PER UNT SYSTEM:-
IN PER UNT SYSTEM WE DISCUSS
INTRODUCTION TO PER UNIT SYSTEM
FORMULAS FOR PER UNT SYSTEM
USES OF PER UNT SYSTEMS
ADVANTAGES OF PER UNIT SYSTEMS
DISADVANTAGES OF PER UNT SYSTEM
PARAMETERS USED IN ELECTRICAL CIRCUIT:-
IMPEDENCE OHM, K.OHMS
PER UNIT SYSTEM
In the per-unit system, the voltages, currents,
powers, impedances, and other electrical
quantities are expressed on a per-unit basis by
Quantity per unit =
Base value of quantity
It is customary to select two base quantities to
define a given per-unit system. The ones usually
selected are voltage and power.
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.
HOW TO CHOOSE A BASE VALUE?
For each system parameter, per-unit value is equal to
the actual value divided by a base value:
Vpu = V / Vbase
Ipu = I / Ibase
Zpu = Z / Zbase
Select rated values as base values, usually rated power
in MVA and rated phase voltage in kV:
Sbase = Srated = 3VlineIline
Ebase = Ephase = Eline/ SQR(3)
FORMULAS OF PER UNIT SYSTEM
TO FIND EQUIVALENT IMPEDENCE OF BOTH SIDES OF
TO FIND EQUIVALENT CKT OF INDUCTION MACHINE
Circuits are simplified.
Same ranges of voltages (EHV to distribution and
Per unit values are same for all machine regarding their
size.(same p.u for both 100 MVA,1000MVA)
P.U reactance for both primary and secondary side remain
This method is ideal for eliminate ideal transformer in ckt
Sqrt (3) factor in the three phase circuit calculations is
P.u value remain same for both ‘Y’ and ‘∆’ connection of
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.
ADVANTAGES OF PER UNIT SYSTEM
For transmission lines, value of impedence and
admittance are same regardless of level of
Equivalent circuits of the components are
modified, making them somewhat more abstract.
Sometimes phase shifts that are clearly presented in
the unscaled circuits varnish in per-unit system
DISADVANTAGES OF PER UNIT