Electrical Auxiliary Systems for Nuclear Power Plants by pharmphresh30

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									              Accountability and Evaluation of Aggregate Effects of Through Faults
                                    On Power Transformers
                               By Kipp Yule, Duane Brock, and Jim Purdy


Keywords – Electrical, Operability, Fault,                      A root cause evaluation was conducted and the
Transformer                                                     most significant contributor to the fault was
                                                                believed to be the aggregate effects of through
Abstract – This paper examines proposed                         fault events since the transformer had undergone
methodologies and recommendations to prevent                    internal inspection and repair in 1996. Due to
failures of power transformers due to the                       the mechanical and electrical stresses and
aggregate affects of through faults on                          physical shock imparted on transformers upon
transformers.                                                   exposure to each through fault, it is theorized
                                                                that these aggregate effects caused by forces over
A 1000MVA Three Phase Generator Step-UP                         time manifested in looseness of the low voltage
(GSU) Transformer at a Generating Station                       bus bar supports enabling the bus bars of two of
suffered a catastrophic electrical fault in October             the low voltage phases to migrate towards each
2005. This resulted in a significant impact to the              other to the degree that the dielectric properties
Operating Companies revenue stream as this                      of the oil alone could not preclude a phase to
base-loaded facility, minus a functional GSU                    phase fault from propagating.
Transformer could not export Power for 17 days
until a temporary (De-rated Output) Transformer                 A broadness review was instituted as a
was installed. Another full rated spare                         component of the root-cause evaluation and it
transformer was installed in early 2006 during                  was discovered that there are recent documented
the scheduled plant shutdown to replace the                     instances where cumulative through faults have
smaller unit. The new replacement Transformer                   led to comparable transformer faults.
(with an up-rated power rating) is scheduled for
installation in early 2008.                                     As a corrective action for the event, the existing
                                                                maintenance program has been improved by
                                                                requiring consideration of the frequency, the
         1.    INTRODUCTION                                     magnitude, and the duration of through faults
                                                                experienced when evaluating maintenance
Power transformers are designed and built by                    indicators that would lead to more detailed
manufacturers to withstand the large mechanical                 inspections and repair actions for transformers.
and electrical stresses imparted on the
transformer due to the maximum electrical fault                          3.   EXTENT OF THIS PROBLEM
energy postulated for that transformer.
Secondary side transformer faults also expose                   As an extension of the aforementioned root cause
the transformer to elevated mechanical and                      evaluation, additional searches were conducted
electrical stresses that are functions of; a) the               to ascertain failure correlations within the
nature of the fault (3 phase fault, ground fault                nuclear industry. There were several instances
etc.), b) the magnitude of the fault (function of               where it was suspected that a power transformer
the equivalent system reactance where the fault                 failure was attributed to aggregate through faults.
occurs), and c) the time necessary for a                        One utility transformer maintenance group has
protective device to clear the fault.                           observed a pattern of transformer failures
Consequently, the challenge is to identify all                  following ice storms that is believed to be highly
transformer through faults, capture the energy                  correlated to an accumulation of through fault
profile of each individual through fault, and                   events on the failing transformers.
ascertain the consequences of the aggregate                     A review of IEEE papers revealed some potential
effects to the transformer.                                     correlations with this phenomenon, however for
                                                                the sake of brevity, the following case study
         2.    ANATOMY OF THE POWER                             entitled ‘Analysis of a Generator Step-Up
               TRANSFORMER FAILURE                              Transformer Failure Following a Faulty
                                                                Synchronization’ is offered to emphasize the


