Doctoral Degrees (Electrical Engineering)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6855

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Analysis of the impact of a facts-based power flow controller on subsynchronous resonance.
(2012) Carpanen, Rudiren Pillay.; Rigby, Bruce S.
Electric power utilities are faced with the challenge of meeting increasing demand for electric power whilst many factors prevent traditional remedies such as the expansion of transmission networks and the construction of new generating facilities. Due to issues of environment, health and rights-of-way, the construction of new generating plants and transmission lines were either excessively delayed or prevented in many parts of the world in past years. An alternative resides in loading the existing transmission network beyond its present operating region but below its thermal limit, which would ensure no degradation of the system. This alternative approach has been possible with the emergence of Flexible AC Transmission Systems (FACTS) technology. The FACTS concept involves the incorporation of power-electronic controlled devices into AC power transmission systems in order to safely extend the power-transfer capability closer of these systems to their stability limits. One member of the family of FACTS series compensators is the Static Synchronous Series Compensator (SSSC), and this thesis considers the use of the SSSC to carry out closed-loop control of AC power flow in a transmission system. Although the SSSC has the potential to enhance the operation of power systems, the introduction of such a device can cause adverse interactions with other power system equipment or existing network resonances. This thesis examines the interaction between high-level power flow controllers implemented around the SSSC and a particular form of system resonance, namely subsynchronous resonance (SSR) between a generator turbine shaft and the electrical transmission network. The thesis initially presents a review of the background theory on SSR and then presents a review of the theory and operation of two categories of SSSC, namely the reactance-controlled SSSC and the quadrature voltage-controlled SSSC. The two categories of SSSC are known to have different SSR characteristics, and hence this thesis considers the impact on the damping of subsynchronous torsional modes of additional controllers introduced around both categories of SSSC to implement AC power flow control. The thesis presents the development of the mathematical models of a representative study system, which is an adaptation of the IEEE First Benchmark system for the study of SSR to allow it to be used to analyse the effect of closed-loop power flow control on SSR stability. The mathematical models of the study system are benchmarked against proven and accepted dynamic models of the study system. The investigations begin by examining the effect of a reactance-controlled SSSC-based power flow controller on the damping of torsional modes with an initial approach to the design of the control gains of the power flow controller which had been proposed by others. The results show how the nature and extent of the effects on the damping of the electromechanical modes depend on both the mode in which the power flow controller is operated and its controller response times, even for the relatively-slow responding controllers that are obtained using the initial controller design approach. The thesis then examines the impact of a reactance-controlled SSSC-based power flow controller on the damping of torsional modes when an improved approach is used to design the gains of the power flow controller, an approach which allows much faster controller bandwidths to be realised (comparable to those considered by others). The results demonstrate that for both of the modes in which the power flow controller can be operated, there is a change in the nature and extent of the power flow controller’s impact on the damping of some the torsional modes when very fast controller response times are used. Finally, the thesis investigates the impact of a quadrature voltage-controlled SSSC-based power flow controller on the damping of torsional modes in order to compare the influence of the design of both Vsssc-controlled and Xsssc-controlled SSSC-based power flow controllers on torsional mode damping for different power flow controller response times. The results obtained indicate that a Vsssc-controlled SSSC-based power flow controller allows a larger range of SSR stable operating points as compared to a Xsssc-controlled SSSC-based power flow controller.
Frequency stability study of interconnected power systems with high penetration of renewable energy in the restructured environment: emulation and control of virtual inertia using intelligent techniques.
(2021) Aluko, Anuoluwapo Oluwatobiloba.; Carpanen, Rudiren Pillay.; Dorrell, David George.; Ojo, Evans.
The main aim of power system operations and control is to ensure reliability and quality of power supply, a key action that helps in achieving this aim is frequency control. Frequency control in power systems is the ability to maintain the system frequency within specified operating limits, i.e., proper coordination between generation and load. The task of frequency control, more importantly, load frequency control (LFC) is becoming a complex control problem in the design and operation of modern electric power systems due to its growing size, changing market structure, newly emerging distributed renewable energy sources with little or no inertia support, evolving regulatory requirements and the increasing interconnectedness of power systems. These developments can lead to a reduction in the active overall inertia in the power system which reduces its frequency response capability by increasing the amplitude of frequency deviation, continuous frequency oscillations and increased settling time after a power mismatch in the system. The potential role of virtual inertia in the task of frequency control has been identified as an integral part of modern power systems. Therefore, in this thesis, novel methods for implementing virtual inertia using intelligent control techniques are proposed in the LFC framework of a multi-area interconnected system with high penetration of renewable energy in the deregulated environment. The first method proposes the novel application of the artificial bee colony (ABC) optimization algorithm in the design of the virtual inertial control in a grid-connected wind energy conversion system (WECS). The WECS operates below the maximum power point to reserve a fraction of active power for frequency response. The proposed ABC-based control method minimizes the first frequency undershoot and active power transients compared to the classical optimization method. Due to the non-storable and variable nature of renewable energy sources, the first method may not be accessible when needed. To tackle this challenge, the second method proposes the application of an energy storage system (ESS) and the type-II fuzzy logic control (FLC) in the development of the virtual inertia control strategy. The proposed type-II FLC method gives a better performance than the type-I FLC and derivative-based control methods with adaptive inertia gain, faster response time for active power injection/discharge, and damped frequency oscillations. Lastly, a novel hybrid LFC scheme is developed to further improve the dynamic response and stability of the system. The hybrid LFC scheme consists of a robust unknown input observer (UIO) for state estimation of the system in the presence of unknown inputs/disturbances, and the interval type-II FLC for the LFC loop. The robust UIO relays the true state of the system frequency to the LFC block in each control area to maintain its frequency and net tie line power flow at scheduled values. The proposed methods are designed and implemented using the MATLAB/Simulink Software.
