Date Code 20020426 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements 3-57
SEL-351-5, -6, -7 Instruction Manual
• V
A
, V
B
, V
C
are all greater than 25 V secondary (wye-connected)
V
AB
, V
BC
, V
CA
are all greater than 43.3 V secondary (delta-connected)
• SEL
OGIC control equation setting FAULT is deasserted
• |V1| is within three percent of the calculated Vbase value (wye connected)
• |V1| is within three percent of the calculated Vbase value/
3 (delta connected)
• At least twelve seconds have elapsed
As soon as the above Vbase initialization conditions are satisfied, the SSI Relay Word bits will be
allowed to change state according to their settings and the present voltage conditions, and the SSI
Recorder will be enabled.
Vbase Tracking Range
The Vbase quantity will track the positive-sequence voltage over a large range of system
voltages. The tracking limits are explained below. In normal relay use, these limits are not likely
to be reached, because one of the Sag, Swell, or Interruption Relay Word bits would most likely
assert for a large voltage deviation, thus blocking the Vbase thermal element from tracking to one
of the range limits.
The minimum value that Vbase can achieve is equivalent to a positive-sequence (V1) value of 25
volts secondary. In primary units, the lowest value depends on the global setting PTCONN:
When PTCONN = WYE: minimum Vbase =
1000
PTR•V25
kV.
When PTCONN = DELTA: minimum Vbase =
1000
PTR•V3.43
kV.
The maximum value that Vbase can achieve is equivalent to 300 volts secondary divided by
VSWELL, so the maximum Vbase in primary kV is
VSWELL
PTR•V30
1000•VSWELL
100•PTR•V300
=
. The upper
limit for Vbase is not affected by the PTCONN global setting.
If the expected higher end of the “normal” system voltage range is close to 300 V, secondary,
then the VSWELL setting may need to be reduced, or turned “OFF,” in order to allow Vbase to
track the actual system voltage and not run into the maximum value limit. For example, if
connecting to an industrial service rated at 277 V
LN
/ 480 V
LL
, using the wye-connection (with no
PTs), and the normal operating range goes up to 285 V
LN
, then the maximum VSWELL setting
that will allow for proper Vbase tracking is 105 percent.
SSI Reset command
After commissioning tests or other maintenance activities that have applied test voltages to the
SEL-351 Relay, the Vbase element may have locked onto a test voltage. Use the SSI R (reset)
command once normal system voltages are restored on the voltage terminals. Powering up the
relay automatically performs this reset.
See Reset the SSI Recorder Logic in Section 12: Standard Event Reports, Sag/Swell/-
Interruption Report, and SER for more details.
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