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

Questions?