2) Vacuum

Excellent insulating and arc quenching properties.

A true vacuum is very difficult to achieve and residual gas pressure of the order of 10-9 to 10-12 bar may exist in vacuum insulated equipment.

Material, shape and surface finish of electrodes, residual gas pressure and contaminating particles are important factors.


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CLASSIFICATION OF INSULATING MATERIALS3) Liquids

High values of dielectric strength, volume resistivity, specific heat, thermal conductivity and flash point plus low values of loss factor, viscosity, pour point and density.

Noncorrosive, nonflammable, nontoxic and chemically stable, good arc quenching as well as gas-absorbing properties.

Mineral oil having alkanes, cyclo-alkanes and aromatics has been used since the last century.

Chlorinated aromatics or askrals, silicone oil, synthetic hydrocarbons and fluorinated hydrocarbons.


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CLASSIFICATION OF INSULATING MATERIALS4) Solids

Must have some of the properties mentioned earlier for gases andliquids as well as having a good mechanical and bonding properties.

Inorganic and organic solid insulating materials are widely used.

Inorganic materials (ceramic and glasses) are used to manufacture insulators, bushings and other HV components.

Organic materials are thermosetting epoxy resins or thermoplastic materials, such as PVC, PE, XLPE, EPR, NR, SR etc.


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CLASSIFICATION OF INSULATING MATERIALS4) Composites

More than one class of insulating materials – Composite or a hybrid type of insulation system. Systems employing solid/gas insulation are transmission line insulators and solid spacers used in GIS.

Solid/liquid insulation : Oil impregnated paper tapes used in HVcables, transformers, capacitors and bushings.

Solid/gas or solid/liquid interface represents the weakest link and has to be carefully designed. Chemically stable and not react with each other under combined thermal, mechanical and electrical stresses, and should have nearly equal dielectric constants.

Liquid insulant should not absorb any impurities from the solid which might adversely affect its dielectric properties.


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ELECTRIC FIELDSProper design of any HV device requires a complete knowledge of the electric field distribution and method to control this field. Some knowledge of the electric field concepts is a prerequisite for an understanding of the insulation failure modes.

The electric field intensity, E (which is related to the electric potential, V) can be obtained from the solution of the Laplace’sequation (if the medium is free of any space charge) or from thePoisson’s equation (if the medium has a space charge density)


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ELECTRIC FIELDS

potentials typermittivi

density charge field electric where,

;

====

−∇==⋅∇

V

E

VEE

o

o

ερ

ερ

0equation sLaplace'

equationsPoisson'

2

2

=∇

−=∇=∇⋅∇

V

VVoερ


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ELECTRIC FIELDS1) Field Distribution Types

2 types; Homogeneous (uniform), nonhomogeneous (nonuniform)

Homogeneous field

E is the same throughout the field region.Uniform or approximate uniform field distributions exist between2 infinite parallel plates, or 2 spheres of equal diameters withgap spacing < sphere radius.“Profiled” parallel plates of finite sizes are also used to simulate homogeneous fields.


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ELECTRIC FIELDS1) Field Distribution Types (cont)

Nonhomogeneous field

E is different at different points in the field region.In the absence of space charges, E usually obtains the maximum value at the surface of the conductor which has the smallest radius of curvature – nonhomogeneous and asymmetrical.Most of the practical HV components have nonhomogeneousand asymmetrical field distribution.In some gaps – will produce nonhomogeneous fields and symmetrical, e.g. rod-rod or sphere-sphere (large distance between spheres) gaps.HV electrode has higher E than the grounded electrode.


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ELECTRIC FIELDS2) Methods of Field Estimation

Simple physical systems (ie. Single/2 parallel conductor above ground, 2 equal diameter spheres, coaxial cylinders, 2 infinitely long parallel plates – possible to find an analytical field solution.

For most HV components, physical systems are complex –extremely difficult to find an analytical field solution. Numerical methods are employed for electric field calculations.

