'SNAP-SHOT' DC CURRENT MEASUREMENT

A Rogowski current transducer can be made to measure a large dc current on an intermittent spot-checking basis. The transducer comprises a very sensitive Rogowski coil (Vs/A) and a low gain active integrator with an extended low frequency 3dB bandwidth.

Rogowski coils do not measure direct currents. In order to make the Rogowski transducer measure a DC current the user of the transducer has to effectively create a current pulse (an ac current) from the DC. This is achieved as shown below

The Rogowski coil sees a change of current from zero to the DC value in a time determined by how quickly the user wraps the coil around the conductor. Once the user has created the pulse of current the peak of the pulse, (the dc level), will begin to drift at a rate determined by the droop of the transducer.

As explained in the CWT application notes.pdf in the section on low frequency perfomance there is a compromise between improving the droop rate and the minimum current that can be measured. Practically this sets the minimum DC current that can be measured to around 1kA.

The integrator must also have a method of zero adjustment which has to be set before a measurement can be taken.

Figure 2. shows a 10kA rated Rogowski DC current transducer made for measuring current in bus-bar sets supplying the electrode current in an aluminium smelting plant. The digital display provides a reading of DC current accurate to ±1%. This reading is valid for 5 seconds, beyond which time the reading will start to change due to the droop of the transducer. The unit must be reset to take another measurement.