Insulation Coordination Procedure and Insulation Strength Characteristics

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Procedure for Insulation Coordination in Four Steps

[IEC 60071-1]

Flow chart acc. to IEC 60071-1 (Figure 1)

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Procedure for Insulation Coordination in Four Steps

Determination of the coordination withstand voltages U

cw

• The coordination withstand voltages are the lowest values of withstand voltages of each overvoltage class, for which the expected low failure rate of the equipment is not exceeded over its full lifetime.

• Derived from the representative overvoltages U_rp by the coordination factor K_c.

[IEC 60071-1]

Typical for Germany:

0.1% per year 1 failure in 1000 years

(3)

Insulation Strength Characteristics

Factors influencing the dielectric strength of the insulation:

• magnitude, shape, duration and polarity of the applied voltage

• electric field distribution in the insulation

• homogeneous or non-homogeneous electric field

• electrodes adjacent to the considered gap and their potential

• type of insulation

• gaseous

• liquid

• solid

• combination of two or all of them

• impurity content and the presence of local inhomogeneities

• physical state of the insulation

• temperature

• pressure

• other ambient conditions

• mechanical stress

• history of the insulation (aging, damage)

• chemical effects

• conductor surface effects

Factors influencing the dielectric strength of the insulation:

• magnitude, shape, duration and polarity of the applied voltage

• electric field distribution in the insulation

• homogeneous or non-homogeneous electric field

• electrodes adjacent to the considered gap and their potential

• type of insulation

• gaseous

• liquid

• solid

• combination of two or all of them

• impurity content and the presence of local inhomogeneities

• physical state of the insulation

• temperature

• pressure

• other ambient conditions

• mechanical stress

• history of the insulation (aging, damage)

• chemical effects

• conductor surface effects

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Insulation Strength Characteristics

Standard atmospheric conditions acc. to IEC 60060-1

Temperature: 20 °C

Pressure: 1013 hPa Absolute humidity: 11 g/m

³

Temperature: 20 °C

Pressure: 1013 hPa

Absolute humidity: 11 g/m

³

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Insulation Strength Characteristics

Topics to be covered in the following:

• Insulators under polluted conditions

• Probability of flashover (Normal and Weibull distributions)

• Behavior of parallel insulation

• Coordination procedure: deterministic and statistical approach

• Correction with altitude of installation

• Clearances in air; "gap factors"

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Pre-conditions Pre-conditions

Performance of Insulators under Pollution

• Surface layers

• dust

• carbon black

• salt (coastal areas)

• chemicals (industry, rural areas: fertilizers)

• dust

• carbon black

• salt (coastal areas)

• chemicals (industry, rural areas: fertilizers)

• no problem in dry condition

• after long rain periods: only moderate effect on flashover performance

• most critical:

Humidification after a long dry period Humidification after a long dry period

“typical“ time of the day for insulator flashovers: morning hours (dew!)

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Development of pollution flashover Development of pollution flashover

dry zone by inhomogeneity of the layer

enlargement of the dry zone by heating of the zone edges (increased current density)

dry band

flashover of the dry band

enlargement of the dry band by arc heating

(max. temperature at foot points)

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Development of pollution flashover Development of pollution flashover

Voltage distribution a) with dry bands b) dry bands bridged

by partial arcs

voltage bridged

voltage drop increased

for further details see HVT 2!

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Performance of Insulators under Pollution

Layer conductivity K is the most important parameter!

