Which contains the information about Armature reaction and parallel operation of an alternator ( synchronous generator)

1. Contents:
 Armature Reaction
 Parallel operation of Alternators
 Conditions for parallel operation
 Procedure for parallel operation
 Methods of Synchronization
 References
Armature reaction and Parallel Operation of Alternators
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

2. Armature reaction
The effects of armature flux on the main flux affecting its value and the distribution is
called armature reaction
Unity Power Factor Load (R Load):
Two fluxes oppose each other on the left half of each pole while assist each other on the right half
of each pole. Such distorting effect of armature reaction under UPF condition of the load is called
cross magnetizing effect of armature reaction
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

3. Armature reaction
Zero lagging Power Factor Load (L Load):
The armature flux and the main flux are exactly in opposite direction to each other. Armature flux
tries to cancel the main flux. Such an effect of armature reaction is called demagnetizing effect
of armature reaction
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

4. Armature reaction
Zero leading Power Factor Load (C Load):
The armature flux and the main flux are in the same direction. They are helping each other. This
result into the addition in main flux. Such an effect of armature reaction due to which armature
flux assist field flux is called magnetizing effect of armature reaction
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

5. Armature reaction
Zero leading Power Factor Load (C Load):
The armature flux and the main flux are in the same direction. They are helping each other. This
result into the addition in main flux. Such an effect of armature reaction due to which armature
flux assist field flux is called magnetizing effect of armature reaction
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

6. Armature reaction
Zero leading Power Factor Load (C Load):
The armature flux and the main flux are in the same direction. They are helping each other. This
result into the addition in main flux. Such an effect of armature reaction due to which armature
flux assist field flux is called magnetizing effect of armature reaction
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

7. Parallel Operation of Alternators
Most of synchronous generators are operating in parallel with other synchronous
generators to supply power to the same power system. Obvious advantages of this
arrangement are:
1. Several generators can supply a bigger load;
2. A failure of a single generator does not result in a total power loss to the load
increasing reliability of the power system;
3. Individual generators may be removed from the power system for maintenance
without shutting down the load;
4. A single generator not operating at near full load might be quite inefficient.
While having several generators in parallel, it is possible to turn off some of
them when operating the rest at near full-load condition.
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

8. Conditions for Parallel Operation
A diagram shows that Generator 2
(Incoming generator) will be connected in
parallel when the switch S1 is closed.
However, closing the switch at an
arbitrary moment can severely damage
both generators!
If voltages are not exactly the same in both lines (i.e. in a and a’, b and b’ etc.), a very large
current will flow when the switch is closed. Therefore, to avoid this, voltages coming from
both generators must be exactly the same. Therefore, the following conditions must be met:
1. The RMS line voltages of the two generators must be equal.
2. The two generators must have the same phase sequence.
3. The phase angles of two a phases must be equal.
4. The frequency of the incoming generator must be slightly higher than the frequency of
the bus-bar (running) system.

9. Conditions for Parallel Operation
If the phase sequences are different, then
even if one pair of voltages (phases a) are
in phase, the other two pairs will be 1200
out of phase creating huge currents in
these phases.
If the frequencies of the generators are different, a large power transient may occur until
the generators stabilize at a common frequency. The frequencies of two machines must be
very close to each other but not exactly equal. If frequencies differ by a small amount, the
phase angles of the oncoming generator will change slowly with respect to the phase angles
of the running system.
If the angles between the voltages can be observed, it is possible to close the switch S1
when the machines are in phase.
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

10. General procedure for paralleling generators
When connecting the generator G2 to the running system, the following steps should be
taken:
1. Adjust the field current of the oncoming generator to make its terminal voltage equal to
the line voltage of the system (use a voltmeter).
2. Compare the phase sequences of the oncoming generator and the running system. This
can be done by different ways:
1) Connect a small induction motor to the terminals of the oncoming generator and
then to the terminals of the running system. If the motor rotates in the same
direction, the phase sequence is the same;
2) Connect three light bulbs across the open terminals of the switch. As the phase
changes between the two generators, light bulbs get brighter (large phase difference)
or dimmer (small phase difference). If all three bulbs get bright and dark together,
both generators have the same phase sequences.

11. General procedure for paralleling generators
If phase sequences are different, two of the conductors on the oncoming
generator must be reversed.
3. The frequency of the oncoming generator is adjusted to be slightly higher than
the system’s frequency.
4. Turn on the switch connecting G2 to the system when phase angles are equal.
The simplest way to determine the moment when two generators are in phase is by observing the
same three light bulbs. When all three lights go out, the voltage across them is zero and,
therefore, machines are in phase.
A more accurate way is to use a synchroscope – a meter measuring the difference in phase angles
between two a phases. However, a synchroscope does not check the phase sequence since it only
measures the phase difference in one phase.
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

12. Three Dark Lamps method
When all the three set of lamps
become dark, the
synchronizing switch can be
closed and thus the alternator
G2 gets synchronized with
alternator G1.
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

13. Two bright and one Dark Lamp method
To find incoming alternator
frequency is higher or lower then
the bus-bar frequency.
The sequence of flickering is L1,
L2 & L3  indicate fincoming >
fBB  N reduced & rate of
flickers brought down to small
The sequence of flickering is L1,
L3 & L2  indicate fincoming <
fBB  N increased.
After brining down the rate
flickering to as small las
possible, the synchronizing
switch can be closed at the
instant when L2 is dark an L1,
L3 are equally bright.

14. Synchro-scope method
The rate of rotation of the
pointer indicates the amount of
frequency difference between the
alternators. The direction of the
rotation indicates whether the
incoming frequency is higher or
lower then the existing
alternator.
The TPST switch is closed to
synchronize the incoming
alternator when the pointer faces
the top thick link marking.
Kongunadu College of Engineering & Technology Armature Reaction and Parallel operation

15. REFERENCES
S.No Book s / Web Sources
1
A.E. Fitzgerald, Charles Kingsley, Stephen. D. Umans, ‘Electric Machinery’, Tata Mc Graw Hill publishing
Company Ltd, 2003.
2 D.P. Kothari and I.J. Nagrath, ‘Electric Machines’, Tata McGraw Hill Publishing Company Ltd, 2002.
3 P.S. Bhimbhra, ‘Electrical Machinery’, Khanna Publishers, 2003.
4 M.N.Bandyopadhyay, Electrical Machines Theory and Practice, PHI Learning PVT LTD., New Delhi, 2009.
5 K. Murugesh Kumar, ‘Electric Machines’, Vikas Publishing House Pvt. Ltd, 2002.
6
Syed A. Nasar, Electric Machines and Power Systems: Volume I, Mcgraw -Hill College; International ed Edition,
January 1995.
7 J. Ganavadivel, ‘Electrical Machines II’, Anuradha publications, Fourth edition, 2015.
8 U.A.Bakshi &M.V.Bakshi, ”Electrical Machines II,” Technical Publications, Second revised edition, 2016.
9 Google and Wikipedia