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Transformer Riddle No. 27 - Power transformer bank capacity
Why cannot we use whole of parallel power transformer bank capacity for power transfer in practice?
Author : Alvina - From: Armenia
 
#1
Thu, July 30th, 2009 - 16:37
For determination of distributed load on many parallel transformers with different rating power and impedance voltage, we can use following equations:

S = Total  load
S1 = Rated  KVA of TR.1
S2 = Rated  KVA of TR.2
S3 = Rated  KVA of TR.3

Uk=Resultant impedance of power transformers bank
Uk1 = Impedance voltage of TR.1
Uk2 = Impedance voltage of TR.2
Uk3 = Impedance voltage of TR.3

S/Uk = S1/Uk1 + S2/Uk2 + S3/Uk3

The power assumed by the individual transformers is :

Sa1 = S1. Uk/Uk1
Sa2 = S1. Uk/Uk2
Sa3 = S1. Uk/Uk3

For example:

S1 = 3500 KVA , Uk1= %6
S2 = 2500 KVA , Uk2= %5
S3 = 1500 KVA , Uk1= %4
S = 6750 KVA

We can write:

Uk= 6750 /( 3500/6 + 2500/5 +1500/4) = % 4.63

Sa1 = 3500 x 4.63/6 = 2700 KVA
Sa2 = 2500 x 4.63/5 = 2315 KVA
Sa3 = 1500 x 4.63/4 = 1736 KVA > 1500 KVA (The individual transformer should not be subjected to overload)

Transformer No.3 is thus overloaded, therefore we cannot transfer whole of actual load with three mentioned transformers.

Available transfer load is:

2700 + 2315 + 1500 = 6515 KVA

 
Author : Hamid - From: Iran
 
#2
Fri, July 31st, 2009 - 16:17

Impedence implies regulations . Different impedence means different regulations i.e different output terminal voltages for same input voltage . When  the 3 trfr.s with differnt impedence i.e  different terminal volatges ,  there will be a circulating current bett the trfr.s that will reduce the outward power flow . If X MVA of power is fed to the individual trfr.s and circulating current owind to variation of terminal voltage calls for y MVA of power , then the total outward power flow will be X-Y MVA . Tht is why normally it is not advisable to put trfr.s with different % impedence in parallel . Morever , the circulating current is likely to increase the thermal loading and load loss of the trfr.s  forming the loop through which the circulating current flows .
In practice , there is always some  difference in impedence of the transformers in parallel causing difference in power flow at input and output.  
Author : R K Mohapatra - From: India
 
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