Theses supervised by Doç. Dr. Ahmet Mete Vural
14 theses · Gaziantep University
Modular multilevel converter design
Modular multilevel converter (MMC) has many submodules in each phase. Hence, it has many operating advantages such as modularity, flexibility, ability to generate high voltage and low harmonic content at the output. Selecting the arm inductance to obtain satisfactory MMC performance is one of the critical design constraints of the MMC. Since it affects the fault/circulating current magnitudes, efficiency as well as the harmonic content at the output. In this thesis, a novel procedure has been conducted to find the optimum value of the arm inductor when considering the factors of DC voltage utilization, harmonics and efficiency of the MMC. On the other hand, capacitor voltage balancing is an important issue in MMC operation. Unequal SM capacitor voltages cause improper operation of the MMC, resulting in high circulating currents, reduced efficiency and low harmonic quality at the output. Carrier swapping method can overcome the voltage balancing problem without any need for voltage and current sensors. In this thesis, a swapping method has also been proposed suitable for applying to different switching modulation schemes for MMC. More specifically, this swapping method has been used for the first time for Nearest Level Modulation (NLM) scheme in the literature. The optimum swapping frequency for each modulation scheme has been determined by performing case studies on the MMC. For all studies, the models are first developed in the simulation environment for verification and finally, the experimental studies are conducted to validate the results.
1MWh battery energy storage system connected to distribution network: Design, modeling, and control
Battery energy storage system (BESS) is recently becoming a strong candidate for being a system component of modern smart grids. In this thesis, a two-stage 1MWh BESS connected to a 34.5 kV distribution grid is designed and simulated. The power stage of the system consists of a three-phase wye-connected 11-level cascaded H-bridge (CHB) converter and a DC-DC converter for each submodule of CHB converter. At first, a recent literature survey is given about BESS components focusing mainly on the battery storage unit in hierarchal order from cell to pack in terms of modeling, design, and wiring diagrams. Then, CHB converter topology is compared with other power electronic interfaces in terms of storage capacity and switching elements. It is shown that CHB converter topology minimizes the number of series connected batteries. The modeling details of the power circuit of the BESS as well as its control system based on decoupled current control to exchange active and reactive power between the BESS and the grid are described. Finally, a nonlinear state feedback exact linearization controller is designed to control the bidirectional buck-boost converter in each submodule to charge/discharge the batteries. It is also shown that this controller can stabilize the dc-link capacitor voltage. Simulation tests is then performed to test the capability of the designed system to exchange active/reactive power with the distribution grid while charging and discharging the battery.
Modeling and control of 1.5 MW rated sparse matrix converter based wind energy conversion system
Sparse matrix converter (SMC) is a matrix converter topology without any storage element like an inductor or a capacitor. There is a fictitious DC link between rectifier and inverter stages in SMC topology. The input is directly connected to the output via a set of bidirectional power semiconductor switches. Such an arrangement decreases the converter size and increases the reliability. Due to these advantages, SMC is used in many applications like AC drives, aerospace industry, electric ships, as well as in wind energy applications. In this thesis, a three-phase SMC is adapted to a grid-connected permanent magnet synchronous generator-based wind energy conversion system (WECS). The IGBT is chosen as the power semiconductor element in the rectifier and inverter stages, controlled by sinusoidal pulse width modulation technique. The switching frequency is changeable in the range between 20-50 kHz. The overall system including the power stage as well as the control sub system are modelled and tested under various operating conditions. An LCL filter is replaced at the output of the SMC to weaken switching harmonics. Moreover, a fuzzy fractional-order PID (FFOPID) controller is designed to control active and reactive power of WECS. By this way, the dynamic response of a classical PID controller with fixed gains is improved by combining the salient features of fractional-order calculus and fuzzy logic theory. The simulation results show that the designed FFOPID controller is superior to classical PID controller in tracking d- and q-axis current references of the SMC at the output. FFOPID controller shows better transient performance when unit step changes are applied to the reference signals.
