MCEER EVALUATION OF TRANSFORMER
BUSHINGS:
CURRENT AND RECOMMENDED PRACTICE
Andrei M Reinhorn, PE, PhD *, Presenter
Andre Filiatrault
Filiatrault, PEng,
PEng PhD
Michael C Constantinou, PE, PhD
Muhammad Fahad, PhD Candidate
Ni h l Oliveto,
Nicholas
Oli t PhD Candidate
Konstantinos Oikonomou, PhD Candidate
Maria Koliou, PhD Candidate
Anshel Schiff, PhD, Consultant/Adviser
MCEER-NEES Quake Summit
Buffalo, June 11, 2011
Introduction
Electrical transformer
Its main body is an oil-filled tank
Oil Conservator High Voltage
Surge Arrester
High Voltage
Bushing
Internal components:

M i core andd coils
Main
il

Small core (small transformer)
Radiators
Tank
External components:

High voltage bushings

High voltage surge arresters

Oil conservator

Radiators
Coils
Introduction
High voltage bushings

Insulate the high voltage conductors

Mounted on the cover plate of the transformer

Made of porcelain or composite material
Introduction
Past seismic performance

Bushings have proven to be
most vulnerable components,
experiencing fracture and oil
leakage

Failure associated to nature
off their
th i design
d i
(i h
(inherently
tl
brittle and relatively long
components)
Introduction
Past seismic performance

Unsatisfactory performance of electrical equipment in
past earthquakes (1989 Loma Prieta, 1994 Northridge,
1995 Kobe, 1999 Izmit, 1999 Chi-Chi, 2010 Chile etc) in
spite of extensive engineering planning

Cost of direct damage in the range of hundreds of
millions of dollars for each event (Schiff 1995)

Collateral
expenses
(replacement
of
damaged
equipment, the restoration of the network operation etc)

Restoration of network operation is significantly
impeded in case of a blackout
Introduction
Qualification of electrical equipment
“IEEE Recommended Practice for Seismic Design of
Substations” (IEEE 693-2005)

3 performance levels (High, Moderate and Low)

Bushings qualified through shake table testing (>161 kV)
 Transformer
bushing tested on a rigid stand at its in-service
slope

Tank amplification and cover flexibility “accounted for” by
testing at an input motion of twice (2 x) the RRS
Qualification Issues
Bushings are designed
and tested according
g
to best knowledge to
date, however,
failures during
earthquakes still
occur.
Why?
MCEER Project on Electrical Substation Equipment
EVALUATION OF TRANSFORMER BUSHINGS:
CURRENT AND RECOMMENDED PRACTICE
MCEER Effort to Evaluate and Improve
Current and Future Practice*
 Identifying shortcomings and developing new standards for qualifications
(CEC, BPA, MCEER, EPRI) – this presentation
 Analytical modeling of transformers and bushings (BPA) – Oikonomou
 Experimental evaluation of full scale bushings (CEC & BPA) – Muhammad
 Modeling of failure and capacity of electrical insulators (BPA) – Oliveto
 Improvements by stiffening of base of bushings (BPA) – Koliou/Filiatrault
 Global protection by base isolations (BPA) - Constantinou
Outline of Current Presentation
 Qualification issues – based on analytical and
experimental study
 Challenges in the current standard
 Motion at the top of transformers – vs
g
ground
 Dynamics of the mounted bushings
 Acceptance criteria – capacity and demand
 Recommendations
 Measuring strength capacity
 Determining strength demand analytically or
b testing
by
t ti
 Qualification by testing – challenges
 Additional notes as a result of current research
Determining challenges in current standards
STUDIES TO IDENTIFY CHALLENGES
Modeling
g for Assessment of Interactions
Actual transformer model
Modeling is explained using a typical transformer model

230 kV Ferranti-Packard
 Three
identical 196/230 kV high voltage bushings
mounted on the transformer’s cover plate

C t l bushing
Central
b hi vertical;
ti l Side
Sid bushings
b hi
i li d 15o
inclined
Modeling
g for Assessment of Interactions
Finite element model

I iti l model
Initial
d l off the
h transformer
f
developed in SAP2000; provided for
the current research by W.
W E.
E Gundy &
Associates, Inc

