Choice of a Transformer For a Generator

Transformer - 2: 35 MVA, 11/132 kV

Yes ,

Transformer -2 Looks more appropriate but any technical justification for Transformer - 2 (Like in transformer-1 may have probability of heating etc)

Transformer #1.

I have absolutely no technical basis for this choice, but there's a 50-50 chance that I will be right.

What do the suppliers say? They probably have some real engineers on staff who may be able to make informed decisions.

You could always flip a coin.

One supplier suggest Transformer-1 while other suggest Transformer-2. Seeking for best Choice ...........Some pros and cons about both choices

Seek a third opinion from a real engineer who can understand what is required of the transformer.

You have provided no information about what it is supposed to do.

if the price is the same and other specifications are the same then transformer 2 is the best option. @ least you have reserve capacity.

Assuming you'll run at 25/31.2 MW/MVA all the time, consider these aspects:

-the efficiency of a transformer starts to drop when the load increases over about 75%. In the end this is wasted energy.

-the external fault current can be another aspect to look for when choosing the transformer. There is one thing to have a high current spike with transformer loaded close to 100% and another one with 75-80% load. (a couple of kA is a lot of current even for just 0.5 seconds and your transformer will have to handle it)

From your generators capacity (25/31.2) you have only one choise which is option -1 (31.5 MVA)

How and Why we have only one option of choosing only 31.5 MVA transformer

What answer did you give and why?

In reality you cannot choose based upon the limited information that you been have provided. You need to know such things as cost, impedance, if the transformer is rated for generator duty (short circuit withstand capability), whether there are taps, winding configuration, where the transformer is in the system (isolated, end of line, grid, etc.), the expected operating point of the generator (generators are rarely operated exactly at their nameplate), etc., etc. The interviewer was trying to determine how much experience you have in this field.

Dear Mr.Salar,

The tendency is to decide on the Higher Capacity of the Transformer. But several Factors are to be taken in to consideration - like No-Load Losses,

I^2 R losses in the windings at full load, over-all efficiency, Life for Insulation, i.e., Estimated Life of Insulation as per "ARRHENIUS LAW" is to be evaluated.

The "ARRHENIUS LAW" is given as V = 10^(a+b)/T,

where V is the Estimated Life Expectancy for the Insulation, "T" is the HOT SPOT Temp. in Deg. Kelvin, in the winding, "a" and "b" are constants for the particular Transformer depends upon he Design.

Further RATING FACTOR for the Transformer is also involved, which varies from 1.45 to 2.18 and it is described as follows,

1. Rating Factor 1.45 termed as TRANSFORMER CRITICALLY LOADED.

2. Rating Factor 1.62 to 1.78 termed as TRANSFORMER OVER LOADED.

3. Rating Factor 1.82 termed as TRANSFORMER NORMALLY LOADED.

4. Rating Factor 2.18 termed as TRANSFORMER UNDER-LOADED.

For further details see here

http://www.google.co.in/url?sa=t&rct=j&q=*selection%20criteria%20for%20power%20transformer*&source=web&cd=7&cad=rja&ved=0CFYQFjAG&url=http%3A%2F%2Fstoa.usp.br%2Fjajardini%2Ffiles%2F1724%2F10388%2FCONG1997107.pdf&ei=WE4aUbe0J4fVrQe5-IDIAQ&usg=AFQjCNGLNF4PchFgu-aeAwIJGfBTEJYc8g&bvm=bv.42261806,d.bmk

DHAYANANDHAN.S

I for got to add, in my Thread No.11, here is, the Estimated Life for the Transformer for different RATING FACTORS are as follows:

For Sl.No. 1, Estimated life of the Transformer is 3 years

For Sl.No. 2, Estimated life of the Transformer is 15 years

For Sl.No. 3, Estimated life of the Transformer is 30 years

For Sl.No. 4, Estimated life of the Transformer is more than 30 years

DHAYANANDHAN.S