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https://noppa.aalto.fi/noppa/kurssi/s-18.3150

LIQUID AND SOLID INSULATION

Lecture 4

S-18.3150 High Voltage Engineering

S-18.3146 Suurjännitetekniikka

https://noppa.aalto.fi/noppa/kurssi/s-18.3150 2

Week Date Lecture Topic Exercises

38 15.9 1 General + Safety + High Voltage Lab Tour

39 22.9 2 Electrostatic Fields + FEM 1 + FEM tasks + Seminar Tasks

40 29.9 3 Gas Insulation

41 6.10 4 Liquid and Solid insulation 2

42 13.10 5 Transients 3

43 20.10 NO LECTURE Partial Discharge Lab

44 27.10 6 Overvoltages & Insulation Coordination 4

45 3.11 7 HV Testing & Measurements 5

46 10.11 8 Generation of High Voltages Seminar Presentations

47 17.11 Seminar Presentations (Seminars)

48 24.11 Excursion Surge Arrestor Lab

49 2.12 NO LECTURE

50 8.12 EXAM (13 – 16, S5)

LIQUID INSULATION

Requirements

Examples

Breakdown


• Protect solid insulation from discharge

• Extinguish arcing

• Efficiently remove heat (cooling)

PURPOSE

• Low viscosity (resistance of fluid - thickness) • Cooling properties

• Liquid fills all voids in solid insulation

• Viscosity must remain small at low temperatures (Finland) • Cannot become solid. Solidification temperature less than -40 °C)

• Chemically stable • Maintain insulating properties through long service life in varying conditions

REQUIREMENTS


REQUIREMENTS DEPEND ON APPLICATION:

Transformers • High withstand voltage – high dielectric strength of liquid

• Good heat transfer capability for cooling

• High resistivity, low loss factor, good PD tolerance

Cables and Bushings • Very good PD tolerance

• Very low viscosity (impregnation)

Switches • High flash temperature so that it can safely extinguish arc

MINERAL OIL


0 0

20

40

60

80

20 40 60

water content [µg/g]

80 100

Ub [kV]

(ii.) same oil

filtered twice

(i.) clean industrial

transformer oil

Most common liquid insulation

• Typical mineral oil is transformer oil

• Easy availability and economical

• Properties defined in IEC 60296

• Good dielectric properties for insulation and low viscosity for cooling

• Prone to oxidization and flammability (flash point over 130 °C)

• Moisture and impurities affect insulating properties

SYNTHETIC LIQUID INSULATION


CONTROLLED CHEMICAL STRUCTURE

DESIRED PROPERTIES MORE EXPENSIVE

Relative

Permittivity εr

[50 Hz, 25 °C]

Kinematic Viscosity

[mm2/s, 20 °C]

Solidification Point

[°C]

Flashpoint

[°C] Comments

Mineral Oil 2.2 16 -50 150-175

Mono/dibenzyl

toluene 2.7 6.5 -50 144

Very toxic for aquatic environment

Silicone Oil 2.9 50 -53 >335

Most environmentally friendly

Non-flammable

Poor heat transfer.

Poor discharge tolerance

(flammable gas by-products from arcing)

Ester 3.3 63 -50 310 Non-toxic,

Environmentally friendly

BREAKDOWN IN LIQUID INSULATION


Due to the density of the liquid, it is hard for electrons to achieve much energy between collisions

In relatively clean and homogenous liquids, the breakdown mechanism is similar to breakdown in gas

Electric field is applied to the charge carriers in the liquid (electrons already present

in the liquid and those released from the cathode through emission and electrochemical processes)

Electrons move in the opposite direction of the field

If the energy of the electrons is sufficient, dissociation of molecules by collisional ionization occurs (ionic compounds split into smaller particles – opposite of

recombination)

Microscopic gas bubbles form in liquid

Townsend avalanches and new free charges can form in the BUBBLES (smaller density)



Mobility of electrons in the liquid is 105 times greater than positive ions

• electrons leave behind a concentration of positive ions

• these charge concentrations enhance electric field in certain regions and further advance ionization

Free charges created by ionization inside the bubble are displaced by the electric field • the bubble is stretched and grows in size

Eventually the bubble (or row of bubbles) will expand across the electrode gap

• an ionized channel is formed and advanced by streamer discharge

• the channel has lower density and smaller dielectric strength than the rest of the liquid

• continuous current flows through the channel



• Impurity moves towards the area of highest electric field

• Impurity enhances the electric field and attracts other particles

• These particles bridge the electrodes leading to breakdown

Impurity.ɛr >

Liquid.ɛr

Fiber bridge

Bridging of impurities and bubble rows can be avoided by

•Inter-layering with solid insulation

•Displacing liquid (flowing)

