FAQ
1. For conventinal CT there is no any factor for calibration. In which theory does the current sensor calibration factor base on ?
The amplitude error of a current sensor is in the practice constant and independent of the primary current. Hence it can be corrected in the secondary device by a calibration factor given separately for each individual sensor. A sensor fulfilling the requirements of e.g. class 3 without the calibration factor can fulfil the requirements of class 1 with the use of the calibration factor. For voltage sensors calibration factors are not used.
Irregularities in material dimensions and variations in the manufacturing process affect the accuracy of a Rogowski coil principally in the same way as a conventional current transformer. However, turn corrections cannot easily be made without disturbing the geometry of the coil. Hence the correction is made, if needed, with a calibration factor, for which modern protection and control IEDs (Intelligent Electronic Device) are prepared.
The formula is as follows:
k= 1 / ( 1+ ( x / 100))
k = calibration factor
x = error on 100% (rated current or rated voltage)
In test reports, there is usually mentioned the accuracy with and without calibration. Marking is as follows:
11 (32)
1 with calibration factor 2 without calibration factor
So this kind of marking means that sensor itself is class 3, but when using it with monitoring unit and calibration factor, accuracy is class 1.
2. How burden is worked out in sensor ?
Sensor doesn't give power, there is only very low voltage signal for the relay.
This signal is all needed for measurement/protection with relay. This enables high standardization level of sensors, and also makes ordering easier, because burden calculation is not needed.
3. Current sensors are linear and there is no saturation. Why there is max. current limitation ?
There are two reasons for giving limits of the primary current:
1. The rated continuous thermal current i.e. the highest steady-state current the primary current part can withstand without too high temperature rise.
2. The highest secondary signal the relay can correctly process. This is important in measurement applications, even if the limitation is not in the sensor.
4. Rogowski coil principle was invented in 1912. Why hasn’t Rogowski coil been used before ?
Rogowski coil couldn’t be used, beacause old relays weren’t advanced enough to handle small signal output of sensor. Improvement of protection and control IEDs (Intelligent Electronic Devices) made the use of Rogowski coil possible.
5. Is there any standards for sensors ?
ABB sensors are designed, manufactured and tested according to electronic voltage transformers standard IEC-60044-7 and electrical current transducers standard IEC 60044-8 DRAFT.