Design and implementation of a modular multilevel converter based wind energy conversion system
The modular multilevel converter (MMC) is an effective power electronic interface due to its modular structure for wind energy conversion system (WECS). Capacitor voltage balancing under nearest level modulation (NLM) method is a critical topic to hold the capacitor voltage of each submodule (SM) of MMC constant. In this thesis, new approaches are discovered by focusing on this topic. Accordingly, firstly, a detailed review on the circuit topology, control and applications of the MMC considering the recent improvements is presented. Secondly, a novel NLM method is proposed with increased output voltage quality for MMC. Thirdly, a generalized SM selection method using the formula of the number of states calculation incorporated into the capacitor voltage balancing scheme under NLM method is developed and sampling frequency analysis is performed. Fourthly, an efficient capacitor voltage balancing scheme under NLM method, DC-link voltage control scheme via hysteresis current control based PI controller and dq reference frame-based load voltage control technique at the point of common coupling are designed and developed for MMC based WECS. Simulation studies are carried out to verify the effectiveness of the proposed capacitor voltage balancing scheme and NLM method using various case studies. Ultimately, experimental studies are presented to demonstrate the applicability of the proposed capacitor voltage balancing scheme under NLM method for the MMC based WECS.
Energy storage technologies and their contribution to damping of low-frequency oscillations within the context of renewable energy sources
This thesis investigates the potential benefits of superconducting magnetic energy storage (SMES) systems and battery energy storage systems (BESS) to mitigate low-frequency oscillations in power systems with renewable energy sources integration, such as photovoltaics and wind energy conversion systems. The proportional integral derivative (PID) controller and fractional-order PID (FOPID) controller are proposed as contributor controllers for the SMES and BESS systems. The parameters of the PID and FOPID controllers are optimized using the particle swarm optimization method. Both inter-area oscillation and local area oscillations are considered in this work. The case studies are conducted in a multimachine power system with multiple renewable energy integration and energy storage systems to verify the designed controllers. The dynamic performances of the designed controllers are presented and compared in detail. The studies are carried out by both time-domain analysis and eigenvalues analysis methods. The results show that the FOPID controller is superior to the classical PID controller.
Tek-faz asimetrik hibrit çok-seviyeli evirici
Inverters are commonly used in renewable energy systems, power supplies, electronic devices and many other areas. A lot of works have been done from past to present about inverters and their control methods. With the studies done, the efficiency of the inverters has been increased and lower harmonic distortions have been tried to be obtained with better output voltage quality. In this study, literature survey about inverter was done firstly, and then information about inverters and modulation techniques are given. An asymmetric hybrid multi-level inverter (MLI) to achieve more output levels by using same number of switching elements was designed by simulation studies. And finally, the designed multi-level asymmetric hybrid inverter was implemented and tested in the laboratory. Key Words: Asymmetric Hybrid Multi-level Inverter, Seven-Level Inverter, Neutral Point Clamped-H-Bridge Inverter, Sinusoidal Pulse Width Modulation, Phase Disposition Pulse Width Modulation.
Comparison of Dynamic Performances of IPFC, UPFC and Back-To-Back HVDC Transmission on Damping of Power System Oscillation
The permanent goal in the field of electrical power systems is to find new ways of transmission and distribution of electrical energy in terms of low cost and reliability. Due to increasing growth in power demand, transmission side of the power systems has faced with stability problems much ever than before. Local and inter-area oscillations are the important types of these stability problems which are the results of insufficient/inadequate damping in the power systems. These oscillations can be observed as the swinging of system parameters such as buses voltages, phase angles, and real/reactive power flow on transmission lines. These oscillations are due to the normal change in the increase or decrease of sudden load or external changes such as a fault occurs in the system. The purpose of this thesis is to research the damping of both local and inter-area oscillations by using three different types of power electronic based compensators. These devices are as follows: "Unified Power Flow Controller (UPFC)", "Interline Power Flow Controller (IPFC)" and "Back to Back High Voltage Direct Current (BTB-HVDC) Transmission". UPFC and IPFC are voltage source converter based FACTS devices. BTB-HVDC is also voltage source converter based one which are mostly preferred in long transmission of bulk power. In the thesis, "Kundur System" and IEEE 9-Bus System" are used as example power systems to test the dynamic performances of the aforementioned devices on damping of both local and inter-area oscillations.