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cumulative effects of through faults. See                                    Three Phase Generator Step-Up (GSU)
Reference 1                                                                  Transformer, which suffered a catastrophic
                                                                             electrical fault in October 2005, prompting this
Abstract - This paper presents the details of an in-depth                    paper.
analysis carried out: to investigate an out-of-phase
synchronizing condition which resulted in the failure of a
725 MVA GSU transformer. Using a technique described in                               4.   SYSTEM CONFIGURATION
this paper, the circuit breaker closing angle was estimated                                AND FAULT TYPES
from oscillograph traces. This information was used with the
Electromagnetic Transients Program (EMTP) to simulate the
disturbance events and thereby produce a complete set of
                                                                             A commonly used station one-line configuration
probable synchronizing currents and generator electrical                     is shown in Figure 12, where the generator and
torques experienced during the disturbance. The analysis of                  the step-up transformer have essentially the same
the simulation results, and further, the analysis of the failure             rating. This is from Reference 3. We should
of the GSU transformer have been viewed in light of the large
number of prior system faults in the vicinity of the
                                                                             recall that the location of a fault will determine
transformer. A brief review of the ANSI/IEEE standards on                    the severity or level of fault current. With a fault
transformer fault withstand capabilities as related to this                  postulated at C1, the fault current through the
type of analysis is also presented.                                          Step-up transformer is mostly due to the
                                                                             generator contribution with auxiliary system
Essentially the authors indicated that in this case                          contribution (generally negligible). In the case of
there were 42 faults below the current rating on                             a fault assumed at B1, the fault current through
phase 2 for the Muskingum River # 5 GSU and                                  the Step-up transformer is mostly due to the
37 prior through faults above the 1.0 per unit                               system contribution but also has additional short
value that individually should not cause a                                   circuit contribution from the auxiliary system.
transformer failure; however the cumulative                                  Yet, the highest available fault current tends to
effect may have been sufficient to loosen the                                be at the Auxiliary system transformer due to the
windings and mechanical blocking to the extent                               contribution of both the System via the Step-Up
that the through fault capability had been                                   Transformer and the direct connected generator.
reduced prior to the out of phase                                            It is noted that the auxiliary system fault will
synchronization.                                                             subject the Auxiliary system transformer to a few
While the out of phase event in this case resulted                           cycles of short circuit forces, while limiting the
in winding failure, we must also include the                                 longer term thermal input associated with
winding to bushing cabling, busses and flexible                              generator coast down, even when the Generator
and solid connectors, as components that are                                 breaker is included in the system configuration.
repeatedly subject to severe mechanical stress as
was the case with the failure of the 1000MVA




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From Reference 3 - IEEE C37-013
                                                                being evenly distributed events where one out of
We also want to briefly consider the possibility                every three (or two out of every six) occurs
that faults may not be randomly distributed                     evenly or randomly across the phases. In the real
across the three phases. In the one case study, it              world case it was observed through the historic
is believed that the area of failure was related to             records that the failed phase had been subjected
one individual phase, which was subjected to                    to multiple faults that involved downed lines and
higher faults and / or experienced the initial                  a high voltage breaker failure. One may postulate
asymmetrical component of the fault. While the                  that if the fault current with a high degree of
IEEE C57.12.90 states:                                          asymmetry occurs repeatedly, or consecutively,
12.3.4 Number of tests - Each phase of the                      on the same phase; then a close inspection and
transformer shall be subjected to a total of six                verification of fitness for service is warranted.
tests satisfying the symmetrical current                        Reference 3 the figures of IEEE C37.013 that
requirement specified in 12.3.1 or 12.3.2, as                   provide good visual references of how the high
applicable. Two of these tests on each phase
shall also satisfy the asymmetrical current
                                                                mechanical stresses are imposed on the phase
requirements specified in 12.3.3.                               that receives the maximum asymmetrical
The real world has shown that the asymmetrical                  component of a through fault.
current does not always follow the pattern of




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                     Figure 7— Asymmetrical system-source short-circuit current
 From Reference 3 - IEEE C37-013




From Reference 3 - IEEE C37-013

        5.   INDUSTRY GUIDANCE                              established rule that is universally adopted by
             REGARDING THROUGH                              transformer manufacturers regarding thresholds
             FAULTS                                         for evaluating the accumulative effects of less
                                                            than maximum through faults. There are some
Although there is specific industry guidance                recommendations recently offered by some
established in IEEE C57.12.00 with respect to               manufacturers as outlined in Section 10 to
the number and magnitude of maximum                         support this paper.
calculated faults that a power transformer must
be designed to withstand, there is no well

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         6.   FAULT TRACKING                                     through fault event and over time help to validate
              PROGRAM ATTRIBUTES                                 recommendations for continued operation.