Breakdown can also be caused by impurities

Moisture and foreign particles



Dielectric strength of liquid is hard to determine:

• not constant

• once oxidation, electrochemical reactions, and impurities affect the liquid, its insulating properties are not the same

• dielectric strength of liquid measured in homogeneous gap even though most insulators are inhomogeneous

Temperature and pressure affect the liquid’s dielectric strength

• As temperature increases, viscosity decrease which increases the speed of electrons between collisions and increase the probability for breakdown

• As pressure increases, the formation of bubbles is more difficult which improves the dielectric strength of the liquid.

SOLID INSULATION

Requirements

Examples

Breakdown


PURPOSE

• Mechanical support for conducting components

• Electrical insulation

REQUIREMENTS

• Mechanical strength

• Dielectric strength

• Heat tolerance


Insulation Breakdown

Field Strength [kV/mm]

Temperature Index TI

(20 000 h) [°C]

Comments

Organic

Paper (dry) 6 90 • easy to handle and machine

• typically good dielectric properties

• insulating properties change during service life

• temperatures above 100 °C deteriorate insulator

• typically porous – absorb liquids, impregnation

• transformers, cables, capacitors

Paper (oil impregnated) 40 – 75 105

Rubber 20 75

Wood (dry) 90

Wood (oil impregnated) 105

Press wood (dry) 6 90 – 120

Inorganic

Porcelain 30 1000 • withstand high temperatures

• excellent dielectric and mechanical properties

• poor machinability, cannot absorb liquids

• overhead lines, bushings, rotating machines

Glass 16 400 – 1000

Mica 80 500 – 700

Synthetic

Polymer

Polythene 20 105 • all industrially produced solid insulation

• excellent electric properties, easy to machine

• thermoplastic / thermoset plastic

• wide range of applications depending on manufacturing process - moisture sealing, tensile strength, flexibility

Polystyrene 100 80 – 90

Phenolic plastic (bakelite) 5 – 16 120 – 155

Epoxy plastic 20 – 40 105 – 155

Melamine 13 – 14 120

Organic

Non-organic

Synthetic Polymer

BREAKDOWN IN SOLID INSULATION


Mechanism of breakdown in solids remains unclear

F12 F21

When voltage stress is increased close to breakdown:

• Current through insulator increases exponentially (similar to gas)

• Assumed to be caused by increasing number of charge carriers in the insulation and electrode surfaces

In practice, other factors affect breakdown besides the increase of electrons:

• Pre-discharge current and dielectric losses causing heat

• Electrostatic forces at interfaces

• Electrochemical reactions

• Water trees, erosion



Breakdown is caused by energy provided by an electric field

Energy transfer can occur by:

Breakdown leads to thermal destruction

of insulator (melting, charring, vaporizing)

Permanent loss of insulating properties

(not self-restoring like gases)

In general:

Collision Ionization (electric breakdown) 1.

Thermal Losses (thermal breakdown) 2.



BREAKDOWN MECHANISMS IN SOLID INSULATION • Electrical Breakdown

• Intrinsic Breakdown

• Partial Discharge

• Electrical Treeing

• Electromechanical Breakdown

• Thermal Breakdown

• Electrolytic Breakdown

ELECTRICAL BREAKDOWN


Multi-stage phenomenon influenced by:

Discharge occurs when critical field strength is

exceeded locally

• assumed to begin at inhomogeneous region of the

insulation or electrode surface

Insulation is destroyed by different chemical

and physical processes

Conducting channel with numerous charge

carriers starts to progress

Complete breakdown occurs when the channel

has bridged the electrodes

• Different ionization mechanisms

• Space charges in discharge channel

• Heating of insulator material

• Molecular structure deformations

INTRINSIC (INTERNAL) BREAKDOWN


When an electric field is applied, the conduction electrons gain energy due to collisions between them and the energy is shared by all electrons.

• For a stable condition, this energy must be somehow dissipated.

• The field raises the energy of the electrons more rapidly than they can transfer it to the lattice.

• Electron temperature increases and conduction continues to increase to a complete breakdown.

