Sel SEL-421 - Single-Phase Voltage Inputs; Voltage Angle Difference in a Paralleled System; Synchronism-Check Voltages for Two Circuit Breakers

R.2.52
SEL-421 Relay Reference Manual Date Code 20111215
Auto-Reclosing and Synchronism Check
Synchronism Check
Figure 2.20 Voltage Angle Difference in a Paralleled System
Alternatively, if the remote sources in each section of the example system
shown in Figure 2.19 are not paralleled beyond the open circuit breakers, the
systems are asynchronous. The corresponding phase voltages of two such
systems are only in phase at infrequent times—when one of the systems slips
by the other. At all other times, the corresponding phase voltages of two such
systems are out of phase (sometimes as much as 180 degrees out of phase) as
the systems continue to slip by each other.
Single-Phase Voltage
Inputs
Figure 2.21 shows single-phase voltage transformers (1 PT) on Bus 1 and
Bus 2. Use these single-phase voltage sources to perform a synchronism
check across the two circuit breakers.
Figure 2.21 Synchronism-Check Voltages for Two Circuit Breakers
Synchronism check occurs on a single-phase voltage basis—see the single-
phase potential transformers (1 PT) shown on each bus in Figure 2.21. The
assumption is that, if the monitored single-phase voltage inputs are in phase
(within a settable voltage angle difference) and are healthy (within a settable
voltage magnitude window), the other phase-to-neutral voltages are likewise
in phase and healthy. The line voltage source is three-phase, but you only need
a single-phase bus voltage to perform a synchronism check across the
corresponding circuit breaker. The relay uses the three-phase voltage from the
line for other functions such as distance protection and metering.
Synchronism-Check
Settings
This example uses a two-circuit breaker arrangement (see Figure 2.21). Set
the synchronism-check enable settings:
E25BK1 := Y Synchronism Check for Circuit Breaker BK1 (Y, N)
E25BK2 := Y Synchronism Check for Circuit Breaker BK2 (Y, N)
If you are using the SEL-421 on a single circuit breaker, enable synchronism
check for only one circuit breaker (E25BK1 := Y and
E25BK2 := N).
Short
distance
Open breaker
80 kilometer
parallel path
Bus 2
Bus 1
Line
BK2
BK1
1
3
1
. . . . . .
. . .