Novatech Bitronics M87X Series, Manual

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ML0021 August 15, 2018
40
Copyright 2018 Bitronics, LLC
32-bit number, some protocols may place further restrictions on the Demand Interval due
to limitations on numerical format. Please refer to the appropriate protocol manual for
details.
4.8 Harmonic Measurements (1-Cycle Update)
M87x instruments continually sample all inputs at 128 samples per cycle, and compute a
128-point Fast Fourier Transform (FFT) every cycle for each input. When combined with
high dynamic-range input of up to 140A
PEAK
and 600V
PEAK
, this allows the M87x to make
extremely accurate measurements of harmonics, regardless of crest factor. All harmonic
and harmonic-based values are calculated every cycle. Both magnitude and phase of
each harmonic are provided. In the following sections, Harmonic 0 indicates DC,
Harmonic 1 indicates the fundamental, and Harmonic N is the nth multiple of the
fundamental.
4.8.1 Voltage Distortion (THD) (1-Cycle Update)
Voltage Harmonic Distortion is measured by phase in several
different ways. The equation for Total Harmonic Distortion
(THD) is given in Equation 1. For Odd Harmonic Distortion,
the summation only uses harmonics where h is odd. For Even
Harmonic Distortion, the summation only uses harmonics
where h is even. Note the denominator is the fundamental
magnitude. For Individual Harmonic Distortion there is no
summation, only one component is used in the numerator.
4.8.2 Current Distortion (THD and TDD) (1-Cycle Update)
Current Harmonic Distortion is measured by phase in several
different ways. The first method is Total Harmonic Distortion
(THD). The equation for THD is given in Equation 2. For Odd
Harmonic Distortion, the summation only uses harmonics
where h is odd. For Even Harmonic Distortion, the summation
only
uses harmonics where h is even. Note the denominator is the
fundamental magnitude.
Alternatively, Current Harmonic Distortion can be measured
as Demand Distortion, as defined by IEEE-519/519A.
Demand Distortion differs from traditional Harmonic Distortion
in that the denominator of the distortion equation is a fixed
value. This fixed denominator value is defined as the average
monthly peak demand. By creating a measurement that is
based on a fixed value, TDD is a "better" measure of distortion
problems. Traditional THD is determined on the ratio of
harmonics to the fundamental. While this is acceptable for voltage measurements, where
the fundamental only varies slightly, it is ineffective for current measurements since the
fundamental varies over a wide range. Using traditional THD, 30% THD may mean a 1
Amp load with 30% Distortion, or a 100 Amp load with 30% Distortion. By using TDD,
these same two loads would exhibit 0.3% TDD for the 1 Amp load and 30% TDD for the
63
2
2
1
% 100%
h
h
I
T H D
I

 

63
2
2
1
% 100%
h
h
V
T H D
V

 

63
2
2
% 100%
h
h
L
I
T D D
I

 

Equation 1 - Voltage THD
Equation 2 - Current
Equation 3 - Current