2–45
Application - 2
81 #3 PICKUP
________ Hz
81 #3 DELAY
________ Cycles
81 #4 PICKUP
________ Hz
81 #4 DELAY
________ Cycles
81 Frequency
When Dispersed Generation (DG) is suddenly
islanded, the frequency will quickly shift from 60.0 Hz
(except for the improbable case of an exact
generation and load match), making the
measurement of frequency an excellent means to
detect the island condition. If the only purpose is to
detect the island condition, the frequency relay 81U
and 81O can be set to operate at 59.5 Hz and 60.5
Hz, respectively (on a 60 Hz system), with a time
delay of about 10 cycles.
A second school of thought advocates that the DG
should definitely not be severed from the utility low
frequency operation while the frequency remains
as high as 59.5 Hz. This concept follows from the
premise that if the drop in frequency is due to a
major loss of system generation, it is at just this
time that all available DG should be kept on-line to
help prevent a complete system collapse. If this is
the objective, it may be useful to set one
underfrequency characteristic at 57.5 to 58.0 Hz
with a very short time delay, but allowing a higher
frequency, say 59.5 Hz, to be maintained for several
seconds.
61.0
60.8
60.6
60.4
60.2
60.0
59.8
59.6
59.4
59.2
59.0
Time (cycles)
81
Over Frequenc
y (Hz)
81
Under Frequenc
y (Hz)
Trip
Trip
Over Frequency
Time Delay #2
Over Frequency
Time Delay #1
Under Frequency
Time Delay #4
Under Frequency
Time Delay #3
Over Frequency
Magnitude #1
Under Frequency
Magnitude #4
Over Frequency
Magnitude #2
Under Frequency
Magnitude #3
Figure 2-33
Typical Settings of 81 Function
81 #1 PICKUP
________ Hz
81 #1 DELAY
________ Cycles
81 #2 PICKUP
________ Hz
81 #2 DELAY
________ cycles
If this function is enabled, the following settings are applicable:
Summary of Contents for M-3520
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