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Abstract-The spectrum-estimation methods based on the Fourier transform suffer from the major problem of resolution. The methods were developed and are mostly applied for periodic signals under the assumption that only harmonics are present and the periodicity intervals are fixed, while periodicity intervals in the presence of interharmonics are variable and very long. A novel approach to harmonic and interharmonic analysis based on the "subspace" methods is proposed. Min-norm and music harmonic retrieval methods are examples of high-resolution eigenstructure-based methods. Their resolution is theoretically independent of the signal-to-noise ratio (SNR). The Prony method as applied for parameter estimation of signal components was also tested in the paper. Both the high-resolution methods do not show the disadvantages of the traditional tools and allow exact estimation of the interharmonic frequencies. To investigate the methods, several experiments were carried out using simulated signals, current waveforms at the output of an industrial frequency converter, and current waveforms during out-of-step operation of a synchronous generator. For comparison, similar experiments were repeated using the fast Fourier transform (FFT). The comparison proved the superiority of the new methods.

The research presented in this paper is aimed at the higher frequency (HF) voltage distortion in public low voltage (LV) grids in the range between 2 kHz and 40 kHz. The particular focus lies on the emission caused by modern photovoltaic (PV) inverters. Based on laboratory measurements, the general characteristics of the emission spectrum of these inverters are discussed and a suitable evaluation method is proposed. The next part presents the results of a long term measurement campaign recording voltage distortion levels at 25 sites up to 40 kHz. Grids without PV installations show very low distortion levels. In grids with PV the level is closely linked to the location of the inverter in relation to the measurement site. In general, the lowest distortion levels were recorded at the transformer LV bus bar. The highest levels were reached at the point of connection (POC) of an inverter. At all sites no customer complaints are known due to HF voltage distortion. The last part of the paper presents a case study, where the connection of a machine with inverter technology caused HF distortion levels, which led to multiple device malfunction and noise in household appliances and subsequently to customer complaints.