FREQUENCY-POWER CHARACTERISTICS
OF SYNCHRONOUS GENERATOR
James Watt’s steam governor
Watt’s fly ball governor
Watt’s fly ball governor
This photograph shows a
flyball governor used on a
steam engine in a cotton
factory near Manchester
in the United Kingdom.
Of course, Manchester was at
the centre of the industrial
revolution. Actually, this
cotton factory is still running
today.
Watt’s fly ball governor
This flyball governor is in the
same cotton factory in
Manchester.
However, this particular
governor was used to regulate
the speed of a water wheel
driven by the flow of the river.
The governor is quite large as
can be gauged by the outline
of the door frame behind the
governor.
Steam Engine Speed Control
Machine
Use
Machine
Operator
Controller
Deviation
Desired
Speed
Negative
(Balancing)
Feedback
Fly-Ball
Governor
Feedback Loop
Actual
Speed
Sensor
Steam
Engine
Steam
Supply
Actuator
• Governors. To provide an equitable and
coordinated system response to
load/generation imbalances, governor droop
shall be set at 5%.
• Governors shall not be operated with excessive
deadbands, and governors shall not be blocked
unless required by regulator mandates
• Assume that all generators on a power grid are
operating in the droop mode with the same 4 percent
speed regulation.
• Assume also that one of the generators is rated at 50
megawatts (call it Unit #1) and is synchronized on a
grid whose total generating capacity is 8000
megawatts.
• The speed governor for Unit #1 will take 50 ÷ 8000
or .625% of any load demand changes that should
occur.
• For example, assume that Unit #1 is currently
generating 37 MW.
• If the grid is operating at 60.00 Hz and an increase in
demand of 5 MW occurs, Unit #1 will increase its
power output by:
(.00625)(5) = .03125 MW. Unit #1 will then be
generating 37.03125 MW.
• The other generators, with their own 4 % droop
characteristic, will share proportionally the remainder of
the load change (that is, 5 MW minus.03125 = 4.96875).
• In the above example, something happens to grid
frequency as well.
• Assume that the frequency is 60.00 Hz when the
additional load of 5 MW came on the grid.
• In this example, the system frequency would droop the
following amount:
60.00 - [(.04) (60) (5) ÷ 8000] = 60.00 - .0015 = 59.9985 Hz
If the operator increases the setpoint on Unit #1 as the
other governor setpoints remain steady, the frequency
will return to 60.00 Hz and all of the new load of 5 MW
will be transferred to Unit #1.
Droop Setting Determines Response
• 5% Droop: 100% change in generator output for a 5%
change in Frequency or Speed.
– A 5% change in frequency, 60 Hz x 0.05 = 3 Hz or
for a 2 pole generator, 3600 rpm x 0.05 = 180 rpm.
• 4% Droop: 100% change in generator output for a 4%
change in Frequency or Speed.
– A 4% change in frequency, 60 Hz x 0.04 = 2.4 Hz or
for a 2 pole generator, 3600 rpm x 0.04 = 144 rpm.
• 4% Droop setting is more sensitive (responsive) than
the 5% Droop setting.
Example of Expected Response
• 150 MW unit at 5% Droop
– 150 / 3 Hz = 50.00 MW/Hz or in tenths of Hz, 5.00
MW/0.1 Hz
– Frequency change from 60.05 to 59.95 should
result in the generator increasing output 5.0 MW’s
• 150 MW unit at 4% Droop
– 150 / 2.4 Hz = 62.50 MW/Hz or in tenths of Hz, 6.25
MW/0.1 Hz
– Frequency change from 60.05 to 59.95 should
result in the generator increasing output 6.25 MW’s
Expected Governor Response
• 5 % Droop:
– Unit Net Capability/30 = MW/0.10 Hz
• 4 % Droop:
– Unit Net Capability/24 = MW/0.10 Hz
Droop Mode Versus Isochronous Mode
• Droop Mode: It is the mode of operation in which the
machine will react to the load variation by changing
its speed. It is utilized when multiple machine run in
parallel so that load is shared between the machines.
For this sharing to be equal the machine need to
have same droop characteristic. Typically if a
machine has droop of 4% it means for a change of
1% in rated speed the machine takes 25% of its rated
load. This is also referred to as load control mode of
operation.
• Isochronous Mode: In this mode the machine is not
affected by load and regardless of load it will
maintain the frequency. This mode is also referred to
as frequency control mode of operation. In case of
systems not connected to grid it is required to run at
least one machine in this mode so as to take care of
the load variation.