Schweitzer Engineering Laboratories SEL-751 - Figure 2.20 Single-Phase Voltage Connections

2.28
SEL-751 Relay Instruction Manual Date Code 20170927
Installation
AC/DC Control Connection Diagrams
Voltage Connections
With the voltage inputs option, you can connect the ac voltages directly, use a
wye-wye VT connection (set DELTA_Y := WYE), an open-delta VT
connection (set DELTA_Y := DELTA), or a single-phase VT (set
SINGLEV := Y). Figure 2.20 and Figure 2.21 show the methods of
connecting single-phase and three-phase voltages.
When the voltage inputs to the relay are connected in wye as shown in
Figure 2.21 (a): Direct Connection, Figure 2.21 (b): Wye-Wye VT Connec-
tions With LEA Inputs) or Figure 2.21 (c): Wye-Wye VT Connection, the relay
computes the zero-sequence voltage as the following:
3V0 = VA + VB + VC
When the voltage inputs to the relay are connected via an open-delta VT as
shown in Figure 2.21 (d)–Figure 2.21 (f), the relay cannot calculate zero-
sequence voltage (3V0). Relay functions that require zero-sequence voltage
may be disabled, unless another 3V0 voltage source is supplied to the relay
via terminal VS-NS and VSCONN is set to 3V0 as in Figure 2.21 (e) and
Figure 2.21 (f). With setting VSCONN := 3V0, voltage input VS (terminals
VS, NS) expects 3V0 voltage (VS = 3V0 = VA + VB + VC) with the polarity
shown in the figures. Setting VSCONN := 3V0 disables the synchronism-
check element. To enable synchronism check element set VSCONN := VS.
Set VSCONN accordingly per your application needs.
Some installations do not have all three voltage phases available to connect to
the relay as shown in Figure 2.20 (a) and Figure 2.20 (b). In such cases set
DELTA_Y to WYE or DELTA according and set SINGLEV := Y. In those
cases the relay will disable some of the voltage based functions as specified in
Table 4.4.
Figure 2.20 Single-Phase Voltage Connections
NOTE: Current limiting fuses in
direct-connected voltage applications
are recommended to limit short-
circuit arc-flash incident energy.
(a) Single Phase-Neutral VT Connection
(DELTA_Y := WYE and SINGLEV := Y)
EO1
EO2
AB
C
ZO9
Z10
Z11
Z12
SEL-751
F1
(b) Single Phase-Phase VT Connection
(DELTA_Y := DELTA and SINGLEV := Y)
AB
C
EO1
EO2
ZO9
Z10
Z11
Z12
SEL-751
F1
VS
NS
VA
VB
VC
N
VS
NS
VA
VB
VC
N