Harmonic impedance measurement of 25 kV single phase AC supply systems

Noninvasive techniques of harmonic impedance measurement require the natural creation of sufficiently high harmonic distortion to enable accurate measurements. Energisation of a power transformer creates such a disturbance. The precision and accuracy of transducers play an important role and the use of a Rogowski coil and a capacitor voltage divider with no transformer at the bottom end give sufficiently accurate results. The accurate calculation of harmonic voltages and currents which are needed to calculate the harmonic impedance requires the intelligent use of discrete Fourier transform and windowing. Also steps must be taken to minimise spectral leakage. Simulations and experimentation show that the proposed method of harmonic impedance measurement by analysing transformer inrush transients is suitable for the determination of low order harmonic impedance values in AC supply systems