The increased sensitivity to the effects of through              There are numerous inputs, which need to be
fault currents on large transformers has                         considered when performing the post through
motivated some individual plant sites as well as                 fault event analysis of transformer health.
the organizations responsible for the utility                    Transformer age, electrical loading, any
distribution system to evaluate the effectiveness                movement due to physical relocation, physical
of their maintenance programs. A major                           design, OE, maintenance and inspection history,
contributor to an effective program for inclusion                as well as the through fault event data and results
of through fault tracking and analysis is to                     of subsequent testing are a few considerations.
identify and obtain the base-line data needed for                The physical design of the transformer will help
future analysis. It is most desirable of course to               to determine the failure modes that might be
obtain this data while the transformer is in a                   possible. Differences between transformer
healthy state. Preferably this will be performed                 designs may necessitate the use of different
in the manufacturers shop prior to delivery. It                  strategies in the specifics of post event testing
can also be established after a transformer has                  and analysis. While comparison of testing
undergone major refurbishment or maintenance.                    results taken after a through fault event to the
Another major aspect of a through fault                          base-line data can show potential movement or
monitoring program is the identification of the                  shifting of the coils, it may not show movement
through fault event and the capturing of the data                of bus bars or other internal components, which
necessary to support the analysis. There can be                  can eventually cause failure of the transformer.
two parts to capturing this data. The primary                    OE has shown that oil sampling and DGA will
source for a power plant is the protective                       not always detect transformer degradation of this
relaying and on-site Fault Recorder data                         type either. OE has also shown that some
acquisition system. It should be verified that the               transformer designs are physically more robust
Fault Recorder data acquisition system is set up                 than others and alternatively others are more
to be triggered (start the data acquisition) at the              sensitive and as such are more likely to
proper level of fault current (recommendations                   experience these failure modes. This will
may be as low as 0.25 per unit, or at a threshold                introduce an additional degree of subjectivity
level commensurate with the transformer design                   into the engineering analysis and weight the
margin) and that adequate voltage and current                    decision for instituting an internal inspection.
readings will be taken for the three phases at the
appropriate locations. A secondary source of                              6.1 Preparation Prior To Fault
valuable data can be secured from the
transmission system group. Operational                           Some of the actions which should be
Experience (OE) has documented cases where                       incorporated into a transformer fault tracking
events on the grid, which happened away from                     program are as follows:
the plant site, contributed to transformer failure
at the site. Formal communication with the                       6.1.1.   Identify the responsible department and
appropriate contacts within the transmission                              individual(s) for implementing a
system group could provide information about                              strategic program designed to foster
grid events where the magnitude of the event as                           long term operation of Power
seen at the site of the transformer is under the                          Transformers as described below to
threshold for triggering the data acquisition                             ensure focus and accountability.
system, but which would be useful in analyzing                   6.1.2.   Determine what testing has been
the accumulated effects of through fault events                           performed on the existing transformers
on a transformer.                                                         and initiate actions to fill any gaps to
                                                                          ensure a compliment of test results are
Supplemental data necessary for the analysis of                           secured as outlined in 6.1.9 below.
the transformer health is the result of periodic oil             6.1.3.   Verify the base-line data that is
samples, Dissolved Gas Analysis (DGA), power                              available for the existing transformers
factor testing, Frequency Response Analysis                               and determine what actions can be
(FRA) testing, and infrared imaging thermograph                           taken to fill any gaps to ensure a
readings. Proper trending of this data should                             compliment of test results are secured
support the analysis performed in response to a                           as outlined in 6.1.9 below.

