Information Notice No. 82-53: Main Transformer Failures at the North Anna Nuclear Power Station
SSINS NO. 6835
IN 82-53
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555
December 22, 1982
Information Notice No. 82-53: MAIN TRANSFORMER FAILURES AT THE NORTH
ANNA NUCLEAR POWER STATION
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or
construction permit (CP).
Purpose:
The purpose of this information notice is to describe seven main transformer
failures, including one that resulted in a fire and one that caused
extensive damage to the main generator, at the North Anna Nuclear Power
Station, to alert other nuclear power facilities to the causes.
Description of Circumstances:
The North Anna main transformers consist of three 33OMVA single-phase
Westinghouse transformers for each unit which are cooled by a forced
oil/forced air cooling system. The 22kv low-voltage windings of these
transformers are supplied from the main unit generator by an isolated phase
bus system. The 500kv voltage windings supply power to the transmission
system by an overhead line to the station switchyard.
The North Anna main transformers have experienced seven failures in the past
two years, the first five of which involved the Unit 2 transformers and the
last two involved the Unit 1 transformers. Of these, the third and seventh
caused the most damage and also posed the greatest threat to the health and
safety of plant personnel. The third failure generated sufficient forces and
heat to rupture the transformer's casing and an oil line. The oil that
erupted from these two breaks ignited and the resulting fire engulfed and
shorted out an overhead three-phase bus system that supplies offsite power
to the normal and emergency buses of the North Anna facility from a reserve
station transformer. The seventh failure also generated sufficient forces to
rupture the transformer's casing; however, the rupture was at the upper
portion of the transformer such that the total oil discharged was
significantly less than that of the third failure. Although no fire ensued
in the immediate vicinity of the transformer, the total damage and risk to
personnel posed by the seventh failure were greater than those of any of the
previous events. For example, the effects of the fault were propagated to
the main generator where significant damage was done to the main generator
and its appendages (e.g., the neutral grounding transformer and its feeder
cable and enclosure were destroyed, the neutral enclosure was severely
damaged with the north side being blown out,
8212060350
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December 22, 1982
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a fire was created in the neutral enclosure but little damage ensued, the
hydrogen lines to the generator around the neutral enclosure were badly
damaged but no leaks occurred).
It has been determined that three of the failures involved both the
high-voltage and low-voltage windings. In addition, it was found that the
failures were either winding to ground faults, as experienced in the first
and fourth failures, or high-voltage bushing to ground failures, as
experienced in the second and third failures. The investigation also
suggests that the following circumstances contributed to the failures:
1. Exposure to Other Faulted Units: At least once before their respective
failures, the transformers involved in the second, third, fourth, fifth
and seventh failures had been used in a bank of transformers where one
of the companion units had failed.
2. Improper Storage: The high-voltage bushings for these transformers were
improperly stored (i.e., in a near-horizontal position). This improper
storage, coupled with subsequent over-voltage conditions, is believed
to be a major contributor to the second and third failures.
3. Overvoltage: Before the first, second, and third failures, the North
Anna Unit 2 main transformer bank had been subjected to several
documented overvoltage conditions. In addition, as a consequence of the
third transformer failure, the transformer associated with the fourth
failure was subjected to an overvoltage condition of unknown magnitude
for a short period of time.
Discussion of Failures and Postulated Causes:
The paragraphs that follow address each failure and highlight factors
believed to have had a bearing on the failure.
1. First Failure: On November 29, 1980, the Phase A transformer of the
North Anna Unit 2 main transformer bank experienced a winding to ground
failure. Although the transformer was operating at 100 percent load,
the hot oil temperature, hot spot temperature, and nitrogen system
trouble annunciators were inoperable. Had these annunciators been
operating, the failure may have been averted. This transformer had
previously failed while on loan to Georgia Power Company. The following
factors contributed to this failure:
o On January 31, 1979 and October 19, 1980, the transformer was
subjected to overvoltage conditions of 31kV and 25kV,
respectively.
o Approximately 6 1/2 hours before the transformer failed, its
mechanical relief device (MRD) operated. Although the station
electricians detected no oil, they found only two coolers running;
therefore, they reset a tripped circuit breaker to start three
other coolers. In addition, the indicated temperature of the oil
on the top of the tank and of the windings were found to be
90ºC and OºC, respectively. The
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December 22, 1982
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winding temperature indication was obviously in error and may have
been the reason for some coolers not operating. Further
examination of the transformer verified MRD operation and detected
oil on the cover of the transformer. Operation of the additional
oil coolers reduced the temperature to 50C at the time of
failure.
