Schweitzer Engineering Laboratories SEL-700G Series, Инструкция по эксплуатации

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4.24
SEL-700G Relay Instruction Manual Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
Step 4. Choose the CTCn settings for the windings.
Because Winding X is the reference, we need no adjustment;
the setting is CTCX = 0. Note that the adjusted Winding Y
inputs coincide exactly with the reference direction; we need
make no adjustment for the 24.9 kV winding either. Therefore,
the setting is CTCY = 0. As mentioned earlier, these two
windings represent a classical DABY application. We can see
this from the fact that the CTCn setting is zero for both
windings. The CT connections themselves perform exactly the
right correction without additional help from the relay. The
process is nearly complete.
Step 5. As a final step, ensure that no wye-connected winding having
wye-connected CTs is set at CTCn = 0 (uncompensated).
Were this the case, zero-sequence currents could appear in
these relay inputs but in no others, and a possible false trip
could occur for external ground faults. Any non-zero value of
CTCn will eliminate the zero-sequence. In this example there is
no wye-connected winding with wye-connected CTs. The
selection is complete. Set ICOM := N (forces the CTCn := 0)
because no compensation is necessary from the relay.
Winding Line-to-Line Voltages.
Enter the nominal line-to-line transformer
terminal voltages. If a load tap changer is included in the transformer
differential zone, assume that it is in the neutral position. The setting units are
kilovolts.
Current TAP.
The relay uses a standard equation to set TAPn, based on
settings entered for the particular winding (n denotes the winding number.)
where:
The relay calculates TAPn with the following limitations:
➤
The tap settings are within the range 0.1 • I
NOMn
and
6.2 • I
NOMn
➤
The ratio of the highest (TAPn/INOMn) to the lowest (TAPn/
INOMn) is less than or equal to 7.5, where n = X, Y
Restrained Element Operating Current Pickup.
The O87P setting range
is 0.1 to 1.0; we suggest an O87P setting of 0.2 to 0.3. The setting must be at a
minimum for increased sensitivity but high enough to avoid operation because
of steady-state errors such as unmonitored station service loads, transformer
excitation current, and relay measuring error at very low current levels. The
setting must also yield an operating current greater than or equal to a
minimum of 0.1 • INOMn/TAPn, where n = X, Y. This restriction applies to
each winding side (irrespective of TAP magnitudes) when 1 A and 5 A
nominal CTs are used in the same relay.
C = 1 if CTCONn setting = Y (wye-connected CTs)
C =
if CTCONn setting = D (delta-connected CTs)
MVA = maximum power transformer capacity setting (must be
the same for all TAPn calculations)
VWDGn = winding line-to-line voltage setting, in kV
CTRn = current transformer ratio setting
TAPn
MVA 1000

3 VWDG n CTR n
 
--------------------------------------------------------- C

=
3