Date Code 20001006 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements 3-43
SEL-351 Instruction Manual
For example, if V
P
is designated as phase input voltage V
B
(setting SYNCP = VB) [or V
BC
(setting
SYNCP = VBC) for delta], then rear-panel voltage input VS-NS is connected to B-phase (or BC
phase-to-phase for delta) on the other side of the circuit breaker. The voltage across terminals VB-
N (or VB-VC for delta) is synchronism checked with the voltage across terminals VS-NS (see
Figure 1.2, Figure 1.3, and Figure 2.7 through Figure 2.10).
System Frequencies Determined from Voltages V
A
(or V
AB
for Delta) and V
S
To determine slip frequency, the system frequencies on both sides of the circuit breaker need to be
determined. Voltage V
S
determines the frequency on one side. Voltage V
A
(for wye-connected
voltage inputs) or voltage V
AB
(for delta-connected voltage inputs) determines the frequency on the
other side. Thus, voltage terminals VA-N (or VA-VB for delta) have to be connected, even if
another voltage (e.g., voltage V
B
for wye or V
BC
for delta) is to be synchronized with voltage V
S
.
In most applications, all three voltage inputs VA, VB, and VC are connected to the three-phase
power system and no additional connection concerns are needed for voltage connection VA-N (or
VA-VB for delta). The presumption is that the frequency determined for A-phase (or AB phase-to-
phase for delta) is also valid for B- and C-phase (or BC and CA phase-to-phase for delta) in a
three-phase power system.
However, for example, if voltage V
B
(or V
BC
for delta) is to be synchronized with voltage V
S
and
plans were to connect only voltage terminals VB-N and VS-NS (or voltage terminals VB-VC and
VS-NS for delta) then voltage terminals VA-N (or VA-VB for delta) will also have to be connected
for frequency determination. If desired, voltage terminals VA-N can be connected in parallel with
voltage terminals VB-N (or voltage terminals VB-VA connected in parallel with voltage terminals
VB-VC for delta; connect voltage terminal VA to VC). In such a nonstandard parallel connection,
remember that voltage terminals VA-N are monitoring Phase B (or voltage terminals VB-VA are
monitoring BC phase-to-phase for delta). This understanding helps prevent confusion when
observing metering and event report information or voltage element operation.
Another possible solution to this example for wye-connected relays (synchronism check voltage
input VS-NS connected to V
B
) is to make setting SYNCP = 120 (the number of degrees that
synchronism check voltage V
S
constantly lags voltage V
A
) and connect voltage input VA-N to V
A
.
Voltage inputs VB and VC do not have to be connected.
For delta-connected relays (synchronism check voltage input VS-NS connected to V
BC
), make
setting SYNCP = 120 (the number of degrees that synchronism check voltage V
S
constantly lags
voltage V
AB
) and connect voltage inputs VA-VB to V
AB
. Voltage input VC does not have to be
connected.
System Rotation Can Affect Setting SYNCP
The solution in the preceding two paragraphs (for a relay with wye-connected voltage inputs):
• voltage input VA connected to Phase A
• voltage input VS connected to Phase B
• setting SYNCP = 120 degrees (V
S
constantly lags V
A
by 120°)
Courtesy of NationalSwitchgear.com
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