The main difference between Rogowski Coils and CTs is that Rogowski Coil windings are wound over an air core (non-magnetic), instead of over an iron core. Therefore, Rogowski Coils are linear since the air core cannot saturate.
The mutual coupling between the primary conductor and the secondary winding in Rogowski Coils is much smaller than in CTs. Therefore, Rogowski Coil output power is small, so it cannot drive current through low-resistance burden like CTs are able to drive. Rogowski Coils can provide input signals for numerical relays that have a high input resistance; therefore, these devices must measure the voltage across the Rogowski Coil secondary output terminals. Relay manufacturers tend not to support technology since the signal processing required to extract the power frequency signal for applications in phasor-based protective relays and must be designed to accept these types of signals. There are other disadvantages to the Rogowski Coil as well, which include:
- The output of the coil must be pass through an integrator circuit to obtain the current waveform
- The integrator circuit requires a power supply of 3 – 24 Vdc
- It cannot measure dc current
- Sensitivity – compared with a CT, the Rogowski Coil has lower sensitivity due to the of a high permeability curve