Rotordynamic instability is present in most or all automotive turbochargers. High subsynchronous amplitudes can cause a variety of problems in areas such as mechanical failures, emissions regulations and rotor design. Self-excited vibrations from sources of damping can lock in at lateral natural frequencies causing dangerously high vibration levels. The resulting high-amplitude conical and bending modes can be reduced in order to achieve a more robust system. This research focuses on the relationship between synchronous and subsynchronous amplitude levels. It is theorized that an increase in unbalance could cause a reduction in subsynchronous vibration amplitudes. Through the use of a custom turbocharger, a series of unbalances were applied to both the turbine and compressor wheels and the resulting amplitudes were recorded off a modified compressor nut. The resulting data were reduced and are presented at the end of this paper. / Master of Science

This thesis assesses the voltage unbalance in distribution networks due to load asymmetry or line asymmetry, based on measurement data from a limited number of monitors. The main outcomes of this research are a probabilistic methodology for estimating both momentary and long term unbalance and an optimal monitor placement providing the highest accuracy for the monitored level of unbalance. With increasing numbers of large single-phase loads and distributed generation integrated into the power system, the future distribution network is expected to be more flexible, robust and “smart”. This results in the requirement for high quality of electricity supply to be delivered to customers and is a challenge for the operation of the system. As the unbalance results in excessive heating, accelerated thermal ageing, reduction of efficiency and financial losses, the unbalance should be regulated to be below the statutory limit. Given the fact that unbalance is a long term phenomenon that may not cause any triggering of protection or faulty response of equipment, it can be only determined from available data such as loading levels of a network and the incomplete monitored voltage of a network. Due to limited monitoring in the network and therefore insufficient data, unbalance may be unobservable. This thesis therefore aims to develop a methodology to increase the observability of unbalance in the network in spite of limited monitoring. This research uses Voltage Unbalance Factor (VUF) to quantify the level of unbalance. The first major part investigates the unbalance caused by asymmetrical loadings. By properly identifying the source of unbalance, the basic patterns of propagation of unbalance under possible scenarios are revealed and a methodology of probabilistic estimation of unbalance can be developed accordingly. Seen from the MV level of distribution networks, the loads are usually in the constant power form. Therefore, the variation in the load can be modelled by changing either active power or reactive power or both of them, depending on the data availability. The combinations of daily loading curve at buses and the normally distributed power factors in three phases of loads are used to create an unbalanced condition at the sources. Realistic assumptions of power factors and reasonable categories of types of loads result in realistic modelling of the unbalanced load. The probabilistic VUFs at different buses in the network are calculated and the weak areas in the network are identified using heat maps. The simulation results match the real VUF levels measured in the distribution network. The second part of the thesis explores the influence of asymmetrical lines in addition to the asymmetrical loading on propagation of unbalance. The last part provides a guideline for optimal monitor placement for unbalance. Two methods, manual ranking of buses and automatic optimization using Genetic Algorithm, are proposed. The two methods indicate the same optimal locations for monitor placement in the network. The developed methodologies enable the assessment of unbalance in the network when monitoring is limited and can be applied to real networks to assess the level of unbalance at non-monitored buses.

