Information Notice 1982-53, Main Transformer Failures at North Anna Nuclear Power Station
SSINS NO. 6835 IN 82-53ý
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555 December 22, 1982 IE 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
failurk's, including one that resulted in a fire and one that caused extensive
damage to thi 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 330MVA single-phase Westing- house 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 I 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 trans- former. The seventh failure also generated sufficient forces to rupture the
transformer's casing; however, the rupturr 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,
0212060350
IN 82-53 December 22, 1982 Page. Z of 7 a fire was created in the neutral enclosure but little damage ensued, the
hydrogen lines to the generator around the nuetral 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, foufth., 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 . main transformer bank had been subjected to several documented over- voltage 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:
Th6 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:
0 On January 31, 1979 and October 19, 1980, the transformer was subjected
to overvoltage conditions of 31kV and 25kV, respectively.
0 Approximately 61/2 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 90%C and O°C, respectively. The
IN 82- December 22, 1982 windinq temperature indication was obviously in error and may have
been the reason for some coolers not operating. Further examination
of the transformer verified MRO operation and detected oil on the
cover of the transformer. Operation of the additonal oil coolers
reduced the temperature to 500 C at the time of failure.
The oil expansion calculations performed by Westinghouse indicated the oil
level was approximately 21/2 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 MRO alarm, and three additional coolers wcre started
manually, thereby increasing the cooling units by 150 percent. As a consequence
the temperatuy, of the oil on the top of the tank dropped about 18*C in the
first 2h hours and an additional 22*C in the next 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. This reduction in
oil temperature caused the oil level to drop approximately 31/2 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:
0 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.
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, respec- tively. During this time, their bushings were stored in shipping
crates at an 80 to 90 angle, but they should have been stored at a
minimum angle of 200.
IN 82-53 December 22, 1982 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 8). On Phase C trans- former, 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 allowid 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 Londi- tions 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 cumula- tive 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 trans- formers 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 emer- gency 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.
IN 82-53 December 22, 1982 '
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.
0 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 en- circling the inside of the tank, indicating that the carbon ring was
probably generated during the July 3 failure of Phase 2 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 systemwas 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 cond'tkr 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 Transforme.rs 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 syschronizing the generator with the
grid in order to restore RSS Transformer C to service. During this period
oil leaked on tho 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.
IN 82-53 December 22, 1982 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 trans- former 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 transr
former 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 recommen- dations 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 instru- ments 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.
ihe 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 genera- tor and its accessories which suffered significant damages. The damage
due to this event included:
0 Phase B Transformer: Tank ruptured in several places; COPS remained
intact, but the connection line from the tank to top of the trans- former 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.
Isolated Phase Bus and Oucting: 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.
IN 82-53 December 22, 1982 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 haidler ducting damaged.
The licensee estimates that repair or replacement of the damaged parts will
be completed by April of 1983.
The licensee is inivestigating 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 experi- enced 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.
v-J
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
ASEE PREVIOUS CONCURRENCES
WPU:JD *EAB:DEQA:IE *C:DEQA:IE *D:DEQA:IE
12/17/82 I VILLALVA WR MILLS EL JORDAN
5520 12/ /82 12/ /82 12/ /82
IN 82- December , 1982 Page 7 of
The licensee is investigating the main transformer failures at the North
Anna nuclear facility. Thent i p tor, a,,d sptcli13sts-4om
tnc Rcgizn II1 10ffiee are monit3l 1 -re3-is f the illce Iiiv's-mt-
-twtt-reiv
arJj ekotiyth 0ll-.
the tfUz-unLvThe
Ldq~c
licensee plans to replace the Unit I 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. Todate, twenty-one failures have been
experienced by these transformers, seven of which have occurred at the
North Anna facility and six at a Georgii Power Company fossil fuel plant.
These failures have been characterized as being winding or bushing
failures similar to those described in this Notice.
t/
S'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
EAB:DEQA:IE C:DEQA:IE 4ý.1E
I VILLALVA WR MILLS EL JORDAN
12/ /82 12/ /82 12//*//82
iN 82- December , 1982 Page 7 of
The licensee is investigating the main transformer failures at the North
Anna nuclear facility. The NRC resident inspector and specialists from
the Region 11 Office are monitoring the progress of the licensee's
investigation and will review the adequacy of the corrective actions. The
licensee plans to replace the Unit I transformers with different make
prior to restart.
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
I VILLALVA WR MILLS EL JORDAN
12/1q /82 12//6/82 12/ /82
Attachment
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Attachment 2 IN 82-53 December 22, 1982 LIST OF RECENTLY ISSUED
IE INFORMATION NOTICES
Information- Date of
Notice No. Subject Issue Issued to
82-52 Equipment Environmental 12/21/82 All power reactor
Qualification Testing Experi- facilities holding
ence ` Updating of Test an OL or CP
Summaries Previously Published
in IN 81-29
82-51 Overexposure in PWR Cavities 12/21/82 All holders of a
Senior Reactor Operator
license; all power
reactor facilities
82-50 Modification of Solid State 12/20/82 A!' pcwer reactor
AC Undervoltage ReWays Type ITE-27 facilities holding
82-49 Correction for Sample Condi- 12/16/82 All power reactor
tions for Air and Gas facilities holding
Monitoring an OL or CP; research
and test reactors;
fuel facilities;
Priority I materials
82-43 Failures of Agastat CR 0095 12/3/82 All power reactor
Relay Sockets facilities holding
82-47 Transporation of Type A 11/30/82 All NRC licensees
Quantities of Non-Fissile
Radioactive Material
82-46 Defective and Obsolete Com- 11/26/82 All facilities
bination padlocks pursuant to 10 CFR
Parts 50 and 70
and Part 95 appli- cable facilities
82-45 PWR Low Temperature Over- 11/19/82 All PWR facilities
pressure Protection holding an OL or CP
82-44 Clarification of Emergency 11/18/82 All power reactor
Plan Exercise Requirements facilities holding
DL = Operating License
CP = Construction Permit
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