User manual

5 Power quality - a guide

This type of current transformer has several distinguishing

features. It may be used to measure very high currents and its

power consumption is low. Magnetizing current causes a phase

shift (tenth of a degree), which may introduce an error in power

measurement (especially at low power factor). The disadvantage

of this type of clamps is the core saturation when very high cur-

rents are measured (above the nominal range). Core saturation

as a result of magnetizing hysteresis leads to significant meas-

urement errors, which may be eliminated only by the core demag-

netization. The core becomes saturated also when the measured

current has a considerable DC component. Undeniable disad-

vantage of hard clamps is their significant weight.

Despite these drawbacks, CT clamps are currently the most widely used non-invasive method for

measuring alternating currents (AC).

Together with the analyzer, you can use the following types of CT clamps for measuring alter-

nating currents:

 C-4, with a nominal range of 1000 A AC,

 C-6, with a nominal range of 10 A AC,

 C-7, with a nominal range of 100 A AC.

5.1.2 Clamps for measuring alternating and direct currents (AC/DC)

In some situations it is necessary to measure the current DC component. For this purpose the

user must apply clamps with a principle of operation different than a traditional current transformer.

Such clamps operate basing on "Hall effect" and include a built-in Hall sensor (called also 'hallo-

tron'). In brief: the effect is based on the occurrence of an electrical voltage on the walls of the

conductor, through which an electric current flows and which is in magnetic field of direction trans-

verse to the induction vector of this field.

Current clamps based on this phenomenon may measure both DC and AC current components.

The conductor with current located inside the clamps generates a magnetic field which concentrates

in its iron core. In the core slot, where the two parts of clamps meet, a semiconductor Hall sensor

is located, and its output voltage is amplified by battery-powered electronic circuit.

Clamps of this type usually have a current-zero adjustment knob. To adjust the current zero,

close the jaws (no conductor inside) and turn the knob until DC indication is zero.

Sonel S.A. offers this type of clamps: C-5 with a nominal range of 1000 A AC / 1400 A DC.

Clamps of this type have a voltage output and for nominal current of 1000 A the provide voltage of

1 V (1 mV/A).

5.1.3 Flexible clamps

Flexible clamps (Flexible Current Probes) operate on a different physical principle than the cur-

rent transformer. Their most important part is Rogowski coil, named after Walter Rogowski - a Ger-

man physicist. It is an air-core coil wound around a conductor with current. Special design of the

coil allows leading out its both ends on the same side, thus facilitating clamp placement around the

conductor (the return end is placed inside the coil at its entire length). The current flowing through

the measured conductor causes centric magnetic field lines which due to the self-induction phe-

nomenon induce the electromotive force at the end of the coil. This voltage, however, is proportional

to the rate of current change in the conductor, and not to the current itself.

Fig. 29. Hard clamps

with voltage output