Model intended for static analysis
expanded for study of dynamics of
g
transformer and bushings
Experimental Investigation
Tested 9 bushings 230 kV and 550 kV
Several porcelain and several composites
Tested fixed (rigid) base bushings – in special set-up
Tested bushings on support structure simulating
transformer tank
 Tests performed at SEESL at University at Buffalo




Bushings tested
Bushings tested
Typical Physical Properties of Bushings Used in Experimental Study
yp
y
p
g
p
y
Bushing#1
Bushing#2
Bushing#3
Bushing#4a
Bushing#4b
Bushing#5
Bushing#6
Bushing#7
Bushing#8
Manufacturer
G.E.
G.E.
G.E.
HSP
HSP
Trench
ABB
ABB
ABB
Provider
CERL
CERL
CERL
LADWP
LADWP
Trench
ABB
ABB
ABB
Material of Insulator
Porcelain
Porcelain
Porcelain
Composite
Composite
Composite
Porcelain
Porcelain
Porcelain
Voltage Capacity (kV )
500
550
550
230
230
500
196/230 550
550
Designation
GE-500 -TypeU
GE‐500 ‐TypeU
GE‐500 ‐TypeU
HSP-230-1200
HSP-230-1200
500D004C_3
196w0800bz
T550W2000UD
T550Z3000SE
Stiff versus flexible base properties
Demand by simulation of field
conditions – “flexible” stand approach
19
Identifying Possible Modes of Failure/Damage
 Rocking of internal segment of bushings (insufficient moment
capacity)
 Sliding segments (insufficient shear friction)
 Porcelain cracking
 Long term impact on prestressing - clamping forces
 Breakage due to impact of other parts
SO FAR, NEVER QUANTIFIED USING ENGINEERING PARAMETERS
UPPER-1
Porcelain
Unit
Rubber
Gasket
Bushing
Mounting
Flange
Turret
Top flange
Challenges in Current Standards
CURRENT PRACTICE
Challenges in the current standard
 Motion at the top of transformers – vs ground
incorrectly considered
 Dynamics of the mounted bushings
completely neglected
 Acceptance criteria – capacity and demand
needed
Challenges in the current standard
 Motion at the top of transformers – vs ground
 Lateral accelerations amplifications vs ground
 Rotational accelerations at base of bushings
 Vertical accelerations at base of bushings
 Dynamics of the mounted bushings
 Dynamic response of bushings
 Frequencies of bushings mounted on a flexible base
 Stiff versus flexible base properties
 Dependency of frequencies on location and inclination
p
criteria – capacity
p y and demand ((see next page)
p g )
 Acceptance
Challenges in the current standard (cont
(cont’d)
d)
 Acceptance criteria – capacity and demand
 Dependency on visual appearance of failure modes
 Engineering parameters describing failure (forces,
moments,
t stresses,
t
deformations,
d f
ti
etc
t
 Current correlation of failure to PGA, ZPA, etc and
calculated
ca
cu ated moments
o e ts aand
d forces
o ces
 Acceptance based on PGA, ZPA, visuals
 Reliable and allowable strength capacity in future criteria
 Strength demands from analysis and testing – rigid
support approach
Challenges in the current standard
 Motion at the top of transformers vs ground
 Lateral accelerations amplifications vs ground
 Rotational accelerations at base of bushings
 Vertical accelerations at base of bushings
 Dynamics of the mounted bushings
 Dynamic response of bushings
 Frequencies of bushings mounted on a flexible base
 Stiff versus flexible base properties
 Dependency of frequencies on location and inclination
p
criteria – capacity
p y and demand ((see next p
page)
g )
 Acceptance
Challenges in Current Standards
MOTIONS AT BASE OF BUSHINGS –
TRANSFORMER COVER EXCITATION
Challenges in the current standard:
Motion at the top of transformers vs. ground
 Lateral accelerations amplifications vs ground
 Rotational accelerations at base of bushings
 Vertical
V ti l accelerations
l ti
att base
b
off bushings
b hi
Lateral accelerations amplifications vs
ground
Acceleration Spectrum at Turret Bottom for 3D Excitation p
(362kV Bushing ‐ Transverse Direction) 16
12
4.5
10
8
6
4
3.5
3
2.5
2
CERL (x2)
CERL (x2)
1.5
1
2
ξ = 2%
CERL
0.5
0
0
0
5
10
15
Frequency (Hz)
20
25
30
0
16
14
5
10
15
20
25
30
Frequency (Hz)
Acceleration Spectrum at Turret Bottom for 3D Excitation (362kV Bushing ‐ Transverse Direction) LANDERS
LANDERS (x2)
Turret Base
18
Amplification at Turret Bottom for 3D Excitation
Amplification
at Turret Bottom for 3D Excitation
(362kV Bushing ‐ Transverse Direction) 5
4.5
4.39
4
12
Am
mplification
Acceleration (g)
A
4.44
4
Am
mplification
Acceleration (g)
A
14
Amplification at Turret Bottom for 3D Excitation
Amplification
at Turret Bottom for 3D Excitation
(362kV Bushing ‐ Transverse Direction) 5
CERL
CERL (x2)
Turret Base
10
8
6
4
3.5
3
2.5
2
LANDERS (x2)
1.5
1
2
ξ = 2%
LANDERS
0.5
0
0
0
5
10
15
Frequency (Hz)
20
25
30
Figure 1 Acceleration Spectra for 500kV Transformer@ Bottom LV Bushing
0
5
10
15
20
25
30
Frequency (Hz)
Figure 1 Acceleration Spectra for 500kV Transformer@ Bottom LV Bushing
Motion at the top of transformers vs
vs.
ground
 Lateral accelerations amplifications vs ground
 Rotational accelerations at base of bushings
 Vertical
V ti l accelerations
l ti
att base
b
off bushings
b hi
Rotational accelerations at base of
bushings – flexible tank cover
Motion at the top of transformers vs
vs.
ground
 Lateral accelerations amplifications vs ground
 Rotational accelerations at base of bushings
 Vertical
V ti l accelerations
l ti
att base
b
off bushings
b hi
Vertical accelerations at base of
bushings
Challenges in Current Standards
CONSIDERING THE DYNAMICS OF
BUSHINGS AS INSTALLED IN
TRANSFORMERS
Challenges in the current standard
 Dynamics of the mounted bushings
 Dynamic response of bushings
 Frequencies of bushings mounted on a flexible base
 Stiff versus flexible base properties
 Dependency of frequencies on location and inclination
Challenges in the current standard:
Dynamics of the mounted bushings




