Study of the generator/motor operation of induction machines in a high frequency link space power system

Static power conversion systems have traditionally utilized dc current or voltage source links for converting power from one ac or dc form to another since it readily achieves the temporary energy storage required to decouple the input from the output. Such links, however, result in bulky dc capacitors and/or inductors and lead to relatively high losses in the converters due to stresses on the semiconductor switches. The feasibility of utilizing a high frequency sinusoidal voltage link to accomplish the energy storage and decoupling function is examined. In particular, a type of resonant six pulse bridge interface converter is proposed which utilizes zero voltage switching principles to minimize switching losses and uses an easy to implement technique for pulse density modulation to control the amplitude, frequency, and the waveshape of the synthesized low frequency voltage or current. Adaptation of the proposed topology for power conversion to single-phase ac and dc voltage or current outputs is shown to be straight forward. The feasibility of the proposed power circuit and control technique for both active and passive loads are verified by means of simulation and experiment

Steady-State Simulation of LCI-Fed Synchronous Motor Drives Through a Computationally Efficient Algebraic Method

Wound-field synchronous motors (WFSMs) fed by load-commutated inverters (LCIs) are widely used for high-power applications in many fields like ship propulsion, oil and gas industry, and pumped-storage hydropower generation. Several design architectures exist for LCI drives, depending on the number of LCIs and their dc-link connection as well as on the number of WFSM phase count. The prediction of LCI drive performance at steady state is important in the design stage, especially in regard to the prediction of the torque pulsations, which can give rise to serious mechanical resonance issues. This paper proposes an algebraic method to simulate the steady-state behavior of LCI drives in all their configurations of practical interest. Compared to conventional dynamic simulation approaches based on differential equation solution, the method is much more computationally efficient and requires a very limited knowledge of system parameters. Its accuracy is experimentally assessed by comparison against measurements taken on a real LCI drive arranged according to various possible schemes. Furthermore, the advantages of the proposed algebraic method over the dynamic simulations are highlighted by comparison against the simulation results on a high-power LCI-fed WFSM drive in MATLAB/Simulink environment

Assessment of novel power electronic converters for drives applications

Phd ThesisIn the last twenty years, industrial and academic research has produced over one hundred new converter topologies for drives applications. Regrettably, most of the published work has been directed towards a single topology, giving an overall impression of a large number of unconnected, competing techniques. To provide insight into this wide ranging subject area, an overview of converter topologies is presented. Each topology is classified according to its mode of operation and a family tree is derived encompassing all converter types. Selected converters in each class are analysed, simulated and key operational characteristics identified. Issues associated with the practical implementation of analysed topologies are discussed in detail. Of all AC-AC conversion techniques, it is concluded that softswitching converter topologies offer the most attractive alternative to the standard hard switched converter in the power range up to 100kW because of their high performance to cost ratio. Of the softswitching converters, resonant dc-link topologies are shown to produce the poorest output performance although they offer the cheapest solution. Auxiliary pole commutated inverters, on the other hand, can achieve levels of performance approaching those of the hard switched topology while retaining the benefits of softswitching. It is concluded that the auxiliary commutated resonant pole inverter (ACPI) topology offers the greatest potential for exploitation in spite of its relatively high capital cost. Experimental results are presented for a 20kW hard switched inverter and an equivalent 20kW ACPI. In each case the converter controller is implanted using a digital signal processor. For the ACPI, a new control scheme, which eliminates the need for switch current and voltage sensors, is implemented. Results show that the ACPI produces lower overall losses when compared to its hardswitching counterpart. In addition, device voltage stress, output dv/dt and levels of high frequency output harmonics are all reduced. Finally, it is concluded that modularisation of the active devices, optimisation of semiconductor design and a reduction in the number of additional sensors through the use of novel control methods, such as those presented, will all play a part in the realisation of an economically viable system.Research Committee of the University of Newcastle upon Tyn

Induction Motors

AC motors play a major role in modern industrial applications. Squirrel-cage induction motors (SCIMs) are probably the most frequently used when compared to other AC motors because of their low cost, ruggedness, and low maintenance. The material presented in this book is organized into four sections, covering the applications and structural properties of induction motors (IMs), fault detection and diagnostics, control strategies, and the more recently developed topology based on the multiphase (more than three phases) induction motors. This material should be of specific interest to engineers and researchers who are engaged in the modeling, design, and implementation of control algorithms applied to induction motors and, more generally, to readers broadly interested in nonlinear control, health condition monitoring, and fault diagnosis

A TWELVE-PULSE LOAD COMMUTATED CONVERTER DRIVE SYSTEM WITH VSI FOR STARTING UP AND ACTIVE POWER FILTERING IN AN LNG APPLICATION

