Appx.22
3-phase 3-wire Measurement
Since the values calculated internally
U
.
3
and
I
.
2
are also applied to the 3-phase total reactive power
Q, apparent power
S
, and power factor
PF
values, these values can also be calculated accurately in
the event of an unbalanced state. [When PF/Q/S setting (p. 66) is RMS]
However, because the three phases are calculated from two power values in 2-wattmeter
measurement, it is not possible to check the power balance between respective phases. If you wish
to check the power balance for individual phases, use 3-wattmeter (3P3W3M) measurement.
Item 3P3W2M
Relative
merits
3P3W3M
Voltage U1
U
.
1
=
U
.
1
=
u
.
1
-
u
.
2
U2
U
.
2
U
.
2
=
u
.
2
-
u
.
3
U3
U
.
3
=
U
.
2
-
U
.
1
U
.
3
=
u
.
3
-
u
.
1
Current I1
I
.
1
=
I
.
1
I2
I
.
3
I
.
2
I3
I
.
2
=
I
.
1
-
I
.
3
I
.
3
Active power
P1
U
.
1
I
.
1
Since the three phases are
calculated from 2-wattmeter,
it is not possible to check
the active power balance for
individual phases.
<
u
.
1
I
.
1
It is possible to check the
active power balance for
individual phases.
P2
U
.
2
I
.
3
u
.
2
I
.
2
P3 -
u
.
3
I
.
3
P
U
.
1
I
.
1
+
U
.
2
I
.
3
=
u
.
1
I
.
1
+
u
.
2
I
.
2
+
u
.
3
I
.
3
See equation (2).
=
u
.
1
I
.
1
+
u
.
2
I
.
2
+
u
.
3
I
.
3
Apparent
power
(When PF/Q/S
setting is RMS)
S1
U
1
I
1
Since calculations are based
on the line-to-line voltage
and phase (line) current,
apparent power values are
not generated for individual
phases.
<
u
1
I
1
Since calculations are
based on the phase
voltage and phase (line)
current, it is possible to
check the apparent power
for individual phases.
S2
U
2
I
3
u
2
I
2
S3
U
3
I
2
u
3
I
3
S
3
3
(U
1
I
1
+U
2
I
3
+U
3
I
2
)
=
3
3
(U
1
I
1
+U
2
I
2
+U
3
I
3
)
In 3P3W2M measurement, the instrument inputs the 3-phase line’s T-phase current as each current’s I2
parameter. For display purposes, a current value of Phase T in the 3-phase line is displayed as the current I2;
and a calculated value of Phase S in the 3-phase line, as the current I3.
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