Storing Power Transformer as Spare
Storing Power Transformer as Spare
(OP)
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!
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!






RE: Storing Power Transformer as Spare
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.
RE: Storing Power Transformer as Spare
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
RE: Storing Power Transformer as Spare
RE: Storing Power Transformer as Spare
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.