ELECTROSTICALLY SHIELDED ROGOWSKI COILS

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 transient voltage disturbances are so large that fitting an electrostatic shield is a necessity. An example of such a shield is shown in the picture below. Care has to be taken to ensure that the screen does not form a shorted turn around the Rogowski coil or the bandwidth will be severely limited to a matter of a few 10's kHz.

Rogowski coil with helically wound copper electrostatic shield

As a general rule an electrostatic screen is only required if the Rogowski coil is adjacent to a source of dV/dt exceeding a few 10's kV/µs.

As an example, fitting a close electrostatic shield to a standard CWT15 with a 700mm coil will reduce the high frequency bandwidth from typically 10MHz to around 3MHz.

PEM has developed a model of the high frequency behaviour of the Rogowski coil and electrostatic shield. This model and related practical results were the subject of a paper at the IEEE-Power Electronic Society 2004 Conference. This paper can be downloaded from Publications.