Power Transformer Tank Rupture and Mitigation- A Summary of Current - PDF by exb38697

VIEWS: 0 PAGES: 79

									         Investigation Report

Power Transformer Tank Rupture and Mitigation
  - Current State of Practice and Knowledge


           by the Task Force of
   IEEE Power Transformer Subcommittee


               March 09, 2010
                Houston, TX.
CONTENT

1.Investigation and Analysis -10 minutes
2.Utility Experience (15 minutes)
3.Manufacturer’s Perspective (25 minutes)
4.PRD (10 minutes)
5.Conclusions (10 minutes )
6.Q&A (5 minutes)
Analysis of Transformer
    Tank Ruptures


Electric Power Research Institute

         Wayne Johnson
       wejohnson@epri.com

         Nick Abi-Samra
       nabisamra@epri.com
Survey On Transformer Rupture



Objectives
• Develop an understanding of the tank rupture process
  associated with internal faults.
• Develop tools, and methods, for evaluating the influence
  of tank designs on rupture characteristics




                            2
 Background

• 42 transformer failures (tanks
  ruptured or deformed without
  rupture)
• 22 utilities
• 10-year period (1980-1990)
• 7 transformer manufacturers
• Different voltage levels
• Different designs: (GSU’s, Auto,
  Phase Shifters, etc.)




                               3
Conclusions

• The arc energy is the critical rupture parameter
• Differences in transformer design and application are not major
  discriminators in the tank rupture
• The fault energy capacity of a tank can be increased by increasing
  the tank rupture pressure limit and tank flexibility.
   – The pressure at which tanks rupture can be increased by local
      strengthening of weak points, while the tank flexibility can be
      increased by replacing large beams with a number of smaller
      beams (which permit greater deflection at a given stress level).
• Venting to conservators or to auxiliary tanks was not found to be
  effective for heavy faults (those with arc power greater than 300 MW)




                                   4
Conclusions

• A long arcing time is not
  necessary for rupture:
   – About 75% of the cases
      occurred with arcing times
      less than 4.5 cycles.
• Since arc energy is
  proportional to I2t, where the t
  is duration in seconds, the
  parameter which offers the
  most opportunity for control of
  the risk of rupture is the
  magnitude of the current, and          Graph of Tank Deformation
                                         Rupture as a function of fault
  specifically, the peak crest            current and fault duration.
  value of the first half cycle of
  the fault current (and the
  associated X/R of the circuit).

                                     5
Power Transformer
Tank Rupture
A Utility's Experience

Marc Foata
IEEE Transformers Committee
Houston, March 2010
           Presentation
Assessment of the risk
  Statistics
  Arc energy
Prevention of tank rupture
  Venting
  Containment
Specification of a Tank pressure withstand
requirement

                                         2
Assessing the risk - Statistics




                                  3
Assessing the risk - Statistics




                                  4
Assessing the risk - Statistics




                                  5
Arc energy - 4 MJ




                    6
Arc Energy - 8 MJ




                    7
Arc Energy - 12 MJ




                     8
Arc Energy - 14 MJ




                     9
Arc energy vs Damage




                       10
    Arc Energy - Calculation

        Earc = 0.9 V I t
I – Arc current: Evaluated from short-circuit level
T – Fault clearing time: Depends mainly on
protection
V – Arc voltage: Very difficult to evaluate
0.9 – Factor introduced for square waveform of V

                                                 11
Arc Energy - Recordings
                     A     100
                                                                                                                          Unité Support et analyses - DESTT




                                                                                                                                   E   BN TT MAIS MICOUA
                                80
                                                                                                                                   E   BN L-7019 MICOUA
                                60                                                                                                 E   BN L-7011 MICOUA
                                                                                                                                   E   BN L-7027 MICOUA
                                40
                                20




                  (kV)
                                 0
                                -20

                                -40
                                -60
                                -80
                                            205       210        215        220        225        230        235         240       245        250         255
                                                                                     (ms rel. à 05:34:32.2870)
                                                  2008-09-12 05:34:32.499 MICOUA       735-02 TENSION RÉSIDUELLE DÉFAUT TRANSFO T8-B

                                12
                                11
                                10
                                 9
                                 8




