Browsing by Author "Oyedokun, David"

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    Open Access
    Design of Low-Frequency acGIC Voltage Stability Laboratory Protocol
    (2022) Sithebe, Ngcebo S; Oyedokun, David
    Geomagnetically Induced Currents (GICs) are formed as a direct consequence of space weather phenomena and have detrimental effects on power systems. GICs enter the power system through transformers' grounded neutrals, causing an array of problems ranging from transformer overheating to voltage instability. There have been extensive studies on the effects of GICs, particularly on power transformers as they are the power systems' most susceptible components to GICs. The effects on transformers then affect the rest of the power system and may lead to voltage instability and blackouts. Recent studies have shown that a real GIC is not necessarily a dc as it has been previously modelled in literature. In reality, GICs are multi-frequency, multi-amplitude currents. At the same time, voltage stability analysis due to GICs with low frequency ac as a GIC representative has not been explored in detail in the literature. This dissertation assesses the effects of GICs on the voltage stability of power systems using a low-frequency ac (acGIC) model as a GIC representative. This is different from the conventional dc model (dcGIC). A laboratory and simulation protocol using a frequency-dependent transmission line with resistive and inductive elements in each phase, and a novel low-frequency ac injection circuit was designed and tested. This single frequency, single amplitude acGIC injection circuit is a first approximation of the real signal GIC. The effects and differences in the response of the power system to dc and low-frequency ac injections are explored and presented. The implementation of the protocol in the laboratory and simulation environments showed that there is a fundamental difference in the response of the power system when subjected to ac injection compared to dc injection. The research showed that the dc model for GIC is a worst-case scenario, constantly at the ‘prospective GIC. Contrarily, low-frequency ac model for GIC is constantly changing, never reaching the prospective GIC and, therefore, the extreme dc settling point. This study shows that with a low-frequency ac model, though being a first approximation, the network parameter response is not constant and the effect of the GIC on the network parameters varies with respect to the magnitude of the GIC at an instance. Furthermore, the implications on voltage stability revealed that the loadability effect on the system due to GIC is also not constant as the dc model depicts. It is, however, dependent on the current flow of GIC at a particular instance during a geomagnetic storm. This research showed that for better power systems modelling with GIC, a varying current injection is necessary to fully understand the effects on the system.
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    Open Access
    Development of a finite element matrix (fem)three-phase three-limb transformer model for Geomagnetically Induced Currents (GIC) experiments
    (2020) Mkhonta, Sizwe; Oyedokun, David; Folly, Komla
    Geomagnetically Induced Currents (GIC) have been a growing concern within power system operators and researchers as they have been widely reported to lead to power system related issues and material damage to system components like power transformers. In power transformers, GIC impacts are evidenced by part-wave saturation, resulting in transformers experiencing increased presence of odd and even harmonics. The three-phase three-limb (3p3L) transformer has been found to be the most tolerant to high dc values compared to other core types. The research was based on a hypothesis which reads “transformer laboratory testing results can be used as a guide towards developing suitable Finite Element Matrix (FEM) models to be used for conducting GIC/DC experiments”. This study thus investigates the response of a 15 kVA 3p3L laboratory transformer to dc current, emulating the effects of GICs. GIC and dc current are the same under steady state conditions, and hence mentioned interchangeably. Laboratory tests conducted identified two critical saturation points when the transformer is exposed to dc. The early saturation point was identified to be at around 1.8 A/phase of dc (18% of rated current), while the deep saturation point was at around 15 to 20 A/phase of dc (about 72% of rated current). Further analysis showed that holes drilled on the transformer can lower the transformer knee-point by about 26%, depending on the size and location of the holes. The holes hence end up affecting the operating point of the transformer due to losses occurring around the holes. A transformer FEM model was developed following the laboratory exercise, where it was concluded that a 2D model leads to grossly erroneous results, distorting the magnetizing current by about 60% compared to the laboratory results. A solid 3D model improved performance by about 30% as it took the transformer's topological structure into consideration. The 3D model was then refined further to include joints and laminations. It was discovered that laminations on the transformer need to be introduced as stacks of the core, with each core step split into two, allocating a 4% air gap space between stacks. Refinement of the T-joints proved that the joints have a relatively high influence on the transformer behaviour, with their detailed refinement improving the transformer behaviour by about 60%. The final FEM model was used for dc experiments. The results of such experiments showed close resemblance to the laboratory results, with saturation points identified in FEM lying within 10% of the laboratory identified saturation points. Overall, the various investigation methods explored showed that the hypothesis was satisfactorily proven true. Laboratory results functioned as a guide in developing the model, offering a reference case.
