3.40
SEL-351A Relay Instruction Manual Date Code 20080213
Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Synchronism Check Elements (Not in SEL-351A-1)
V
P
Phase input voltage (V
A
, V
B
, or V
C
for wye-connected voltages; V
AB
,
V
BC
, V
CA
for delta-connected voltages), designated by setting SYNCP
(e.g., if SYNCP = VB, then V
P
= V
B
)
V
S
Synchronism check voltage, from SEL-351A rear-panel voltage input
VS
For example, if V
P
is designated as phase input voltage VB (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 2.12–Figure 2.15, Figure 2.24, and Figure 2.25).
System Frequencies Determined From Voltages V
A
(or V
AB
for Delta) and
V
S
To determine slip frequency, first determine the system frequencies on both
sides of the circuit breaker. 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.
Courtesy of NationalSwitchgear.com
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