VTechWorks Repository :: Browsing by Author "Liu, Yilu"

Browsing by Author "Liu, Yilu"

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Accurate magnetic field sensor and method for wireless phasor measurement unit
(United States Patent and Trademark Office, 2013-05-07)
A phasor measurement unit and method including a transducer for transduction measurement of an electrical characteristic of a current carrying element in electrical power generation or distribution systems, the transducer generating a transducer output signal representative of the electrical characteristic; an amplifier receiving the transducer output signal and generating an amplifier output signal; a filter receiving the amplifier output signal, low pass filtering the amplifier output signal, and generating a filter output signal; an analog to digital converter receiving the filter output signal and generating a digital output signal; and a processor receiving the digital output signal, calculating phasor data from the digital output signal and generating a data output signal, wherein the calculated phasor data is at least as accurate as phasor data from a phasor measurement unit or a frequency data recorder having a transducer for nontransduction measurement of the same electrical characteristic.
Adaptive out-of-step relaying with phasor measurement
Zaldivar, Virgilio Antonio Centeno (Virginia Tech, 1995)
This work describes the development of an adaptive out-of-step relay, from the formulation of its concept to its field implementation and one year testing at the Florida- Georgia interface. This dissertation describes the theory of such a relay, its hardware configuration, the system as it was installed in the field, the major results and improvements obtained after the one year field test, and the adaptive features developed after the analysis of the collected data. Most of the adaptive concepts applied on this relay were used on field application for the first time and proved their value through the one year field test. Synchronized phasor measurements were used for the first time for relaying application, proving their ability to detect and analyze system disturbances through the measurement of angle differences between any two points in a system. It is shown that for a system that behaves primarily as a two-machine power system, the out-of-step relay could be enhanced and made more secure by applying the principle of equal area criterion. The main contribution of this dissertation is the use and application of old and new adaptive concepts as well as new technology to the solution of the out-of-step problem for a system that behaves like a two machine system. This work provides a solution for the basic stability problem with currently available technology and knowledge. In addition, the data collected during this research has been and will be of great help for those studying the power system stability problem and those developing new adaptive relaying techniques.
Advanced Integrated Single-Stage Power Factor Correction Techniques
Zhang, Jindong (Virginia Tech, 2001-03-15)
This dissertation presents the in-depth study and innovative solutions of the advanced integrated single-stage power-factor-correction (S2PFC) techniques, which target at the low- to medium-level power supplies, for wide range of applications, from power adapters and computers to various communication equipment. To limit the undesirable power converter input-current-harmonic's impact on the power line and other electronics equipment, stringent current harmonic regulations such as IEC 61000-3-2 have already been enforced. The S2PFC techniques have been proposed and intensively studied, in order to comply these regulations with minimal additional component count and cost. This dissertation provides a systematic study of the S2PFC input-current-shaping (ICS) mechanism, circuit topology generalization and variation, bulk capacitor voltage stress and switch current stress, converter design and optimization, and evaluation of the state-of-the-art S2PFC techniques with universal-line input. Besides, this presentation also presents the development of novel S2PFC techniques with a voltage-doubler-rectifier front end to both improve the performance and reduce the cost of S2PFC converters for (international voltage range) universal-line applications. The calculation and experimental results show that the proposed techniques offer a more cost-effective and efficient solution than industries' current practice, with universal-line input and converter power level up to 600 W. Finally, further improved technique is also presented with reduced filter inductor size and increased power density.
Advanced Semiconductor Device and Topology for High Power Current Source Converter
Xu, Zhenxue (Virginia Tech, 2003-12-02)
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.