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6.1.4.  Review the existing maintenance                        occurs, a recommended course of action would
        procedures to insure that the desired oil              be as follows:
        analysis, DGA, thermal readings, are
        being taken in accordance with                         6.2.1.  Determine what relay actuations were
        manufacturers recommendations and/or                           received on protection systems.
        governing IEEE recommendations.                        6.2.2. Obtain the data from the Fault Recorder
6.1.5. Verify that the onsite Fault Recording                          data acquisition system.
        data acquisition system is capable of                  6.2.3. Obtain data from any other data
        securing all of the necessary data                             acquisition system available.
        required to facilitate a complete                      6.2.4. Obtain the operational history for the
        evaluation.                                                    transformer, including through fault
6.1.6. Establish a communication protocol                              legacy information.
        with the proper individual(s) that                     6.2.5. Contact the appropriate transmission
        triggers communication between                                 group to determine what other transients
        transmission and the designated plant                          may have been experienced on the grid
        representative(s) for incidents that could                     at that time.
        adversely influence the health of the                  6.2.6. To the degree possible, determine the
        transformer.                                                   initiating event, the location, and the
6.1.7. Implement procedural requirements to                            magnitude of the fault as seen by the
        track faults on the system, identifying                        transformer.
        the location, magnitude, etc.                          6.2.7. Pull maintenance history and trending
6.1.8. Review the maintenance history and                              analysis for the transformer.
        through fault events experienced by the                6.2.8. Secure vendor information for the
        existing transformers and initiate                             transformer.
        actions to perform any future testing or               6.2.9. If the protective relaying isolated the
        inspections as necessary.                                      transformer from the system, perform
6.1.9. For new transformers, ensure that the                           the following tests as dictated by the
        procurement specifications include                             analysis of the event data:
        requirements for execution of relevant                              • DGA
        baseline testing such as, Insulation                                • Oil Power Factor
        Resistance, Winding Resistance, 10kV                                • Insulation Resistance
        single phase excitation (Doble), Induced                            • Insulation Power Factor
        Voltage, Turns Ratio, Frequency                                     • FRA
        Response Analysis (FRA), and                                        • Winding Resistance
        Insulation Power Factor, to be
                                                                            • Turns Ratio
        conducted in the manufacturer’s shop.
                                                               6.2.10. If the transformer remained in service,
6.1.10. Once the new transformer is received,
                                                                       review the data secured in step 1
        perform a 10kV single phase excitation
                                                                       through 8 above to determine if the Unit
        (Doble)test, Ratio test, and FRA test.                         should be removed from operation and
        Also, if there is any indication based on
                                                                       the above testing performed.
        external testing, conduct an internal
                                                               6.2.11. Perform an engineering analysis using
        visual inspection or when assembling                           the specific transformer design and
        bushings or other components removed
                                                                       considering the data obtained in the
        for shipment.. If the transformer is
                                                                       steps above. This analysis may
        stored and subsequently moved again to                         recommend continued operation as is,
        another location, the appropriate subset
                                                                       more testing, or an internal inspection
        of these tests and internal inspection
                                                                       of the transformer.
        should be repeated depending on the                    6.2.12. Consult with the transformer
        details of the move.
                                                                       manufacturer if available and
                                                                       communicate the findings and
         6.2 Post Fault Actions                                        recommendations to gain a consensus
                                                                       opinion.
All of the above recommendations of course are
just the preparation for dealing with a
transformer through fault event. Once one


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    7.   CURRENT TECHNOLOGIES TO                                could be reasonably long periods of time
         SUPPORT COMPREHENSIVE                                  between faults, say five to ten years, or more,
         THROUGH FAULT PROGRAM                                  without significantly consuming the original
                                                                manufactured ability to withstand through faults.
There are many relay platforms that are capable                 This requires a transformer lifetime commitment
of waveform capture of system faults on both the                by the end user to record and track significant
high and low side of large power transformers.                  events from the perspective of the transformer.
There is increasing use of digital fault recording              In the world of analogies, this is the equivalent
on the transmission side as well as the generation              of our own lifetime medical records whereby the
side. However, one key aspect of all the data                   input of multiple care givers, doctors, parents,
collection available from the various data                      spouses and children are involved across
acquisitions and event capture equipment is the                 generations to maintain an accurate historic
organizational and management structures that                   record of the significant events in our lives.
are required to be present to analyze and act on                Similarly, the health of the transformer should be
the collective aspect of information that is                    recorded and reviewed to allow for preventative
derived from the analyzed data.                                 measures and to assess the projected remaining
                                                                life. The second obstacle is the multiple parties,
    7.1. Data Management over Lifecycle                         groups, or even companies that may be involved
                                                                in providing data to a possibly virtual data
The first obstacle to overcome is the long life                 Owner. Refer to Figure 7A depicting an
spans that many large power transformers are                    organizational arrangement of data collection
capable of achieving. Many in service units in                  groups that may be typical due to the specialized
North America are twenty, thirty and in some                    functions of these groups and their inherent
well established infrastructures even forty years               charter. There is a distinct possibility that no one
old. Considering the IEEE C57.12.00 standard                    individual, group, or assigned entity is required
(Reference 2) regarding short circuit capability, a             to compile and analyze the collective data,
new transformer is expected to be able to                       although that should be the objective to ensure
withstand at least six (6) through faults. So there             program accountability.