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    Open Access
    Development of a Network Design Tool for the Herman-Beta Extended Transform
    (2024) Khan, Isma-Eel; Oyedokun, David; Gaunt Charles
    The Herman-Beta method has been one of the most important network planning tools used in South Africa. Over decades, it has transformed from being able to perform probabilistic load flow studies for LV passive feeders to an algorithm capable of performing PLF studies for passive/active feeders of any voltage. Furthermore, the new algorithm reformulated the equations used to dispose of the underlying assumptions used in the original method. This reformulation came to be known as the Herman-Beta Extended Transform (HBET), a powerful network planning tool which could prove vital to network planners as the uncertainty in power systems increase. Previously, the HBET was implemented using MATLAB programming software. In this dissertation, a set of user requirements and data structure to enable efficient handling of input and output data in the HBET was developed for an open-source platform. Python programming language was chosen as the most suitable programming language to implement the program, due to its array manipulation capabilities and the plethora of information and help available online. Four scenarios were used to test the accuracy with which the tool was created. The four scenarios included a 12-bus passive feeder, a 12-bus active feeder, a 33-bus passive feeder with laterals and a 33- bus active feeder with laterals. These results were tested against the results produced by the MATLAB tool, where it was previously proven to be accurate. It was found that the percentile voltages, mean voltages and voltage standard deviations for all systems simulated in Python were identical to those simulated using the MATLAB tool, indicating that the tool had been implemented accurately, thereby validating the hypothesis.
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    Open Access
    Development of a Network Design Tool for the Herman-Beta Extended Transform
    (2024) Khan, Isma-Eel; Oyedokun, David; Gaunt Charles
    The Herman-Beta method has been one of the most important network planning tools used in South Africa. Over decades, it has transformed from being able to perform probabilistic load flow studies for LV passive feeders to an algorithm capable of performing PLF studies for passive/active feeders of any voltage. Furthermore, the new algorithm reformulated the equations used to dispose of the underlying assumptions used in the original method. This reformulation came to be known as the Herman-Beta Extended Transform (HBET), a powerful network planning tool which could prove vital to network planners as the uncertainty in power systems increase. Previously, the HBET was implemented using MATLAB programming software. In this dissertation, a set of user requirements and data structure to enable efficient handling of input and output data in the HBET was developed for an open-source platform. Python programming language was chosen as the most suitable programming language to implement the program, due to its array manipulation capabilities and the plethora of information and help available online. Four scenarios were used to test the accuracy with which the tool was created. The four scenarios included a 12-bus passive feeder, a 12-bus active feeder, a 33-bus passive feeder with laterals and a 33- bus active feeder with laterals. These results were tested against the results produced by the MATLAB tool, where it was previously proven to be accurate. It was found that the percentile voltages, mean voltages and voltage standard deviations for all systems simulated in Python were identical to those simulated using the MATLAB tool, indicating that the tool had been implemented accurately, thereby validating the hypothesis.
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    Open Access
    Post-implementation review of the net metering policy in Namibia and design of distributed generation hosting capacity algorithm
    (2021) Sam, Angula Markus; Oyedokun, David
    Global campaigns against fossil fuels to reduce the emission of greenhouse gases and combat climate change has compelled the electricity supply industry (ESI) around the globe to explore environmentally friendly sources of electricity. The concept of distributed generation (DG) has gained momentum and is emerging as a promising source of clean energy, with immense potential to maximize the shares of renewable energy in the global energy mix. Like the rest of the world, Namibia is witnessing an unprecedented growth of DG courtesy of governmental efforts to ensure a speedy transition to low-carbon generation technologies. In 2016, the Namibia government developed the net metering (NM) policy known as the Net Metering Rules (NMR) as a consumer-focused approach to achieve the low-carbon objective. To date, there has been no rigorous post-implementation review of the NMR to assess its effectiveness, despite rising concerns from distribution network operators (DNOs) about whether the NMR is suited for long-term application in the fast-growing market of prosumers. This study conducts a broad appraisal of the status quo on DG integration into distribution networks in Namibia and an in-depth assessment of the technical and financial impacts of the NMR using the Erongo Regional Electricity Distributor (Erongo RED) as a case study. The findings indicate that most prosumers export over 60% of generated energy to distribution networks and achieve