Advanced System Monitoring with Phasor Measurements
Zhou, Ming (Virginia Tech, 2008-05-19)
Phasor Measurement Units (PMUs) are widely acknowledged as one of the most promising developments in the field of real-time monitoring of power systems. By aligning the time stamps of voltage and current phasor measurements that are consistent with Coordinated Universal Time (UTC), a coherent picture of the power system state can be achieved through either direct measurements or simple linear calculations. With the growing number of PMUs planned for installation in the near future, both utilities and research institutions are looking for the best solutions to the placement of units as well as to the applications that make the most of phasor measurements. This dissertation explores a method for optimal PMU placement as well as two applications of synchronized phasor measurements in state estimation. The pre-processing PMU placement method prepares the system data for placement optimization and reduces the size of the optimization problem. It is adaptive to most of the optimal placement methods and can save a large amount of computational effort. Depth of un-observability is one of the criteria to allow the most benefit out of a staged placement of the units. PMUs installed in the system provide synchronized phasor measurements that are highly beneficial to power system state estimations. Two related applications are proposed in the dissertation. First, a post-processing inclusion of phasor measurements in state estimators is introduced. This method avoids the revision of the existing estimators and is able to realize similar results as mixing phasor data with traditional SCADA with a linear afterwards step. The second application is a method to calibrate instrument transformers remotely using phasor measurements. Several scans of phasor measurements are used to accomplish estimating system states in conjunction with complex instrument transformer correction factors. Numerical simulation results are provided for evaluation of the calibration performance with respect to the number of scans and load conditions. Conducting theoretical and numerical analysis, the methods and algorithms developed in this dissertation are aimed to strategically place PMUs and to incorporate phasor measurements into state estimators effectively and extensively for better system state monitoring. Simulation results show that the proposed placement method facilitates approaching the exact optimal placement while keep the computational effort low. Simulation also shows that the use of phasor measurement with the proposed instrument transformer correction factors and proposed state estimation enhancement largely improves the quality of state estimations.
Alternative structures for integrated electromagnetic passives
Liu, Wenduo (Virginia Tech, 2006-04-19)
The demand for high power density keeps driving the development of electromagnetic integration technologies in the field of power electronics. Based on planar homogeneous integrated structures, the mechanism of the electromagnetic integration of passives has been investigated with distributed-parameter models. High order modeling of integrated passives has been developed to investigate the electromagnetic performance. The design algorithm combining electromagnetic design and loss models has been developed to optimize and evaluate the spiral winding structure. High power density of 480 W/in3 has been obtained on the prototype. Due to the structural limitation, the currently applied planar spiral winding structure does not sufficiently utilize the space, and the structure is mechanically vulnerable. The improvement on structures is necessary for further application of integrated passives. The goal of this research is to investigate and evaluate alternative structures for high-power-density integrated passives. The research covers electromagnetic modeling, constructional study, design algorithm, loss modeling, thermal management and implementation technology The symmetric single layer structure and the stacked structure are proposed to overcome the disadvantages of the currently applied planar spiral winding structure. Because of the potential of high power density and low power loss, the stacked structure is selected for further research. The structural characteristics and the processing technologies are addressed. By taking an integrated LLCT module as the study case, the general design algorithm is developed to find out a set of feasible designs. The obtained design maps are used to evaluate the constraints from spatial, materials and processing technologies for the stacked structure. Based on the assumption of one-dimensional magnetic filed on the cross-section and linear current distribution along the longitudinal direction of the stacked structure, the electromagnetic field distribution is analyzed and the loss modeling is made. The experimental method is proposed to measure the loss and to verify the calculation. The power loss in the module leads to thermal issues, which limit the processed power of power electronics modules and thus limit the power density. To further improve the power handling ability of the module, the thermal management is made based on loss estimation. The heat extraction technology is developed to improve the heat removal ability and further improve the power density of integrated passives. The experimental results verify the power density improvement from the proposed stacked structure and the applied heat extraction technology. The power density of 1147 W/in3 (70 W/cm3) is achieved in the implemented LLCT module with the efficiency of 97.8% at output power of 1008W.
