Current Measurements; Neutral Current (Residual Current) For Wye Connections; Voltage Measurements; Changing Transformer Ratios - Novatech Bitronics M87X Series Manual

7.6 Current Measurements

The M871 has four current inputs, and the M872 has 6 current inputs, with an internal CT
on each channel. These inputs can read to a maximum of 100A (symmetrical), or
RMS
141A , for the S10 and S13 and S16 (bus 2) input modules under all temperature and
PEAK
input frequency conditions. No range switching is used, allowing a high dynamic range.
Refer to section 7.3 for the current measurement input ranges and specifications for the
signal input modules that are offered.

7.6.1 Neutral Current (Residual Current) for WYE Connections

The M871 directly measures the neutral current from the neutral CT. Residual current is
calculated from the vector sum of the 3 per-phase currents on both the M871 and the
M872.

7.7 Voltage Measurements

The M87x inputs are a unique advancement in voltage measurement technique. All three
phases, and the neutral, are measured relative to a common reference point (essentially
panel ground). Because all signals are sampled at the same instant in time, common
mode signals can be removed by subtraction of samples in the DSP, instead of the more
traditional difference amplifier approach. This greatly simplifies the external analog
circuitry, increases the accuracy, and allows measurement of the Neutral-to-Ground
voltage at the panel. The 7kV input divider resistors are accurate to within +/-
25ppm/DegC, and have a range of 600V , from any input to panel ground.
PEAK
A signal must be present on any of the phase-voltage or current inputs for the M87x to
attain Frequency lock.

7.8 Changing Transformer Ratios

The M87x has the capability to store values for Current Transformer (CT) and Potential
Transformer (VT) turns ratios. The VT and CT values are factory set to 1:1 CT and 1:1 VT.
These values can be entered into the M87x over the network or via the Configurator
software, and will be stored in internal non-volatile memory located on the Signal Input
Module. All measurements are presented in primary units, based on these ratios. Refer to
the appropriate protocol manual for information on changing transformer ratios.

7.9 User (External Transformer) Gain and Phase Correction

It is possible to correct for both gain and phase errors in external current and voltage
instrument transformers connected to the M87x, when these errors are known. These
Correction Factors can be entered via a protocol or by using the 70 Series Configurator
Software.
User Gain Correction is a multiplier (from -2 to +2) that can be used to adjust for known
gain errors in the system. User Gain Correction is "1" by default. For example, a gain
correction of 1.01 would increase the effective ratio by 1%. Entering a negative number
will reverse the phase of an input.
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ML0021 August 15, 2018 Copyright 2018 Bitronics, LLC