73 research outputs found

    Modeling and Control of Single Phase Grid-Tie Converters

    Get PDF
    The penetration of renewable energy into the electric utility grid is growing worldwide. At the heart of these renewable sources is the power electronic systems used to convert the renewable source to an output that can be connected to the grid. In recent years, there has been a great deal of work in designing converters for grid-tie applications and is continuing to grow. With recent smart grid activities, it is not likely that this work will cease in the short term. Most of the recent research is in ancillary services that the converter can offer in addition to the normal energy transfer. With more advanced converters, the ability to provide reactive power and harmonic compensation has triggered many researchers to look at more advanced control schemes. The work in this thesis focuses on modeling and control of single phase grid connected converters with an emphasis on grid interactions and ancillary services. While there has been a great deal of work in the modeling and control area for general converter operation, there has been little analysis in the converter’s response to grid disturbances. There are very few resources that discuss the controller design as it relates to power quality. However, these are issues that must be considered in a real design and what separates the research and commercial level converters. In addition to control and modeling work, the author suggests two new transformerless converter topologies for photovoltaic applications. In general, these converters can be viewed as a hybrid converter topology comprised of a two level and multi-level structure. Both converters show conducted emissions improvements over the standard commercial transformerless converters while also meeting leakage current requirements

    Fault detection in a three-phase inverter fed circuit: Enhancing the Tripping capability of a UPS circuit breaker using wave shape recognition algorithm

    Get PDF
    Uninterruptible power supplies (UPS) are electrical devices that protect sensitive loads from power line disturbances such as source side overcurrents caused by overvoltage and power surges. The critical load in a double conversion UPS system is supplied from an invert-er. When overcurrents occur on the load side of double conversion UPS systems, both the UPS system’s inverter and the critical load connected to it stand a high risk of damage. Load side overcurrents due to short circuits, ground faults and motor/transformer start-up are very damaging to power electronic components, electrical equipment and cable connections. There exists circuit breakers on the load side designed to trip when a huge overcurrent occurs, thereby clearing the fault. A circuit breaker is normally sized and installed based on the maxi-mum capacity of the host system and trips when a predetermined overcurrent is recorded within a specific period of time. The UPS system’s inverter has a pre-set current limit value to protect insulated-gate bipolar transistors (IGBTs) from damage. During an overcurrent, invert-ers can supply a fault current whose peak value is limited to the IGBT current limit value. This inverter supplied fault current is not high enough to trip the circuit breaker. After an extended period of overcurrent, UPS internal tripping will be activated and all loads lose power. Opera-tion of the UPS in bypass mode supplies the required fault current but exposes the sensitive load to power line distortions. Therefore, it is desired to always supply the critical load via the inverter. This study targets to design a detection algorithm for short circuits and ground faults with a detection time faster than the UPS system’s internal tripping in order to isolate the faulted ar-ea, when the inverter is supplying the critical load. To achieve this, first, a MATLAB model was designed to aid in preliminary studies of fault detection through analysing the system behaviour. Secondly, literature review was conducted and a fault detection method selected with the help of the MATLAB model. Next, laboratory tests on a real UPS system were carried out and compared to the MATLAB results. Lastly, the detection algorithm was designed, im-plemented and tested on a real double conversion UPS system. The test results indicate that the implemented detection algorithm successfully detects short circuits and ground faults well within the desired time. It also successfully distinguishes short circuits and ground faults from other sources of overcurrents such as overloading and transformer inrush current. Future development of this study includes additional features such as a fault classification method proposed for implementation to improve the UPS debugging process during maintenance. Moreover, the detection algorithm will also be refined and devel-oped further to activate a circuit that discharges a current pulse to increase the fault current fed to the circuit breaker

    Voltage Sag Ride-Through and Harmonics Mitigation for Adjustable Speed Drives using Dual-Functional Hardware