Analysis and Design for a High Power Density Three-Phase AC Converter Using SiC Devices
Lai, Rixin (Virginia Tech, 2008-12-10)
The development of high power density three-phase ac converter has been a hot topic in power electronics area due to the increasing needs in applications like electric vehicle, aircraft and aerospace, where light weight and/or low volume is usually a must. Many challenges exist due to the complicated correlations in a three-phase power converter system. In addition, with the emerging SiC device technology the operating frequency of the converter can be potentially pushed to the range from tens of kHz to hundreds of kHz at higher voltage and higher power conditions. The extended frequency range brings opportunities to further improve the power density of the converter. Technologies based on existing devices need to be revisited. In this dissertation, a systematic methodology to analyze and design the high power density three-phase ac converter is developed. All the key factors of the converter design are explored from the high density standpoint. Firstly, the criteria for the passive filter selection are derived and the relationship between the switching frequency and the size of the EMI filter is investigated. A function integration concept as well as the physical design approach is proposed. Secondly, a topology evaluation method is presented, which provides the insight into the relationships between the system constraints, operating conditions and design variables. Four topologies are then compared with the proposed approach culminating with a favored topology under the given conditions. Thirdly, a novel average model is developed for the selected topology, and used for devising a carrier-based control approach with simple calculation and good regulation performance. Fourthly, the converter failure mode operation and corresponding protection approaches are discussed and developed. Finally, a 10 kW three-phase ac/ac converter is built with the SiC devices. All the key concepts and ideas developed in this work are implemented in this hardware system and then verified by the experimental results.
Analysis of Electric Disturbances from the Static Frequency Converter of a Pumped Storage Station
Rosado, Sebastian Pedro (Virginia Tech, 2001-05-11)
The present work studies the disturbances created in the electric system of a pumped storage power plant, which is an hydraulic generation facility where the machines can work as turbines or pumps, by the operation of a static frequency converter (SFC). The SFC is used for starting the synchronous machines at the station when in the pump mode. During the starting process several equipment is connected to the SFC being possible to get affected by the disturbances generated. These disturbances mainly include the creation of transient overvoltages during the commutation of the semiconductor devices of the SFC and the introduction of harmonics in the network currents and voltages. This work analyzes the possible effects of the SFC operation over the station equipment based on computer simulations. For this purpose, the complete system was modeled and the starting process simulated in a computer transient simulator program. The work begins with a general review of the effects of electric disturbances over high voltage equipment and in particular of the disturbances generated by power electronics conversion equipment. Then the models for the different kind of equipment present in the system are discussed and formulated. The control system that governs the operation of the SFC during the starting process is analyzed later as well as the operation conditions. Once the model of the system is set up, the harmonic analysis of the electric network is done by frequency domain and time domain methods. Time domain methods are also employed for the analysis of the commutation transient produced by the SFC operation. Finally, the simulation results are used to evaluate the impact of the SFC operation on the station equipment, especially on the generator step up transformer.
Analysis of power system disturbances due to relay hidden failures
Tamronglak, Surachet (Virginia Tech, 1994)
This research analyzes the linkage between power system disturbances and failures in relaying systems. The annual disturbance reports prepared by the North American Electric Reliability Council were examined. It has been found that relaying system failures plays very important role in power system cascading outages. The type of relaying system failures that are the most troublesome are the ones that have a potential to remain hidden until being exposed by some abnormal power system states to trigger relay misoperations. Each commonly used relaying scheme in transmission system is examined for any hidden failures that can lead to relay misoperations and multiple power system contingencies. Each hidden failure mode has a region, called region of vulnerability. Inside this region, some abnormal power system states can expose the hidden failure. The reach of the region depends largely on the settings of the relay in question. A method of computing the relative importance of each region of vulnerability, called vulnerability index, was proposed. The calculation of the index can be based on some measurements of power system performances. In this research, the stability measurements of the system following some contingencies that may occur in the region are chosen. With this approach, vulnerable relays can be identified. A preventive method was proposed so that the number of relay misoperations due to hidden failures and, ultimately, the number of power system disturbances can be reduced.
