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Date Code 20011026
Applications
5-17
SEL-321/321-1 Instruction Manual
The echo logic includes timers for qualifying the permissive signal as well as timers for blocking
the echo logic for certain conditions. The Echo Block Time Delay (EBLKD) is used to block the
echo logic after dropout of local permissive elements. The recommended time setting for the
EBLKD timer is the remote faulted terminal breaker time, plus communications channel round
trip time, plus some safety margin. Assume a typical breaker operation time of three cycles and
communications channel round trip time of two cycles, and a safety margin of 5 cycles to give a
conservative EBLKD timer setting of 10 cycles.
An echo delay ensures that the reverse elements at the receiving end have sufficient time to
operate and block the echo repeat for faults behind that terminal. It also insulates the Echo and
Weak-Infeed logic (discussed later) against noise bursts which can occur on the communications
channel during close-in external faults. Typically, these noise bursts coincide with faults
external to the line section.
Since the pickup time of the reverse elements and duration of these noise bursts are very short,
the system requires a received permissive signal to be present for a short time before the permis-
sive signal can be echoed back to the remote terminal or used in the Echo Conversion To Trip
(ECTT) logic. The Echo Time Delay Pickup (ETDPU) timer specifies the time a permissive trip
signal must be present. The ETDPU setting depends upon your communications equipment, but
a typical setting for this timer is approximately 2 cycles.
The Echo Duration Time Delay (EDURD) limits the duration of the echoed permissive signal.
Once the echo signal is initiated, it should be maintained for a minimum period. The echo signal
should be cut off after some time, even if the permissive signal is continuously received. This
prevents permissive trip signal lockup between the two line ends. Assume a three-cycle breaker
at the remote terminal and half-cycle channel delay. The summation of remote breaker operation
time plus channel delay gives an EDURD setting of 4 cycles.
The EWFC setting enables or disables the weak-infeed feature of the relay. When the EWFC
setting is Y, the Echo Conversion To Trip (ECTT) logic is enabled. When the setting is N,
ECTT logic is disabled.
When weak-infeed tripping is used, typical single-phase undervoltage settings (27L) are 70–80%
of the lowest expected system operating voltage. The residual overvoltage setting should be set
to approximately twice the expected standing V
0
voltage. With the 59N element set at twice the
nominal standing V
0
voltage, the instrument measures only fault induced zero-sequence voltage.
These elements are set in the voltage element settings (EVOLT = Y).
In the sample application, the weak infeed feature is not required.
EPOTT = Y
Y
Y
Y
Z3RBD = 5.00
5.00
5.00
5.00
EBLKD = 10.00
10.00
10.00
10.00
ETDPU = 2.00
2.00
2.00
2.00
EDURD = 4.00
4.00
4.00
4.00
- Other Settings Affected
KEY, ECTT, and EKEY elements.
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