Transformer Fan operation

[pwrengrds]

What are the best practices for settings the fan control temperatures on a 50-100 MVA, 65C rise, ONAN/ONAF/ONAF, mineral oil transformer? What temperatures should the fans be set to operate at?

Usually I have seen 65C/75C or 75C/85C. We are in process of startup and the manufacture is recommending 90C/95C.

The trips are also a bit higher than I normally see. Top oil is alarming at 100C, trip at 110C, Winding alarming at 120C, trip at 130C.

David

 

[Gr8blu]

pwrengrds: Might want to review an older thread or two, such as

https://www.eng-tips.com/viewthread.cfm?qid=431771

Not sure it has your exact answer, but it might point you in the correct direction. Specifically - how is the transformer rated in terms of insulation class (Hint: could be a letter or a numeric designation).

Converting energy to motion for more than half a century

 

[prc]

The logic for the recommendations from the manufacturer:
-Maximum ambient temperature- 40C
Maximum winding average temperature rise - 65C
Max winding hot spot rise - 78C (as per IEC)
Normally winding temperature indicators are programmed to show hot spot temperature and not average temperature rise. Hence maximum winding hot spot at full load and maximum ambient = 78+40= 118 C. So selected 120C for the alarm and 130C for the trip, considering some emergency overloading
The top oil rise for this size transformer is usually 20C lower than the hot spot temperature. Accordingly oil alarm and trip, are 20 C lower than winding.

 

[bacon4life]

Transformer temperature readings are rather imprecise. I have seen adjacent temperature wells on a transformer read several degrees different using the same sensor. Including sensor drift and winding temperature compensation could result in more than a 15 C error.

I turn on fans at lower temperature than the MFR recommends. Most of my transformers have very low fan run times because the transformers typically carry N-0 loads but are sized for N-1 loads. The suggestion might be different for a transformer with constant load where the energy used by the fans is non-trivial or the fans might not last the whole transformer lifetime. I use 80C/90C winding temperature. If the fans are based on oil rather than winding temperature, the settings would be 10-15 C lower.

I would allow at least a 10 C margin trip between the calculated winding temperature at the transformer MVA rating and the trip setting. Although IEEE transformers are based on 30 C ambient + 65 C rise + 15 C winding rise, keep in mind that the 30 C ambient value is a 24 hour average. The standards actually allows a peak ambient of up to 40 C, leading to a meaning an expected normal life expectancy winding temperature of up 120 C. If the transformer is going to be loaded beyond nameplate, the trip would be set even. If excess loading will bring the wind temperature near the 140 C, the risk from high temperature changes from cumulative paper damage to bubble formation, so a different approach may be needed at high levels of allowed overloads.

In general PRC-023-4 requires a 15% margin between relay settings and the Facility Rating. Although high temperature trips work slightly differently, I assume best practice would be to allow a 15% margin in terms of MVA. For certain BES transformers, PRC-023-4 R1.10 prohibits setting top oil supervision below 100 C and prohibits setting winding temperature supervision below 140 C.

My comments are based on the assumption that system operators will be monitoring alarms and will make adjustments to keep the transformer with the published Facility Ratings. Decades ago substations were monitored by customer outage calls and monthly station checks instead of real-time SCADA monitoring. Without active monitoring, I can see why old installations had winding trips set at 110 C.

One other item to consider is whether there needs to be a few second time delay on the trip to prevent inadvertent trips. Such trips could be caused by power cycling a temperature monitor with contacts programmed as failsafe, a loose wiring connection to the RTD, or a momentary fault current that tricks the temperature monitor.

 

[crshears]

My utility's relationship and history with its transformers' cooling fans can be interesting . . .

Since we mostly are a dual-element transmission owner/operator, we tend to set the cooling fan start/stop set points rather conservatively, the main reason being that if one transformer suffers a contingency there will be a bit of a thermal "hole" that buys the operators some time to perform load transfers etc. before excessive winding temperatures are reached, as and if required by the nature of the contingency.

Some / a few of our big city / downtown transformer cooling fans have a parallel control circuit that places all fans in service immediately if the total secondary current exceeds 2000 A, without regard for ambient temperature, again in way of extending the life of the thermal "hole" / minimizing the duration of any high oil/winding temperatures, especially over peak or on very hot summer days.

One other thing we do is place all cooling fans in service during the bird nesting season in an effort to discourage our avian friends from nesting in the rads.

This effort is only partially successful, as at stations located in dense urban centres where the fans generate a considerable noise, people without air conditioning who reside close to the station and try to sleep with their windows open will register noise complaints about fans roaring all night.

Conversely there have been numerous reports of birds choosing to build their nests directly on the tops of the cooling fan shrouds in the outside centre of the cluster while these are all in continuous operation, with the speculation being that their bird-brained idea is that predators won't take a chance on getting injured by all those whirling blades . . .

This latter policy has frequently led to the failure of individual cooling fans well before the trafo EOL.



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]