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M-3410A Instruction Book
3–46
81 Over/Under Frequency
Generator Protection
: The Over/Under Frequency
function (81) provides overfrequency or
underfrequency protection of the generator. It has
four independent pickup and time delay settings.
The overfrequency mode is automatically selected
when the frequency setpoint is programmed higher
than the nominal frequency (50 or 60 Hz), and the
underfrequency mode selected when the setpoint
is programmed below the nominal frequency.
The prime mover is usually considered to be more
restrictive than the generator at reduced frequencies
because of possible natural mechanical resonance
in the many stages of the turbine blades. If the
generator speed is close to the natural frequency of
any of the blades, there will be an increase in
vibration. Cumulative damage due to this vibration
can lead to cracking of the blade structure.
Sample settings of the 81 Function are shown in
Figure 3-52, Example of Over/Under Frequency
(81) Trip Characteristics. The frequency functions
are automatically disabled when the input voltage
(positive sequence) is less than about 5 V.
These magnitude and time settings describe a
curve (as shown in Figure 3-52) which is to be
coordinated with the capability curves of the turbine
and generator as well as the system underfrequency
load-shedding program. These capabilities are given
by a description of areas of prohibited operation,
restricted time operation, and continuous allowable
operation.
The underfrequency function is usually connected
to trip the machine whereas the overfrequency
function is generally connected to an alarm.
In order to prevent mis-operation during switching
transients, the time delay should be set to greater
than five cycles.
Intertie Protection
: 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 delay
of about 10 cycles.
A second school of thought advocates that the DG
should definitely not be severed from the utility at
the slow side 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
59.0 Hz, to be maintained for several seconds.
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