Dynamic
D
i response off bushings
b hi
Frequencies of bushings mounted on a flexible base
Stiff versus flexible base properties
Dependency of frequencies on location and inclination
m,EI
H
k
a)
Dynamic response of bushings
Amplification factor (transversal)
3.5
Elastic Acceleration Spectrum (transversal)
7
3.00
Pseudo Acceleratio
on (g)
3
2.5
2
1.5
1
0.5
0
IEEE (1g PGA)
Corner
IEEE (2g PGA)
6
5
4
3
2
1
ξ = 2%
2%
0
0
5
10
15
20
25
30
0
5
10
Frequency (Hz)
Amplification factor (transversal)
5
4.5
15
20
25
Elastic Acceleration Spectrum (transversal)
9
Pseudo
o Acceleration (g)
4.32
4
3.5
3
2.5
2
1.5
1
0.5
0
IEEE (1g PGA)
Corner
IEEE (2g PGA)
8
7
6
5
4
3
2
1
ξ = 2%
0
0
5
10
15
Frequency (Hz)
30
Frequency (Hz)
20
25
30
0
5
10
15
20
Frequency (Hz)
Figure 1 - Acceleration Spectra for 230 kV Transformer at Bottom of HV Bushing (f
FB=21.0Hz, fAI=11.25 Hz)
25
30
Frequencies of bushings mounted on a
flexible base (Oliveto et al, 2011)
 Lumped Mass Model
f k  f fixed
1
1  3
  EI / kH
where
H
mEI
,
 Distributed Mass Model
f k  f fixed
1
1  
where

 3.516 
3
2
k
4.12
 Combined Distributed Lumped Mass Model
f k  f fixed
fi d
1
1  
where
   
3  1
  (1 /  )
where
  M / mH and   4.12 ,
Stiff versus flexible base properties
Dependency of frequencies on location
and inclination (Kong 2010)

