Effects of Distorted Voltages on the Performance of Renewable Energy Plant Transformers

The significant global growth in renewable energy production has led to increasing concerns about the problems associated with electrical equipment in power plants connected with this type of energy. The crucial electrical components of renewable energy generation are step-up transformers, with respect to which, gassing problems and premature insulation failures have been extensively reported in recent years. One of the factors related to the reported problems is the presence of high-frequency high-dV/dt voltages that are created by switching operations in wind energy plants. Addressing this challenge necessitates the investigation of transformer insulation systems under high-dV/dt pulse voltages. This thesis presents research undertaken in order to examine the performance of wind turbine step-up transformers under distorted voltages, with consideration of internal resonance phenomena and high-frequency dielectric effects. To evaluate the effect of distorted voltage on the acceleration of the ageing process in transformers, model transformers have been aged under distorted power converter voltage as well as under pure sinusoidal power grid voltage. Relevant parameters are monitored as indicators of the condition of the transformer insulation, and the results are compared throughout the ageing period. In addition, to study the transformer behavior under high-frequency high-dV/dt voltages, a detailed high-frequency model is necessary. High frequency modeling of large power transformers is complex and time-consuming due to their sizes. Therefore, as an alternative approach, this work proposes a modeling method that considers the high-frequency behavior of a scale down model transformer, and then relates it to the behavior of the actual size transformer. To verify the proposed modelling method, an experimental study investigates the correlations between the frequency responses of the two model transformers of different power ratings and sizes. Comparison of the frequency responses of the scaled and original transformers validates the proposed approach of scaling transformers for high-frequency study. High-frequency measurements are performed on an actual wind turbine transformer to represent a linear wideband black-box model of wind turbine step-up transformer in an electromagnetic transient study. Using the simulation results for switching impulsive waveforms imposed on wind turbine transformers due to the adjacent breakers operations, this work evaluates the effects of impulsive voltage parameters such as rate of rise and repetition frequency on inception voltage and intensity of partial discharge, generally assumed to be the main long-term cause of insulation deterioration. Partial discharge parameter measurement under high-dV/dt voltages is challenging due to interferences from fast oscillations, and difficulties of PD energy measurements. To avoid such issues, which are related to electromagnetic detection methods under pulse energization, this work uses a chemical approach to compare PD, based on the rate of hydrogen generation in a controlled test chamber with oil/paper samples. Gas monitoring of the oil containing impregnated paper samples show good linear correlation between the amount of hydrogen detected and PD energy level. Transformers installed in renewable energy plants require specific design considerations in order to protect them from the adverse effects of abovementioned voltage distortions. A number of manufacturers practice the implementation of electrostatic shields in transformer windings to filter the transferred voltages. Although this method has shown some improvements, effectiveness of the electrostatic shielding for a broad frequency range requires further studies. The effectiveness of electrostatic shielding in alleviating the transfer of high-frequency distortions from LV winding to HV winding and vice versa is evaluated with an experimental study. To compare the internal field enhancement at different frequencies in the presence and absence of an electrostatic shield, the frequency response of the voltage distribution inside the transformer’s winding is also measured and analyzed. Internal short circuit is one of the far-reaching incidents that has been recently reported for many wind-farm transformers. Detecting the location of an internal short circuit in a transformer winding is beneficial in improving future designs by defining the critical spots for target oriented insulation reinforcement. In an effort to identify trends with inter-turn fault locations and frequency responses, this research investigates the effect of the location of deliberately initiated internal faults on parameters such as transfer voltages and input impedance by means of frequency response analysis. Finally, alternative approaches are suggested for wind-farm design based on a method for recognizing the transformer compatibility with its surrounding devices such as power converters, breakers and cables

Evaluation of all-electric secondary power for transport aircraft

This report covers a study by Douglas Aircraft Company (DAC) of electrical power systems for advanced transport aircraft based upon an all-electric design concept. The concept would eliminate distributed hydraulic and pneumatic secondary power systems, and feature an expanded secondary electrical power system redesigned to supply power to the loads customarily supplied by hydraulic or pneumatic power. The initial study was based on an advanced 20-kHz electrical power transmission and distribution system, using a system architecture supplied by NASA-Lewis Research Center for twin-engine aircraft with many advanced power conversion concepts. NASA-LeRC later requested DAC to refocus the study on 400-Hz secondary power distribution. Subsequent work was based on a three-engine MD-11 aircraft, selected by DAC as a baseline system design that would provide data for the comparative cost/benefit analysis. The study concluded that the 20-kHz concept produced many expected benefits, and that the all-electric trijet weight savings on hardware redesign would be 2,304 pounds plus a 2.1-percent fuel reduction and resized for a total weight reduction of 11,000 pounds. Cost reductions for a fleet of 800 aircraft in a 15-year production program were estimated at 76.71 million for RDT&E; 2.74 million per aircrat for production; $9.84 million for nonrecurring expenses;$120,000 per aircraft for product support; and $300,000 per aircraft per year for operating and maintenance costs, giving a present value of$1.914 billion saved or a future value of $10.496 billion saved

