Schweitzer Engineering Laboratories SEL-751 - Page 340

5.4
SEL-751 Relay Instruction Manual Date Code 20170927
Metering and Monitoring
Metering
In either case, UBI% is not calculated if the average phase current magnitude
is less than 25 percent of the I
NOM
current. Voltage unbalance percent is
calculated in a similar manner.
The SEL-751 calculates the voltage unbalance percent in one of two ways,
depending on the magnitude of the average voltage. When the average voltage
(V
av
) is greater than the rated voltage Vnm, where Vnm = VNOM/1.732 when
Wye, VNOM when Delta, the relay calculates the percent unbalance as shown
in Equation 5.3.
Equation 5.3
When the average voltage is less than Vnm, the relay calculates the percent
unbalance as shown in Equation 5.4.
Equation 5.4
In either case, the UBV% is not calculated if the average voltage magnitude is
less than 25 percent of the Vnm voltage.
where:
UBI% = Current unbalance percentage
I
m
= Maximum deviation of I
av
from highest and lowest
magnitudes of the phase currents
I
av
= Magnitude of the average phase current
I
NOM
= CT rated current
where:
UBV% = Voltage unbalance percentage
V
m
= Maximum deviation of Vav from highest and lowest
magnitudes of the phase voltages
V
av
= Magnitude of the average voltage
(|VAN| + |VBN| + |VCN|)/3 when Wye;
(|VAB| + |VBC| + |VCA|)/3 when Delta
Vnm = VNOM/1.732 when Wye, VNOM when Delta
UBV% 100
V
m
V
av
---------=
UBV% 100
V
m
V
nm
-----------=