                    (MJoules)
                                 7
                                 6
                                 5                                                                                 ÉNTERGIE     TOTALE    (TT MAIS)
                                 4                                                                                 ÉNTERGIE     TOTALE    (TTC 7019)
                                 3                                                                                 ÉNTERGIE     TOTALE    (TTC 7011)
                                 2                                                                                 ÉNTERGIE     TOTALE    (TTC 7027)
                                 1
                                 0
                                -1
                                            205       210        215        220        225        230        235         240       245        250         255
                                                                                     (ms rel. à 05:34:32.2870)
                                        2008-09-12 05:34:32.499 MICOUA        735-02 ESTIMÉ DE L'ÉNERGIE TOTALE DANS LE DÉFAUT TRANSFO T8-B

                   20000

                   15000

                   10000

                      5000
            (A)




                                 0

                    -5000

                  -10000
                                                                                                                               COURANTS CÔTÉ 735 kV
                  -15000                                                                                                       COURANTS APROX. CÔTÉ 315 kV

                  -20000
                                      190           200            210             220             230             240             250              260            270
                                                                                     (ms rel. à 05:34:32.2870)
                                                          2008-09-12 05:34:32.499 MICOUA       735-02 COURANTS DE DÉFAUT TRANSFO T8-B


             Lundi 15 Septembre, 2008
                                                                                                                                                       12
                                             \\VECTEUR\DONNEES\PARTAGE\@AUTOMATISMES\COMPORTEMENT\RAPPORT\RAPP_2008\TOSC_ANA\20080912_053432499_ENERGIE_T8-B_MICOUA.TOS
Arc Energy - Evaluation of Arc
          Voltage
60-100 V/cm range is often referred
For 40 kV, this means an arc length of
more than 4 m !!




                                         13
Arc Energy - Pressure Effect

        V = 55L P
Constant 55 V/cm
L is arc length (m)
P is absolute pressure (atm)
Pressure in the gas bubble at arc ignition
can reach extremely high values
                                             14
Prevention of tank rupture –
   Venting Simulations




                               15
Prevention of tank rupture –
   Venting Simulations




                               16
Prevention of tank rupture – Vented
     vs Non-vented Example




                                  17
 Prevention of tank rupture –
   Conclusions on venting

Pressure reduction from a single 25 cm
aperture is low and becomes negligible
when the arc is more than 1 meter away.
An effective pressure venting strategy
would require either a very large venting
duct or numerous small apertures in the
close vicinity of the arc.

                                            18
Prevention of tank rupture -
      Containment




                               19
 Prevention of tank rupture -
       Containment

Present design can contain up to 10 MJ
for the largest tanks (735 kV)
More resistant tank design can be
achieved
Need to implement specifications with
minimum energy requirement to meet.
Energy requirement will be a compromise
between the feasibility and the likelihood.

                                              20
New Specification - Philosophy
Priority is given to the protection of the
workers
Worst energy levels may not always be
containable by the tank
First rupture point must be the cover
Required calculation tools must be
accessible to transformer designers
Must take into account the highly dynamic
phenomena involved
Must be easily verified
                                             21
New specification - Formula

            ⎡               ⎤
   Ps= F
                 1   kE
            ⎢100   +    − 50⎥
            ⎣    4 100C     ⎦

Ps – Calculated tank pressure withstand
F – Dynamic (time & location) amplification
E – Fault energy level to withstand
K – Arc energy conversion factor
C – Tank expansion coefficient
                                         22
New specifications – Dynamic
            factor
        3

       2,5

        2
   F




       1,5

        1
             0   20   40   60    80     100    120    140   160   180   200


                                C/V (x 10 -5 kPa-1)




Time related dynamic factor (pressure and
deformation)
Proximity related dynamic factor
Takes into account tank volume          23
New specification : Hydro-Québec's
Energy Containment Requirements

    Voltage Class   Arc Energy




                                     24
New specification - Implementation


 All transformer suppliers since 1992 have shown
 adequate tank withstand calculation capabilities.
 Since implementing energy level requirements
 (2006), manufacturers have been forced to
 improve their tank design.
 Detailed analysis by a number of manufacturers
 confirmed that all the specified energy
 requirements can be met.

