A3 304 2014

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A3_304_2014
CIGRE 2014
Study of seismic design and guideline of substation equipment based on the
Great East Japan Earthquake
I. OHNO
T. ITO
H. NAKAKOJI
Tokyo Electric Power Co.
T. KOBAYASHI
H. SATO
Central Research Institute of Electric Power Industry
Japan
SUMMARY
The Great East Japan Earthquake, which occurred on 11-Mar-2011, was one of the most severe
earthquakes in the world with a magnitude 9.0. A large area of East Japan experienced the strong
quake for a long time. In the earthquake, a high level seismic force was observed which was twice as
high as that assumed by JEAG 5003, 848 units of equipment in 200 substations were damaged, and
275 units in 80 substations were forced out of service by the earthquake. However the population of
damaged equipment was small (1 % or less) among all equipment in the substations and electricity
supply was recovered in a week. Seismic design specification in Japan, JEAG 5003, was established to
prevent serious damage to equipment as a result of general seismic motion, and/or to prevent a serious
electrical outage due to high level seismic conditions. The limited duration and area of the interruption
in power supply suggests that JEAG 5003 could satisfy the expected purpose even in a massive
earthquake. However, the definition of an individual high level specification may be necessary for
equipment for which the influence of failure has a significant impact or which would result in a long
time for the restoration of damage. This paper reviews the design process by JEAG 5003 and discusses
in detail the following issues about seismic design for high level seismic forces.
Ground surface accelerations during the earthquake were much higher than the values
estimated by JEAG 5003. This caused the acceleration at a centroid of equipment to be two
times higher than the value expected by JEAG 5003.
Magnification factor of equipment and base amplification defined in JEAG-5003, by which
ground surface accelerations are multiplied to obtain the seismic design force in JEAG 5003,
were found to be appropriate.
The seismic design for a high level seismic force was studied. TEPCO has applied a guideline
of the design condition being twice as high as the seismic design force of JEAG 5003 to
equipment from the 1970’s or 1980’s. Damage by the earthquake was observed mostly among
the old equipment to which the guideline was not applied. This suggests that applying twice the
high seismic design force as the value defined by JEAG 5003 is adequate. It can cover more
than 90 % of recent earthquakes exceeding 300 gal.
Consideration based on the rare types of damage by the earthquake was discussed. Since the
damage of equipment due to non-linear motion, such as deformation of steel parts or collision
of moving parts by the high level seismic force, were observed in the earthquake, verification
of these phenomena by seismic testing is important for the design to be able to withstand high
level seismic forces. The design as a total system considering the influence of the conductor
should also be the challenge.
KEYWORDS
The Great East Japan Earthquake, High level seismic condition, Seismic design guideline
[email protected]
1. Introduction
The Great East Japan Earthquake, which occurred on 11-Mar-2011, was one of the most severe
earthquakes in the world with a magnitude 9.0. However, the damage ratio (population of equipment
which were forced out of service) was as small as 1 % or less among all equipment in the substations
which experienced the heavy quake (recorded ground surface acceleration 300 gal and higher). But, in
the analysis according to equipment type, the ratio of some categories was a little high. Seismic
damage causes were bending stresses on porcelain insulators in past massive earthquakes, but in this
earthquake, the equipment with a higher damage ratio experienced other failure modes or received
extreme loads. In this paper, failure processes and damage causes are examined from these damage
situations or dynamic analysis results. In addition, a comparison between the Great East Japan
Earthquake and the specified seismic condition in the seismic design guideline of Japan (JEAG 50032010 “Guideline for seismic design for electric equipment at substations, etc.”[1]), and the
effectiveness of earthquake resistant measures are considered.
2. Review of the equipment damage and the seismic design guideline of Japan
2-1. Review of the equipment damage
The Great East Japan Earthquake occurred at 14:46 on 11-Mar-2011, in a border between the North
American and Pacific Plates. The scale was estimated to be a moment magnitude 9.0, and the focal
depth was 24km. It was a massive earthquake with a focal region of 450km × 150km and a fault
rupture length of approximately 30m maximum.
