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4.53
Date Code 20081022
Instruction Manual
SEL-787 Relay
Protection and Logic Functions
Voltage-Based Protection
Volts Per Hertz
Elements
Overexcitation occurs when a transformer magnetic core becomes saturated.
When this happens, stray flux is induced in nonlaminated components,
causing overheating. In the SEL-787 Relay a volts/hertz element detects
overexcitation. The SEL-787 provides a sensitive definite time volts/hertz
element, plus a tripping element with a composite operating time. The relay
calculates the present transformer volts/hertz as a percent of nominal, based
on the present and nominal voltages and frequencies. The settings VNOM and
FNOM define the nominal transformer voltage and frequency, respectively.
shows logic diagram of the volts/hertz elements. If the torque-
control 24TC SEL
OGIC
control equation is true and the present volts/hertz
exceed the 24D1P setting, the relay asserts the 24D1 Relay Word bit and starts
the 24D1D timer. If the condition remains for 24D1D seconds, the relay
asserts the 24D1T Relay Word bit. Typically, you should apply this element as
an overexcitation alarm.
Figure 4.31
Overvoltage Element Logic
For volts/hertz tripping the relay provides a time-integrating element with a
settable operating characteristic. You can set the element to operate as an
inverse-time element; a user-defined curve element (using the SEL-5806 PC
Software); a composite element with an inverse-time characteristic and a
definite-time characteristic; or as a dual-level, definite-time element. In any
case, the element provides a linear reset characteristic with a settable reset
time. This element also is supervised by the 24TC torque-control setting.
The volts/hertz tripping element has a percent-travel operating characteristic
similar to that employed by an induction-disk time-overcurrent element. This
characteristic coincides well with the heating effect that overexcitation has on
transformer components.
The element compares the three phase voltages and uses the highest of the
values for the volts/hertz magnitude calculations. The relay asserts 24C2
Relay Word bit without time delay when the transformer volts/hertz exceeds
the element pickup setting, and asserts 24C2T Relay Word bit after a delay
determined by the characteristic setting. The relay tracks the frequency over
the range 20 to 70 Hz.
+
-
24D1D
0
Settings
24D2P1
24D1
24D1T
24C2
24C2T
24CR
(V/Hz)
V/Hz
24D2P1
24D2P2
24IP
24D2D2
24CCS=DD
24IP
24D2P2
V/Hz
24CCS=ID
Relay
Word
Bits
24TC
24D2D2
24CCS
V/Hz
24CCS=U
24D2D1
24TC
24D1P
24CCS=I
24D2D1
24IP
24IC
24ITD
24D2P2
24D2D2
SEL
OGIC
Setting
24CR
t
t
t
Switch closed
when 24TC=1
Voltx/Hertz Element
Curve Timing and
Reset Timing
Summary of Contents for SEL-787
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