Browsing by Author "Mokeke, Sebota"

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    Adaptive, robust, and fault-tolerant control strategies for grid-connected renewable energy systems
    (National University of Lesotho, 2024) Nkhabu, Tsitso; Makhele, Molefe; Mokeke, Sebota
    Lesotho is currently facing a power generation-demand imbalance. The 2023/24 annual report by the Lesotho Electricity and Water Authority highlights a peak demand of 222.12 MW, which is nearly three times the country’s installed generation capacity of 74.7 MW. Consequently, the nation relies on costly power imports from Mozambique and South Africa to cover the shortfall. To address this issue, Lesotho plans to integrate renewable energy sources, specifically wind and solar, into its national grid. However, integrating these low-inertia and intermittent renewable power sources introduces grid stability challenges, as they are vulnerable to disturbances like load changes or grid faults. This research focused on designing control strategies capable of not only adjusting system voltage to accommodate uncertainties from power system dynamics, internal changes, and external disturbances but also stabilizing the system under small disturbances. The analysis was conducted on a grid-connected 38 MW wind power plant, using five control strategies: Constant Voltage (Const. V), Constant Reactive Power (Const. Q), Constant Power Factor (Const. cosɸ), Voltage Droop based on Reactive Power (voltage Q-droop), and voltage droop based on reactive power in the q-axis (voltage Iq-droop). These strategies were evaluated for their adaptability, robustness, and fault tolerance under two operational scenarios: variations in wind speed and fluctuations in the Point of Connection (POC) bus voltage. With a three-phase short circuit created and cleared on the high voltage (HV) bus bar, all control strategies are within a 5 % voltage deviation upon service restoration and fault clearance within a period of 0.015 seconds. A Supervisory Control and Data Acquisition (SCADA) system was designed for the monitoring and control of the wind power plant. Determined from the closeness of the HV bus voltage to the POC voltage, the most adaptive control strategy is used for each operation scenario, taking into account the wind speed as well as the POC voltage. It was observed that the evaluation by the SCADA system is consistent with the result obtained from the DIgSILENT PowerFactory software. For further exploration, research is recommended for the implementation of Artificial Neural Networks (ANNs) in machine learning to accommodate any wind speed and POC voltage levels for the control strategies under investigation.
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    Economic comparison between solar PV and diesel operated system for irrigation application in Lesotho
    (National University of Lesotho, 2023) Tukula, Mafa; Makhele, Molefe; Mokeke, Sebota; Hove, Tawanda
    Access to electricity is improving in Lesotho although the use of energy for economic growth is still stagnant. Sustainable economic growth is essential for improving livelihoods; however, this is difficult to achieve in rural areas where agriculture is the main industry. Hence in this study, an innovative economic analysis of solar and diesel-operated pumping devices for irrigation application in Lesotho is put forth. The main objective of the study is to scrutinize the concepts related to the effective characteristics of irrigation pumps, their design, selection, installation, and possible diagnosis of their problems in order to assist farmers and stakeholders. Furthermore, the aim is to compare the economic costs of solar and diesel-pumping systems. Several novel solar photovoltaic models and solutions have been proposed in an effort to circumvent some of the challenges. In addition, a recent intervention that serves as an example of the new study technique is discussed. A thorough technique for sizing and performance forecasting of photovoltaic (PV) solar pumps and a diesel generator is also given. The empirical data on the performance of the solar pump, the prescribed model of sizing, and the performance extrapolation approach made use of data on solar radiation and ambient temperature of the location. The empirical functions of the flow rate, as opposed to solar power, were similarly derived for different pipe size diameters of 63 mm, 75 mm, 90 mm, and 110 mm. Additionally, depending on the pump, pipe diameter, and PV array size that resulted in the lowest pumping cost per unit of energy, the optimum solar pumping system was chosen. This method of designing solar pumping systems was advised because it produced significantly different and more accurate results than the frequently employed straightforward method, which ignored the fact that the total dynamic head (TDH) fluctuated as the solar irradiance deviated. The simulation results drawn indicated that the best system configuration that resulted in the least unit cost of pumping is comprised of a 4 kW Lorentz PS2-4000-CS-F32-20-2 centrifugal solar surface pump, a 110 mm pipe size, and a 2110 watt PV array. As a stipulation, it is important to point out that solar photovoltaic (PV) seems to be a promising energy alternative to support irrigation development in Lesotho. In that matter, the unit cost of pumping for a solar PV-operated pump for irrigation application is 3.58 USD cents/ m³ while for a diesel generator it is 16.1 USD cents/ m³. Based on the life cycle cost analysis (LCCA) of both systems, the annualized cost of solar PV at a 10% discount rate is $1263.00 and that of a diesel generator is $5517.00, with 35314 m³ of water pumped per annum. The cost of solar PV per watt, including installation at the initial stage is 0.42, $/watt, while for the diesel generator it is 0.41 $/watt. However, for a long run, solar PV is more cost effective as compared to diesel generator. The proposed system was also found to be not only cost-effective but similarly environmentally friendly, as it emits zero amounts of greenhouse gases (GHG). The amount of greenhouse gases to be emitted as per simulation when using a diesel generator for irrigation purposes is 32.3 tons of carbon dioxide (𝐶2) per year.
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    The Impact of Intermittent Renewable Energy Generators on Lesotho National Electricity Grid
    (National University of Lesotho, 2020) Mokeke, Sebota; Prof Thamae, L. Z.
    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.
  • Thumbnail Image
    ItemOpen Access
    The impact of intermittent renewable energy generators on Lesotho National Electricity Grid
    (National University of Lesotho, 2020) Mokeke, Sebota; Thamae, R
    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 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.