OpenUCT :: Browsing by Author "Folly, Komla"

Browsing by Author "Folly, Komla"

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Open Access
Computer simulation programs to systematically screen protective relay applications for electric power distribution
(2006) Craib, Andrew Murray Alexander; Folly, Komla
[Pages 28,52,G:24,G:48 missing] Using EMTDC, an advanced closed-loop simulator, a set of network application tests is created in COMTRADE file format. The COMTRADE files created may be used to control an injection set or sets, which, in-turn, automatically inject protective devices. Part of the aim of the research has been to standardise the application tests, so that the testing of each device does not require a large number of calculated settings. The set of application tests simulate various network configurations, faults, loads, sources and devices. The tests are designed to highlight limitations within the protective schemes or their application within real networks. These limitations may take the form of incorrect settings and/or masking to certain applications. The application tests do in no way seek to supersede the necessity for qualification and/or acceptance testing. The tests are repeatable on any number of protective schemes and are independent of the test engineer. Although the test engineer is required to calculate and apply settings to the device being tested and to set up the test facility correctly, he/she cannot alter the test program in any way. In this way, protective devices are screened in a scientific manner to reveal their limitations in certain applications. A simulated nominal network model is developed by using a real network with its connected plant as the basis for the simulation model input data.
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.
Open Access
Investigation of mitigation and detection methods of Open Phase Conditions (OPCs) in nuclear power plants based on the operating experience
(2018) Bass, Cindy Allison; Folly, Komla
Safety in a Nuclear Power Plant (NPP) is of utmost importance, as the implications of a nuclear event have dire consequences on people, animals, and the environment and on unborn generations. This dissertation is about Open Phase Conditions (OPCs), which occur when one or two of the three electrical phases are lost or open circuited, e.g. one circuit breaker phase doesn’t open or close. The study of this condition is important, as it can affect important-to-safety equipment, which is critical to the safe shutdown systems of the reactor in a nuclear power plant. The OPC can occur in the transmission (TX) switchyard or at the transformers connected to the nuclear plant. It also has the capability to make the offsite supply inoperable, which is the alternate source of power for the nuclear safety systems. As many as sixteen (16) cases of Open Phase events have occurred in different countries worldwide from 1994 till 2015. An event also occurred in the Koeberg Nuclear Power Station (KNPS) in South Africa on the 11th November 2005. When these events occurred, the protection schemes did not “see” nor isolate the condition, as this was discovered to be a design vulnerability. This was due to oversight in the design of the protection to detect this condition. This dissertation seeks to answer the following research question: Is it possible to prevent or mitigate an open phase condition from occurring in the switchyard of a nuclear power plant? Previous work has attempted to address the lack of awareness of people working in close proximity to a Nuclear power station i.e. amongst staff working in the switchyard and operators in a nuclear plant, by sharing the operating experience (OE) of OPC occurrences. The approach to answer the research question lies in the analysis of the awareness of the people involved, as the condition cannot be prevented if it is not known. Case studies of the documented OE were categorised and analysed using a simplified root cause analysis method. A survey was conducted to assess the OPC awareness and perceptions of people in the system operator, i.e. TX division and at the Koeberg nuclear power station, within the Eskom Holdings utility. The results demonstrate that there is insufficient overall knowledge and understanding of this condition within the system operator. Operators in the nuclear plants all over the world have been required by the US Nuclear regulator, to be trained and to be aware of this condition. The results of this dissertation highlight the focus areas in people’s awareness that need attention. And that educating the system operator through training will strengthen the relationship between transmission and the nuclear plant within Eskom holdings.
