Reconstruction of transient waveforms from phasors sampled at the fundamental frequency

The concept of Wide-Area Measurement Systems (WAMS) envisages the transmission of current and voltage phasor samples from remote Phasor Measurement Units (PMUs) to control centres, relays or controllers. Compression of the high frequency samples of sinusoidal waveforms into phasors is usually done using Discrete-Fourier Transform(DFT)-like algorithms. The phasor data is normally useful only when the system is in sinusoidal steady state, or nearly so. It is worthwhile to explore means to get reasonably accurate transient information as well, even if it means that some additional compressed information needs to be transmitted from the PMUs. Transient reconstruction has practical utility in the analysis of disturbances (at a remote control centre) and for flagging the initiation and clearing of network disturbances, which may be required for some WAMS analytics. In this paper, it is shown that one-cycle-delayed reconstruction of power system transient waveforms can be done with samples of the dc, fundamental and harmonic phasors reported at a rate of one sample per cycle obtained from the DFT algorithm. For power system transients, a few harmonics are usually adequate to reconstruct the transient waveforms with reasonable accuracy. The paper explains the reconstruction method, presents illustrative examples and discusses possible applications and practical issues.