4.8
SEL-351A Relay Instruction Manual Date Code 20080213
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Load-Encroachment Logic (Not in SEL-351A-1)
Refer to Figure 3.14. To prevent phase time-overcurrent element 51PT from
operating for high load conditions, make the following SEL
OGIC
®
control
equation torque control setting:
51PTC = !ZLOAD * !LOP + 50P6 (= NOT[ZLOAD] * NOT[LOP] + 50P6)
As shown in Figure 4.2, load-encroachment logic is a positive-sequence
calculation. During LOP conditions (loss-of-potential; see Figure 4.1),
positive-sequence voltage (V
1
) can be substantially depressed in magnitude or
changed in angle. This change in V
1
can possibly cause ZLOAD to deassert
(= logical 0), erroneously indicating that a “fault condition” exists. Thus,
!ZLOAD should be supervised by !LOP in a torque control setting. This also
effectively happens in the directional element in Figure 4.21, where ZLOAD
and LOP are part of the logic.
In the above setting example, phase instantaneous overcurrent element 50P6 is
set above any maximum load current level—if 50P6 picks up, there is
assuredly a fault. For faults below the pickup level of 50P6, but above the
pickup of phase time-overcurrent element 51PT, the !ZLOAD * !LOP logic
discriminates between high load and fault current. If an LOP condition occurs
(LOP = logical 1), the pickup level of 50P6 becomes the effective pickup of
phase time-overcurrent element 51PT (51PT loses its sensitivity when an LOP
condition occurs):
51PTC = !ZLOAD * !LOP + 50P6 = !ZLOAD * NOT[LOP] + 50P6 =
!ZLOAD * NOT[logical 1] + 50P6 = 50P6
If phase time-overcurrent element 51PT is used in a directional application,
then this special torque control logic is not used and the corresponding torque
control setting is set directly to logical 1 (51PTC = 1), unless additional
control is desired.
Embedded logic handles load encroachment concerns for directional phase
overcurrent elements. Directional control for phase overcurrent elements
comes from Figure 4.24, which refers back to Figure 4.22, which in turn
refers back to Figure 4.20 and Figure 4.21. In Figure 4.21, notice that the
!ZLOAD * !LOP condition is effectively embedded in the positive-sequence
voltage-polarized directional element logic. The 32QE Relay Word bit input
into the negative-sequence voltage-polarized directional element logic
(Figure 4.20) also has LOP control embedded in it (see Figure 4.6). And both
these directional elements (Figure 4.20 and Figure 4.21) have overcurrent
fault detectors (50QF/50QR and 50P23P, respectively).
Use SEL-321 Relay
Application Guide for
the SEL-351A
The load-encroachment logic and settings in the SEL-351A are the same as
those in the SEL-321 Relay. Refer to Application Guide 93-10 (SEL-321
Relay Load-Encroachment Function Setting Guidelines) for applying the
load-encroachment logic in the SEL-351A. Note that Application Guide
AG93-10 discusses applying the load-encroachment feature to phase distance
elements in the SEL-321. The SEL-351A does not have phase distance
elements, but the principles and settings example are still applicable to the
SEL-351A.
Courtesy of NationalSwitchgear.com
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