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

Development of a scaled doubly-fed induction generator for assessment of wind power integration issues

Years of experience have been dedicated to the advancement of thermal power plant technology, and in the last decade the investigation has focused on the wind energy conversion system (WECS). Wind energy will play an important role in the future of the energy market, due to the changing climate and the fossil fuel crisis. Initially, wind energy was intended to cover a small portion of the energy market, but in the long term it should compete with conventional fossil fuel power generation. The movement of the power system towards this new phenomena has to be investigated before the wind energy share increases in the network. Therefore, the wind energy integration issues serve as an interesting topic for authors to improve the perception of integration, distribution, variability and power flow issues. Several simulation models have been introduced in order to resolve this issue, however, the variety in types of wind turbines and the network policies result in these models having limited accuracy or being developed for specific issues. The micro-machine is introduced in order to overcome the challenges of simulation models and the costs involved in field tests. In the past, the grid integration issue of large turbo-alternators was solved by the micro-machines. A variety of tests are possible with the micro-machines and they also increase the flexibility of the system. The increased accuracy as well as the ability to carry out real-time analysis and compare actual field test data are strengths worth utilizing. This project involves the designing and the prototyping of a scaled doubly-fed induction generator (micro-DFIG). The machine is also analysed and tested. The scaling of the micro-machine is achieved by means of a dimensional analysis, which is a mathematical method that allows machines and systems to be downscaled by establishing laws of similitude between the reference model and its scaled model. MATLAB/SIMULINK, Maxwell and Solid Work are employed to achieve the objectives of this project

Life cycle energy cost assessment for pump units with various types of line-start operating motors including cable losses

The paper presents a comparative analysis of life-cycle energy consumption for three different types of 4 kW line-start motors used in a pump unit with throttling: the most widely used induction motor with IE3 efficiency class, line start permanent magnet synchronous motor with IE4 efficiency class and line start synchronous reluctance motor with IE4 efficiency class. The operating cycle for pump units with constant flow is considered for the above-mentioned types of motors taking into account not only the losses in the pump and motor, but also in the power supply cable. It is shown that the line start synchronous reluctance motor without magnets has the highest efficiency over the entire considered loading range. However, its power factor is lower than that of the synchronous motor with magnets and therefore it has more significant losses in power supply cable. Despite this disadvantage, the line-start reluctance motor is a good alternative to widespread induction motor since it allows saving of approximately 4000 euro more than the latter during the 20 years life cycle. It also provides similar savings in comparison to the permanent magnet synchronous motor, but unlike it, it does not have costly rare-earth materials in the rotor. © 2020 by the authors

Stability studies of Sasol Synfuels Transmission and Distribution network under fault conditions and N-1 supply contingency

Includes abstract.Includes bibliographical references (leaves 129-130).The aim of the work is to perform a transient stability analysis of the Sasol Synfuels Transmission and Distribution network when the power system is subjected to fault conditions and N-1 supply contingency conditions. The work provides an overview of the problem of power system stability as well as discussing issues related to the problem of power system stability; which include power system control and power system modelling

Performance analysis of a linear motor with HTS bulk magnets for driving a prototype HTS maglev vehicle

This paper presents the performance analysis of a linear synchronous motor which employs high-temperature superconducting (HTS) bulk magnets on the mover and normal copper windings on the stator. The linear motor is designed to drive a prototype HTS maglev vehicle in which the mover is suspended by the levitation force between HTS bulks on the mover and permanent magnets on the ground. Finite element magnetic field analysis is conducted to calculate the major parameters of the linear motor and an equation is derived to calculate the electromagnetic thrust force. Theoretical calculations are verified by the measured results on the prototype. © (2013) Trans Tech Publications, Switzerland

Voltage and frequency regulation to support synchronous generators operation by grid feeding distributed energy resources

Research on new grid topologies and control configurations to support distributed energy resources is being carried out in order to improve electric service reliability and better power quality to the end consumer. Besides, due to more restrictive environmental policies and economical incentives for the deployment of new renewable energy resources, the energetic scenario seems to be moving towards a more sustainable one. With the increasing proliferation of renewable energies and distributed energy resources, however, the challenges that future grids will have to confront can only escalate. Before dealing with these new challenges, it is first necessary to fully comprehend how a standard grid is regulated and to embrace the fundamentals on grid operation and management from a technical perspective. By understanding how current grids function, the effect of these new actors on the grid namely distributed energy resources can be isolated and addressed either individually as a new phenomenon never encountered before or extrapolated from a well-known challenge of the mains. In this thesis, the operating of the standard grid is depicted together with these forthcoming technologies such as microgrids and distributed energy resources. The synchronous generator together with its regulator and its excitation system prove to be key actors in terms of frequency and voltage regulation thus special emphasis is given to them. Simulations regarding the control of the synchronous generator and its influence on the grid stability are performed to support the many literature that attribute the synchronous generator as the par excellence regulator of the grid. Finally, the interaction between an inverter-based distributed generation and a diesel-based distributed generation is studied to identify its effects on both the dynamic response of the grid and its stability. The realized simulations provide scenarios in which to test the importance of the synchronous generation inasmuch as the regulation of the grid is concerned. In addition, the introduction of an inverter-based distributed generation in the simulations is particularly interesting to present the benefits that the support from distributed generation on the grid can bring about

Stability of distribution networks connected with distributed generation

Bibliography: pages 164-167.This thesis describes an investigation into the stability of distribution networks that are connected with distributed generators. Due to the restructuring of the electricity industry in the region as well as environmental concerns, distributed generation is bound to increase at a higher rate in the Southern African region in the near future. Southern Africa, like many other developing regions, is dominated by electrically weak distribution networks that have relatively high impedance lines. These networks suffer extreme voltage fluctuations when a transient disturbance occurs on the network. The distributed generators are connected onto distribution networks that were designed to operate without any generation, but were designed to receive power from the transmission networks. Once distributed generators are connected to distribution networks, a number of technical challenges are presented. One of the technical challenges includes investigating the stability of distribution networks connected with distributed generation. It would be beneficial to know what effect the connection of distributed generators onto distribution networks would have on the system stability. This is because if the connection of distributed generators onto distribution networks increases instability on the network, the quality of supply of that network would be degraded, therefore the connection of distributed generators must be limited or methods of improving the stability must be implemented. It is important to establish the measures that can be taken to make sure that the generators react in a stable manner when subjected to disturbances and to make sure that the local system stability is not compromised. The first objective of this thesis was to identify the types of generators that are likely to be connected to Southern African distribution networks and investigate their stability. The next objective was to design model distribution networks that would be utilised to highlight key stability issues that are raised when distributed generation is connected to distribution networks. The third objective was to conduct and analyse stability studies on model as well as existing Southern African distribution networks connected with distributed generation, including the assessment of the implications of potential instability such as on the quality of supply. The last objective was to identify various ways of improving the stability of distribution networks that are connected with distributed generation