                                                25
       Transformer Tanks
- Some Factors Related to Rupture

  A Manufacturers Perspective

    IEEE Transformers Committee
       Houston, March 9, 2010

        by Bill Darovny, P.Eng.
           Siemens Canada
 Facts about Liquid Filled Transformer Tanks

• C57.12.00 and C57.12.10 define the operating
  pressures for transformer tanks
  – full vacuum = -101.4 kPa (-14.7 psig)
  – pressure 25% above the normal operating
    pressure


• Transformer tanks are not pressure vessels
  – are not required to be designed to the ASME
    Boiler and Pressure Vessel Code.
     • ASME code is mandatory when operating
       pressure exceeds 2 atmospheres (203 kPa).
 Facts about Liquid Filled Transformer Tanks

• C57.12.10 requires a pressure relief device to
  be mounted on the tank cover
  – Typically activate at 34.5 to 69 kPa (5 to 10 psig)


• Most rectangular tanks will sustain internal
  pressures of 140 to 210 kPa (20 to 30 psig)
  before rupture

• A tank on its own cannot be made strong
  enough to resist all magnitudes of internal
  pressure
Transformers have been saved from
rupture by common protection devices

  • Pressure Relief Devices
  • Gas Detector Relays
  • Rapid Pressure Rise Relays
  • Real time Gas Monitors

Provided the alarm signals are quickly
   recognized and the transformer is
             de-energized
Pressure Relief Device in Action
   Not Every Tank Rupture Results in a Fire
 Unit gassing, GDR alarmed, long delay in response to alarm,
ambient -30°C, reaction force to rupture shifted unit on the pad.
Some Units Burn After Tank Rupture- 750 MVA 500kV
Same Rating Different Supplier
 The Location Where a Tank will Rupture is a
               Function of:

1. the rate of change of the pressure increase
     Slow rate:
     •   Pressure has time to distribute throughout the tank
    Fast rate:
     •   Results in a pressure build-up at the source


2. the co-ordinates of the pressure source
   inside the tank

3. the closest weak spot to those co-ordinates
          Slow Rate of Pressure Increase
- Failed at 2 locations on the Cover
- Tensile end reactions tore the welds at the ends of the stiffeners
- Cover end angle rotated and the weld to tank flange cracked
     Moderate Rate of Pressure Increase

• Tank failed at
the cover joint
• Some stiffeners
on the tank wall
were permanently
deformed
• There were no
cracks in the oil
containment
welds in the tank
body
      Rapid Rate of Pressure Increase

• Co-ordinates of
pressure source was
about 1/3 tank height

• Wall plate fracture
started at the corner
welds and ran almost
full height of the tank


• Tank wall to bottom
weld joint also failed
        Rapid Rate of Pressure Increase

• Cover weld did not fail


• Tank failed at the high
stress points in welds at
the tank corners and
wall penetrations then
propagated through the
wall plates


• Unit was returned to
the factory and rebuilt
    Weakest Points of a Rectangular Tank

• Main cover to tank wall flange - weld joint

• Tank wall corners - weld joint

• Tank wall to base plate - weld joint

• High stress points at throats and large
  penetrations through the tank plates

• High stress points at ends of stiffeners where the
  end reaction force is transmitted to the tank plate
Main Cover to Tank Wall Flange
     Mode of Weld Failure

              Typical Cover Welded Joint

              Upward and outward forces
              due to internal pressures



              Stress is concentrated at the
              weakest point - the root of
              the weld.

              Weld crack progresses
              outward from the root
Typical Rectangular Tank Corner Joints



                  Stresses are on the face of
                  the weld.


                  The face of the weld is
                  stronger than the root of
                  the weld.


                  Adding corner gussets will
                  reinforce this joint
  Cylindrical Tanks are Inherently Stronger
           than Rectangular Tanks

• 110MVAR 735 kV
shunt reactor
• The tank wall is
stressed in hoop
tension
• Typically the
cylinder walls can
sustain pressures
> 350 kPa (50 psig)
• Weakest point is
the cover weld
     Design to Help Reduce Tank Rupture

• Design in service systems to detect faults
  early & de-energize quickly

• Use detection & relief accessories
  – Real time gas monitors
  – Gas detector relays - ensure gas collection
    system / piping functions as intended
  – Rapid pressure relays
  – Pressure relief devices
     • To be effective, relief devices must be located close to
       the pressure source
     • Standard size relief devices may not prevent all tank
       ruptures
      Design to Help Reduce Tank Rupture

