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Chair of Energy Distribution and High-Voltage Engineering Prof. Dr.-Ing. Harald Schwarz

High-Voltage Technique and Insulating Materials Page 63

Breakdown Field Strength

Assumption: αeff * s = k and homogeneous field

From Aeff p E

B A

pη α α

α −== and

/exp*

follows⎟⎟ ⎠ ⎞⎜⎜

⎝ ⎛

+

=

A

d

ss p A

p B E

η κ ***ln

*

Calculated breakdown field strength E d as function of the sparking distance d for different

gases under normal pressure (p = 1 bar) and normal temperature (T = 293 K).

- Ed depends on the sparking distance s.

- Volume effect: tests at scaled-down models are not suitable for theassessment of high-voltage devices.

- High-voltage tests of the real device (actual physical dimensions) arenecessary.





Breakdown Voltage (Paschen Curve)

Ud = Ed * s →

⎟ ⎠ ⎞⎜

⎝ ⎛

+

=

Ask

s p As p B

d U

η ln

**

Ud = f (p * s)

Calculated breakdown voltages as function of pd (Paschen curve) for different gases: (1)

Helium and (2) air; (3) Air and (4) Sulfur hexafluoride; (5) Sulfur hexafluoride

- for large values of ps: fan out into several curves

- for very small values of ps→ mechanism of vacuum breakdown





Paschen’s law as analytical approximation formula (thin curve) and as real curve (broad curve)

- Another breakdown mechanism works above 18...14=∗d eff α .





4.5. Streamer Mechanism

- The Townsend mechanism depends on an interaction between ava-lanche and electrode. For long sparking distances or in case thatthere are no electrodes at all this model is no longer valid.

- The constantly growing size of the avalanche results in a distortion of the basic field distribution.

Avalanche in homogeneous field and field distribution E along the central axis of theavalanche for the critical number of electrons at the avalanche head: E g is the field

distribution of the base field.

- The highest alteration of field strength can be found at the avalanchehead.

- If the avalanche reaches a critical length of

20...18

0

=∫ dx

L

eff

κ

α

(determined experimentally) photon radiation is emitted.

- The photons can generate secondary avalanches.





Streamer mechanism: Fusion of independent avalanches into a

conductive channel during the avalanchetransit time

Model for discharge reactions with space charges or channels discharge according to

Raether respectively (Streamer mechanism)

- The streamer grows with high velocity

vst = 10 …. 100 cm/µs,

because the photons can bridge long distances with the velocity of light.




High-Voltage Technique and Insulating Materials 8

Avalanche growth in homogeneous field

Streamer growth against the field direction





5. Discharge Reactions in Gases (technical details)

5.1. Breakdown of Mixed Gases

In technical devices mixed gases can occur (intentional for certain tech-nical applications or unintentionally caused by leakiness). The most im-portant mixed gases for high-voltage applications are:

a) Air as a mixture of O2, N2, CO2 …b) SF6 as a mixture of

b1) SF6 + N2 technical application for lowtemperatures

b2) SF6 + Air unintentionally caused by leakiness

The phase diagram for pure SF6 is given as an example for b1).

Phase diagram of SF 6

The typical gas pressure in gas insulated switchgear is 2 … 4 bar. For temperatures below -40 °C the insulating gas becomes liquid. By addingN2 the temperature for liquefaction can be lowered considerably.





Leakiness can also result in a mixture of SF6 and N2. The followingpicture shows the breakdown characteristic of this mixed gas.

AC breakdown voltage for a mixture of SF 6 and N 2 depending on the volume percentage V SF6

of SF 6 for constant striking distance s = 15 mm and varying pressure

5.2. Influence of the Electrode Roughness

A certain surface roughness can not be avoided during the manufactur-ing process of technical devices. Even polished surfaces have a

roughness of about 3 µm, drawn material of about 6 µm.These micro peaks cause an increase of the electric field strength.





Increase of field strength by surface roughness of the electrode

The increased field strength can exceed the dielectric strength of theinsulating gas.

Field distribution E/p, effective ionization coefficient ∗α and number of electrons N e in an

avalanche near the rough surface for air, SF 6 and SF 6 with increased pressure

A breakdown can occur if the value of ∫ eff α exceeds the above men-

tioned limiting values.





5.3. Breakdown in Inhomogeneous Fields

- In weakly inhomogeneousfields reaching the ignitioncondition results in abreakdown immediately.

