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configure the M57x for 2-1/2 element mode and which phase voltage is missing, a "0" is
written to the phase-to-neutral VT Ratio for the missing phase voltage.
The average of the 3 voltage phases ((Va + Vb + Vc)/3) is also available. The Average 3phase Volts for bus 1 and bus 2 are calculated and made available on a per cycle basis.
6.4 Power Factor (1-Cycle Update)
The per-phase Power Factor measurement is calculated using the "Power Triangle", or the
per-phase WATTS divided by the per-phase VAs. The Total PF is similar, but uses the
Total WATTS and Total VAs instead. The sign convention for Power Factor is shown in
Figure 10 (pg. 65). Note that the Total PF calculation depends on the Total VA calculation
type chosen.
6.5 Watts / Volt-Amperes (VAs) / VARs (1-Cycle Update)
On any power connection type (2, 2-1/2, and 3 element), the M57x calculates per-element
Watts by multiplying the voltage and current samples of that element together. This
represents the dot product of the voltage and current vectors, or the true Watts. The perelement VAs are calculated from the product of the per-element Volts and Amps. The perelement VARs are calculated from fundamental VARs.
In any connection type, the Total Watts and Total VARs is the arithmetic sum of the perelement Watts and VARs. The sign conventions are shown in Figure 10 (pg. 65).
When used on 2-element systems, the reference phase voltage (typically phase B) input,
is connected to the Neutral voltage input, and effectively causes one of the elements to be
zero. It is not required to use any particular voltage phase as the reference on 2element systems. When used on 2-element systems the per-element Watts, VARs,
and VAs have no direct physical meaning, as they would on 2-1/2 and 3 element
systems where they represent the per-phase Watts, VARs, and VAs.
When used on 2-1/2 element systems, one of the phase-to-neutral voltages is fabricated,
as described in Section 6.3. In all other respects, the 2-1/2 element connection is identical
to the 3 element connection.
The M57x may be configured to calculate Total VAs in one of several different ways. The
calculation method may be selected either by sending a command to the M57x via a
network and protocol (refer to the specific protocol manual for details) or by using the 70
Series Configurator. The three methods, Arithmetic, Geometric, and Equivalent (both for
WYE and DELTA), all yield the same results when used on balanced systems with no
harmonics present. The differences are illustrated below:
6.5.1 Geometric VA Calculations
GEOMETRIC
=
VATOTAL
ML0032
2
WattsTOTAL + VARsTOTAL
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