Fault analysis and protection for wind power generation systems

Wind power is growing rapidly around the world as a means of dealing with the world energy shortage and associated environmental problems. Ambitious plans concerning renewable energy applications around European countries require a reliable yet economic system to generate, collect and transmit electrical power from renewable resources. In populous Europe, collective offshore large-scale wind farms are efficient and have the potential to reach this sustainable goal. This means that an even more reliable collection and transmission system is sought. However, this relatively new area of offshore wind power generation lacks systematic fault transient analysis and operational experience to enhance further development. At the same time, appropriate fault protection schemes are required. This thesis focuses on the analysis of fault conditions and investigates effective fault ride-through and protection schemes in the electrical systems of wind farms, for both small-scale land and large-scale offshore systems. Two variable-speed generation systems are considered: doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) because of their popularity nowadays for wind turbines scaling to several-MW systems. The main content of the thesis is as follows. The protection issues of DFIGs are discussed, with a novel protection scheme proposed. Then the analysis of protection scheme options for the fully rated converter, direct-driven PMSGs are examined and performed with simulation comparisons. Further, the protection schemes for wind farm collection and transmission systems are studied in terms of voltage level, collection level wind farm collection grids and high-voltage transmission systems for multi-terminal DC connected transmission systems, the so-called “Supergrid”. Throughout the thesis, theoretical analyses of fault transient performances are detailed with PSCAD/EMTDC simulation results for verification. Finally, the economic aspect for possible redundant design of wind farm electrical systems is investigated based on operational and economic statistics from an example wind farm project

Intelligent voltage dip mitigation in power networks with distributed generation

Includes bibliographical references.The need for ensuring good power quality (PQ) cannot be over-emphasized in electrical power system operation and management. PQ problem is associated with any electrical distribution and utilization system that experiences any voltage, current or frequency deviation from normal operation. In the current power and energy scenario, voltage-related PQ disturbances like voltage dips are a fact which cannot be eliminated from electrical power systems since electrical faults, and disturbances are stochastic in nature. Voltage dip tends to lead to malfunction or shut down of costly and mandatory equipment and appliances in consumers’ systems causing significant financial losses for domestic, commercial and industrial consumers. It accounts for the disruption of both the performance and operation of sensitive electrical and electronic equipment, which reduces the efficiency and the productivity of power utilities and consumers across the globe. Voltage dips are usually experienced as a result of short duration reduction in the r.m.s. (r.m.s.- root mean square) value of the declared or nominal voltage at the power frequency and is usually followed by recovery of the voltage dip after few seconds. The IEEE recommended practice for monitoring electric power quality (IEEE Std. 1159-2009, revised version of June 2009), provides definitions to label an r.m.s. voltage disturbance based upon its duration and voltage magnitude. These disturbances can be classified into transient events such as voltage dips, swells and spikes. Other long duration r.m.s. voltage variations are mains failures, interruption, harmonic voltage distortion and steady-state overvoltages and undervoltages. This PhD research work deals with voltage dip phenomena only. Initially, the present power network was not designed to accommodate renewable distributed generation (RDG) units. The advent and deployment of RDG over recent years and high penetration of RDG has made the power network more complex and vulnerable to PQ disturbances. It is a well-known fact that the degree of newly introduced RDG has increased rapidly and growing further because of several reasons, which include the need to reduce environmental pollution and global warming caused by emission of carbon particles and greenhouse gases, alleviating transmission congestion and loss reduction. RDG ancillary services support especially voltage and reactive power support in electricity networks are currently being recognized, researched and found to be quite useful in voltage dip mitigation

Distributed photovoltaic systems: Utility interface issues and their present status. Intermediate/three-phase systems

The interface issues between the intermediate-size Power Conditioning Subsystem (PCS) and the utility are considered. A literature review yielded facts about the status of identified issues

Investigation of the cumulative impact of alkaline electrolysers on electrical power systems

