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Transformer Riddle No.31 - Enhancement of 20MVA to 25MVA
A 20 MVA transformer can be change to 25 MVA is possible without change in the winding?
Author : A.Selvakumar - From: India
 
#1
Mon, September 21st, 2009 - 19:48
It is possible, as you know the rating capacity of power transformers can be increased by their cooling system modification.
Tank-mounted pressed-steel radiators now represent the most widely used arrangement for cooling smaller transformers for which tank surface alone is not adequate. These can now be manufactured so cheaply and fitted so easily that they have totally replaced the arrangement of tubes which were commonly used for most distribution transformers. They are available in various patterns but all consist basically of a number of flat ‘passes’ of edge-welded plates connecting a top and bottom header. Oil flows into the top and out of the bottom of the radiators via the headers and is cooled as it flows downwards through the thin sheet-steel passes. The arrangement is most suited to transformers having natural oil and natural air circulations, i.e. ONAN cooling, as defined in BS 171.
For larger units it is be possible to suspend a fan below or on the side of the radiators to provide a forced draught, ONAF arrangement. This might enable the transformer rating to be increased by some 25%, but only at the extra cost and complexity of control gear and cabling for, say, two or four fans. Achievement of this modest up rating would require that the radiators be grouped in such as way as to obtain optimum coverage by the fans.
For your concerned subject, we can obtain 25 MVA output with improvement of transformer cooling systems from ONAN system to ONAF system (25 = 20*1.25) without any change in other parts.
 
Author : Hamid - From: Iran
 
#2
Wed, October 7th, 2009 - 15:32
If you do not change the windings and want to operate at 25 MVA ,
(a) The load losses will increase by 56 %

(b) The voltage drop will be more on the secondary side . This may ceate problem in ,load sharing if the transformer is operating in  parallel  with other transformers

( c ) The hot spot temperature of winding will go up if there is no enhanvement in the cooling system. The amount will go up depending upon the type of cooling employed .

( d ) The overlaoding of the transformer can  not abide  the IEC norms

(e ) As a side effect the transformer will be ageing faster than expected .
 
Author : R K Mohapatra - From: India
 
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