Non-magnetic current sensor or (Rogowski coil named after its inventor) is recently used by modern protection and metering purpose in replacement of conventional current transformer. Its greatest advantage is that- it contains no magnetic material.
Why non-magnetic current transformer is better than conventional current transformer
Conventional current transformer made of magnetic material has this limitation of non linear behavior in saturation with high current out of range but Non-magnetic current sensors give an output voltage signal proportional to the derivative of the primary current. Because of this they can be used across a wide range of currents.
Working principle of non magnetic current transformer
r: adjustment resistor, E: output voltage, RCA: non-magnetic current receptor
Non-magnetic current sensors work according to Rogowski’s principle:The primary current i produces a magnetic field in each secondary turn the flux variations of which are opposed by an e.m.f. “e” induced by the cause that produces them (the variation in i).
Construction of non magnetic current transformer
Rogowski coil current transformer is an advance CT without magnetic core. All it has only helical coil of wire with the lead from one end returning through the centre of the coil to the other end, so that both terminals are at the same end of the coil. The measured conductor is place in the center of this
So in rogowski coil current a helical coil is coiled around the plastic or rubber tube and then the second terminal is passed back through the center of the toroid core (a plastic or rubber tube) and connected along the first terminal.
The principle of this sensor was described by Rogowski in 1912. As opposed to magnetic current transformers, it contains no ferromagnetic material; a fact that gives it perfect linearity across a
range of currents.
Advantage of Non-magnetic current transformer
Rogowski coil current transformer is advantageous due its absence of magnetic core, as magnetic core material has its own disadvantages of saturation, non linear behavior, high inductance, slow response. Advanced CTs wit hout a magnetic core (Rowgowski coil) have been developed to save space and reduce the cost of GIS. The output signal is at a low level, so it is immediately conver ted by an enclosuremounted device to a digital signal. It can be transmitted over long distances using wire or fiber optics to the control and protective relays.
Disadvantage Non-magnetic current transformer
The only disadvantage of Rogowski coil current transformer is that it need an electrical (or electronic) integrator circuit which need external power.