2.20
SEL-351S Relay Instruction Manual Date Code 20080103
Installation
Making Rear-Panel Connections
Throughout this instruction manual, relay functions, specifications, or features
that are different for delta-connected and wye-connected PTs are identified,
either with “wye-connected” and “delta-connected” text, or with
“PTCONN = WYE” and “PTCONN = DELTA” text.
Synchronism Check VS Connection (Global Setting VSCONN = VS)
When setting VSCONN = VS, voltage input VS-NS operates in its traditional
role of voltage input for the synchronism check elements. Figure 2.17,
Figure 2.20, Figure 2.29, and Figure 2.30 show examples of synchronism
check voltage inputs applied to voltage input VS-NS. See Synchronism Check
Elements on page 3.36.
SEL-351S relays with firmware revisions R107 and earlier do not have a
VSCONN setting. In these relays, voltage input VS-NS operates in its
traditional role of voltage input for synchronism check (as if setting
VSCONN = VS).
Broken-Delta VS Connection (Global Setting VSCONN = 3V0)
In SEL-351S relays with firmware revision R108 or higher, global setting
VSCONN = 3V0 adjusts the relay to accept a 3V
0
zero-sequence voltage
signal connected to voltage input VS-NS. This signal is usually derived from
PTs connected wye (primary)/broken-delta (secondary):
V
S
= V
A
+ V
B
+ V
C
= 3V
0
.
This signal is passed to certain relay functions that require zero-sequence
voltage, such as zero-sequence voltage-polarized ground directional elements
or wattmetric and incremental conductance elements (for Petersen Coil
grounded systems). Because setting VSCONN = 3V0, these elements use the
3V
0
zero-sequence voltage measured by the VS-NS voltage input, even if wye-
connected PTs are connected to voltage inputs VA-VB-VC-N (PTCONN = WYE;
see Figure 2.13). With this wye-connection, 3V
0
zero-sequence voltage is
calculated from voltages V
A
, V
B
, and V
C
(3V
0
= V
A
+ V
B
+ V
C
) for metering,
but this calculated 3V
0
is not used elsewhere when VSCONN = 3V0.
To prevent a broken-delta voltage source from exceeding the rated voltage of
the relay voltage inputs, some applications require an external step-down
transformer. Figure 2.13, Figure 2.15, and Figure 2.28 show the PT wiring,
including an instrumentation step-down transformer, for using voltage input
VS-NS as a zero-sequence voltage source. Group setting PTRS accommodates
the ratio of the step-down transformer. See Settings Explanations on page 9.48
for an example setting of PTRS when VSCONN = 3V0. For a complete listing
of the changes caused by setting VSCONN = 3V0, see Table 9 .8 , Table 9.9,
and related discussions.
Selecting global setting VSCONN = 3V0 disables the synchronism check
element. Therefore, input terminals VS-NS cannot be used for 3V
0
zero-
sequence voltage measurement and as a synchronism check voltage input, at
the same time.
Polarity Check for VSCONN = 3V0
Refer to Figure 2.13 (wye-connected PTs) or Figure 2.15 (open-delta
connected PTs). With setting VSCONN = 3V0, voltage input VS-NS expects
3V
0
voltage (V
S
= 3V
0
= V
A
+ V
B
+ V
C
) with the polarity shown. However, in
a nonfault, balanced system condition, voltage V
S
= 3V
0
≈ 0. The result is that
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