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5.48
SEL-421 Relay
Instruction Manual
Date Code 20171021
Protection Functions
Series-Compensation Line Logic
SIR is defined as follows:
Use the Zone 1 distance element reach (Z1MP, Z1MG, XP1, or XG1) because the
CVT transient detection logic only supervises Zone 1 distance protection.
Series-Compensation Line Logic
NOTE:
The SEL-421-4 does not
provide series-compensated line
protection logic.
The SEL-421 includes logic to detect when a fault is beyond a series capacitor (a
series capacitor can possibly cause Zone 1 overreach). The relay blocks the
Zone 1 elements until the series-compensation logic determines that the fault is
between the relay and the series capacitor (i.e., the fault is on the protected line
section).
The value that you enter for setting XC depends on the position of the series-
compensation capacitor(s) relative to the relay potential transformers. Capacitors
can be on either end of a line, in the middle of a line, or at both ends of a line.
Capacitors that are external to a protected line section can have an effect if infeed
conditions are present.
In applications where there is a series capacitor on an adjacent line, for any
SEL-421 Relays on non-compensated lines, set ESERCMP := Y and XC := OFF.
This allows the Zone 1 element to be set to the desired sensitivity, yet still be
secure during the voltage reversal that will occur when a neighboring compen-
sated line experiences a fault.
For more information on setting the relay for series-compensated lines see SEL
Application Guide AG2000-11,
Applying the SEL-321 Relay on Series-Compen-
sated Systems
.
Table 5.42
CVT Transient Detection Logic Relay Word Bit
Name
Description
CVTBL
CVT transient blocking active
Figure 5.29
CVT Transient Detection Logic
where:
Z
1S
= positive-sequence source impedance
Z
R
= distance element reach
0
1.5
CYC
Distance Calculation
Smoothness
Relay
Word
Bits
3PO
CVTBL
SOTFE
ECVT := Y
SIR > 5 During
Zone 1 Fault
Setting
SIR
Z
1S
Z
R
-------
=
Summary of Contents for SEL-421-4
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