Improving power transfer capability of SCIG based fixed speed WECS

Wind power generation is regarded as the most promising source of all renewable energy system. In this thesis, a concept to advance the knowledge in the possibility to improve the power transfer of the fixed speed Wind Energy Conversion System based on a Squirrel Cage Induction Generator is presented. The proposed system endeavour to increase power extracted from the wind energy system through the utilization of variable capacitors and tap changing transformer in tandem

Enhanced decoupling current scheme with selective harmonic elimination pulse width modulation for cascaded multilevel inverter based static synchronous compensator

This dissertation is dedicated to a comprehensive study and performance analysis of the transformer-less Multilevel Cascaded H-bridge Inverter (MCHI) based STATic synchronous COMpensator (STATCOM). Among the shunt-connected Flexible AC Transmission System (FACTS) controllers, STATCOM has shown extensive feasibility and effectiveness in solving a wide range of power quality problems. By referring to the literature reviews, MCHI with separated DC capacitors is certainly the most versatile power inverter topology for STATCOM applications. However, due to the ill-defined transfer functions, complex control schemes and formulations were emerged to achieve a low-switching frequency high-bandwidth power control. As a result, adequate controller parameters were generally obtained by using trial and error method, which were practically ineffective and time-consuming. In this dissertation, the STATCOM is controlled to provide reactive power (VAR) compensation at the Point of Common Coupling (PCC) under different loading conditions. The goal of this work is to enhance the performance of the STATCOM with the associated proposed control scheme in achieving high dynamic response, improving transient performance, and producing high-quality output voltage waveform. To evaluate the superiority of the proposed control scheme, intensive simulation studies and numerous experiments are conducted accordingly, where a very good match between the simulation results and the experimental results is achieved in all cases and documented in this dissertation

Modelling and control of hybrid LCC HVDC System

A novel hybrid HVDC system is proposed based on the traditional LCC HVDC system. The proposed system is able to achieve full elimination of commutation failures which cannot be achieved in traditional LCC HVDC systems. In addition, reactive power controller is designed for the hybrid HVDC system. The controller is able to achieve zero reactive power exchange with the connected AC system at inverter side. It can also facilitate a faster fault recovery. Finally, the black start capability of the hybrid system is investigated. The black start sequence and inverter AC voltage controller are designed to achieve smooth and reliable black start of inverter AC system. The performances of the proposed system and controller are validated through detailed simulations in Real Time Digital Simulator (RTDS)

Multi-busbar sub-module modular multilevel converter

Modular multilevel converter (MMC) plays increasingly significant roles in large scale power electronics system including high voltage direct current (HVDC) system, static synchronous compensator (STATCOM), large scale energy storage, motor control, and so on, thanks to its advantages including modular configurations, reduced dv/dt, low total harmonic distortions, and low power losses. The classic sub-module (SM) topologies (e.g. half or full bridge types) all have in common their single connection arrangement between each SM in their series connection within a stack; i.e. a single busbar. This single busbar arrangement does come however with some drawbacks in terms of performance, reliability and flexibility. The lack of redundant switching states limits the potential optimization for the whole MMC. To solve the above mentioned issue, this thesis presents the control and performance of a new topology of SMs for MMCs, which uses multiple parallel connections between SMs and is referred to as multi-busbar sub-module (MBSM). Stacks made entirely of MBSMs can see improved functionalities such as pre-charging capability, capacitor paralleling, lower power losses, improved reliability, and a rational bypass mechanism in the event of SM failure. The soft-parallel mechanism is proposed to maintain voltage balancing without the requirement of additional spike current inductors. Despite the fact that the number of semiconductors in MBSM MMC has been doubled, semiconductor losses have been reduced to 80% of those in its counterpart. Simulation results have verified the characteristics of a FB MBSM MMC in an HVDC scenario. Several advanced control schemes for the control of the MBSM MMC are also investigated, including an algorithm to automatically generate independent variables state space models from linear electrical circuits, a model predictive control-based start-up controller to simplify the SM pre-charge procedure and at the same time improve the transient performance, and a reinforcement learningbased low-level controller to achieve low switching frequency operation of the MBSM MMC. The control schemes are validated by detailed theoretical analysis and simulation results. Besides, some MBSM applications in the operation scenarios of STATCOM are studied. Two topologies of delta-configured, partially rated energy storage (PRES) MBSM STATCOM and their corresponding low-level controllers are presented to improve the active power output capability. The soft parallel of MBSM is more effective in reactive power mode than active power mode due to the location of ES, which sees their current circulation limited to their own SM capacitor. The proposed controller for the MBSM STATCOM dynamically switches between two operation modes to reduce the converter losses over the extended range of active power. Simulation results confirm the earlier point, in that PRES-MBSMSTATCOM performs better at pure reactive power set-points and marginally better at high active power. This is explained by the fact that MBSM operates more frequently in soft-paralleling mode when the ES releases less power, i.e. reactive power set-points. Then the MBSM concept is further extended to a structure with more busbars, named multi-H-bridge SM, aiming at solving the current sharing issue of paralleled discrete SiC MOSFETs in large current applications. When compared to conventional FBSM constructed directly paralleled SiC MOSFETs, simulation results show that the current sharing performance against on-state resistance mismatch is improved and the switching loss is reduced. The same converter rating can be achieved with fewer MHSMs compared with Si IGBT SMs. Finally, the designing process of a benchtop-scale, low-voltage, open-source, and affordable hardware prototype of a MMC, the μMMC, is presented with a case study of a three-phase inverter-mode MMC. The proposed μMMC is configured as full bridge SMs type in the experiment, yet the flexible structure makes it capable to be configured as other SM types, including MBSMs. The cost for a single μMMC could be around 50 pounds. The control framework and concrete implementation are presented in detail. With the application of the μMMC, the STM32Cube Hardware Abstraction Layer, and the MATLAB/Simulink hardware support packages, it is possible to shorten the transition process from simulation to hardware realization to several hours. The experiment setup and results of a three-phase inverter mode MMC validate the proposed μMMC’s effectiveness, scalability, and convenience

