The Impact of Intermittent Renewable Energy Generators on Lesotho National Electricity Grid

Abstract

Lesotho is confronted with huge challenge of low electricity access, with 63.9 % of the population lacking access to electricity. Lack of electricity impedes both economic and social development. However, Lesotho has abundant renewable energy resources that can be exploited through large integration of renewable energy sources. The inherent variability and uncertainty of renewable energy sources (solar-PV and wind) creates both operational and planning challenges for the power system. This results in the reluctance of the power system operators integrating largescale renewables to the national grid due to the power system stability problems. The characteristics of the intermittent renewable energy generators mandates that careful grid impact studies be performed in ensuring that the power grid is operated stably. The thesis focuses on the impact of the Intermittent Renewable Energy Generators (IREGs) on the power stability of Lesotho electrical grid considering both solar photovoltaic (PV) and wind generation at Ha-Ramarothole and Letseng respectively. The integration of IREGs involves both steady state and dynamic analysis of the electrical network. To this aim, the thesis assesses the impact of the IREGs on the stability of Lesotho electrical network at transmission level. In addition, maximum allowable penetration levels were determined at the point of interconnection. Load flow simulations were performed to assess the steady state performance of the electrical network. Furthermore, the transient analysis was performed by applying the 3-phase short circuit at the critical points of the network and observing how voltage, frequency and rotor angle stability were affected and evaluated against grid code of Lesotho. The simulations were performed using DigSILENT PowerFactory software, which was used to model the electrical network of Lesotho. The maximum allowable penetrations for solar was about 19 % at substation at Ramarothole while for the wind it was found to be 27 % at Letseng substation. The simulations revealed that increased penetration of the IREGs led to grid instability. For all the simulations, frequency stability was observed except for the penetration of 36 MW for solar farm. The voltage ii violations at the Tlokoeng substation of 1.051 p.u. resulted from penetration limit of 52 MW capacity of the wind farm at Letseng. The solar penetration limit resulted from the rotor angle instability as increased penetration resulted in large rotor angle oscillations.