Characteristics of this earthquake were, because fault rupture occurred three times with time lag, a
strong quake in the wide area of East Japan with a long duration of about 3 minutes. Figure 2 shows
the waveform of this earthquake observed at Tokyo Electric Power Company’s (TEPCO) ShinFukushima substation where there was significant equipment damage.
848 units of equipment in 200 substations were damaged and 275 units in 80 substations were forced
out of service by the earthquake. Table 1 shows the numbers and ratio of damaged main equipment.
Overall, the population of inoperable equipment was as small as 0.2~ 1.0% among all equipment in the
substations which experienced the heavy quake with a ground surface acceleration of 300 gal and
higher.
Ground surface acceleration [gal]
Center of
earthquake
Focal region
Area of electricity
supply by TEPCO
Fig. 1 Peak acceleration contour map of the
Great East Japan Earthquake [2]
Fig. 2 Earthquake waveform at ground surface
(TEPCO Shin-Fukushima S/S.)
Table 1 Number and ratio of damaged main equipment
Forced out of
service
Equipment
Population*1
Circuit-breakers
7,284
15
0.2%
Disconnectors
Instrument
transformers
15,363
133
0.9%
4,986
15
0.3%
Arresters
3,574
37
1.0%
*1:Number of equipment in the substations which recorded
300 gal and higher ground surface acceleration.
*2:Table 1 does not include the numbers about GIS.
(Damage case of GIS was very few)
1
2-2. The specified seismic condition in the seismic design guideline of Japan
JEAG 5003, which is a seismic design standard in Japan, is established to prevent serious damage to
equipment as a result of general seismic motion (which the equipment will be expected to experience 1
or 2 times during their expected service period), and/or to prevent a serious electrical outages (i.e. long
time or wide area) due to high level seismic conditions. It defines “300 gal resonant 3 cycles sine wave
sudden input (applying at the bottom of equipment)” as the seismic design condition for equipment
with porcelain insulators. The defined input provides the acceleration as 1830 gal to the center of
equipment, which corresponds to 680 gal resonant 1 cycle sine wave at the bottom (based on a
damping factor 5%).
The calculation method for seismic design force by JEAG 5003 is shown in Table 2 and Figure 3,
which enable a rational design and simple verification for many kinds of substation equipment and
various ground conditions.
Table 2 Calculation method of seismic design force by JEAG 5003
Item
Acceleration
Ground surface
motion
Wave form
(a)
(Response factor *1)
Amplification of base (b)
Uncertainty factor (c)
(vertical acceleration, connected conductors, etc)
Definition
300 gal
Resonant 2 cycles sine wave
(4.7)
1.2
1.1
(a)×(b)×(c) = 396gal: resonant 2 cycles sine wave
→300gal: resonant 3 cycles sine wave *2
*1 Response factor = equipment centroid acceleration / input acceleration
*2 Conversion from ”Resonant 2 cycles sine wave” to ”Resonant 3 cycles sine wave” : 4.7/ 6.1 = 1/1.3
Response factor for ”Resonant 2 cycles sine wave” = 4.7, Response factor for ”Resonant 3 cycles sine wave” = 6.1
Design seismic force
Equipment centroid acceleration
1830gal
Response of
equipment
Supporting
structure
Response of equipment
(3) x1.1 (Uncertain factor)
(1) x4.7
(Resonant 2 cycles sine wave)
x 6.1 (Resonant 3 cycles sine wave)
Base surface
Base
(2) x1.2 (Influence of foundation)
Input at ground surface
300gal Resonant 2 cycles sine wave
Input at lower end of
supporting structure
Supporting
structure
[Response characteristics]
300gal Resonant 3 cycles sine wave
[Seismic design force]
Fig. 3 Response characteristics and seismic design force of equipment with porcelain insulator
TEPCO has applied a design condition twice as high as the seismic design force of JEAG 5003 to 500
kV equipment from the 1970’s and to 275 kV equipment from the 1980’s, which is higher than the
IEC standard.
