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5-16
Applications
Date Code 20011026
SEL-321/321-1 Instruction Manual
Permissive Overreaching Scheme Logic
For our application, the tripping scheme is permissive overreaching transfer trip. Setting
EPOTT = Y enables the Permissive Overreaching Transfer Trip logic and settings. With the
permissive tripping logic selected, no settings are required for the Directional Comparison
Blocking logic.
The Zone 3 Reverse Block Delay (Z3RBD) timer extends the block signal issued by reverse
looking elements after a current reversal occurs and Zone 3 elements drop out. Zone 3 relay
elements do not encompass the origin. The recommended time setting for the Z3RBD timer is
the remote faulted terminal breaker time plus communications channel reset time, plus remote
Zone 2 relay reset time. Assume a typical breaker operation time of three cycles,
communications channel reset time of one cycle, and remote Zone 2 relay reset time of one cycle.
The sum of these times gives a conservative Z3RBD timer setting of 5 cycles.
The SEL-321 Relay includes logic which echoes the received permissive signal back to the
remote end after certain conditions are met. This is used in conjunction with the weak infeed
logic so that the relay at the strong terminal may trip even if the relay at the weak terminal does
not have any protective elements picked up.
In some applications, with all sources in, one terminal may not contribute enough fault current to
operate the protective elements. If the fault lies within the Zone 1 reach of the strong terminal,
the fault currents may redistribute after the strong terminal line breaker opens to permit sequen-
tial tripping of the weak-infeed terminal line breaker. If currents do not redistribute sufficiently
to operate the protective elements at the weak-infeed terminal, it is still desirable to open the
local breaker. This prevents the low level currents from maintaining the fault arc and allows
successful auto reclosure from the strong terminal. When the fault location is near the weak
terminal, the Zone 1 elements of the strong terminal do not pick up and the fault does not clear
rapidly. This is because the weak terminal protective elements do not operate. Note that while
the weak-infeed terminal contributes little fault current, the phase voltage(s) are depressed.
The SEL-321 Relay provides additional logic for weak-infeed terminals to permit rapid tripping
of both line terminals for internal faults near the weak terminal. The strong terminal is permitted
to trip via the permissive signal echoed back from the weak terminal. The weak terminal is
tripped by converting the echoed permissive signal to a trip signal after the following conditions
are met:
1. No reverse looking elements have picked up. For faults behind the relay location, the
reverse elements operate to inhibit the operation of the weak-infeed and echo logic.
2. LOP is deasserted and ELOP = Y1
3. At least one single-phase undervoltage or the residual overvoltage element operates.
4. The line terminal breaker is closed.
5. A permissive trip signal is received for ETDPU time period. The received PT signal is
initiated by the overreaching remote terminal Zone 2 elements.
After these four conditions are met, the weak-infeed logic sets the Echo Conversion To Trip
(ECTT) element. This element may be masked in the MTU logic mask to permit tripping the
local breaker under the conditions described above.
Содержание SEL-321
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