1. Parallel Operation of
Synchronous Generators
NOORNABI SHAIKH
DEPARTMENT OF ELECTRICAL ENGINEERING
MUET

2. N.N.SHAIKH 2
An isolated synchronous generator supplying its own load
independently from other generators is very rare. This happens only in
emergency generators. [standby generators]
For all usual generator applications, there is more than one generator
operating in parallel to supply the power demand by the loads.
Parallel operation of ac generators
There are several major advantages of generators operating in parallel
 Several generators can supply a bigger load than one machine
by itself.
 Increased reliability of power system, since failure of any one of
them does not cause a total power loss to the load.
 For preventive maintenance, one or more of them can be
removed form the system.
 More efficient then single generator operating alone.

3. N.N.SHAIKH 3
Conditions Required For Paralleling
Above figure shows a generator G1 supplying power to a load, with
another generator G2 about to paralleled with G1 by closing the switch
S1.
 If switch S1 is closed randomly at some moment, the generators are
liable to be severely damaged, and the load may lose power.
 If voltages are not exactly the same in each conductor being tied
together, there will be a very large current flow when the switch is
closed.

4. N.N.SHAIKH 4
To avoid problems, following paralleling 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 the two a phases must be equal.
4. The frequency of the new generator, called oncoming
generator, must be slightly higher than the frequency of
the running system.

5. N.N.SHAIKH 5
Condition 1
obvious – in order for two sets of voltages to be identical, the voltage in
phases a & a’ will be completely identical. If not then very large current
will flow when the switch is closed???
Condition 2
This ensures that the sequence in which the phase voltages peak in the
2 – generators is same. If phase sequence is different as shown then
even one pair voltages are in phase, the other two pairs of voltages are
120o
out of phase.
If generators are connected in this
manner, there would be no
problem with phase a but huge
current would flow in phase b & c,
damaging both machines.

6. N.N.SHAIKH 6
Condition 4
If frequencies are not very nearly equal then, large power transients
will occur until generators stabilize at a common frequency. The
frequencies of the 2 – machines must be very nearly equal, but not
exactly equal. [???]

7. N.N.SHAIKH 7
General Procedure for Paralleling Generators
Suppose G2 is to be connected to the running
system shown in figure. The following steps should
be taken to accomplish the paralleling.
STEP – 1
Using voltmeters & ammeters, the IF of the oncoming generator should
be adjusted until its terminal voltage is equal to the line voltage of the
running system.
STEP – 2
Phase sequence of the oncoming generator must be compared to the
phase sequence of the running system. It can be checked by:
a. Phase sequence meter. It is a small 3 – phase induction motor. First
connect it with the running system and check its direction of rotation.
Then connect it with the oncoming generator and check the direction. If
direction of rotation of two separate systems is equal, then the phase
sequence is same. If not then interchange the 2 – phases of oncoming
generator to make same the phase sequence same.
b. Three-lamp method [HW].

8. N.N.SHAIKH 8
STEP – 3
Frequency of the oncoming generator is adjusted to be slightly higher
than the frequency of the running system. This is done by watching a
frequency meter until the frequencies are close and then by observing
changes in phase between the systems. [why oncoming generator’s
frequency should be slightly higher than running system???]
STEP – 4
Phase difference must be same. A Synchroscope is a meter that
measures the difference in phase angle between the a phases of the
two systems.