The integration of distributed generators (DGs) into the distribution grid has exacerbated voltage unbalance issues leading to greater risks of reducing equipment lifetime, equipment damages, and increased ohmic losses. Most approaches to regulating voltage in distribution systems only focus on voltage magnitude and neglect phasor discrepancies and do little to remedy voltage unbalance. To combat this, a novel Optimal Power Flow (OPF) is designed to help operate these resources in a manner that curtails voltage unbalance using the reactive power compensation capabilities of inverters. The OPF was run for a wide variety of loading conditions on a pair of systems using MATLAB and was shown to improve the voltage profile of the system in addition to minimizing losses in most cases. However, it is noted that the OPF loses exactness in highly stressed conditions and is unable to provide meaningful solutions / Master of Science / With the power grid getting greener and smarter by the day, a slew of new challenges arise to overcome. Distributed sources of energy like solar panels and batteries are being added to the grid right from the household level. While they are desirable for reducing our need for traditional sources of energy, the addition of these resources has been shown to cause issues in the quality of the power grid. This is particularly observed at the low-voltage domestic part of the grid where the resources cause issues with the voltage quality. The distribution grid is unbalanced by nature and adding these resources only amplifies this problem. To help mitigate voltage quality issues grid operators are starting to require voltage regulation capabilities from resources to be connected to the grid and a lot of work has been conducted to find the optimal strategies for operating these resources. However, existing paradigms for these sources only focus on fixing the voltage magnitude part of the power quality and neglect phasor relationships. This thesis aims to bridge this gap by developing a method to determine the optimal operation of these resources by using the voltage regulation capability to address both voltage magnitude and voltage unbalance issues in addition to optimal operation.

This dissertation covers three levels of issues and solutions dealing with unbalanced and/or nonlinear situations in power electronic systems, namely power converter level, power converter system level, and large-scale power electronics system level. At power converter level, after review of traditional PWM methods, especially two-dimensional space vector modulation schemes, three-dimensional space vector modulation schemes are proposed for four-legged voltage source converters, including inverters and rectifiers. The four-legged power converters with three-dimensional space vector modulation schemes have a better DC link voltage utilization and result in a low distortion. It is an effective solution to provide the neutral point for a three-phase four-wire system and to handle the neutral current due to unbalanced load or source and nonlinear loads. Comprehensive design guidelines for a four-legged inverter are presented. The four-legged rectifier is also presented which allows not only fault tolerant operation, but also provides the flexibility of equal resistance, equal current, or equal power operation under unbalanced source. Average large-signal models of four-legged power converters in both the a-b-c and d-q-o coordinates are derived. Small signal models are obtained in the d-q-o rotating coordinates. Voltage control loops are designed in the d-q-o coordinates for a high power utility power supply. Performance is studied under various load conditions. At the power converter system level, the load conditioner concept is proposed for high power applications. A power converter system structure is proposed which consists of a high-power low-switching frequency main inverter and a low-power high-switching frequency load conditioner. The load conditioner performs multiple functions, such as active filtering, active damping, and active decoupling with a high current control bandwidth. This hybrid approach allows the overall system to achieve high performance with high power and highly nonlinear loads. At the large-scale power electronics system level, the nonlinear loading effect of load converters is analyzed for a DC distribution system. Two solutions to the nonlinear loading effect are presented. One is to confine the nonlinear load effect with the sub-converter system, the other is to use a DC bus conditioner. The DC bus conditioner is the extension of the load conditioner concept. / Ph. D.

Interfacing and operating AC power electronic systems requires rapid and accurate estimation of the phase angle of the power source, and specifically of the positive sequence of the three-phase utility grid voltage. This is needed to ensure reliable operation of the power control devices and of the resulting power flow. However, the quality of this information is undermined by various distortions and unbalanced conditions of the three-phase grid voltage. Phase estimation and power control can both be performed in real time by a DSP, but a DSP typically has limited computational resources, especially in regards to speed and memory, which motivates the search for computationally efficient algorithms to accomplish these tasks. In contrast to conventional PLL techniques, recent approaches have used adaptive amplitude estimation to enhance the acquisition of the phase information, resulting in faster response and improved performance. This thesis presents a novel technique to estimate the phase of the positive sequence of a three-phase voltage in the presence of frequency variations and unbalanced conditions, referred to as hybrid negative sequence adaptive synchronous amplitude estimation with PLL, or H-NSASAE-PLL. The key feature consists of a feedback structure which embeds a positive sequence PLL and an adaptive synchronous negative sequence estimator to enhance the performance of the PLL. The resulting benefits include faster estimation of the phase of the positive sequence under unbalanced conditions with zero steady state error, simplified tuning of PLL parameters to address a wide range of application requirements, robust performance with respect to distortions and PLL parameters, a structure of minimal dynamical order (fifth) to estimate the main signal parameters of interest, simplified discretization, and reduced computational costs, making the proposed technique suitable for real time execution on a DSP. The H-NSASAE-PLL is developed in the Matlab/Simulink environment, and a specialized test signal generator is developed to evaluate its performance. The overall system is executed, and experimental results are produced, in real time, on a dSPACE DS1104 controller board. / Power Engineering and Power electronics

No description available.

