SEISMIC PERFORMANCE
OF INSULATORS IN
ELECTRIC SUBSTATIONS
Stu Nishenko, Khalid Mosalam,
Shakhzod Takhirov, and Eric Fujisaki
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Used in almost every substation equipment
Apparatus, e.g., bushings, circuit breaker
interrupter housings, surge arresters,
instrument transformers
Posts, e.g., bus supports, capacitor racks, air
core reactors, disconnect switches
Porcelain—Traditional material of choice;
long history of use
Brittle and massive—often a weak link
during earthquakes
Insulators in Substation Equipment
Bushings
Circuit breaker bushings,
interrupter housings, and
support columns
Interrupter
4
Insulators in Substation Equipment
Surge
arrester
Bushing
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Transformer bushings,
Surge arresters
Insulators in Substation Equipment
6
Instrument transformers
Insulators in Substation Equipment
Bus supports
7
Insulators in Substation Equipment
Post
insulator
Air disconnect switches
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Insulators in Substation Equipment
Post
insulator
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Circuit switchers
Insulators in Substation Equipment
Post
insulator
10
Capacitor racks/ platforms
Insulators in Substation Equipment
Post
insulator
Air core reactors
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Insulators in Substation Equipment
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Cable terminations
Characteristics of Porcelain Post Insulators
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Typical mechanical properties
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Physical configuration
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Elastic Modulus: 10,000 – 14,000 ksi
Modulus of Rupture: 7 – 16 ksi, COV = 0.06 - 0.15
Unit weight: 140 – 170 lb/ft3
Load carrying cores: 3” – 8” dia
Lengths depend on insulation level required:
14” at 12kV service – 152” at 500kV service
Sheds used to increase surface length and
prevent flashover event
Porcelain Post Insulators
Sheds
Load-carrying
porcelain core
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Ductile iron end fitting
with Portland cement
grout in joint
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Rated for cantilever load capacity (fixedbase, load at tip)
Also rated for tension, compression, torsion
Quasi-static, monotonic load tests
Assign load rating as dependable breaking
strength
Typically rating = Mean – 2σ, or -3σ
Sometimes rated according to ANSI
Technical Reference Standard
Seismic Design of Substation Insulators
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Governed by IEEE 693 Std.
Qualified by test or analysis as part of
the equipment
Designed for elastic behavior
Allowable Strength = 50% of
dependable capacity at 0.5g Required
Response Spectrum
Often the controlling element in an
equipment qualification
Circuit breaker support
columns
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Transformer bushings
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Surge arresters
Instrument transformers
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Bus supports (posts)
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Air disconnect switches (posts)
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Circuit switchers (posts)
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Capacitor racks (posts)
Industry Needs
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Better understanding of effects of cyclic
loading
Simple, reliable damage detection
techniques for post-shake test
inspection/ assessment
Improved insulator analysis models
Better understanding of failure
mechanisms
Methods for seismic qualification testing
with varied support characteristics
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Post insulator cyclic load testing
Development of finite element analysis
models
Hybrid simulation of disconnect switch on
support
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Obtained static break test data from
insulator manufacturer
Tested 6 posts of 2 different cross sections
Tested with cyclic load reversals, increasing
magnitude
Used hammer blows at intermediate points,
to attempt to detect damage
0.59*Mean Static
Number of
Cycles
6
0.66*Mean Static
6
0.72*Mean Static
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0.78*Mean Static
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0.86*Mean Static
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0.93*Mean Static
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1.00*Mean Static
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Monotonic to failure
1
Load Step
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Two types of failures observed
Cross-section #1: Cyclic Test Mean
Breaking Strength = 0.84*Static Test Mean
Cross-section #2: Cyclic Test Mean
Breaking Strength = 1.21*Static Test Mean
Hammer blows unable to detect damage
Sheds
Grout
Separation,
Fracture
No
No
No
No
Beam: lower porcelain
section extends to top
No
No
No
No
SAP2000
Beam elements with variable
cross section
Iron
No
No
No
M4
DIANA
Solid elements with variable
cross section
Iron
No
No
No
M5
DIANA
Solid elements with variable
cross section
Iron
Yes
No
No
M6
DIANA
Solid elements with variable
cross section
Actual
Yes
Yes
No
M7
DIANA
Solid elements with variable
cross section
Actual
Yes
Yes
Yes
Name
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Method
Modeling details
M1
Hand
Calcs.
Beam: lower porcelain
section extends to top
M2
SAP2000
M3
Caps
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Further development in progress
Parametric studies and comparisons
with test data
Frequency
Force/ displacement
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Varied supports may be used by different
utilities for same equipment
Repeated tests are costly
Test of equipment on full-scale support is
generally required
Lead time is long
Jaw
Post
Braced
frame
support
structure
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Insulator
550 kV Switch Test
0
10
10
1.2
0.6
0
-0.6
-1.2
0
1.2
acc. (g)
X acc. (g)
0
0
Physical Substructure
(switch jaw end with
blade open)
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0.8
Support structure
response or from
shake table test
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20
30
30
Earthquake
10
20
30
40motion
50
60
10
20
40
50
60
70
40
50
60
70
40
50
60
70
70
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Time (sec)
Computational
Substructure
Physical
Insulator Substructure
(assumed 1D)
Dynamic
Actuator &
Load Cell
550 kV Switch Test
Displacement (in)
10
5
Movable platform
0
-5
-10
0
Fixed tracks Calculated support structure response
applied to movable platform
10
20
30
40
30
40
100
Velocity (in/sec)
Force feedback
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Displacement command
0
-50
-100
Acceleration (in/sec2)
1.2 Earthquake motion
0.6
0
-0.6
-1.2
0
10 20 30 40 50
1.2
(g)
X acc. (g)
Dynamic DOF i
50
0
10
20
4
2
Computational
Substructure
0
-2
-4
0
10
20
30
40
Time (sec)
60
70
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Co-Authors
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Stu Nishenko, Sr. Seismologist, PG&E
Khalid Mosalam, Professor of Civil and
Environmental Engineering, UC Berkeley
Shakhzod Takhirov, Sr. Development
Engineer, UC Berkeley
Bonneville Power Administration
California Energy Commission
Pacific Gas and Electric Company