Analysis of the impact of a facts-based power flow controller on subsynchronous resonance.

Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2012.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

Enhancing transient stability of power systems using a thyristor controlled series capacitor.

Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2005.The continuously growing demand for electric power requires transmitting larger amounts of power over long distances. An economically attractive solution to increase the power transfer through a long interconnection (up to a limit) without building new parallel circuits is to install series capacitor compensation on the transmission line. Large disturbances which constantly occur in power systems may disrupt the synchronous operation of the generators and lead to out-of-step conditions. Coordinated insertion and removal of the compensating capacitors in series with a transmission line is an approach that has been known for many years to be capable of enhancing the transient stability of power systems as well as providing additional damping to the power system oscillations. The relatively recent emergence of the thyristor controlled series capacitor (TCSC) has now made this method of transient stability enhancement practically feasible. This thesis compares a range of different strategies that have been proposed in the literature for control of series compensating reactance to enhance transient stability. Initially a simple swing-equation model of a single-generator power system, including an idealised controllable series compensator (CSC) is used to study the fundamental characteristics of the variable impedance control and its impact on transient stability. Subsequently, a detailed model of a small study system is developed, including a detailed representation of a TCSC, for more in-depth analysis. This detailed study system model is then used to compare three different transient stability control schemes for the TCSC, namely: generator speed-deviation based bang-bang control, discrete control based on an energy-function method, and nonlinear adaptive control. Time-domain results are presented to demonstrate the impact of the TCSC on first swing stability of the SMIB system with the above control schemes for various fault scenarios. The performance of each control scheme is also compared by evaluating the extent to which it extends the transient stability margin of the study system. For each of the three different TCSC control approaches considered, the results show that variable impedance control of the TCSC provides further improvement in the transient stability limits of the study system over and above the improvement that is obtained by having a fixed-impedance TCSC in the system. In the case of the bangbang and discrete control approaches, it is shown that a combination of a large steady state value of the TCSC compensation, together with a relative small range of variable TCSC reactance under transient conditions, offers. the best improvement in the transient stability limits for the studied system. The results also show that there is little difference in the extent to which the energy function method of TCSC control improves the transient stability limits over the improvement obtained using speed-deviation bang-bang control of the TCSC for the study system considered

Small signal stability analysis of a four-machine system with placement of multi-terminal high voltage direct current link

Inter-area oscillation caused by weak interconnected lines or low generator inertia is a critical problem facing power systems. This study investigated the performance analysis of a multi-terminal high voltage direct current (MTDC) on the damping of inter-area oscillations of a modified two-area four-machine network. Two case studies were considered, utilising scenario 1: a double alternating current (AC) circuit in linking Bus_10 and Bus_11; and scenario 2: a three-terminal line commutated converter high voltage direct current system in linking Bus_6 and Bus_11 into Bus_9. It was found that scenario 2 utilising MTDC link with a robust controller provided quick support in minimising the network oscillations following a fault on the system. The MTDC converter controllers’ setup offered sufficient support for the inertia of the AC system, thus providing efficient damping of the inter-area oscillation of the system

Impact of LCC–HVDC multiterminal on generator rotor angle stability

Multiterminal High Voltage Direct Current (HVDC) transmission utilizing Line Commutated Converter (LCC-HVDC) technology is on the increase in interconnecting a remote generating station to any urban centre via long distance DC lines. This Multiterminal-HVDC (MTDC) system offers a reduced right of way benefits, reduction in transmission losses, as well as robust power controllability with enhanced stability margin. However, utilizing the MTDC system in an AC network bring about a new area of associated fault analysis as well as the effect on the entire AC system during a transient fault condition. This paper analyses the fault current contribution of an MTDC system during transient fault to the rotor angle of a synchronous generator. The results show a high rotor angle swing during a transient fault and the effectiveness of fast power system stabilizer connected to the generator automatic voltage regulator in damping the system oscillations. The MTDC link improved the system performance by providing an alternative path of power transfer and quick system recovery during transient fault thus increasing the rate at which the system oscillations were damped out. This shows great improvement compared to when power was being transmitted via AC lines

Balancing of Low-Voltage Supply Network with a Smart Utility Controller Leveraging Distributed Customer Energy Sources

