shubham kumar(k12139) btech 5th sem
Post on 06-Apr-2017
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Career Point UniversityAlaniya(kota)
Submitted to:-Mr.Kamal AroraDeanCareer Point University
Sumitted by:-Shubham Kumar(K12139)B.tech 5th sem (Electrical)
Power TranSfOrmer
Per Unit System
Content:-
Introduction To Per Unit SystemFormulas for per unit systemUses Of Per Unit systemAdvantages Of Per Unit SystemDisadvantage Of Per Unit System
INtroduction
Parameters used in electrical circuit:-Voltages KV,MVCurrents A,KAImpedence OHM,K.OHMSPower KW,KVA,MVA
Per Unit SystemIn the per unit system,the voltages,currents,powersImpedances , and other electrical quantities are Expressed on a per –unit basis by the equations:
It is customary to select two base quantities to defineA given per-unit system. The ones usually selected are Voltage and power.
Quantity per unit system=actual value/base value of quantity
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 givenWhen base value is not given
If base value is not given then largest value of the Among the quantities is chosen as base.
FORMULAS OF PER UNIT SYSTEMFor each system parameter, per unit value is to actual value divided by a base value:Vpu =V/Vbase
Ipu=I/I base
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)
FORMULAS
FOLLOWING FORMULLAS ARE USED:BASE CURRENT=(BASE,KVA)(BASE KV)BASE ’Z’=(BASE,KV)(BASE,I)BASE ‘Z’=(BASE,KV)2 /(BASE KVA)*1000BASE ‘Z’=BASE,KV)2 /(BASE,MVA)BASE ‘Z’=(BASE,V)2 /(BASE,VA)
PER UNITIMPEDENCE RELATIONPer unit has relation with kva and V Per unit impedence a base kvaPer unit impedence a 1/(base v)
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
USESTO FIND EQUIVALENT IMPEDENCE OF
BOTH SIDES OF TRANSFORMER..TO FIND EQUIVALENT CKT OFINDUCTION
MACHINE
ADVANTAGES OF PER UNIT SYSTEMCircuit 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.
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.
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.
thank you
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