Comparison of dynamic performances of TCSC, SSSC and STATCOM on Inter-Area Oscillation Damping
In power systems, the effective control of the power flows on transmission lines is very important. This control capability aids to transfer the power in full capacity and reduces power losses to minimum values. There is a necessity of studying the problems that effect on power system stability for ensuring optimum operation conditions. One of these problems is the oscillations of low frequency. Inter-area oscillations are one type of these low frequency oscillations which are critical and should be detected and damped efficiently before not to cause a blackout in the power systems. With this respect, it is of great importance to deal with inter-area oscillations so that power quality of the power systems is continuously supplied by the power system operators. The objective of this thesis is to investigate the inter-area oscillations and damp these oscillations effectively. In this thesis, three different "Flexible Alternating Current Transmission (FACTS) devices" are used to damp inter-area oscillations in an example power system which is called "Kundur System". Kundur System has two separate generation areas which is very suitable to research about inter-area oscillations. These FACTS devices are Thyristor-Controlled Series Compensator (TCSC), Static Synchronous Series Compensator (SSSC), and Static Synchronous Compensator (STATCOM). Aforementioned FACTS devices are allocated to a selected bus/transmission line of Kundur System in a simulation environment and their capabilities on inter-area oscillation damping are observed and compared to each other. The simulation results are presented and the conclusions are given at the end of the work.
Three-phase modular multilevel converter based unified power flow controller
The power system needs to be flexible to cater for the increasing demand by its consumers. Flexible AC Transmission System (FACTS) devices use power electronic devices to achieve this by controlling some power system parameters. The Unified Power Flow Controller (UPFC) is the most versatile and flexible of FACTS devices. Modeling of UPFCs is not an easy task especially with the recent introduction of modular multilevel converters (MMC) as a new type of voltage source converter (VSC). Most models are average or equivalent circuit models which do not really depict the situation when UPFCs are implemented practically. This thesis provides a detailed model of an MMC based UPFC made up of half-bridge converter submodules. An MMC has been designed and tested in a simulation environment before being connected in back to back mode with one end in series and the other in parallel with a transmission line (UPFC configuration). A DQ real and reactive power flow control scheme has been proposed and results obtained from the numerous simulations will be presented. UPFC is able to control real and reactive power flows on the chosen line and at the same time, it can control the bus voltage dynamically while the power flow control loops are active. Key Words: Flexible AC Transmission Systems (FACTS), Unified Power Flow Controller (UPFC), Voltage Source Converter (VSC), Modular Multilevel Converter (MMC), Half Bridge Sub-module, Real and Reactive Power Flow Control and Bus Voltage control.
Design and implementation of a low cost, low loss energy quality regulator for energy distribution systems
The Energy Quality Regulator (EQR) is the solution for reactive power compensation, neutral current compensation, load balancing and harmonic current mitigation. In this thesis, design and simulation of a distribution type, voltage source inverter (VSI) based having two level inverter topologies have been carried out. The thesis is dedicated to detailed analysis, design, control, simulation and implementation of EQR for a three-phase four-wire nonlinear, unbalanced, and non-resistive loads. The power stage of VSI based EQR is composed of naturel air forced discrete IGBT modules switch at 5 kHz to reduce switching losses. The three-leg four-wire VSI topology utilizes a standard three phase PWM inverter bridge and the neutral wire is connected to the midpoint of the dc-link. Properly designed, the LCL supply filter is used instead of L type filter in this work since it provides a much better suppression at the modulation frequency. Whole compensation reference currents are extracted in the synchronous reference frame. Control has been carried out in selective manners. The proposed VSI based EQR has been implemented after the PQ measurement for the reactive power compensation, neutral current compensation, load balancing and 3., 5., 7., harmonics current elimination of the distribution type transformer station which is 400kVA, %4 uk belongs to the Mediterranean Electricity Distribution Company. Keywords: Flexible AC Transmission Systems (FACTS), Power Quality (PQ), Reactive Power Compensation, Neutral Current Compensation, Load Balancing, Zero and Negative Sequence Compensation, Harmonic Current Mitigation, Synchronous Reference Frame, Proportional-Resonant Controller (PR)
Design and implementation of a single-phase cascaded H-bridge multi-level converter based statcom
Reactive power control is very crucial in terms of voltage stability and power quality of electrical power systems. In steady-state, during heavy loading conditions, the system voltage may be lower than its reference value, or it might be observed an increase in system voltage during light loading conditions. In either of these cases, especially shunt reactive power support is necessary to restore the system. STATCOM is one of the most versatile member of FACTS devices that is used for VAR compensation and voltage control. When compared to old compensation technologies such as fixed capacitor or SVC, STATCOM has many advantages such as faster response and small size. In this thesis, a low-power single-phase STATCOM prototype has been modelled, designed and practically implemented. The proposed STATCOM comprises of a multilevel converter having three cascaded full H-bridges. It is aimed that the proposed STATCOM can operate both in capacitive and inductive modes. It is shown by both simulations and experiments that the STATCOM injects and absorbs reactive power into/from single-phase 220V grid. A closed loop control scheme has been implemented so that the STATCOM can generate the desired amount of capacitive/inductive reactive power. Some optimization methods have been used to determine many controller parameters which provide good dynamic performance to STATCOM.
