4.8
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
age related pickup settings accordingly to convert the settings from 8 V to
300 V base. Refer to Example 4.6. With LEA voltage inputs the relay sees
8 Vac on the voltage inputs as 300 Vac secondary (8 • 37.5 = 300 V). Further,
to realize accurate primary voltage metering, the relay uses the derived LEA
scales, LEA_SC and LEA_S_SC. These scale factors are autocalculated by
the relay based on LEA_R and LEA_S_R settings, respectively. The LEA
scale is derived as follows:
LEA_SC = LEA_R • (8/300)
where LEA_R is the marked LEA sensor ratio.
The ratio of V
pri
to V
sec
of the voltage divider shown in Figure 4.1 is referred
to as true ratio. Ideally, the marked LEA sensor ratio should equal the true
ratio. If not, apply the following additional ratio correction factors.
For example, if an LEA sensor has an LEA sensor ratio of 1400:1,
LEA_R = 1400
LEA_SC = 1400 • 8/300 = 37.33
EXAMPLE 4.6 Voltage Setting Conversion to 300 V Base
A voltage divider (10000 ratio) is connected between a 12.47 kV system
(7.2 kV line-to-neutral) and the LEA inputs.
7200 V / 10000 = 0.72 V
(actual voltage divider output to the 8 Vac LEA inputs;
8Vbase)
0.72 V • (300/8) = 27 V
(the relay thinks it is looking at 27 V on a 300 V base, not
0.72 V on an 8 V base)
27 V is the nominal adjusted secondary voltage—adjusted by the 300/8
factor from an 8 V base to a 300 V base. For this same example, if a 0.8 V
output of the 8 Vac LEA (8 V base) is deemed an overvoltage condition,
then an overvoltage element pickup setting (e.g., 59P1P) could be set at
59P1P = 0.8 V • (300/8) = 30 V (300 V base).
LEA Ratio and Angle Correction Factors (Global Settings)
In the SEL-751 with LEA voltage inputs, Global settings VARCF, VBRCF,
VCRCF, VSRCF, VAPAC, VBPAC, VCPAC, and VSPAC are applied to their
respective voltage inputs, VA, VB, VC, and VS. These normalized secondary volt-
ages are used throughout the SEL-751. Refer to Tabl e 4 .6 9 under Global Set-
tings for these settings.
Ratio Correction Factors (RCF) for Voltage Inputs
The ratio correction factor (RCF) settings minimize the magnitude error by
compensating for the irregularities (on a per-phase basis) of the voltage divid-
ers connected between the primary voltage system and the LEA inputs. The
derivation of the RCF value for a voltage divider for a particular phase is
defined as follows:
Equation 4.1
RCF
true ratio
marked ratio
------------------------------
V
pri
V
sec
LEA_R
------------------------
V
pri
V
sec
LEA_R
----------------------------------===
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