Schweitzer Engineering Laboratories SEL-751 - Figure 2.32 SEL-751 Provides Overcurrent Protection for an Ungrounded System (Open-Delta Connected Pts, Broken-Delta 3 V0 Connection)

2.41
Date Code 20170927 Instruction Manual SEL-751 Relay
Installation
AC/DC Control Connection Diagrams
A core-balance current transformer is often referred to as a zero-sequence, ground fault, or window current transformer.
Directional control for an ungrounded system is selected with setting ORDER := U (see Table 4.19—Table 4.21).
Nondirectional sensitive earth fault (SEF) protection is also available.
The voltage inputs can accept open-delta PT (three-wire) connection (as shown) when setting DELTA_Y := DELTA.
VB must be externally tied to N, as shown.
The zero-sequence voltage 3V
0
(from the “broken-delta” connection) is shown coming from a step-down
instrumentation transformer, and connecting to voltage input VS-NS. To use this connection, make setting VSCONN :=
3V
0
. Make group setting PTRS as shown in Section 4: Protection and Logic Functions.
The step-down transformer is required when the maximum expected residual voltage exceeds the relay voltage channel
rating.
The polarity of voltage input VS-NS connection should be verified prior to placing the relay into service.
Figure 2.32 SEL-751 Provides Overcurrent Protection for an Ungrounded System (Open-Delta Connected PTs,
Broken-Delta 3V0 Connection)
TC
Trip
Coil
52A
(+)
(—)
Trip
Circuit
IA
C
B
A
SEL-751 RELAY
VS
NS
OUT103
Forward Tripping Direction
Core-Balance CT
CC
Close
Coil
52B
(+)
(—)
Close
Circuit
IB OUT102
86
Lock
Out
86B
(+)
(—)
Breaker
Failure
Trip
Circuit
IC OUTxxx
OUT101
FEEDER
ABc
52
(+)
Step-Down
Transformer
(Some
Applications)
ALARM
(+)
(—)
Breaker Status
INxxx
IN
Ungrounded
Broken-Delta
TRANSFORMER BANK
Open-Delta
52A
+
+
to Annunciator, RTU,
or SEL Communications Processor
VCNVBVA