Open Access
Modelling and Tracking of the Global Maximum Power Point in Shaded Solar PV Systems Using Computational Intelligence
(2019) Sagonda, Arnold Farai; Folly, Komla
Solar Photovoltaic (PV) systems are renewable energy sources that are environmentally friendly and are now widely used as a source of power generation. The power produced by solar PV varies with temperature, solar irradiance and load. This variation is nonlinear and it is difficult to predict how much power will be produced by the solar PV system. When the solar panel is directly coupled to the load, the power delivered is not optimal unless the load is properly matched to the PV system. In the case of a matched load the variation of irradiance and temperature will change this matching so a maximum peak power point tracking is therefore necessary for maximum efficiency. The complete PV system with a maximum power point tracking (MPPT) includes the solar panel array, MPPT algorithm and a DC-DC converter topology. Each subsystem is modelled and simulated in MATLAB/Simulink environment. The components are then combined with a DC resistive load to assess the overall performance when the PV panels are subjected to different weather conditions. The PV panel is modelled based on the Shockley diode equation and is used to predict the electrical characteristic curves under different irradiances and temperatures. In this dissertation, five MPPT algorithms were investigated. These algorithms include the standard Perturb and Observe (PnO), Incremental conductance (IC), Fuzzy Logic (FL), Particle Swarm Optimisation (PSO) and the Firefly Optimisation (FA). The algorithms are tested under different weather conditions including partial shading. The Particle Swarm and Firefly algorithm performed relatively the same and were chosen to be the best under all test conditions as they were the most efficient and were able to track the global maximum power point under partial shading. The PnO and IC performed well under static and varying irradiance, the PnO was seen to lose track of the MPP under rapid increasing irradiance. The PnO was tested under partial shaded conditions and it was seen that it is not reliable under these conditions. The Fuzzy logic performed better than the PnO and IC but was not as good as the PSO and FA. Since the fuzzy logic requires extensive tuning to converge it was not tested under partial shaded conditions. A DC-DC boost converter interface study between a DC source and the DC load are performed. This includes the steady state and dynamic analysis of the Boost converter. The converter is linearised about its steady state operating point and the transfer function is obtained using the state space averaged model. The simulation results of the complete PV system show that PSO and Firefly algorithm provided the best results under all weather conditions compared to other algorithms. They provided less oscillations at steady state, high efficiency in tracking (99%), quick convergence time at maximum power point and where able to track global power under partial shaded weather conditions for all partial shaded patterns. The Fuzzy logic performed well for what it was tested for which are static irradiance and rapid varying irradiance. The PnO and IC also performed relatively well but showed a lot of ringing at steady state. The PnO failed to track the MPP at certain instances under rapid increasing irradiance and the IC was shown to be unstable at low irradiance. The PnO was not reliable in tracking the global maximum power point under partial shaded conditions as it converged at local maximum power points for some partial shaded patterns.
Open Access
Power System Transient Stability Analysis Considering Load And Wind Power Generation Variability
(2023) Ncwane, Siyanda; Folly, Komla
This research investigated the transient stability of a power system that integrated wind power generators (WPGs) by considering the variability of the system load and power they produced. The power produced by the WPGs was modelled using wind speed, while the variability of the power system load and wind speed was modelled using distribution functions. Power system transient stability was investigated using the deterministic and probabilistic methods. The deterministic method was used to investigate the power system transient stability based on 9 scenarios consisting of low, moderate, and peak system loading and wind power generation conditions. The probabilistic method was also used to investigate power system transient stability using scenarios consisting of low, moderate, and peak system loading conditions. During each of the three system loading conditions, the variable power produced by the WPGs was modelled using 5000 samples that were randomly sampled using Monte-Carlo Simulations. It was found that selecting distribution functions based on their fit alone did not ensure that they modelled the load and wind speed range. Their data range modelling ability ensured that they synthesised the low and high load and wind speed values. It was also found through power system simulations that the transient stability was negatively impacted when the power system's net-load reduced because of increased wind power generation penetration levels. These findings highlight that distribution functions should not be selected based only on their fit to load and wind speed data. In addition to a good fit, they should also be assessed to determine whether they synthesise the data range. The research findings also highlight that power system transient stability should be investigated using scenarios in which the power produced by WPGs is high, during periods when the system's netload is low
Open Access
Saturation Effects in 3p3L Power Transformers due to Geomagnetically Induced Currents
(2023) Kerr, Evan; Folly, Komla; Gaunt Charles
