Advanced and Innovative Optimization Techniques in Controllers: A Comprehensive Review

New commercial power electronic controllers come to the market almost every day to help improve electronic circuit and system performance and efficiency. In DC–DC switching-mode converters, a simple and elegant hysteretic controller is used to regulate the basic buck, boost and buck–boost converters under slightly different configurations. In AC–DC converters, the input current shaping for power factor correction posts a constraint. But, several brilliant commercial controllers are demonstrated for boost and fly back converters to achieve almost perfect power factor correction. In this paper a comprehensive review of the various advanced optimization techniques used in power electronic controllers is presented

Improvement of voltage and power flow control in the GCC power grid by using coordinated FACTS devices

This work presents HVDC/FACTS control device implementation framework in the Gulf cooperative council’s countries. It comprises of five layers of FACTS control devices (STATCOM, SSSC, UPFC, HVDC and centralized/De-centralized Control). This five-layer architecture is designed in order to configure and produce the desired results; based on these outcomes, GCC power system network control and operational problems can be identified and addressed within the control architecture on the GCC power grid. In the context of power FACTS-FRAME, this work is to identify and determine a number of power systems operational and control problems which are persistent on the GCC power grid e.g. poor voltage quality (SAG-Swell), poor load flow control, and limited power transfer capacity issues. The FACTS-FRAME is configured and synthesized by integrating multiple FACTS control devices (STATCOM, SSSC, UPFC) in parallel at different locations on the GCC power grid in order to meet stringent power system control and operational requirements with improved power transfer capacity, controllability and reliability. The mathematical models are derived to indentify and determine operational constraints on the GCC power grid by incorporating real-time and estimated data and the acquired desired results. Herein, FACTS-FRAME is designed to handle distributed computation for intensive power system calculation by integrating multiple FACTS devices on multiple networks within the GCC power network. Distributed power flow algorithms are also derived in order to understand and implement centralized and decentralized control topologies as appropriate. The simulation results indicate the feasibility of FACTS devices implementation and their potential benefits under current operating conditions on the GCC power grid.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Improvement of Voltage and Power Flow Control in the GCC Power Grid by using Coordinated FACTS Devices

This work presents HVDC/FACTS control device implementation framework in the Gulf cooperative council’s countries. It comprises of five layers of FACTS control devices (STATCOM, SSSC, UPFC, HVDC and centralized/De-centralized Control). This five-layer architecture is designed in order to configure and produce the desired results; based on these outcomes, GCC power system network control and operational problems can be identified and addressed within the control architecture on the GCC power grid. In the context of power FACTS-FRAME, this work is to identify and determine a number of power systems operational and control problems which are persistent on the GCC power grid e.g. poor voltage quality (SAG-Swell), poor load flow control, and limited power transfer capacity issues. The FACTS-FRAME is configured and synthesized by integrating multiple FACTS control devices (STATCOM, SSSC, UPFC) in parallel at different locations on the GCC power grid in order to meet stringent power system control and operational requirements with improved power transfer capacity, controllability and reliability. The mathematical models are derived to indentify and determine operational constraints on the GCC power grid by incorporating real-time and estimated data and the acquired desired results. Herein, FACTS-FRAME is designed to handle distributed computation for intensive power system calculation by integrating multiple FACTS devices on multiple networks within the GCC power network. Distributed power flow algorithms are also derived in order to understand and implement centralized and decentralized control topologies as appropriate. The simulation results indicate the feasibility of FACTS devices implementation and their potential benefits under current operating conditions on the GCC power grid.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Application of Unified Power Flow Controller to Improve the Performance of Wind Energy Conversion System

This research introduces the unified power flow controller (UPFC) as a means to improve the overall performance of wind energy conversion system (WECS) through the development of an appropriate control algorithm. Also, application of the proposed UPFC control algorithm has been extended in this research to overcome some problems associated with the internal faults associated with WECS- voltage source converter (VSC), such as miss-fire, fire-through and dc-link faults

Damping subsynchronous resonance using supplementary controls around the static synchronous series compensator.