Module-level autonomous settingless protection and monitoring for standalone and grid-connected photovoltaic array systems using quadratic integration modeling

This research applies a recently developed dynamic state-estimation based protection scheme, the settingless protection, to the photovoltaic (PV) industry for the first time. At this time, the proposed protection algorithm has been implemented on traditional protection zones for individual power system devices, but this research extends this protection to a microgrid, specifically, a system of PV network composed of several PV modules. Several illustrative examples on various anomalies such as high impedance faults and shorted-out PV modules have been provided to demonstrate the effectiveness of this protection scheme. The detection of these anomalies has been demonstrated in the presence of changing atmospheric conditions, and with the operation of maximum power point tracking (MPPT) equipped dc-dc converters. This protection scheme requires an accurate model of the PV module, therefore, a two-diode PV model has been developed using quadratic integration modeling. In this PV model development, a scaling factor is applied to the Taylor series expansion of the exponential terms of the model of the PV module. Then the higher order terms of the Taylor series expansion are reduced to at most second order terms using the quadratization technique. Furthermore, a novel approach for extracting the PV parameters, namely, the ideality constants, leakage currents, PV module internal current, shunt and series resistances, has been presented. A comparison was performed between numerically generated data using the determined PV module parameters and data measurements from a physical PV module. It was shown that the maximum error from this comparison was below 0.12A, and less than 0.05A around the maximum power point region of the PV modules used for this research. The residual data from the PV array protection scheme has been used to develop a method for identifying the location of faulted PV modules. Also, condition-based monitoring of the PV array system has also been presented with examples. From the PV array system monitoring, the shading and underperformance of a PV module have been identified. From the contributions of this research, an accurate module of the PV array has been developed in a form that can be integrated with other power system devices. This accurate module can be used for state estimation of the PV array, load flow analysis, short circuit analysis, and other power system analytical studies. Also, by determining the location of the faulted PV module, the time to identify this faulted PV module in a large PV installation is drastically reduced. Lastly, by identifying shading conditions and underperforming PV modules, the PV system operator can quickly bring the underperforming module or modules to optimal performance, thereby, maximizing the power yield of the PV array, and maximizing the revenue of the PV system owner.Ph.D

NASA Space Engineering Research Center Symposium on VLSI Design

The NASA Space Engineering Research Center (SERC) is proud to offer, at its second symposium on VLSI design, presentations by an outstanding set of individuals from national laboratories and the electronics industry. These featured speakers share insights into next generation advances that will serve as a basis for future VLSI design. Questions of reliability in the space environment along with new directions in CAD and design are addressed by the featured speakers

Advances in Computer Science and Engineering

The book Advances in Computer Science and Engineering constitutes the revised selection of 23 chapters written by scientists and researchers from all over the world. The chapters cover topics in the scientific fields of Applied Computing Techniques, Innovations in Mechanical Engineering, Electrical Engineering and Applications and Advances in Applied Modeling

Runtime Monitoring for Uncertain Times

In Runtime Verification (RV), monitors check programs for correct operation at execution time. Also called Runtime Monitoring, RV offers advantages over other approaches to program verification. Efficient monitoring is possible for programs where static checking is cost-prohibitive. Runtime monitors may test for execution faults like hardware failure, as well as logical faults. Unlike simple log checking, monitors are typically constructed using formal languages and methods that precisely define expectations and guarantees. Despite the advantages of RV, however, adoption remains low. Applying Runtime Monitoring techniques to real systems requires addressing practical concerns that have garnered little attention from researchers. System operators need monitors that provide immediate diagnostic information before and after failures, that are simple to operate over distributed systems, and that remain reliable when communication is not. These challenges are solvable, and solving them is a necessary step towards widespread RV deployment. This thesis provides solutions to these and other barriers to practical Runtime Monitoring. We address the need for reporting diagnostic information from monitored programs with nfer, a language and system for event stream abstraction. Nfer supports the automatic extraction of the structure of real-time software and includes integrations with popular programming languages. We also provide for the operation of nfer and other monitoring tools over distributed systems with Palisade, a framework built for low-latency detection of embedded system anomalies. Finally, we supply a method to ensure program properties may be monitored despite unreliable communication channels. We classify monitorable properties over general unreliable conditions and define an algorithm for when more specific conditions are known

First Annual Workshop on Space Operations Automation and Robotics (SOAR 87)

Several topics relative to automation and robotics technology are discussed. Automation of checkout, ground support, and logistics; automated software development; man-machine interfaces; neural networks; systems engineering and distributed/parallel processing architectures; and artificial intelligence/expert systems are among the topics covered