K = κ ·ds κ ... specific layer conductivity ds ... thickness of layer

K = 5 µS "light to medium pollution"

K = 10 µS "medium to heavy pollution"

K = 40 µS "very heavy pollution"

Influence of layer conductivity

Influence of layer conductivity for details see IEC 60507

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Determination of layer conductivity from

measured conductance and insulator geometry Determination of layer conductivity from

measured conductance and insulator geometry

2 d

d d d / 2

r s K

G

l l r

π κ

π

⋅ ⋅

= =

Measurement of conductance G of the full insulator

shed core creepage

distance l

k

insulator length l

G V

l κ

general: =

(11)

k

0

d 2

l

G K

l r π

=

∫

k

0

d 2

l

F

π r

= ∫

G ... conductance of total insulator surface

“form factor"

K = F·G K = F·G

(IEC 60507

^*)

)

form factor to be determined by graphical procedure, described in IEC 60507

Determination of layer conductivity from

measured conductance and insulator geometry Determination of layer conductivity from

measured conductance and insulator geometry

*) IEC 60507, 2^ndEd. 1991-04: "Artificial pollution tests on high-voltage insulators to be used on a.c. systems"

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Decrease in flashover voltage by conductive layers Decrease in flashover voltage by conductive layers

û

_{fo, rain}

≈ 0.7 ... 0.9 · û

_{fo, dry}

U

fo, polluted

≈ 0.2 ... 0.3 · U

_fo,dry

An overhead line insulator must be desigend about five times as long as required to withstand operating stresses under dry conditions!

U

_m

= 123 kV û

_L-E

= 100 kV

û

_d

= 5 kV/cm l = 20 cm would be sufficient (dry!)

Actual length: ca. 1100 mm

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Countermeasures

p

Countermeasures

Sheds Sheds

s ... flashover or arcing distance l

k

... creepage distance

l

i

... insulator length

p ... shed overhang

t ... shed spacing

Terms ...

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p

l

i

derived from required standard lightning impulse voltage strength (u

_{d, LI}

ca. 5.5 kV/cm)

l

k

from requirement on specific creepage distance (IEC 60815

^*)

)

31 mm/kV for „very heavy" pollution severity (IV) 25 mm/kV for „heavy" pollution severity (III)

20 mm/kV for "medium" pollution severity (II) 16 mm/kV for "light" pollution severity (I)

∆ = 20%

Reference value is U

_m

, i.e.

the phase to-phase-voltage!

Reference value is U

_m

, i.e.

the phase to-phase-voltage!

Countermeasures Countermeasures

Note: IEC 60815 applicable to porcelain insulators; so far no standard on polymeric insulators available

*) IEC 60815, 1^st Ed. 1986: "Guide for the selection of insulators in respect of polluted conditions"

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Performance of Insulators under Pollution

Countermeasures Countermeasures

Correction of these values necessary depending on insulator's average diameter D_m ^*) Correction factor k_D (derived from service experience):

*) for definition of D_m see IEC 60815

D

_m

(mm) k

_D

< 300 1

300 - 500 1.1

> 500 1.2

Pollution performance gets worse

with increasing diameter!

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Pollution level Examples of typical environments

I - Light

- Areas without industries and with low density of houses equipped with heating plants - Areas with low density of industries or houses but subjected to frequent winds and/or rainfall

- Agricultural areas ¹⁾ - Mountainous areas

All these areas shall be situated at least 10 km to 20 km from the sea and shall not be exposed to winds directly from the sea ²⁾

II - Medium

- Areas with industries not producing particularly polluting smoke and/or with average density of houses equipped with heating plants

- Areas with high density of houses and/or industries but subjected to frequent winds and/or rainfall

- Areas exposed to wind from the sea but not too close to the coast (at least several kilometres distant) ²⁾

III - Heavy

- Areas with high density of industries and suburbs of large cities with high density of heating plants producing pollution

- Areas close to the sea or in any case exposed to relatively strong winds from the sea ²⁾

IV - Very heavy

- Areas generally of moderate extent, subjected to conductive dusts and to industrial smoke producing particularly thick conductive deposits

- Areas generally of moderate extent, very close to the coast and exposed to sea-spray or to very strong and polluting winds from the sea

- Desert areas, characterized by no rain for long periods, exposed to strong winds carrying sand and salt, and subjected to regular condensation

1) Use of fertilizers by spraying, or the burning of crop residues, can lead to a higher pollution level due to dispersal by wind.

2) Distances from sea coast depend on the topography of the coastal area and on the extreme wind conditions.