The energy gained by electrons from the electric field exceeds the amount of energy that electrons can transfer, resulting in the collapse of the entire molecular lattice

Intrinsic strength is a property of the MATERIAL and TEMPERATURE only

In pure homogeneous dielectric materials the conduction and valence bands are separated by a large energy gap and at room temperature the electrons cannot acquire sufficient thermal energy to make transitions from valence to conduction band.

metal semiconductor insulator

conduction band

valence band

Fermi level

elec

tro

n e

ner

gy

band gap

PARTIAL DISCHARGE BREAKDOWN


Insulators usually have voids or gas bubbles

If the void has lower permittivity than its

surrounding material, voltage

stress is higher inside the void

Once the void’s withstand strength is

exceeded, partial discharge begins

Positive ions and electrons released by the discharge collide with the

void wall causing slow erosion

(discharge by-products can also

cause chemical erosion)

Irregularities in the void

wall enhance the electric field locally

As further discharges

concentrate on this area, the erosion

path is advanced

ELECTRICAL TREEING


If stress is continuous, eventually discharges will advance through a solid insulator in a branching erosion path (electric tree) along which a complete breakdown can occur Treeing commences at impurities on the electrode or in the insulation (pre-breakdown phenomenon)

Water treeing – moisture advances in insulator under the influence of the electric field

E.g. Chemical degradation of polymeric insulation such as XLPE or EPR that only occurs in the presence of water and electrical stress (initiates from inhomogenities within the insulation or at insulation/screen interface)

Electrical treeing Vented water treeing Bow tie water treeing


ELECTROMECHANICAL BREAKDOWN

The pressure forces the

insulator to compress

Electrode distance

decreases causing the

electric field to further increase

pressure

Localized heating and softening of the insulator

leads to mechanical

collapse


Mechanical force between charges at electrodes causes pressure (force of attraction between surface charges)

2

2

1Ep

(not very common for typical insulators)

ELECTROMECHANICAL BREAKDOWN


Mechanical collapse when electrostatic compressive forces exceed mechanical compressive strength:

Stress (E = U/d0) required for collapse (d < 0.6d0)

21

0

6.0

r

YE

21

0

0

0

2

2

0

2 ln2

ln2

1

2

1

d

dYdU

d

dY

d

UEp

r

r

Y = Young’s modulus of elasticity d0 = original distance d = compressed distance

PET (polyethylene terephthalate, thermoplastic polymer resin) ɛr = 3, Y = 3.1 x 109 Pa E = 6480 kV/mm!

BaTiO3 (barium titanate, dielectric ceramic used in capacitors) ɛr = 18500, Y = 167.4.1 x 109 Pa E = 607 kV/mm!

THERMAL BREAKDOWN


An electric field causes the insulator to experience heat produced by conductivity and dielectric losses

• as temperature of the insulator increases conductivity increases more heating

• the dissipation factor tan δ increases with temperature more dielectric losses

Field strength E1: insulator temperature

stabilizes at t1

no thermal breakdown

1. Field strength E2:

temperature increases to t2

unstable

(small temporary fluctuation can lead to

thermal breakdown)

2. Stabilization point

unreachable

thermal breakdown

Heat produced at E3 always exceeds cooling

3.

If heat is being produced in the insulator faster than it is removed by cooling thermal breakdown

t 1

t 2

specimen temperature

3 ( E 3 )

1 ( E 1 )

2 ( E 2 )

ELECTROLYTIC BREAKDOWN

Electrolyte

Any substance containing free

ions that behave as an electrically

conductive medium

Ions transport conducting

material from the electrodes

into the interior of the dielectric

Ions (oxygen, chlorine) present in the insulator migrate to the

electrodes causing a chemical reaction


Insulators with moisture and impurities

are prone to electrolytic breakdown

Highly dependant on

local environmental

conditions

Gradual build up (years)

resulting in sudden

breakdown



10 2

10 3

10 4

10 5

10 6

10 6 10 3 10 0 10 - 3 10 - 6 10 - 9 10 9 t

[s]

[V/m

m]

b E

Water Treeing

Thermal Breakdown

Partial Discharge

Electrical Treeing

Electrical Breakdown

Electromechanical Breakdown

Breakdown strength and

discharge processes are

highly dependent

on stress duration

PRACTICAL INSULATORS

OHL Components

Cables

Bushings


Above certain temperature hard plastic becomes soft and its mechanical and dielectric properties degrade

As temperature decreases plastic becomes brittle and its impact strength weakens

• Voltage withstand strength • Surface distance • Dirt accumulation prevention • Self-cleaning (aerodynamic shape) • Water sealing • Mechanical strength – arcing, erosion, UV radiation, thermal reaction

All outdoor insulators need to consider:

Insulators are needed to support live (voltage) components so that their distance to grounded parts and other devices is maintained.

All organic insulators are influenced by temperature and stress duration


a. b.

c. d.


