hey there.. been googling all night.. but can't find any formula for
voltage regulation for a load condition of half-load efficiency(power factor is given)..
can anybody share their ideas..
is it simply 1/2V.R?

 

You probably haven't provided sufficient information.

I would pose the problem in more concrete terms.

Perhaps your question might be something like ....

A 1kVA transformer has rated load power factor of 0.95. If the (purely resistive) load voltage at rated load is 230V rms what will the load voltage be when the load resistance value is increased so as to reduce the load power to half the full load power? Assume the primary source voltage is constant. Neglect copper and iron losses in the calculation.

Is this the sort of thing you have in mind?

 

A 10 kVA, 2200/220 V, 50 Hz single phase transformer is tested and the following results are obtained.​

​

Open circuit test (LV)​

Short circuit test (HV)​

Voc = 220 V​

Vsc = 150 V​

Ioc = 2.5 A​

Isc = 4.55 A​

Poc = 100 W​

Psc = 215 W​

Calculate:
(i) The transformer parameters and draw the equivalent circuit referred to the high voltage side.

answered​

(ii) The transformer parameters and draw the equivalent circuit referred to the low voltage side.

answered​

(iii) The approximate value of secondary voltage at full load of 0.85 lagging power factor. Draw the phaser diagram.

answered​

(iv) Determine the voltage regulation in percentage for a load condition of half-load efficiency of 0.6 power factor lagging. Draw the phaser diagram.
i don't know how to calculate V.R for half load efficiency..
should i calculate the half load efficiency first?

 

Have a look at this ....

http://books.google.com.au/books?id=paPKsOXn5FMC&pg=PA844&lpg=PA844&dq=Power+transformer+regulation+equation&source=bl&ots=LlIiQuBR7j&sig=VrbITlSPl9_NFAtQLzHklJ6yXbo&hl=en&ei=cgXIS_eDHM2HkQWnkLieCQ&sa=X&oi=book_result&ct=result&resnum=4&ved=0CBEQ6AEwAw#v=onepage&q=Power%20transformer%20regulation%20equation&f=false

 

And also this .....

http://nptel.iitm.ac.in/courses/IIT-MADRAS/Electrical_Machines_I/pdfs/1_9.pdf

 

(iv) Determine the voltage regulation in percentage for a load condition of half-load efficiency of 0.6 power factor lagging. Draw the phaser diagram.
i don't know how to calculate V.R for half load efficiency..
should i calculate the half load efficiency first?

Yes - I've decided I don't really know what the question is stating as the load condition. Does it mean a load of half the KVA rating with 0.6 power factor or an efficiency of 60% at an unspecified lagging power factor condition? Or perhaps something else ....? It's rather cryptic.