United States Nuclear Regulatory Commission - Protecting People and the Environment

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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                                                           IN 82-53 
                                                           December 22, 1982
                                                           Page 2 of 7 

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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                                                           IN 82-55  
                                                           December 22, 1982 
                                                           Page 3 of 7 

          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. 
.

                                                           IN 82-53 
                                                           December 22, 1982
                                                           Page 4 of 7 

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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                                                           IN 82-53  
                                                           December 22, 1982 
                                                           Page 5 of 7 

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. 
.

                                                           IN 82-53  
                                                           December 22, 1982 
                                                           Page 6 of 7 

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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                                                           IN 82-53  
                                                           December 22, 1982 
                                                           Page 7 of 7 

     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  

Page Last Reviewed/Updated Tuesday, November 12, 2013