Design of modular multi-level converter and permanent magnet synchronous generator-based wind energy conversion system
Wind is the important type of clean energy originated from sun. Wind energy conversion systems are important actors in developing smartgrids and microgrids. On the other hand, wind energy is a premier resource which has been one of the world's fastest growing renewables for producing clean electrical energy. Due to higher efficiency, reliability, and controllability, nowadays, variable speed wind turbines employing permanent magnet synchronous generators (PMSG) have been extensively adopted as the major source for power generation in wind energy conversion systems. As the power and voltage levels increase, utilizing multi-level converters are inevitable due to the salient features such as redundancy, high flexibility, low switching losses, low harmonic content, and less electromagnetic interference. Among multi-level converter topologies, modular multi-level converter (MMC) topology is a promising candidate which has been originally proposed for high voltage direct current transmission systems. The purpose of the thesis is to design, simulate, and validate a three-phase wind energy conversion system connected to the electric power grid. The overall system consists of a full-scale MMC and PMSG with no gearbox. In MMC topology, the capacitor voltage balancing problem is solved by a closed-loop control. Various case studies have been conducted in a simulation platform to verify the overall model of the proposed system.
Design and simulation of a wind energy conversion system based on doubly fed induction generator and back-to-back modular multilevel converters
Wind energy is one of the most growing renewable energy types in the world. Doubly fed induction generator (DFIG) based wind turbines have been used for more than two decades. Modular multilevel converters (MMC), on the other hand, have been initially proposed for high voltage direct current transmission for the purpose of long distance huge power transmission. Voltage levels even up to 100 or more at the output can be obtained using many cascaded power semiconductor bridges in each phase. The major benefits of MMC are well-dropped harmonic distortion at the output and significant voltage rating reduction of power semiconductors. In this thesis, the potential benefits of utilizing a back-to-back MMC system for a DFIG based wind energy conversion system connected to main grid has been investigated by simulation studies. The dynamic performance of the current vector control scheme has been tested by applying unit step-changes which applied to the reference signals of the controllers. Many simulation studies have been designed to verify the simulation model of back-to-back MMC based DFIG wind energy conversion system.
Design and control of three phase grid connected pv system with 1 mw power capacity
The analysing and designing of a three phase grid connected photovoltaic (PV) system with 1 MW power capacity for electricity consumption of University of Gaziantep with three different inverter control techniques have been comparatively completed in this thesis. The classical PI control, model predictive control (MPC) and sliding mode control techniques have been been applied in this project. The simulations of designed PV system with three different inverter current control techniques have been carried out and compared with obtained each other under the grid connected and standalone operating conditions. According to the obtained steady state results under the grid connected operating conditions, the performances of all three inverter control techniques are almost same to satisfy the requirements of the connected loads and grid connection. Based on the obtained steady state results under the standalone operating conditions, the performances of sliding mode current control and MPC techniques satisfy the requirements of the connected loads and grid connection. According to the obtained dynamic results under the grid connected operating conditions, the performances of all three inverter control techniques are almost same to satisfy the requirements of the connected loads and grid connection. However, the dynamic performances of PI current control technique are a bit better than the dynamic performances of other two inverter control techniques.