Fault tolerant data management system

Described in detail are: (1) results obtained in modifying the onboard data management system software to a multiprocessor fault tolerant system; (2) a functional description of the prototype buffer I/O units; (3) description of modification to the ACADC and stimuli generating unit of the DTS; and (4) summaries and conclusions on techniques implemented in the rack and prototype buffers. Also documented is the work done in investigating techniques of high speed (5 Mbps) digital data transmission in the data bus environment. The application considered is a multiport data bus operating with the following constraints: no preferred stations; random bus access by all stations; all stations equally likely to source or sink data; no limit to the number of stations along the bus; no branching of the bus; and no restriction on station placement along the bus

Digital computer simulation of inductor-energy-storage dc-to-dc converters with closed-loop regulators

The simulation of converter-controller combinations by means of a flexible digital computer program which produces output to a graphic display is discussed. The procedure is an alternative to mathematical analysis of converter systems. The types of computer programming involved in the simulation are described. Schematic diagrams, state equations, and output equations are displayed for four basic forms of inductor-energy-storage dc to dc converters. Mathematical models are developed to show the relationship of the parameters

A Hybrid Method of Performing Electric Power System Fault Ride-Through Evaluations on Medium Voltage Multi-Megawatt Devices

This dissertation explores the design and analysis of a Hybrid Method of performing electrical power system fault ride-through evaluations on multi-megawatt, medium voltage power conversion equipment. Fault ride-through evaluations on such equipment are needed in order to verify and validate full scale designs prior to being implemented in the field. Ultimately, these evaluations will help in reducing the deployment risks associated with bringing new technologies into the marketplace. This is especially true for renewable energy and utility scale energy storage systems, where a significant amount of attention in recent years has focused on their ever increasing role in power system security and stability. The Hybrid Method couples two existing technologies together - a reactive voltage divider network and a power electronic variable voltage source - in order to overcome the inherent limitation of both methods, namely the short circuit duty required for implementation. This work provides the background of this limitation with respect to the existing technologies and demonstrates that the Hybrid Method can minimize the fault duty required for fault evaluations. The physical system, control objectives, and operation cycle of the Hybrid Method are analyzed with respect to the overall objective of reducing the fault duty of the system. A vector controller is designed to incorporate the time variant nature of the Hybrid Method operation cycle, limit the fault current seen by the power electronic variable voltage source, and provide regulation of the voltage at the point of common coupling with the device being evaluated. In order to verify the operation of both the Hybrid Method physical system and vector controller, a controller hardware-in-the-loop experiment is created in order to simulate the physical system in real-time against the prototype implementation of the vector controller. The physical system is simulated in a Real Time Digital Simulator and is controlled with the Hybrid Method vector controller implemented on a National Instruments FPGA. In order to evaluate the complete performance of the Hybrid Method, both a synchronous generator and a doubly-fed induction generator are modeled as the device under test in the simulations of the physical system. Finally, the results of the controller hardware-in-the-loop experiments are presented which demonstrate that the Hybrid Method is a viable solution to performing fault ride-through evaluations on multi-megawatt, medium voltage power conversion equipment