significant financial savings by offsetting on-site demand with their generation in real-time, as well as, by offsetting a portion of their electricity bills through NM compensations for grid exports. NM compensations at the avoided cost makes grid exports in Namibia a cheaper alternative source of energy to DNOs as compared to the national utility, which charges other energy service charges i.e. reliability charge, transmission losses charge etc. on top of the avoided cost. Additional findings indicate that prosumers are subjecting DNOs to revenue losses because of reduced volumetric energy sales caused by the reduction of prosumers' on-site energy requirements from the grid. With the deployment of DG growing rapidly in Namibia, increasing grid exports and associated technical constraints are envisaged in distribution networks. This dissertation recommends adaptations to existing regulatory policies to mitigate envisaged financial and technical risks associated with DGs. These adaptions include a DG hosting capacity (HC) assessment methodology for consumer-side photovoltaic (PV) DG in existing distribution network where a high and uniform uptake of DG is anticipated. The methodology captures time dependency correlations between load and generation profiles, which increase the accuracy of HC results. The uniqueness of the methodology is the concept of calculating monthly HC, which aids the optimal integration of DG into distribution networks to meet consumers' daily energy requirements throughout the year without comprising the network's quality of supply. The methodology was tested on a residential and business distribution network. Results confirm that HC in distribution networks varies monthly. However, the practical implementation of monthly HC require upgrades to existing inverter technology, which currently contains a single export limit functionality. This opens the possibility to drive innovation in the inverter technology, to develop a date-based multiple export limit functionality. The results also demonstrated the importance of considering phase unbalance when conducting HC studies for residential distribution networks. Applications and limitations of the methodology were discussed.
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    Open Access
    Power System Grid Planning with Distributed Generation
    (2021) Kakaza, Mnikeli; Oyedokun, David
    Distributed Generation (DG) is one of the technologies approved by the South African government for the country's generation expansion to meet future load demand and to support economic growth. DGs change the conventional power flow (generation, transmission to distribution) by injecting real and reactive power at distribution voltage levels. The change in the conventional power flow creates complexity in the power system grid planning due to the conversion of the power system from a passive network to an active network. Introduction of bi-directional power flow on the power system can, among other benefits reduce local power demand which opens opportunities for capital investment deferrals on the transmission and distribution sectors. Consequently, DG impact on the transmission and distribution grid planning has been studied by other researchers. However, previous studies evaluated DG integration on a regulated market and assumed a certain level of generation availability during network peaking period. None of the studies have yet evaluated the benefits on an unregulated market using real measured data. Furthermore, SA distribution network expansion is also being planned without incorporating DGs on the network because of unreliability of wind and solar energy and the network operator's inability to influence the size, location and penetration level of DGs. This planning approach forces the network operator to do more to ensure high network strength. This approach can also result in network overdesign and unnecessary capital expenditure due to the potential benefits that can be deduced from DGs. This dissertation therefore aims to investigate whether incorporating future DG integration in distribution network planning can alleviate financial ramifications of grid code compliance requirements. The data used in the simulations was obtained from the distribution network operator and comprises of both real and reactive power values with a sampling time of 60 minutes for a period of a year. Simulations were conducted for both low and high load conditions to cover the extreme ends of the network and the parameters that were assessed are thermal rating, voltage regulation and network grid losses. Results showed that thermal constraints that are expected on the network when DGs are not considered are not evident when DGs are considered. Results further revealed that there are undervoltage improvements on the network when DGs are considered, and this reduces the capital expenditure that would have otherwise been incurred without DGs to result in a grid code compliant network. Furthermore, there is evidence of reduction in losses under high load conditions and increase in losses under low load conditions in the simulation results. Reduction in losses is caused by supplementary generation from wind and solar plants while increase in losses is due to excessive generation from wind plants which necessitate transportation over long distances to the nearest load centres. In addition to location, size and penetration levels as described in the literature, technology selection for a particular load type is also of utmost important to maximise the DG benefits on the network.