- In strongly inhomogeneousfields partial discharges

occur at the electrode withthe stronger curvature. Inelectronegative gases(SF6, air) these dischargesare stabilized by absorptionof charge carriers in re-gions of lower fieldstrength.

Difference between the voltage for initiation of coronaand the breakdown voltage in a strongly

inhomogeneous field for constant sparking distance





Polarity Effect

Polarity effect in a strongly inhomogeneous field for positive point electrode (left) and negative point electrode (right) Above: Formation of streamers in regions of high electric field strength and positive

effective ionization coefficient.

Middle: Formation of a positive space charge by left-over positive ions (left and right)and formation of a negative space charge by trapping of electrons in the regionof lower field strength (right).

Bottom: Field strength E(x) along the x-axis for an electric field without space charge(thin lines) and the resulting electric field with space charges (broad lines) withshifting of ionization boundaries.





Positive point electrode:

- Positive ions in front of the electrode „decrease“ the field strength;

- Electrons start in the field space;

- After formation of the first avalanche a stable glow discharge isgenerated by photo ionization (dim bluish glowing);

- Field strength in the gas space is increased;

- “Remaining sparking distance” is shortened;

- Breakdown processes are favoured

Negative point electrode:

- Strong increase of field strength in the vicinity of the point electrode;

- Large statistical time variation until an initiating electron is available;emission of charge carriers from the point electrode;

- Single statistically varying corona impulses;

- Field strength in the gas space gets lower and more uniform.

In general: Ud, pos < Ud, neg





5.4. Streamer and Leader discharge

5.4.1. Positive Streamer Discharge

- Avalanches form in the gas space and travel towards the anode(analogous to chapter 5.3.). A positive space charge is left over.

- If the field distortion is sufficient photons are emitted (similar to chap-ter 4.5.).

Formation of positive streamer discharge: 1 Streamer head, old; 2 Streamer channel; 3Photon path; 4 Initiating electron; 5 Avalanche; 6 Streamer head, new

- These photons generate secondary electron avalanches, which aremoving towards the positive space charge and get neutralized.

- The electrode avalanches leave a new positive space charge, whichis situated closer to the opposite electrode.

- A passive, weakly conducting zone develops between the anode andthe positive space charge.

- The streamer growth needs a voltage gradient of about 4-5 kV/cm.

- The propagation velocity is about several 10 cm/s.





5.4.2. Negative Streamer Discharge

The statements from chapter 4.5 are valid.

A voltage gradient of about 10 kV/cm is needed. The propagation veloc-ity of the negative streamer discharge is smaller than the velocity of thepositive streamer.

5.4.3. Leader Discharge

- For large striking distances only.

- The positive leader discharge is discussed, because it is the mostcritical case.

Formation of a leader discharge: 1 Leader channel; 2 Leader head; 3 Leader corona;4 Streamer head of corona; 5 Ionization boundary

- Up to now the physical mechanism of the leader discharge is notcompletely clear.

- The heating-up of the leader channel up to 5000 °C is important for the formation of a leader discharge.

- The density of charge carriers is strongly increased by thermo-ionization and the voltage demand decreases from 5 kV/cm auf 1.5kV/cm, resulting in a growing of the leader.

- The heating-up is caused by collision processes of the numerousstreamer avalanches in the leader head.





Existence of different discharge types in an air insulated point-plate-arrangement: Inceptionvoltage U e and breakdown voltage U d as function of the sparking distance d for positive and

negative point electrodes (left). Discharge types and the accompanying relative voltagedemand (right).





5.5. Breakdown Behaviour for Transient Voltages

The breakdown time can be divided into three periods:

ts = statistically varying time for the appearance of astarting electron for anavalanche at the rightlocation

ta = build-up time for theformation of a high-resistive channel

tz = breakdown time for thetransition from high- to low-resistive channel

For slowly varying voltages the breakdown occurs after slightly exceed-ing the permissible voltage Uo.

Ignition delay time for a voltage step (a) and a voltage impulse (b)





For fast varying voltages the voltage can rise considerably in the timet = ts + ta after exceeding the value Uo.

Therefore the impulse volt-time characteristic of a high-voltage device isvery important, giving the limiting curve of the additional voltage increasein dependence on the impulse slope.

Voltage-time area

- Determination of the static breakdown voltage Ud ∞ - Measurement of breakdown voltages for impulse voltages

Lower value = build-up time characteristicUpper value = variance zone

- Determination of area A

- For a given arrangement the area A is constant (independent of thechosen voltage curve).