Some of the methods used; finite difference, finite element, theMonte Carlo, moment method, method of images, charge and surface charge simulation, etc.


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ELECTRIC FIELDS3) Field Enhancement Factor

Knowledge of the maximum value of the electric field to which the insulation is likely to be subjected and the location of such a maximum gradient point.

Concept of field enhancement factor (field factor) is of considerable use.

gap in the field average :

f

factor;t enhancemenField

max

av

av

EEE

=

Dr M. Afendi M. Piah, MIEEE, CMCIGRE 21

Field Enhancement Factor for some common configurations


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ELECTRIC FIELDS3) Field Enhancement Factor (cont)

Sometimes the Field Utilization Factor, (µr = 1/f) is used. The larger value of µr represents a more compact equipment.

In a multidielectric media, the field computations become complicated. Considering the boundary conditions at the interface of the two dielectrics. Needs numerical computations for the analysis. General values of f for such cases cannot be given.


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ELECTRIC FIELDS4) Field Intensification at Protrusions

At the tip of a protrusion, the field line converge.

The microscope field at the protrusions tip, Ep = f E becomes greater than the macroscopic gap field E. The field enhancement factor, f depends upon the shape and size of the protrusion.

The protrusions and surface defects play a prominent role in the initiation of partial discharges and ultimate breakdown of insulation.


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ELECTRIC FIELDS5) Field at the Interface of Composites

At the interface of 2 different dielectrics (see Figure), the tangential electric field and the normal flux density must be continuous.

density charge surface :

interface at the discharge surface is thereif

s

sBnBAnA

BnBAnA

BtAt

EE

EEEE

ρρεε

εε

=−

==


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ELECTRIC FIELDS5) Field at the Interface of Composites (cont.)

For the case of solid/gas interface (εA > εB), the field is distorted at the boundary and the net field on the material B of the interface becomes larger ⇒ makes such an interface the weakest link in the system.

For DC applications, the insulating materials can get charged ⇒ may be charging of insulator due to corona or other types of discharges giving rise to a surface charge density ⇒ enhance the total surface field on the gas side.

Solid/gas, solid/vacuum, solid/liquid or solid/solid interfaces needs careful consideration.


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ELECTRIC FIELDS6) Field Inside Cavities

Cavities can be generated inside solid or liquid during manufacture or operation.

Gas spaces in side the cavities experience higher electric stress than the bulk liquid or solid media.

Generally, large stress values occur inside cavities under DC voltages than AC applied voltages.

In the case for the insulation subjected to both AC and DC voltages simultaneously, the cavity stress could be estimated by superposition principle.


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ELECTRIC FIELDS7) Field at Free Particles

In liquids, gaseous or vacuum insulation, free conducting or insulating particles may be introduced during manufacture, installation or operation.

These particles acquire some charge as a result of various mechanisms and may drift in the insulating medium.

The electric stress at the ends of such particles can be enhanced and consequently trigger breakdown of the insulation medium.


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ELECTRIC FIELDS8) Electric Stress Control

Electric stress values have to be controlled in the design of HVequipment.

Higher value of electric stress may trigger or accelerate the degradation and failure of the insulation.

Electrical stresses are controlled in cable terminations, HV bushings, potential transformers, etc.

In addition, corona-free connections are desired during the testing of some HV equipment, thereby limiting the stress to values below critical values.


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DESIGN PARAMETERS OF HV EQUIPMENTInsulating materials play a critical role in the design and performance of HV power equipment.

These materials must not only meet the dielectric requirements, but must also meet all other performance specifications including mechanical and thermal requirements, reliability, cost, ease of manufacture as well as environmental concerns .

Generally, for economic reasons, the operating electric stressincreases with system voltage whereas the design electric stressdecreases with system voltage .

As the system voltage increases, the equipment operates closer to its limits and presence of any defects can have serious consequences for the equipment’s life expectancy.

HV Insulators

electrical insulation in power systems - …portal.unimap.edu.my/portal/page/portal30/lecturer...

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