Detection and suppression of parasitic DC voltages in 400V AC grids

Abstract

Grid connected static power converters inject parasitic DC currents due to the offset in current sensing, control imperfections, asymmetries in power switches and other secondary effects. Ever growing number of grid connected converters contributes to an increase of DC bias in AC grids, and this brings the cores of distribution transformers closer to saturation and increases their power losses. This paper provides sensitivity analysis of distribution transformers to the DC bias, and considers solutions for detecting and compensating the parasitic DC components in AC grids. Active compensation methods can be advantageously used in suppressing the DC bias at grid connection point of the power converter. The sensing approach proposed in this paper makes use of saturable ferromagnetic cores and a low cost DSP for signal analysis and processing. Proposed algorithm uses distortion of the magnetizing current of a parallel connected saturable core due to the bias. Experimental results demonstrate the capability for detecting and compensating the bias voltages far below 1 mV in 0.4 kV grids. The paper describes the principles of DC bias detection and it provides the guidelines for the proper design of magnetic components. High precision of the proposed DC bias sensing is thoroughly verified on the experimental setup connected to a 0.4 kV grid.

Characterization of nonlinear loads in power distribution grid

“…New subsystem in the power production, transport, and consumption emerge named micro-grids and the overall system is supposed to become a smart-grid. For example, due to the rise of the number of different kind of electricity sources even the frequency of the grid voltage may be considered as "unknown" asking for algorithms and software to be implemented in real time to extract the frequency value [2] and, based on that, to compute the amplitudes of the harmonics [3,4,5].…”

Experimental Detection of Transformer Excitation Asymmetry through the Analysis of the Magnetizing Current Harmonic Content

“…Among even harmonics of an AC excitation current in a steady state with DC bias, the second order harmonic exhibits some unique properties -an almost constant phase shift relative to the fundamental component and the phase jump of  as the DC bias changes the sign [8]- [10].…”