Seimic Qualification of Transformers - Pradeep ProSIM

SEISMIC QUALIFICATION OF
TRANSFORMER
Pradeep S
ProSIM R&D Pvt Ltd.,
Transformer Technology Symposium 2016

Source of
Seismic Vibrations
•Earthquakes
•Tsunami
•Volcanoes
•Explosions
•Rock burst (mines)
Significance
Substations are critical components in power systems. Systems handling electrical energy
for critical applications:
•Nuclear power plants
•Hospitals
•Data centers
•Process industries
Turkey Earthquake,17 Aug 1999

Physical Testing
•Shake table simulating ground
motions
Methods
TYPICAL FLOOR RESPONSE SPECTRA
Software based analysis & qualification
•Equivalent static method
•Response Spectrum Method
•Time history

Equivalent Static Method
• The floor Zero Period Acceleration (ZPA) is
used if it is observed that the fundamental
frequency is high, typically well above 33 Hz.
• Equivalent static load is determined by
multiplying the component masses by
acceleration equal to the peak of the input
response spectrum.
• Base moment is determined by using an
acceleration equal to 1.1 times the peak of
the applicable response spectrum.
The equivalent-static method is a simplified method, as compared to other more rigorous
analysis methods
• Load is be applied at the structure's center of gravity.

• Rich in energy content in the frequency range of
1 to 20 Hz
• Above 20 Hz, the energy content reduces
considerably and is very less beyond 33 Hz.
• RSA is performed when fundamental frequency
is well within 33Hz.
• Most of the damage is done by resonance,
resulting in dynamic amplification.
Response Spectrum Method
• Each direction analyses is done independently.
• Modal combination
• Spatial Combination
• Combined with Static Loads
Mode Freq (Hz)
1 9
2 20
3 26

Seismic Qualification (Earthquake)
Rules
•IEEE document #344-1987:
•Operating Basis Earthquake (OBE)
•Design basis earthquake (DBE)
•DBE is usually considered to produce accelerating force 1.8 times as great as the
OBE - the basis of the intensity multiplier used.
•IEEE 693 recommends that equipment should be qualified on the support structure that
will be used at the final substation.
• Transformer body is assumed to be fairly rigid
•The supporting structure of the bushing, turret and transformer frame, amplifies the
ground acceleration.
•IEEE 693 standard assumes that the motion at the base of the bushing is equal to the
ground motion multiplied by a factor of 2.
• IEC 61463 for Seismic qualification of bushings

CASE STUDY - 1
Demonstrate the procedure for RSA of transformer
Objective

•SST auxiliary transformer
•Rated Power 36/60 MVA
•Equipment is mounted on railway carriage
•The rails are installed on reinforced
concrete foundation
Response Spectrum Analysis

Conservator Active Part
• Conservator modeled by plane elements.
• Active part is modeled by beam elements
• Bushings modeled by beam elements. Mass of the bushings is equally distributed along the
element length
• Radiators modeled by beam elements. Mass is distributed in the three vertical elements
• Reinforced concrete foundation is modelled by rigid elements

NATURAL FREQUENCIES AND MODES
Mode No. Frequency
(Hz)
1 4.95
13 5.96
14 7.09
15 7.56
20 12.85

Qualification
• Normal (tensile or compressive) and shear stresses are calculated for every critical cross
section of the elements.
• The shear capacity of the bolt is checked.
• Overturning of the transformer (also can be checked)

CASE STUDY - 2
• Indentify the dynamic characteristics of a typical transformer and the interactions
between transformer components and HV bushings
• Investigate the sensitivity of the interactions and the amplifications between the base
and bushings
Objective

Software based analysis & qualification
Discretized model
• Three identical high voltage bushings mounted on the transformer’s cover plate
• Central bushing vertical, side bushings inclined 15 degrees.
Modeling

Modeling for assessment of interactions
• Modeling of high voltage bushings dynamic
properties
• Modeling of dynamic properties & boundary
conditions of the core-coil assembly
• Modeling of Oil in the transformer (oil mass
distribution)
• Detailed meshing of the cover plate of the
transformer

Modeling of bushings

Modeling of Cover Modeling of Oil mass distribution
Oil mass divided between the tank walls
and the core to account for the oil-core
assembly dynamic interaction

Critical Regions
• Centroids of high voltage bushings
• Base of bushing
• Cover plate corners

Fixed base versus as-installed bushing frequency
Dynamic characteristics

Remarks
The analysis results suggest that interaction occurs
between the HV bushings and the following
components
• HV surge arresters
• Radiators
• Conservator
• Core
Dynamic characteristics

Structural modifications
Structural modifications necessary for:
•Assess sensitivity of interactions
•Assessment of amplification factors from base to top of transformer
Modifications implemented on the detailed FE model:
•Bracing components
•Radiator
•Conservator
•Core

Sensitivity to Structural modifications
Components bracing

Structural modifications
Components bracing

Modified model
Analysis Procedure
• Modified model subjected to ground motion in three linear directions
• Specific response components measured (accelerations at corners, base of turrets,
bushings CG, displacements at top & bottom of bushings, moments at base of
bushings)
• Acceleration spectra were produced at the corners of the plate and base of the bushing
in three linear directions
• Amplification factors obtained through comparison with spectra as specified by IEE 693.

Result: Structural modifications
Base of bushing amplifications
• Acceleration spectrum and amplification factor at the base of the central bushing
• Amplification greater that 2 (around 2.9 at bushing frequency & 4.1 max)

Remarks: Structural modifications
Remarks
•Modifications that alter the dynamic behavior of the cover are the most unfavorable
for the bushing response
•No clear conclusion can be drawn for the other modifications as the response of the
bushings was in some cases attenuated and in others worsened, though not
significantly.
•Amplifications factors from the base to bushings are generally greater than 2.

SUMMARY
 Seismic qualification of transformers is mandatory before being manufactured & installed.
 Physical testing & virtual testing (simulation) are the methods adopted.
 Response Spectrum Method (RSA) is one of the favored methods in software based
qualification.
 Case Study-1 :
 From RSA, Normal and shear stresses are calculated.
 Shear capacity of the bolt is also checked
 Case Study-2 :
 The ground to cover and ground to bushing base amplification factors exceeded the
value of 2.
 The analyses on the structurally modified models indicate that for this transformer the
most important effect on the bushing response is the cover flexibility and the cover
plate dynamics.

REFERENCES
• IEEE 693,1997 – Recommended Practices for Seismic Design of Substations
• Chopra, AK (2000) – Dynamics of structures, Theory and Applications to Earthquake Engineering,
Prentice Hall
• Gilani AS, Whittaker AS and Fenves – Seismic evaluation of 550kv porcelain transformer bushings
• Gilani AS, Whittaker AS and Fenves – Seismic evaluation of 230kv porcelain transformer
bushings, report No PEER 1999/14, for Pacific Earthquake Engineering Research Center
• Ersoy S, Saadeghvaziri MA (2004) – Seismic response of Transformer bushing system.
• Modeling of transformers & seismic performance of high voltage bushings – Univ of Buffalo, SUNY

SIMULATED DESIGNED DELIVERED

Seimic Qualification of Transformers - Pradeep ProSIM

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