A novel compensation scheme based on a virtual air gap variable reactor for AC voltage control

Voltage control based on reactive power compensation is a fundamental aspect of the operation of ac electric power systems. This paper presents a novel shunt compensation scheme based on a virtual air gap variable reactor. The scheme is fully developed, from the adaptation of the virtual air gap principle to high-voltage applications and the determination of its expected performance, to the proposal of a digital cascade control using internal model and proportional-integral controllers. The suitability and flexibility of the device, and the voltage control and reactive power compensation scheme are verified by means of laboratory tests performed in a small-scale prototype. Measured results show that the proposed device and its control provide a robust load compensation scheme for ac systems

Improve the Flexibility of Power Distribution Network by Using Back-to-back Voltage Source Converter

Employing increasing distributed generations (DGs) into existing distribution networks is an inevitable trend of the development of modern electric power systems because of the benefits including the environmentally friendly generation, higher efficiency and improved flexibility and reliability. However, high DG penetration level could pose various issues among which the voltage violation and fault level increase are the most concerned. According to the current situation of UK distribution networks, voltage violation is likely to be the first constraint to be met when DG penetration level is increased to certain level. Therefore, compensators are considered to be implemented to regulate the voltage. The reactive power compensators that widely used in transmission systems appear less effective in distribution networks thus active power compensation is desired. Soft-open points (SOPs) are power-electronic devices used replacing the normally-open points which can control active power transfer between two feeders and/or provide reactive power compensation. The back-to-back voltage source converter (B2B-VSC) is preferred as the SOP because of its capability of restricting fault current despite that it has higher power loss and associated capital cost. Two types of controller are developed for the B2B-VSC-based SOP: one is based on the PI control theory and the other is based on the concept of synchronverters. For the former type, the controller design is introduced comprehensively including system modelling and parameters selection. The precise selection of the damping ratio for nonstandard second-order system is derived, and a technique of resetting integrator in output voltage controller loop to achieve fast and smooth islanding transition is proposed. For the latter type, modifications are made to adapt the synchronverter idea to the application of an SOP. Simulations and experiments are carried out to validate the controller designs and both the controllers are verified to be able to provide sufficient performance on voltage regulation, fault current restriction and independent load supply in island mode. In general, the controller based on PI control theory has better performance in fault condition thanks to the current control loop, and the controller based on synchronverter owns better reliability because it does not require additional detections and signal switches inside the controller. At last, the use of an SOP in a dynamic load dominated network after the loss of mains is further investigated. Torque-speed characteristic is used to analyse the influence of the VSC’s filter impedance on the stability margin of an induction motor. Though the filter impedance can significantly decrease the stability margin, the output impedance of the VSC can be mitigated by properly designing the output voltage controller. Simulation and experiment are carried out to validate the analyses and controller design. The results show that the VSC is capable of supplying an induction motor in island mode

Industrial and Technological Applications of Power Electronics Systems

The Special Issue "Industrial and Technological Applications of Power Electronics Systems" focuses on: - new strategies of control for electric machines, including sensorless control and fault diagnosis; - existing and emerging industrial applications of GaN and SiC-based converters; - modern methods for electromagnetic compatibility. The book covers topics such as control systems, fault diagnosis, converters, inverters, and electromagnetic interference in power electronics systems. The Special Issue includes 19 scientific papers by industry experts and worldwide professors in the area of electrical engineering

Symmetry in Renewable Energy and Power Systems

This book includes original research papers related to renewable energy and power systems in which theoretical or practical issues of symmetry are considered. The book includes contributions on voltage stability analysis in DC networks, optimal dispatch of islanded microgrid systems, reactive power compensation, direct power compensation, optimal location and sizing of photovoltaic sources in DC networks, layout of parabolic trough solar collectors, topologic analysis of high-voltage transmission grids, geometric algebra and power systems, filter design for harmonic current compensation. The contributions included in this book describe the state of the art in this field and shed light on the possibilities that the study of symmetry has in power grids and renewable energy systems

Improving power transfer capability of SCIG based fixed speed WECS

Wind power generation is regarded as the most promising source of all renewable energy system. In this thesis, a concept to advance the knowledge in the possibility to improve the power transfer of the fixed speed Wind Energy Conversion System based on a Squirrel Cage Induction Generator is presented. The proposed system endeavour to increase power extracted from the wind energy system through the utilization of variable capacitors and tap changing transformer in tandem

