3-2
Input and Output Functions
BE1-951
Zero Sequence Voltage. When four-wire VT connections are used, the BE1-951 calculates the zero
sequence voltage (3V
0
). Zero sequence voltage measurement is not available when single-phase or three-
wire VT connections are used. The 27X, and 59X can be set to monitor the zero sequence voltage.
Negative Sequence (V
2
) Voltage. Negative sequence voltage is calculated from the fundamental component
of the three-phase voltage inputs. It is only available on three-phase, three-wire or three-phase, four-wire
systems. V
2
is calibrated to the phase-to-neutral base. Negative sequence measurements can
accommodate either ABC or ACB phase sequence
Frequency Measurement
Power system frequency is monitored on the A phase voltage input or the AB voltage input when in three-
wire mode. When the applied voltage is greater than 10 volts, the BE1-951 measures the frequency. The
measured frequency is used by the 81 function and applies to all measurements and calculations .
Frequency Compensation. After measuring the frequency, the BE1-951 varies the sampling rate to maintain
12 samples per cycle over a frequency of 10 to 75 hertz. If the voltage is too low for accurate frequency
measurement, or if the measured frequency is out of range, the ADC defaults to a sampling rate appropriate
for the relay nominal frequency setting. The sampling rate is adjusted every 250 milliseconds.
Nominal Frequency. Nominal frequency is set for 50 hertz or 60 hertz power systems. When the voltage
is too low for reliable frequency measurement, the ADC defaults to operation at the nominal frequency
setting.
Power Measurement
The measured fundamental component of current and voltage as described previously in this section, is
used to calculate the power per the following equations.
For four-wire VT connection:
Equation 1
WATTs
3
-
V
A
I
A
cos(
-
A
)
V
B
I
B
cos(
-
B
)
V
C
I
C
cos(
-
C
)
Equation 2
VARs
3
-
V
A
I
A
sin(
-
A
)
V
B
I
B
sin(
-
B
)
V
C
I
C
sin(
-
C
)
where:
3
x
5
V
x
5
I
x
For three-wire VT connection, with ABC phase sequence:
Equation 3
WATTs
3
-
V
AB
I
A
cos(
-
A
)
V
CB
I
C
cos(
-
C
)
Equation 4
VARs
3
-
V
AB
I
A
sin(
-
A
)
V
CB
I
C
sin(
-
C
)
where:
3
x
5
V
xy
5
I
x
For three-wire VT connection, with ACB phase sequence:
Equation 5
WATTs
3
-
V
AC
I
A
cos(
-
A
)
V
BC
I
B
cos(
-
B
)
Equation 6
VARs
3
-
V
AC
I
A
sin(
-
A
)
V
BC
I
B
sin(
-
B
)
where:
3
x
5
V
xy
5
I
x
For AN, BN, or CN VT connection:
Equation 7
WATTs
3
-
3 × V
x
I
x
cos(
-
x
)