An Analysis of the Financial Incentives Impact on the Utility Demand-Side Management Programs
Prastawa, Andhika (Virginia Tech, 1998-07-10)
Many utilities implement the financial incentive plans in promoting their Demand-Side Management (DSM) programs. The plans are intended to reduce the customer investment cost for a high efficiency equipment option, so that to make the investment more attractive. Despite its potential to increase customer participation, the financial incentives could cause a considerable increase in program cost to the utility. An analysis of financial incentive impact on the utility DSM program is conducted in this thesis. The analysis uses the combination of the customer participation modeling and the cost-benefit analysis of a DSM program. A modeling of customer participation by a discrete choice model is presented. The model uses the logistic probability functions. The benefit and cost of DSM programs are explored to develop the analysis methodology. Two typical energy conservation options of DSM programs are taken for case studies to demonstrate the analysis. The analysis is also conducted to see the effect of financial incentives on the performance of DSM programs in a fluctuating marginal energy cost. The result of this research shows that the financial incentive could induce the customer participation, thus provide an increase of benefit and costs. However, this research also reveals that, in certain circumstances, the financial incentive may result in a decrease of net benefit due to significant increase of cost. These imply that utilities must carefully evaluate the financial incentive plan in their DSM programs, before the programs are implemented.
Artificial Intelligence Applications in the Diagnosis of Power Transformer Incipient Faults
Wang, Zhenyuan (Virginia Tech, 2000-08-08)
This dissertation is a systematic study of artificial intelligence (AI) applications for the diagnosis of power transformer incipient fault. The AI techniques include artificial neural networks (ANN, or briefly neural networks - NN), expert systems, fuzzy systems and multivariate regression. The fault diagnosis is based on dissolved gas-in-oil analysis (DGA). A literature review showed that the conventional fault diagnosis methods, i.e. the ratio methods (Rogers, Dornenburg and IEC) and the key gas method, have limitations such as the "no decision" problem. Various AI techniques may help solve the problems and present a better solution. Based on the IEC 599 standard and industrial experiences, a knowledge-based inference engine for fault detection was developed. Using historical transformer failure data from an industrial partner, a multi-layer perceptron (MLP) modular neural network was identified as the best choice among several neural network architectures. Subsequently, the concept of a hybrid diagnosis was proposed and implemented, resulting in a combined neural network and expert system tool (the ANNEPS system) for power transformer incipient diagnosis. The abnormal condition screening process, as well as the principle and algorithms of combining the outputs of knowledge based and neural network based diagnosis, were proposed and implemented in the ANNEPS. Methods of fuzzy logic based transformer oil/paper insulation condition assessment, and estimation of oil sampling interval and maintenance recommendations, were also proposed and implemented. Several methods of power transformer incipient fault location were investigated, and a 7Ã 21Ã 5 MLP network was identified as the best choice. Several methods for on-load tap changer (OLTC) coking diagnosis were also investigated, and a MLP based modular network was identified as the best choice. Logistic regression analysis was identified as a good auditor in neural network input pattern selection processes. The above results can help developing better power transformer maintenance strategies, and serve as the basis of on-line DGA transformer monitors.
Bidirectional DC-DC Power Converter Design Optimization, Modeling and Control
Zhang, Junhong (Virginia Tech, 2008-01-30)
In order to increase the power density, the discontinuous conducting mode (DCM) and small inductance is adopted for high power bidirectional dc-dc converter. The DCM related current ripple is minimized with multiphase interleaved operation. The turn-off loss caused by the DCM induced high peak current is reduced by snubber capacitor. The energy stored in the capacitor needs to be discharged before device is turned on. A complementary gating signal control scheme is employed to turn on the non-active switch helping discharge the capacitor and diverting the current into the anti-paralleled diode of the active switch. This realizes the zero voltage resonant transition (ZVRT) of main switches. This scheme also eliminates the parasitic ringing in inductor current. This work proposes an inductance and snubber capacitor optimization methodology. The inductor volume index and the inductor valley current are suggested as the optimization method for small volume and the realization of ZVRT. The proposed capacitance optimization method is based on a series of experiments for minimum overall switching loss. According to the suggested design optimization, a high power density hardware prototype is constructed and tested. The experimental results are provided, and the proposed design approach is verified. In this dissertation, a general-purposed power stage model is proposed based on complementary gating signal control scheme and derived with space-state averaging method. The model features a third-order system, from which a second-order model with resistive load on one side can be derived and a first-order model with a voltage source on both sides can be derived. This model sets up a basis for the unified controller design and optimization. The Δ-type model of coupled inductor is introduced and simplified to provide a more clearly physical meaning for design and dynamic analysis. These models have been validated by the Simplis ac analysis simulation. For power flow control, a unified controller concept is proposed based on the derived general-purposed power stage model. The proposed unified controller enables smooth bidirectional current flow. Controller is implemented with digital signal processing (DSP) for experimental verification. The inductor current is selected as feedback signal in resistive load, and the output current is selected as feedback signal in battery load. Load step and power flow step control tests are conducted for resistive load and battery load separately. The results indicate that the selected sensing signal can produce an accurate and fast enough feedback signal. Experimental results show that the transition between charging and discharging is very smooth, and there is no overshoot or undershoot transient. It presents a seamless transition for bidirectional current flow. The smooth transition should be attributed to the use of the complementary gating signal control scheme and the proposed unified controller. System simulations are made, and the results are provided. The test results have a good agreement with system simulation results, and the unified controller performs as expected.
