Faculty of Engineering and Built Environment

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Investigation and mitigation of temporary overvoltage caused by harmonic filters for variable speed drives
(2021-12-01) Ayar, Prenushka; Ojo, Evans Eshiemogie; Akinrinde, Ajibola O.
The introduction of variable speed drives (VSD’s) has become very popular in industrial operations, as it allows for smoother and more efficient operations of different processes. The installation of a VSD in an electrical system reduces the operational costs and if well implemented, it prolongs the life span of electrical equipment in the power system. Despite the fact that the use of VSD’s offers these benefits, the main disadvantage is that its operations produce harmonics. However, the implementation of harmonic filters to eliminate these harmonics can cause temporary overvoltage during the filter switching which can negatively affect the systems operations. Temporary overvoltage and its mitigation are a matter of concern; thus, this study is aimed at investigating and mitigating the temporary overvoltage caused by the harmonic filters used to control the operations of variable speed drives. The investigations conducted comprised of analytical modelling, computer simulations, and case studies. For all these aspects of the study, four harmonic filters were modelled, simulated, analysed, and were also implemented for the case studies. Firstly, was the analytical investigation where a network of VSDs of with and without filters was designed, modelled, and simulated to ascertain the harmonics produced. Secondly, was the computer simulation of the network in the ATP software. Thirdly, were the case studies, in which the harmonic analyser was installed for three different industries to measure the harmonic distortion and the power quality of the system. From the results, it is found that the 3rd and 5th harmonic orders were the highest in all three studies conducted. Harmonic filters may be used to reduce harmonic distortion to levels detailed in IEEE 519-1992 standards. The single-tuned, double-tuned, CType, and high pass filters were proposed for the purpose of mitigation. According to the results from the simulation, the double-tuned, and single-tuned filters were the most successful in mitigating the 3rd and 5th harmonics. Overvoltage’s for both energization and de-energization of the harmonic filters were found to be greater than the power frequency withstand limit. To avoid the undesirable effects that overvoltage has on the system, pre-insertion resistors, surge arresters, and controlled switching was considered for the mitigation of the overvoltages produced during the switching of the harmonic filters. Based on the simulation results, the surge arrester greatly mitigated the overvoltage caused by the energization, followed by the use of a pre-insertion resistor. Controlled switching was the least effective mitigation method in all three case studies and is therefore not the best choice to limit the temporary overvoltage.
Power flow and faults analysis of a hybrid DC Microgrid : PV system and wind energy
(2021-12-01) Zulu, Musawenkosi Lethumcebo Thanduxolo; Ojo, Evans E.; Akinrinde, Ajibola O.
Rural electrification has become a very important means of improving the standard of living of rural dwellers, a process which also helps in the electrification of remote and isolated regions. Presently, the electrification of such regions can be achieved through the use of renewable energy. The use of renewable energy sources such as PV and wind energy is gaining popularity as the solution to achieving the electrification of rural areas, such as the use of the microgrid, which can be in the form of an AC or DC microgrid. The DC microgrid can be used to connect distributed energy resources and its energy storage is considered to be an economical system to meet consumer demand due to its benefits, namely environmental friendliness, reliability and good performance in load distribution. The power system may experience many faults when transferring power via overhead transmission lines to the load. When these faults occur, it is important to detect the location and isolate the part that had experienced the fault quickly, without de-activating the whole microgrid. The main aim of this study was to conduct a power flow and faults analysis on a hybrid DC microgrid model with battery storage. The hybrid energy sources for the DC microgrid are the PV system and wind energy system. Firstly, this research conducted a power flow analysis for the hybrid DC microgrid. Secondly, a fault analysis was carried out on the system and both the power flow and the fault analysis were formulated through implementation in a MATLAB/Simulink environment under various conditions in order to ascertain the stability and reliability of the system. Various MATLAB/Simulations were carried out, including the DC single-line-ground fault and DC line-line fault and are analysed in a designed hybrid DC microgrid power system. The results showed that DC line-to-line and DC line-to-ground faults lead to the imbalance of DC voltage, which is difficult to re-balance and stabilize in the system after the existence of these faults. When these faults occurred in the system, there was immense fluctuation and unsteadiness of output load power delivered to consumers. Moreover, wind-generated power on the generation side was severely affected. Based on the results and analysis of those results, the hybrid DC microgrid is seen as a satisfactory and optimum concept for the generation and transmission of power for rural and isolated area electrification, i.e. it can provide power to remote areas that cannot be reached by the national grid. The study revealed, based on the analysis of results, that it has an effective response under fault conditions. Results for a hybrid DC microgrid revealed that high quality of power is experienced in load distribution. Also based on the results, when DC faults occurs there is disturbance to output.