The MOS-gated thyristors (MGT) refer to the class of power devices that combine the ease of a MOS gate control with the superior current carrying capability of a thyristor structure for high-power applications. The MOS-controlled thyristor (MCT) is a typical MGT device. A comprehensive investigation of the reverse-biased safe operating area (RBSOA) characteristics of the MCT has been undertaken. The electrical failure mechanisms of the MCT are discussed, and the relationship between the dynamic avalanche limited RBSOA boundary of the MCT and the lower open-base transistor is identified. An analytical model based on the dynamic current gain concept is proposed to characterize the open-base transistor. For the first time, a RBSOA characteristic equation is developed for the MCT and a unified view of the RBSOA characteristics of the MCT is presented. The fundamental characteristics of the MCT are compared to those of the insulated gate bipolar transistor (IGBT) at two levels: unit-cell and multi-cell. The investigation of the unit-cell level focuses on the tradeoff between the on-state voltage drop, the turn-off loss, and the RBSOA characteristic. The investigation of the multi-cell level reveals the fundamental difference between the MCT and the IGBT in handling the non-uniform turn-off caused by the internal propagation gate delay of a large-area device. Lack of current saturation capability is identified as the main reason for the severe degradation of the turn-off capability of a large-area multi-cell MCT. The current saturation and controlled turn-on capabilities can be realized in the MGT devices with dual operation modes. For the first time, a dual operation mode MCT developed with superior current saturation capability is used to demonstrate how the dual operation device can be beneficial in the switching circuit application. The maximum controllable current density (Jmcc) is the most important characteristic of the dual operation mode MGT devices. A first-order analytic model is developed to characterize the Jmcc of the dual operation mode MGT structures compatible with the IGBT fabrication process. A new device structure with improved Jmcc characteristics is proposed and verified by both simulation and experimental results. The dissertation also carries out a comprehensive investigation of the development of power diodes. A new power diode, called the Trench Bipolar Junction Diode (TBJD), which has superior dynamic characteristics over the conventional P-i-N diode, is proposed. The TBJD controls the anode injection efficiency of the diode by the action of a reverse active transistor structure integrated into its anode junction. The reverse active transistor helps tailor an optimized on-state carrier profile to improve the diode switching characteristics. A novel self-aligned process is developed to fabricate the TBJD. Experimental characterization of the fabricated TBJD devices shows that the TBJD achieves superior dynamic characteristics without sacrificing the on-state voltage drop and the leakage current characteristics. / Ph. D.

This dissertation presents the analysis and development of an innovative semiconductor device and topology for the high power current source converter (CSC). The CSC is very attractive in high power applications due to its lower output dv/dt, easy regeneration capability and implicit short-circuit protection. Traditionally, either a symmetrical gate turn-off (GTO) thyritor or an asymmetrical GTO in series with a diode is used as the power switch in the CSC. Since the GTO has a lower switching speed and requires a complicated gate driver, the symmetrical GTO based CSC usually has low dynamic response speed and low efficiency. To achieve high power rating, fast dynamic response speed and low harmonics, an advanced semiconductor device and topology are needed for the CSC. Based on symmetrical GTO and power MOSFET technologies, a symmetrical emitter turn-off (ETO) thyristor is developed that shows superior switching performance, high power rating and reverse voltage blocking capability. The on-state characteristics, forced turn-on characteristics, forced turn-off characteristics and the load-commutated characteristics are studied. Test results show that although the load-commutation loss is high, the developed symmetrical ETO is suitable for use in high power CSC due to its low conduction loss, fast switching speed and reverse voltage blocking capability. The snubberless turn-on capability is preferred for a semiconductor device in a power conversion system, and can be achieved for devices with forward biased safe operation area (FBSOA). The FBSOA of the ETO is investigated and experimentally demonstrated. The ETO device has excellent FBSOA due to the negative feedback provided by the emitter switch. However, the FBSOA for a large area ETO is poor. A new ETO concept is therefore proposed for future development in order to demonstrate the FBSOA over a large area device. To improve the turn-on performance of the large area ETO, a novel concept, named the transistor-mode turn-on, is proposed and studied. During the transistor-mode turn-on process, the ETO behaves like a transistor instead of a thyristor. Without a snubber, the transistor-mode turn-on for the ETO is hard to achieve. Through the selection of a proper gate drive and di/dt snubber, the transistor-mode turn-on can be implemented, and the turn-on performance for the ETO can be dramatically improved. To increase the power rating of the CSC without degrading the utilization of power semiconductor devices, a novel multilevel CSC, named the parallel-cell multilevel CSC, is proposed. Based on a six-switch CSC cell, the parallel-cell multilevel CSC has the advantages of high power rating, low harmonics, fast dynamic response and modularity. Therefore, it is very suitable for high power applications. The power stage design, modeling, control and switching modulation scheme for a parallel-cell multilevel CSC based static var compensator (STATCOM) are analyzed and verified through simulation. / Ph. D.