Unbalanced operation of distribution feeders not only affects equipment utilization, voltage level and system protection, but it also increases extra energy losses. This leads to a deterioration of service quality, reliability and operation efficiency of a distribution system. This dissertation analyzes the problems of unbalanced three-phase distribution feeders, and offers potential solutions. Due to the voluminous data involved in a distribution system, analyzing the system by retrieving system data from paper maps is tedious and difficult. Thus, this dissertation uses data from the already constructed Outage Management System (OMS) of Taiwan Power Company (Taipower) to support distribution feeder three-phase unbalance analysis. The distribution feeder network was obtained by retrieving the connectivity table and attribute data of distribution components from the database of OMS. The topology process and node reduction were executed to identify the network configuration and to prepare the input data for load flow analysis. The hourly loading of each distribution transformer was derived using data of monthly energy consumption of customers served by each transformer, as retrieved from the Customer Information System (CIS), and the typical daily load patterns of customer classes. By performing three-phase load flow analysis, phase currents and neutral current of each primary trunk line section and each lateral could be calculated. Finally, an expert system is proposed to establish the rephasing strategy of laterals and distribution transformers to improve the imbalance of the three phases of the unbalanced distribution feeders. The heuristic rules adopted by distribution engineers are incorporated in the knowledge base of the expert system in the problem-solving process. The neutral current reduction algorithm is developed to support the inference engine to derive the rephasing strategy to reduce the neutral current of distribution feeder. In doing so, customer service interruption due to unexpected tripping of low energy over current relay (LCO) can be prevented, and furthermore the customer service interruption costs and labor costs to implement the rephasing strategy can be justified by the reduced power loss. To demonstrate the effectiveness of this proposed methodology in improving the three-phase unbalance of the distribution feeders, two actual distribution feeders in the Taipower Fengshan District were selected for computer simulation. After Taipower engineers implement the proposed rephasing strategies, the data of phase currents and neutral current of test feeders were collected from the SCADA system of Distribution Dispatch Control Center (DDCC). By comparing to the neutral current of test feeders before rephasing, it is concluded that the proposed rephasing strategy is effective in achieving three-phase balance of the distribution feeders after executing the rephasing of laterals and distribution transformers.

No description available.

No description available.

