Transformer forced cooling 1

[siva9]

Hello everybody,

Can anyone help me on the following power transformers forced cooling questions -
1.how to calculate the rating of oil circulating pump for a power transformer? are there any guidelines for the transformer oil flow?
2.How to calculate the watts dissipated by a oil to air heat exchanger (used in case of a power transformer)?

Please give me the reference to the articles on oil circulating pumps and heat exchangers.

Thanks in advance.
siva9

 

[7anoter4]

hi siva9
There are some general informations:
The watts dissipated by a oil to heat exchanger for a steady state load of transformer has to be theoretically the transformer losses-magnetic losses mainly in magnetic core losses and copper losses in the windings. A good approximation is P=Po+Pk
Po=no load losses
Pk= shortcircuit losses
If the transformer is outside located the sunshine could be taken into consideration.
A significant part of these losses is dissipated in the ambient air through radiation convection and conduction.
Srad is approx. the rough surface [H*[W+D]*2 +W*D].
H= average height of transformer
W = average width of transformer
D=average depth of transformer
The dissipation from radiation may be:
Prad=ar*Srad*DT
DT=10-15 degrees Celsius [difference between transformer wall temperature and maximum ambient]
The dissipation from convection-conduction may be:
Sc is the total exterior surface[including radiators if any] Pcc=acc*Sc*DT
Approximatly the factor ar=6[w/oC/m^2]; and acc=8 [w/ oC /m^2]
The remaining heat transmitted to oil depends upon the inner surface of the core and windings of the transformer [and in a great measure upon the height of this surface].The transmitting factor depends in turn upon oil entering temperature , oil heat capacity and the difference between oil temperature and transformer exposed to oil surface.As an average 80W/ oC /m^2 could be considered.The inner surface exposed to oil could be more or less equal to exterior surface.
So if we have the finally the Q[W] the power transmitted to oil we need the oil temperature,viscosity and specific gravity,the maximum oil temperature and the enter oil temperature.
A quick and dirty method of estimating oil mass flow rate is to assume that all of the electrical energy entering a process is converted to heat. To determine the heat load more accurately, the equation m=Q/Cp/DTcan be used where:
Q = heat load (BTU/hr) or (KW)
m = mass flow rate (lb/hr) or (KG/HR)
Cp = specific heat (BTU/lb °F) or (KJ/KG/oC)
?T = change in temperature (°F) or (oC)
There is an example:
transformer data
S= 10 MVA
Po=16.5 kw
Pk=69 kw
Q=16.5+69=85.5 kw
Cp[oil]=1.93 kJ/kg/oC
DT=70-20=50oC
Q=16.5+69=85.5 kw 85.5 kw=291737.97 (BTU/hr)
Cp[oil]=1.93 J/g/oK 1.93kJ/kg/oC= 0.46097258(BTU/lb °F)
DT=70-20=50 50 oC 90 oF
m= 3189.6 KG/HR or 7031.94(lb/hr)
For conversion you may use:

http://www.unitconversion.org/unit_converter/specific-heat-capacity.html

calculator
in order to select a suitable heat exchanger you could be assisted by manufacturer.
For information see:

http://www.lytron.com/heatexchangers/ApplicationNotesDetail.aspx?id=2922

But from Google you could find more others.
Best Regards

 

[siva9]

Thankyou very much for the response.
Can you please tell me how to select the type of cooling for the power transformers?
what factor will decide the switching of cooling between fans,pumps and heat exchangers?

Thanks in advance.
Siva9

 

[7anoter4]

Hi siva9
Usually the transformer Manufacturer decides what to provide .For instance, if your transformer is 10/15 MVA rated type OFAF[Oil Forced Air Forced] that means may be that OFAN [Oil Forced Air Natural] will be for 10 MVA rated and OFAF[Oil Forced Air Forced] would be for 15 MVA. That means "OF" the pump will be activated always but the ventilation only for upper load. Sometime may in the contrary, depends upon Manufacturer design.
If, for some reason the Manufacturer must delete the radiators then a heat exchanger has to be provided instead. Usually inside location of a substation power transformer requires heat exchangers. Manufacturer may also upgrade the transformer and by adding a heat exchanger may lower the oil temperature and so increase the maximum load. But I think this is limited due to limit of maximum specific permitted load of the oil as cooling liquid.
The main goal of a good design is to obtain for a minimum price, the lowest losses [including electric losses in pumps, fans and heat exchangers].
Best Regards

 

[siva9]

Hi 7anoter4,

Thank you very much for the response. this helps alot.

Best regards,
siva9