The oil expansion calculations performed by Westinghouse indicated the oil
level was approximately 2 1/2 inches below the top of the tank at the time
the MRD operated. Since the inert gas system was isolated, this oil
expansion would produce a gas pressure of about 19 psig in the absence of
MRD operation; however, the MRD apparently opened at approximately 10 psig
and relieved the pressure. Soon after the reseal, the inerting gas cylinder
was isolated in an effort to silence the MRD alarm, and three additional
coolers were started manually, thereby increasing the cooling units by 150
percent. As a consequence the temperature of the oil on the top of the tank
dropped about 18C in the first 2 1/2 hours and an additional 22C
in the next 4 hours. This reduction in oil temperature caused the oil level
to drop approximately 3 1/2 inches, and since the inert gas pressure system
was inoperable, a partial vacuum was created.
It is postulated by Westinghouse that the vacuum condition resulted in the
release of previously dissolved nitrogen gas from the oil, and that this
released gas rose as bubbles. It is also postulated that these gas bubbles
entered the cylindrical insulation structure around the high-voltage bushing
and distorted the electrostatic field surrounding the high-voltage lead. The
distorted electrostatic field, coupled with the reduced dielectric strength
caused by the gas within the high-voltage bushing, allowed electrical arcing
which resulted in the low-and high-voltage windings being shorted to ground.
2. Second and Third Failures: On June 19 and July 3, 1981 respectively,
the Phase C and Phase B transformers of the North Anna Unit 2 main
transformer bank experienced high-voltage bushing to ground failures.
Before these failures, these two units had been exposed to overvoltage
stresses of 31kV on one occasion and 25kV on four other occasions.
Other factors that may have contributed to these failures include:
o These units and their companion unit were initially installed at
North Anna on July, 1974. On April of 1976 they were shipped to a
Georgia Power Co. facility where, on May 6, 1976, they were
exposed to the aforementioned failure of the companion unit. After
the failure of the companion unit, they were shipped to the
Westinghouse factory in Muncie, Indiana for inspection and test
before being returned to the North Anna facility. While at North
Anna, these units were again exposed to the failure of their
companion unit described above in Item 1.
o From September 25 to October, 1976, the Phase B and C transformers
were stored out of the oil at North Anna for 5 and 6 months,
respectively. During this time, their bushings were stored in
shipping crates at an 8 to 9 angle, but they should have
been stored at a minimum angle of 20.
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December 22, 1982
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Initial inspections of the failed transformers disclosed internal failures
of the high-voltage bushings; burning was more severe on the bushing of
Phase B transformer. Although no other causes for failure were evident,
examination of Phase B transformer revealed secondary flashover points and a
ruptured cover that raised the high-voltage bushing and lead. Phase C
transformer on the other hand, evidenced no tank distortion.
Factory inspections of these bushings (see attached Figure 1) disclosed
burning of the tap and ground foils on the straight portion of the condenser
where the flange and ground sleeve are attached (Region A of Figure 1). The
insulating paper between these foils was wrinkled and burned on the edges in
this region and across the surface of the condenser from Region A to the
bottom end of the condenser taper. Arc marks were found on the lower bushing
corona end shield and the ground sleeve of Phase C transformer, and on the
HV lead tank wall of Phase B transformer. Based on the factory inspections,
it was concluded that the failures originated as arcs between the ground and
tap foils in the straight portion of the condenser (Region A). These arcs
then propagated across the surface of the tapered portion of the condenser
(Region B). On Phase C transformer, the gas generated by this burning
disintegrated the lower portion of the bushing and established an arc path
from the lower corona shield to the steel ground sleeve. The arc path on the
Phase B transformer included the high voltage lead connected to the bottom
of the bushing, the tank wall, and windings.