Adaptive Single-Phase Reclosing in Transmission Lines

This research work is mainly concerned about dealing with temporary short circuit faults in power system transmission lines. In fact, there are two types of electrical faults in power systems, namely temporary and permanent. When a fault is permanent, the only way to clear it is to de-energize the transmission line by opening the associated circuit breakers. However, in many cases the fault is not solid and is caused by objects such as flying birds or broken branches of trees. For these cases, electrical arc plays a major role. For such fault cases, it is also possible to de-energize the faulted phase, temporarily, and re-energize it after a short delay by reclosing the opened circuit breakers. This operation is called single-phase reclosing. There is a chance that the fault becomes clear by natural extinction of the arc after the faulted phase isolation in case the fault is temporary. There are two considerable challenges regarding traditional single-phase reclosing in transmission lines. The first challenge is the determination of the fault type, i.e., permanent or temporary, as there is no guarantee that the fault is temporary. This is crucially important as reclosing-on-fault, i.e., reclosing the opened breakers while the fault still stands, is harmful for both power system stability and power system equipment. The second challenge which is regarding temporary faults only, is that there is still no guarantee that the arc is extinguished by the moment of reclosing. In such cases, reclosing leads in re-striking of arc and therefore, an unsuccessful reclosing. This research work is conducted in two phases. At the first phase, two adaptive methods are developed to improve the traditional reclosing method upon the two challenges mentioned in the second paragraph. The developed methods are capable of recognition of the fault type in a reasonable amount of time after single-phase isolation of the line. Therefore, the protection system will be able to block the reclosing function in case the fault is recognized as permanent and to issue three-phase-trip signal as the next action. For temporary faults, re-energizing of the isolated phase by reclosing the opened breakers is the next action which has to be performed after the arc extinction. The developed methods also have the capability of detection of the arc extinction and therefore, a better performance for temporary fault cases is guaranteed. This is the second feature required for an adaptive reclosing method. The second phase of the research project is to estimate the arc extinction time well in advance in case the fault is temporary. The idea is that three-phase tripping could be the right action if the arc extinction time is too long as working under unbalanced conditions for an unnecessarily long time duration is harmful for the power system. Both of the proposed adaptive single-phase reclosing methods in this research work employ local voltage information. Therefore, communication facilities are not needed for implementation of the proposed methods. It is shown in the thesis that the proposed methods are able to quickly detect the fault type and also the arc extinction if the fault is temporary. Also, the two proposed arc extinction time prediction methods are capable of prediction of the arc extinction time well in advance and with acceptable precision. All four proposed methods are effective for various system configurations including ideally-transposed, untransposed and partially-transposed transmission lines and also for transmission lines with different compensation conditions including with and without shunt reactor. Superior performance of the proposed methods have been verified using 550 case studies simulated in PSCAD and Matlab, and also a field recorded temporary fault case associated with a 765 kV transmission line. The 550 simulated case studies include 100 ideally-transposed, 240 untransposed and 210 partially-transposed line cases. The performances of the two proposed reclosing methods are also compared with two of the existing adaptive reclosing methods where considerable improvements are observed

Controlled Switching of Reactive Loads and Commisioning Regimes

Switching is a vital task in any power system for ensuring its safe and reliable operation. Switching may be necessary for fault clearance, to ensure wider system stability and to prevent damage to plant. It is essential for isolation, to allow technicians to carry out maintenance tasks safely. Also, switching of reactive loads such as shunt capacitor banks and shunt reactors, is crucial for controlling system voltage. Switching of some loads however, may produce voltage transients and heavy transient inrush currents which can impact on wider system power quality, impact customers and cause damage or deterioration of the insulation of HV equipment. Therefore, it is important to provide some form of measure to control or mitigate transients caused by switching. The main control measures include: metal oxide surge arrestors, pre-insertion resistors, current limiting reactors and synchronised or controlled switching. Controlled switching is the favoured solution for frequently switched loads such as reactive plant, for economic benefits and as it reduces transients in the first instance. Controlled switching is defined as the use of electronic equipment to control the making or breaking of high voltage circuit breakers at pre-determined points on the system voltage and current waveforms. It has been implemented in Ireland for over 30 years for the energisation of shunt capacitor banks. Over the last two years, the benefits of controlled switching for different applications has become ever more apparent, with increased use such as switching of transmission shunt reactors and the energisation of large power transformers, particularly in remote areas of the network such as wind farm interfaces. The aim of this thesis is to provide a complete overview of the stages concerned in implementing controlled switching schemes, from examining the impacts of switching certain loads, to performing systems studies, up to site commissioning stage. The research in this thesis looks at both the theory and practice. It draws together the published work, manufacturers guidelines, international standards and simulation results, to give the total awareness of the issues involved in reactive load switching and commissioning regimes. The various solutions and strategies associated with controlled switching schemes are examined, to ensure that the best and most economical solution has being implemented. Several recent projects where controlled switching has been implemented for switching of transmission reactors and power transformers are also investigated