Advances in Theoretical and Computational Energy Optimization Processes

The paradigm in the design of all human activity that requires energy for its development must change from the past. We must change the processes of product manufacturing and functional services. This is necessary in order to mitigate the ecological footprint of man on the Earth, which cannot be considered as a resource with infinite capacities. To do this, every single process must be analyzed and modified, with the aim of decarbonising each production sector. This collection of articles has been assembled to provide ideas and new broad-spectrum contributions for these purposes

Measurements, Models, Systems and Design

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Embedded Sensors and Controls to Improve Component Performance and Reliability Conceptual Design Report

The objective of this project is to demonstrate improved reliability and increased performance made possible by deeply embedding instrumentation and controls (I&C) in nuclear power plant (NPP) components and systems. The project is employing a highly instrumented canned rotor, magnetic bearing, fluoride salt pump as its I&C technology demonstration platform. I&C is intimately part of the basic millisecond-by-millisecond functioning of the system; treating I&C as an integral part of the system design is innovative and will allow significant improvement in capabilities and performance. As systems become more complex and greater performance is required, traditional I&C design techniques become inadequate and more advanced I&C needs to be applied. New I&C techniques enable optimal and reliable performance and tolerance of noise and uncertainties in the system rather than merely monitoring quasistable performance. Traditionally, I&C has been incorporated in NPP components after the design is nearly complete; adequate performance was obtained through over-design. By incorporating I&C at the beginning of the design phase, the control system can provide superior performance and reliability and enable designs that are otherwise impossible. This report describes the progress and status of the project and provides a conceptual design overview for the platform to demonstrate the performance and reliability improvements enabled by advanced embedded I&C

Protection of Future Electricity Systems

The electrical energy industry is undergoing dramatic changes: massive deployment of renewables, increasing share of DC networks at transmission and distribution levels, and at the same time, a continuing reduction in conventional synchronous generation, all contribute to a situation where a variety of technical and economic challenges emerge. As the society’s reliance on electrical power continues to increase as a result of international decarbonisation commitments, the need for secure and uninterrupted delivery of electrical energy to all customers has never been greater. Power system protection plays an important enabling role in future decarbonized energy systems. This book includes ten papers covering a wide range of topics related to protection system problems and solutions, such as adaptive protection, protection of HVDC and LVDC systems, unconventional or enhanced protection methods, protection of superconducting transmission cables, and high voltage lightning protection. This volume has been edited by Adam Dyśko, Senior Lecturer at the University of Strathclyde, UK, and Dimitrios Tzelepis, Research Fellow at the University of Strathclyde

Mitigation of DC Current Injection in Transformerless Grid-Connected Inverters

PhD ThesisWith a large number of small-scale PV plants being connected to the utility grid, there is increasing interest in the use of transformerless systems for grid-connected inverter photovoltaic applications. Compared to transformer-coupled solutions, transformerless systems offer a typical efficiency increase of 1-2%, reduced system size and weight, and reductions in cost. However, the removal of the transformer has technical implications. In addition to the loss of galvanic isolation, DC current injection into the grid is a potential risk. Whilst desirable, the complete mitigation of DC current injection via conventional current control methods is known to be particularly challenging, and there are remaining implementation issues in previous studies. For this reason, this thesis aims to minimize DC current injection in grid-connected transformerless PV inverter systems. The first part of the thesis reviews the technical challenges and implementation issues in published DC measurement techniques and suppression methods. Given mathematical models, the performance of conventional current controllers in terms of DC and harmonics mitigation is analyzed and further confirmed in simulations and experiments under different operating conditions. As a result, the second part of the thesis introduces two DC suppression methods, a DC voltage mitigation approach and a DC link current sensing technique. The former method uses a combination of a passive attenuation circuit and a software filter stage to extract the DC voltage component, which allows for further digital control and DC component mitigation at the inverter output. It is proven to be a simple and highly effective solution, applicable for any grid-connected PV inverter systems. The DC link sensing study then investigates a control-based solution in which the dc injection is firstly accurately determined via extraction of the line frequency component from the DC link current and then mitigated with a closed loop. With an output current reconstruction process, this technique provides robust current control and effective DC suppression based on DC link current measurement, eliminating the need for the conventional output current sensor. Results from rated simulation models and a laboratory grid-connected inverter system are presented to demonstrate the accurate and robust performance of the proposed techniques. This thesis makes a positive contribution in the area of power quality control in grid-connected inverters, specifically mitigating the impact of DC injection into the grid which has influences on the network operating conditions and the design and manufacture of the PV power converter itsel