Since the bushings were improperly positioned (i.e., in the near-horizontal
position) during their transportation and storage periods, portions of the
condenser were not completely covered by oil. The absence of oil allowed the
gas in the expansion cap to permeate the exposed paper layers. Once gas
permeates the paper region the busings are degraded electrically and long
time periods are required to completely reimpregnate the paper with oil.
Such degraded bushings are susceptible to corona discharges on overvoltage
conditions such as those experienced by these bushings before their ultimate
failures.
Corona discharges are usually extinguished when the voltage returns to
normal; however, the dielectric quality of the bushing is reduced each time
a corona discharge occurs. As a result, new corona discharges will occur at
lower voltage levels than those of the previous discharges. This process is
cumulative and can continue until the corona extinction voltage level is
less than the normal operating level, in which case disruptive failures will
occur. Unlike previous disruptive events at North Anna, the transformer
failure of July 3 generated such great internal forces that the transformer
box and one oil pump discharge pipe ruptured. The oil that erupted from
these two breaks ignited, and the resultant fire shorted out the overhead
bus bars from a three-phase reserve station service (RSS) transformer. The
affected RSS transformer is one of three transformers that supply Unit 1 and
2 with offsite power(startup power) whenever a unit generater is not
available. These transformers are also the preferred sources of power for
the Class 1E loads whenever a unit generator is not available. Underground
cables serve the 4160-V emergency buses from these RSS transformers, and the
overhead bus bars and cables serve the normal service station buses;
however, there are no isolation breakers near the RSS transformer for the
overhead bus bars.
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December 22, 1982
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3. Fourth Failure: On July 25, 1981, while being backfed from the 500-kV
switchyard for-preoperational testing, Phase C transformer of the North
Anna Unit 2 main transformer bank failed. The failure was a high
voltage to ground fault which ruptured the tank and resulted in the
loss of cooling oil. No fire ensued. Factors which contributed to this
failure include:
o This transformer was purchased by another utility in 1968 and
placed in service in 1970; its two companion transformers failed
in 1976. It was then shipped to the Westinghouse factory in Muncie
for retest, where it was subjected to its basic insulation level
test. The unit was shipped from Muncie to the other utility and in
June of 1981 it was shipped to North Anna.
o On July 3, 1981 it was exposed to the failure experienced by Phase
B transformer.
It appears that the low-voltage coil edge of the transformer experienced an
incipient failure as a result of the high-voltage to low-voltage failure of
Phase B transformer on July 3, 1981. Evidence leading to the assumption
includes:
The inside of the tank top above the end frames was coated with carbon.
The corner nearest the failed points was darker than other walls.
The original oil level was clearly defined by a dark carbon ring
encircling the inside of the tank, indicating that the carbon ring was
probably generated during the July 3 failure of Phase B transformer
(e.g., the ruptured tank associated with the July 25 failure depleted
the oil too quickly for a carbon ring to form).
4. Fifth Failure: On August 22, 1982, Phase B transformer of the North
Anna Unit 2 main transformer bank experienced an electrical insulator
bushing failure. This failure caused a turbine trip and reactor trip
from 30 percent power; as a result hot transformer oil was sprayed
around the failed unit and the fire protection water deluge system was
activated. Although no fire ensued, the oil emanating from the bushing
also sprayed the bus bars of RSS Transformer C. After the plant was
brought to a static condition and to ensure that no fire would occur
from the sprayed oil, RSS Transformer C was taken out of service until
the oil was removed.
Offsite power to most of the plant auxiliary loads remained available
via RSS Transformers A and B; however, the Class 1E buses normally
served from RSS Transformer C required the running of the onsite
emergency diesel generator dedicated to said buses. While in the
process of restoring RSS Transformer C to service, an unrelated event
occurred. Load on the diesel generator was being increased while
synchronizing the generator with the grid in order to restore RSS
Transformer C to service. During this period oil leaked on the hot
exhaust system of the diesel generator and a fire ensued. The fire was
quickly extinguished and no damage was done to the diesel generator.
