I had a fault in the switch of a 1000KVA dry-type transformer during some rain when water got in the switch. It flashed over in the switch and took the OCB out upstream (13.8kV) and the main OCB(115kV).
This transformer is one of two paralleled 1000kVA units that are over 50 years in age.
After disconnecting this substation 2-1000kVA units, re-energizing the 15kV circuit and powering the other subs in line, I wanted to get some testing of these 1000 kVA units before bringing them back online after the switch is repaired. It looks to have just 'marked' up the insulators and other minor damage. I think the OCB caught the fault in time.
Well these test on these 2 units came back fine as far as the Turns ratio tests (within 0.5% calculated) and the Winding resistance tests (1.45-1.49 M-ohm).
The problem was the Insulation resistance testing.
The 2 units had sat OFF for about 8 days and it has been wet outside. Even though these units are indoors, I feel that they have cooled and taken on a bit of moisture. They are dusty so I am cautious about blowing air on them and further take the chance of damage. The Last time we did this to one, it failed when energized.
The results came back in the 1-2 M-ohm range @5kV and less then 1 M-ohm @1kV. I then had the tester, unhook both the primary AND secondary to totally isolate the transformer and retest. The results changed a little to 3-4 M-ohms @5kV but that does not come close to the needed 1 per KV + 1 general rule. I need to see 10-15M-ohm to be comfortable but I doubt I would ever see this. The only records I can find dates from 1997 and 2003. The 2003 results have some issues with the way the tester present the info.
(2003)
H-L & GND (30-sec)
He lists the values as (1)1350k and (2)7000k
I read this as 1.35 G-ohm and 7 G-ohm
(I really think these values are junk, but who knows? I mean 50 years and 7 G-ohms?)
(1997)H-L & GND (30-sec)
The values are listed (2)15M-ohm and (1)1000M-ohm
The results of my testing now are just 1-4 M-ohm for the same test. I am worried because I can not see the fact that they have been off 8 days and picked up some moisture could have that large of an effect. Since the other test results came out fine, this one puzzles me and I do not feel comfortable in re-energizing them, but time is not on my side. I do not have the funds to replace them as of now. I have temp power run to the critical loads that are fed from these units so I have some headroom.
Does anyone have some advice as to what these numbers mean? I can not seem to find anything online for older units other than the kV rating plus 1 for a result.
I plan to put some heaters in the rooms and 'cook' the transformers and re-test, but I doubt it would raise the results to 15M-ohms...
Thank you for your help
[ November 12, 2004, 01:30 PM: Message edited by: jryan ]
This transformer is one of two paralleled 1000kVA units that are over 50 years in age.
After disconnecting this substation 2-1000kVA units, re-energizing the 15kV circuit and powering the other subs in line, I wanted to get some testing of these 1000 kVA units before bringing them back online after the switch is repaired. It looks to have just 'marked' up the insulators and other minor damage. I think the OCB caught the fault in time.
Well these test on these 2 units came back fine as far as the Turns ratio tests (within 0.5% calculated) and the Winding resistance tests (1.45-1.49 M-ohm).
The problem was the Insulation resistance testing.
The 2 units had sat OFF for about 8 days and it has been wet outside. Even though these units are indoors, I feel that they have cooled and taken on a bit of moisture. They are dusty so I am cautious about blowing air on them and further take the chance of damage. The Last time we did this to one, it failed when energized.
The results came back in the 1-2 M-ohm range @5kV and less then 1 M-ohm @1kV. I then had the tester, unhook both the primary AND secondary to totally isolate the transformer and retest. The results changed a little to 3-4 M-ohms @5kV but that does not come close to the needed 1 per KV + 1 general rule. I need to see 10-15M-ohm to be comfortable but I doubt I would ever see this. The only records I can find dates from 1997 and 2003. The 2003 results have some issues with the way the tester present the info.
(2003)
H-L & GND (30-sec)
He lists the values as (1)1350k and (2)7000k
I read this as 1.35 G-ohm and 7 G-ohm
(I really think these values are junk, but who knows? I mean 50 years and 7 G-ohms?)
(1997)H-L & GND (30-sec)
The values are listed (2)15M-ohm and (1)1000M-ohm
The results of my testing now are just 1-4 M-ohm for the same test. I am worried because I can not see the fact that they have been off 8 days and picked up some moisture could have that large of an effect. Since the other test results came out fine, this one puzzles me and I do not feel comfortable in re-energizing them, but time is not on my side. I do not have the funds to replace them as of now. I have temp power run to the critical loads that are fed from these units so I have some headroom.
Does anyone have some advice as to what these numbers mean? I can not seem to find anything online for older units other than the kV rating plus 1 for a result.
I plan to put some heaters in the rooms and 'cook' the transformers and re-test, but I doubt it would raise the results to 15M-ohms...
Thank you for your help
[ November 12, 2004, 01:30 PM: Message edited by: jryan ]
