PECO Energy Company`s Peach Bottom nuclear generation plant was able to assure the summer capacity of their six 390 MVA single phase main generator step-up transformers by installing upgraded transformer oil coolers and, thereby, assure power plant generation capacity during summer peak load periods. The transformer coolers designed and manufactured by Unifin International Inc. (London, Ontario) lowered the temperature of the transformer cooling oil by up to 23 C from previous conditions. This will allow for normal plant operation even for the loss of a transformer cooler.
Before the cooler replacement, because of inefficient transformer cooling caused by cooler degradation and fouling, PECO sometimes operated during periods of the summer with excessive transformer oil and hotspot temperatures. On several rare occasions over the years, plant generation even had to be temporarily reduced for short periods to avoid excessive loss of transformer life before the coolers could be cleaned. Continuing to operate with the existing coolers would have resulted in further cooler degradation over time to the point where the transformers might have been unable to carry plant generation during peak summer conditions even when cleaned. In addition, operating under present conditions, a major reduction in generation capacity would have occurred for the loss of a transformer cooler during peak summer loads.
Transformer cooling systems in North America are designed and rated for 30 C average ambient temperature although in many locations of the U.S. the daily average temperature exceeds this significantly. To avoid greater than normal loss of insulation life, which leads to earlier transformer failure, transformers should in many cases be derated during the hot summer days when load requirements are usually the highest.
Transformer oil temperature increases at a higher rate than ambient temperature increase. This is because air density decreases as air temperature increases, but fans on coolers, move a fixed volume of air irrespective of air temperature. The result is less air mass flowing through the cooler and, therefore, less capability to dissipate heat on hotter days. The reduction in air mass translates into a higher temperature difference between oil and air. This further increases the top oil temperature beyond the ambient increase alone since top oil temperature is equal to the sum of the increase in ambient temperature and the increase in temperature rise.
Finned tube coolers
PECO`s 390 MVA transformers coolers are of the forced-oil forced-air type, known as FOA in the USA, and OFAF in most other countries.
FOA transformer oil coolers operate on the principle of pumping the transformer oil through the inside of fintubes while air is blown over the fins by propeller fans. The heat transfer surface consists of multiple rows of finned aluminum tubes. The Unifin design is produced from one piece of metal from which the fins are extruded or formed by means of rotary dies under extreme pressure.
This one piece or integral construction provides maximum heat transfer efficiency coupled with long service life. The PECO project fintube includes spirally wound turbulators inside the tubes that can further increase the heat transfer efficiency of the fintube.
The original coolers built in 1971 were of the platefin type. Platefin is manufactured with sheets of aluminum which are punched to have collared holes. The plates are then stacked and tubing is inserted through the holes and then mechanically or hydraulically expanded into the fin collars. Although platefin coolers have low capital cost there are disadvantages to this type of cooler. Thermal stresses from expansion and contraction cause the bond between fin and tube to be degraded, and the problem is exacerbated by dirt lodging between the bond.
Another platefin problem is galvanic corrosion that exists at every fin/tube joint. Galvanic corrosion will occur between dissimilar metals on heat exchangers when both water and electrolyte (SOx, NOx) are present. The electrolytic solution will accelerate corrosion of the anodic material. When there is corrosion of the tube, there is both the risk of oil containment failure and the destruction of the heat transfer bond.
Platefin also has a tendency to become easily fouled due to the close stacking of the aluminum sheets. In addition, the cooler is difficult to clean because of the plate being thin and not always being able to withstand high pressure cleaning procedures. PECO was forced to institute special cleaning procedures to clean the coolers on-line between refueling outages in order to maintain acceptable cooling efficiencies. To reduce fouling, Unifin supplied the new coolers with an increase in tube spacing which allows particulates in the air to go straight through to the core. As an extra assurance, cleaning ports were provided on the front casings, which allow ease of entry of spray cleaning equipment.
A site specific problem PECO had with the original coolers was the presence of firewalls close to the transformer. There was a noticeable recirculation of hot air, which further cut down on the cooler efficiency. To address the recirculation problem, updraft cabinets were installed. With the fans mounted up at an angle of 30 degrees, the heated air is directed up the firewall away from the transformer, ensuring that cooler air is used in the heat transfer process.
The installation of the new coolers was completed by PECO`s own maintenance staff. The new coolers were 24 inches longer, so pipe cutting had to be done. The top spool piece was reused and a dresser coupling was used for the bottom connection. A 30 ton all terrain crane was used to lift the coolers which weighed 3,200 lbs. each. New braces had to be attached from the coolers to the transformer. The new cooler fans had a higher inrush current draw and the existing breakers had to be analyzed for this increased duty before the fans were wired.
The entire installation of 18 coolers (three single phase transformers), not including oil handling and vacuum filling time, could be achieved in nine days (three days per transformer) with two shifts per day.
A field verification test was performed on May 13, 1999 to confirm that Top Oil Temperature Rise (TOR) above ambient was as designed for the coolers. A hot wire anemometer was used for measuring ambient temperatures and air entering the cooler. The oil temperature readings were taken from a recently calibrated temperature gauge on the 17,250 gallon transformer main tank.
The original transformer test report showed tested load and no load losses at tested amperes and these were used to calculate dissipated watts for the platefin coolers. The results from the new coolers were compared to similar data obtained on May 7, 1998 on the degraded platefin coolers.
The Unifin coolers were designed for dissipating 254 kW (full load) with inlet air temperature at 35 C for a TOT of 66.7 C at 100 percent efficiency. Since the coolers were calculated to have a TOT of 50.3 C at 18.3 C ambient with 31,000 amperes load, the field TOT of 49.0 C was lower than expected, proving that the coolers were performing as designed.
It was calculated that the platefin coolers had been operating at 63 percent efficiency due to fouling, thermal degradation, galvanic corrosion, and poor air recirculation.
Unifin International Inc.
Inquire R.S. 101