Multi-Level Medium Voltage Inverter for Dc Distributed Wind Farm to Establish Grid Interface and Provide Ancillary Support

Wind energy has gained in popularity in recent years due to cost, security and environmental concerns associated with conventional energy sources like fossil fuels. However, the utilization of wind energy in power systems creates many technical and non-technical challenges that need to be addressed for successful integrations. The main technical issues related to wind energy are its uncertainty and variability and their impacts on stability, reliability and quality of the electric power. In systems with high wind energy penetrations, unlike conventional generations, sudden changes in active and/or reactive power demand cannot be supported by wind energy. This lack of demand support may create unwanted voltage and frequency variations in the grid. On the hand, the existing AC distributed wind farms have several drawbacks including complexity, higher cost, and lower efficiency. In this dissertation, a medium voltage direct current (MVDC) distribution system for wind farms is investigated. The proposed system offers higher reliability, lower cost, higher efficiency and more importantly grid support. It also allows for easier integration of energy storage systems at DC level. Design, control, implementation, and testing of a three-level medium voltage inverter are presented. The inverter can provide active and reactive power support to the grid in case of frequency and voltage droops. Simulation and experimental results are presented to verify the viability of the proposed system and control techniques

Comprehensive loss optimization of induction motor drives

Extensive use of power electronics-controlled induction motor drives over the past few decades has enabled the development of loss minimization control algorithms. With the technological advancements in power semiconductor switching devices such as insulated gate bipolar transistors and gate commutated thyristors, induction motor drives are increasingly used in applications, ranging from automotive traction to more-electric aircraft, which have widely varying speed, torque and power requirements. Advances in control technology have enabled the development of various sophisticated controllers for motor drives aimed at performance enhancement. Substantial energy savings may be obtained when drive controllers are optimized for loss reduction under varying operating conditions. This dissertation addresses loss optimization opportunities in induction motor drives from system perspectives. First, a constrained loss optimization method is developed. Past work on loss minimization has focused on specific drive components such as the machine stator and rotor windings, inverter and dc-link. Component-level loss minimization, however, will not guarantee minimum total loss in the drive system. So, a system-level loss minimization method is proposed using a comprehensive loss model, to achieve true minimum total loss. Next, a lossless damping controller is proposed to suppress undesirable resonant oscillations in the machine voltages and currents due to the use of LC filters between the inverter and motor terminals. Passive damping methods employing physical resistors to suppress these oscillations, contribute to additional losses. Lossless active damping methods with virtual resistors have been explored in the literature. Conventionally, this resistance value is fixed, based on empirical rules, and left unchanged for all operating conditions. Choosing incorrect resistance values for the damping controller can result in degraded system behavior. A small-signal transfer function approach based on operating conditions and dynamic adjustment of the virtual resistance, is developed for the damping controller. The controller is designed to allow a flexible differential damping approach. Finally, power electronics loss reduction is investigated in a voltage source inverter (VSI)-based induction motor drive. It is known that low drive speeds will result in poor bus utilization and increased power electronics loss for higher link voltages. Losses can be reduced by dynamically varying the dc link voltage according to operating conditions. In addition to reducing losses, varying the link voltage also reduces the switched voltage magnitude across the inverter switches, potentially increasing inverter reliability. In the proposed method, the link voltage is varied using a front-end dc-dc buck converter according to a loss minimization algorithm. The effect of additional loss from the front-end converter on the total loss is also studied. Benefits of the proposed methods are verified by simulations and experiments

Modelling And Simulation Of Lossless Damping Reduction By Vector Controlled Ac Motor Drive With An Efficient Lc Filter

In this paper Active damping technique isused to damp out the unwanted resonant frequencyoscillations and proposed for lossless damping of vectorcontrolledac drives with an efficient LC-Filter.  Thispaper mainly concentrated on protection of Inductionmotor and Synchronous motor under any balanced orunbalanced load conditions and this proposed techniqueis simulated with the combination of Voltage SourceInverter (VSI), LC Filter and AC Drives. However, theLC-Filter created unwanted oscillations due to internalresistance at system resonant frequency. This resistancedrop is emulated by controlling terminal voltage. Theproposed technique neither affects the dynamicresponse of the drive nor changes the design of thestandard vector control loops. This proposed techniqueis carried out in three phase domain for better accuracyof control. This paper has been implemented andsimulated by using MATLAB/ SIMULINK 7.8 (R2009a)version

Investigations of LC Filter Unbalance in an Inverter-Fed Permanent Magnet Synchronous Motor Drives

