Effect of varying excitation on an alternator connected to infinite bus

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-20

2. 2
Learning Outcomes: - (Previous Lecture_19)
 Numerical on Effect of varying field excitation on two alternators running in
parallel:

3. 3
Learning Outcomes: - (Today’s Lecture_20)
 Synchronous generator (Alternator) on an infinite bus.
 Effect of change in varying excitation on an alternator connected to infinite
bus at no load.

4. 4
Alternator on an Infinite Bus: -
Infinite Bus: -
 A bus-bar that maintains constant voltage and constant frequency irrespective of the
load variation on it is called an infinite bus.
 A large number of alternators interconnected together to form a supply system
may be regarded as an infinite bus.
 So, an infinite bus is a voltage source having zero internal impedance and infinite
rotational inertia.
 Therefore, a synchronous machine switched on to or disconnected from the infinite
bus, can not change the voltage and frequency of the supply system.

5. 5
YB R
LOAD
G2
Prime Mover-2
G1
Prime Mover-1
Te
Tm
Tm
Te
1fI
2fI
Field Excitation
of Alternator-1
Field Excitation
of Alternator-2
Two Alternators running in parallel: -
 Increase in excitation of one
alternator, increases the terminal
voltage of the system. To maintain
constant voltage, excitation of the
other alternator has to be decreased.
 Increase in mechanical power input
to one alternator, increases the
system frequency. To regain the
nominal frequency, mechanical
input to the other alternator must be
decreased.

6. 6
Effect of varying excitation:-
a. No load operation: -
 Initially, assume that |E| = |V| and are in phase opposition in the local circuit
formed by interconnection of incoming alternator and infinite bus (load angle δ =
0 at no load).
 Expression for active and reactive power delivered/phase by the alternator are
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
( )
, , 0 0, ( )
, 0.
s s
s
EV V
P sin and Q Ecos V
X X
V
So At noload P and Q E V
X
and as E V Q
 

  
    
 

7. 7
 Voltage equation of the alternator is:
 As δ = 0, and E = V => Ia = 0. So, the active and
reactive power delivered by the alternator:
 So at no load, no power is delivered to or, received
from the infinite bus. Therefore the alternator is said to
be in floating condition.
 Now, if the excitation of alternator is increased,
induced emf of alternator ‘E’ becomes more than bus-
bar voltage ‘V’.
 So, active and reactive power becomes:
0 0a aP VI cos and Q VI sin    
a sE V j I X
  
 
( )
, , 0 0,
, ( )
s s
s
EV V
P sin and Q E V
X X
So At noload P
V
Wehaveincreased theexcitation so E V Q E V ve
X


  
  
     
V
E E V
f
r a sE I X
aI
E
V
E
V
r a sE I X
'f f

8. 8
 As ‘Q’ is positive, the alternator delivers lagging reactive power to the infinite bus.
 The resultant emf ‘Er = E – V = IaXs’ gives rise to armature current ‘Ia’, which lags ‘E’ by
900 and leads bus voltage ‘V’ by 900.
 This armature current produces demagnetizing effect to maintain constant air-gap flux (as
the alternator is connected to a constant voltage and constant frequency source its air-gap
flux is always constant).
 Under this condition,
0 0
(90 ) 0 (90 )a a aP VI cos and Q VI sin VI   

9. 9
 Now, if the excitation of alternator is decreased,
induced emf of alternator ‘E’ becomes less than bus-
bar voltage ‘V’.
 So the active and reactive power becomes:
 Decrease in excitation of alternator, reduces its
induced emf causing armature current ‘Ia’.
( )
, , 0 0,
, ( )
s s
s
EV V
P sin and Q E V
X X
So At noload P
V
Wehavedecreased theexcitation E V Q E V ve
X


  
  
     
aI
E
V
r a sE I X
'f
V
E
f

10. 10
 As ‘Q’ is negative, the alternator receives lagging reactive power or delivers
leading reactive power to the infinite bus.
 Now, in this case armature current ‘Ia’ leads induced emf (E) by 900 which
increases the air-gap flux, due to magnetizing effect of armature reaction. This
magnetizing effect again maintains a constant air-gap flux.
 Under this condition,
 So any change in field excitation, automatically sets in an armature current
‘Ia’ which helps in regaining the synchronism.
0 0
( 90 ) 0 ( 90 )a a aP VI cos and Q VI sin VI      

11. 11
Thank you