BECKWITH ELECTRIC M-3425A, Инструкция

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4–50
M‑3425A Instruction Book
24 Overexcitation Volts/Hz
The Volts-Per-Hertz function (24) provides
overexcitation protection for the generator and unit-
connected transformers. This function incorporates
two definite time elements which can be used to
realize traditional two-step overexcitation protection.
In addition, the relay includes an inverse time
element that provides superior protection by
closely approximating the combined generator/unit
transformer overexcitation curve. Industry standard
inverse time curves may be selected along with
a linear reset rate which may be programmed to
match specific machine cooling characteristics. The
percent pickup is based on the Nominal Voltage
setting and the nominal frequency. The V/Hz function
provides reliable measurements of V/Hz up to 200%
for a frequency range of 2–80 Hz. The ranges and
increments are presented in Figure 4-32.
Setting this relay function involves determining
the desired protection levels and operating times.
The first step is to plot the combined generator
and associated unit transformer overexcitation
capability limits. This data is typically available
from the manufacturer and should be plotted on
the same voltage base. Depending on the resulting
characteristic, one of the four families of inverse
time curves (as shown in Appendix D,
Inverse Time
Curves
) can be matched to provide the protection.
The two definite time elements can be used to
further shape the protection curve or provide an
alarm.
Figure 4-31 illustrates a composite graph of
generator and transformer limits, a chosen inverse
time curve and pickup, and a definite time pickup
and delay.
24DT #1 PICKUP
%
24DT #1 DELAY
Cycles
24DT #2 PICKUP
%
24DT #2 DELAY
Cycles
24IT PICKUP
%
24IT CURVE
crv#1 crv#2 crv#3 crv#4
24IT TIME DIAL
24IT RESET RATE
Seconds
Definite time setpoint #1 establishes the V/Hz level above
which the protection operating time will be fixed at the
definite time delay #1.
Delay time #1 establishes the operation time of the
protection for all V/Hz values above the level set by definite
time setpoint #1.
Definite time setpoint #2 could be programmed to alarm,
alerting the operator to take proper control action to
possibly avoid tripping.
Time to operation at any V/Hz value exceeding Definite
time setting #2.
The pickup value is the V/Hz value at which the chosen
inverse curve begins protective operation. Typical value is
105%.
Allows the user to designate the appropriate curve family
for this protection application. These curves are shown in
Appendix D,
Inverse Time Curves
.
The appropriate curve in the family is designated by the
associated “K” value of the curve.
The value entered here should be the time needed for the
unit to cool to normal operating temperature if the V/Hz
excursion time was just under the trip time.