6.13
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
230 kV Overhead Distribution Line Example
Trip Equations
Set these two SELOGIC control equations for tripping:
➤ TR (unconditional)
➤ TRSOTF (SOTF)
TR
The TR SELOGIC control equation determines which protection elements cause
the relay to trip unconditionally. You typically set all direct tripping and time-
delayed protection elements in the SEL
OGIC control equation TR. Direct tripping
and time-delayed protection elements include step distance protection elements,
plus instantaneous and time-overcurrent protection elements.
Set TR equal to Zone 1 instantaneous protection (Z1T), time-delayed Zone 2 dis-
tance protection, and the inverse-time overcurrent element (51S1T). For informa-
tion on setting 51S1T, see Selectable Operating Quantity Time Overcurrent
Element 1 on page 6.10.
TR := Z1T OR Z2T OR 51S1T. Trip (SELOGIC Equation)
TRSOTF
The TRSOTF SELOGIC control equation defines which protection elements cause
the relay to trip when the SOTF scheme is active. Assertion of these protection
elements during the SOTFD time causes the relay to trip instantaneously (see
SOTF Scheme on page 6.8). Set instantaneous Zone 2 distance protection (Z2P
and Z2G) and Level 1 phase instantaneous overcurrent element (50P1) in the
TRSOTF SEL
OGIC control equation.
TRSOTF := Z2P OR Z2G OR 50P1. Switch-Onto-Fault Trip (SELOGIC Equa-
tion)
Trip Unlatch Options
Unlatch the control output you programmed for tripping (OUT101) after the cir-
cuit breaker 52A contacts break the dc current. The SEL-421 provides two meth-
ods for unlatching control outputs following a protection trip:
➤ ULTR—all three poles
➤ TULO—phase selective
ULTR
Use ULTR, the Unlatch Trip SELOGIC control equation, to unlatch all three
poles. Use the default setting, which asserts ULTR when you push the front-panel
TARGET RESET pushbutton.
ULTR := TRGTR. Unlatch Trip (SELOGIC Equation)
TULO
Use TULO (Trip Unlatch Option) to select the conditions that cause the SEL-421
to unlatch the control outputs that you programmed for tripping. Table 6 .5 shows
the four trip unlatch options for setting TULO.
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