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                                             High Voltage Side

                                             Grid faults monitored by
                                              Transmission Group or
                                                 wheeling utility.




                                                GSU XFMR

                                                     HV -- Grid
                                                 LV – Generator
                                             Plant loads via Isophase
                                                 Bus to Aux Xfmr

                Generator                                                         AUX XFMR(s)
                                                                                Plant auxiliary load faults,
            Generator & Iso Phase Bus
                                                                                    including Auxiliary
               faults monitored by
                                                                                 Transformers monitored
                Generator Group
                                                                                by Plant Electrical Group.




Figure 7A -- Radial
The ability of the transformer to fully withstand              Figure 7B. Asset managers and those that may
the next through fault is clearly dependant on the             require external companies, or even outsourced
history and present health of the transformer.                 services, should ensure that the data is collected,
While the technology to capture and accumulate                 analyzed and actions implemented to maintain or
data may best be served by the radial functional               restore the power transformers ability to
groups that interface with a power transformer,                withstand the next and the requisite accumulated
the ownership and guardianship must also be a                  through faults.
circular one of empowerment as shown in




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                     Generator                                                           GSU XFMR

                  Generator & Iso Phase                                                      HV -- Grid
                 Bus faults monitored by                                                 LV – Generator
                    Generator Group                                                  Plant loads via Isophase
                                                                                         Bus to Aux Xfmr



                                          MULTIPLE GROUP
                                             INTERFACE
                                           REQUIRED FOR
                                            CUMULATIVE
                                          FAULT TRACKING

                  AUX XFMR(s)                                                        High Voltage Side

                Plant auxiliary load faults,                                         Grid faults are monitored
                    including Auxiliary                                              by Transmission Group
                 Transformers monitored                                                 or wheeling utility.
                by Plant Electrical Group.




Figure 7B – Circular

    7.2. Fault Levels and Remnant Fault                           levels to just be a percentage of full fault level,
         Capacity                                                 say three quarters (75%), half (50%) and one
                                                                  fourth (25 %). The question or expectation that is
When attempting to find concise and / or                          attempting to be confirmed is: Assuming that
consensus type guidelines in the industry and via                 short circuit forces are basically a squared
the standards regarding the degree of effect of                   function of the current, then can one postulate
accumulated faults ranging from a full design                     that the transformer can be reasonably expected
basis or type test fault, several manufactures                    to successfully endure a numerous
provided helpful guidance, design practice and                    mathematically proportional number of lesser
their experience based on end user feedback. For                  faults that would constitute a full design basis
the purposes of this paper we considered or                       fault level. In a more classical mathematical
defined a full design basis or type test fault as the             relationship, if the transformer could accept one
one hundred percent fault (100%) level located at                 100% through fault, then could it be expected to
the transformer terminals and the lesser fault