3. Review of the process of seismic design force in JEAG 5003
Ground surface acceleration, magnification factor and amplification of base assumed in JEAG 5003
for the calculation of the seismic design force, and the equipment centroid acceleration are compared
with those of the Great East Japan Earthquake. Table 3 shows the results of comparison by calculation
of the data at Shin-Fukushima S/S and Shin-Motegi S/S of TEPCO.
Table 3 Results of comparison between JEAG 5003 and the Great East Japan Earthquake *3
Great East Japan
Comparison results
Earthquake
(1) Ground surface acceleration
300 gal
428-1069gal
Excess of expected level
(2) Response factor of equipment
4.7
1.7-3.7
Within expected value
(3) Amplification of base
1.2
1.0-1.3
As expected on standard ground
(4) Equipment centroid acceleration
1830 gal
1752-3720gal
Excess of expected level
*3 Analysis of data at Shin-Fukushima S/S and Shin-Motegi S/S where significant damage to equipment was observed.
Items
JEAG 5003
2
Shin-Fukushima 500 kV DS
Shin-Fukushima 275 kV ABCB
Shin-Fukushima 500 kV IT
Shin-Fukushima 275 kV SA
Shin-Motegi 500 kV DS
Shin-Motegi 275 kV DS
Shin-Motegi 500 kV IT
Resonant 3 cycles sine wave
Magnification factor
(1)Acceleration at ground surface:
Ground surface accelerations by the Great East
Japan Earthquake were from 428 to 1069 gal,
which exceed the acceleration level of 300gal
expected in JEAG 5003.
Resonant 2 cycles sine wave
4.7
(2)Magnification factor of equipment:
Resonant 1 cycle sine
The magnification factor of equipment is defined
as the ratio between the equipment centroid
Cycle
Freq.(Hz)
acceleration and the input acceleration. A
Fig. 4 Magnification factor of equipment
magnification factor 4.7 is used in JEAG 5003,
which covers 93% of the response factors of earthquakes in the past. The magnification factors of
damaged equipment by the Great East Japan Earthquake range from 1.7 to 3.7 and were as expected
in JEAG 5003.
1.35
Amplification factor > 1.2
Amplification factor > 1.2
Dead tank GCB
Amplification factor > 1.2
66 kV equipment
(excluding dead tank GCB)
Vs 150m/s
Amplification factor for foundation
Proportion of equipment with
amplification factor exceeding 1.2
(3)Amplification of base:
The seismic force at the lower end of structures is amplified by the influence of the ground and base.
The amplification factor caused by base is assumed to be 1.2 in JEAG 5003 for standard ground
with S-wave velocity of >150m/s to enable a rational design and simple verification as shown in
Figure 5. Sufficiently small amplification factors of damaged equipment were confirmed as shown
in Figure 6. However in the case of bad ground condition (Vs<150m/s), the amplification factors
become higher than that of standard ground condition and attention is necessary on the arrangement
of equipment.
Initial value of Vs(Vs for foundation
under normal condition)
Convergent value of Vs(Vs after decline
of stiffness due to earthquake)
1.3
1.25
1.2
1.15
1.1
1.05
1
Proportion of each Vs (%)
Fig. 5 Results of survey of amplification of
base in past earthquakes
80
100 120 140 160 180 200 220 240 260 280 300 320
Vs(m/s)
Fig. 6 S-wave velocity of the ground and amplification
of base of damaged equipment
(4)Equipment centroid acceleration:
JEAG 5003 defines 300gal resonant 3-cycles sine wave as the seismic design force. The equipment
centroid acceleration 1830 gal is defined at 5% damping. Accelerations at the centroid of damaged
equipment were calculated from 1752 to 3720 gal by dynamic analysis which exceed nearly twice
the value of 1830 gal defined in JEAG 5003.
