ABB PQFA Instruction Manual page 54

PQFA Instructions Manual
On the scopemeter screen, two waveforms should appear. The voltage
1
waveform should be approximately a sine wave and the current waveform
would normally be a well distorted wave because of harmonic distortion.
Usually, it is quite easy to extrapolate the fundamental component as it is the
most important one (Figure 9.3).
I
I1
Figure 9.3. Extrapolation of fundmental component from a distorted
waveform.
From the fundamental component of both signals, the phase shift must then
be evaluated (Figure 9.4). The time ?T between zero crossing of the rising
(falling) edge of both traces must be measured and converted to a phase shift
? by the following formula:
?
T
? ? ?
* 360
T
1
where T is the fundamental period duration.
1
For an inductive and non regenerative load, the current signal should lag the
voltage by a phase shift lower than 90°.
U
? T
I1
T
1
Figure 9.4. Phase shift evaluation between two waveforms.
1
If the earthing of the system is bad, the phase to ground voltage may appear like a very distorted waveform. In this
case, it is better to measure the phase to phase voltage (move the black clamp to the phase L2) and substract 30° on
the measured phase shift.
Asea Brown Boveri Jumet S.A.
54