2.15
Date Code 20080213 Instruction Manual SEL-351A Relay
Installation
Making Rear-Panel Connections
System frequency is determined from voltage connected to voltage input VA-N.
Additionally, voltage input VS-NS measures frequency on the other side of an
open breaker for synchronism check applications. See subsections
Synchronism Check Elements (Not in SEL-351A-1) and Frequency Elements.
Throughout this Instruction Manual, relay functions, specifications, or
features that are different for delta-connected and wye-connected PTs are
identified, either with “wye-connected” and “delta-connected” text, or with
“PTCONN = WYE” and “PTCONN = DELTA” text.
Synchronism Check VS Connection (Global setting VSCONN = VS)
NOTE: The VS channel is not
supported in the SEL-351A-1.
When setting VSCONN = VS, voltage input VS-NS operates in its traditional
role of voltage input for the synchronism check elements. Figure 2.12–
Figure 2.15, Figure 2.24, and Figure 2.25 show examples of synchronism
check voltage inputs applied to voltage input VS-NS. See Synchronism Check
Elements (Not in SEL-351A-1) on page 3.36.
SEL-351A relays with firmware revisions R104 and earlier do not have a
VSCONN setting. In these relays, voltage input VS-NS operates in its
traditional role of voltage input for synchronism check (as if setting
VSCONN = VS).
Broken-Delta VS Connection (Global setting VSCONN = 3V0)
In SEL-351A relays with firmware revision R105 or higher, global setting
VSCONN = 3V0 adjusts the relay to accept a 3V
0
zero-sequence voltage
signal connected to voltage input VS-NS. This signal is usually derived from
PTs connected wye (primary)/broken-delta (secondary):
V
S
= V
A
+ V
B
+ V
C
= 3V
0
.
This signal is passed to certain relay functions that require zero-sequence
voltage, such as zero-sequence voltage-polarized ground directional elements
or Wattmetric and incremental conductance elements (for Petersen Coil
grounded systems). Because setting VSCONN = 3V0, these elements use the
3V
0
zero-sequence voltage measured by the VS-NS voltage input, even if wye-
connected PTs are connected to voltage inputs VA-VB-VC-N (PTCONN = WYE;
see Figure 2.8). With this wye-connection, 3V
0
zero-sequence voltage is
calculated from voltages V
A
, V
B
, and V
C
(3V
0
= V
A
+ V
B
+ V
C
) for metering,
but this calculated 3V
0
is not used elsewhere when VSCONN = 3V0.
To prevent a broken-delta voltage source from exceeding the rated voltage of
the relay voltage inputs, some applications require an external step-down
transformer. Figure 2.8, Figure 2.10, and Figure 2.23 show the PT wiring,
including an instrumentation step-down transformer, for using voltage input
VS-NS as a zero-sequence voltage source. Group setting PTRS accommodates
the ratio of the step-down transformer. See Settings Explanations on page 9.34
for an example setting of PTRS when VSCONN = 3V0. For a complete listing
of the changes caused by setting VSCONN = 3V0, see Table 9 . 7, Table 9 .8,
and related discussions.
Selecting global setting VSCONN = 3V0 disables the synchronism-check
element. Therefore, input terminals VS-NS cannot be used for 3V
0
zero-
sequence voltage measurement and as a synchronism check voltage input, at
the same time.
Courtesy of NationalSwitchgear.com
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