Replacing a large transfomer connected to a generator 2

[bdn2004]

We're replacing a step up transformer directly connected to a generator. There is a spare available but I want to verify that it can handle the load. I know the peak MW over the past year - don't I also require the power factor info? Also are there rules of thumbs on how much to load that transformer, like 80%?

For example, the peak loading over the past year is 322MW. The transformer available is a 386MVA unit.

 

[waross]

You need the peak current. Max MW may not coincide with max current nor with max voltage nor with worst case power factor.
Your 386 MVA transformer will handle 322 MW at a power factor of 83%, at rated voltage.
The bottom line is the current. If your transformer voltage is above nominal at peak MVA loading then the current will be less than it would be at the same MVA at nominal voltage.
I am surprised that that an installation of that size does not have more data available, unless this is a homework problem.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ruggedscot]

Just my thoughts waross - but if you get the right answer who cares!?

alls fair in education!

Rugged

 

[jghrist]

I'd size it for the capacity of the generator, not the load.

 

[bdn2004]

Real numbers:

The peak current is 10,000A then
(10kA)(20.9kV)(1.732) = 362 MVA, not overloaded.

Per your comment, I don’t need to know the peak voltage? What if then the peak voltage is 5% higher…(10kA)(22kV)(1.732) = 381 MVA, almost overloaded.

The power factor is the cos(theta) right? - theta being the phase angle of the voltage vs. current.

So if you knew the power factor at the time of the maximum MW, then the current angle is already taken into account right? 322MW/.83 = 388 MVA

What am I missing here?

 

[waross]

The amount of heat a transformer can safely disipate determines the safe VA or KVA or MVA of the transformer.
Current determines the heating of a transformer.
100% voltage times 100% current = 100% VA.
95% voltage times 105.6% current = 100% VA but 5.6% overload.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

You can't accurately calculate overload capability without knowing a little more about the transformer losses and how they are split into iron losses and copper losses. To a first approximation iron losses are voltage dependent, copper losses are current dependent. Depending on how the transformer design was optimised the transformer could deal better with excess voltage than excess current or vice-versa.




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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi.
In additional to Scotty's post.
Large losses is copper reactive losses.
Is about ten time more then copper active losses and more more then iron losses.
reactive losses of trafo are function of %impedance of trafo and (Iactive/Inom)^2.
Regards.
Slava

 

[BJC]

Does the transformer have extended ratings. ie: FA/FO/FAO
If so what are they?
If the existing transformer is bad what choice do you have?
Lead time to get the correct (if you determine the one you have isn't it) transformer is years, not months.
The most practical thing to do may be to see if you have connections for additional radiators and check on adding bigger and/or more fans.

 

[PiotrM]

Hi,

you should looking for loading parameters in your transformer documentation.
There should be some loading parameters, e.g.:
1. normal load cyclical, daily
2. emergency, long-time cyclical daily load
3. emergency, short-time overload

It specify direcly load current for HV, LV windings and temperatures:
1. oil temperature
2. windings hot-spot temperature
3. ambient temperature (at rated power)

Regards,
PiotrM