Methods to reduce the starting current of an induction motor.
(2022) Habyarimana, Mathew.; Carpanen, Rudiren Pillay.; Dorrell, David George.
Power system loads that have high starting currents are a serious source of concern in smaller grids or remote locations on the main grid. This problem is envisaged to be exacerbated by the rollout of smart microgrids. When a high power induction motor is turned on in such a power system, its inrush current can be up to about ten times the full-load current. This transient current can cause problems when attached to weak grids. The increased current is due to the power required to start the load and the increased reactive power demand during the starting process. To protect the grid connection as well as the load, energy storage units can be used to compensate for the increased power requirement. A more pragmatic approach is to reduce the reactive power requirement using tuned compensation capacitors in order to reduce the inrush current. The aim of this research is to address the selection, calculation and switching of the capacitor bank for reactive power compensation. The capacitors are calculated and switched on to compensate the starting transient and disconnected when the machine has run up to speed using a point-on switching approach that reduces the switching transient.
PMSG-based wind power integration-modelling and analysis of impacts on the dynamic performances of a power system and mitigation under stochastic wind disturbances.
(2017) Legesse, Ayele Nigussie.; Saha, Akshay Kumar.; Carpanen, Rudiren Pillay.
Because of the ever-growing demand for electrical energy and environmental challenges of fossil fuel consumption, a priority has been given to the development of wind energy systems, among which, currently, permanent magnet synchronous generator (PMSG)-based wind power is receiving much attention from researchers, engineers, and turbine manufacturers. However, high PMSG-based wind power integration into a power system brings several challenges to transmission system operators. One of the challenges is its impacts on the dynamic performances of a power system due to the presence of stochastic wind disturbances. Thus, for a thorough investigation of the influences of stochastic wind speed disturbances, a proper wind speed model should be adopted. Therefore, this thesis proposes the use of Markov chain model for modelling wind speed series in dynamic simulations of wind turbines. In this regard, comparison of statistical quantities of measured wind speed data from Durban and Markov model generated ones confirms the accuracy of the model adopted. The results have shown that the dynamic performances of a power system deteriorate with the presence of stochastic wind speed disturbances, and thus the need for improving poor dynamic performances. Wind gusts cause stress, over currents, over voltages and instability in a power system. This thesis, therefore, introduces novel mitigation techniques based on virtual controls stemming from real resistors, compensators, and damper windings, and supplementary controllers to enhance the dynamic performances of a wind turbine directdriven PMSG, the main component of a PMSG-based wind farm. In the proposed schemes, the virtual controllers adjust the terminal d- and q-axis reference voltages in the generator side converter controller and their influences on the dynamic performances of the wind turbine are investigated. MATLAB/Simulink simulations on a wind turbine connected to an infinite bus show that virtual controls are effective in enhancing the dynamic performances of the PMSG. Local oscillations caused by wind disturbances are efficiently suppressed. Overall, the proposed mitigation techniques smooth the rotor speed and power of a PMSG, and hence reducing the influences of the stochastic wind speed disturbances. Furthermore, the results have demonstrated that stochastic wind speed disturbances affect the dynamic performances of a power system containing a PMSG-based wind farm as the dynamics of synchronous machines within the system depend on power balance, which is influenced by the power response of the wind farm. Finally, investigations in this thesis have confirmed that virtual controls and FACTS devices such as STATCOM and SVC are efficient in improving the dynamic performances of a power system containing PMSG-based wind farms under stochastic wind disturbances.
Reliability study under the smart grid paradigm using computational intelligent techniques and renewable energy sources.
(2022) Onaolapo, Adeniyi Kehinde.; Carpanen, Rudiren Pillay.; Dorrell, David George.; Ojo, Evans Eshiomogie.