A comprehensive overview, behavioral model and simulation of a Fault Current Limiter
Verma, Manish (Virginia Tech, 2009-06-29)
Distribution systems across most parts of the globe are highly radial in nature. As loads are gradually increased on a particular distribution system, a higher operating current state leading to increased fault current levels is attained. Hence, the relay co-ordination is disturbed and equipments such as feeders and circuit breakers need to be replaced with higher rating so that they can handle the new currents often leading to expensive retrofit costs. The use of fault current limiter (FCL) is proposed to mitigate the effects of high current levels on a distribution system. A comprehensive and up-to-date literature review of FCL technologies is presented. Detailed efforts of an in-house developed behavioral superconducting FCL model are delineated, including FCL control algorithm and its implementation in PSCADÂ®/EMTDC environment. Results from simulation studies are investigated and compared to an actual FCL commissioned by Z-energy to highlight the effectiveness of a generic model without having to access proprietary details. Extending those concepts, a solid-state and hybrid type of limiter is also modeled and it results discussed. Finally, an impact assessment is conducted on the distribution protection scheme, due to the FCL being inserted and subsequently operated in the distribution system.
Computer relaying for EHV/UHV transmission lines
Yang, Lifeng (Virginia Tech, 1994)
As the power systems grow, system connections become more complex. Due to cost and environmental concern, more and more parallel lines and series compensated lines may be installed in the system. In order to efficiently use the transmission network, more nonlinear flexible devices such as the phase shifter and the advanced compensation system will be put into use. Once a fault occurs on such a system, a delay in clearing the fault is usually not permissive. This requires a new generation of relays which have high security and dependability and high operating speed. With the advent of high speed microprocessor and fiber optic communication technologies, it is possible to develop high performance relays. In this dissertation, a new generation of pilot relays and non-pilot relays were developed for a transmission line. The pilot relays include the instantaneous percentage current differential (IPD), the phase comparison and the phasor based percentage current differential (PPD) principles. In the pilot protections the synchronized phasor measurement techniques are employed and digital CT saturation detectors are incorporated. All these schemes in primary protections feature charging current (or shunt current ) compensation. The phasor based principles are designed to work within one and a half cycles; while others based on sample by sample comparison are assumed to work in less than a cycle. The non-pilot relays to be investigated in this dissertation include the fault location based and phase angle based directional distance relays. One cycle data window is used in the phasor calculation. Both the distance relays are assumed to make a trip decision in about one and a half cycles. All algorithms were simulated against different fault conditions using EMTP outputs. The simulation results show all the pilot relay algorithms work well for EHV IUHV transmission lines including the series compensated lines. The fault location based distance relay works well in most cases, but it may give a wrong decision for the close-in fault with the fault resistance and may have a singularity problem. The phase angle based distance relay works very well for different fault conditions and is insensitive to fault resistance. The modified phase angle based distance relay was also developed for the series compensated line. It would not lose the direction for faults with or without fault resistance, either for a compensated system in forward direction, or in an adjacent line in reverse direction. The overreach is within 20% of the protected zone. This relay algorithm is also based on the one-cycle data window DFT, and it can give a reliable trip decision in about two cycles. All pilot relays with a fiber optic link and the phase angle based distance relay can constitute a new generation of protection systems for EHV IUHV transmission lines.
