Power Transformer Emergency Overload Condition: Keeping Temp Low
Power Transformer Emergency Overload Condition: Keeping Temp Low
(OP)
If there's a situation where a transformer must be loaded 125% or more above the top ONAF rating for a short period of time, what are some options for preventing excessive temperature rise?
Aside from additional fans, has there been any experimentation with other ways to provide additional cooling that does not require taking the transformer out of service? For example, would multiple large industrial air conditioner units blowing directly on the radiators have any effect on an outdoor transformer? Or, some sort of water mist cooling unit?
I realize these are a bit off-the-wall, but I was wondering what options are available if you're ever in a bind.
Thanks
Aside from additional fans, has there been any experimentation with other ways to provide additional cooling that does not require taking the transformer out of service? For example, would multiple large industrial air conditioner units blowing directly on the radiators have any effect on an outdoor transformer? Or, some sort of water mist cooling unit?
I realize these are a bit off-the-wall, but I was wondering what options are available if you're ever in a bind.
Thanks






RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
old field guy
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
However, if the transformer is a large and new one, it could be a good idea to foresee an integrated real-time temperature / dissolved gas monitoring system.
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RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
That's fine for outdoor-style bushings and a droplet or mist application. I've been told by fire-fighting 'professionals' that the stream from a firehose is safe to 69 kV. However, none of them accepted my offer to view a demonstration.
I remember, however, a client's transformer that suffered a poor foundation, allowing it to tip to the point that the upper tubes of the radiators were no longer in oil. This killed the natural thermosiphon effect through the radiators. The transformer heated up rapidly.
It was the sole power source for critical environmental equipment and could not be removed from service, so we cooled it by flowing water over the case. The high voltage (13.8 kV) bushings were inside a termination cabinet, a fortunate thing because the long-term flow of water over the case resulted in long beards of algae growing from the transformer. Clearly, as a long term solution, this had some issues. That transformer was still in service with its long green beard when I moved to another service area.
old field guy
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
Every time when the insulation temperature exceeds its rated value, its life reduces and the reduction in the insulation life keeps on accumulating. Hence in my view cooling the transformer as proposed by “111R” shall not be advised by an Electrical Engineer.
I have seen the mechanical guys in a process plant cooling the heat exchanger plates by continuously spraying water.
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
The rating of transformer, age of it and period of overloading in hours and ambient at the time of overload will make the estimation easier. As per IEC Power Transformers can be overloaded (normal cyclical loading) up to 130 % provided,absolute temperatures of oil/winding and winding hot spot do not exceed 105/120 &140 C. The understanding is that loss of life at the time of overloading is compensated when the unit works under load or below normal average ambient temperatures.
For every 6C rise above the nominal winding hot spot temperature of 98C ( based on weighted annual ambient of 20C and max ambient of 40C as per IEC 60076-2) the paper life halves.
External additional cooling by way of extra fans or showering with water are all good to reduce the oil temperature rise, but still the winding rise will be more than normal and to that extent additional ageing must be expected.
Another point is that if the transformer is old, it is dangerous to overload the unit. Paper insulation in old transformers will contain 2-5 % of water in it which will be vaporized at temperatures above 100 resulting in bubbles in electric field which is a sure way for dielectric failure. Anyway a complete insulation dry out before the overloading cycle is recommended. When moisture exceeds 1 %level, insulation ageing goes up multifold for the same insulation temperature.
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
The underlying physics are based on Arrenhius's equation (which describes the relation of a rate of a chemical reaction to absolute temperature, the chemical reaction being the degrading of the paper in our case). As such, it is the absolute temperature of the hot-spot that is important for the insulation aging, nominal temperatures, winding gradients and temperature rises are only important in relation to the absolute hot-spot temperature.
Therefore anything that will lower the absolute hot-spot temperature will have positive effect on the degrading of the paper. Including lowering the ambient and adding thermal convection from the radiators (which will result in lower bottom oil temp and thus lower hot-spot although the temp rise stays unchanged).
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
Bill
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RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low
Depending on the transformer, it maybe at outside conditions, or in maybe under a N2 blanket. In either case if it is of signifent size it would have a chenical dryer attached.
The small transformers may not have much at all, and may be consitered expendable.
It is possible to increase the number of fans, cooling fins, add water cooling, but that dosen't change what is happening at the core. The oil can only draw heat away so fast.
What I saw once was to leave the cooling fans on all the time to allow the transformer to cool to an extra amount at night, so it stayed cooler during the day.
Really, water cooling? How many substations have water plumed to them?
RE: Power Transformer Emergency Overload Condition: Keeping Temp Low