Effect of varying mechanical power input to an alternator

1. Electrical Machines-II
6th Semester, EE and EEE
By
Dr. Binod Kumar Sahu
Associate Professor, Electrical Engg.
Siksha ‘O’ Anusandhan, Deemed to be University,
Bhubaneswar, Odisha, India
Lecture-22

2. 2
Learning Outcomes: - (Previous Lecture_21)
 Synchronous generator (Alternator) on an infinite bus.
 Effect of change in varying excitation on an alternator connected to infinite
bus under loaded condition.

3. 3
Learning Outcomes: - (Today’s Lecture_22)
 Effect of varying field excitation: V-Curve and inverted V-curve of an
alternator.
 Effect of change in varying mechanical power input to an alternator
connected to an infinite bus under loaded condition.

4. 4
G1 G2 G3 Gn
…. Incoming
Alternator
Infinite Bus
XSXS XS XSXS
Infinite BusAlternator
XS
V = Constant
f = Constant
Ia
E
Field Excitation
of Alternator
Prime Mover-1
Te
Tm

5. 5
Effect of change in varying field excitation Phasor Diagram: -
VIa
jIa1X
s
E1EE2
Ia1
Ia2
jIaXs
jIa2
Xs
 1
1
2
2
1 1
2 2
a
a
a
I cos
I cos
I cos





0aI sin 
1 1aI sin
2 2aI sin
1 1
2 2
Esin
E sin
E sin






6. 6
The following important points may be noted for the phasor diagram:
 The alternator current is minimum at unity power factor and starts increasing, if the field
excitation is changed. This mode of operation is called normal excitation.
 If the alternator is operating at lagging power factor, increase in field excitation increases
the induced emf ‘E’, decreases the load angle ‘δ’, increases the armature current ‘Ia’ and
decreases the power factor ‘cosφ’ and vice-versa.
 When the alternator is operating at lagging power it is said to be over excited.
 If the alternator is operating at leading power factor, decrease in field excitation reduces
the induced emf ‘E’, increases the load angle ‘δ’, increases the armature current ‘Ia’ and
decreases the power factor ‘cosφ’ and vice-versa.
 Change in excitation changes the reactive component of current Iasinφ keeping its active
component Iacosφ constant.

7. 7
 So, change in field excitation changes the reactive power of the alternator keeping
its active power constant.
 When the alternator is operating at leading power it is said to be under excited.
,
,
If Ecos V Alternator is normallyexcited
If Ecos V Alternator isoverexcited
If Ecos V Alternator isunderexcited







8. 8
Plotted by varying the
excitation from 0.5 pu to
1.7 pu,
and taking V=1.0 pu,
Xs=0.3 pu

9. 9
Plotted by varying the
excitation from 0.5 pu
to 1.7 pu,
and taking V=1.0 pu,
Xs=0.3 pu

10. 10
Plotted by varying the
excitation from 0.5 pu
to 1.7 pu,
and taking V=1.0 pu,
Xs=0.3 pu

11. 11
Plotted by varying the
excitation from 0.5 pu
to 1.7 pu,
and taking V = 1.0 pu,
Xs= 0.3 pu
a
s
EV
P sin VI cos
X
  
Active Power
  a
s
V
Q Ecos V VI sin
X
   
Reactive Power

12. 12
 At point ‘x’, active power supplied by the alternator
is +ve, so it is operating at lagging reactive power
factor. The alternator is said to be over excited.
Complex power, S1 = P +j Q
 Now, if the excitation is decreased, the lagging
reactive power delivered by the alternator
decreases.
 At point ‘y’ no reactive power is delivered to the
infinite bus. The alternator is said to be operating at
unity power factor or normal excitation.
Complex power, S2 = P
 If the excitation is decreased further, at point ‘z’,
the alternator delivers leading reactive power i.e. Q
is -ve. So it will be operating at leading power
factor and said to be under excited.
Complex power, S3 = P - j Q1
P
Q
Q
+ve
-ve
x
y
z
S1
S2
S3
Lagging
pf
Unity pf
Leading
pf

13. 13
Thank you