Geomagnetically induced currents (GIC's) caused by solar storms or other sources of dc excitation can impact the normal operation of power transformers in transmission networks. The impact is caused by the dc current introduced into the windings of a transformer energised with ac, which leads to a simultaneous dc magnetomotive force (mmf) resulting in part-cycle saturation of the core and so, heat related ageing and failures. A generally accepted theory on the impact of GIC's on transmission networks is that three-phase three-limb (3p3L) power transformers are not affected by this phenomenon. The basis is that a three-limb core forms balanced return paths which oppose the dc flux evenly in each limb, thereby neutralising the effect. The challenge is this theory only remains true if the circuit resistance of the three-phase lines connected to the transformer are balanced, which has proven difficult to achieve in practice. This study set up laboratory experiments to evaluate the effect of simultaneous ac and dc in the windings of an unloaded 3p3L transformer while simulating realistic unbalanced conditions. To ensure a valid test protocol, the transformer was designed as a scaled-down model of a power transformer with a core constructed using four step-lap mitred joints and a high quality electrical steel. This was to closely resemble the performance of a normal power transformer. The source supply for the test circuit was carefully designed and tested to ensure that it was sufficiently robust that it would not affect the results of the transformer under test (TuT). The initial testing was of symmetrical ac conditions to establish a performance benchmark at normal operation and the TuT nameplate rating, and to emulate core saturated conditions by over-exiting the core for exploratory core stray flux recordings. The dc testing that followed required adjustable levels of dc current and the unbalance to emulate a range of GIC conditions. At each setting of dc and unbalance, the excitation parameters such as THD were recorded using a power analyser. Simultaneously, the core stray flux was recorded using search coils, while tank ‘hot-spot' temperatures from stray flux eddy current heating were recorded. Finally, the audio levels of the 3p3L transformer in balanced and unbalanced GIC conditions were analysed. The results of the tests challenge the assumption that 3p3L power transformers are immune to GIC primarily because it is not possible to ensure ideally balanced conditions in the field. The importance of the transformer core design was highlighted by showing its significant contribution to the GIC vulnerability. The core joints of a four step-lap core was shown to saturate before the bulk core areas that emphasised the need for continual careful attention to core design and construction in an effort to reduce inherent design vulnerabilities.
Open Access
Short-term wind power forecasting using artificial neural networks-based ensemble model
(2020) Chen,Qin; Folly, Komla
Short-term wind power forecasting is crucial for the efficient operation of power systems with high wind power penetration. Many forecasting approaches have been developed in the past to forecast short-term wind power. In recent years, artificial neural network-based approaches (ANNs) have been one of the most effective and popular approaches for short-term wind power forecasting because of the availability of large amounts of historical data and strong computational power. Although ANNs usually perform well for short-term wind power forecasting, further improvement can be obtained by selecting suitable input features, model parameters, and using forecasting techniques like spatial correlation and ensemble for ANNs. In this research, the effect of input features, model parameters, spatial correlation and ensemble techniques on short-term wind power forecasting performance of the ANNs models was evaluated. Pearson correlation coefficients between wind speed and other meteorological variables, together with a basic ANN model, were used to determine the impact of different input features on the forecasting performance of the ANNs. The effect of training sample resolution and training sample size on the forecasting performance was also investigated. To separately investigate the impact of the number of hidden layers and the number of hidden neurons on short-term wind power forecasting and to keep a single variable for each experiment, the same number of hidden neurons was used in each hidden layer. The ANNs with a total of 20 hidden neurons are shown to be sufficient for the nonlinear multivariate wind power forecasting problems faced in this dissertation. The ANNs with two hidden layers performed better than the one with a single hidden layer because additional hidden layer adds nonlinearity to the model. However, the ANNs with more than two hidden layers have the same or worse forecasting performance than the one with two hidden layers. ANNs with too many hidden layers and hidden neurons can overfit the training data. Spatial correlation technique was used to include meteorological variables from highly correlated neighbouring stations as input features to provide more surrounding information to the ANNs. The advantages of input features, model parameters, and spatial correlation and ensemble techniques were combined to form an ANN-based ensemble model to further enhance the forecasting performance from an individual ANN model. The simulation results show that all the available meteorological variables have different levels of impact on forecasting performance. Wind speed has the most significant impact on both short-term wind speed and wind power forecasting, whereas air temperature, barometric pressure, and air density have the smallest effects. The ANNs perform better with a higher data resolution and a significantly larger training sample size. However, one requires more computational power and a longer training time to train the model with a higher data resolution and a larger training sample size. Using the meteorological variables from highly related neighbouring stations do significantly improve the forecasting accuracy of target stations. It is shown that an ANNs-based ensemble model can further enhance the forecasting performance of an individual ANN by obtaining a large amount of surrounding meteorological information in parallel without encountering the overfitting issue faced by a single ANN model.
