ck364
Electrical
- May 1, 2009
- 1
Could anybody shed some light on some possible causes for transformer failure on a 3 phase 7500/12470 y primary with floating ground to a 480/277 low resistance grounded secondary. When we started the installation we found that 2 of the single phase cans had leaks around the bushings which we repaired then replaced then filled the oil back to the correct level. We let the cans sit for 2 days before they went back up to the sub made our y to y connections and blew the first 2 20 amp fuses on the primary when we put the second transformer of the bank on line. ABB calculated the current @ 23 amps so we jumped up the fuse size to 40 amps. We again sticked in the fuses and this time they all held. The secondary voltages were spot on but the only secondary connection X2 all tied together on subtractive polarity transformers. Problem solved the rest of the plant happy time to take the generator off line so we connected all the paralled secondary wires that were isolated from the switchgear and each other and left them energized overnight.In the morning the center phase fuse was blown on the primary so we meggered out all the secondary conductors to each other and to ground with a solid 20 gigohm reading on a 30 second test with a 1kv setting. We also shorted the secondary and primary to each other and then ground and megged those connections with another 15gigohm reading. The primary was done @ 5kv and the secondary @1kv. With all of thse things checking out we then tryed to put the fuses back in and once again one the second primary fuse was closed they both blew. I plan to do a polarity test and a turns test although we had a company do this before we installed them and then a ratio test with the megger with a 10 minute base. Any suggestions as to causes or advice about further testing procedures would be greatly appreciated because the lets see if the fuse will blow now method is not how we want to troubleshoot this problem. Thank You
