3. Introduction To Per Unit System
Formulas for per unit system
Uses Of Per Unit system
Advantages Of Per Unit System
Disadvantage Of Per Unit System
4. Parameters used in electrical circuit:-
Voltages KV,MV
Currents A,KA
Impedence OHM,K.OHMS
Power KW,KVA,MVA
5. In the per unit system,the voltages,currents,powers
Impedances , and other electrical quantities are
Expressed on a per –unit basis by the equations:
It is customary to select two base quantities to define
A given per-unit system. The ones usually selected are Voltage
and power.
Quantity per unit system=actual value/base value of quantity
6. HOW TO CHOOSE A BASE VALUE?
We choose only voltages and power as a base
Value and base current ,base impedance can b
Found as
Either base value given
When base value is not given
If base value is not given then largest value of the
Among the quantities is chosen as base.
7. FORMULAS OF PER UNIT SYSTEM
For each system parameter, per unit value is
to actual value divided by a base value:
Vpu =V/Vbase
Ipu=I/Ibase
Zpu=Z/Zbase
Select rated value as base value, usually rated
Power in MVA and rated phase voltage in KV:
Sbase=Srated=3VlineIline
Ebase=Ephase=Eline/SQR(3)
8. FOLLOWING FORMULLAS ARE USED:
BASE CURRENT=(BASE,KVA)(BASE KV)
BASE ’Z’=(BASE,KV)(BASE,I)
BASE ‘Z’=(BASE,KV)2 /(BASE KVA)*1000
BASE ‘Z’=BASE,KV)2 /(BASE,MVA)
BASE ‘Z’=(BASE,V)2 /(BASE,VA)
9. PER UNITIMPEDENCE RELATION
Per unit has relation with kva and V
Per unit impedence a base kva
Per unit impedence a 1/(base v)
10. CHANGE OF BASES.
IF A P.U VALUE IS GIVEN AND WE WANT TO FIND
P.U VALUE FOR NEW BASES THEN
P.U IMPEDENCE(new)=
(P.U Z (given))((BASE,KV(g)/(BASE,KV(n))2
((BASE,KVA(n))/(BASE,KVA(g)))2
11. USES
TO FIND EQUIVALENT IMPEDENCE OF BOTH
SIDES OF TRANSFORMER..
TO FIND EQUIVALENT CKT OFINDUCTION
MACHINE
12. ADVANTAGES OF PER UNIT SYSTEM
Circuit are simplified.
Same ranges of voltages(EHV to distribution and
utilization).
Per unit value are same for all machine regarding
their size (same p.u for both 10 MVA,1000 MVA)
P.U reactance for both primary and secondary side
remain same.
This method is idea for eliminated ideal transformer
in ckt.
Sqrt(3) factor in the three phase circuit calculation is
eliminated.
13. p.u value are same for both ‘Y’ and ‘ ‘ ‘ connection of
T/F.
Manufacturer specify values mostly in per unit.
P.U values are same for both single phase and three
phase.
Multiplication of two per unit is also per unit.
14. DISADVANTAGES OF PER UNIT
SYSTEM
For transmission lines ,value of impedence and
admittance are same regardless of level of voltages
(KVA,MVA).
Equivalent circuits of the components are modified
making them somewhat more abstract.sometimes
phase shifts that are clearly presented in the unscaled
circuit varnish in per unit system.
