Novatech Bitronics M87X Series, Руководство пользователя

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ML0021 August 15, 2018
34
Copyright 2018 Bitronics, LLC
are sampled simultaneously (including currents), these signals can be easily combined to
form true Phase-to-Phase and Phase-to-Neutral differential measurements with extremely
low amplitude and phase errors. It also allows accurate calculation of Bus-to-Bus
differential voltages and angles. Each sample is corrected for offset and gain using factory
calibration values stored in non-volatile memory on the board. Additionally, a continuous
DC removal is performed on all inputs, except the AUX Voltages.
The M87x calculates voltages in PRIMARY units, based on the VT Ratios entered. There
are separate VT Ratios for each input. Ratios can be entered via a network and protocol
(refer to the specific protocol manual for details) or by running the M87x Configurator.
The advantages of this method of voltage measurement are apparent when the M87x is
used on the common 2, 2-1/2, and 3 element systems (refer to Section 4.4). The M87x is
always calculating Line-to-Neutral, Line-to-Line, and Bus-to-Bus voltages with equal
accuracy. On 2 element connections, any phase can serve as the reference phase.
Further, the M87x can accommodate WYE connections on one Bus, and DELTA
connections on the other Bus.
On 2-1/2 element systems, one of the phase-to-neutral voltages is missing, and the M87x
must create it from the vector sum of the other two phase-to-neutral voltages. In order to
configure the M87x 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 AUX Voltage input measurements (M871 only) are similar to the phase voltages
except they do not have continuous DC removal. Both input-to-ground and input-to-input
differential voltages are measured. Because they do not have continuous DC removal,
they can be used to measure either AC or DC signals, making these measurements useful
for Substation battery monitoring, generator field voltages, local bus voltage
measurements, etc.
The average of the 3 voltage phases ((Va + Vb + Vc)/3) is also available. The Average 3-
phase Volts for bus 1 and bus 2 are calculated and made available on a per cycle basis.
4.3 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 7 (pg. 33). Note that the Total PF calculation depends on the Total VA calculation
type chosen (Section 4.4).
4.4 Watts / Volt-Amperes (VAs) / VARs (1-Cycle Update)
On any power connection type (2, 2-1/2, and 3 element), the M87x 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 per-
element VAs are calculated from the product of the per-element Volts and Amps. The per-
element VARs are calculated from fundamental VARs.