(5)Results of review:
The ground acceleration by the Great East Japan Earthquake exceeded the 300 gal value specified in
JEAG 5003 and was a main cause for the centroid acceleration of damaged equipment being
approximately twice as high as that in JEAG 5003. On the other hand, magnification factors of
equipment and amplification of the base were smaller than those in JEAG 5003, which indicates the
sufficient margin in the equipment design.
The next chapter discusses seismic design and design standards for high level seismic forces by
using acceleration response spectrum.
3
4. Consideration of seismic design and seismic standard to high level seismic force
Figure 7 shows a comparison of acceleration response spectrum between the Great East Japan
Earthquake and other past big earthquakes. Predominant frequencies in the wave of the Great East
Japan Earthquake were within natural frequencies of porcelain-type equipment. Also, this earthquake
was quite severe compared with big earthquakes in the past. The maximum acceleration was nearly
two times higher than that in JEAG 5003 and also exceeds IEC and IEEE spectrum values.
resonant frequency range of damaged equipment
resonant frequency range of damaged equipment
10000
spectrum of acceleration response [gal]_
spectrum of acceleration response [gal]_
10000
JEAG 5003×2
IEEE693 PL
JEAG 5003
1000
IEC 62271-207
IEEE693 RRS
100
0.01
Great East Japan Earthquake in 2011
Niigataken Chuetsu-oki Earthquake in 2007
M id Niigata prefecture Earthquake in 2004
Hyogoken Nanbu Earthquake in 1995
The Off-M iy agi Pref. Earthquake of 1978
0.1
period [sec]
JEAG 5003×2
IEEE693 PL
JEAG 5003
1000
100
1
0.01
10
IEC 62271-207
IEEE693 RRS
Great East Japan Earthquake in 2011
1989 Loma Prieta
1992 Cape Mendocino
1994 Northridge (Sylmar,USA)
1999 Duzce, Turkey
0.1
period [sec]
1
10
Fig. 7 Comparison of acceleration response spectrum among the past big earthquakes in Japan
Safty factor (with respect to JEAG 5003)
4
Figure 8 shows safety factors of equipment
with and without damage by the Great East
circuit breakers
(no damage)
Japan Earthquake versus JEAG 5003. Safety
arresters
factor is defined as the ratio of the allowable
: conductor interaction
(no damage)
3
stresses of materials and the produced stress
circuit breakers
against the input specified in JEAG 5003.
TEPCO has applied a design condition twice
disconnectors
as high as the seismic design force of JEAG
2
arresters
5003, safety factor of 2 against JEAG 5003, to
equipment from the 1970’s or 1980’s. Safety
current
transformers
factors of most damaged equipment were less
1
than 2 and those of equipment without damage
400
600
800
1000
1200
were higher than 2. The safety factor of 2
Peak value of acceleration at foundation [gal]
correlates with the observation that the
Fig. 8 Safety factors with respect to of JEAG 5003 of
acceleration level of the Great East Japan
equipment which were damaged and not damaged
Earthquake was nearly two times than seismic
design force based on JEAG 5003.
The damaged equipment is classified into two types. One is old-type equipment without seismic
countermeasures and equipment whose seismic performance appears to decrease from the initial stage.
The other is the influence of connecting leads.
The characteristic damage of this equipment is described in Chapter 5.
JEAG 5003 was established to prevent a serious electrical outage due to high level seismic conditions.
The limited duration and area of the interruption in power supply suggests that JEAG 5003 could
satisfy the expected purpose even in a massive earthquake. However, for equipment whose influence
of failure has significant impact or that needs a long time for restoration, it may be required to define
an individual high level specification.
According to these considerations, the study of seismic design and seismic standards against the high
level earthquakes was conducted among Japanese utilities. The main items of the study are as follows.
Consideration of requirements which can cover the high level seismic forces based on the
analysis of recently observed earthquake waves.
Introduction of seismic force calculated from acceleration response spectrum and testing with
artificial seismic wave (time-history vibration test) into JEAG 5003 standard.
4
These issues are under discussion in the Japanese
electric power industry by taking into account other
seismic standards, IEEE 693 and IEC.
