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Most existing power flow calculation methods use a swing bus as a reference node for the whole system. Increasingly, new distributed generation resources (DGRs) are being added to the grid. Sometimes, local demand or failure of the grid can result in independent micro-grids forming, which are known as `islanding' systems. However, current DGRs are often limited such that there is no single DGR which can balance the power demand and stabilize the frequency of the micro-grid, meaning that there is no swing bus from which the micro-grid can be managed. According to existing research, a DGR coupled with a dedicated energy storage system and suitable control strategy (here termed a distributed generation (DG) system) has the ability to adjust its output. This means that a DG system can respond dynamically to grid events. This means that a DG system can respond dynamically to grid events. In this paper, a new power flow calculation method (based on Newton-Raphson power flow solution) with good convergence is proposed that can accommodate the lack of a swing bus in an islanding system. This addresses power flow results and the frequency of the whole system. The method proposed is discussed in detail with examples of different DG systems with various adjustment coefficients and load models. The results are compared with those of a traditional power flow calculation method based around the use of a swing bus. In conclusion, this paper shows that the improved method is more appropriate for islanding systems with mesh topology and for micro-grid management with no swing bus.