1. Subject :ELECTRICAL POWER SYSTEM - I(2150908)
A.D.PATEL INSTITUTE OF TECHNOLOGY
Subject :ELECTRICAL POWER SYSTEM - I( )
Topic: POWER SYSTEM COMPONENTS
Prepared by:
GOHIL BHAVIN ( )
KHICHADA BHAVIN( )
PATEL KARAN ( )
Guided by:
CHOTANI SIR

2. Actual value in any units
Per Unit System
per-unit system is the expression of system quantities as fractions of a defined base unit quantity.
In the per-unit system, the voltages, currents, powers, impedances, and other electrical quantities are expressed on a per-unit basis by the equations:
Consider single phase system
Base volt-ampere = (VA)B VA
Base voltage = VB V
Then
Base current = (VA)B / VB A
Quantity per unit =
Actual value in any units
Base or reference value in same units

3. Base impedance ZB = VB / IB = (VB) 2 /(VA)B ohms
If the actual impedance is Z (ohms), its per unit value is given by
Zpu = Z / ZB = Z (ohms) × (VA)B / VB ohms
Consider three phase system
Three-phase base megavolt amperes = (MVA)B
Line-to-line base kilovolts = (kV)B
Assuming star connections (equivalent star can always be found).
Base current IB = 1,000 ×(MVA)B / √3(kV)B A
Base impedance ZB = 1,000 × (kV)B / √3 IB
= (kV)B 2 / (MVA)B
= 1,000 × (kV)B 2 / (kVA)B ohms
Per unit impedance
Zpu = Z (ohms) × (MVA)B / (kV)B ohms

4. ADVANTAGES OF PER UNIT SYSTEM
Circuits are simplified.
Per unit values are same for all machine regarding their size.(same pu for both 100 MVA,1000MVA)
P.U reactance for both primary and secondary side remain same.
This method is ideal for eliminate ideal transformer in circuit.
PU value remain 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.

5. Per unit representation of transformer
The ohmic values of impedance of a transformer depend on whether they are measured on the high or low tension side of transformer. In the per unit system, the per unit impedances of a transformer is the same regardless of whether it is determined from ohmic values referred to the high tension or low tension side of the transformer.
Z1=N12/N22 (Ohm) Z1=V12/V22 (Ohm) The base impedance on the two sides of the transformer are Z1b = KV12/MVAb Where KV1 is the 1st side base voltage. Z2b = KV22/MVAb Where KV2 is the 2nd side base voltage.

7. Per Unit Impendence Diagram Of a Power System
From a one-line diagram of a power system we can directly draw the impedance diagram by following the steps given below:
Choose an appropriate common MVA (or KVA) base for the system.
Consider the system to be divided into a number of section by the transformer. Choose an appropriate kV base in one of the sections. Calculate kV base of other sections in the ratio of transformation.
Calculate per unit value of voltage and impedance in each section and connect them up as per the topology of the one-line diagram. The result is the single-phase per unit impedance diagram.

8. Example: Obtain the per unit impedance (reactance) diagram of the power system of the following figure :
Solution: The per phase impedance diagram of the power system of figure has been drawn in figure 4.6. We shall make some further simplifying assumptions.
1. Line capacitance and resistance are neglected so that it is represented as a series reactance only.
2. We shall assume that the impedance diagram is meant for short circuit studies Current drawn by static loads A and B are therefore ignored

9. Thank You 