170 146

OVERHEAD LINE COMPONENTS


Pin Insulator (porcelain)

Disk Insulator, Cap and Pin Insulator (glass or porcelain)

Line Post Insulator (porcelain)

Long-Rod Insulator

Composite Insulator

Cast Plastic

Insulators


• Typically porcelain

Weaker mechanical and electrical strength compared to toughened glass.

Different constituents in porcelain have different thermal expansion at varying temperatures

• No shattering and easier to manufacture large insulators

CERAMIC INSULATORS

• Toughened glass Improved mechanical strength

Smaller size

• Microscopic fractures on glass surface during manufacturing Shattering caused by mechanical impact or erosion (surface impurities)

GLASS INSULATORS


Δce

x

l

U 0

• Capacitance chain with highly non-linear voltage distribution

• Each individual unit has:

self capacitance Δcs

capacitance to ground Δce

capacitance to conductor Δcv

xcxl

Cc i

ii

x

c

x

lCc s

ss

i = e (earth) or v (voltage conductor)

l = length of insulator string

Δx = insulator unit length

Insulator String

Δce

Δce

Δce

Δcs

Δcs

Δcs

Δcs

Δcs

Δcv

Δcv

Δcv

Δcv


Voltage distribution depends on capacitance ratio Cv/Ce

Cv/Ce < 1 voltage stress concentrates on conductor side

Cv/Ce = 1 voltage distribution is symmetrically

Cv/Ce > 1 voltage stress concentrates on tower beam side

Beam (GND)

Conductor

10 20 n

5 15

0.5

1.0

0

N = 7 N = 15 N = 22

10 20 n

5 15

0.5

1.0

0

U [p.u.]

N = 7 N = 15, 20

N = number of units

n = unit number

Cv/Ce ≈ 0 Cv/Ce =1.3


Emax = 4.6

kV/mm

Emax = 3.75

kV/mm

Emax = 4.5

kV/mm

U

GND

GND

GND

U

U



COMPOSITE INSULATORS


Protect core from moisture/chemicals/UV radiation/surface discharge

• PD can release hydrogen which forms acid when combined with moisture leading to fracture

+ mechanical strength + light weight + elastic (hard to vandalize) – cost – uncertain long term stability

• Used in overhead lines as insulator strings, phase separators and external insulation for surge arresters, bushings, transformers

Manufactured from at least two different insulating materials

CABLE INSULATION


AC: plastic – PVC, PE, XLPE (PEX)

DC: HVDC transmission (> 150 kV) with oil-paper insulation

plastic – polarization state does not change fast enough when polarity is changed or voltage transients are applied to the cable

critical field strength is exceeded – breakdown.

Cable

Cable Termination

Cable Joint

PAPER INSULATION (IN CABLES)


10 – 50 mm wide paper tape wrapped around the conductor with small gaps left between adjacent tapes

• next layer is positioned to avoid continuous radial layering

• the positioning of the tape is called registration and describes how much the layers are on top of each other

50% registration 75% registration

Inhomogeneous construction:

paper enables formation of thin, high dielectric strength oil sheet layers and prevents impurities from traveling between layers

defines mechanical properties – cannot bend cable so that the layers connect

CABLE DESIGN


1. stranded conductor

2. conductor screen

3. insulation

4. insulation screen

5. metallic screen

6. bedding

7. metallic sheath

8. seperation sheath

9. armouring

10. outer sheath

• Water blocking powder can be used between the strands to prevent water migration.

CONDUCTOR STRANDS

• Standard in most cables.

BASIC LAYERS (2 – 4)

• Vary depending on cable type and intended application • E.g. submarine, underground, etc.

ADDITIONAL LAYERS (5 – 10)

• Serves multiple purposes • E.g. protection against short circuit currents, external

fields, mechanical stress, and moisture.

GROUNDED COMPONENTS (5, 7, 9)

• Used for cushioning and water blocking.

BEDDING (6)

• Can be implemented to separate different metallic layers.

SEPARATION SHEATH (8)

• Serves as external protection.

THE OUTER SHEATH (10)


Screens help to maintain the cylindrical symmetry of the field

Small cavities and impurities are unavoidable in the cable

Screens (semiconducting layers) are placed between the conductor and insulation layer and also between the insulation layer and the sheath.

1. conductor/insulation – conductor screen (conductor cover) 2. insulation/sheath – insulation screen (corona shield)

2.