- Drawing of volt-time characteristic based on a few measurements.





5.6. Spark Discharge and Arc Discharge

During the breakdown the gas path is bridged by a streamer if thesparking distance is below about 1 m. The time necessary for the forma-tion of the discharge is called the build-up time ta. After this time the insulating ability breaks down. The time for thebreakdown of the voltage from 90% to 10% is called the breakdown timeor spark build-up time. A low-energy discharge is called a spark; a high-energy discharge iscalled an arc.

5.6.1. Spark Discharge

The charge carrier multiplication (dn) caused by collision ionization alongthe path (dx) is given by:

dxndneff

**α =n = number of initiating electrons

eff α = effective ionization coefficient

By taking into account the drift velocity

dt

dxv= it follows vn

dt

dneff

**α =

By assuming that the total current density equals the electron currentdensity one gets:

evn I I e

**==

or

e

I

dt

dneff

α =

respectively.





From there it follows:

( )t Qe A

dt ie A

dt I e

n

F

eff

t eff

t eff

(

*

*0

0

α

α

α

=

=

=

∫

∫

A = cross-sectional area of the spark

)(t QF = quantity of charge flowing

during time t

With:

tyConductivienb

spark of Length LF

→=

→

** χ

one gets the spark resistance ( )t RF

( )( )t Qb

L

Aenb

L

A

Lt R

F eff

F F F F

****** α χ ===

→ ( )

( )∫

==t

F T

F

F T F

dt i

L

t Q

Lt R

0

** κ κ

T κ = Toepler constant

Toepler constant for different gases

Air =T

κ 0.5 … 0.6 * 10-4 Vs/cm

Nitrogen =T

κ 0.4 * 10-4

Vs/cm

Argon =T

κ 0.85 * 10-4

Vs/cm

SF6 =T κ 0.4 ... 0.8 * 10

-4

Vs/cm





The spark build-up time is estimated using the discharge of a capacitor as analogy.

Spark resistance, build-up time, voltage and current for the discharge of a capacitance(schematic)

It holds:

( ) ( ) ( )

( )

( )t i

dt t i

Lt it Rt u

t

F T F

∫==

0

*κ

and

( ) dt

duC t i −=

Because the sum of the residual charge C ∗u (t ) of the capacitor and the

quantity of charge flown through the spark equals the initial charge C ∗ U d of the capacitor it holds:

( ) ( )

∫+=∗

t

d

t uC dt t iU C

0





By insertion of variables and solution of the differential equation onegets:

( )

⎥⎦

⎤⎢⎣

⎡+

=

t L

U

U t u

F T

d

d

κ exp1

Using the voltage values 0.1∗Ud and 0.9∗Ud gives

d

T

d

F T

F E U

Lt κ κ 4,4*4,4 ==

With the above mentioned values of T

κ one gets

nst F

8...7≈ (Air with Ed ≈ 30 kV/cm)

nst F 3...2≈ (SF6 with Ed ≈ 90 kV/cm)





5.6.2. Arc Discharge

If the current density in a spark discharge increases sufficiently, thermo-ionization starts. In this case we talk about an arc discharge.

Gas-discharge characteristic for one example

Possible operation points at theintersection points of gas-dischargecharacteristic and resistance line:

(1) stable; (2) instable and (3) stableoperating point

Operating characteristic of the arc with seriesresistor





For a stable arc it is necessary that the supplied electric power Pzu equals the dissipated thermal power Pab:

U * I = Pzu = Pab.

The voltage demand of the arc can be determined empirically

n

BU Ι

=l

Bl = arc length

n = 0,5 … 0,25

i.e. the voltage demand increases with the arc length and decreases for higher currents.

The quenching of the arc can be accomplished according to the

A.C. voltage principle: At the zero transition of the current the arc isshortly interrupted. If the discharge gap is deionised during this phase,no reignition is possible.

D.C. voltage principle: If the voltage demand is increased beyond thefeeding voltage (e.g. by lengthening the arc) there is no stable operationpoint and the arc is interrupted.





5.7. Surface Discharges

5.7.1. Flash-Over

Breakdown: Discharge between electrodes in the designedinsulation arrangement (discharge in solid material,Gas discharge).

