Schweitzer Engineering Laboratories SEL-351-5, Instruction Manual

Page: 55 / 706

2.15
Date Code 20070117
Instruction Manual SEL-351-5, -6, -7 Relay
Installation
Making Rear-Panel Connections
Polarity Check for VSCONN = 3V0
Refer to Figure 2.6 (wye-connected PTs) or Figure 2.8 (delta-connected PTs).
Voltage input
VS
(terminals
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 a polarity problem with
voltage input
VS
, such as when secondary wires on terminals
VS-NS
are on the
wrong terminals, will not necessarily be apparent until a ground fault occurs
or testing is performed.
Wye-Connected PT Example
Figure 2.6 Broken-Delta Secondary Connection to Voltage Input VS, Wye-
Connected PTs
To verify the correct polarity on voltage input VS , perform the following test
on the primary side of one of the PTs connected in broken-delta secondary
(refer to
Figure 2.6 ) and observe the resultant voltage phase angle differences:
Open circuit the primary side of the PT connected to power system phase
A. With the resultant collapse of secondary voltage V
A
(V
A
= 0) in the
broken-delta secondary circuit, the voltage at voltage input VS is:
V
S
= 3V
0
= V
A
+ V
B
+ V
C
= V
B
+ V
C
Figure 2.7 shows the resultant voltage V
S
, with respect to the wye-
connected power system voltages connected to the voltage inputs VA , VB ,
VC (ABC rotation used in this example). For this scenario of the collapse
VS
NS
Setting
VSCONN = 3V0
Open Circuit
for Test
VB
N
Setting
PTCONN = WYE
A B C
VA
V
AN
V
BN
V
CN
VC
Step-Down
Transformer
SEL-351 Relay
Broken-Delta
Wye
V
A
V
B
V
S
V
C