Best practice for testing before re-energizing a dry type MV 13.2-480 transformer 2

[tdepuy]

Can anyone speak to the dangers of having a Dry type MV 13.2-480 transformer down for any amount of time and then bringing it up without testing? Should you dry them out like a motor? What is the best way to do it? What testing is adequate (Meggering)? What does good look like? What voltage is recommended? Is 1000VDC good enough? Should it be 500VDC or higher? What is the best practice for testing before re-energizing?

 

[collies99]

Has it been stored properly ? (was it stored in a heated environment), was it protected from environmental contamination, was it stored from any possible physical damage ?).

Least amount of testing is PI test, you can use NETA test voltage levels.

hv to lv/gnd @ 5kv, 10 minutes
lv to hv/gnd @ 1kv, 10 minutes

for surety of winding integrity, for a little more extra time.

ratio tests and winding resistance tests at as found tap.

compare to factory test records or last test records. If no deviation from previous date, xformer is ready for energization.

 

[tdepuy]

Thanks for your reply. Its environment is not the best. Its enclosure is inside a transformer room, but it gets its fresh air intake and return from outside. The air is moist. It was down for three days to do work on another piece of equipment. It rained during this time so the relative humidity was high and it was cold-ish (40-50deg). When re-energized, the secondary switch had not been thrown so there was no load. The Primary shorted to ground and it failed. It was not Pi’d or Meg’d prior to energizing. At present we do not have a policy to test before energizing a transformer after having it down for some length of time, but I’m trying to determine if we should develop one as an action item for the RCFA, along with getting the room environment improved. I will review the Meg reading over the past few years to see if the insulation resistance has been going down.

 

[collies99]

You are correct, similar to a motor.

A big percentage of dry-type transformer failure do occur after a prolonged shutdown when the unit cools down and condensation penetrates the insulation. Keeping it dry, cleaned and vacuumed is recommended and the megger test is very sensitive to moisture presence and temperature changes.

 

[collies99]

IEEE C57.12.01 Recommended practice for dry-type

 

[collies99]

Oops! C57.94 please.

 

[tdepuy]

A Summary of what I found concerning “Best practice for testing before re-energizing a dry type MV 13.2-480 transformer” composed from this thread "collies99" and from other sources. Thanks to collies99 & A.Mc.

A big percentage of dry-type transformer failure does occur after a prolonged shutdown when the unit cools down and condensation penetrates the insulation. Keeping it dry, cleaned and vacuumed is recommended and the megger test is very sensitive to moisture presence and temperature changes.

C57.94-1982 - IEEE Recommended Practice for Installation, Application, Operation, and Maintenance of Dry-Type General Purpose Distribution and Power Transformers

Get the past PM data for all the transformers. Form a baseline and trend them. If you continue to trend them Y/Y you may be able to see a fault coming.

At a minimum install local “ELECTRIC” heater in transformer compartment. Must be large enough to provide full heating. You may need two one on each end front to back. Use Electrical duct heater and blower.

You may want to have a policy in place for energizing a transformer after having it down for any length of time. Typically if any electrical equipment is kept down more than 48 hours, you should megger at a minimum. If it is located within an uncontrolled environment you may want to consider PI testing and trending Y/Y.

For any transformers, you should TTR the transformer as well as megger to ground.
1. TTR transformers to get baseline
2. Megger to get baseline
3. PI test them if critical and keep the data Y/Y. The trend will show you insulation issues.

Least amount of testing is PI test; you can use NETA test voltage levels.

hv to lv/gnd @ 5kv, 10 minutes
lv to hv/gnd @ 1kv, 10 minutes

For surety of winding integrity, for a little more extra time.

Ratio tests and winding resistance tests at as found tap.

Compare to factory test records or last test records. If no deviation from previous date, xformer is ready for energization.