The impact of intermittent renewable energy generators on Lesotho national electricity grid

Abstract

Utility-scale integration of solar photovoltaic (PV) and wind farms has gained momentum as countries pursue sustainable power systems. Increased penetration of solar PV and wind alters the operation of power grids as they have different electrical properties from conventional power plants. The paper assesses the dynamic and steady state impacts of intermittent (variable) renewable energy generators (IREGs) on the Lesotho power grid. Maximum allowable penetration of both solar PV and wind generators were determined for Ha-Ramarothole and Letseng sub-stations, respectively. Dynamic impact studies were performed by applying the short circuit fault at the bus bar with the least critical clearing time (CCT) and observing voltage, frequency and rotor angle as the renewable energy capacity was varied. Steady state voltage analysis was performed based on the hourly load of 2018 and generation of both IREGs and Muela Hydropower. Voltage, frequency and rotor angle stability were evaluated against the Grid Code of Lesotho. The impact studies were done using DigSILENT PowerFactory software. Maximum allowable penetrations for solar PV was about 19% at Ramarothole substation while for wind it was found to be 27% at Letseng substation. Simulations revealed that increased penetration of the IREGs led to grid instability. The solar farm penetration of 36 MW connected at 132 kV resulted in grid instability mainly from the rotor angle instability while the wind farm penetration of 52 MW connected at 88 kV resulted from an overvoltage of 1.051 p.u. at the neighboring 33 kV substation of Tlokoeng.