The Impact of Intermittent Renewable Energy Generators on Lesotho National Electricity Grid
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Date
2020
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National University of Lesotho
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
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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.