Schweitzer Engineering Laboratories SEL-311C - Residual Ground Time-Overcurrent Element

3.36
SEL-311C Relay Instruction Manual Date Code 20060320
Distance, Out-of-Step, Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Time-Overcurrent Elements
The Torque Control Switch closes and phase time-overcurrent element
51PT is enabled and nondirectional.
51PTC = M2P
The 51P/51PT uses the Zone 2 mho phase distance element to provide
forward directional control.
Other SEL
OGIC control equation torque control settings may be set to provide
directional control. See Overcurrent Directional Control Provided by Torque
Control Settings on page 4.28.
Reset Timing Details (51PT Element Example)
Refer to Figure 3.24.
Any time current I
ABC
goes above pickup setting 51PP and the phase time-
overcurrent element starts timing, Relay Word bit 51PR (reset indication) =
logical 0. If the phase time-overcurrent element times out on its curve, Relay
Word bit 51PT (curve time-out indication) = logical 1.
Setting 51PRS = Y
If electromechanical reset timing setting 51PRS = Y, the phase time-
overcurrent element reset timing emulates electromechanical reset timing. If
maximum phase current, I
ABC
, goes above pickup setting 51PP (element is
timing or already timed out) and then current I
ABC
goes below 51PP, the
element starts to time to reset, emulating electromechanical reset timing.
Relay Word bit 51PR (resetting indication) = logical 1 when the element is
fully reset. See Time-Overcurrent Curves on page 9.5 for reset curve
equations.
Setting 51PRS = N
If reset timing setting 51PRS = N, element 51PT reset timing is a 1-cycle
dropout. If current I
ABC
goes above pickup setting 51PP (element is timing or
already timed out) and then current I
ABC
goes below pickup setting 51PP,
there is a 1-cycle delay before the element fully resets. Relay Word bit 51PR
(reset indication) = logical 1 when the element is fully reset.
Residual Ground
Time-Overcurrent
Element
To understand the operation of Figure 3.25, follow the explanation given for
Figure 3.24, substituting residual ground current I
G
(I
G
= 3I
0
= I
A
+ I
B
+ I
C
)
for maximum phase current I
ABC
and substituting like settings and Relay
Word bits.