<p>In this thesis, controllers for power oscillation damping, transient stability improvement and power flow control by means of a Controlled Series Compensator (CSC) and and a Dynamic Power Flow Controller (DPFC) are proposed. These devices belong to the group of power system components referred to as Flexible AC Transmission System (FACTS) devices. The developed controllers use only quantities measured locally at the FACTS device as inputs, thereby avoiding the risk of interrupted communications associated with the use of remote signals for control.</p><p>For power systems with one dominating, poorly damped inter-area power oscillation mode, it is shown that a simple generic system model can be used as a basis for damping- and power flow control design. The model for control of CSC includes two synchronous machine models representing the two grid areas participating in the oscillation and three reactance variables, representing the interconnecting transmission lines and the FACTS device. The model for control of DPFC is of the same type but it also includes the phase shift of the internal phase-shifting transformer of the DPFC.</p><p>The key parameters of the generic grid models are adaptively set during the controller operation by estimation from the step responses in the FACTS line power to the changes in the line series reactance inserted by the FACTS device. The power oscillation damping controller is based on a time-discrete, non-linear approach which aims to damp the power oscillations and set the desired power flow on the FACTS line by means of two step changes in the line reactance separated in time by half an oscillation cycle.</p><p>A verification of the proposed controllers was done by means of digital simulations using power system models of different complexities. The CSC and DPFC controllers were shown to significantly improve the small-signal- and transient stability in one four-machine system of a type commonly used to study inter-area oscillations. The CSC controller was also tested for 18 different contingencies in a 23-machine system, resulting in an improvement in both the system transient stability and the damping of the critical oscillation mode. </p>

This thesis studies different aspects of Flexible AC Transmission System (FACTS) devices which are used to improve the power transfer capability and increase the controllability in electric power systems. In the thesis, different aspects on the usage and control of Dynamic Power Flow Controllers (DPFC) and related FACTS devices are studied. The DPFC is a combination of a Phase Shifting Transformer (PST) and a Thyristor Switched Series Capacitor (TSSC)/Thyristor Switched Series Reactor (TSSR). The thesis proposes and studies a new method, the Ideal Phase-Shifter (IPS) method, for selection and rating of Power Flow Controllers (PFC) in a power grid. The IPS method, which is based on steady-state calculations, is proposed as a first step in the design process for a PFC. The method uses the Power controller plane, introduced by Brochu et al in 1999. The IPS method extends the usage of decoupling methods in the Power controller plane to a power system of arbitrary size. The IPS method was in the thesis used to compare the ratings of different PFC:s required to improve the power transfer capability in two test systems. The studied devices were here the PST, the TSSC/TSSR and the DPFC. The thesis treats control of ideal Controlled Series Capacitors (CSC), TCSC, TSSC/TSSR, and DPFC. The goals of the FACTS controllers which are developed are Power Oscillation Damping (POD), fast power flow control, and transient stability improvement in the power system. New adaptive control strategies for POD and power flow control are proposed and studied in different models of power systems by time-domain simulations. A strategy for transient stability improvement is also proposed and studied. Additionally, different methods for study of Subsynchronous Resonance (SSR), which is associated with series compensation in power systems, are investigated. Here, four of the most common methods for frequency scanning to determine the electrical damping of subsynchronous oscillations in a power grid are studied. The study reveals significant differences of the electrical damping estimates of the studied standard methods when applied to a four-machine test system. / QC 20110819

