Bitronics M651, User Manual

Page: 60 / 80

60
ML0039 November 14, 2013 Copyright 2013 Bitronics, LLC
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 100 Amp load (if the Denominator was
set at 100 Amps). In the M651, Current Demand Distortion is implemented using
Equation 3. The TDD equation is similar to Harmonic Distortion (Equation 2), except
that the denominator in the equation is a user-defined number. This number, I
L
, is
meant to represent the average load on the system. The denominator I
L
is different for
each phase and neutral, and is set by changing the denominator values within the
M651.
Note that in Equation 3, if I
L
equals the fundamental, this Equation becomes Equation
2 - Harmonic Distortion. In the instrument this can be achieved by setting the
denominator to zero amps, in which case the instrument will substitute the
fundamental, and calculate Current THD.
Note that there is a separate, writeable denominator for each current input channel.
The TDD Denominator Registers are set by the factory to 5 Amps (secondary), which
is the nominal full load of the CT input with a 1:1 CT. These writeable denominators
can be used in conjunction with the distortion measurements to obtain the magnitudes
of harmonics, in other words, convert from percent to amps. This is simply done by
multiplying the percent TDD by the TDD Denominator for that phase, and the result
will be the actual RMS magnitude of the selected harmonic(s). This technique can
also be used if the THD mode (denominator set to zero) is used, by multiplying the
percent THD by the Fundamental Amps for that phase.
6.11.3 Fundamental Current
Fundamental Amps are the nominal component (50/60 Hz) of the waveform. The
M651 measures the magnitude of the fundamental amps for each phase. These
measurements can be used in conjunction with the distortion measurements to obtain
the magnitudes of harmonics, in other words, convert from percent to amps. As was
mentioned previously, this is simply done by multiplying the percent THD by the
63
2
2
% 100%
h
h
L
I
TDD
I
=
= ×
∑
Equation
3
- Current TDD