The effect of electrostatic screening of Rogowski coils designed for wide-bandwidth current measurement in power electronic applications

Rogowski current transducers are ideal for monitoring pulsed and transient currents in power electronic applications as they have a high bandwidth and low insertion impedance. It is sometimes necessary to fit an electrostatic shield to the Rogowski coil. Large dV/dt disturbances in close proximity to the coil produce measurement interference due to capacitive coupling. A shield eliminates this coupling. However Rogowski coils are not normally fitted with an electrostatic shield since the resulting increase in coil capacitance reduces the high frequency bandwidth. Also the coil is bulkier, less flexible and more difficult to manufacture. There are applications where local dV/dt disturbances are so large that fitting a screen is a necessity. This paper examines the compromise between the high frequency performance of the Rogowski transducer and the effective electrostatic shielding of dV/dt interference. A theoretical model for the mechanism by which large close coupled sources of dV/dt affect the behaviour of the Rogowski coil is presented. In addition the paper describes a model for the high frequency performance of a shielded Rogowski coil. These models are verified using a purpose built test rig capable of generating large dV/dt disturbances in excess of 40 kV/us.