In this thesis, controllers for power oscillation damping, transient stability improvement and power flow control by means of a Controlled Series Compensator (CSC) and and a Dynamic Power Flow Controller (DPFC) are proposed. These devices belong to the group of power system components referred to as Flexible AC Transmission System (FACTS) devices. The developed controllers use only quantities measured locally at the FACTS device as inputs, thereby avoiding the risk of interrupted communications associated with the use of remote signals for control. For power systems with one dominating, poorly damped inter-area power oscillation mode, it is shown that a simple generic system model can be used as a basis for damping- and power flow control design. The model for control of CSC includes two synchronous machine models representing the two grid areas participating in the oscillation and three reactance variables, representing the interconnecting transmission lines and the FACTS device. The model for control of DPFC is of the same type but it also includes the phase shift of the internal phase-shifting transformer of the DPFC. The key parameters of the generic grid models are adaptively set during the controller operation by estimation from the step responses in the FACTS line power to the changes in the line series reactance inserted by the FACTS device. The power oscillation damping controller is based on a time-discrete, non-linear approach which aims to damp the power oscillations and set the desired power flow on the FACTS line by means of two step changes in the line reactance separated in time by half an oscillation cycle. A verification of the proposed controllers was done by means of digital simulations using power system models of different complexities. The CSC and DPFC controllers were shown to significantly improve the small-signal- and transient stability in one four-machine system of a type commonly used to study inter-area oscillations. The CSC controller was also tested for 18 different contingencies in a 23-machine system, resulting in an improvement in both the system transient stability and the damping of the critical oscillation mode. / QC 20101112

Este trabalho analisa a necessidade da aplicação dos compensadores estáticos de reativos em sistemas elétricos industriais e em sistemas de potência. Vários tipos de compensadores de reativos são apresentados e também é feita uma comparação entre eles. O trabalho está centralizado no estudo dos compensadores estáticos tiristorisados, principalmente o Reator Controlado a Tiristores (RCT) e o Capacitor Chaveado a Tiristores (CCT). São discutidas as influências de alguns parâmetros nas formas de onda do RCT. É feita a dedução das equações das componentes harmônicas de corrente e tensão do RCT. São também analisados os efeitos das harmônicas e os meios de atenuá-las, de modo a mantê-las dentro de níveis razoáveis. Modelos para a simulação de um RCT, para vários tipos de fenômenos em circuitos monofásicos e trifásicos, são apresentados. São Mostrados gráficos e tabelas de algumas simulações utilizando o programa CSMP (Continuous System Modeling Program) e um programa desenvolvido ciclo a ciclo, utilizando o método passo a passo. As simulações permitem avaliar com razoável precisão o comportamento do RCT associado a um capacitor fixo. Sugestões de estudos afins são apresentadas no final do trabalho. / This work analyses the need of using Var systems in industrial and in power electric systems. Various types of VAr systems are presented and also a comparison among them is done. The work is centralized in the study of thyristor static VAr systems. The thyristor controlled reactor (TCR) and the thyristor switched capacitor (TSC) are analysed in more details. Parametric influences in the wave forms of TCR are discussed. TCR voltage and current harmonic equations are deduced. Harmonic effects and also the methods of producing their attenuation are analysed. TCR simulation models for multiple types of phenomena in one phase and three phase circuits are presented. Some graphics and tables using the CSMP (Continuous System Modeling Program), and a cycle by cycle program that uses the step method are showed. The simulations can evaluate with a reasonable precision the behavior of a TCR with a fixed capacitor (FC). Suggestions of other studies are presented at the end of the work.

