insulator design

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AS/NZS7000 STANDARD

ELECTRICAL AND MECHANICAL DESIGN OF INSULATORS

Colin Lee – Network Systems Development Manager,

ENERGEX

Insulation Basics

Cigre Australian Panel B2- Overhead Line Seminar - AS/NZS7000:2010 Design of

Overhead LinesSydney 28-29 March 2011

• Insulation is subjected to electrical and mechanical stresses

•Electrical stresses include:

•Power frequency voltage

•Switching impulse voltage

•Lightning impulse voltage

•Mechanical stresses include:

•Tension loads (suspension and tension strings)

•Compressive loads (braced post insulators)

•Cantilever loads (post insulators)

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Line and Substation Coordination



• Impulse flashover strength of overhead line insulator usually exceeds BIL of substation plant insulation

•132 kV line insulation – 650 to 700 kV, 132 kV BIL of substation plant – 550 kV

•Line insulator is self restoring, substation insulation may result in destructive failure

•Measures need to be employed to protect substation plant from lightning surges

•Installation of OH earthwire

•Low tower footing resistance close to substation

•Installation of surge arresters

More on Electrical Stresses



•Power frequency voltage

•Normal operating – 1.1 p.u

•Maximum dynamic – 1.4 p.u

•Switching impulse overvoltages

•Up to 3 p.u peak phase to ground

•Can be higher with single phase auto-reclosing

•Usually design for 3 p.u and when auto-reclosing is employed, install surge arresters

•Lightning performance

•Overvoltages will vary depending on the lightning current (average of 30 kA) and usually exceed line insulation

•Design for acceptable lightning flashover rate – insulation influences shielding failure and backflashover rate

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More on Electrical Stresses



Design for Pollution Performance

Mechanism of Contamination Flashover

• Build up of contamination on insulator surface

• Light wetting of the pollution on the insulator surface

• build up of leakage current on the surface• formation of scintillating arcs and

associated audible noise level increases• formation of dry bands and build up of

electrical stress• sparking discharges across the dry bands

• the joining of the discharges to form a complete flashover



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Ceramic Insulator Profiles



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GUIDE FOR SELECTING INSULATORS IN CONTAMINATED ENVI RONMENTS

Contamination severity ESDD range(1) Minimum nominal specific creepage distance(2)

g/m mm/kV

Light 0 to1.2 16

Medium 1.2 to 2.0 20

Heavy 2.0 to 3.0 25

Very Heavy Above 3.0 31(1) ESDD is the equivalent salt deposit density.(2) Ratio of leakage distance measured between phase and earth over the r.m.s phase to phasevoltage of the highest voltage of the equipment.(3) Consideration should be given to increasing the creepage distances is areas where there are longperiods without rainfall or very close to the marine coast

References1. AS 1824.2—1985, Insulation coordination, Part 2: Application guide.2. IEC 60815, Guide for the selection of insulators in respect of polluted conditions.3. AS 4436 Guide for the selection of insulators in respect of polluted conditions. (Identical to ISO Report 815).




Example:

Select a suitable disc insulator string for a 132 kV line subject to heavy contamination. Use normal or fog disc profiles where the creepage length is 292 mm normal and 432 mm for fog.

System Highest Voltage = 145 kV (1.1 p.u)Minimum nominal specific creepage distance = 25 mm /kV for heavy contaminationRequired creepage distance for 145 kV = 3625 mmNumber of normal discs = 3625/292 = 12 discsNumber of fog discs = 3625/432 = 8 → 9 discs

Note: Need to check switching surge (3 p.u) and pow erfrequency (wet and dry) overvoltage conditions as well. Generally pollution performance governs.

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Composite Long Rod Insulator – Design



Composite Insulator – Design Aspects



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INSULATOR MECHANICAL DESIGN



TABLE CC1 - INSULATOR LOADING CONDITIONS

State Tension insulator condition

Suspension and vee string insulator

condition

Post and pin insulator condition

Everyday - Weight span, 0 Pa wind

Weight span, 0 Pa wind

Serviceable –working wind1

- Resultant load at serviceable wind or 500 Pa transverse

load

Resultant load with serviceable wind or 500 Pa transverse +

longitudinal unbalance load

Serviceable -Maintenance

Construction and maintenance loads

Resultant load for construction and

maintenance

Resultant load for construction and

maintenance

Ultimate load Ultimate load Resultant load for ultimate conductor

wind transverse load or failure

containment load

Resultant load with ultimate transverse wind + longitudinal unbalance load or

failure containment load

Notes: The criteria for serviceable working wind is damage or deflection limit

Porcelain or glass cap and pin string insulator units

Strength

0.95

(verified from statistical testing)

0.8

(unverified)

(electro-mechanical strength tested)

AS 3608

Porcelain or glass insulators other than cap and pin string insulator units

Strength 0.8 AS 3608

Synthetic composite suspension or strain insulators (See Note 2)

Strength

0.5

Long term

0.7

Short term ultimate (for one minute

mechanical strength)

AS 4435.1

Synthetic composite line post insulators (See Note 2)

Strength 0.9

(maximum design cantilever load)

AS 4435.4

Other synthetic composite insulators

Strength Subject to further

research



φφφφRn > Wn + 1.1 Gs +1.25 Gc +1.25 Ft

• Calculate the applied limit state loads (LST – at ul timate wind)• Apply appropriate LSD multiplier (1.25 for conducto r loads and 1.25 for for tension loads)• Apply appropriate component strength factor

Limit State Design

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Example: 132 kV Suspension Insulator

Calculate the strength of a suspension composite long rod used to support Oxygen conductor with a weight of 0.925 kg/metre, weight and wind span of 400 metres, and strung to everyday tension of 20% CBL. For broken conductor condition assume a wind of 0.25 of ultimate wind (1300 Pa).