The increase in the demand for a reliable electricity supply by the utilities and consumers has necessitated the evaluation of the reliability of power systems. A reliable electricity supply is characterized by no or minimal duration and frequency of supply outages. Current power systems are changing due to increasing power demand and depletion of fossil fuel deposits. These changes are related to smart grids which are intelligent electric networks that are capable of using demand management methods, supporting communication devices and monitoring of consumer energy consumption. They can also integrate renewable energy sources thereby reducing reliance on fossils fuel sources. The main objective of this study is to optimize power systems operations and improve reliability. Different optimization methods are proposed in this study to address the issues of power systems operations. These optimization problems consider different constraints for maximum operations of the power systems. Case studies are used to confirm the proposed methods using the historical and climatic data for the City of Pietermaritzburg (29.37°S and 30.23°E), and Newcastle (27.71°S, 29.99°E) South Africa. Firstly, the implementation of the back-propagation algorithm method of the artificial neural networks (ANNs) for designing a predictive model for power system outage is proposed. The results obtained are found to be satisfactory. In situations where there is the problem of accessibility to large system data and presence of multiple system constraints, another method is proposed. This second technique proposes the application of a maximum entropy function-based multi-constrained event-driven outage prediction model, using the collaborative neural network (CONN) algorithm. The outcome is better than the conventional event-driven methods. Lastly, an adaptive model predictive control (AMPC) method with the integration of renewable energy sources (RESs) and a battery energy storage system (BESS) is proposed to further improve the reliability of the power system. The developed method uses a modified Roy Billinton Test System (RBTS) to implement the reliability improvement of the power system. The proposed computational intelligent techniques fulfil the necessities of operation robustness, implementation simplicity and reliability improvement of the power systems.
Stability of the grid incorporating multi terminal HVDC: case study of a south African network.
(2021) Oni, Oluwafemi Emmanuel.; Swanson, Andrew Graham.; Carpanen, Rudiren Pillay.
Transmission lines make one of the significant parts of power systems; faults or disturbances along any of the transmission medium often transcend to both the generating ends and the loads' end. Besides, the strength of any particular grid depends solely on the impedance of the tie-lines of that grid. Therefore, in this thesis, the line commutated converter (LCC) multiterminal high voltage direct current (MTDC) system is modelled and improved for the stability of an AC network. The converter control architecture and modelling are emphasized and explained. The effective short circuits ratio (ESCR) of the interconnecting AC lines is first described and analyzed as well. The existing CIGRE control techniques for a point-to-point LCC HVDC system have been enhanced and adapted for this study. The control and the filter parameters have also been calculated to generate a better and efficient result during a steady-state and dynamic analysis of the study. The work carried out in this study is divided into four sections, with each section focusing on each of the research objectives. In the first section, dynamic modelling and control of LCC MTDC systems were carried out with consideration to the ESCR of the inverter side of the AC substation. The impact of large-disturbance at the inverter is investigated. This analysis has been proposed to study the impact of AC short circuit fault on the three substations. The results from this study, which are shown on a subplot, show that the system experienced a large transient overcurrent and non-severe commutation failures. Also, a voltage dip at the faulted inverter station was recorded; however, the efficacy of the converter controller disallowed the transfer of such voltage dip to the other two converters. The second section of this study focuses on the application of MTDC system. We have carried out a comparative analysis of MTDC and AC transmission line on a single machine infinite bus (SMIB) network. The main focus of the investigation was on the transient and rotor angle stability of the SMIB network with or without MTDC link. The study also carried out a power-angle curve with the use of equal area criterion. The third section focuses on the interarea oscillation reduction in a power system. Kundur's two-area four-machine network was adapted to suit the scenarios of this study. Different fault analysis was carried out, and the response of the generator active power, frequencies, and DC-bus voltages are recorded. The results in this study show the better performance of the MTDC implemented in this study over the other well-known method of AC transmission medium. Also, the integration of the MTDC link is constrained by the variation of the current order of the overall power controller. The result is observed in the damping rate of the interarea oscillation of the network. The final section of this study carried out dynamic modelling of the South African grid, and detailed dynamic response to different stability studies was carried out. An auxiliary controller for the MTDC system capable of reducing the active power oscillation by generating a new current order is proposed. This secondary control for the MTDC system is based upon dynamic sensitivity analysis of the oscillations, and thereby generate a DC current compensation for the reduction of active power oscillations in the MTDC converters' station. Two network configurations were considered in this section. System disturbance during the first configuration shows a loss of synchronizing effect from both the AVR and PSS, which causes the generator to lose synchronism with subsequent oscillations. A negative damping torque for the rotor angle and negative synchronizing torque for the interarea oscillations was also observed. Meanwhile, the results during the second configuration recorded quick damping of the interarea oscillations with a significant improvement to the voltage profile. Among all of these benefits, the power carrying capacity at a reduced loss and cost stood out. The conclusion from this section is that the implementation of the MTDC link on the South African grid provided a better system performance. Therefore, the adoption of this research into South African transmission network will surely help enhance the stability margin of the grid. The proposed secondary controller also provided potential mitigation of excessive active power dip of the MTDC link during the system disturbance.

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Browsing Doctoral Degrees (Electrical Engineering) by Author "Carpanen, Rudiren Pillay."