The turbocharger industry is booming recently, and there is an urgent need for new evaluations of the overall design. As the oil prices continue to rise, along with the new emissions regulations strictly enforced for the in-road as well as the off-road vehicles, the transition to turbocharged engines, and especially for diesel engines, has become irresistible. Higher power, smaller engines, reduced emissions, and overall better efficiency are the main concerns. By means of the recent development in the computational tools, a new era of the product development has emerged. Most diesel engine turbochargers incorporate floating-ring bearings that use the engine's oil for lubrication. The high-speed turbocharger is known to have subsynchronous vibrations at high amplitudes for a wide speed range that could reach 150,000 rpm. The bearing fluid-film whirl instability is the main source of the subsynchronous vibration. The nonlinear reaction forces inside the bearings are usually causing the rotor to whirl in a limit cycle but may become large enough to cause permanent damages. Additionally, the lubrication oil may leak at higher rates through the seals into the engine or the exhaust emissions. This dissertation investigates methods for improving the dynamic stability of the high-speed automotive turbochargers, especially designed for heavy-duty diesel engines that are used for example in heavy machinery, trucks, tractors, etc. The study utilizes the available modern computational tools in rotor-dynamics in addition to the locally developed supportive computer codes. This research is a major part of the turbocharger dynamic analysis supporting the current extensive experimental tests in the Virginia Tech Rotor Dynamics Laboratory for the product development of different high-speed diesel engine turbochargers. The study begins with the method of enhanced-performance hydrodynamic bearings. The aim is to modify the inner surface of the bearing for better dynamic characteristics. The finite-element model of the turbocharger rotor shaft with linearized bearing dynamic coefficients is developed. The system is solved for eigenvalues and eigenvectors in order to evaluate the dynamic stability. The first phase of the study demonstrated that there are two modes of instability that persist during much of the operating speed range, and one of the modes exhibits serious subsynchronous vibration levels at the higher speeds. The first unstable mode builds up at very low speeds forming a conical shape, where both rotor shaft ends whirl forward out-of-phase. The second unstable mode has a cylindrical shape with slight bending, where both rotor ends whirl forward in-phase. The outcome of the study is that the inner surface of the bearing has direct influence on the turbocharger dynamic stability. However, a fixed hydrodynamic bearing may not give total linear stability of the system if it is used without additional damper. The second method is to analytically design flexible damped bearing-supports in order to improve the dynamic characteristics of the rotor-bearing system. The finite-element model of the turbocharger rotor with linearized bearing dynamic coefficients is used to solve for the logarithmic decrements and hence the stability map. The design process attempts to find the optimum dynamic characteristics of the flexible damped bearing-support that would give best dynamic stability of the rotor-bearing system. The method is successful in greatly improving the dynamic stability of the turbocharger and may also lead to a total linear stability throughout the entire speed range when used besides the enhanced-performance hydrodynamic bearings. The study also presents a new method for improving the dynamic stability by inducing the turbocharger rotor unbalance in order to suppress the subsynchronous vibrations. The finite-element model of the turbocharger rotor with floating-ring bearings is numerically solved for the nonlinear time-transient response. The compressor and the turbine unbalance are induced and the dynamic stability is computed. The turbocharger model with linearized floating-ring bearings is also solved for eigenvalues and eigenvectors to predict the modes of instability. The linear analysis demonstrates that the forward whirling mode of the floating-ring at the compressor end becomes also unstable at the higher turbocharger speeds, in addition to the unstable forward conical and cylindrical modes. The numerical predictions are also compared to the former experimental results of a typical turbocharger. The results of the study show that the subsynchronous frequency amplitude of the dominant first mode is reduced when inducing either the compressor or the turbine unbalance at a certain level. In addition to the study of the stability improvement methods, the dissertation investigates the other internal and external effects on the turbocharger rotor-bearing system. The radial aerodynamic forces that may develop inside the centrifugal compressor and the turbine volutes due to pressure variation of the circulating gas are numerically predicted for magnitudes, directions, and locations. The radial aerodynamic forces are numerically simulated as static forces in the turbocharger finite-element model with floating-ring bearings and solved for nonlinear time-transient response. The numerical predictions of the radial aerodynamic forces are computed with correlation to the earlier experimental results of the same turbocharger. The outcomes of the investigation demonstrated a significant influence of the radial aerodynamic loads on the turbocharger dynamic stability and the bearing reaction forces. The numerical predictions are also compared to the former experimental results for validation. The external effect of the engine-induced vibration on the turbocharger dynamic stability is studied. The engine-induced excitations are numerically simulated as time-forcing functions on the rotor-bearings of the turbocharger finite-element model with floating-ring bearings in order to solve for the nonlinear time-transient response. The compressor radial aerodynamic forces are combined to the engine-induced excitations to numerically predict the total nonlinear transient response. The results of the study show that there are considerable amplitudes at the engine-excitation frequency in the subsynchronous region that may also have similar amplitude at the second harmonic. Additionally, the magnitudes of the engine-induced vibration have an effect on the turbocharger dynamic stability. The numerical predictions are compared to the former experimental tests for turbocharger dynamic stability. / Ph. D.