Electrical Power Systems

Power System Components and Per Unit System 101
The impedance of generators, transformers and motors supplied by the manufacturer are
generally given in per-unit values on their own ratings. or power system analysis, all impedances
must be expressed in per-unit values on a common base.
When base quantities are changed from (MVA) B, old to (MVA) B, new and from (KV) B, old to
(KV) B, new, the new per-unit impedance can be given by
Zpu, new = Zpu, old (MVA)
(MVA)
B, new
B, old
( KV)
×
2
( KV)
2
B,old
B, new
5.4 PER-UNIT REPRESENTATION O TRANSORMER
It has been stated in section–5.2, that a threephase
transformer can be represented by a
single-phase transformer for obtaining per phase
solution of the system. igure 5.8 shows a singlephase
transformer in terms of primary and
secondary leakage reactances Z p and Z s and
transformation ratio 1 : a.
...(5.16)
Let us choose voltage base on the primary side V pB and on the secondary side V SB. Also
choose a common volt-ampere base of (VA) B.
Now
V
V
pB
SB
= 1
a
...(5.17)
As the (VA) B is common, we can also write
IpB
= a ...(5.18)
I
SB
ZpB = VpB
I
pB
ZSB = VSB
ISB
...(5.19)
...(5.20)
rom ig. 5.8, we can write,
VS = ES – ZS IS ...(5.21)
Ep = Vp – ZpIp ...(5.22)
Also Es = a.Ep ...(5.23)
Substituting Es from eqn. (5.23) into eqn. (5.24), we obtain
VS = aEp – ZSIS Substituting Ep from eqn. (5.22) into eqn. (5.24), we get,
VS = a(Vp – ZpIp) – ZSIS Eqn. (5.25) can be converted in per-unit form, i.e.,
ig. 5.8: Representation of single phase
transformer (magnetizing impedance neglected).
...(5.24)
...(5.25)