Flash-over: Discharge between electrodes outside of thedesigned insulation arrangement (device flash-over)

There are two cases

a) Field lines parallel to surfaceb) Field lines nearly perpendicular to surface

Flash-over at boundary surfaces: a Field lines parallel to open boundary surface; b Field lines nearly perpendicular to open boundary surface

Case a): A gas discharge occurs. The insulator surface has a limited

influence on the discharge process (trapping and releasing of electrons).Very often the breakdown field strength at the boundary surface is lower than in the solid insulating material and in the gas space due to pollutionlayers at the boundary surface (see next section).

Case b): Sliding discharge (see section 5.7.3)





5.7.2. Flash-Over Caused by Pollution Layers

Conductive layer at the insulator surface caused by

- atmospheric humidification and- pollution.

As a result leakage currents of about 10 mA … 100 mA occur.

Phases of pollution layer flash-over:

a) Displacement of leakagecurrent by dry zone with localtemperature rise.

b) Growing of dry zone causedby Joule heat, accelerateddrying.

c) Interruption of current after drying of complete insulator periphery.

d) Flash-over bridging the dryzone, formation of an arc.

e) Growing of dry zone and arclength by further drying.





Increase of creeping distance at real insulators by sheds

Insulator for open-air application with sheds for increasing the creeping distance

The creeping distance depends on the pollution.

There are several classes of pollution defined for transmission networks.Ub is the operating voltage.





Pollu-tionclass

According toGlöyer andVogelsang [9.9]

SpecificcreepingdistanceIk /U b

Degree of pollution

IEC TC 28 (C.O.) 42November 1973 [17.72]

a Agriculture andforestry, noindustry, cleanatmosphere

(1,7 … 2,0)cm/kV

Nosignificantpollution

Areas without industry andlow density of buildings withhouse heating. Areas with few industrialcompanies but withfrequent wind and rain. All areas have to be distant

from sea shore or at highaltitudes (no sea winds inany case).

b Light pollution(periphery of large industrialareas), frequentlystrong fog (river valleys)

(2,2 … 2,5)cm/kV

Lightpollution

Areas with industrialcompanies without specialdust and exhaust gaspollution or areas withaverage density of buildings. Areas with high density of

buildings and heavyindustry, but with cleaningwind and rain.Distance to the sea shorehas to be at least 1 km.

c Heavy industrialpollution (e.g.Ruhr area,industrial areaaround Köln and

Mannheim)

(2,6 … 3,2)cm/kV

Heavypollution

Areas with many industrialcompanies and periphery of large cities (high density of house heating). Areas close to the sea

shore or with strong seawinds.

d Very heavyconductivepollution (vicinityof large power stations, chemicalplants, iron andsteel industry)

≥ 3,8 cm/kV Very heavypollution

Smaller areas with heavyindustrial pollution, causingthick conductive layers atthe insulators. Areas close to the seashore with strong salt winds(salt mist).





5.7.3. Sliding Discharge

Sliding discharges along surfaces are critical at cable ends and bush-ings.

Typical electrode arrangement for sliding discharges with equipotential lines (without taking into account the refraction of field lines)

The following equivalent networks can be used for determination of thefield distribution:

Equivalent networks for determination of the initial tangential field distribution for different

voltages:Left: Impulse voltage and A.C. voltage (dielectric displacement field only).Middle: Taking into account conductive pollution layers for A.C. voltage.Right: D.C. voltage (steady-state electric field).





After exceeding the inception voltage Ue first partial discharges occur atthe electrode corners.

r

e

s pk U

ε

*=

k = 8 discharge in air k = 21 discharge in SF6 k = 30 discharge in oil

- After first partial discharges –initiation of glow discharge of

formation of streamers

- Because of high transversecapacitances high currents are

flowing in the streamers ⇒ possibilityof thermo-ionization even for shortstreamer lengths

- Formation of leader discharges andbridging of large sparking distances

- Formation of leaders for

44,02

/

/8,25 ⎥

⎦

⎤⎢⎣

⎡

∆∆=

AC

cm pF kV U

g

Formation of sliding discharges at the surface of a cylindrical insulator

Positive sliding discharge just before the formation of a flash-over





6. Breakdown Reactions in Solid and Fluid Insulating

Materials

6.1. Purely Electrical Breakdown

• The purely electrical breakdown occurs in solid insulating materials atvery high field strength.

• High-polymer plastics consist of macromolecules and show semi-

crystalline behaviour.

Structure of semi-crystalline high-polymers with crystal lamellas:

1 Crystal lamellas, 2 amorphous regions with a) Interconnection chains, b) Chain ends,c) Folding back of chains





• The energy band model for crystals, giving an energetic description of the mobility of electrons, can be used.