Supportt st
Suppo
structure
uctu e

Re-locatable mounting plate
 Edge distance can be considered

Turret system
 To consider various ways of bushing mount
8 FT
 Tilted turret
 Vertical turret
8 FT
PL 127 X 127 X 3/4
L 5 X 5 X 3/4
20°
See D1. ~ D4. on Sheet #8
See D1
D1. ~ D4.
D4 on Sheet #8
8 FT
TS 5 X 5 X 1/2
See D1. ~
D4.D5.
on on
Sheet
#7 #8
See
Sheet
See D5. on Sheet #8
TableExtension
Extension
Table
Extension
Table
17”
Front View
Top View w/ Mounting Plate
Shake
Shake Table
Table
Shake
Tilted
Vertical
Noturret
turret
turret
/ At
/ At/center
At
center
center
Dependency
epe de cy o
of frequencies
eque c es o
on location
ocat o
and inclination
Dependency
epe de cy o
of frequencies
eque c es o
on location
ocat o
and inclination
W4: Y Freq at center w /o turret
W6: Z Freq at center w /o turret
1
1
0.6
0.6
0.6
0.2
G
1
G
G
W2: X Freq at center w /o turret
0.2
-0.2
0.2
-0.2
0
1
2
3
4
5
6
7
8
9
-0.2
10
0
1
2
3
4
Hertz
5
6
7
8
9
10
0
2
4
6
8
Hertz
10 12 14 16 18 20
Hertz
a) Bushing at center w/o turret
W10: Y Freq at side w /o turret
W12: Z Freq at side w /o turret
1
1
0.6
0.6
0.6
0.2
G
1
G
G
W8: X Freq at side w /o turret
0.2
-0.2
0.2
-0.2
0
1
2
3
4
5
6
7
8
9
10
-0.2
0
1
2
3
4
Hertz
5
6
7
8
9
10
0
2
4
6
8
Hertz
10 12 14 16 18 20
Hertz
b) Bushing at side w/o turret
W16: Y Freq at corner w /o turret
W18: Z Freq at corner w /o turret
1
1
0.6
0.6
0.6
0.2
G
1
G
G
W14: X Freq at corner w /o turret
0.2
-0.2
0.2
-0.2
0
1
2
3
4
5
Hertz
6
7
8
9
10
-0.2
0
1
2
3
4
5
6
7
8
9
10
0
Hertz
c) Bushing at corner w/o turret
Figure 1 Frequency dependence on location – No turret
2
4
6
8
10 12 14 16 18 20
Hertz
Dependency
epe de cy o
of frequencies
eque c es o
on location
ocat o
and inclination
Challenges in Current Standards
CURRENT ACCEPTANCE CRITERIA
Challenges in the current standard:
Current acceptance criteria
 “The acceptance criteria requires that there shall be no
slippage, visible oil leaks, or broken support flanges. The
qualification
lifi ti criteria
it i requires
i to
t monitor
it stresses
t
in
i the
th
flange to determine the integrity of the flange”
 “Bushings
Bushings are to be qualified by shake
shake-table
table acceleration
accelerationhistory testing to withstand a "Performance Level" defined
as twice the magnitude of the spectrum for the High
Required Response Spectra (RRS)”
 “The bushings shall be mounted on a rigid stand and the
bushing
g flange
g shall be subjected
j
to the excitation
corresponding to the "Performance Level"
Challenges in the current standard:
Current acceptance criteria
 “The acceptance criteria requires that there shall be no
slippage, visible oil leaks, or broken support flanges. The
qualification
lifi ti criteria
it i requires
i to
t monitor
it stresses
t
in
i the
th
flange to determine the integrity of the flange”
 “Bushings
Bushings are to be qualified by shake
shake-table
table acceleration
accelerationhistory testing to withstand a "Performance Level" defined
as twice the magnitude of the spectrum for the High
Required Response Spectra (RRS)”
 “The bushings shall be mounted on a rigid stand and the
bushing
g flange
g shall be subjected
j
to the excitation
corresponding to the "Performance Level"
Acceptance based on PGA,
PGA ZPA,
ZPA visuals
Current correlation of failure to PGA,
PGA
ZPA, - calculated moments and forces
H
V  ao m  z  z dz 
0
W
W
ab  aCG
g
g
m  z  z dz W

