Sel 551 - Page 52

3.14
SEL-551 Relay Instruction Manual Date Code 20110408
Relay Elements and Logic
Time-Overcurrent Elements
Curve Timing/Reset Timing
In addition to SELOGIC control equations setting 51P1TC, phase time-
overcurrent element 51P1T curve timing/reset timing are subject to settings:
51P1P . pickup
51P1C . curve type
51P1TD . time dial
51P1RS electromechanical reset timing? (Y/N); see Time-Overcurrent
Element Setting Reference Information on page 4.2.
If reset timing setting 51P1RS = Y, element 51P1T reset timing emulates
electromechanical reset timing. If current I
P
goes above pickup setting 51P1P
(element 51P1T is timing or already timed out) and then current I
P
goes below
pickup setting 51P1P, element 51P1T starts to time to reset, emulating
electromechanical reset timing. Relay Word bit 51P1R (reset indication) =
logical 1 when element 51P1T is fully reset.
If reset timing setting 51P1RS = N, element 51P1T reset timing is a one-cycle
dropout. If current I
P
goes above pickup setting 51P1P (element 51P1T is
timing or already timed out) and then current I
P
goes below pickup setting
51P1P, there is a one-cycle delay before element 51P1T fully resets. Relay
Word bit 51P1R (reset indication) = logical 1 when element 51P1T is fully
reset.
Any time current I
P
goes above pickup setting 51P1P and element 51P1T
starts timing, Relay Word bit 51P1R (reset indication) = logical 0. If the curve
times out, Relay Word bit 51P1T (curve timeout indication) = logical 1.
Disable Time-Overcurrent Element With Pickup Setting
If pickup setting 51P1P is set 51P1P = OFF, phase time-overcurrent
element 51P1T is disabled all the time. Relay Word bits 51P1, 51P1T, and
51P1R all equal logical 0 at all times.
Applications for Time-Overcurrent Element Relay Word Bits
The second phase time-overcurrent element in Figure 3.9 (51P2T) and the
other time-overcurrent elements operate similarly.
Neutral Ground Time-
Overcurrent Element
One neutral ground time-overcurrent element (51N1T) is available (see
Figure 3.10). Its pickup setting (51N1P) is compared to the magnitude of the
neutral ground current (I
N
). This current is from separate neutral current input
channel IN (see Figure 1.2).
Relay
Word
Bit
Relay Word Bit
Definition
Application
51P1 pickup indication Primarily for testing or other SELOGIC control
equation applications. See Trip Logic on
page 3.17 (setting ULTR). See Reclosing Relay
on page 3.22 (setting 79BRS).
51P1T curve timeout indication Primarily for tripping or other SEL
OGIC control
equation applications. See Trip Logic (setting
TR).
51P1R reset indication Primarily for testing.