Este trabalho analisa a necessidade da aplicação dos compensadores estáticos de reativos em sistemas elétricos industriais e em sistemas de potência. Vários tipos de compensadores de reativos são apresentados e também é feita uma comparação entre eles. O trabalho está centralizado no estudo dos compensadores estáticos tiristorisados, principalmente o Reator Controlado a Tiristores (RCT) e o Capacitor Chaveado a Tiristores (CCT). São discutidas as influências de alguns parâmetros nas formas de onda do RCT. É feita a dedução das equações das componentes harmônicas de corrente e tensão do RCT. São também analisados os efeitos das harmônicas e os meios de atenuá-las, de modo a mantê-las dentro de níveis razoáveis. Modelos para a simulação de um RCT, para vários tipos de fenômenos em circuitos monofásicos e trifásicos, são apresentados. São Mostrados gráficos e tabelas de algumas simulações utilizando o programa CSMP (Continuous System Modeling Program) e um programa desenvolvido ciclo a ciclo, utilizando o método passo a passo. As simulações permitem avaliar com razoável precisão o comportamento do RCT associado a um capacitor fixo. Sugestões de estudos afins são apresentadas no final do trabalho. / This work analyses the need of using Var systems in industrial and in power electric systems. Various types of VAr systems are presented and also a comparison among them is done. The work is centralized in the study of thyristor static VAr systems. The thyristor controlled reactor (TCR) and the thyristor switched capacitor (TSC) are analysed in more details. Parametric influences in the wave forms of TCR are discussed. TCR voltage and current harmonic equations are deduced. Harmonic effects and also the methods of producing their attenuation are analysed. TCR simulation models for multiple types of phenomena in one phase and three phase circuits are presented. Some graphics and tables using the CSMP (Continuous System Modeling Program), and a cycle by cycle program that uses the step method are showed. The simulations can evaluate with a reasonable precision the behavior of a TCR with a fixed capacitor (FC). Suggestions of other studies are presented at the end of the work.

In this research work, the control system of 16 MVAr, 13.8 kV TCR will be designed and digitally implemented. A Real-Time Control System (NI CompactRIOTM Reconfigurable I/O) and a Digital Platform (NI LabVIEWTM Gcode) are used in the digital implementation of TCR control system. The digital control system is composed of reactive power calculation, firing angle determination and triggering pulse generation blocks. The performance of control system will be tested in the field. The simulation results will also be compared with test data.

This research work deals with the analysis, design and implementation of thyristor switched plain capacitor banks and thyristor switched shunt filter banks. Performances of various thyristor switched capacitor (TSC) topologies are also investigated by simulations. The theoretical findings have been verified by carrying out experimental work on two prototypes implemented within the scope of this research work, one is a wye-connected laboratory prototype and the other is a delta-connected application prototype integrated to some of the SVCs existing in Turkish Coal Enterprise s Plants. The advantages of back-to-back connected thyristor switches over conventional electromechanical contactors are also made clear by conducting an intensive experimental work in the laboratory. A good correlation have been obtained between theoretical and experimental results.

This thesis describes a new concept, applicable to high-power current-sourced conversion (CSC), where a controllable firing-angle shift is introduced between series and parallel converters to enable independent active and reactive power control. The firing-shift concept solves a difficult problem, by giving thyristor based CSCs the control flexibility of pulse-width modulated (PWM) converters, but without a loss in efficiency or rating. Several configurations are developed, based on the firing-shift concept, and provide flexible, efficient solutions for both very high power HVDC transmission, and very high current industrial processes. HVDC transmission configurations are first developed for 4-quadrant high-pulse operation, based on the series connected multi-level current reinjection (MLCR) topology. Independent reactive power control between two ends of an HVDC link are proven under firing-shift control, with high-pulse operation, and without on-load tap changing (OLTC) transformers. This is followed by application of firing-shift control to a bi-directional back-to-back HVDC link connecting two weak systems to highlight the added dc voltage control flexibility of the concept. The fault recovery capability of an MLCR based ultra-HVDC (UHVDC) long distance transmis-sion scheme is also proven under firing-shift control. The scheme responds favourably to both ac disturbances and hard dc faults, without the risk of commutation failures and instability experienced during fault recovery of line-commutated conversion. The two-quadrant capability of very high current rectification is also proven with configurations based on phase-shifted 12-pulse and MLCR parallel CSCs. The elimination of the electro-mechanical OLTC/satruable reactor voltage control, the high-current CSC’s biggest shortcoming, greatly improves controllability and with firing-shift control, ensures high power-factor for all load conditions. This reduces the reactive power demands on the transmission system, which results in more efficient power delivery