Overloaded Transformer Question

[Hv&Lv]

A few years ago a local municipal utility actually did run hoses up a few of their poles to cool overloaded transformers.

We had a 2500 KVA PMT at a mill that was overloaded to the point of being close to tripping out. We had rotating shifts watching the temperature while washing down the XF until the weekend when we could change it. Strange what some will do when a contract guarantees power...
(person related, not natural)

I agree with some here, It wont do anything to bring down the currents, but it allows the XF to hold the loads longer. Turning off the equipment wasn't an option.

 

[T.M.Haja Sahib]

That a pole top transformer with excess cooling capacity can sustain 200% overload is given in section 4.6.3 distribution transformers of the following
http://books.google.co.in/books?id=...cON7TO4&hl=en&sa=X&ei=wY0sULbtKcTqrAeiloCQDg&

 

[T.M.Haja Sahib]

When they do allow a transformer to be loaded that heavily they take into consideration how long it is expected to be loaded, at what level, how long of a cool down period expected before loaded again, and ambient temperature.

But the pole top transformer appears to be a different breed.

A British standards institution publication CP1010-1959 states a maximum of 150% overload only for 3 hours for oil cooled transformer (it will be naturally less for dry type transformer).

 

[kwired]

But the pole top transformer appears to be a different breed.

A British standards institution publication CP1010-1959 states a maximum of 150% overload only for 3 hours for oil cooled transformer (it will be naturally less for dry type transformer).

May be a different breed, but you still can't assume it will take any load you put on it, it will have limitations. Amount of loading over time with cool down considerations are still used, just at different levels.

 

[Hv&Lv]

But the pole top transformer appears to be a different breed.

A British standards institution publication CP1010-1959 states a maximum of 150% overload only for 3 hours for oil cooled transformer (it will be naturally less for dry type transformer).

We have 10's that will handle a 20 kW house in the winter but not in summer. ( I love the new meters that give demand) Point is, what the book says and actual will be determined on different variables, ambient temp. and overload time just to name two. At what point does the coil act more like a resistor than an inductor...

 

[T.M.Haja Sahib]

May be a different breed, but you still can't assume it will take any load you put on it, it will have limitations. Amount of loading over time with cool down considerations are still used, just at different levels.

Agreed. Please provide any data on overload characteristics of pole top transformer with respect to time,cooling period thereafter etc., if you have any. Thanks.

 

[mivey]

Agreed. Please provide any data on overload characteristics of pole top transformer with respect to time,cooling period thereafter etc., if you have any. Thanks.

You can run them up to about 300% for very short periods. I have been amazed at some of the transformers I have found that have survived for a very long time with severe overloads. I know the older ones are quite durable but I suspect the newer ones are not over-built as much.

Anyway, you have to factor in maximum loading prior to the overload, initial loading profile, temperature ambients and ratings, duration of overload, acceptable loss of life, etc. There are publications that cover the specifics but some generic examples:

Normal life expectancy with 1 hour overload:
For 90% initial load: 140% loading permitted
For 70% initial load: 154% loading permitted
For 50% initial load: 160% loading permitted

Normal life expectancy with 4 hour overload:
For 90% initial load: 112% loading permitted
For 70% initial load: 117% loading permitted
For 50% initial load: 119% loading permitted

0.25% life loss per occurance with 50% Initial loading:
1 hour overload: 191% loading permitted
4 hour overload: 138% loading permitted

1.0% life loss per occurance with 50% Initial loading:
1 hour overload: 200% loading permitted
4 hour overload: 153% loading permitted

0.25% life loss per occurance with 100% Initial loading:
1 hour overload: 169% loading permitted
4 hour overload: 132% loading permitted

1.0% life loss per occurance with 100% Initial loading:
1 hour overload: 192% loading permitted
4 hour overload: 148% loading permitted

 

[T.M.Haja Sahib]

You can run them up to about 300% for very short periods. I have been amazed at some of the transformers I have found that have survived for a very long time with severe overloads. I know the older ones are quite durable but I suspect the newer ones are not over-built as much.

Anyway, you have to factor in maximum loading prior to the overload, initial loading profile, temperature ambients and ratings, duration of overload, acceptable loss of life, etc. There are publications that cover the specifics but some generic examples:

Normal life expectancy with 1 hour overload:
For 90% initial load: 140% loading permitted
For 70% initial load: 154% loading permitted
For 50% initial load: 160% loading permitted

Normal life expectancy with 4 hour overload:
For 90% initial load: 112% loading permitted
For 70% initial load: 117% loading permitted
For 50% initial load: 119% loading permitted

0.25% life loss per occurance with 50% Initial loading:
1 hour overload: 191% loading permitted
4 hour overload: 138% loading permitted

1.0% life loss per occurance with 50% Initial loading:
1 hour overload: 200% loading permitted
4 hour overload: 153% loading permitted

0.25% life loss per occurance with 100% Initial loading:
1 hour overload: 169% loading permitted
4 hour overload: 132% loading permitted

1.0% life loss per occurance with 100% Initial loading:
1 hour overload: 192% loading permitted
4 hour overload: 148% loading permitted

Thanks, mivey. Please give the cooling air temperature reference for the above figures for the above data seem to suggest inferior performance for pole mount transformers. The maximum overload of 150% for oil cooled transformers, I gave earlier, is for operation of 3 hours and for remaining 21 hours at 105% load with cooling air temperature at 0 degree C.

 

[mivey]

Thanks, mivey. Please give the cooling air temperature reference for the above figures for the above data seem to suggest inferior performance for pole mount transformers. The maximum overload of 150% for oil cooled transformers, I gave earlier, is for operation of 3 hours and for remaining 21 hours at 105% load with cooling air temperature at 0 degree C.

0?C would be an unusual ambient. I would think the data I gave was based somewhere between 20?C to 40?C. I'll try to remember to look.

 

[ERIC LEDBETTER]

tun off the lights and go to sleep

 

[mivey]

Please give the cooling air temperature reference for the above figures

30?C

 

[T.M.Haja Sahib]

30?C

The book stated 200% overloading of pole top transformer for hours. So, that may be so for an ambient temperature less than 30?C. So some more information is required.