Permanent magnet synchronous machines (PMSMs) are usually controlled using two-level inverters. The output voltage of the inverter is in the form of the switching pulses between the positive DC-bus voltage and the negative DC-bus voltage. Such voltage waveforms have several adverse effects on the motor. These include, higher stress on winding insulation, higher eddy current losses and acoustic noise. Thus, to overcome these problems, different types of filters, typically LC-filters are used between the inverter and motor terminals to smooth the pulse width modulation (PWM) output voltages of the motor drives. Theoretically, the inductance and capacitance used for the filters are considered identical in each phase. However, in a practical scenario, it is difficult to have identical filter elements for all three phases. This non-ideal condition of filter elements amongst the three phases is considered as filter unbalance. This thesis investigates the impacts of filter unbalance on the PMSM drive system. Specifically, a comprehensive model of the motor drive system considering filter unbalance is proposed and developed at first. With the developed model, conventional field oriented control (FOC) is implemented to investigate the impact of this filter unbalance. A range of filter parameter variation and the corresponding impact on the motor drive including the motor current, torque and speed ripples is then studied in detail. Thereafter, the results obtained from the proposed model are validated through both circuit simulations and experimental tests. Based on the investigation results, this thesis will discuss the allowable parameter variation in the LC filters to limit the motor performance deterioration within the required bounds, which will be beneficial to engineering practice in motor drive area. In addition, this investigation shows that a conventional FOC with proportional integral (PI) controller might not be capable of mitigating the negative impact on the motor due to filter unbalance, for example, the negative sequence current. Therefore, this thesis implemented an adaptive proportional resonant (PR) controller to address negative sequence current and the corresponding impacts. A detailed mathematical framework to develop this proposed controller will also be presented in the thesis. Finally, the proposed adaptive PR controller is extensively evaluated on a laboratory PMSM drive system under different operating conditions

Optimised design of isolated industrial power systems and system harmonics

This work has focused on understanding the nature and impact of non-linear loads on isolated industrial power systems. The work was carried out over a period of 8 years on various industrial power systems: off-shore oil and gas facilities including an FPSO, a wellhead platform, gas production platforms, a mineral processing plant and an LNG plant. The observations regarding non-linear loads and electrical engineering work carried out on these facilities were incorporated into the report.A significant literature describing non-linear loads and system harmonics on industrial power systems was collected and reviewed. The literature was classified into five categories: industrial plants and system harmonics, non-linear loads as the source of current harmonics, practical issues with system harmonics, harmonic mitigation strategies and harmonic measurements.Off-shore oil and gas production facilities consist of a small compact power system. The power system incorporates either its own power generation or is supplied via subsea cable from a remote node. Voltage selection analysis and voltage drop calculation using commercially available power system analysis software are appropriate tools to analyse these systems. Non-linear loads comprise DC rectifiers, variable speed drives, UPS systems and thyristor controlled process heaters. All nonlinear loads produce characteristic and non-characteristic harmonics, while thyristor controlled process heaters generate inter-harmonics. Due to remote location, harmonic survey is not a common design practice. Harmonic current measurements during factory acceptance tests do not provide reliable information for accurate power system analysis.A typical mineral processing plant, located in a remote area includes its own power station. The power generation capacity of those systems is an order of magnitude higher than the power generation of a typical off-shore production facility. Those systems comprise large non-linear loads generating current and voltage interharmonics. Harmonic measurements and harmonic survey will provide a full picture of system harmonics on mineral processing plants which is the only practical way to determine system harmonics. Harmonic measurements on gearless mill drive at the factory are not possible as the GMD is assembled for the first time on site.LNG plants comprise large non-linear loads driving gas compressor, however those loads produce integer harmonics. Design by analysis process is an alternative to the current design process based on load lists. Harmonic measurements and harmonic survey provide a reliable method for determining power system harmonics in an industrial power system

Control of synchronous motor drives with an LC filter

Electric motors can experience voltage stress over the motor terminals due to the short rise time of the voltage pulse at the inverter output and the impedance mismatch between the lead cables and the motor. This overvoltage degrades the motor insulation, thus reducing the motor lifespan. The problems can be avoided by using a sinusoidal LC filter in the inverter output, limiting the overvoltage and dampening high-order harmonics. However, the existing control methods for LC-filtered synchronous motors are infeasible for plug-and-play drives, in which the motor data or user input are not required. This is because the methods either contain several cascaded control loops, require cumbersome parameter tuning, are sensitive to parameter errors, or the range of operating speeds is limited. Nevertheless, recently developed observer-based volts-per-hertz control shows advantages through relatively low sensitivity to parameter errors, simplicity and generality of the control algorithm, and reliance on common control gains for all synchronous motor types. These attributes indicate that the observer-based volts-per-hertz control can be used for medium-performance drives ensuring robust and stable operation at a wide range of speeds. This thesis develops observer-based volts-per-hertz control for synchronous motor drives with an LC filter. The two designed methods are based on two different observer types (reduced-order and full-order) with two state feedback control laws. The methods are further linearized by means of small-signal linearization, the control strategies are simulated in Simulink, and experimental measurements validate the simulations. The measurement results show a satisfactory performance of the permanent magnet synchronous motor with both methods. However, the control performance of the synchronous reluctance motor is poor when the full-order observer is used. The thesis subsequently provides several suggestions for future work improvements