Parallel Operation of Governors
If Unit A had been operating alone carrying full load, the
system frequency would be 60 cycles as shown by Line
"A" at 60 cycles and 100% load. Now, if the frequency
of Unit B is adjusted by the engine speed control to be
equal to that of Unit A and the circuit breaker of Unit B is
closed, the system would be operating under the
following conditions:
1. Unit A is at 60 cycles and 100% load.
2. Unit B is at 60 cycles and 0% load. The
characteristic of the governor of Unit B at this time is
shown by the dotted line B.
In order for Unit B to carry load, it is necessary to
advance the speed setting of the governor. If it is
advanced to the full load position, the governor
characteristic of B will coincide with the characteristic of
A. Since the load on the system was 100% of one unit,
no change was made in total load, and the available
capacity is now 200%, the system will operate at 50%
load on each unit, and 60.9 cycles for two units of equal
capacity (Point E). For units of unequal capacity, the
load will be divided in proportion to the ratio of the
capacity of each to the total capacity. The system
frequency will be determined by the points on the
governor characteristics corresponding to these loads.
The frequency will be the same for both units since
paralleled alternators must operate at the same speed.
If Unit A had been a 60 KW unit fully loaded and a 100
KW Unit B was paralleled with it and the governor
adjusted to the full load position, the final load division
and frequency would be determined as follows:
a. System load - 60 KW
b. System capacity - 160 KW
System Load
KW rating for unit
c. The 60 KW unit will carry 60 x 60 KW = 22.5 KW
160
System capacity
KW load for unit
d. 100 KW unit will carry 60 x 100 KW = 37.5 KW
160
e. The system frequency can be determined readily
from step c or step d. The load carried by each
unit is 22.5 or 37.5 which figures out to be
60
100
37.% of the capacity of either unit Again, using Figure 1
for the governor characteristic of the 100 KW unit
and reading up from the value of load (37.5% to point F),
we find the system frequency to be 61.125 cycles.
Figure 2-Paralleling two units, one with a 3% governor
and the other with a hydraulic governor set for
isochronous operation.
Example
2-One
3%
Governor
and
one
0%
(Isochronous) Governor.
The characteristics of the 3% governor (Unit A) is shown
by line A of Figure 2 and the characteristics of the
isochronous governor (Unit B) is shown by line B. Only
at full load, 60 cycles, do the frequencies of the units
have the same value. It is customary to operate a
system of this type with a system load greater than the
capacity of Unit A. In this way Unit A carries its full load
at 60 cycles and the additional load and load swings are
handled by Unit B, also at 60 cycles. The system can
maintain constant frequency by this method of operation.
The system described in example 1 cannot maintain
constant frequency with load changes because of the
speed droop characteristics of the governors.
In the system described in this example, if the load is
less than the capacity of Unit A (which has the 3%
governor) and can be carried by Unit B, the governor
setting of Unit A can be reduced to give a governor
characteristic such as A, so Unit A will still carry the
steady part of the load and Unit B will carry the load
swings. If the system load is reduced to the point where
Unit A is not operating at the 60 cycle point of its
governor characteristic, Unit A will try to motor Unit B
and the system frequency may be greater than 60
cycles. The reason for using an isochronous governor in
a power system is to maintain constant frequency above
approximately 40% load.
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