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accept a mathematically equivalent number of              basis event as the inverse of the percentage
lesser through fault events as shown in Table 1?          squared of full fault current levels, it must be
                                                          emphasized that the manufacturer included a
Table 1                                                   minimal time interval between repetitive through
Fault Fault     Square    Inverse    Postulated           faults at higher levels. The recommendation is
level level     of per    of per     mathematical         for through faults at levels of 50% and above
(%)    at       unit      unit       capability of        there should be at least 15 live relaxation days
       per      value     value      equivalent           between events. While there must be a time
       unit               squared    through fault        constant for a deformable but resilient
       value                         events               component to return to a point of mechanical
100    1        1         1          1                    stress equilibrium after having a force exerted
75     ¾        .5625     1.7777     ~2                   upon it, it is reassuring to know that some
50     ½        .25       4          4                    manufacturers are able to quantify such
25     ¼        .0625     16         16                   relaxation or recovery time between higher level
                                                          through fault events. This relaxation time is most
After performing some research of various                 likely obvious for the manufacturers and industry
standards, reviewing various operating and                consultants, and does make good practical sense.
maintenance manuals, and discussing the topic             The question that users should ask is: Is the
with a consultant, we believed there was a basis          relaxation time between events, appropriate due
to expect the assumption to have some validity.           to higher through fault level accounted for in
                                                          operating procedures? We know there are
Upon contacting several manufacturers, one                thermal time constants (recover time) associated
strong point of consensus offered was that the            with numerous hot starts of large motors, and
winding clamping force, and the original (as              that some of the intelligent relays have
manufactured) condition of the windings,                  algorithms that will block the next restart based
insulation, key spacers, blocking, and other              on the thermal characteristics and recent
cellulose products that are subject to short circuit      operating history of a motor. For some specific
forces during a through fault must be within the          situations, should a user similarly consider
design margin values for the specific design for          operating procedures that recommend a minimal
any transformer to successfully withstand a               relaxation time prior to re-energizing a unit that
through fault event. There are services such as           has had a severe fault, or numerous consecutive
the Mature Transformer Management Program in              lesser faults? This question can only be answered
Reference 5 and in conjunction with Advanced              by the responsible entity in charge based on a
Diagnostic Testing Services offered by                    multitude of considerations. The question is only
Reference 5 that can provide unit specific                asked to: a) provide a possible mechanism to
transformer design and condition assessment for           reduce operating risk by adopting conservative
shell and core form transformers. The short               operating practices, b) to further illustrate the
circuit strength analysis would consider the              possible adverse consequences of not factoring
various stresses such as, buckling, crushing,             this variable into the decision making process,
hoop, and tipping, acting on the radial and axial         and c) to highlight that there are service life
directions of the winding for a specific design in        reductions that might be accompanied with
combination with the historical loading,                  immediate attempts to re-energize a recent
operational history and diagnostic data and               faulted unit.
would assign a relative risk category
proportional to the design margin.                            7.3. ECONOMIC CONSIDERATIONS
One major manufacturer confirmed that their
design rules for short circuit withstand have a           Considering the current robustness of the Power
section relating to this matter, and based on these       Industry, typical quotes for new large power
rules the following number of short circuits per          transformers are in the range of 15 to 18 months
year would be acceptable for one of their designs         (and for some specific large capacity and high
from a specific manufacturing location:                   voltage, or special application factories, the lead
100% level - 1 per year; or at the 75% level - 5          times are over 22 months) for GSU Transformers
per year; or at the 50% level - 20 per year; or at        and 12 to 15 months for auxiliary and station
the 25% level - 100 per year. While this supports         service transformers. Some larger base load
the postulated mathematical relationship of the           Units are subject to revenue losses of
probable number events at less than a full design         approximately 1 million dollars a day after a

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transformer failure. Consequently, if the Utility
does not have a spare power transformer readily
available, the economic consequences are
substantial.

     7.4. CONCLUSIONS

Based on a review of industry data, it is apparent
that there are opportunities for original
equipment manufactures and industry
organizations to establish a formal program to
better predict when a Power Transformer is at
risk. This paper illustrates the salient attributes
of such a dedicated and structured program to
assist the operating and maintenance departments
in protecting themselves from significant
economic risks due to failed power transformers
as a consequence of the accumulative affects of
through faults.
In other words, instead of implementing a plan to
ensure operability and reliability of the power
transformers, one could find themselves working
a plan for an unplanned outage.

     7.5. REFERENCES
1.   Analysis of a Generator Step-Up
     Transformer Failure Following a Faulty
     Synchronization by B.M. Bernard, J. H.
     Provanzana, and L. B. Wagenaar – IEEE
     Transactions on Power Delivery, Volume 3,
     No 3, July 1988
2.   IEEE Std C57.12.00 -- IEEE Standard
     General Requirements for Liquid-Immersed
     Distribution, Power, and Regulating
     Transformers
3.   IEEE Std C37.013 - IEEE Standard for AC
     High-Voltage Generator Circuit Breakers
     Rated on a Symmetrical Current Basis
4.   ABB Service Handbook for Transformers
5.   ABB Mature Transformer Management
     Program (MTMProgram™)


     7.6. ACKNOWLEDGEMENTS
We wish to acknowledge the contributions,
opinions, and guidance provided by the
following companies and their technical experts
with respect to this paper:
1. ABB
2. AREVA
3. Hyundai
4. Southern Nuclear Company
5. Bechtel Power Corporation




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