OpenUCT :: Browsing by Author "Folly, Komla A"

Browsing by Author "Folly, Komla A"

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Open Access
Application of differential evolution to power system stabilizer design
(2012) Mulumba, Tshina Fa; Folly, Komla A
In recent years, many Evolutionary Algorithms (EAs) such as Genetic Algorithms (GAs) have been proposed to optimally tune the parameters of the PSS. GAs are population based search methods inspired by the mechanism of evolution and natural genetic. Despite the fact that GAs are robust and have given promising results in many applications, they still have some drawbacks. Some of these drawbacks are related to the problem of genetic drift in GA which restricts the diversity in the population. ... To cope with the above mentioned drawbacks, many variants of GAs have been proposed often tailored to a particular problem. Recently, several simpler and yet effective heuristic algorithms such as Population Based Incremental Learning (PBIL) and Differential Evolution (DE), etc., have received increasing attention.
Open Access
Application of evolutionary algorithms for optimal directional overcurrent relay coordination
(2014) Stenane, Ndabeni Moses; Folly, Komla A
Relay coordination is necessary to ensure that while protection relays operate as fast as possible, they are also able to isolate only the faulted parts of the system from the network, ensuring that a power system disturbance does not result in interruption of the power supply to a larger part of the power system network. Optimal relay coordination for overcurrent relays depends on two parameters, namely, Time Multiplier and Pickup Current Setting. The conventional method of setting these two parameters for overcurrent relays applied on the power system network is to first determine the main and backup relay pairs which form part of the clockwise and anti-clockwise loops around the power system network. The relays are then set through an iterative process to ensure coordination. Initially, a general rule of setting relays to operate in 0.2 seconds for faults in the primary zone, to ensure fast operation, and in 0.2 seconds plus additional grading time, to ensure coordination, for faults in the backup zone is applied. The next relay in the loop is tested to check if it fulfils the requirements of the initial general rule. If the conditions of the general rule are not met, the previous relay’s setting is adjusted to meet the requirements. This process is repeated until all the relays around the loop are set. Conventional relay coordination process has a limitation in the sense that it is deterministic and the settings of subsequent relays depend on the initial guess of the settings of the initial relay. Therefore, this method does not necessarily provide solutions which guarantee optimal relay coordination but the best of the solutions tried.
Open Access
Application of IEC61850 protocol to implement peer-to-peer communication on General Electric (GE) relays : evaluation of IEC61850 performance
(2013) Moerane, Christina Matankiso; Folly, Komla A
SA is said to be improving protection performance due to the Generic Object Oriented Substation Event (GOOSE) messages that can be rapidly transmitted in a network to perform protection and other functions. Based on the fact that the GOOSE messages only occupy three layers of the Open System Interconnection (OSI) model, in contrast to hardwired relay contacts which must traverse additional communication layers; the GOOSE messages are claimed to be faster than hardwired contacts on the relays. The objective of this research is to prove whether the GOOSE messages are indeed faster than the hardwired contacts. The main questions that guide the thesis are, firstly, is the speed of GOOSE messages always faster than the speed of hard wired contacts of the relays? If not, what are the factors that can affect the performance of GOOSE messages? Secondly, is a nonconventional Breaker fail function using GOOSE messages possible, without using a conventional bus zone panel? Additionally, the research sought to establish the interoperability of IEDs from different vendors using the GOOSE message system.
Open Access
A comparative study of unloaded transmission line switching with disconnectors using ATP
(2010) Arigye-Mushabe, Sharon; Folly, Komla A
Disconnectors exist in most sub-transmission networks where high voltage equipment is used. The primary function of disconnectors is to electrically isolate equipment from the power system, such as breakers, for maintenance or repair. They are also used to interrupt negligible currents such as capacitive currents associated with unload transmission lines.
Open Access
Comparison of DigSILENT, Matlab PST and PSAT for steady state and stability studies on HVAC-HVDC systems
(2012) Ubisse, Albino Virgílio; Folly, Komla A
It is said that the electric power system is the most complex system ever built by mankind. Over the past few decades, many software packages focusing on the study of this complex system have been developed. These software packages range from academic/ research based to industrial/commercial based software. Before any component/device is installed in the power system, it undergoes rigorous research, simulation and testing.