Figure 9 shows the results of analysis of observed
waves [2] of recent earthquakes exceeding 300 gal. The
results show that the level covering 95 % of the waves
corresponds to the level of two times the high seismic
design force as defined by JEAG 5003. Although it can
be suggested that 2 times higher acceleration may be
adopted as the seismic design specification for high
level seismic force, further discussion is necessary to
build a consensus in Japan.
Fig. 9 Response spectra observed in resent
earthquake in Japan exceeding 300 gal
5. Verification test and considerations on seismic design based on the distinctive damage
of the Great East Japan Earthquake
Although seismic damage was caused by bending stresses on porcelain insulators in past massive
earthquakes, other mechanisms of damage were observed in this earthquake as follows.
Damage due to high stress caused by non-linear displacement of structures.
Non-linear displacement of structures could not be taken into account during the development
process because it did not appear within design seismic duty twice as high as JEAG 5003.
Damage by extreme loads through conductors.
In the following sections, verification tests and considerations to ensure the seismic performance of
equipment are discussed based on the distinctive damage by the Great East Japan Earthquake.
5-1. Damage due to high stresses caused by non-linear displacement of structure:
(1) Damage of parts other than porcelain insulator (deformation of steel parts):
Figure 10 shows damage of 275kV air blast circuit-breakers. The supporting insulators for main- or
sub-breaking-units were broken. It is observed that stay-insulators became slack. The stay-insulators
had been added to reinforce the support-insulators by their tension in three directions based on past
seismic experience. However, the tension was lost due to deformation of the steel bases by 6mm.
Since the estimated PGA (peak ground acceleration) at the substation was as large as 1069gal with a
long duration, the steel bases of the stay-insulators were deformed. Consequently, the stay-insulators
lost their function to reinforce the support-insulators and they were damaged by the seismic force.
Main-breaking-units
Main-units
broken
Subbreakingunits
Sub-units
broken
Stay-insulators
became slack
Stay-insulators
*pulling head from
three directions
to stay vibration
Steel bases
transformed upward
Support-insulators
Fig. 10 Damages of 275kV air circuit-breaker
(2) Damage by displacement and collision of moving structures:
A large amount of damage was observed around flexible joint parts in the middle of the operatinginsulators of 500kV disconnectors as shown in Figure 11. Based on prior experience, 500kV
disconnectors are equipped with flexible joints in the middle of the operating-insulators to avoid
bending stresses at high seismic condition. However, collision marks were observed at upper and
5
lower parts of the pin joints of the damaged disconnectors as shown in Figure 11. It is considered
that collision of moving parts occurred due to vertical displacement exceeding the allowable limit of
100 mm to the axis.
[Moving structure of operating-insulators]
The top end or bottom end of operating-insulators was pin connection. The middle point
is equipped flexible joint to avoid bending stress at high seismic condition.
Support insulators
[upper parts]
Moving parts
Pin connection
Operating-insulators
At the displacement limit,
collision of upper and
lower metal parts
Collision marks
(plastic deformation)
Moving parts
(flexible joints)
Max. displacement
100mm
Operating insulator
was broken
Moving parts
(displacement limit)
Pin connection
[Movable structure of control-insulators]
[lower parts]
[collision marks]
Fig. 11 Damages of 500kV disconnectors
(3) Verification tests and considerations to ensure the seismic performance of equipment:
Although non-linear displacement, such as deformation of steel parts and collision of moving parts
due to a high level seismic force makes seismic performance decrease in some equipment, it has not
been taken into account during the development process. This suggests the importance of
qualification by shake-table tests at actual high level acceleration in the development of new types of
equipment. Testing with artificial seismic waves (time-history vibration test) should be effective for
the qualification of the equipment which has multiple natural vibration modes or for the verification
how the deformation of a steel part affects the seismic performance by a long duration wave.
Performance evaluation using extrapolation from the results of tests with reduced conditions may be
necessary since tests with high level condition are difficult due to test facility limitations and/or
safety reasons. However, conditions of sufficient acceleration and duration should be applied for the
test of equipment which may show non-linear displacement.
