ITech IT9121 Series, User Manual

Page: 44 / 60

Harmonic
Measurement Function
Copyright © Itech Electronics Co., Ltd. 36
denominator.
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
( )
( )
Total U
k U
( )
( )
1 U
k U
Harmonic distortion factor of
current
( )
( )
Total I
k I
( )
( )
1 I
k I
Harmonic distortion factor of
active power
( )
( )
Total P
k P
( )
( )
1 P
k P
Total harmonic distortion rate
of voltage
( )
( )
Total U
k U
k
∑
=
max
2
2
( )
( )
1
max
2
2
U
k U
k
∑
=
Total harmonic distortion rate
of current
( )
( )
Total I
k I
k
∑
=
max
2
2
( )
( )
1
max
2
2
I
k I
k
∑
=
Total harmonic distortion rate
of active power
( )
( )
Total P
k P
k
∑
=
max
2
( )
( )
1
max
2
P
k P
k
∑
=
NOTE
Total harmonics are calculated as follows:
( ) ( )
∑
=
=
max
0
2
k
k U Total U
,
( ) ( )
∑
=
=
max
0
2
k
k I Total I
,
( ) ( ) ∑
=
=
max
0 k
k P Total P
. 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, IT9121 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
results may not be obtained. When the test object is a switch type power supply