IEC 60815, Table 1

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Shed profiles Shed profiles

Some typical shed profiles (from IEC 60815; explanation of the parameters see there).

From left to right: normal shed profile, alternating shed profile, underrib sheds (fog profile), cap-and-pin insulators

Some typical shed profiles (from IEC 60815; explanation of the parameters see there).

From left to right: normal shed profile, alternating shed profile, underrib sheds (fog profile), cap-and-pin insulators

IEC 60815

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Performance of Insulators under Pollution

Recommendations of IEC 60815 - Example

c ≥ 30 mm p₁ – p₂ ≥ 15 mm

s/p₁ ≥ 0.65 (in case of plain, non-underripped sheds) l_x/d_x < 5

C.F. ≤ 3.5 (pollution classes I + II)

≤ 4 (pollution classes III + IV) C.F.= creepage factor = l_t/s_t

l_t = total creepage distance

s_t= arcing distance (arcing horns not considered)

IEC 60815

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Recommendations of IEC 60815 - Example

α ≥ 5 °specified

IEC 60815

No specification for bottom side angle;

however, ≥ 2 °"advisable" in case of sheds without underribs

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Performance of Insulators under Pollution

Shed profiles

Example of user's

experience

From:

Raouf Znaidi: "Service Experience and Maintenance Requirements for Different Types of Insulators in Tunisia", World Congress on Insulators, Arresters and Bushings, Hong Kong, Nov.

27-30, 2005

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Cleaning, greasing, coating of insulators Cleaning, greasing, coating of insulators

Some particular sites require regular cleaning of the insulators.

Extreme situation:

„Maritime desert climate with industrial pollution“

(e.g.: petrochemical facilities in Saudi-Arabia) Extreme situation:

„Maritime desert climate with industrial pollution“

(e.g.: petrochemical facilities in Saudi-Arabia)

But also in Middle Europe in the vicinity of industrial facilities (steel works, petrochemistry)

for further details see HVT 2!

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Semi-conducting glazing Semi-conducting glazing

Idea: to avoid dry-band arcing by resistive bypass

No flashover due to bypass current

surface current

Drawback: stable semiconducting glazing difficult to produce

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Semi-conducting glazing Semi-conducting glazing

Under development: for composite insulators by coating filled with micro-varistors

µ-varistors

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Composite insulators Composite insulators

• introduced in the beginning of the 1970s

• today „virtually" state of the art

• „problems": long time performance not yet clear,

"brittle fracture", animal attacks

Shed material:

• EPDM (Ethylene-Propylene- Diene-Monomer)

only in distribution

• Silicone rubber (SIR)

FRP core extruded SIR sheath

push-over SIR sheds

crimped-on metal end fitting

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From an EPRI Questionnaire in North America (publ. in 2003)

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From an EPRI Questionnaire in North America (publ. in 2003)

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compare this with the

"bathtub curve"of failure

no evidence for aging

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From an EPRI Questionnaire in North America (publ. in 2003)

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Hydrophobicity Hydrophobicity

One of the most important properties of composite insulators

with regard to pollution performance is Hydrophobicity Hydrophobicity

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Hydrophobicity Hydrophobicity

Advancing angle

Receding angle

= most important for characterization of hydrophobicity

Receding angle

= most important for characterization of hydrophobicity

Properties change under the influence of electrical field actual research!

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Hydrophobicity Hydrophobicity

Draft IEC 62073

Hydrophobicity classes

^*)

*) Based on the "STRI Guide" (of STRI, Ludvika/Sweden)

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Silicone rubber as insulator material Silicone rubber as insulator material

Hydrophobicity transfer to pollution layers Hydrophobicity transfer to pollution layers

Hydrophobicity only with silicone rubber Hydrophobicity only with silicone rubber

Dynamics of hydrophobicity Dynamics of hydrophobicity

Excellent service record so far (only few exceptions where silicone rubber is not optimal, e.g. under extreme coastal conditions, i.e. heavy salt layers)