For the following equipment, the studies of high level vibration tests (including time-history
vibration tests) are being conducted among Japanese utilities and manufacturers from the viewpoint
of performing the test and accurate evaluation of the test results.
Air insulated disconnector (with the steel parts for the supporting structure or with multiple
natural vibration modes)
Center clamp type bushings of 275 kV or less for transformers (exhibiting non-linear
behavior)
5-2. Damage by extreme loads through conductors:
(1) Interaction between equipment through conductors:
Figure 12 shows damage of 500kV current transformers. Porcelain bushings were broken and
connecting terminals were damaged.
Some conductors consisted of stranded wires with spacers to prevent contact with each other due to
electromagnetic force of short circuit current. In recent years, to cope with the increase of short
circuit current caused by expansion of the system, spacers were added (i.e. distances of spacers
were reduced) including the damaged lines in this time. Though construction of the conductors was
considered to have the necessary slack to avoid stresses from contiguous equipment, it is suspected
that stresses should be transposed from contiguous equipment by stiffer conductors as a result of
addition of spacers.
6
(2) Load by vibration of line
Figure 13 shows damage of arrestors ≤275 kV that are directly connected to the conductors from
lines. Porcelain bushings were broken and terminals were deformed. It is suspected that
displacement of the lines generated high stresses to the arrestor’s terminal.
Reduced distance between spacers
(from 2.7m(50kA) to 1.5m(63kA) )
Deformed terminals
Steel structures
Lines
Bushings were broken,
the inside was exposed
Arrestors
Damaged terminals
(small porcelain insulators)
Fig. 12 Damages of 500kV current transformers
Broken bushings
at botom end
Conductors connected from lines to
arrestors directly
Fig. 13 Damages of 275kV or lower class arrestors
(3) Future challenges based on the analysis of the damage in the earthquake:
For seismic design concerning the conductor, the necessary length was studied based on the damage
to equipment in the past earthquakes, characteristics of the conductor wire, and cross-interactions
between equipment [3] [4]. However, damage was observed in this earthquake even among
equipment with the sufficiently long conductors. Since high stiffness of the conductor or vibration of
lines presumably has an influence on the seismic performance of the equipment, Japanese utilities
and manufacturers are studying the following issues.
Gathering and reviewing measurement data of conductor stiffness.
Evaluation of the interaction behavior of equipment connected by conductors
6. Conclusion
The Great East Japan Earthquake was one of the most severe earthquakes in the world. Maximum
equipment centroid accelerations approximately twice as high as the level defined in JEAG 5003
standard were observed in the earthquake.
Almost all damaged equipment was confirmed to have shown higher stress than 50% of allowable
stress at seismic conditions specified by JEAG 5003. It can be suggested that 2 times higher
acceleration may be adopted as the seismic design specification for equipment whose influence of
failure has significant impact or that needs a long time for restoration, though further discussion is
necessary to build a consensus in Japan.
The importance of qualification by shake-table tests with high acceleration or artificial seismic motion
in the development of new types of equipment was recognized through the experience of damage of
parts other than porcelain insulators due to deformation and collision. It is necessary to consider nonlinear behaviour of parts in evaluation test, even in the case that the tests may be performed by
reduced acceleration because of limitation of the facility or safety reason.
Improvement of evaluation of equipment including connected conductors as a whole system is also the
further challenge.
BIBLIOGRAPHY
[1]
[2]
[3]
[4]
JEAG 5003-2010 “Guideline for seismic design for electric equipment at substations, etc.”
HOMEPAGE of NIED, “National Research Institute for Earth Science and Disaster Prevention”,
“http://www.bosai.go.jp/e/”
Electric Technology Research Association, Vol.38, No.3 “Seismic design of connecting leads
between two neighbouring apparatuses” (in Japanese)
CIGRE Paris International Conference in 1986, No.23-04, “Seismic Design of Connecting
Leads in Open―air Type Substations”
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