Short Notes: Per Unit System | Short Notes for Electrical Engineering - Electrical Engineering (EE) PDF Download

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                In pu system, energy quantity is expressed as a ratio of some based value. 
                          
Absolute value or Actual value
pu value=
Base value
 
                           Percentage value = pu value x 100% 
Base value 
 1 - ? System 
? 
base base base base
S , V , I , Z 
? Out of these, 2 value must be known, to convert entire system into pu system. 
? 
base base base
S V I ?? 
2
base base base
base base
base base base
S V V
I ,    Z
V I S
? ? ?         
? Usually, we assume 
base
S & 
base
V as known. 
 
3 - ? System 
? 
base
S & 
base
V are assumed 
? 
base
base
base
S
I
3V
?
?
 
base
V = line to line voltage 
base
S = 3 – phase power 
? For start connection 
? ?
? ?
2
base base base
base
base base base
V ph V / 3 V
Z
I ph I S
? ? ? 
? For delta connection 
? ?
? ?
2
base base base
base
base base base
V ph V 3V
Z
I ph S I / 3
? ? ? 
 
In per unit system, equivalent impedance of transformer referred to primary or 
secondary in same.