Excellent tracking resistance

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Natural Test Sites Natural Test Sites

Weather Aging Tests for Polymeric Insulators

^*)

Example: Koeburg, RSA Realistic test conditions, but no acceleration

factors

long test times necessary (several years)

"(In)famous" test sites:

• Koeburg, RSA (Atlantic Ocean)

• Dungeness, UK (The Channel)

• Martiguez, F (Mediterranean Sea)

*) NOTE: often the term NCI = non ceramic insulators is being used

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Natural Test Sites Natural Test Sites

Example: Dungeness, UK (Excursion 2002)

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The "Tracking and Erosion Test" acc. to IEC 61109 The "Tracking and Erosion Test" acc. to IEC 61109

Similar test procedures specified e.g. for surge arresters (IEC 60099-4) and for polymeric insulators (IEC document 36/213/CDV: Project IEC 62217)

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The "Tracking and Erosion Test" acc. to IEC 61109 The "Tracking and Erosion Test" acc. to IEC 61109

Examples of test chambers

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Tracking

Erosion

The "Tracking and Erosion Test" acc. to IEC 61109

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Cyclic Tests Cyclic Tests

Cyclic tests usually consist in applying, in addition to voltage stress, various stresses in a cyclic manner:

- solar radiation simulation;

- artificial rain;

- dry heat;

- damp heat (near saturation);

- high dampness at room temperature (saturation has to be obtained);

- salt fog at low concentration.

Furthermore, temperature variations may cause some degree of mechanical stress, especially at the level of insulator interfaces and also give rise to condensation phenomena, which are repeated several times in the course of a cycle.

For power frequency test voltage, a test transformer shall be used. The test circuit when loaded with a resistive current of 250 mA (r.m.s.) on the high voltage side shall experience a maximum voltage drop of 5 %. The protection level shall be set at 1 A (r.m.s.).

Problem: no general agreement on one particular test!

Problem: no general agreement on one particular test! Examples next slides:

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Cyclic Tests Cyclic Tests

Practical test problem: rain and solar

radiation at the same time!

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Cyclic Tests Cyclic Tests

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Cyclic Tests Cyclic Tests

"EPRI" cycle: a year in service is considered to be represented by 10 days of summer cycle and 11 days of winter cycle. A duration of 5040 h is required for the whole test, 10 summer/winter cycles of 21 days each.

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Wheel Test acc. to IEC 62217 Wheel Test acc. to IEC 62217

Other Aging Tests for Polymeric Insulators

The test specimens shall be cleaned with de-ionized water before starting the test. The test specimens are mounted on the wheel as shown in Figure A.1 below. They go through four positions in one cycle. Each test specimen remains stationary for about 40 s in each of the four positions. The 90° rotation from one position to the next takes about 8 s. In the first part of the cycle the insulator is dipped into a saline solution. The second part of the test cycle permits the excess saline solution to drip off the specimen ensuring that the light wetting of the surface gives rise to sparking across dry bands that will form during the third part of the cycle. In that part the specimen is submitted to a power frequency voltage. In the last part of the cycle the surface of the specimen that had been heated by the dry band sparking is

allowed to cool.

Electrical stress: The power frequency test voltage in kV is determined by dividing the actual creepage distance in millimetres by 28,6.

NaCl content of de-ionized water: 1,40 kg/m³ ± 0,06 kg/m³ Ambient temperature: 20 °C ± 5 K

Test duration: 30 000 cycles

The test is regarded as passed, if on both test specimens:

• no tracking occurs

• for composite insulators: erosion depth is less than 3 mm and does not reach the core; if applicable

• for resin insulators: erosion depth is less than 3 mm;

• no shed, housing or interface is punctured.

Extremely severe test!