• The best location for a tank to fail is at the welded
  cover joint as this will minimize fluid loss

• Strengthen the tank below the cover joint
   – Reinforce tank corners and wall to bottom joints
     with plates / gussets

   – Distribute stiffener end reaction forces with
     reinforcements or by connecting to stiffeners on
     adjacent walls

   – Reinforce around wall penetrations to reduce the
     high stress points
  Transformer Tank Rupture and Mitigation
                 Tutorial
               March 9, 2010


Mitigation Research and Example Techniques
           Presented by Craig Swinderman
       Mitsubishi Electric Power Products, Inc.
    Research on Transformer Tank Rupture
                  Mitigation
•   Joint research performed in the mid-1980’s by three large Electric
    Utilities in Japan, Tokyo University, and several transformer
    manufacturers.

•   Goal was to study ways of reducing the risk of transformer tank
    explosion in urban substations/ underground substations.

•   Full scale model testing performed.

•   Arc energies calculated and gas generation rate observed.

•   Pressure rise models developed.

•   Tank construction countermeasures developed.
                       Summary of Research

Tank Explosion                  Study Process for Internal Fault                     Verification
Process                                                                              Tests
                                 Estimation of Fault Condition
  Internal Fault                                                                    Arc Test in Oil Tank
                                 ① Arc Current ② Arc Voltage ③ Time
   Dissolved Gas
    Generation                    ④ Dissolved Gas Generation Volume



  Internal Pressure                 Pressure Rise Analysis                          Pressure Rise Test
       Increase
                             Dynamic Analysis considering Oil Motion                using Full Scale Tx



Internal Pressure                Comparison between Internal
                                 Pressure and Tank Strength
> Tank Strength

                                         Countermeasure
     Tank
                      ① Tank Strength Improvement ② Protection Relay Improvement
   Explosion
                      ③ System Improvement        ④ Pressure Restrained Structure
                              Dynamics of Internal Fault (1)
                                                                                                           10000
                Arc Test in Oil Tank
                Arc Test in Oil Tank




                                                                                         Arc Voltage (V)
                                                                                                             1000
                                             Electrode




                                                                                                             100
                                                                        Arc                                         10         100          1000
                                                                                                                          Arc Length (mm)
                                                                       Oil                                                Gas Volume
                                                                                                               1




                                                                                           Gas Volume (m3)
             Arc Current : 1.3 to 40.9 kA
             Time             : 3Cycle (0.05sec)
                                                                                                              0.1
             Arc Energy : 0.11 to 2.64 MJ
             Gap Length : 100 to 300 mm

*Reference – T. KAWAMURA, M. UEDA, K. ANDO, Y. MAEDA, Y. ABIRU, M. WATANABE, K.
MORITSU “Prevention of Tank Rupture Due to Internal Fault of Oil Filled Transformers”,                       0.01
CIGRE, 12-02, 1988.
                                                                                                                 0.1           1            10
                                                                                                                         Arc Energy (MJ)
                     Dynamics of Internal Fault (2)
Pressure Rise Test using Full Scale model for verifying pressure rise
Pressure Rise Test using Full Scale model for verifying pressure rise
- Pressure rise at internal fault can be simulated by powder combustion, considering nozzle area of
  container and powder amount.
- Dead space (steel tank) was set for simulating internal parts(Core,Coil).in the tested tank.
                                                                                               Measured Results
   Transformer : 275kV 300MVA
                                                                                               Analytical Results
   Arc Energy : 142,000kW、 Time : 80msec




                                                                         Pressure
     (Single Line-Ground Fault at Upper Tank)                                                           Upper Tank



                                                                                                                 Time
                                         Combustion Container
                                                 Φ300,L570                                              Middle Tank




                                                                        Pressure
   Upper Tank
                                   Gas Outlet
                                   (Φ12*72)
                                                                                                                 Time


                                                                                                         Lower Tank




                                                                        Pressure
                                                                                                                  Time
                         Middle Tank                         *Reference – T. KAWAMURA, M. UEDA, K. ANDO, Y. MAEDA, Y.
Lower Tank
                                       Cartridge increment   ABIRU, M. WATANABE, K. MORITSU “Prevention of Tank
                                          (Max.7500g)        Rupture Due to Internal Fault of Oil Filled Transformers”, CIGRE,
                                                             12-02, 1988.
                             Results of Analysis
•   Decomposed gas generation calculated to be around
    0.5L/kW sec for larger 275 kV class transformers at HV lead.*

•   Dynamic oscillation of fault pressure wave (kinetic energy)
    has a considerable influence on the pressure rise. (Dynamic
    Load Factor approx. 1.3 was recommended)*

•   Tank expansion characteristics and tank strength are
    important in determining the transformer’s capability to
    resist rupture.

