Schweitzer Engineering SEL-387-0, Instruction Manual

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3.51
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Overcurrent Element
51Pn–Phase Inverse-Time Element
Figure 3.21 shows the logic for the 51P n element. The logic compares the
magnitudes of phase input currents IAW n , IBW n , and ICW n to pickup setting
51P n P. If one or more current magnitudes exceed the pickup level, a logical 1
asserts at one input to the AND gate at the center. The torque-control
SEL
OGIC
control equation 51P n TC determines the other AND input. If
51P n TC is true, Relay Word bit 51P n asserts and the inverse curve begins
timing.
Figure 3.21 51Pn Phase Inverse-Time O/C Element, Torque Controlled
Four settings define an inverse-time curve: the pickup setting, 51P n P, acts as a
horizontal scaling factor, because the curve formula uses current multiple of
pickup as an input; the curve setting, 51P
n C, defines the particular curve
equation, of which there are 10 (five U.S. and five IEC); the time-dial setting,
51P
n TD, defines the time dial, which scales the curve in a vertical direction to
vary the output timing for a given multiple of pickup; and the reset setting,
51P
n RS, defines whether the curve resets slowly like an electromechanical
disk or instantaneously when current drops below pickup. The phase inverse-
time curve looks at all three phase current magnitudes and times on the basis
of the greatest current of the three. It updates this maximum phase current
selection every quarter-cycle.
If the curve times out, Relay Word bit 51P n T asserts. When all phase currents
drop below pickup, with or without a curve time-out, 51P n deasserts and the
element resets according to setting 51P n RS. At the completion of the reset,
Relay Word bit 51P n R asserts. This bit normally will be at logic state 1, when
the element is at rest during normal system operation. Use the TAR command
via a serial port or the front panel to verify the state of this bit. You can use the
Level 2 serial port command
RES or the front-panel RESET51 function under
the
{OTHER}
pushbutton to force this bit to logical 1 during element testing.
This saves time if you have chosen electromechanical reset.
50Qn1 and 50Nn1–Sequence Current Definite-Time Element Logic
Figure 3.22 shows the logic for the definite-time 50Q n 1 negative-sequence
element and the definite-time 50N n 1 residual element.
+
–
51PnT
(curve timeout)
–
+
+
–
51Pn
(n = Winding 1, 2, 3, or 4)
|
IAWn
|
|
IBWn
|
|
ICWn
|
51PnP
51PnP (pickup)
51PnC (curve)
51PnTD (time dial)
51PnRS (E/M reset)
(1 cycle if RS = N,
curve if RS = Y)
Phase
Currents
Timing
51PnR (reset)
51PnTC (SEL
OGIC
control equation TRUE)
Relay
Word
Bits
Relay
Word
Bit