Open Access
Technical analysis of renewable energy integration with battery energy storage systems on the Eskom grid
(2023) Gongxeka, Jacqueline; Folly, Komla
The Eskom fleet of coal-fired power plants is ageing and will be retired within 30 years, if not sooner. As a clean alternative towards the decarbonization of the South African electrical industry, Eskom is progressively incorporating renewable energy sources into its infrastructure. However, the intermittent nature of renewable energy sources provides a challenge for supply security and system stability. There seems to be a need for Eskom to integrate large-scale energy storage into its grid to maintain energy security during the transition from fossil fuels to clean energy sources. Even though load shedding has been looming since 2008 and is getting worse as the fleet of power plants ages. Eskom is routinely curtailing wind energy in the early morning due to low demand, thereby wasting energy that could have been used during peak hours. In addition, Eskom's limited financial resources necessitate the delay of certain network strengthening projects. This dissertation examines the effects of Battery Energy Storage System (BESS) on the Eskom Distribution grid. This was accomplished by selecting two Eskom BESS sites, Skaapvlei (80 MW) which will be used for ancillary services (AS) and energy support (ES) and Hex (20 MW) which will be used for AS, ES and Load shaving (LS). Both networks were modelled using DigSilent Powerfactory and hourly load data from MV90 and the Eskom data tool. Eskom contractors' BESS models were used to simulate the future network, allowing for a comparison between the current network without BESS and the future network with BESS.
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.
Open Access
Voltage stability analysis of a power system network comprising a nuclear power plant
(2018) Boesak, Dawid John Johannes; Folly, Komla
As recently as 2016, the performance of South Africa’s power utility has shown that it is not resilient enough to withstand the consequences of a power system blackout. Blackouts are defined as being a form of power system instability that can be brought about by a variety of abnormal network scenarios. The most common modes of failure are grouped under the term power system stability. In this dissertation, the different modes of power stability that can affect a nuclear power station will be investigated and discussed. The particular phenomenon that will be focused on, however, is the effect that voltage instability has on the ability of generators and loads to perform their standard functions, thus ensuring a secure power system. To investigate the effect that voltage instability has on a nuclear power station, this dissertation will look at relevant literature on the topic. In addition, by extracting from common examples of national and international occurrences of voltage stability, this dissertation will record the effects that this phenomenon has on the security of a power system, in particular on nuclear power plants. To model the network containing a nuclear power plant for the evaluation of voltage stability, the different mathematical models of the generation plant are presented, which include: the automatic voltage regulator, power system stabilizer, governor, nuclear reactor, and excitation system. Also presented are mathematical models of network equipment such as under voltage tap changers and the dynamic loads that are of interest when evaluating voltage stability. The models used for evaluation of the voltage stability phenomenon affecting a nuclear power plant and the surrounding integrated power system are built in the Digsilent PowerFactory® software. The scenario for evaluation is based on a voltage stability event that occurred around at the Koeberg nuclear power system situated in the Western Cape province on South Africa on 15 October 2003. It is commonly accepted that voltage stability can be evaluated at a steady state level by performing power versus voltage (PV) analysis to determine the voltage buses vulnerable to voltage collapse, and reactive power versus voltage (QV) analysis to determine the critical reactive devices required to avert a voltage instability event. The scenarios that are evaluated for voltage stability are divided into two sections: i) a PV and QV analysis as per the event that occurred on 15 October 2003 and ii) present-day voltage stability indices for PV and QV if mixed with a generation such as renewable energy sources that include wind, solar, biomass and concentrated solar power (CSPs). The result reveals the vulnerabilities of the nuclear power plant and the surrounding integrated power system due to a voltage instability event. Some of the solutions proposed include a review of the typical power system protection schemes — such as under and overvoltage detection scheme — that are used. In the study, PV and QV curves provide v good indications of the state of critical busbars and the reactive power reserve margins available before instability can potentially settle in. Simulations confirmed the effectiveness of critical equipment installed in the Western Grid and the effect on their electrical parameters such as torque and the slip on motors.

Browsing by Author "Folly, Komla"

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