In South Africa, there has been a rapid adoption of solar power, particularly inverter-based solar sources, in low-voltage (LV) networks due to factors such as load shedding, rising electricity costs and greenhouse gas emissions reduction. In residential LV networks, the alignment between solar supply and energy demand is less precise, necessitating larger battery storage systems to effectively utilize solar energy. Residential areas experience peak energy demand in the morning and evening when solar irradiance is limited. As a result, substantial energy storage is important to fully utilize the potential of solar energy. However, increasing inverter-based, customer-generated power creates an imbalance in the utility supply. This is because utility LV supply transformers have three phases, while individual customers have single-phase connections and no load balancing control mechanism. This supply imbalance adversely affects the overall power quality, causing energy losses, damage to devices and other issues. To address these problems, the paper proposes a smart control approach to minimize power imbalances within utility LV supply transformers. The controller uses customer battery storage in residential areas to balance the utility transformer phases. A laboratory model was built to simulate a three-phase low-voltage network with single-phase customers, both with and without a smart controller. The results show that closely monitoring and controlling individual inverters through a central controller can significantly improve the balance of the supply network

Implementation of a Multiterminal Line Commutated Converter HVDC Scheme with Auxiliary Controller on South Africa’s 765 kV Corridor

The deployment of a 765-kV transmission line on Eskom’s South African Grid marks the beginning of a new era in power industries. The integration of renewable energies by independent power producers (IPPs) leads to an infrastructural change in the stability performance of the entire grid. These developments are expected to bring about a multiterminal direct current (MTDC) system for practical implementation on this grid. Therefore, this study focuses on the dynamic response of the South African transmission grid during a system disturbance. In the carrying out of this study, the South African grid was modeled on PSCAD, and its performance was evaluated. The impact of the MTDC link on the grid’s interarea oscillation was also investigated. An additional current order controller for the MTDC link was developed, and its impact on the MTDC power transfer was analyzed. The results show a better system performance and reduced interarea power swings with the inclusion of the MTDC link

A Comprehensive Review: Study of Artificial Intelligence Optimization Technique Applications in a Hybrid Microgrid at Times of Fault Outbreaks

The use of fossil-fueled power stations to generate electricity has had a damaging effect over the years, necessitating the need for alternative energy sources. Microgrids consisting of renewable energy source concepts have gained a lot of consideration in recent years as an alternative because they use advances in information and communication technology (ICT) to increase the quality and efficiency of services and distributed energy resources (DERs), which are environmentally friendly. Nevertheless, microgrids are constrained by the outbreaks of faults, which have an impact on their performance and necessitate dynamic energy management and optimization strategies. The application of artificial intelligence (AI) is gaining momentum as a vital key at this point. This study focuses on a comprehensive review of applications of artificial intelligence strategies on hybrid renewable microgrids for optimization, power quality enhancement, and analyses of fault outbreaks in microgrids. The use of techniques such as machine learning (ML), genetic algorithms (GA), artificial neural networks (ANN), fuzzy logic (FL), particle swarm optimization (PSO), heuristic optimization, artificial bee colony (ABC), and others is reviewed for various microgrid strategies such as regression and classification in this study. Applications of AI in microgrids are reviewed together with their benefits, drawbacks, and prospects for the future. The coordination and maximum penetration of renewable energy, solar PV, and wind in a hybrid microgrid under fault outbreaks are furthermore reviewed

A Comparative Assessment of Conventional and Artificial Neural Networks Methods for Electricity Outage Forecasting

The reliability of the power supply depends on the reliability of the structure of the grid. Grid networks are exposed to varying weather events, which makes them prone to faults. There is a growing concern that climate change will lead to increasing numbers and severity of weather events, which will adversely affect grid reliability and electricity supply. Predictive models of electricity reliability have been used which utilize computational intelligence techniques. These techniques have not been adequately explored in forecasting problems related to electricity outages due to weather factors. A model for predicting electricity outages caused by weather events is presented in this study. This uses the back-propagation algorithm as related to the concept of artificial neural networks (ANNs). The performance of the ANN model is evaluated using real-life data sets from Pietermaritzburg, South Africa, and compared with some conventional models. These are the exponential smoothing (ES) and multiple linear regression (MLR) models. The results obtained from the ANN model are found to be satisfactory when compared to those obtained from MLR and ES. The results demonstrate that artificial neural networks are robust and can be used to predict electricity outages with regards to faults caused by severe weather conditions