Conditioning of FNET Data and Triangulation of Generator Trips in the Eastern Interconnected System
Gardner, Robert Matthew (Virginia Tech, 2005-07-29)
Using data from the frequency disturbance recorders (FDRs) that comprise the nation-wide frequency monitoring network known as FNET, disturbances in the eastern interconnected system (EI) have been monitored and recorded over the past several years. Analysis of this and other data by a wide variety of research scientists and engineers has rendered the idea that frequency disturbances from generator trips, transmission line trips, load trips, and other events, travel with finite speed as electromechanical waves throughout any power system (in this case the EI). Using FNET data as a tool, it is possible to measure and output the arrival times of these disturbance waves with a time resolution of 100 ms. To observe with certainty the arrival time of the frequency disturbance waves, field data collected by the FDRs must first be conditioned in a robust manner. The current method that uses the moving mean of raw FDR data is analyzed and two computationally efficient robust methods are suggested in this report. These new methods that rely on robust statistics are more resistant to the effect of outliers contained within the raw FDR data. Furthermore, like the moving mean, these methods smooth the raw data without removing the general trend. Having recorded and conditioned the FDR data, three conventional triangulation techniques taken from the field of seismology are proposed and analyzed. This study reconfirms the fact that the EI is not a medium of continuous elasticity though which the frequency perturbations travel but rather a discontinuous patchwork of varying elasticities. Within this report, nine generator trip events are analyzed and the aforementioned triangulation methods are applied. The advantages and disadvantages of each method are discussed. To conclude, axioms of future research are proposed and delineated.
Confidence Interval Estimation for Distribution Systems Power Consumption by Using the Bootstrap Method
Cugnet, Pierre (Virginia Tech, 1997-07-15)
The objective of this thesis is to estimate, for a distribution network, confidence intervals containing the values of nodal hourly power consumption and nodal maximum power consumption per customer where they are not measured. The values of nodal hourly power consumption are needed in operational as well as in planning stages to carry out load flow studies. As for the values of nodal maximum power consumption per customer, they are used to solve planning problems such as transformer sizing. Confidence interval estimation was preferred to point estimation because it takes into consideration the large variability of the consumption values. A computationally intensive statistical technique, namely the bootstrap method, is utilized to estimate these intervals. It allows us to replace idealized model assumptions for the load distributions by model free analyses. Two studies have been executed. The first one is based on the original nonparametric bootstrap method to calculate a 95% confidence interval for nodal hourly power consumption. This estimation is carried out for a given node and a given hour of the year. The second one makes use of the parametric bootstrap method in order to infer a 95% confidence interval for nodal maximum power consumption per customer. This estimation is realized for a given node and a given month. Simulation results carried out on a real data set are presented and discussed.
Configurable, Coordinated, Model-based Control in Electrical Distribution Systems
Hambrick, Joshua Clayton (Virginia Tech, 2010-04-28)
Utilities have been planning, building, and operating electrical distribution systems in much the same way for decades with great success. The electrical distribution system in the United States has been consistently reliable; an impressive feat considering its amazing complexity. However, in recent years, the electrical distribution system landscape has started to undergo drastic changes. Emerging applications of technologies such as distributed generation, communications, and power electronics offer both opportunities and challenges to power system operators as well as customers and developers. In this work, Graph Trace Analysis along with an integrated system model are used to develop algorithms and analysis methods necessary to facilitate the implementation of these new technologies on the electrical distribution system. A penetration limit analysis is developed to analyze the impact of distributed generation on radial distribution feeders. The analysis considers generation location, equipment rating, voltage violations, and flicker to determine the amount of generation that can be safely attached to a circuit. A real-time, hierarchical, model-based control method is developed that coordinates the operation of all control devices on electrical distribution circuits. The controller automatically compensates for changes in circuit topology as well as the addition or removal of control devices from the active circuit. Additionally, the controller allows the integration of modern, "smart" equipment with legacy control devices to facilitate incremental modernization strategies. Finally, a framework is developed to allow the testing of new analysis and control methodologies for electrical distribution systems. The framework can be used to test scenarios over multiple consecutive hourly or sub-hourly time points. The framework is used to demonstrate the effectiveness of the model-based controller versus existing operating methods for a distribution circuit test case.
