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 meter, or transformer harmonic derating factor meter. This innovative instrument is based on the recently developed time domain method for K-factor determination. K-factor is measured from 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 current transformer, or Rogowski coil, to acquire the time derivative of a current and then numerical integration, of this digitized signal, to obtain the current waveform itself. This method avoids the noise of the numeric differentiation, characteristic of the time domain approach to K-factor measurement, as well as the computational effort of the fast Fourier transform in the frequency domain approach. 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 prototype meter, using a MC68HC16 microcontroller, was built and tested with a variety of actual and synthesized power system waveforms.