•   Reinforcements can be made at the joining flange between
    the tank and cover (or flange between upper and lower tank
    for shell-form) to significantly improve the tank strength
    against rupture.


*Reference – T. KAWAMURA, M. UEDA, K. ANDO, Y. MAEDA, Y. ABIRU, M. WATANABE, K. MORITSU
    “Prevention of Tank Rupture Due to Internal Fault of Oil Filled Transformers”, CIGRE, 12-02, 1988.
          Example of Tank Strength Improvement


            Conventional Type                      Improved Type




Connecting Part
                       Alleviation of Stress   Improvement of Connecting Part
                       Concentration           Strength by Tie Reinforcement
              Pressure Reducing Space
•   Diaphragm type Conservator Tanks can be used as
    effective pressure reducing space if the connection duct to
    the main tank is short, the cross sectional area of the duct
    is large (approx. 1.4 m dia.), and the air space in the
    conservator diaphragm is adequate

Diaphragm (bladder)                            Conservator Tank

    Connection duct



                       Transformer main tank




      •Tests on 300 MVA, 230 kV units have verified ability to
      withstand 15,000 MVA short circuit capacity without
      rupture of the tank.
      •Still requires operation of protective relays and circuit
      breakers to clear fault within approx. 60 - 80 ms.
 Gas Insulated Power Transformers

•Use SF6 Gas as the insulating and cooling medium
instead of insulating oil.

•First units produced in 1967.

•Several thousand units of various sizes now in
service worldwide, several manufacturers.

•Transformer applications: From Distribution class
units up to 400 MVA, 345 kV ratings.

•Primarily used in substations located in urban
areas (including inside buildings, underground) due
to safety benefits.
  Features of Gas Insulated Transformers



• Use SF6 Gas as insulating and cooling
  medium, instead of oil.

• Typically use special internal insulation
  materials such as plastics, special paper,
  and pressboard.

• SF6 has excellent dielectric properties, but
  not as good for heat transfer.
                       Benefit of Gas Insulated Transformers
 SF6 Gas Insulation:                      non-flammable, compressible gas

 Pressure rise during an internal fault is slower than oil-immersed (non-
 compressible fluid), thus SF6 gas reduces the chances of tank explosion
Pressure Rise (%)




                    100
                     100                               Tank Strength
                     80
                      80
                     60
                                Oil-Immersed
                      60
                                Transformer
                     40
                      40                                      Gas Insulated
                     20
                      20
                                                              Transformer
                      00
                           00     0. 22
                                   0.     0. 44
                                           0.         0. 66
                                                       0.        0. 88
                                                                  0.     11
                                                  Fault Time (sec)
                Application Example



                             15MVA, Three phase, 50Hz,
                             Continuous Rating,
                             Core-Form,
                             Forced-Gas Natual Air,
                             with On-Load Tap Changer
                             GNAN/GFAN

                             H.V.   64.5kV +10/-10%   Star
                             L.V.    6.6kV            Delta



For underground substation
  beneath office building
A VERY BRIEF HISTORY:

PRESSURE RELIEF DEVICES
(PRDs) AND THEIR USE ON
POWER AND DISTRIBUTION
TRANSFORMERS


          Transformer Tank Rupture and
         Mitigation - March 9, 2010 J.Herz,
                       Qualitrol
BEFORE THERE WERE RE-SEALABLE PRDs…
Rupture Discs

  GOOSENECK CAN ADD TO BACK PRESSURE
  HAD TO BE REPLACED AFTER OPERATION - LEFT
  THE TRANSFORMER OPEN TO ATMOSPHERIC
  MOISTURE IN THE INTERIM
  TYPICALLY WITH NO ALARM

