What are Rogowski Coils and how to use them?
In the Blog we discuss How Rogowski Coils work and how the sensitivity output is related to the Integrator output
Output of Rogowski Coils (dv/dt)
Rogowski Coils output a dv/dt signal which is a velocity signal around mV/kA. for example LEM Rogowski Coils have sensitivity of 22.5mV/kA (22.5 mV per 1 thousand amps)
In order to get around this issue, and give a linear output like a Current Transformer (CT) a mathematical integration process is required. The two brands we deal with have the following output versions
Magnelab
RCS Series 0.333V (AC) Fixed measurement (set at factory for up to Infinite range)
RCS-VAR Series 5 Amp (AC) Multirange measurement. 4 Selectable ranges 500,1000,2500, and 5000 Amp.
LEM
AI-PMUL Series 0.333V Multirange measurement. 12 Selectable ranges 100, 200, 300, 400, 500, 600, 800, 1000, 1500, 2000, 4000, and 5000.
AI-P1A Series 1 Amp Multirange measurement 12 Selectable ranges 100, 200, 300, 400, 500, 600, 800, 1000, 1500, 2000, 4000, and 5000.
Fixed vs Multirange Integrators
Fixed type means that you need to know the current you wish to measure much like standard Current Transformer (CT)
The response factor is defined for amplitudes of sinusoidal steady state signals of the given frequencies (50Hz and 60Hz).
Then the quotient of the given response factors 2.4 / 2.0 = 1.2 is exactly what you expect from the formula 60Hz/50Hz = 1.2.
Direct Connect (Internal CT) Type Meters
Rogowski Coils can not be used on Direct connect meters of meters which require to take the voltage reference from the CT input.
Direct Rogowski Coil Connected Meters
Manufacturers of Meters and Rogowski Coils offer the ability to do the integration function within the meter itself. Note the coil sensitivity needs to be matched to the meter and in many case the manufacturer does not publish this information although you may be able to work it out by trial and error and with a current clamp meter.
Long cable runs using Rogowski Coils
Due to the low VA/Burden for 1A/5A output integrators, and the high losses using 0.333V output integrators, cables may be extended only on the coil side of the integrator. in the case of Magnelab the readings have been tested for up to 1.5 km cable extension and the error was within class 1 specifications.
Q: Why can we extend the coil cable so long?
A: The output of the coil is a dV/dt or rate of change based on the current being measured. The absolute value is not important, only the rate with which the signal is changing is important. Therefor the integrator will read the correct dV/dt regardless and give the correct linear measurement on the output. The same as you expect from a standard current transformer.
For further clarification please contact us.