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Wheel Test Wheel Test

Other Aging Tests for Polymeric Insulators

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"Silicone Bonus"

For NCIs with permanent (recovering) hydrophobic characteristics a "silicone bonus" may be applied as a reduction factor of creepage distance (C.D.) compared with ceramic insulators:

Class 1 : 70 % … 75 % of C.D. of ceramic insulators

Class 2 : 80 % of C.D. of ceramic insulators (not applicable in coastal areas!) Class 3 : same C.D. as for ceramic insulators

Class 4 : in general, application of NCI should be carefully checked for each individual application

Class 1 : 70 % … 75 % of C.D. of ceramic insulators

Class 2 : 80 % of C.D. of ceramic insulators (not applicable in coastal areas!) Class 3 : same C.D. as for ceramic insulators

Class 4 : in general, application of NCI should be carefully checked for each

individual application

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Performance of Polymeric Insulators

Animal attack (parrots, cockattoos, termites) Example: Australia

Example: Australia

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Brittle fracture Brittle fracture

From:

M. Kuhl: "FRP Rods for Brittle Fracture Resistant Composite Insulators",

http://www.lappinsulator.com/downloadcenter/technical.asp

Countermeasures:

• ECR glass (electro-chemical resistant)

• quality of sealing at triple point

• field stress reduction by grading rings

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Performance of Insulators under Pollution

2 different methods:

Salt fog method Salt fog method

Solid layer method Solid layer method

Artificial pollution tests Artificial pollution tests

IEC standard 60507

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Salt fog method Salt fog method

Test specimen energized at operating voltage under conductive salt fog exposure Test specimen energized at operating voltage under conductive salt fog exposure

Salt mass concentration between 2.5 kg/m

³

und 224 kg/m

³

Salt mass concentration between 2.5 kg/m

³

und 224 kg/m

³

(1 kg/m

³

corresponds to 1 g/l)

Classification by withstand salt mass concentration Classification by withstand salt mass concentration

Test specimen must not flash over within a specified time of exposure

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Solid layer method Solid layer method

Test specimen is energized in a cold fog chamber and then exposed to humidity

Layer conductivity between 3 µS and 80 µS Layer conductivity between 3 µS and 80 µS

Classification by withstand layer conductivity or withstand salt deposit density Classification by withstand layer conductivity or withstand salt deposit density

Solid layer of specified conductivity is applied in wet condition and dried Solid layer of specified conductivity is applied in wet condition and dried

Test specimen is exposed to humidity in a cold fog chamber and then energized

Test specimen must not flash over within a specified time of exposure Test specimen must not flash over within a specified time of exposure

Salt Deposit Density (SDD) between 0.03 mg/cm

²

and 0.60 mg/cm

²

Salt Deposit Density (SDD) between 0.03 mg/cm

²

and 0.60 mg/cm

²

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Artificial pollution tests Artificial pollution tests

Correlation between pollution level, recommended creepage distance and artificial pollution test parameters:

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Radial Field Stress under Pollution Radial Field Stress under Pollution

MO column

Conductive layer

Gas or solid Solid

U

axial, int

U

_radial

Arises if there is an internal active part with a given, constant axial voltage distribution; risk of

• internal PD in case of internal gas volume

• puncture in case of pure solid insulation MO-Scheiben

Porzellangehäuse-Innenwand MO discs

porcelain housing, inner wall

Photo: PD in a porcelain housed surge arrester

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Dielectric and Thermal Effects Dielectric and Thermal Effects

Internal partial discharges

⇒ changes in internal atmosphere

⇒ risk of deterioration of all internal parts Internal partial

discharges

⇒ changes in internal atmosphere

⇒ risk of deterioration of all internal parts

Risk of partial heating of internal active elements Risk of partial heating of internal active elements

Risk of external flashovers Risk of external flashovers

Outer surface discharges

⇒ Risk of partial heating of internal active elements Outer surface discharges

⇒ Risk of partial heating of internal active elements

Example: 800-kV surge arrester

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Emerging Insulator Standards

From:

Claude de Tourreil: "New IEC standards: their Impact on future Selection of Composite

Insulators", World Congress on Insulators, Arresters and Bushings, Hong Kong, Nov. 27- 30, 2005