Rogowski Coil Test

A Rogowski coil is a flexible air-cored current transducer (CT) used for measuring AC current. Because it has no ferromagnetic core, it does not saturate. The raw output of the coil is typically proportional to di/dt (i.e., the induced voltage), therefore it is normally supplied together with an integrator so that the output becomes proportional to the current waveform. The transfer ratio is commonly expressed in mV/A. Consequently, when testing relays that use Rogowski-coil inputs, only accurate voltage sources should be used.

An important consideration when testing such relays with the AMT205 is that the unit’s main voltage sources do not provide sufficient accuracy for these applications. This is because their voltage range is ±420 V (840 V peak-to-peak), which reduces achievable resolution/accuracy.

Therefore, the voltage sources to be used are limited to the device’s Cross Voltage sources, which have a smaller range and higher accuracy.

To convert current sources to voltage sources (i.e., to use the Cross Voltage sources), in Hardware Configuration,

set the value in the Type column to Voltage.

Note that with this configuration alone, functions such as Distance and Differential testing cannot be performed directly. This is because these functions require outputs with defined Targets; if the selected source is originally a current source, the test execution may become challenging due to the way targets and outputs are allocated.

Therefore, to test relays of this type using the Vebko device and software, the software configuration must be adjusted as described below. Consider the Distance protection function as an example.

Testing this function requires three voltage sources with targets VL1-E, VL2-E, VL3-E and three current sources with targets IL1, IL2, IL3. Also note that when the relay input is provided via a Rogowski coil, all test signals must be voltage signals. module

Hence, we convert the three phases of Current Group A into Cross Voltage sources with targets VL1-E, VL2-E, VL3-E.

The Display Type shall be set to Voltage

and the Coefficient shall be entered according to the coil transfer ratio (e.g., 0.1). This means any value entered for the test will be multiplied by this coefficient and will appear as a voltage at the corresponding voltage output.

Next, the current channels must be configured. For this purpose, set Display Type to Current,

set the targets to IL1, IL2, IL3,

and enter the Coefficient according to the coil transfer ratio (e.g., 0.1).

The most critical point here is changing the Type from Current to Voltage, because the “current” input is provided by the Rogowski coil as a voltage signal.

With this configuration, the three phases of Current Group A are cross-assigned and converted to voltage outputs to feed the voltage input of the relay’s Distance function, and the three phases of Current Group B are cross-assigned and converted to the required “current” channels to feed the current input of the relay’s Distance function.