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Maintenance and Testing
Date Code 20011026
SEL-321/321-1 Instruction Manual
For general testing, use the instantaneous and time delayed elements that respond to any phase-
to-phase fault within a given zone such as M1P, M2P, etc. You can record more detailed and
specific test results using the instantaneous element associated with the faulted phase pair and
zone under test.
Phase Distance Element Supervisory Conditions
There are a number of supervisory conditions that must be fulfilled before the relay allows a
phase distance element to pick up. These supervisory conditions are described below.
Phase-to-Phase Nondirectional Overcurrent Element, 50PPn
Each phase-to-phase distance element is supervised by a nondirectional phase-to-phase
overcurrent element. For example, the relay cannot assert the Zone 3 A-B phase-to-phase
distance element if the vector difference between A-phase current and B-phase current is less
than the 50PP3 setting.
Negative-Sequence Directional Element
The phase distance protection function is supervised by the negative-sequence directional
element to provide improved directional security. Forward-reaching phase distance elements
may not assert unless the 32QF element is asserted. Reverse-reaching phase distance elements
may not assert unless the 32QR element is asserted. The relay operates properly for three-phase
faults where no negative-sequence current is present using alternative logic to detect three-phase
faults.
Loss-of-Potential Logic
If ELOP = Y, Y1, or Y2 the relay supervises the phase distance element logic using the Internal
Loss-of-Potential condition (ILOP). When ELOP = Y, Y1, or Y2 the relay disables the phase
distance elements when a loss-of-potential is detected, regardless of other signals applied to the
relay. If you test the phase distance elements using low voltage signals, set ELOP = N to
simplify the test.
Single-Pole Open Logic
In single-pole trip applications, the relay cannot properly polarize or operate the phase distance
elements associated with an open breaker pole. The relay Single-Pole Open logic disables the
two phase-to-phase distance elements associated with an open breaker pole. For instance, if Pole
A is open, the relay may not operate the A-B or C-A phase-to-phase distance elements. To
simplify distance element tests, apply control voltage to any inputs assigned to perform the 52
auxiliary functions, or disable the Single-Pole Open logic by setting ESPO = N.
Load-Encroachment Logic
The relay includes Load-Encroachment logic to help prevent the relay phase distance elements
from operating improperly under heavy load conditions. Forward and reverse load regions are
defined using impedance and angle settings. The relay calculates the positive-sequence
impedance. If the calculated impedance falls within the set load area, the phase distance
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