    Get PDF
    Great portion of today's industry are Adjustable Speed Drives (ASD's) operated in order to fulfill certain processes. When these processes are critical ones or sensitive to voltage disturbances, that might take place due to inserting high load in an area near to the Point of Common Coupling (PCC) of the process or due to a short term outage, few tens of thousands up to millions of dollars will be lost once such interruptions (voltage sags) take place as a result of the process failure. On the other hand, a distorted voltage waveform at the PCC for some sensitive process might malfunction as a result of the high harmonic content of the voltage waveform. Utilities are required to deliver as pure as possible sinusoidal voltage waveform according to certain limits; thus, they might apply fines against the consumers who are responsible for producing high amounts of current harmonics that affect the voltage wave shape at the PCC in order to force them to improve the consumer's load profile by adding filters at PCC for instance. Utilities are charging the consumers who are drawing power at poor power factor as well. This thesis presents an ASD retrofitted with a dual-functional piece of hardware connected in series to its DC-link that is capable of handling the previously two mentioned problems. In other words, hardware that is capable of providing voltage sag ride-through during the voltage sag conditions on one side, on the other side, during the normal operating conditions, it is capable to mitigate the harmonic contents of the drawn current by the ASD's rectifier and to improve the power factor. Survey on voltage sag ride-through for ASD's approaches are presented in the literature has been made. Approaches are classified as the topology utilized; first, topologies that utilizes energy storage elements that store energy to compensate the DC-link voltage with during the voltage sags, second, topologies retrofitting the DC-link itself with additional hardware to compensate the DC-link voltage. The first group is capable to provide voltage compensating during the full outages while the second can't. The presented voltage sag ride-through work of this thesis belongs to the second group. Boost converter has been used as the hardware to compensate the DC-link voltage because of its simplicity and cheap price. An adaptive linear network (ADALINE) is investigated as the detection system to detect the envelope of the input voltage waveform. Once the envelope of the voltage goes below a certain level, the boost converter is activated to compensate the difference between voltage set point and the actual DC-link voltage. Simulation results supporting the proposed configuration are presented. A third-harmonic current injection approach is utilized in this work in order to achieve total harmonic distortion (THD) mitigation from 32% to 5. 125% (theoretically). Two third-harmonic current injection networks have been investigated; one utilizes a real resistor, the other utilizes a resistor emulator to reduce the energy dissipated. The proposed controller for the resistor emulator does not require a proportional-integral (PI) controller. As a result of the common devices between the voltage sag ride-through circuitry and the harmonic mitigation one, they can be integrated together in one circuitry connected in series with the DC-link of the ASD. And hence, the dual functionality of the hardware will be achieved. Simulation results supporting the theoretical results have been presented

    Unified Control Strategy for Microgrid Solid-State Transformers

    Get PDF
    Solid-state transformers (SST) are particularly useful components in distributed generation systems (DG). This research approaches the control of the SST in a more comprehensive and an organized way. It proposes a compact, versatile and an efficient unified control strategy. This proposal gives rise to three more proposals. i) A method to mitigate the current harmonic distortion which is uniquely software-based and ii) An efficient low-voltage ride-through (LVRT) scheme. Both of those functions come at no extra cost using the proposed unified control scheme. These proposals further demonstrate the proposed strategy’s ability to accommodate further features and modifications. A further contribution to this research addresses the unbalanced load conditions. It proposes a simple, cost-free modification to a resonant filter - making it suitable for the proposed control strategy thus maintaining its simplicity without compromising its practicality. All the proposals of this research have been validated through simulation in Simulink

    Unified Control Strategy for Microgrid Solid-State Transformers

    Get PDF
    Solid-state transformers (SST) are particularly useful components in distributed generation systems (DG). This research approaches the control of the SST in a more comprehensive and an organized way. It proposes a compact, versatile and an efficient unified control strategy. This proposal gives rise to three more proposals. i) A method to mitigate the current harmonic distortion which is uniquely software-based and ii) An efficient low-voltage ride-through (LVRT) scheme. Both of those functions come at no extra cost using the proposed unified control scheme. These proposals further demonstrate the proposed strategys ability to accommodate further features and modifications. A further contribution to this research addresses the unbalanced load conditions. It proposes a simple, cost-free modification to a resonant filter - making it suitable for the proposed control strategy thus maintaining its simplicity without compromising its practicality. All the proposals of this research have been validated through simulation in Simulink

    Advanced Solutions for Renewable Energy Integration into the Grid Addressing Intermittencies, Harmonics and Inertial Response