Open Access
Comparison of three power system software packages for small-signal stability analysis
(2009) Mudau, Dovhani Selby; Folly, Komla A; Awodele, Kehinde
Many power system simulation tools exist for small-signal stability analysis. This is due to the rapid development of computer systems, higher industrial growth and the need for reliable power system simulation tools for efficient planning and control of electric power systems. Three power system small-signal stability simulation tools have been selected for comparison and these are: PSAT 2.1.2, MatNetEig and PacDyn 8.1.1. These combine both open and closed source code industrial-grade power system analysis tools. The objective of this thesis is to compare three simulation tools on power system small-signal stability analysis. Input formats, data output flexibility, dynamic components and synchronous machine saturation modelling in all three simulation tools were amongst other features investigated for comparative studies.
Open Access
Cost of Electricity Interruption to Commercial and Industrial End-Users
(2019) Akpeji, Kingsley Oladipo; Folly, Komla A; Awodele, Kehinde O
The question ‘what is the cost of electricity interruptions?’ is fraught with lots of complexities as electricity interruption is not a tradable commodity. A closely associated question is ‘from whose perspective should this cost be assessed – the electric utility or its customers?’ Extant research has shown that the primal focus should be on the electricity customer as the electric utility’s revenue loss after an electricity interruption event is significantly less than customers’ interruption cost (CIC). Existing methods of assessing the cost of electricity interruptions are not always consistent, because analysts make different assumptions, primarily in the incorporation of key parameters of electricity interruptions and customer characteristics in their analyses. However, one thing is important: the chosen assessment method should suit the decision-making context in which the cost data will be applied. In this dissertation, both micro- and macro-level approaches were applied to the assessment of the cost of electricity interruptions to commercial and industrial electricity customers. However, the central investigation is the micro-level assessment of the direct financial cost of electricity interruptions to suit value-based reliability planning and power system operations management. The cost assessment was done from the business customer’s viewpoint via a firm-level survey of commercial and manufacturing businesses in Cape Town. Three CIC models were developed from an analysis of the survey data viz. a time-invariant average interruption cost (TIAIC) model, a time-varying average interruption cost (TVAIC) model, and a time-varying probabilistic interruption cost (TVPIC) model. All three models were applied in an assessment of reliability worth indices for a case study distribution system to demonstrate the practical application of the cost data. The results showed that the TVPIC model is more effective for describing CIC as it accounts for the time-dependencies and uncertainty in CIC estimates. The TVPIC allows for an evaluation of the impact of different confidence levels in decision-making. Reliability worth indices like ECOST derived based on the TVPIC can be expressed as Rands@Risk in different season-time windows. This allows for optimal implementation of contingency measures like load shedding or reliability improvement programs like switch/disconnect placement on distribution feeders. An exploratory macroeconomic analysis was also done using an input-output (IO) model that allowed the investigation of the effect of the removal of the electricity sector from intersectoral interactions in South Africa’s economy. Based on the model’s framework and assumptions, the potential economy-wide cost of a day-long blackout was estimated to be approximately R2.2 billion. Compared to estimates of the economic cost of past load shedding events, this figure seemed to be a very optimistic estimate and a potential lower bound of a day-long blackout in South Africa. Also, the relationship between the firm-level survey and the macroeconomic IO approaches to estimating the cost of electricity interruptions was assessed via a case study of the weekly cost of load shedding to South Africa’s trade and manufacturing sectors. The ensuing discussions show that caution must be exercised in quoting blanket figures of the cost of load shedding to the South African economy without appropriate description of the basis for estimation.