• Anode and cathode are metallic electrodes with free electrodes, sym-

bolised by the Fermi level of anode and cathode respectively (FNK /FNA).

• The difference between FNK or FNA and the valence band of theinsulating material is the work function, which is necessary for re-leasing electrons from the metal surface.

• In the valence band of the insulating material all electrons are fixed, inthe conduction band the electrons can move freely. There are no

electrons in the ‚forbidden zone’.

• The energy bands of the insulating material shift if a voltage is appliedto the electrodes.





• In order to initiate a breakdown electrons have to be elevated into theconduction band of the insulating material. This can be accomplished

by:

Thermo-ionization

For high cathode temperature electrons can get over the poten-tial wall.

Outer field emission

For high field strength at the cathode electrons can get over thepotential wall.

Inner field emission

Electrons can be elevated from a trap level (defect of insulatingmaterial) to the conduction band with lower energy.

• Outer field emission occurs especially in very inhomogeneous fields.





Energy band model inhomogeneous fields

Energy band model ininhomogeneous fields

• The breakdown is governed by an avalanche mechanism, similar togas discharge processes.

• A permanent damage of the insulating material occurs if the power density exceeds the value 10

-5w/mm

3.

• The breakdown field strength Ed lies above 1000 kV/cm.

• That value can only be reached for:

- dielectric stress < 1 µs- very thin insulating material < 1 mm- very pure insulating materials

e. g. PE foil d = 100 µmEd ≈ 800 kV/mm at 20 °CEd ≈ 300 kV/mm at 100 °C





6.2. Global Thermal Breakdown

Current density ( ) E j J w ωε σ +=

Loss factor ωε

σ δ =tan

specific dissipation power: P ν = σ E 2 = ωε 0 ε r tan δ E 2

δ tan ~ exp ( )( )ou

υ υ σ −





Heating of an insulating material by ohmic and dielectric losses

zuP ~ tan δ , u2

~ exp ( )ν , u2

Pab ~ uν ν −

Curves for explanation of thermal stability conditions

Stationary case: Pzu = Pab (Points A1 and B1

or A2 and B2)

Stable operating point:ν ν d

dP

d

dPab zu ⟨ (Points A1 / A2)

The stability limit is reached for ν ν d

dP

d

dPab zu

= (Point C)

At the stability limit any disturbance of the system (without further in-

crease of the voltage) results in an increase of temperature ⇒ destruc-tion.

The breakdown voltage is a parameter of the insulating material:

about 0.1 ... 20 MV depending on type and purity of the material.

The breakdown voltage is independent of the sparking distance,because the complete insulation is thermally overloaded.





6.3. Masked Gas Breakdown

- Of similar importance for insulating fluids as the purely electric break-down for solid insulating materials.

- In very pure liquid insulating materials (e.g. LN2) the electrons can notreach the ionization energy, because the free path length is too small.

- Experimental investigations show that the energy of the electrons issufficient for evaporation.

- Formation of micro bubbles with larger free path length.

- Ionization processes with formation of avalanches similar to gas dis-charge processes.





6.4. Local Thermal Breakdown

Pab s

- Channel with higher conductivitybetween electrodes;

- Heating-up of channel.

- After reaching the breakdown voltage ⇒ self-maintained temperaturerise (similar to global thermal breakdown);

- Breakdown voltage Uk ∼ s from practical experience;

- Especially critical for tangential field strength at boundary surfaces of

laminated dielectrics.

Example: Solid-type cable

Belted insulation cable Radial-field cable

→ No belted insulation cables for higher voltages.





6.5. Fibre-Bridge Breakdown

- Mixed dielectric oil - paper – pressboard – wood

- Fibres in oil are hygroscopic, i.e. they attract water molecules from thesurrounding environment.