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    Reactive power compensation of fixed speed wind turbines using a hybrid wind turbine technology
    (2022) Apata, Oluwagbenga; Oyedokun, David
    There has been significant growth in the use of wind energy as an alternative form of energy by many countries globally. This is in direct response to calls to reduce environmental pollution from the usage of fossil fuels in energy generation. Due to the intermittency of wind energy, the integration of wind energy into an existing power system grid can lead to increased exposure to instability. This has prompted power systems operators to revise the connection requirements for grid codes that require wind turbine generators to contribute to power control and stay online during a network disturbance. These revisions have resulted in technical migrations from the fixed speed wind turbine (FSWT) to the variable speed wind turbines (VSWTs). The fixed speed induction generator (FSIG) powers the FSWT while the VSWT is based on the permanent magnet synchronous generator (PMSG) or the doubly-fed induction generator (DFIG). Irrespective of this technical migration in wind power systems, the FSWTs still represent a considerable percentage of globally installed wind turbines (WTs). Furthermore, some wind turbine (WT) manufacturers have introduced life expansion programs for these FSWTs to increase their operating life to as much as thirty years. The FSWT is common in wind power systems due to its robustness, mechanical simplicity, and low production cost. Its major setback is the inability to compensate for its reactive power need and improve voltage stability during a fault condition. In steady-state conditions, these WTs experience large fluctuations in the generator terminal voltage because of uncontrolled consumption of reactive power. During grid fault conditions, the FSWT consumes large quantities of reactive power to stay grid-connected and prevent the rotor from over-speeding, thereby losing synchronization. It is therefore imperative to provide voltage and reactive power support for the FSWT-power system. This is important to enable the wind power system to fulfill the grid codes as prescribed by the system operator and successfully go through a fault condition. Flexible AC transmission system (FACTS) devices are often used in enhancing the voltage stability and reactive power control in the FSWT- power system. It has been shown however from the available literature that the installation of these devices alongside the WT inflates the total cost of the wind power system thereby making the overall wind system more expensive. The PMSG-WT is becoming more attractive for wind energy systems. This class of WTs has fully-rated converters that control the active and reactive power of the WT, enabling the WT system to ride through a grid fault condition successfully. This improves the low voltage ride through (LVRT) ability during a grid disturbance. This characteristic makes it a very suitable choice for grid-connected operations. A unique feature of this category of wind system is the possibility of controlling its fully rated converters to support a nearby induction generator WT system, making it a suitable choice for the development of a hybrid wind farm. This research proposes exploring this characteristic of the PMSG-WT in developing a hybrid wind system of the PMSG-WT and FSWT. The proposed hybrid wind system utilizes the ability of the PMSG in supporting a nearby wind farm. Therefore, systematic control of the PMSG and FSWTwind system is proposed using the fully rated converters of the PMSG-WT system in providing the required voltage support and reactive power needed by the FSWT-wind system during a grid disturbance. The proposed strategy, therefore, eliminates the need for FACTS devices. To develop the proposed coordinated control strategy of the PMSG and FSWT-hybrid wind power system, a current allocation strategy is first developed for the grid side converter (GSC) of the PMSG. This is based on the converter capacity and current capability of the GSC in providing reactive power needed by the FSWT-based wind system in the event of a grid fault condition. With this approach, the GSC capacity of the PMSG is utilized efficiently to improve the LVRT capability of the FSWT-based wind farm and voltage support of the hybrid wind farm. In a steadystate, the control priority ensures that both wind systems operate efficiently and reliably. The proposed solution offers both technical and economic advantages compared to the traditional voltage support methods available to the FSWT-wind system under grid fault conditions. This method can be applied to existing wind power systems operating with the FSWTs and can be further applicable to any new wind farm, which would be established with this hybrid configuration. Results from the proposed strategy show an improvement in the grid voltage of the hybrid wind farm, there is a reduction in grid voltage sags while the FSWT-based wind farm experiences an improvement in its output power and reactive power profile while it rides through a fault condition.