Compensación de Potencia Reactiva en Turbinas eólicas basadas en Máquina de Inducción directamente conectadas a la Red

En este trabajo de grado se propone un esquema de compensación de potencia reactiva en turbinas eólicas tipo-A, mediante la teoría ABC, formulada como un de problema de optimización matemática, donde a partir de las restricciones, se pueden trazar diferentes objetivos de compensación y reducción de armónicos en la corrientes de linea. Para ello, se utiliza un convertidor de tres ramas VSC (Voltage source converter) construido con IGBTs (Insulate Gate Bipolar Transistor), modulado por histéresis y controlado con los esquemas de compensación estudiados, basados en la teoría de compensación ABC. El esquema de compensación propuesto se probó en una red que presenta armónicos de bajo orden en tensión, y que se conecta a un sistema constituido por una carga no lineal y a un generador eólico tipo-A, bajo un modelo de viento que presenta alteraciones en el perfil de velocidad. Además, se presentan los diferentes tipos de generadores eólicos, sistemas de compensación de potencia reactiva y convertidores, para as presentar de mejor manera la implementación del sistema de compensación

Protection of Renewable Energy Systems

The recent progress in renewable energy (RE) technologies has led to the erection of RE power plants (REPPs) up to the order of several hundred megawatts. Unlike their predecessors, which generally appeared in the form of dispersed generation (DG) coupled mainly with distribution systems, such large REPPs are naturally part of high-voltage transmission networks and hold non-negligible proportions of the generation. On the other hand, RE-based DGs are becoming pervasive in modern distribution systems. As a result, the fault ride-through (FRT) requirement has become an essential part of modern grid codes. This dissertation investigates the challenges brought about by the FRT requirement now affecting protection of systems with which REPPs are integrated. On the transmission level, it explores the performance of distance relays that are installed at an REPP substation and protect the neighboring line. The analyses are founded upon time-domain simulation of detailed REPP models with FRT capability. The studies include squirrel cage induction generator and doubly-fed induction generator (DFIG)-based wind farms, as well as full-scale converter-interfaced REPPs. The exclusive fault behavior of REPPs is scrutinized to identify possible relay maloperations and their root causes. The relay malfunctions revealed by this dissertation are restricted to systems with REPPs, and are not among the known distance relay failures that can occur in conventional power systems. If a communication link with minimal bandwidth requirement is in place, distance relays provide non-delayed fast tripping over the entire length of the line. This feature is retained by devising modified relaying algorithms. On the distribution level, the dissertation examines the effects of RE-based DGs on directional relays and on fault type classification methods. DFIG-based wind turbines constitute an appreciable portion of today's DG power. Conventional directional elements are shown to be adversely affected when a distribution system incorporates DFIG-based wind DG. An effective method is proposed to identify the fault direction using the waveshape properties of fault signals. Microgrids are the building blocks of future smart distribution systems. Protective devices of smart and fault-resilient microgrids are not expected to trip the healthy phases during unbalanced short-circuits. Thus, some utilities as well as relay manufacturers have started contemplating single- and double-pole tripping for distribution systems. Selective phase tripping demands fault type classification. This dissertation reveals that existing industrial methods that exploit the phase difference between sequence currents and the magnitudes of phase and sequence currents misidentify the fault type in microgrids that include photovoltaic and/or Type IV wind DGs. Using phase and sequence voltages, two new classifiers are proposed to determine the fault type for not only microgrids with different DGs, but for any three-phase system.1 yea