Assessment of Quasi-Stationary Power Quality Phenomena in DC Power Systems

• The paper focuses on the main quasi-stationary PQ (Power Quality) disturbances commonly encountered in dc power systems, i.e., dc magnitude deviation and ripple. • For the most common indices used for quantifying the intensity of these phenomena (average and rms dc magnitude, rms and peak-to-peak value of ripple, spectral component amplitudes of ripple), measurement procedures, commonly adopted, are analyzed and discussed, through analytical modeling and experimental applications, considering important aspects that affect the accuracy and reliability of results in field measurements: reciprocal effects of the presence of PQ phenomena, desynchronization of the analyses, presence of noise, and quantization due to analog-to-digital conversion. • Then, advanced signal processing methods, suitable to be adopted in standards for dc PQ compatibility level surveys, are proposed to improve the accuracy and reliability of results. • For analysis in the time domain, the presented procedures are aimed at the mitigation of the effects of spikes, noise, and slow variations. • For analysis in the frequency domain, a practical method of grouping spectral components, similar to that adopted for harmonics in ac systems, is proposed to cope with spectral dispersion of ripple components. • The presented procedures are applied to a concrete measurement situation of a real LVDC (Low Voltage Direct Current) grid. The adoption of dc architectures for electricity distribution has been increasingly considered to reduce the number of energy conversions and to exploit their inherent ability to cope with peaks in energy supply and demand. Furthermore, dc systems have been commonly and widely adopted for high-voltage transmission systems and public transport for many years. Despite this, the Power Quality (PQ) assessment in dc power networks is still unstandardized. In the scientific literature, various indices have been proposed for the quantification of dc PQ. However, not all the aspects encountered in practical applications are adequately accounted for, and their effects regarding the impact on the measurement results are not fully analysed. This paper focuses on signal processing for the analysis of PQ of dc networks in stationary or quasi-stationary conditions aimed at compatibility level assessment. The influence quantities that affect the measurement results of the most commonly adopted procedures for determining the PQ index determination are identified and modelled, and their impact is analytically calculated. Advanced signal processing methods, suitable to be adopted in standards for dc PQ compatibility level surveys, are proposed to improve the accuracy and reliability of results. The rationality and effectiveness of the proposed procedures are shown with analytical calculations and in real measurement situations.