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Harmonic Measurement Function
Copyright © Itech Electronics Co., Ltd. 37
The distortion factor calculation formulas and methods of different
measurement functions are as follows:
Measurement
function
Calculation formula and method
%r
%f
Harmonic distortion
factor of voltage
TotalU
kU
1U
kU
Harmonic distortion
factor of current
TotalI
kI
1I
kI
Harmonic distortion
factor of active power
TotalP
kP
1P
kP
Total harmonic
distortion rate of
voltage
TotalU
kU
k
max
2
2
1
max
2
2
U
kU
k
Total harmonic
distortion rate of
current
TotalI
kI
k
max
2
2
1
max
2
2
I
kI
k
Total harmonic
distortion rate of active
power
TotalP
kP
k
max
2
1
max
2
P
kP
k
NOTE
Total harmonics are calculated as follows:
max
0
2
k
kUTotalU
,
max
0
2
k
kITotalI
,
max
0k
kPTotalP
. k indicates the harmonic order, and max indicates the upper limit of
analysis orders. max is determined by the frequency of PLL source automatically, 50 orders
at most.
PLL source
In the harmonic mode, IT9121E adopts PLL to multiply the frequency of the
input signal. The frequency multiplication output signal is used as the A/D
sampling clock in the instrument so as to achieve the purpose of synchronous
sampling. Therefore, the frequency of the signal of the PLL source can
determine the A/D sampling clock in the instrument. However, the quality of the
signal of the PLL source may affect the stability and accuracy of sampling data.
PLL source selection
The voltage or current of the input unit can be selected as the PLL source. Used
for determining the fundamental wave cycle as the reference for analysis of
harmonic orders. The fundamental wave frequency of the PLL source is 10Hz to
1.2kHz.
Considerations for selection of PLL source
Please select the signal with the same cycle as that of the harmonic
measurement object signal. In order to stably measure harmonics, select the
input signal with little distortion as the PLL source. If the fundamental wave
frequency of the PLL source changes or the fundamental wave frequency
cannot be measured as a result of waveform distortion, accurate measurement
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