2.30
SEL-751 Relay Instruction Manual Date Code 20170927
Installation
AC/DC Control Connection Diagrams
Potential Transformer
Ratios and PT
Nominal Secondary
Voltage Settings
The relay setting PTR is the overall potential ratio from the primary system to
the relay phase voltage inputs VA-VB-VC-N. For example, on a 12.5 kV
phase-to-phase primary system with wye-connected 7200:120 V PTs (setting
DELTA_Y := WYE, and the relay wired as shown in Figure 2.21 (c)), the
correct PTR setting is 60. For the same 12.5 kV system connected through
12470:115 V PTs in an open-delta configuration (setting
DELTA_Y := DELTA, and the relay wired as shown in Figure 2.21 (d),
Figure 2.21 (e) or Figure 2.21 (f)), the correct PTR setting is 108.44.
Single-phase voltage connections follow the same rationale. Refer to
Figure 2.20 (a) and Figure 2.20 (b). For example, with a single-phase voltage
connection to the VA-N terminals (DELTA_Y := WYE and SINGLEV := Y)
from a 12.5 kV phase-to-phase primary system with a line-neutral connected
7200:120 V PT, the correct PTR setting is 60. For the same 12.5 kV system
connected through 12470:115 V PTs in a line-to-line configuration
(DELTA_Y := DELTA and SINGLEV := Y) the correct PTR setting is
108.44.
The relay setting PTRS is the overall potential ratio from the synchronizing or
broken-delta voltage source to the relay VS-NS voltage inputs. For example,
in a synchronism-check application (setting VSCONN := VS), with phase-to-
ground voltage connected from a 12.5 kV phase-to-phase primary system
through a 7200:120 V PT, the correct PTRS setting is 60.
In an application that uses a broken-delta PT connection to create a 3V0 zero-
sequence voltage signal (setting VSCONN := 3V0 and the relay VS-NS termi-
nals wired as shown in Figure 2.21 (e) or Figure 2.21 (f)) with three PTs con-
nected wye (primary)/broken delta (secondary) with ratios of 7200:120, the
correct PTRS setting is 60. If the application includes a step-down trans-
former in addition (not show in the figure), it must also be included in the
overall PTRS ratio calculation. If a 400:250 step-down instrumentation trans-
former is in the circuit, the correct PTRS setting would be 60 • 1.6 = 96.00.
Settings PTR and PTRS are used in the event report and METER commands
to report power system values in primary units. Settings PTR and PTRS are
also used when the setting VSCONN := 3V0, to scale the measured VS volt-
age into the same voltage base as voltage inputs VA-VB-VC-N for certain
directional functions in Section 4: Protection and Logic Functions. If no VTs
are connected to voltage inputs VA-VB-VC-N, make setting PTR the same
value as setting PTRS. The ratio of the PTRS and PTR settings (PTRS/PTR)
must be less than 1000 and greater than 0.001 when VSCONN := 3V0. The
relay setting VNOM is the nominal secondary voltage connected to voltage
inputs VA-VB-VC-N. For wye-connected or delta-connected PTs, VNOM is
the phase-to-phase secondary voltage value.
For example, for a 10 kV (phase-to-phase) system with wye-connected VTs
rated 7200:120 V (PTR := 60), the setting for VNOM would be:
10000 V/60 = 166.66 V. For a 12.5 kV (phase-to-phase) system with open-
delta connected VTs rated 14000:115 V (PTR := 121.74), the setting for
VNOM would be 12500 V/121.74 = 102.68 V.
The SEL-751 automatically sets VNOM := OFF and hides the setting when
the setting SINGLEV := Y. In Tab le 4.4, a setting of SINGLEV := OFF is
shown to disable/turn-off a number of features. Effectively, VNOM := OFF
signifies that a full three-phase voltage source is not connected to voltage
inputs VA-VB-VC-N. Even with VNOM := OFF, voltage can still be con-
nected to voltage inputs VA-VB-VC-N (for example, single-phase voltage
connected to voltage input VA-N), as discussed.
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