Variable Frequency Drives (VFDs) are an integral component of the industry in today’s age. VFDs provide a great range of control for electrical machines, and can be integrated in a variety of applications to meet the desired objectives of operation with improved reliability and efficiency. This thesis presents the Load-Commutated Converter (LCC) drive, which belongs to the Current Source Converter (CSC) based drive system family. Such drives are widely used in high power applications, due to power handling capabilities and the maturity of the drive system. The application under study is that of a helper/starter motor for a turbine compressor in a Liquefied Natural Gas (LNG) plant. Primarily, the thesis presents real-life scenarios of drive system operation such as constant/variable speed operation at constant/varying torque. The respective controllers for the LCC drive are presented alongside their results. In addition to simulating the drive system in this LNG application, current harmonic mitigation measures are presented in this study. The typical converter topology presented in this thesis is the 12-pulse type, however comparisons with different topologies (6, 18, and 24-pulse) have also been presented. Finally, a dual-purpose external Voltage Source Inverter (VSI) is used both as a starter and an Active Power Filter (APF), therefore addressing the issues of drive/load induced harmonics and LCC starting. As a conclusion, a controlled LCC drive model is simulated in SIMULINK to emulate the drive operation in actual plant conditions. The controlled drive is further studied for the presence of harmonics and their subsequent mitigation, by using passive as well as active power filters. The results obtained present the adequacy of the control system as well as the efficacy of the filters used for harmonics mitigation. Future work revolves around improving the efficiency of the APF, and the drive control system to be more robust and reliable. The system can further be investigated for enhancements as per operational requirements

The control and operation of the five level diode clamped inverter

This thesis describes an investigation of three and five level diode clamped inverters for motor drive applications. The work was completed as a PhD project at the University of Nottingham with funding from EPSRC and Heenan Drives Ltd. The investigation of the three level converter describes the design, development, control and operation of an 11kW prototype. Included in the design is a review of typical switching strategies employed for control of the output voltage. New improvements to the sub-harmonic pulse width modulation method are presented which allow an improved output waveform to be obtained. The problem of DC link capacitor voltage balancing (Neutral Point Control) is addressed and a novel balancing control method is presented based on the addition of a DC offset to the modulation pattern. This method is verified through mathematical analysis and experimental operation. The operational limits of the control are analysed. Improvements to the technique are presented to expand its operating limits. The development of a prototype five level converter is then described. The design again features improvements to the sub-harmonic modulation strategy to provide enhanced output waveform generation, particularly for transient operation. The current demands on the DC link capacitors for the five level arrangement are analysed and it is concluded that the capacitors cannot be regulated by simple modifications to the output switching pattern. A novel circuit is presented to achieve capacitor balancing within the DC link. The circuit behaviour is described and analysed. Operation is confirmed through simulation and experimental implementation. High dynamic performance is demonstrated via the use of a vector controlled induction motor. Neutral point control is successfully achieved through a similar method to that used for the three level inverter. Having demonstrated the principle of operation of the three and five level inverters on low voltage prototypes, the thesis concludes with a review of the main considerations required to implement the configurations as medium voltage drives

Collected Papers on Wind Turbine Technology

R and D projects on electricity generating wind turbines were conducted at the NASA Lewis Research Center from 1973 to 1988. Most projects were sponsored by the U.S. Department of Energy (DOE), a major element of its Federal Wind Energy Program. Another large wind turbine project was by the Bureau of Reclamation of the U.S. Department of Interior (DOI). From 1988 to 1995, NASA wind energy activities have been directed toward the transfer of technology to commercial and academic organizations. As part of these technology transfer activities, previously unpublished manuscripts have been assembled and presented here to share the wind turbine research results with the wind energy community. A variety of wind turbine technology topics are discussed: Wind and wake models; Airfoil properties; Structural analysis and testing; Control systems; Variable speed generators; and acoustic noise. Experimental and theoretical results are discussed

The control and operation of the five level diode clamped inverter

This thesis describes an investigation of three and five level diode clamped inverters for motor drive applications. The work was completed as a PhD project at the University of Nottingham with funding from EPSRC and Heenan Drives Ltd. The investigation of the three level converter describes the design, development, control and operation of an 11kW prototype. Included in the design is a review of typical switching strategies employed for control of the output voltage. New improvements to the sub-harmonic pulse width modulation method are presented which allow an improved output waveform to be obtained. The problem of DC link capacitor voltage balancing (Neutral Point Control) is addressed and a novel balancing control method is presented based on the addition of a DC offset to the modulation pattern. This method is verified through mathematical analysis and experimental operation. The operational limits of the control are analysed. Improvements to the technique are presented to expand its operating limits. The development of a prototype five level converter is then described. The design again features improvements to the sub-harmonic modulation strategy to provide enhanced output waveform generation, particularly for transient operation. The current demands on the DC link capacitors for the five level arrangement are analysed and it is concluded that the capacitors cannot be regulated by simple modifications to the output switching pattern. A novel circuit is presented to achieve capacitor balancing within the DC link. The circuit behaviour is described and analysed. Operation is confirmed through simulation and experimental implementation. High dynamic performance is demonstrated via the use of a vector controlled induction motor. Neutral point control is successfully achieved through a similar method to that used for the three level inverter. Having demonstrated the principle of operation of the three and five level inverters on low voltage prototypes, the thesis concludes with a review of the main considerations required to implement the configurations as medium voltage drives