Open Access
Decision theory process for making a mitigation decision on harmonic resonance in power systems
(2003) Atkinson-Hope, Gary; Folly, Komla A
Decision analysis is a scientific tool that is traditionally applied in business and not to electrical engineering decisions. The reason for this research is to show how to use decision analysis to make a decision on the size of a power factor correction capacitor to be installed in an end-user plant in an electrical power system, that has the potential for harmonic resonance. How to make a decision as to whether or not mitigation is needed is also researched. The two-stage decision theory process, developed by management scientists, to assist decision makers on making a decision when uncertainty, risk and certainty situations-exist, is reviewed in this thesis. To understand the application of decision theory, worked examples are included to improve understanding and to provide a foundation for the new work introduced. The addition of capacitors to a harmonic carrying system can result in resonance. Harmonic levels can be magnified well above accepted limits and this can cause damage to system components, especially capacitors. Recognizing and correcting a harmonic resonance problem before disastrous consequences arise is essential for system designers. Traditionally, when considering harmonic resonance, power factor correction capacitors are sized heuristically and a power factor of 0.95 is taken as a starting point. Usually, a harmonic analysis software package is used and a frequency scan study is conducted to generate a resonance curve. Resonant points are then compared to the harmonics in the system. If there is coincidence, the technique of de-tuning is applied to overcome overlapping and to choose the capacitor size. For utilities to maintain system efficiencies at acceptable levels, they encourage end-users to use a capacitor size so that the power factor has a value greater than 0.9 and as a rule of thumb, correction is not done to unity. This traditional technique is subjective and lacks decision structure. A new three-stage decision theory process for making a harmonic resonance mitigation decision in an end-user plant is developed. Two new indices are developed to assist in making the decision. The first index assesses the severity of resonance and the second is used to make a mitigation decision. In Stage 1, a quantitative model is developed to structure and represent the decision problem with the harmonic resonance severity index as the objective function. The model uses a fixed capacitor based on full load rating as this represents the worst case. In Stage 2, Utility Theory is used as the decision criterion to select the most desirable capacitor size. In Stage 3, the mitigation index is applied to assess if mitigation is needed or not for the chosen capacitor. Three case studies, based on deterministic models are conducted and they demonstrate the effectiveness of this newly developed decision theory process.
Open Access
Design of power system stabilizers using evolutionary algorithms
(2010) Sheetekela, Severus Panduleni Napandulwe; Folly, Komla A
Over the past decades, the issue of low frequency oscillations has been of major concern to power system engineers. These oscillations range from 0.1 to 3Hz and tend to be poorly damped especially in systems equipped with high gain fast acting AVRs and highly interconnected networks. If these oscillations are not adequately damped, they may sustain and grow, which may lead to system separation and loss of power transfer.
Open Access
Geomagnetically induced currents (GIC) in large power systems including transformer time response
(2015) Oyedokun, David Temitope; Gaunt, C Trevor; Folly, Komla A
Geomagnetically induced currents (GIC) are the result of changing geomagnetic fields which are a consequence of a geomagnetic disturbance (GMD). The flow of GIC through transmission lines and transformers across the power network could have severe consequences, if the magnitudes of the GIC are high enough. Problems that could arise from the flow of GIC in transmission networks include an increase in the amount of reactive power demand by GIC-laden transformers, half-wave saturation, excessive heating in transformers, incorrect operation of transmission line protection schemes and voltage collapse in affected sections of the network. In the past, GIC were calculated without taking the transformer's response time into account. The limitation of this approach is that the size and core type of the transformer is neglected. This may affect the assessment of GIC in the power network as the flux pattern and winding inductance distribution are not uniform across all transformer core structures. This thesis postulates that these characteristics could have far-reaching effects on the GIC that flows through a transformer as a function of time. Based on this assumption, a novel way of calculating GIC is introduced in this thesis. This method combines the uniform plane wave model and the network Nodal Admittance Matrix (NAM) method and incorporated for the first time, the transformer time response, which does not appear to have been considered in previous calculation methods. A general formula, which describes the transformer's time response to GIC was derived, followed by the derivation of the electric field induced in each transmission line. A key input to the prospective GIC with transformer time response calculation, is a set of piecewise linear equations derived from a laboratory test and PSCAD simulations. These suitably characterise the response of three transformer core structures, namely: bank of single phase (3(1P-3L)), three-phase three-limb (3P-3L) and three-phase five-limb transformers (3P-5L). Each of these core types were considered as a Generator Step-up Unit (GSU) and a Transmission Transformer (TT). The results of the laboratory experiment and simulations in PSCAD led to the conclusion that the transformer time response to GIC is irregular across the transformer cores that were tested. The 300 VA transformer core structure with the shortest response time is the 3P-3L, followed by the 3P-5L and the 3(1P-3L). For the 500 MVA transformers, the order was: 3P-3L; 3(1P-3L); and 3P-5L. The 3P-3L transformers permit the flow of GIC through the windings of the transformer over a shorter length of time. Therefore based on the order in response time, during GMDs leading to higher GIC, the prospective GIC with or without transformer time response flowing through 3P-3L transformers will be similar. Furthermore, the response time to GIC in 3P-3L, 3P-5L and 3(1P-3L) transformer core types are load-dependant. The 3(1P-3L) and 3P-5L transformers operating as TT's (modelled as transformers at 40 % load) have the longest response time to GIC, while 3P-3L transformers operating as a GSU (modelled as transformers at full load) have the longest response time to DC. The shortest response time to DC was with a GSU at light load (modelled as transformers at 80 % load), which was consistent across the three transformer core types. This correlates well with the notion that power networks could stand a better chance of surviving a high GMD when all generating units and loads are online. Three different core structures were modelled with a variation of DC current levels and load conditions, both in PSCAD and in the laboratory. These results are unique to the transformer models used, but are representative of major types of core configurations used on power networks. These results provide an indication that it is incorrect to lump the responses of all transformers and transformer time response should be taken into consideration, especially when sampling at intervals as low as 2 seconds.
