The design of a hand-held K-factor meter using an air-core current transformer

This paper describes in detail the design of a portable microcontroller-based K-factor, or transformer harmonic derating factor, meter. This innovative instrument is based on the time-domain method for K-factor determination which relies on the ratio of the RMS value of the time derivative of a current to the RMS value of this same current. The meter employs the properties of an air-core CT or Rogowski coil to provide the current time derivative and subsequent numerical integration to obtain the current. It thus avoids both the noise associated with differentiation usually associated with the approach as well as the computational effort associated with the FFT in the frequency domain approach to K-factor measurement. In addition the instrument incorporates thermal demand models for both K-factor and RMS current to assess transformer harmonic heating over a user selected time interval. A MOTOROLA MC68HC16-based prototype of the meter has been built and tested with a variety of actual and synthesized power system waveforms