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    Synchronous generator and excitation system response to GIC
    (2021) Jankee, Pitambar; Oyedokun, David
    Solar cycle 25 has started catching the attention of space scientists, physicists, and power engineers. The resultant Geomagnetically Induced Currents (GIC) lead to periodic part wave saturation of transformers, harmonic pollution, increased non-active power demand and potential voltage collapse. While most power system studies have focussed on the transformer's response to GIC, there has been very little research investigating the response of synchronous generators and excitation systems to such disturbances. Moreover, conventional GIC modelling assumes a dc voltage induced in the neutral or along transmission lines, but this may not adequately represent the actual dynamic power system's response to GIC. Using a 4-bus and a multimachine power system modelled in MATLAB Simulink, this project shows that the power system's response to low frequency GIC (acGIC) is different to the conventional dc approximation (dcGIC). In both cases, harmonic analysis was carried out. It was observed that under unbalanced system conditions, harmonics do not follow the conventional phase sequence. This means that multiples of the third harmonic no longer have a zero sequence. Such harmonics may affect the generator despite delta winding connections of generator step-up transformers (GSUs). The increased var demand due to saturating transformers was compared using an IEEE1459 meter and a General Power Theory meter. The analysis showed that conventional power theories using the term “reactive power” underestimate the increased var demand. The differences arise in the definitions of conventional reactive power and non-active power defined by IEEE1459. Hence, the burden of var on generators and var compensators can be higher than expected. Additionally, the responses of different complexities of generator models were compared. The results showed that ac equivalent sources might not represent the true dynamic power system's response to GIC. The multi-machine simulation results revealed that voltage dip below the 10 % limit, due to GIC, can be reduced by up to 8.75 % using proper excitation system models and control parameters. Moreover, it was shown that while one generator might not be able to provide enough non-active power to meet the increased demand, a group of generators pushing out var within the generator's capability limits, can help to reduce this deficiency of non-active power. This is because the total var output from the generators is higher. The findings of this study aim to raise concern on the conventionally used dc model of GIC which are not representative of a realistic GIC profile. Excitation system control appears to play a significant role in voltage drop reduction. As such, contingencies such as Geomagnetic Disturbances (GMDs) can be used to determine and tune optimal parameters of the excitation system control.
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    The Formulation of a Novel Control Framework for Regulation of an Active Low Voltage Network
    (2022) Windapo, Mobolaji Oladipupo; Oyedokun, David
    The low voltage (LV) Network has become more complex due to the addition of loads like Electric Vehicles (EVs) and generation from Renewable Energy Sources (RES). These additions will result in power quality issues arising from excess supply or load unbalance. As LV networks and power systems were not designed with these entities in mind, scalable and flexible mitigation strategies will be needed to tackle these problems. This general conclusion was determined in the literature. This dissertation presents the implementation of a novel framework established to solve these problems. The proposed framework consists of a Multi-Agent Control System (MAS) to coordinate the various independent entities (agents) and a Thévenin Equivalent Impedance (TEI) based estimator to measure real-time load unbalance towards determining optimal currents in real-time and adjust supply/demand optimally to minimize losses. A test network was developed to compare systems making use of the same MAS-generated charging scheme (for the ESS and EV) but with different modes of phase power injection, BPI (Balanced Power Injection), and OPI (Optimal Power Injection). The study reveals OPI minimizes transmission losses by exploiting the transmission lines with lower impedance. Also, the impact of OPI on the network voltage is minimal as seen in phase voltage unbalance rate (%PVUR) figures, this means the extra unbalance introduced by OPI is negligible. Based on the findings the study concludes that the integration of MAS and OPI (guided by TEI parameter estimations) is a feasible combination and an improvement on the existing frameworks as it minimizes losses.
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    Open Access
    Three-phase five limb transformer responses to geomagnetically induced currents
    (2021) Murwira, Talent Tafadzwa; Oyedokun, David; Folly, Komla
    Geomagnetically induced currents (GIC) are quasi-DC currents that result from space weather events arising from the sun. The sun ejects hot plasma in a concept termed ‘coronal mass ejections' which is directed towards the earth. This plasma interferes with the magnetic field of the magnetosphere and ionosphere, and the magnetic field is subsequently distorted. The distortions in these regions results in the variation of potential on the earth's surface and distortions in the earth's magnetic field. The potential difference between two points on the earth's surface leads to the flow of direct current (DC) of very low frequency in the range 0.001 ~ 0.1 Hz. Geomagnetically induced currents enter into the power system through grounded neutrals of power transformers. The potential effects of GIC on transformers are asymmetrical saturation, increased harmonics, noise, magnetization current, hot spot temperature rise and reactive power consumption. Transformer responses to GIC was investigated in this research focussing on a three-phase fivelimb (3p5L) transformer. Practical tests and simulations were conducted on 15 kVA, 380/380 V, and 3p5L transformers. The results were extended to large power transformers in FEM using equivalent circuit parameters to show the response of grid-level transformers. A review of literature on the thresholds of GIC that can initiate damage in power transformers was also done and it was noted that small magnitudes of DC may cause saturation and harmonics to be generated in power transformers which may lead to gradual failure of power transformers conducting GIC. Two distinct methods of measuring power were used to measure reactive power consumed by the transformers under DC injection. The conventional method and the General Power Theory were used and the results show that the conventional method of measuring power underestimates reactive power consumed by transformers under the influence of DC injections. It may mislead system planners in calculating the reactive power reserves required to mitigate the effects of GIC on the power system.