Open Access
Impact assessment of large-scale penetration of permanent magnet synchronous generators on power quality
(2017) Ntsadu, Ntlahla; Folly, Komla A; Khan, Mohamed Azeem
Wind power generation has gained a large share in the renewable energy market over the past few years. This study investigates the impact of large scale penetration of permanent magnet synchronous generator (PMSG) based wind turbines on power quality of the grid. PMSGs are attractive due to the absence of a gearbox in the drive-train, which results in lower maintenance costs and higher reliability. Moreover, the advancements in power electronics have facilitated PMSGs to generate optimal power at varying wind speed conditions. This is achieved through the use of maximum power point tracking algorithms. The drawbacks of PMSG-based wind energy systems are that they inject harmonics into the network and cause flicker as well as other power quality issues. Despite these disadvantages, the grid code requires that PMSGs stay connected to the grid even under grid disturbances. This is because the reactive power control capability of PMSG-based wind energy systems can actually assist with voltage support. It will be shown in this study that disconnecting large scale PMSGs based wind turbines during grid disturbances has a detrimental effect on transient stability of the grid. This study will show that PMSG-based wind energy systems improve transient stability and assist in voltage support through reactive power control. Moreover, the impacts of large scale PMSG based wind turbines on power quality of the grid can be reduced by various means, which are also addressed in the study.
Open Access
The impact of wind generators on a Powe system's transient stability
(2009) Khomari, Moloantoa; Folly, Komla A
This thesis discusses the investigations carried out on the different types of wind generators and how these would affect the transient stability of a hypothetical power network as presented in this report. Focus was on the transient responses of the conventional synchronous generator’s rotor angle and terminal voltage when connected to different types of wind generators. The three different wind generator technologies explored were the squirrel cage induction generator (SCIG), doubly-fed induction generator (DFIG) and the converter driven synchronous generator (CDSG).
Open Access
Implementation of wide area protection system (WAPS) for electrical power system smart transmission grids
(2021) Tetteh, Bright; Awodele, Kehinde O; Folly, Komla A
The planning, operation and control of the power system has been evolving since its inception. These changes are due to the advancement in science and technology, and changes in energy policy and customer demands. The envisioned power system - smart grid (SG) - is expected to have functional and operational capabilities that maximize the reliability, minimize generation deficit, and cost issues in the power system. However, many power systems in the world today still operate traditionally, with one-way communication and one-way power flow. Transitioning to a smart grid influences the protection schemes of the power system, as the smart grid is to leverage distributed energy resources (DERs) using distributed generation (DG) units and allow for bi-directional flow of power and information. Therefore, there is a need for advanced protection schemes. Wide-area protection (WAP) techniques are proposed as one of the solutions to solve the protection challenges in the smart grid due to their reliance on wide-area information instead of local information. This dissertation considered three WAP techniques which are differentiated based on the data used for faulted zone detection: (A) Positive sequence voltage magnitude (PSVM), (B) Gain in momentum (GIM) and (C) Sum of positive and zero sequence currents (SPZSC). The dissertation investigated their performances in terms of accuracy in detecting the faulted zones and the faulted lines, and fault clearing time. The investigation was done using three simulation platforms: MATLAB/Simulink, Real-Time (Software in the Loop (SIL)) and Hardware-in-the-Loop (HIL) implementation using Opal-RT and SEL-351A relay. The results show that, in terms of detecting the faulted zones, all the techniques investigated have 100% accuracy in all the 36 tested fault cases. However, in terms of identifying the faulted line in the faulted zone, the algorithms were not able to detect all the 36 tested cases accurately. In some cases, the adjacent line was detected instead of the actual faulted line. In those scenarios, the detected line and the faulted line present similar characteristics making the algorithms to detect the wrong line. For the faulted line detection accuracy, the algorithm (A) has an accuracy of 86%, (B) has an accuracy of 94% and (C) has an accuracy of 92%. The fault clearing times of the algorithms were similar for both the MATLAB/Simulink and realtime simulation without the actual control hardware which was the SEL-351A relay. When the simulation was done with the control hardware through Hardware-in-the-loop, a communication delay was introduced which increased the fault clearing times. The maximum fault clearing time for the techniques investigated through the HIL simulation are 404 ms, 256 ms, and 150 ms for the techniques (A), (B) and (C) respectively and this variation is due to the different fault detection methods used in the three algorithms. The fault clearing time includes communication between the Opal-RT real-time simulator and SEL-351A relay using RJ45 ethernet cable, these fault clearing times can change if a different communication medium is used. From the performance data presented, it is evident that these algorithms will perform better when used as backup protection since the common timer settings for backup protection schemes range from 1200 ms to 1800 ms, while primary protection is expected to respond almost instantaneously, that is, with no initial time delay.