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December 22, 1982
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5. Sixth Failure: On November 16, 1982, while Unit 1 was being heated up
following a refueling outage, the Phase C Transformer of the main
transformer failed. Since the Unit was not generating power at the time
of the failure, the main transformer was being used to provide power to
the auxiliary, loads from the grid. Although this is the sixth main
transformer failure experienced at the North Anna nuclear facility, it
is the first failure of a Unit 1 main transformer.
As a result of this failure, a small hole was blown in the
transformer's case, causing oil to be sprayed into the adjacent area.
The oil was contained in the concrete basin surrounding the transformer
and no fire ensued.
Subsequent to this failure, Westinghouse recommended that the North
Anna Unit transformers be modified. Shortly thereafter the licensee
modified the Unit 1 transformers as recommended by Westinghouse. The
recommendations included incorporating a constant oil pressure system
(COPS) that separates the nitrogen gas from the transformer oil by use
of a diaphragm. The intent of this modification was to eliminate the
previously described adverse affect of dissolved nitrogen gas in the
oil.
6. Seventh Failure: On December 5, 1982, with Unit 1 operating at 30
percent power, its Phase B main transformer failed. The transformer
failure caused an automatic trip of the turbine and the reactor. Prior
to the trip, but not related to the transformer failure, two steam flow
instruments had been tripped because of erratic indications. Subsequent
to the trip, the reactor coolant temperature (Tave) reached a low
setpoint which initiated emergency safeguards functions, including
safety injection which was terminated after seven minutes.
The forces associated with this failure caused the transformer's case
to rupture; however, since the rupture occurred in the upper part of
the transformer, the oil discharged was limited to approximately 1,500
gallons. The oil was contained within the transformer's concrete basin.
The effects of the failure were not confined to the immediate area of
the transformer. Rather, the effects were propagated to the main unit
generator and its accessories which suffered significant damages. The
damage due to this event included:
o Phase B Transformer: Tank ruptured in several places; COPS
remained intact, but the connection line from the tank to top of
the transformer ruptured. Cooling fans and enclosures were blown
off and fell against the fire lines breaking the lower ring of the
deluge system; low-voltage bushings were both broken; low-voltage
connection box split; high-voltage flexible cable to the bushing
had failed and had, gone to ground and to the secondary winding.
o Isolated Phase Bus and Ducting: Oil found on the B Phase duct;
breaker G-12 was coated with aluminum dust and its access cover on
Phase A was bowed out; Phase A and B duct and bus work at the
elbow on the generator side of Breaker G-12 and six standoff
insulators for the bus were damaged sufficiently to require
replacement.
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o Main Unit Generator: Neutral grounding transformer, feeder cable
and enclosure destroyed; generator neutral enclosure was damaged
and the north side was blown out (a fire was created in the
enclosure but only minimal damage resulted); hydrogen lines to the
generator around the neutral enclosure were badly damaged, but no
leaks occurred (had a leak occurred or had the line ruptured, the
health and safety of plant personnel could have been endangered);
Phase C in the neutral enclosure bushing was cracked and others
received splatter; stator damaged and copper found in the lead
box; air handler ducting damaged.
The licensee estimates that repair or replacement of the damaged parts will
be completed by April of 1983.
The licensee is investigating the main transformer failures at the North
Anna nuclear facility. The licensee plans to replace the Unit 1 transformers
prior to restart and the Unit 2 transformers during the next refueling
outage with different make transformers.
We have been informed that about 300 similar transformers were placed in
service between 1965 and 1976. To date, twenty-one failures have been
experienced by these transformers, seven of which have occurred at the North
Anna facility and six at a Georgia Power Company fossil fuel plant. These
failures have been characterized as being winding or bushing failures
similar to those described in this Notice.
If you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office or this office.
Edward L. Jordan, Director
Division of Engineering and
Quality Assurance
Office of Inspection and Enforcement
Technical Contact: I. Villalva, IE
(301) 492-9635
Attachments:
1. Figure 1, "Outline - High Voltage Bushing"
2. List of Recently Issued IE Information Notices
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