4-8 Loss-of-Potential, Load Encroachment, and Directional Element Logic Date Code 20020426
SEL-351-5, -6, -7 Instruction Manual
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-351 Relay
The load-encroachment logic and settings in the SEL-351 Relay 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-351 Relay.
Note that Application Guide AG93-10 discusses applying the load-encroachment feature to phase
distance elements in the SEL-321 Relay. The SEL-351 Relay does not have phase distance
elements, but the principles and settings example are still applicable to the SEL-351 Relay.
DIRECTIONAL CONTROL FOR NEUTRAL GROUND AND RESIDUAL GROUND
OVERCURRENT ELEMENTS
The directional control for overcurrent elements is enabled by making directional control enable
setting E32. Setting E32 and other directional control settings are described in the following
subsection Directional Control Settings.
Six directional elements are available to control the neutral ground and residual ground
overcurrent elements (not all available simultaneously). These six directional elements are:
• Negative-sequence voltage-polarized directional element
• Zero-sequence voltage-polarized directional element
• Channel IN current-polarized directional element
• Zero-sequence voltage-polarized directional element (low-impedance grounded system)
• Wattmetric and incremental conductance directional elements (Petersen Coil grounded
system)
• Zero-sequence voltage-polarized directional element (ungrounded/high-impedance
grounded system)
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