 Home Page News Search Contact Us      Transformer Riddle No. 27 - Power transformer bank capacity
Why cannot we use whole of parallel power transformer bank capacity for power transfer in practice?

#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:

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.

2700 + 2315 + 1500 = 6515 KVA

#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.     