Voltage Stability Assessment of System with High share of Inverter Interfaced Generation - Long Term Voltage Stability Assessment using DIgSILENT Powerfactory simulations

Abstract

Abstract A significant amount of renewable energy-based generators are being connected to the distribution grid and provide reactive power support to the transmission grid in a less effective way compared to traditional centralized synchronous machine-based generation. Consequently, the voltage stability phenomenon is affected in the transmission buses. In this Master thesis, the impact of distributed generation (DG) on voltage stability at the transmission grid is studied. The analyses are done in a simplified two-bus system and in the IEEE/CIGRE Nordic32 benchmark model using DigSILENT PowerFactory simulations. The impact on the PV curves of the transmission buses has been evaluated considering different control modes of DGs at different control points. One of the synchronous generators in South Sweden is replaced by DG, and its impact on the maximum power transfer from North to Central Sweden has been studied. Furthermore, the loss of one transmission line between North and Central Sweden has also been studied. When one of the synchronous generators is replaced by DG in South Sweden, the control of the voltage at the DG terminal causes a reduction in transfer capacity compared to the current scenario of the synchronous generator. This is caused by an increase in impedance between the DG and the TSO bus. With voltage control at the 135 kV bus to 1 p.u., the transfer capacity is worse than the voltage control in the DG terminal scenario. DG tries to keep the voltage to 1 p.u. at the 135 kV bus by supplying Q appropriately, causing more reactive power demand compared to DG voltage control at its terminal. So, DG reaches its Q limit earlier. This results in reduced transfer capacity. In contrast, with Q control on the 135 kV bus, it is observed that the reactive power of the DG is not fully utilized, as the transfer capacity reaches its limit even before the reactive power limit of the DG is reached.