m  z  dz  m z dz  g a Z
  
H
H
M  ab m  z  z dz  ab
0
H
0
H
0
b
CG
W
 aCG Z CG
g
0
Such estimate can be severely non-conservative
Actual correlation of failure to PGA,
PGA
ZPA, - calculated moments and forces
(Oliveto,
(Oliveto Koliou et al)
VBr ,max  L1r Sa ( f r )  H L2 r S  f r  
H
 m( z )  ( z ) dz
r
0
M Br ,max  L1r S a ( f r )  H L2 r S  f r  
H
 z m( z )  ( z ) dz
r
0
Challenges in the current standard:
Current acceptance criteria
 “The acceptance criteria requires that there shall be no
slippage, visible oil leaks, or broken support flanges. The
qualification
lifi ti criteria
it i requires
i to
t monitor
it stresses
t
in
i the
th
flange to determine the integrity of the flange”
 “Bushings
Bushings are to be qualified by shake
shake-table
table acceleration
accelerationhistory testing to withstand a "Performance Level" defined
as twice the magnitude of the spectrum for the High
Required Response Spectra (RRS)”
 “The bushings shall be mounted on a rigid stand and the
bushing
g flange
g shall be subjected
j
to the excitation
corresponding to the "Performance Level"
Testing on Rigid Stand
Why not rigid stand approach ?
(Neglected components)
Lateral
Rotational
VBr
B ,max  
L1r Sa ( f r )  H L2 r S  f r  
Dynamic
Properties
H
 m( z )  ( z ) dz
r
0
M Br ,max  L1r S a ( f r )  H L2 r S  f r  
Transversal
I
Input
t IImproper
Evaluated
H
 z m( z )  ( z ) ddz
r
0
Dynamics
Rotational
properties of
Input Neglected bushings as
installed
neglected
Why not rigid stand approach ?
(Neglecting dynamics of bushings)
Amplification factor (transversal)
3.5
Elastic Acceleration Spectrum (transversal)
7
3.00
Pseudo Acceleration (g)
3
2.5
2
1.5
1
0.5
0
IEEE (1g PGA)
Corner
IEEE (2g PGA)
6
5
4
3
2
1
ξ = 2%
0
0
5
10
15
20
25
30
0
5
10
Frequency (Hz)
Amplification factor (transversal)
5
4.5
15
20
25
Elastic Acceleration Spectrum (transversal)
9
Pseudo Accceleration (g)
4.32
4
3.5
3
2.5
2
1.5
1
0.5
0
IEEE (1g PGA)
Corner
IEEE (2g PGA)
8
7
6
5
4
3
2
1
ξ = 2%
0
0
5
10
15
Frequency (Hz)
30
Frequency (Hz)
20
25
30
0
5
10
15
20
Frequency (Hz)
Figure 1 - Acceleration Spectra for 230 kV Transformer at Bottom of HV Bushing (f
FB=21.0Hz, fAI=11.25 Hz)
25
30
Challenges in Current Standards
NEEDED ACCEPTANCE CRITERIA
Challenges in the current standard:
Needed acceptance criteria:
Capacity vs Demand
 Required: Definition of reliable and allowable capacity
 Required: Determining realistic demand from analysis or
testing – flexible support approach
Develop acceptance criteria based on
strength
 Force and moment demand in critical sections smaller than
force and moment capacity
Vd ≤ SF * Vc
and
Md ≤ SF * Mc
………
 Define the strength capacity of equipment (bushings) in
terms of forces, moments, stresses and/or deformations
associated with the lowest unacceptable failure mode which
may not allow continuous operations of transformers.
Define and determine (reliable)
allowable strength capacity
 By static (or dynamic fragility) testing. Static may provide a
more conservative figure
 Manufacturer to specify the “reliable
reliable strength capacity”
capacity or
other name
 Reduce measured capacity by safety factor to account for
uncertainties, measurement errors, etc
Reliable and allowable strength capacity
in future criteria (Muhammad, 2011)
 Definition of critical sections where failure occurs
 Definition of the engineering parameters describing failure
 Shear
Sh force
f
– slippage,
li
cracking,
ki gasket
k t extrusion,
t i etc
t
 Moments – cracking, overturning, oil leak, etc
 Stresses …. all of the above
 Deformations …. All of the above
 Definition of the lowest unacceptable
p
mode of failure ((not
absolute) for functionality
 Definition of uncertainties and safety factors
Current definitions of the above are not clear in the standard
Define and determine strength demand
- in situ considerations
 The strength demand for the bushings and other related
equipment must consider:
 the amplifications of base motions - lateral and vertical
 the additional rotational motions of the support (cover of the
transformer) dependent on:
 the cover construction
 the location of the bushings in respect to the walls of the
transformer
 the
th dynamic
d
i characteristics
h
t i ti off the
th b
bushing
hi as mounted
t d on th
the
transformer – vertical, horizontal and rotational properties
RECOMMENDATIONS
Define and determine strength demand
- in situ considerations
 The strength demand expressed in the same engineering
parameters as the strength capacity (i.e. forces, moments,
stresses and or deformations)
 Should be determined based on the envelope seismicity
according to the current standard, or according to any other
more specific and documented requirement related to the
local seismicity.
 The current Required Response Spectra scaled adequately
for the performance levels for the ground motion, specified
for the lateral and vertical effects, should be used as
minimum.
i i
RECOMMENDATIONS FOR
QUALIFICATION PROCEDURES
a)
Install the bushing to simulate the worst “field conditions” from point of
view of base flexibility. This can be achieved with a flexible plate
installed in a test frame.
b))
Provide a motion at the base of the bushing
g to include horizontal and
rotational accelerations. This can be done with suitable shake table
motions.
c)
Provide an amplification of base motion derived from the earthquake
motion with proper amplification functions. Increase factor of 2 (or
g ) for possible
p
flexible tanks
higher)
RECOMMENDATIONS FOR
QUALIFICATION PROCEDURES
d)
Request of (c) can be eliminated if the transformer manufacturers are
requested to “miss-match” frequency of tank and of bushings
e)
Measure the shear and moment demands directly in testing. Do not rely
on “mass-acceleration” approximation.
pp
Measurements can be done
using strain gauge or load cells properly calibrated.
Recommendation for Future Standard:
Lab Qualifications