Limit state condition - everyday load

Conductor weight load = 0.925 * 9.806* 400 N = 3628 NConductor weight multiplier = 1.25Limit state conductor weight load = 4535Assume no longitudinal load due to free swinging insulatorComponent strength factor for long rod insulator = 0.5 (Table 6.2 and long term strength)

Insulator specified mechanical load = 4535 / 0.5 = 9070 N



Example: 132 kV Suspension InsulatorLimit state condition - ultimate strength state unde r 1300 Pa wind

Conductor weight = 0.925 * 9.806* 400 N = 3628 NConductor weight multiplier = 1.25Limit state conductor weight load = 4535Assume no longitudinal load due to free swinging insulatorTransverse load = 0.0238 * 1300 * 400 = 12376 NTension load multiplier = 1.25Limit state transverse load = 15470 N Resultant load = SQRT (4535^2 + 15470^2) = 16120 NComponent strength factor for long rod insulator = 0.7 (Table 6.2 and short term strength)Insulator specified mechanical load = 16120 / 0.7 = 23028 N

Limit state condition - failure containment load und er broken conductor

Longitudinal load = 18600 N (at 325 Pa wind)Residual Static Load = 0.7 Longitudinal load with load relief = 13000 NTension load multiplier = 1.25Limit state tension load = 16300 NComponent strength factor for long rod insulator = 0.7 (Table 6.5 and short term strength)Insulator specified mechanical load = 16300 / 0.7 = 23300 N

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Example: 132 kV Suspension Insulator

Comments:

1. The determining state is the ultimate strength state under 1300 Pa wind or broken conductor condition

2. The minimum recommended size for the suspension insulator is 111 kN (specified mechanical load). The SML is a one minute withstand load.

3. If a ceramic disc insulator would be used, then the recommended minimum size is 70 kN (minimum breaking load).

4. The minimum recommended strengths are based on the requirement to achieve a design life comparable to other line components

Composite Long Rod Insulator – Design



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Composite Long Rod Insulator – Rating





Example: 132 kV Post Insulator

Calculate the strength of a composite line post insulator used to support Oxygen conductor in a clamp top with a weight of 0.925 kg/metre, weight span of 200 metres, and strung to everyday tension of 20% CBL. Servicable wind is 500 Pa and Failure Containment 1300 Pa.

Limit state load condition - everyday load

Conductor weight force = 0.925 * 9.806* 200 = 1814 NConductor load multiplier = 1.25 Limit State Vertical Load = 2270 NLongitudinal load for 3:1 adjacent span ratio, and max operating temperature of 75 deg C = 5200 NConductor tension multiplier = 1.25Limit state longitudinal design load = 6500 NResultant bending moment load = SQRT (2270^2 + 6500^2) = 6900 NComponent strength factor for composite post insulator = 0.9 (Table 6.2)Insulator ultimate design cantilever load = 6900 / 0.9 = 7650 N

Note: The maximum design cantilever load of a post insulator is typically 40 to 50% of the ultimate strength.

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Limit state condition - serviceable wind load at 500 Pa


Transverse compressive load = 0.0238*500*200 = 2380 NCompression load multiplier = 1.0 Limit state transverse compressive load = 2380 NCombining bending and compressive loads - simplified method:Compressive strength of 2.5 inch line post = 50 kNNeed for derating for combined bending and compression loadsDerating factor = 1-2380 / 50000 = 0.95Insulator ultimate design cantilever load with transverse load = 7650 / 0.95 = 8050 N




Limit state condition - failure containment or ultim ate load at 1300 Pa


Transverse compressive load = 0.0238*1300*200 = 6188 NCompression load multiplier = 1.00 Limit state transverse compressive load = 6188 NCombining bending and compressive loads - simplified method:Compressive strength of 2.5 inch line post = 50 kNDerating factor = 1- 6188 / 50000 = 0.88Insulator ultimate design cantilever load = 7650 / 0.88 = 8690N

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Comments:

The determining state is the failure containment lo ad where the factored ultimate design cantilever load is 8690 N.

A 2.5 inch post insulator is typically rated at 12. 5 kN ultimate cantilever strength and is recommended for this ult imate load

For spans much higher than 200 metres, the combined loads may exceed the 12.5 kN ultimate design cantilever stren gth. Design options to support the failure containment load for the long spans include:

• Brace 2.5 inch post with a long rod insulator• Limit the line layout to an adjacent span ratio of 2 or less• Use a 3 inch post which has a MDCL of around 9 kN

Composite Line Post Insulator – Rating



Braced Post

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Ceramic Disc Failures



Composite Insulator Failures



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Composite Insulator Failures



Composite Insulator Failures – UV Damage and Brittle

Fracture



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Questions

