Beoordeling van foutstroombijdragestrategieën door vermogenelektronische gedistribueerde bronnen

摘要

It is a clear trend that the share of Distributed Generation (DG) in the grid increases. Many DG units are Converter Based DG (CBDG) units and it is expected that the number of CBDG units will increase in the future. Due to the variable nature of most of these CBDG units, there will be occasions when the share of CBDG units is much higher than their average share in the energy production. During these periods, several of the conventional synchronous generators will be disconnected from the grid and scenarios with a large number of CBDG units and very few synchronous generators arise.The goal of this dissertation is to evaluate the impact of these scenarios on the fault currents and voltages during balanced and unbalanced faults. On the one hand, fault currents have a direct impact on the protection system of the grid. On the other hand, the grid voltages during a fault determine the impact of the fault on the loads and the generation units. Therefore, both the fault currents and fault voltages strongly influence the reliability of the grid.First, it is demonstrated that existing control systems of CBDG units are able to flexibly control the fault current injections of CBDG units. Thus, their fault behaviour is a design parameter. Several specific control aspects related to the injection of negative sequence currents during faults are explained.Since detailed electromagnetic transient simulations are considered computationally too intensive for scenarios with multiple CBDG units, this dissertation investigates which simplified method is able to evaluate the fault currents and fault voltages in these scenarios. As the fault behaviour of CBDG units is a design parameter, it is clear that any method has to take into account the control objectives of the CBDG units. Based on these requirements, a simplified calculation framework is developed and validated. This framework is then used to evaluate different current contribution strategies during balanced and unbalanced faults in scenarios with a high share of CBDG.For balanced faults, the influence of different voltage support settings on the short-circuit power in the grid is investigated. It is shown that CBDG units, with the appropriate voltage support settings, can contribute to the short-circuit power of the grid. This way, the CBDG units limit the drop in short-circuit power at the higher voltage levels when they replace conventional generation. When this reduction of the short-circuit power at the higher voltage levels is limited, the fault currents at the lower voltage levels, supplied by the higher voltage levels, do not change significantly. This avoids a complete redesign of the existing protection systems of typical European medium and low voltage grids in scenarios with a high share of CBDG and little conventional generation, as these protection systems rely on the magnitude of the fault currents. Locally, the voltage support of CBDG units can result in a (limited) increase of the short-circuit power. The voltage support can then be applied until the short-circuit power limits of the local grid are reached.For unbalanced faults, scenarios with a high share of CBDG and little conventional generation are evaluated. When only positive sequence voltage support is applied, and negative sequence currents are blocked by the CBDG units, this can lead to very low fault currents during unbalanced faults. These low fault currents would require a complete redesign, including huge investment costs, of the existing protection systems at the lower voltage levels of the grid, as these protection systems rely on the magnitude of the fault currents. In addition, very low fault currents result in a reduced reliability of the grid, as faults at lower voltage levels then have a significant impact on the higher voltage levels. The remaining synchronous generators in the grid also experience additional stress when CBDG units only provide positive sequence voltage support during unbalanced faults. When CBDG units provide both positive and negative sequence voltage support, these drawbacks are avoided: the existing protection systems of typical European medium and low voltage grids do not require a complete redesign, faults on the lower voltage levels do not have a significant impact on the higher voltage levels and the remaining synchronous generators don't experience additional stress.