Schweitzer Engineering Laboratories SEL-351A - Page 90

3.18
SEL-351A Relay Instruction Manual Date Code 20080213
Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Time-Overcurrent Elements
Torque Control Switch Operation (51PT Element Example)
Torque Control Switch Closed
The pickup comparator in Figure 3.14 compares the pickup setting (51PP) to
the maximum phase current, I
P
, if the Torque Control Switch is closed. I
P
is
also routed to the curve timing/reset timing functions. The Relay Word bits
logic outputs operate as follows with the Torque Control Switch closed:
Torque Control Switch Open
If the Torque Control Switch in Figure 3.14 is open, maximum phase current,
I
P
, cannot get through to the pickup comparator (setting 51PP) and the curve
timing/reset timing functions. For example, suppose that the Torque Control
Switch is closed, I
P
is:
I
P
> pickup setting 51PP
and the phase time-overcurrent element is timing or is timed out on its curve.
If the Torque Control Switch is then opened, I
P
effectively appears as a
magnitude of zero (0) to the pickup comparator:
I
P
= 0 A (effective) < pickup setting 51PP
resulting in Relay Word bit 51P deasserting to logical 0. I
P
also effectively
appears as a magnitude of zero (0) to the curve timing/reset timing functions,
resulting in Relay Word bit 51PT also deasserting to logical 0. The phase
time-overcurrent element then starts to time to reset. Relay Word bit 51PR
asserts to logical 1 when the phase time-overcurrent element is fully reset.
Control of Logic Point TCP
Refer to Figure 3.14.
The Torque Control Switch is controlled by logic point TCP. Logic point TCP
is controlled by directional control (not available in SEL-351A-1) and
SEL
OGIC control equation torque control setting 51PTC.
If logic point TCP = logical 1, the Torque Control Switch is closed and
maximum phase current, I
P
, is routed to the pickup comparator (setting 51PP)
and the curve timing/reset timing functions.
51P = 1 (logical 1), if I
P
> pickup setting 51PP and the phase
time-overcurrent element is timing or is timed out on its
curve
= 0 (logical 0), if I
P
≤ pickup setting 51PP
51PT = 1 (logical 1), if I
P
> pickup setting 51PP and the phase
time-overcurrent element is timed out on its curve
= 0 (logical 0), if I
P
> pickup setting 51PP and the phase
time-overcurrent element is timing, but not yet timed out
on its curve
= 0 (logical 0), if I
P
≤ pickup setting 51PP
51PR = 1 (logical 1), if I
P
≤ pickup setting 51PP and the phase
time-overcurrent element is fully reset
= 0 (logical 0), if I
P
≤ pickup setting 51PP and the phase
time-overcurrent element is timing to reset (not yet fully
reset)
= 0 (logical 0), if I
P
> pickup setting 51PP and the phase
time-overcurrent element is timing or is timed out on its
curve
Courtesy of NationalSwitchgear.com