Open Access
An improved algorithm for phase-based voltage dip classification
(2007) Jattiem, Mogamad Shaheed; Folly, Komla A
In this thesis, a new phase-based algorithm is developed, which overcomes the shortcomings of the Bollen algorithms. The new algorithm computes the dip type based on the difference in phase angle between the measured voltages.
Open Access
Integration of renewable energy into Nigerian power systems
(2017) Awodiji, Olurotimi Olakunle; Folly, Komla A
Many countries are advancing down the road of electricity privatization, deregulation, and competition as a solution to their growing electricity demand and other challenges posed by the monopolistic nature of the existing structure. Presently, Nigeria has a supply deficit of electricity as a result of the growing demand. This imbalance has negatively affected the economy of the country and the social-economic well-being of the population. Hence, there is an urgent need to reform the power sector for greater efficiency and better performance. The objectives of the reform are to meet the growing power demand by increasing the electric power generation and also by increasing competitiveness through the participation of more private sector entities. The renewable energy integration is one way of increasing the electricity generation in the country in order to cater for the growing demand adequately. Examples of the renewable energy that is available in the country include wind, geothermal, solar and hydro. They are considered to be environmentally friendly, replenishable and do not contribute to the climate change phenomena. The country presently generates the bulk of its electricity from both thermal (85%) and hydroelectric (15%) power plants. While electricity generation from the thermal power stations constitutes the largest share of greenhouse emission, this is mostly from burning coal and natural gas. The effect of this high proportion of greenhouse emission causes climate change which is referred to as a variation in the climate system statistical properties over a long period of time. It has been observed that many of the activities of human beings are contributory factors to the release of these greenhouse gases (GHG). But, as the traditional sources of energy continue to threaten the present and future existence on the planet earth, it is, therefore, imperative to increase the integration of the variable renewable energy sources in a sustainable and eco-friendly manner over a long period of time. The variability and the uncertainties of the renewable energy source's output, present a major challenge in the design of an efficient electricity market in a deregulated environment. The system deregulation and the use of renewable sources for the generation of electricity are major changes presently being experienced in power system. In a deregulated power system, the integration of renewable generation and its penetration affects both the physical and the economic operations. The main focus of this research is on the integration of wind energy into Nigerian power systems. Up till now, research on the availability of the wind energy and its economic impacts has been limited in Nigeria. Generally, the previous study of wind energy availability in Nigeria has been limited in scope. The wind energy assessment study has not been detailed enough to be able to ascertain the wind energy potential of the country. To cope with this shortcoming, a detailed statistical wind modeling and forecasting methodology have been used in this thesis to determine the amount of extractable wind energy in six selected locations in Nigeria using historical wind speed data for 30 years. The accuracy test of the statistical models was also carried using the Mean Absolute Error (MAE), Root Mean Square Error (RMSE), and Chi-Square methods to determine the inherent error margin in the modeling and analysis. It is found that the error margin of the evaluations falls within the expected permissible tolerance range. For a more detailed wind assessment study of the Nigeria weather, the seasonal variation of the weather conditions as it affects the wind speed and availability during the two major seasons of dry and rainy was considered. A Self-Adaptive Differential Evolution (SADE) was used to solve the economic load dispatch problem that considers the valve-point effects and the transmission losses subject to many constraints. The results obtained were compared with those obtained using the "standard" Differential Evolution (DE), Genetic Algorithm (GA), and traditional Gradient Descent method. The results of the SADE obtained when compared with the GA, DE, and Gradient descent show the superiority of SADE over all the other methods. The research work shows that the wind energy is available in commercial quantity for generation of electricity in Nigeria. And, if tapped would help reduce the gap between the demand and supply of electricity in the country. It was also demonstrated that the wind energy integration into the power systems affects the generators total production cost.