Capacity by static testing – issues
Demand by simulation of field conditions – “flexible” stand, not rigid
Consider vertical and rotational distortions
Use enveloped motions
Direct measurements of strength capacity and demand
Approximations based on accurate approaches
Direct measurements of strength
capacity and demand (Koliou et al, 2010)
 M
Measurement off force
f
andd moments can be
b done
d
b
by converting
i the
h
components of bushings as load measuring devices
 Strain measurements – straingages, fiber optic, clip gages, etc can
be used.
 Place on porcelain (preferred), or on steel flange (if geometry
allows).
 Possibly use four 45o straingage rosettes with the mid gage in
longitudinal direction of bushing.
bushing Use longitudinal to calibrate and
measure moments. Use diagonal gages to measure shear force.
 Calibrate by horizontal pull test
See presentation by Koliou
Approximated calculations of bushings
behavior based on accurate approaches
 See Report by Reinhorn et al, 2011 (to CEC and BPA)
presenting
ti d
detailed
t il d analytical
l ti l evaluation
l ti off b
bushing
hi and
d
approximation based on rigorous analyses with high
accuracyy p
prepared
p
based on Oliveto,, 2011.
Additional Notes:
PROTECTIVE SOLUTIONS
FOR TRANSFORMERS AND
TRANSFORMER BUSHINGS
Additional Notes from the Current
Research: Stiffening transformer covers
 Stiffening the transformer cover may eliminate
 base rotations,
 vertical
ti l amplifications,
lifi ti
 differences between as-installed and fixed base,
 reduce base moments and shears for “rigid”
rigid bushings,
bushings
qualified equipment,
q p
, mayy be used without
 Currentlyy q
additional qualification or modification
See presentation by Koliou
Additional Notes from the Current Research:
Base isolation for global protection
 Base isolate the transformer
 reduce base accelerations,
 narrow the
th frequency
f
b
band
d th
thatt goes tto transformer
t
f
away from sensitive equipment,
 control displacements through friction or damping
 reduce forces in bushings and other components
 Develop a demonstration test for the industry (underway)
See presentation by Constantinou
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Thank you for your attention!