                  More Recently
  THEY HAVE BEEN USED IN MULTIPLE SETS ALONG
  THE TOP AND BOTTOM OF A SINGLE TRANSFORMER
  TO MAXIMIZE THE PRESSURE RELIEF AREA AND TO
  BE LOCATED AS CLOSE AS POSSIBLE TO ANY
  POTENTIAL FAULT LOCATION.
  THEY ARE THE RELIEF MECHANISM FOR
  COMBINATION PRESSURE RELIEF/FIRE
  SUPPRESSION SYSTEMS
THERE WERE SEVERAL DIFFERENT
DESIGN APPROACHES TO RE-SEALING
PRDs MADE BY VARIOUS
MANUFACTURERS

IN THE LATE 1950’s THE INDUSTRY
SETTLED OVERWHELMINGLY ON ONE
DESIGN.
THE DESIGN IS STILL IN FREQUENT
USE TODAY. IT OFFERS SIMPLICITY
AND DURABILITY IN ADDITION TO
RE-SEALABILITY.

REFINEMENTS SINCE HAVE TO DO
WITH IMPROVING THE SEAL,
SHIELDING AND PROTECTION,
SWITCH CAPACITY, CORROSION
RESISTANCE, ETC
                          WITH OPERATING PRESSURE
WHEN OPERATING PRESSURE   ACTING ON THE LARGER AREA
IS REACHED, TOP SEAL      CIRCUMSCRIBED BY THE SIDE
OPENS WHILE SIDE SEAL     SEAL, THE SPRING IS RAPIDLY
REMAINS BRIEFLY CLOSED    COMPRESSED AND THE VALVE
                          EXHAUSTS QUICKLY
8400 SCFM AT 50% OVERPRESSURE ON A 10 PSI PRD WAS
TYPICAL. NOW THERE ARE DEVICES WHICH GO TO 12,600 SCFM.
Most frequent question: Will it protect?

Most frequent answer is: Depends.


       • location of fault
       • magnitude of fault
       • duration of fault
TEST 1: 1958 AT GE SCHENECTADY


    The test tank was approximately 6 ft in diameter and 4 ft deep, with the
    PRD mounted in the center of the 6 ft. diameter cover.
    The gas space was 10 inches below the cover which resulted in about 700
    gallons of oil and 23 cu. ft. of gas (air).
    Ball nosed copper electrodes (2) were threaded with a small copper wire to
    trigger the arc, the highest of which was 25K amps and 20K volts. Oil,
    smoke, mist, spray blasted out of the relief device over a radius of about 40
    feet.

TEST 2: 1958 AT GE SCHENECTADY

   In tests performed by Jim Barr on a transformer with NO COVER , the fault
   was introduced near the bottom of the tank and the bottom of the tank BLEW
   OUT, at 10K amps and 10K volts.
There are thousands of events where rupture
discs and re-sealable PRDs have successfully
protected transformers:

“Internal arcing, breaker insulation break
down, load tap changer problems, phase
angle regulator problems, and internal
winding problems” are some of the more
common.

                 Transformer Tank Rupture and
                Mitigation - March 9, 2010 J.Herz,
                              Qualitrol
                            CONCLUSIONS

TTR is a complex problem. The severity is a function of arc location, arc I and T
as well as oil volume and tank expansion characteristics.

It’s possible to reduce the risk of TTR by performing modifications to the tank.

PRDs help to protect the tank against low energy internal arcing faults.

Fluids with high fire point will reduce the consequences of a tank rupture;
however it is not yet proven if these fluids will prevent tank rupture.

GITs will eliminate the risk of tank rupture.

Improved electrical protection and electrical system design can also help
prevent TTR.

The IEEE currently has no standards that provide guidance on TTR mitigation.
IEEE TRANSACTIONS ON POWER DELIVERY
Volume: 24 Issue: 4 Date: Oct. 2009
Page(s): 1959 - 1967


       Power Transformer Tank Rupture and Mitigation
  - A Summary of Current State of Practice and Knowledge
by the Task Force of IEEE Power Transformer Subcommittee
Nick Abi-Samra, Javier Arteaga, Bill Darovny, Marc Foata, Joshua Herz, Terence Lee, Van Nhi Nguyen,
   Guillaume Perigaud, Craig Swinderman, Robert Thompson, Ge (Jim) Zhang, and Peter D. Zhao
        Action Next –
Planning to Generate an IEEE Std
   You are all welcome to join

								
To top