Open Access
Integration of wind energy systems into the grid: power quality and technical requirements
(2010) Madangombe, Taruziwa; Folly, Komla A
The integration of wind energy into the utility network has increased significantly over the past years largely as a result of the increasing environmental concerns arising from the use of fossil fuels, coupled with the anticipated global increase in oil. In South Africa, the wind energy industry is still in its infancy, with the Klipheuwel (about 3.2 MW) and Darling (about 4.2 MW) wind farms being the only grid connected projects in the country. However, grid integration studies carried out in [1] have shown that there are over 7 000 MW potential ideas for wind power in the Western Cape alone and this is a clear indication that there is a growing interest in wind development locally. The Government has also set a 4% target for the development of the renewable energy in the country by 2013. In light of the above, this thesis discusses some of the technical requirements and power quality issues that need to be addressed in order to fully integrate wind power into the network without adversely affecting the operation of the grid. These have been researched through reviewing the various standards and grid codes for wind power that have been implemented in other leading countries, in order to identify some of the requirements that can be adapted to suit our local integration process. Some of the main technical issues that are discussed in this thesis include the strength of the grid (fault levels), permitted penetration levels, choice of wind turbine and the reactive power requirements of the network. All these issues contribute towards the resolution of the impact of wind turbines on the power quality of the network, especially at the point of common coupling or connection (PCC). Various power quality phenomena were discussed in the literature but the ones that were further investigated included the voltage level profile, harmonic distortions as well as reactive power requirements from the wind turbines. These were determined both during the steady operation of the network and during a network disturbance.
Open Access
An investigation into the capabilities of three simulation tools for small-disturbance stability analysis
(2007) Ntombela, M; Folly, Komla A
This MSc thesis forms part of more comprehensive research being conducted at the University of Cape Town comparing power system simulation tools. Part of the ongoing research at the University of Cape Town, which looks at solution methodology, power system component models and software flexibility using simulation tools not discussed in this thesis can be found in [4] and [7]. The objectives of this research are to investigate the capabilities of three power system simulation tools for small-disturbance angle stability analysis, namely PST, MatNetEig and CPAT Computers today have become fast, efficient with high memory capacities and advanced in data processing capabilities. Many power system simulation tools are available on the market and making a decision about which simulation tool to purchase has become complicated, especially with existing high competition between vendors. Once a decision has been made and the tool has been purchased, most users do not want to change to a new tool for several reasons. Firstly, learning how to use power system simulation tools is time consuming and secondly, the tools are very expensive.
Open Access
Investigation into transient stability of a nuclear power plant using DIgSILENT
(2016) Emmanuel, Paul; Folly, Komla A
The current electricity crisis, coupled with the lack of generation, has led to a major focus on continuity of supply. The Western Cape has been severely affected, as it only has a limited number of generation sources, namely Koeberg Nuclear Power Station (base supply) along with other power stations used for peak load running such as Ankerlig, Palmiet, etc. Koeberg Nuclear Power Station is located at the end of a long transmission line with no other base-load generation for 1500km, between the power station and the mass pool of coal-fired generation in Mpumalanga. Koeberg Nuclear Power Station (herein referred to as "Koeberg") plays a significant role in ensuring the stability of the Western Cape's electricity network. Without this power stattion, the network power flow is greatly affected, and is placed under severe strain. Koeberg is the determining factor when the system operators control the Western Cape network. The network is modelled having one of Koeberg's unit offline, using contingency analysis N-1. The one Koeberg unit being the N-1 contingency factored into the transmission network. The network should be able to withstand a disturbance with one Koeberg unit out of service and maintain stability after the disturbance. With the Eskom's decision to increase Koeberg power plant's electrical output power, it became apparent that the impact of this upgrade needed to be assessed. In the past, various hand calculations and assumptions were made before implementation of these types of changes could occur. With the advent of technology, modern computer-based software simulation tools have reduced the time to analyse such changes and aid engineers to quickly assess the impact it would have on various components. A load flow and short circuit studies of the Koeberg internal networks were performed and verified against plant data. The original data was compared to simulated data using a computer-based simulation package. The simulation software package used to validate the results is the DIgSILENT software package. This is one of the standard software packages used by Eskom to validate models on the network. Load flow studies for Koeberg have been completed in the past, but many changes have since been made to the plant. There was thus a requirement to re-calculate the original load flow studies and ensure that all plant and protection settings are within an acceptable criterion. This new study found that there were minor errors in the on-site documentation and that the software is compatible with the plant data.
