Schweitzer Engineering Laboratories SEL-351 - Torque Control Switch Closed; Torque Control Switch Open; Torque Control Switch Operation (51 PT Element Example)

Date Code 20001006 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements 3-19
SEL-351 Instruction Manual
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:
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
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.
Courtesy of NationalSwitchgear.com