Quantifying the harmonic content of interference currents in traction applications resulting from VSDs


A customised RCTi installed on a rail measuring 30-60kHz interference currentsA common problem in electrical engineering is the measurement of small high frequency interference currents or low strength higher order harmonics in the presence of a large fundamental current.

With traditional methods of current measurement the primary transducer, often a Current Transformer or Hall Effect device, has to be of sufficiently large rating to not saturate in the presence of the fundamental component. Thus the primary transducer is expensive, bulky and in combination with the power analyser or oscilloscope often without the resolution necessary to measure these small higher frequency currents accurately.

The Rogowski current transducer (either RCTi or CWT) offers a solution to this problem:

  • the coil size can be specified independently of measured current
  • the fundamental current component cannot damage the transducer and the Rogowski coil does not suffer from magnetic saturation
  • it is a simple modification to tune the CWT or RCTi bandwidth to attenuate the fundamental components and enhance the sensitivity to provide a large signal to noise ratio at those frequencies of interest.
  • and the RCTi has a sufficiently high (-3dB) cut-off to measure accurately into the MHz range

PEM Ltd has supplied a number of such transducers applications in the traction industry. For example to facilitate EMC assessment of a new national railway PEM has supplied a customised RCTi transducer. The railway signalling system passes coded messages between a trackside cable and the train antenna at 30-60kHz. The EMC issue concerns the traction harmonic currents flowing in the third rail and running rails which can couple inductively into the signalling cable and also cause interference at the antenna. It is necessary to provide a certain signal to noise ratio for the transmitted data. Therefore an optimised RCTi transducer which is flexible and clip-around to easily wrap around the rail track is required. The low frequency cut-off is optimised to give a cut-off of 4.2kHz with a flat response at 10kHz but -40dB of attenuation at 1kHz. Thus the RCTi can measure small interference currents in the rail of the order of a few mA’s in the range 30-60 kHz yet reject much larger components (of the order of A’s)  from power frequency sources.