Open Access
Investigation into voltage and angle stability of a hybrid HVAC-HVDC power network
(2010) Azimoh, Leonard Chukwuma; Folly, Komla A; Chowdhury, Shyama Pada
This study investigates the power stability problems of HVDC and VSC-HVDC interactions on their hybrid networks with HV AC link, with the intention of bringing out their weaknesses and strengths. The knowledge of this will assist network planners to be informed on ways of improving the efficiency and quality of power systems network. The simulations for this study was done using DIgSILENT Powerfactory software version 14.0.515. This study encapsulates the three major stability problems affecting power systems network, namely, the voltage stability, transient stability and small signal stability. The voltage stability study was conducted using series of load flows at various levels to plot the VP and QV curves, and the results were used to analyze the systems proximity and sensitivity to voltage collapse, as well as the maximum loading point (MPL) of the network. Furthermore, the voltage angle, and terminal voltage responses during a three-phase short circuit disturbance was also used to analyze the voltage stability of the networks. For the transient stability study, several case studies were investigated and their dynamic performances during three-phase short circuit perturbations were analyzed. The small signal stability investigation was done using modal analysis to determine the small signal stability of the three transmission schemes mentioned above. The transient and small signal stability, which are both subsets of rotor angle stability, were further investigated to show the effect of power systems stabilizer (PSS) and automatic voltage regulator (A VR) on rotor angle stability. The results of the analyses show that the HVDC transmission scheme provides the best alternative for bulk power transmission over a long distance. The VSC-HVDC transmission network is suitable for interconnections where the tie with HV AC networks have a low short circuit ratio (SCR). Other conclusions reached with the investigations are explained in chapter ten.
Open Access
Investigation of the impact of demand elasticity and system constraints on electricity market using extended Cournot approach
(2015) Yan, Jun; Folly, Komla A
In today's electricity supply industry, demand side participation is considered an important factor that can influence the market performance and output effectively. Demand elasticity shows the sensitivity of demand side to the market price, and thus can provide potential adjustment of demand in the market. The purpose of this research is to study the impact of demand elasticity on power producers 'market competition output. An analytical model, called "Extended Cournot model" is developed in this thesis based on the classical Cournot model. Through the integration with conjectural variation model, in which power producers consider both the generation and price level, the extended Cournot model can analyze electricity market results under the conditions of different constraints. In the classical Nash Cournot model, capacity withdrawal exists in most cases especially when transmission constraint occurs. In contrast, the newly developed analytical model ensures that demand is always satisfied at all time. Demand elasticity is incorporated directly into the market results calculation instead of using the market clearing price. This approach enables the load demand to directly obtain the market results by tuning its demand elasticity. The intention is to show that demand side should be more encouraged to participate in the market competition. In the classical economic dispatch, the load demand is highly inelastic. From the load curve, there is only a change of physical volume of demand. The demand responsiveness, which is represented by demand elasticity, has been understated. In this thesis, the hypothesis is that demand elasticity and system constraint have critical influence on the power producers' competition results in terms of market clearing price, individual output and profit. Load demand can make use of demand elasticity to affect its final payment to the market. Such ability is expected to be limited in the case where system constraints, i.e. generation limits and transmission limits, exist. For simplicity, a small network and number of power producers are used in this thesis to investigate the effectiveness of the Extended Cournot model. However, this model can be applied to more complex networks with different market environments.