Storing Power Transformer as Spare

[111R]

Are there any IEEE standards or other technical papers that describe various methods used to store power transformers?

I've heard that it's a good idea to leave the cabinet heaters on for condensation control.

I've also heard of circulating current through the windings to keep moisture down in the paper and oil. With a 138kV/12.5kV, 20 MVA transformer with ~10% impedance, I'm not sure if it would be possible to get optimal current flow. 208V/3Ø is available, but connected to the low side with high side shorted will be too much current. Connected to the high side, the current will likely be too low to have any effect.

I'm assuming that 1/10th of the voltage on a transformer winding with 10% impedance will cause 100% of load current to flow. So, on the low side, 1250V will cause full load current to flow with the high side shorted. 208V will cause 1/6th of full load current to flow which will be ~150A...way above what is practical or possible. Connecting to the high side will reduce this by a factor of the turns ratio squared, so that will be probably be too low. Please let me know if I'm off on my math or understanding of how this works.

Thanks!

 

[dpc]

If it has a nitrogen system, make sure it is functional and properly adjusted. I'd consider wiring out the low gas pressure alarm so they get empty bottles swapped out. Any space heaters should be put into service but on a thermostat.

I don't think I'd bother with trying to energize the main windings - a lot of things could go wrong with that not to mention consuming a lot of power.

Maybe the transformer experts will have better advice.

 

[crshears]

Standards for this I am not aware of, but the ones I've seen stored included but were not limited to the following:

1] Particularly when stored in proximity to other energized high-voltage electrical equipment, a light-guage draining wire was commonly / usually wrapped daisy-chain fashion from ground to each bushing to prevent static charge build-up.

2] Grounding conductor was bonded from the station ground grid to the transformer tank.

3] An auxiliary AC supply was normally wired into the mechanism box or boxes and cabinet heaters were kept in service for anti-condensation purposes. Oil circulating pumps were not normally operated.

4] Low oil level alarms were commonly connected into some sort of remote telemetry / supervisory scheme, particularly at unattended locations, so that a prompt dispatch for inspection could be initiated upon receipt of this alarm.

5] In the absence of the transformer being kept pressurized with a nitrogen 'pad,' a desiccant canister was piped between the conservator vent and atmosphere, and same was regularly and diligently maintained.

I've never seen any attempt made at keeping a low-grade current flowing through any windings for warmth purposes.

CR

 

[squeeky]

I am very interested in this subject and I am trying to convince management about the need to look after these spare transformers. We have up to 20 transformers sitting in an outdoor yard. They sit in the sun with shade temperatures about 35 degrees Celcius and night 12. Static build up is definitely a problem and in my opinion they should never be emptied. Gas build up inmidland the risk of a spark is too great. All these transformers are oil filled. I along with the wiring of all the alarm systems and the earthing. The oil is often drained a d mixed with the diesel to run vehicles. Trying to get them onto the maintenance program is my next step so that the silica gel breathers are checked on a regular basis. I believe that DGA on the oil on an annual basis is required. Being in a bunded area is also critical. Generally they are stored next to each other without fire walls. If one were to catch fire, I believe the rest would join them. This is an ideal place for people to hide and have a smoke. These areas are generally out of the way and there is no fire fighting equipment kept near by as they become mobile and immigrate. I feel that they need to be included on the security schedule and the Keyalert system extended to include this area.

 

[Millbo]

The manufacturers O&M manual will give guidance on putting their units in long term storage.

A few key things from memory:
Keep the bushings/windings shorted and grounded
If it's a conservator type unit keep up with the condition of the desiccant in its breather
Power up the heaters in the control panel, usually 120 VAC and don't bypass its circuit breaker or thermostat
Ensure the tank is properly grounded.
Seal up any knockouts in the control panel to keep varmints out.