Characterization Methodology, Modeling, and Converter Design for 600 V Enhancement-Mode GaN FETs

Gallium Nitride (GaN) power devices are an emerging technology that have only become available commercially in the past few years. This new technology enables the design of converters at higher frequencies and efficiencies than those achievable with conventional Si devices. This dissertation reviews the unique characteristics, commercial status, and design challenges that surround GaN FETs, in order to provide sufficient background to potential GaN-based converter designers.Methodology for experimentally characterizing a GaN FET was also presented, including static characterization with a curve tracer and impedance analyzer, as well as dynamic characterization in a double pulse test setup. This methodology was supplemented by additional tests to determine losses caused by Miller-induced cross talk, and the tradeoff between these losses and overlap losses was studied for one example device.Based on analysis of characterization results, a simplified model was developed to describe the overall switching behavior and some unique features of the device. The impact of the Miller effect during the turn-on transient was studied, as well as the dynamic performance of GaN at elevated temperature.Furthermore, solutions were proposed for several key design challenges in GaN-based converters. First, a driver-integrated overcurrent and short-circuit protection scheme was developed, based on the relationship between gate voltage and drain current in GaN gate injection transistors. Second, the limitations on maximum utilization of current and voltage in a GaN FET were studied, particularly the voltage overshoots following turn-on and turn-off switching transients, and the effective cooling of GaN FETs in higher power operation. A thermal design was developed for heat extraction from bottom-cooled surface-mount devices. These solutions were verified in a GaN-based full-bridge single-phase inverter

Inverter-converter automatic paralleling and protection

Electric control and protection circuits for parallel operation of inverter-converte

Study of switching transients in high frequency converters

As the semiconductor technologies progress rapidly, the power densities and switching frequencies of many power devices are improved. With the existing technology, high frequency power systems become possible. Use of such a system is advantageous in many aspects. A high frequency ac source is used as the direct input to an ac/ac pulse-density-modulation (PDM) converter. This converter is a new concept which employs zero voltage switching techniques. However, the development of this converter is still in its infancy stage. There are problems associated with this converter such as a high on-voltage drop, switching transients, and zero-crossing detecting. Considering these problems, the switching speed and power handling capabilities of the MOS-Controlled Thyristor (MCT) makes the device the most promising candidate for this application. A complete insight of component considerations for building an ac/ac PDM converter for a high frequency power system is addressed. A power device review is first presented. The ac/ac PDM converter requires switches that can conduct bi-directional current and block bi-directional voltage. These bi-directional switches can be constructed using existing power devices. Different bi-directional switches for the converter are investigated. Detailed experimental studies of the characteristics of the MCT under hard switching and zero-voltage switching are also presented. One disadvantage of an ac/ac converter is that turn-on and turn-off of the switches has to be completed instantaneously when the ac source is at zero voltage. Otherwise shoot-through current or voltage spikes can occur which can be hazardous to the devices. In order for the devices to switch softly in the safe operating area even under non-ideal cases, a unique snubber circuit is used in each bi-directional switch. Detailed theory and experimental results for circuits using these snubbers are presented. A current regulated ac/ac PDM converter built using MCT's and IGBT's is evaluated

Communication Subsystems for Emerging Wireless Technologies

The paper describes a multi-disciplinary design of modern communication systems. The design starts with the analysis of a system in order to define requirements on its individual components. The design exploits proper models of communication channels to adapt the systems to expected transmission conditions. Input filtering of signals both in the frequency domain and in the spatial domain is ensured by a properly designed antenna. Further signal processing (amplification and further filtering) is done by electronics circuits. Finally, signal processing techniques are applied to yield information about current properties of frequency spectrum and to distribute the transmission over free subcarrier channels