User manual

PQM-702, PQM-703 Operating Manual
64
Rogowski coil has some undeniable advantages
compared with current transformers. As it does not have a
core, the core saturation effect is eliminated; thus being a
perfect instrument to measure high currents. Such coil has
also an excellent linearity and a wide pass band, much
wider than a current transformer, and its weight is much
smaller.
However, until recently the wider expansion of flex-
ible clamps in the current measurement area was difficult.
There are some factors that hinder the practical implemen-
tation of the measurement system with a Rogowski coil.
One of them is a very low voltage level which is induced
on the clamps (it depends on geometrical dimensions of
the coil). For example, the output voltage for 50 Hz fre-
quency of the F-series flexible clamps (to be used with the
analyzer) is approx. 45µV/A. Such low voltages require the
use of precise and low-noise amplifiers which of course
increase the costs.
As the output voltage is proportional to the current
derivative, it is necessary to use an integrating circuit; gen-
erally, the flexible probes comprise a Rogowski coil and
an analogue integrator circuit (characteristic battery-pow-
ered module). On the integrator output the voltage signal
is available and proportional to the measured current and suitably scaled (for example 1mV/A).
Another problem concerning Rogowski coil, is its sensitivity to external magnetic fields. A per-
fect coil should be sensitive only to the fields closed within its area and should totally suppress
external magnetic fields. But this is a very difficult task. The only way to obtain such properties is
very precise manufacturing of the coil, with perfectly homogeneous windings and impedance as low
as possible. It is the high precision which results in a relatively high price of such clamps.
The user may connect the analyzer to the following flexible clamps offered by Sonel S.A.:
 F-1, with coil perimeter equal to 120 cm,
 F-2, perimeter of 80 cm,
 F-3, perimeter of 45 cm.
All these clamps (probes) have identical electrical parameters. Peak current that may be meas-
ured by connecting the analyzer is 10 kA (this restriction results from the properties of input chan-
nels, and not the probe itself).
5.2 Flicker
In terms of power quality, 'flicker' means a periodical changes of light intensity as a result of
fluctuations of voltage supplied to light bulbs.
The flicker measurement function appeared in the power quality analyzers when it turned out
that this phenomenon causes discomfort, irritation, sometimes headache, etc. The luminous inten-
sity fluctuations must have a specified frequency, they cannot be too slow, as the human pupil is
able to adapt to changes in illumination; they cannot be too fast because the filament inertia elimi-
nates these fluctuations almost totally.
Studies have shown that the maximum discomfort occurs for frequencies around 9 changes per
second. The most sensitive light sources are the traditional light bulbs with a tungsten filament.
Halogen bulbs, which filaments have much higher temperature, have also much higher inertia,
which reduces the perceived brightness changes. Fluorescent lamps have the best flicker "re-
sistance", as due to their specific properties they stabilize the current flowing through the lamp dur-
ing the voltage changes, and thus reduce the fluctuations.
Flicker is measured in perceptibility units, and there are two types of flicker: short-term P
st
,
Fig. 30. Rogowski coil