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

High-Voltage Technique and Insulating Materials Page 63

Breakdown Field Strength

Assumption: eff* s = k and homogeneous field

From AeffpE

BA

p

== and

/exp*

follows

+

=

A

d

sspA

pBE

***ln

*

Calculated breakdown field strength Ed 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.

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

High-Voltage Technique and Insulating Materials Page 64

Breakdown Voltage (Paschen Curve)

Ud = Ed * s

+

=

spAspB

dU

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

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

High-Voltage Technique and Insulating Materials Page 65

Paschens law as analytical approximation formula (thin curve) and as real curve (broadcurve)

- Another breakdown mechanism works above 18...14=deff .

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

High-Voltage Technique and Insulating Materials Page 66

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 For long sparking distances or in case thatthere are no electrodes at all this model is no longer valid.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 ofthe 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: Eg 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

= dxL

eff

(determined experimentally) photon radiation is emitted.

- The photons can generate secondary avalanches.

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

High-Voltage Technique and Insulating Materials Page 67

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/s,

because the photons can bridge long distances with the velocity oflight.

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

High-Voltage Technique and Insulating Materials Page 68

Avalanche growth in homogeneous field

Streamer growth against the field direction

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

High-Voltage Technique and Insulating Materials Page 69

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 SF6

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

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

High-Voltage Technique and Insulating Materials Page 70

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 SF6and N2depending on the volume percentage VSF6of SF6 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 m, drawn material of about 6 m.These micro peaks cause an increase of the electric field strength.

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

High-Voltage Technique and Insulating Materials Page 71

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 and number of electrons Ne in an

avalanche near the rough surface for air, SF6and SF6with increased pressure

A breakdown can occur if the value of eff exceeds the above men-tioned limiting values.

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

High-Voltage Technique and Insulating Materials Page 72

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

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

High-Voltage Technique and Insulating Materials Page 73

Polarity Effect

Polarity effect in a strongly inhomogeneous field for positive point electrode (left) andnegative 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.

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

High-Voltage Technique and Insulating Materials Page 74

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

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

High-Voltage Technique and Insulating Materials Page 75