A continuum Approach to Power system simulation
Donolo, Marcos A. (Virginia Tech, 2006-09-04)
The behavior of large and tightly interconnected power systems resembles, in certain circumstances, the behavior of a continuously distributed system. This resemblance motivated the derivation of continuum models, which were used to explain and predict disturbance propagation, un-damped power oscillations, and the stability of power systems. In this dissertation, we propose a one-dimensional continuum representation suitable for meshed power systems. Previous continuous representations of meshed power systems used two-dimensional spatial domains. Thus our approach has the potential to provide better resolution for comparable computational burden. It is important to note that, the computational burden required to obtain solutions for PDEs involved in the continuum representation varies notably with the solver implementation. The contributions of this dissertation are: a) Reviewing a previous continuum model and providing a detailed derivation for the one-dimensional version of it. b) Providing and describing in detail a parameter distribution technique adequate for the continuum approach. c) Identifying and documenting limitations on the continuum model voltage calculation. e) Providing a procedure to simulate the behavior of meshed power systems using the one dimensional continuum model. And f) Identifying and applying a numerical PDE solver for the continuum approach.
A decision making tool for evaluating uncertainties in electric power system planning
Osareh, Ali Reza (Virginia Tech, 1994)
Planning of today's electric utilities demand careful consideration of issues such as environment, demand-side management, non-utility generation, and new technologies which are subject to different constraints and uncertainties. Utilities have long developed and used models for their short and long-term planning, most of which are single purpose, large, data intensive, and do not fully account for uncertainties. New techniques have emerged to deal with uncertainties in utility planning. Among them, the Analytic Hierarchy Process (AHP) has been more successful in assessing uncertainties, and found to be well structured and applicable to individual as well as group decision makers. However, the results of this method are merely point estimate values. It is the objective of this research to identify a methodology which is capable of evaluating uncertainties with relative ease and accuracy without the need for a large volume of data and complicated software packages. The Analytic Hierarchy Process has been extended to estimate the variance of the error in judgments and therefore the confidence interval of values instead of point estimate values. A simulation study was carried out to check the accuracy of error variance (QI) in confidence interval calculations. The results showed that QI has a linear relationships with the variance of weights. The extended AHP method is applied to three case studies, including 1) Third party generation bidding evaluation criteria, 2) Identification and evaluation of different load management programs on utility peak reduction, and 3) Oil price prediction for electric utilities. This method promises to be an effective decision making tool for evaluating uncertainties in electric power system planning.
Design and Analysis of a Grid Connected Photovoltaic Generation System with Active Filtering Function
Leslie, Leonard Gene Jr. (Virginia Tech, 2003-03-14)
In recent years there has been a growing interest in moving away from large centralized power generation toward distributed energy resources. Solar energy generation presents several benefits for use as a distributed energy resource, especially as a peaking power source. One drawback of solar energy sources is the need for energy storage for the system to be utilized for a significant percentage of the day. One way of avoiding adding energy storage to a solar generation system while still maintaining high system utilization is to design the power conversion subsystem to also provide harmonic and reactive compensation. When the sun is unavailable for generation, the system hardware can still be utilized to correct for harmonic and reactive currents on the distribution system. This system's dual-purpose operation solves both the power generation need, and helps to improve the growing problem of harmonic and reactive pollution of the distribution system. A control method is proposed for a system that provides approximately 1 kW of solar generation as well as up to 10 kVA of harmonic and reactive compensation simultaneously. The current control for the active was implemented with the synchronous reference frame method. The system and controller was designed and simulated. The harmonic and reactive compensation part of the system was built and tested experimentally. Due to the delay inherent in the control system from the sensors, calculation time, and power stage dynamics, the system was unable to correct for higher order harmonics. To allow the system to correct for all of the harmonics of concern, a hybrid passive - active approach was investigated by placing a set of inductors in series with the AC side of the load. A procedure was developed for properly sizing the inductors based on the harmonic residuals with the compensator in operation.

Browsing by Author "Liu, Yilu"

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