1.

eriste

ru

rs

s

x

r

x

r

Et

a

b

Er

johdin

CABLE JOINTS


CONICAL CONSTRUCTION

Should be logarithmic but straight conical is satisfactory

• Some cables have pre-molded joints – insulation does not need to be altered

• No voids or impurities

insulation

r outer

r inner

a

a

D s

D x

D r

x

r

E t

E r

conductor


CABLE JOINTS

insulation

ground

conductor

CABLE TERMINATIONS


Local concentration of electric field occurs where a conducting electrode continues and the grounded electrode terminates

aim is to maintain a small and constant field

the stress cone has a conducting layer on its surface or in its interior

grading is done by adding a stress cone on top of the cable insulation

Grading of the electric field is necessary (FI: kentänohjaus)

a.) no grading b.) geometric grading c.) refractive grading


Conductive surface of stress cone displayed as darker bold line.

The design is inserted through a hollow core (porcelain) insulator in this figure.

d .) stress cone placed directly on conductor

e.) stress cone placed on top of cable insulation

stress cone (solid

insulation)

gas/liquid insulation

solid insulation

(mechanical)

E E

• Coat electrode edges with appropriately shaped insulation having different permittivity than the surrounding insulation.

• A high permittivity insulating cylinder can be fastened over the section of the cable that has been stripped of insulation. Capacitance grows between the edge of the metal sheath

and the bare section of insulation.

Forces the field in the surrounding air to be distributed fairly uniformly along the surface of the insulation.

Permittivity-based grading (refractive stress control, ɛ control, or resistive and capacitive grading)


CABLE TERMINATIONS

100%

75%

50%

25%

0%

without grading

100% 75%

50%

25%

0%

capacitive grading

Folio sheets

inserted into

insulation

BUSHING (FEED-THROUGH INSULATOR)


A live conductor is fed through a grounded enclosure

To avoid electric field concentration:

• Electrodes and insulation shape is considered

• Grading ring can be applied (kentänohjausrengas)

• Capacitive grading – field electrodes added






Voltage level

Insulator string length

Number of insulator

units

110 kV ~ 1 m 6-8

220 kV ~ 2 m 10-12

400 kV ~ 4 m 18-21

(in Finland, approx. 1 m/100 kV)


110/20 kV Substation (Espoo)

110 kV switching yard and line terminals - Main transformer (1-3 units),

busbar, circuit breakers, disconnectors, current transformers, surge arresters

20 kV panels and line terminals - Busbar, circuit breakers, disconnectors, current

transformers

Control and monitoring equipment - Measurement, protection, data transfer, local

automation, remote application, interlocking devices, backup electrical system, storage batteries


110/20 kV Substation (Levi)

Instrument transformer VT (voltage) (USA:

potential transformer PT) Disconnector

Circuit breaker

Instrument transformer CT

(current) Disconnector

Disconnector

(Bypass) Main transformer

(31,5 MVA, 110 kV)


Electropedia: The World’s Online Electrotechnical Vocabulary www.electropedia.org

switching device a device designed to make or break the current in one or more electric circuits

switchgear a general term covering switching devices and their combination with associated control, measuring,

protective and regulating equipment, also assemblies of such devices and equipment with associated

interconnections, accessories, enclosures and supporting structures, intended in principle for use in

connection with generation, transmission, distribution and conversion of electric energy

(mechanical) switch a mechanical switching device capable of making, carrying and breaking currents under normal circuit

conditions which may include specified operating overload conditions and also carrying for a specified time

currents under specified abnormal circuit conditions such as those of short circuit.

Note – A switch may be capable of making but not breaking short-circuit currents.

disconnector (isolator) a mechanical switching device which provides, in the open position, an isolating distance in accordance with

specified requirements.

Note – A disconnector is capable of opening and closing a circuit only when either negligible current is

broken or made, or when no significant change in the voltage across the terminals of each of the poles of the

disconnector occurs. It is also capable of carrying currents under normal circuit conditions and carrying for a

specified time currents under abnormal conditions such as those of short circuit.

circuit-breaker a mechanical switching device, capable of making, carrying and breaking currents under normal circuit

conditions and also making, carrying for a specified time and breaking currents under specified abnormal

circuit conditions such as those of short circuit.

instrument transformer a transformer intended to transmit an information signal to measuring instruments, meters and

protective or control devices Note – The term "instrument transformer" encompasses both

current transformers and voltage transformers.

http://www.electropedia.org/

Next time:

MONDAY 13.10.2014, 12-16, S2

• Lecture 5: Transients

• Exercise 3: For voluntary bonus points, submit answers BEFORE the beginning of the exercise session

• Work on your FEM assignment!


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