    Get PDF
    Numerous countries are trying to reach almost 100\% renewable penetration. Variable renewable energy (VRE), for instance wind and PV, will be the main provider of the future grid. The efforts to decrease the greenhouse gasses are promising on the current remarkable growth of grid connected photovoltaic (PV) capacity. This thesis provides an overview of the presented techniques, standards and grid interface of the PV systems in distribution and transmission level. This thesis reviews the most-adopted grid codes which required by system operators on large-scale grid connected Photovoltaic systems. The adopted topologies of the converters, the control methodologies for active - reactive power, maximum power point tracking (MPPT), as well as their arrangement in solar farms are studied. The unique L(LCL)2 filter is designed, developed and introduced in this thesis. This study will help researchers and industry users to establish their research based on connection requirements and compare between different existing technologies. Another, major aspect of the work is the development of Virtual Inertia Emulator (VIE) in the combination of hybrid energy storage system addressing major challenges with VRE implementations. Operation of a photovoltaic (PV) generating system under intermittent solar radiation is a challenging task. Furthermore, with high-penetration levels of photovoltaic energy sources being integrated into the current electric power grid, the performance of the conventional synchronous generators is being changed and grid inertial response is deteriorating. From an engineering standpoint, additional technical measures by the grid operators will be done to confirm the increasingly strict supply criteria in the new inverter dominated grid conditions. This dissertation proposes a combined virtual inertia emulator (VIE) and a hybrid battery-supercapacitor-based energy storage system . VIE provides a method which is based on power devices (like inverters), which makes a compatible weak grid for integration of renewable generators of electricity. This method makes the power inverters behave more similar to synchronous machines. Consequently, the synchronous machine properties, which have described the attributes of the grid up to now, will remain active, although after integration of renewable energies. Examples of some of these properties are grid and generator interactions in the function of a remote power dispatch, transients reactions, and the electrical outcomes of a rotating bulk mass. The hybrid energy storage system (HESS) is implemented to smooth the short-term power fluctuations and main reserve that allows renewable electricity generators such as PV to be considered very closely like regular rotating power generators. The objective of utilizing the HESS is to add/subtract power to/from the PV output in order to smooth out the high frequency fluctuations of the PV power, which may occur due to shadows of passing cloud on the PV panels. A control system designed and challenged by providing a solution to reduce short-term PV output variability, stabilizing the DC link voltage and avoiding short term shocks to the battery in terms of capacity and ramp rate capability. Not only could the suggested system overcome the slow response of battery system (including dynamics of battery, controller, and converter operation) by redirecting the power surges to the supercapacitor system, but also enhance the inertial response by emulating the kinetic inertia of synchronous generator

    Power Quality

    Get PDF
    Electrical power is becoming one of the most dominant factors in our society. Power generation, transmission, distribution and usage are undergoing signifi cant changes that will aff ect the electrical quality and performance needs of our 21st century industry. One major aspect of electrical power is its quality and stability – or so called Power Quality. The view on Power Quality did change over the past few years. It seems that Power Quality is becoming a more important term in the academic world dealing with electrical power, and it is becoming more visible in all areas of commerce and industry, because of the ever increasing industry automation using sensitive electrical equipment on one hand and due to the dramatic change of our global electrical infrastructure on the other. For the past century, grid stability was maintained with a limited amount of major generators that have a large amount of rotational inertia. And the rate of change of phase angle is slow. Unfortunately, this does not work anymore with renewable energy sources adding their share to the grid like wind turbines or PV modules. Although the basic idea to use renewable energies is great and will be our path into the next century, it comes with a curse for the power grid as power fl ow stability will suff er. It is not only the source side that is about to change. We have also seen signifi cant changes on the load side as well. Industry is using machines and electrical products such as AC drives or PLCs that are sensitive to the slightest change of power quality, and we at home use more and more electrical products with switching power supplies or starting to plug in our electric cars to charge batt eries. In addition, many of us have begun installing our own distributed generation systems on our rooft ops using the latest solar panels. So we did look for a way to address this severe impact on our distribution network. To match supply and demand, we are about to create a new, intelligent and self-healing electric power infrastructure. The Smart Grid. The basic idea is to maintain the necessary balance between generators and loads on a grid. In other words, to make sure we have a good grid balance at all times. But the key question that you should ask yourself is: Does it also improve Power Quality? Probably not! Further on, the way how Power Quality is measured is going to be changed. Traditionally, each country had its own Power Quality standards and defi ned its own power quality instrument requirements. But more and more international harmonization efforts can be seen. Such as IEC 61000-4-30, which is an excellent standard that ensures that all compliant power quality instruments, regardless of manufacturer, will produce of measurement instruments so that they can also be used in volume applications and even directly embedded into sensitive loads. But work still has to be done. We still use Power Quality standards that have been writt en decades ago and don’t match today’s technology any more, such as fl icker standards that use parameters that have been defi ned by the behavior of 60-watt incandescent light bulbs, which are becoming extinct. Almost all experts are in agreement - although we will see an improvement in metering and control of the power fl ow, Power Quality will suff er. This book will give an overview of how power quality might impact our lives today and tomorrow, introduce new ways to monitor power quality and inform us about interesting possibilities to mitigate power quality problems. Regardless of any enhancements of the power grid, “Power Quality is just compatibility” like my good old friend and teacher Alex McEachern used to say. Power Quality will always remain an economic compromise between supply and load. The power available on the grid must be suffi ciently clean for the loads to operate correctly, and the loads must be suffi ciently strong to tolerate normal disturbances on the grid

    An Update on Power Quality

    Get PDF
    Power quality is an important measure of fitness of electricity networks. With increasing renewable energy generations and usage of power electronics converters, it is important to investigate how these developments will have an impact to existing and future electricity networks. This book hence provides readers with an update of power quality issues in all sections of the network, namely, generation, transmission, distribution and end user, and discusses some practical solutions
    • …
    corecore