- Orientation and stringing together of fibres (strong forces caused by

εr = 81 of H2O)

Formation of fibre bridges under the influence of an electric field (right) in insulating oil contaminated with dielectric fibres (left)

- Travelling of the fibres into regions of high field strength

- Formation of a channel with higher conductivity

- Special case of the local thermal breakdown

- Remedy: Barriers for subdivision of free oil spaces





Life characteristics for different dielectrics (A.C. voltage):PE: Polyethylene foil in air, SF 6 and silicone oil

EP 1: Epoxy resin in model arrangement (d = 1mm)EP 2: Insulation sample with local increase of field strength by corrugated

metal foil inlaysEP 3: Same as EP 2 for large –scale insulations

Life tests with insulation samples produced under real manufacturingconditions

Life law (Montsinger’s law)

k

o

d

o

d

t

t

E

E /1

ˆ

ˆ−

⎟⎟ ⎠

⎞

⎜⎜⎝

⎛

=

k = Lifetime exponent





Orientation values for the short-time dielectric strength (1 minute),lifetime exponents and operating field strength (30 years) for severalinsulations at f = 50 Hz, T = 20 °C

Dielectric Application kd

E ˆ

(1 minute)kV/mm

b E ˆ

(30 years)kV/mm

Polyethylene Cables 140 3 … 7

PE + SF6 Foils 9 > 200

PE + oil Foils 30 > 200 < 40Paper + oil Capacitors 30 … 40 180 < 40

Cables 30 … 40 55 … 80 < 20

Transformers 20 … 30 3 … 7

Porcelain Insulators 125 1 … 3

Expoxy resin 12 125 1.5 … 4





6.6. Erosion Breakdown

Aging of solid insulating materials by:

- mechanical, chemical, thermal stress,- Weather and radiation,- Partial discharges and leakage currents,- Humidity

Breakdown field strength E d depending on stress duration

There are three ranges for insulating materials with high tan δ (Pertinax,

some cast resins, PVC):

- Electric breakdown,- Thermal breakdown,- Erosion breakdown.





Breakdown field strength E d depending on stress duration for polyethylene

For high-performance insulating materials with small tan δ (PE, PTFE,epoxy resin) one gets a lifetime line (double-logarithmic scaled diagram).

The region of thermal breakdown is missing, because the dielectriclosses in the insulating materials are not high enough.

Therefore it is possible, to determine the permissible operating fieldstrength for an expected lifetime of 25-40 years (= 200.000 ... 350.000hours).





6.7. Partial Discharge

A partial discharge (PD) is a localised dielectric breakdown of a smallportion of a electrical insulation system under high voltage stress. Itbelongs to the most substantial mechanisms for erosion discharges.

PD can occur in gaseous, liquid or solid insulating materials. It is ofteninitiated within gas voids enclosed in solid insulation, or in bubbles withina liquid insulating material, such as voids in an epoxy insulator, or gasbubbles dissolved within transformer oil.

The PD level is a measure for the number of imperfections inside theinsulating material and thus for the quality of high-voltage devices.

E

x

0ε r ε

Ed(x)

E(x)

E

x

0ε r ε

Ed(x)

E(x)





• In small gas voids and gaps the local electric field strength is higher

than in the surrounding insulating material. Usually the breakdownfield strength is lower than in the solid insulation too.

• Partial discharges can occur at the following exemplary locations:

1)

2)

3) 4)

5)

1) Surface defect of insulation2) Gas void in insulating mate-

rial

3) local destruction caused bypreceding stress

4) inadequate electrode adhe-sion

5) Gas gap at electrode edge

Besides these inner partial discharges outer PD can occur. For both

types the following equivalent networks were developed:





The current in both circuits is determined by the series impedance beforethe defect:

1

2

1

C R

ω >>

for outer PD

12C C << for inner PD





From there it follows:

Outer PD

( )dt

t U d C

R

t U t i 1

1

2

)()( ==

→

( ) ( )

12

1

C R

t U

dt

t dU =

Inner PD ( )( ) ( )

dt

t dU C

dt

t dU C t i 1

12==

( ) ( )dt

t dU

C

C

dt

t dU

1

21

=

( )dt

t dU 1

∼

( )dt

t dU





It is possible to sketch the voltage at C1, which is temporarily short-circuited by the spark gap.

Impulses at the peak of the testvoltage

Impulses at the zero passage of the test voltage

• The partial discharge is an extreme long-time mechanism.

• PD causes high-frequency field variations at the defect, resulting inhigh local dielectric losses and possibly a local thermal breakdown.

• Inside the defect ions are accelerated, which erode the insulationsurface and cause needle-shaped cracks.

• Accelerated electrons penetrate into the insulating material and oc-cupy trap levels (energy band model). Thereby the inner field emis-sion is intensified.

• PD can cause an intesified generation of NOx at the defect. If there isresidual humidity inside the void, nitric acid is generated, which cor-rodes the insulation.





PD channels (electrical tress) in polyethylene (LDPE):a bush-like-tree; b tree-like-tree