Masters Degree. University of KwaZulu-Natal, Durban.The demand for electric power increases rapidly with the growth in human population whereas expansion of existing power transmission infrastructure is restrained by difficulties in obtaining rights of way, resource scarcity and environmental policies inter alia. This has called for better utilization of existing transmission facilities which, for many years has been achieved through series compensation of transmission lines using conventional series capacitor banks. However, during major system disturbances, these conventional series capacitors weaken the damping of torsional oscillations in the neighboring turbine-generator shafts, which may lead to the failure and damage of the shafts concerned; a phenomenon known as subsynchronous resonance (SSR). Alternative means of series compensation using high-speed semiconductor switches has been realized following introduction of Flexible AC Transmission Systems (FACTS) in power systems. This research work focuses on damping of SSR using damping controls around the second-generation series device of the FACTS family namely the static synchronous series compensator (SSSC). The SSSC is designed to inject voltage in series with the transmission line and in quadrature with line current to emulate capacitive reactance in series with the transmission line. In this research work, a model of the SSSC is developed in Power System Computer Aided Design (PSCAD) and the IEEE First Benchmark Model (FBM) is used for SSR analysis. Initially, the resonant characteristics of the SSSC compensated transmission line is studied to determine whether this device has a potential to excite SSR on its own. The results confirm earlier work by other researchers using a detailed model of the SSSC, showing that introduction of a SSSC can indeed excite SSR, although not to the same extent as conventional series capacitors. The research work then considers the addition of supplementary damping controllers to the SSSC to add positive damping to subsynchronous oscillations caused by the SSSC itself as well as by a combination of conventional series capacitors and a SSSC in the IEEE FBM. Finally, the research work considers a more complex transmission system with an additional transmission line that incorporates conventional series capacitors. Time-domain simulation results and Fast Fourier Transform analyses show that a damping controller around the SSSC can be used to mitigate SSR either due to the SSSC itself, or due to conventional series capacitors in the same line as the SSSC or due to conventional series capacitors in an adjacent line of an interconnected transmission network

Damping interarea and torsional oscillations using FACTS devices

A problem of interest in the power industry is the mitigation of interarea and torsional oscillations. Interarea oscillations are due to the dynamics of interarea power transfer and often exhibit poor damping when the aggregate power transfer over a corridor is high relative to the transmission strength. These oscillations can severely restrict system operations and, in some cases, can lead to widespread system disturbances. Torsional oscillations are induced due to the interaction between transmission system disturbances and turbine-generator shaft systems. The high torsional stresses induced due to some of these disturbances reduce the life expectancy of the turbine-generators and, in severe cases, may cause shaft damage. This thesis reports the development of novel control techniques for Flexible AC Transmission System (FACTS) devices for the purpose of damping power system interarea and torsional oscillations. In this context, investigations are conducted on a typical three-area power system incorporating FACTS devices. The Genetic Algorithm (GA) and fuzzy logic techniques are used for designing the FACTS controllers. Although attention is focused in the investigations of this thesis on the Unified Power Flow Controller (UPFC), studies are also conducted on two other FACTS devices, a three voltage-source converter Generalized Unified Power Flow Controller (GUPFC) and a voltage-source converter back-to-back HVdc link. The results of the investigations conducted in this thesis show that the achieved control designs are effective in damping interarea oscillations as well as the high torsional torques induced in turbine-generator shafts due to clearing and high-speed reclosing of transmission system faults. The controller design procedures adopted in this thesis are general and can be applied to other FACTS devices incorporated in a power system. The results and discussion presented in this thesis should provide valuable information to electric power utilities engaged in planning and operating FACTS devices

Frequency deviations stabilizations in restructured power systems using coordinative controllers

Modern restructured power system faces excessive frequency aberrations due to the intermittent renewable generations and persistently changing load demands. An efficient and robust control strategy is obligatory to minimise deviations in the system frequency and tie-line to avoid any possible blackout. Hence, in this research, to achieve this target, automatic generation control (AGC) is utilized as a secondary controller to alleviate the changes in interconnected restructured systems at uncertainties. The objective of AGC is to quickly stabilize the deviations in frequency and tie-line power following load fluctuations. This thesis addresses the performance of AGC in two-area restructured power systems with many sophisticated control strategies in the presence of renewable and traditional power plants. As per literature of research work, there are quite a few research studies on AGC of a restructured system using optimized coordinative controllers. Besides, investigations on advanced optimized-based coordinative controller approaches are also rare to find in the literature. So, various combinations of two degrees of freedom (2DOF) controllers are utilized as supplementary controllers to diminish the frequency deviations. Nevertheless, the interconnected tie-lines are typically congested in areas with huge penetration of renewable sources, which may reduce the tie -line capability. Therefore, distinct FACTS controllers and ultra-capacitor (UC) are integrated into two-area restructured systems for strengthening the tie-line power and frequency. Further, new optimization techniques such as cuckoo search (CS), bat algorithm (BA), moth-flame optimization (MFO) are utilized in this work for investigating the suggested 2DOF controllers and compared their performance in all contracts of restructured systems. As per the simulation outcomes, the amalgamation of DPFC and UC with MFObased 2DOF PID-FOPDN shows low fluctuation rate in frequency and tie-line power. Besides, the settling times (ST) of two areas are 9.5 S for ΔF1, 8.2 S for ΔF2, and 10.15 S for ΔPtie. The robustness of the suggested controller has been verified by ±25% variations in system parameters and loading conditions

Dynamic Modeling and Control of Multi-Machine Power System with FACTS Devices for Stability Enhancement

