Power control strategies for grid-connected converters during low voltage ride through operation

Abstract

Momentary voltage sags are very common phenomena in electrical power systems. Thus, the transmission system operators require that the generation units remain connected to the transmission mains while the voltage is within the values outlined by the grid codes, preventing the sudden loss of large generation energy blocks. In addition to non-disconnection, the current grid codes establish that wind and photovoltaic generators must assist in the voltage recovery through the positive sequence reactive current injection, increasing the positive sequence voltage at the point of common coupling. Grid-connected converters have greater current control flexibility in relation to conventional synchronous machines allowing, for example, unbalanced currents synthesize or with specific harmonic components. Hence, the calculation of reference currents can be modified to mitigate other quality issues during the fault occurrence. The present work evaluates different power control strategies, i.e., the calculus of the reference currents values for injecting certain power set-points, focusing on three-phase threewire systems. Where they will be compared in relation to the power oscillation and currents profiles, in order to study how these strategies can attenuate the effects of the voltage sags to the grid converters, reducing the voltage fluctuation and current ripple in the d.c. bus capacitors. In addition, is investigated how these strategies affect the voltage support at the point of common coupling. For this, equations, that are the basis of the analysis, are proposed, and they are validated through computational simulations. Two case studies are performed, the first one of a 2(MW) wind energy conversion system, to evaluate the impacts to the gridconnected converter and the second one of a 118(MW) generation plant to investigate the voltage support at the point of common coupling.