Schweitzer Engineering Laboratories SEL-751 - Page 135

4.9
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
The marked LEA sensor ratio of the voltage divider is always provided by the
manufacturer and often the per phase RCF values are also provided.
If the voltage divider is perfect, then
V
pri
/ V
sec
= LEA_R and RCF = 1.000
Therefore, the measured voltage divider performance equals the marked ratio
of the voltage divider, as given by the manufacturer. But such perfect condi-
tions are usually not the case.
If the voltage divider is putting out more voltage (V
sec
) than nominally
expected for an applied voltage input (V
pri
), then
V
pri
/ V
sec
< LEA_R and RCF < 1.000
An example of an RCF value less than 1.000 is found in Example 4.7. In this
example, setting VBRCF = 0.883 brings down the too-high voltage on voltage
input VB (0.82 V is brought down to nominal 0.72 V).
If the voltage divider is putting out less voltage (V
sec
) than nominally
expected for an applied voltage input (V
pri
), then
V
pri
/ V
sec
> LEA_R and RCF > 1.000
EXAMPLE 4.7 Normalizing Voltages With Ratio Correction Factors
A voltage divider is connected to the 8 Vac LEA voltage inputs (see
Figure 4.1). The RCF values per phase for the voltage divider are given as
follows:
V
ARCF
= 1.078 (voltage input VA)
V
BRCF
= 0.883 (voltage input VB)
V
CRCF
= 1.112 (voltage input VC)
The marked ratio of the voltage divider is given as:
LEA_R = 10000
What are the true ratios of each phase of the voltage divider?
true ratio = V
pri
/V
sec
V
pri
and V
sec
are measured in manufacturer tests to derive RCF values as
shown in Equation 4.1 and accompanying explanation. From Equation 4.1:
RCF • LEA_R = V
pri
/V
sec
= true ratio
1.078 • 10000 = 10780 (true ratio for voltage input
VA)
0.883 • 10000 = 8830 (true ratio for voltage input
VB)
1.112 • 10000 = 11120 (true ratio for voltage input
VC)
Note that these true ratios vary from 8830 to 11120, while the marked ratio
of the voltage divider is given as 10000.
where:
V
pri
= test voltage applied to the primary side of the volt-
age divider
V
sec
= resultant voltage measured on the secondary side of
the voltage divider
True ratio = V
pri
/ V
sec
Marked
ratio
= LEA_R = effective nominal LEA sensor ratio of the
voltage divider connected between the primary
voltage system and the LEA input.