Due to environmental and economical constraints, it is difficult to build new power lines and to reinforce the existing ones. The continued growth in demand for electric power must therefore to a great extent be met by increased loading of available lines. A consequence of this is reduction of power system damping, leading to a risk of poorly damped power oscillations between generators. This thesis proposes the use of controlled active and reactive power to increase damping of such electro-mechanical oscillations. The focus of this thesis is a FACTS device known as the Unified Power Flow Controller (UPFC). With its unique capability to control simultaneously real and reactive power flows on a transmission line as well as to regulate voltage at the bus where it is connected, this device creates a tremendous quality impact on power system stability. These features turn out to be even more significant because UPFC can allow loading of the transmission lines close to their thermal limits, forcing the power to flow through the desired paths. This providdes the power system operators much needed flexibility in order to satisfy the demands. A power system with UPFC is highly nonlinear. The most efficient control method for such a system is to use nonlinear control techniques to achieve system oscillation damping. The nonlinear control methods are independent of system operating conditions. Advanced nonlinear control techniques generally require a system being represented by purely differential equations whereas a power system is normally represented by a set of differential and algebraic equations. In this thesis, a new method to generate a dynamic modeling for power network is introduced such that the entire power system with UPFC can be represented by purely differential equation. This representation helps us to convert the nonlinear power system equations into standard parametric feedback form. Once the standard form is achieved, conventional and advanced nonlinear control techniques can be easily implemented. A comprehensive approach to the design of UPFC controllers (AC voltage control, DC voltage control and damping control) is presented. The damping controller is designed using nonlinear control technique by defining an appropriate Lyapunov function. The analytical expression of the nonlinear control law for the UPFC is obtained using back stepping method. Then, combining the nonlinear control strategy with the linear one for the other variables, a complete linear and nonlinear stabilizing controller is developed. Finally, an adaptive method for estimating the uncertain parameters is derived. This relaxes the need for approximating the uncertain parameters like damping coefficient, transient synchronous reactance etc., which are difficult to be measured precisely. The developed controller provides robust dynamic performance under wide variations in loading condition and system parameters, and provides a significant improvement in dynamic performance in terms of peak deviations. The proposed controller is tested on different multi-machine power systems and found to be more effective than existing ones

Optimisation of Smart Grid performance using centralised and distributed control techniques

A massive change is currently taking place in the manner in which power networks are operated. Traditionally, power networks consisted of large power stations which were controlled from centralised locations. The trend in modern power networks is for generated power to be produced by a diverse array of energy sources which are spread over a large geographical area. As a result, controlling these systems from a centralised controller is impractical. Thus, future power networks will be controlled by a large number of intelligent distributed controllers which must work together to coordinate their actions. The term Smart Grid is the umbrella term used to denote this combination of power systems, artificial intelligence, and communications engineering. This thesis focuses on the application of optimal control techniques to Smart Grids with a focus in particular on iterative distributed MPC. A novel convergence and stability proof for iterative distributed MPC based on the Alternating Direction Method of Multipliers is derived. Distributed and centralised MPC, and an optimised PID controllers' performance are then compared when applied to a highly interconnected, nonlinear, MIMO testbed based on a part of the Nordic power grid. Finally, a novel tuning algorithm is proposed for iterative distributed MPC which simultaneously optimises both the closed loop performance and the communication overhead associated with the desired control

AC Voltage Control of a Future Large Offshore Wind Farm Network Connected by HVDC

The offshore wind resource around the seas of the UK is a very large renewable energy resource. Future offshore wind farm sites leased by the Crown Estate for Round 3 development will need high power capacity grid connection, but their remote location presents a challenge for the electrical connection to the grid. Long distance AC cable transmission is not practical due to the large cable capacitance which leads to reactive power loss. This thesis considers the voltage source converter and high voltage direct current (VSC-HVDC) technology as the future grid connection for the offshore wind farm network, which is more controllable and has better transmission efficiencies for long distance and high power cable transmission applications. The offshore AC network is weak with very little inertia and has limited rating at the HVDC converter substation. The dynamics in key variables in the offshore wind farm AC network and how they affect certain components in the system were studied. Without proper control, the offshore voltage and the frequency will be sensitive to change. The capacitor of the AC filter at the offshore VSC-HVDC station was found to be vulnerable to over-voltage, therefore a closed loop AC voltage controller was proposed here to maintain a constant capacitor voltage and to prevent tripping or over-voltage damage. The tuning of the control gains were optimised with a pole placement design method and small signal analysis for observing the system eigenvalue damping. The control parameters were then tuned for a fast and well damped controller. The AC voltage controller was evaluated and compared to an open loop system. The controller was able to limit the resonance in the LC filter that can be triggered by large and fast disturbances in the current, voltage and frequency. Current saturation could be implemented within the control structure for device protection from over-currents. Insight on how the wind turbine fully rated frequency converters and controllers may interact with the VSC-HVDC converter station is also discussed