Hydrogen could be the best candidate fuel for our future, especially in the transportation sector. It could be generated using water electrolysers running with power from carbon-free, renewable resources, since this is zero emission at the point of use, and so can help transition from the energy infrastructure available today into an energy world with a growing renewable electricity supply.This work models a highly distributed electrolyser system e.g. an urban hydrogen filling station network, and explores the Demand Side Management (DSM) potential of these electrolysers to improve the performance of the power system operating under the impact of intermittent renewable power generation.A comprehensive literature review has been carried out on the hydrogen economy, electrolysers and the potential role of storage devices in power systems. Three main areas related to alkaline electrolysers working within power systems were identified for further exploration. - Potential role of electrolysers in the existing distribution networks to increase the integrated wind power capacity - Potential role of electrolysers to stabilise the frequency of the power system - Potential role of electrolysers to absorb any surplus, carbon free, generation within the UK electricity networkThe first item of archival value within this work is the identification, presentation and discussion of electrolyser characteristics which are relevant to the introduction of an acceptable control strategy to integrate such electrolyser loads within the power system and thus provide improved performance of the network when exposed to the highly time variable energy supply from renewable sources. Two types of electrolyser made by NEL Hydrogen are detailed: atmospheric and pressurised. Their characteristics are reported in this thesis using the results from experiments designed by the author. In addition, an experiment has also been carried out on a PEM electrolyser available at Strathclyde University to compare its results with the characteristics of the commercial alkaline units. Second, a novel algorithm for sizing, placing and control of electrolysis based hydrogen filling stations operating within radial distribution networks has been proposed and its performance is assessed using a United Kingdom Generic Distribution System (UKGDS) case study. The controller objective is to dispatch alkaline electrolysers appropriately to increase the amount of integrated wind power capacity and reduce the grid losses within the network while satisfying the network constraints and respecting the electrolyser characteristics.In addition, a MATLAB Simulink model has been developed to investigate the impact of alkaline electrolysers as dynamically controlled loads for the stabilisation of system frequency in the case of a sudden loss of generation and also when the power system has high penetrations of wind power. The electrolysers are controlled according to a droop control strategy. A novel approach to determine the aggregate nominal electrolysis demand for frequency stability purposes has also been proposed in this work, and the financial viability of the proposed strategy to control electrolysers has been assessed.Finally, several scenarios have been modelled to investigate the role of electrolysers to absorb surplus power and produce hydrogen for the fuel cell vehicles in the UK in the year 2050. Different wind, solar and nuclear power generation capacities have been considered. On the demand side, different penetration levels of electric vehicles and hydrogen fuel cell cars have been modelled. The results are discussed and analysed.Keywords: Alkaline electrolysers, Renewable power, Active Network Management, Distribution network, Power system stability, Hydrogen economy, Power system losses, Demand side management, Load Frequency Control, Energy storage.Hydrogen could be the best candidate fuel for our future, especially in the transportation sector. It could be generated using water electrolysers running with power from carbon-free, renewable resources, since this is zero emission at the point of use, and so can help transition from the energy infrastructure available today into an energy world with a growing renewable electricity supply.This work models a highly distributed electrolyser system e.g. an urban hydrogen filling station network, and explores the Demand Side Management (DSM) potential of these electrolysers to improve the performance of the power system operating under the impact of intermittent renewable power generation.A comprehensive literature review has been carried out on the hydrogen economy, electrolysers and the potential role of storage devices in power systems. Three main areas related to alkaline electrolysers working within power systems were identified for further exploration. - Potential role of electrolysers in the existing distribution networks to increase the integrated wind power capacity - Potential role of electrolysers to stabilise the frequency of the power system - Potential role of electrolysers to absorb any surplus, carbon free, generation within the UK electricity networkThe first item of archival value within this work is the identification, presentation and discussion of electrolyser characteristics which are relevant to the introduction of an acceptable control strategy to integrate such electrolyser loads within the power system and thus provide improved performance of the network when exposed to the highly time variable energy supply from renewable sources. Two types of electrolyser made by NEL Hydrogen are detailed: atmospheric and pressurised. Their characteristics are reported in this thesis using the results from experiments designed by the author. In addition, an experiment has also been carried out on a PEM electrolyser available at Strathclyde University to compare its results with the characteristics of the commercial alkaline units. Second, a novel algorithm for sizing, placing and control of electrolysis based hydrogen filling stations operating within radial distribution networks has been proposed and its performance is assessed using a United Kingdom Generic Distribution System (UKGDS) case study. The controller objective is to dispatch alkaline electrolysers appropriately to increase the amount of integrated wind power capacity and reduce the grid losses within the network while satisfying the network constraints and respecting the electrolyser characteristics.In addition, a MATLAB Simulink model has been developed to investigate the impact of alkaline electrolysers as dynamically controlled loads for the stabilisation of system frequency in the case of a sudden loss of generation and also when the power system has high penetrations of wind power. The electrolysers are controlled according to a droop control strategy. A novel approach to determine the aggregate nominal electrolysis demand for frequency stability purposes has also been proposed in this work, and the financial viability of the proposed strategy to control electrolysers has been assessed.Finally, several scenarios have been modelled to investigate the role of electrolysers to absorb surplus power and produce hydrogen for the fuel cell vehicles in the UK in the year 2050. Different wind, solar and nuclear power generation capacities have been considered. On the demand side, different penetration levels of electric vehicles and hydrogen fuel cell cars have been modelled. The results are discussed and analysed.Keywords: Alkaline electrolysers, Renewable power, Active Network Management, Distribution network, Power system stability, Hydrogen economy, Power system losses, Demand side management, Load Frequency Control, Energy storage

Distribution System Voltage Management and Optimization for Integration of Renewables and Electric Vehicles: Research Gap Analysis

California is striving to achieve 33% renewable penetration by 2020 in accordance with the state’s Renewable Portfolio Standard (RPS). The behavior of renewable resources and electric vehicles in distribution systems is creating constraints on the penetration of these resources into the distribution system. One such constraint is the ability of present-­‐‑day voltage management methodologies to maintain proper distribution system voltage profiles in the face of higher penetrations of PV and electric vehicle technologies. This white paper describes the research gaps that have been identified in current Volt/VAR Optimization and Control (VVOC) technologies, the emerging technologies which are becoming available for use in VVOC, and the research gaps which exist and must be overcome in order to realize the full promise of these emerging technologies

Optimisation of stand-alone hydrogen-based renewable energy systems using intelligent techniques

Wind and solar irradiance are promising renewable alternatives to fossil fuels due to their availability and topological advantages for local power generation. However, their intermittent and unpredictable nature limits their integration into energy markets. Fortunately, these disadvantages can be partially overcome by using them in combination with energy storage and back-up units. However, the increased complexity of such systems relative to single energy systems makes an optimal sizing method and appropriate Power Management Strategy (PMS) research priorities. This thesis contributes to the design and integration of stand-alone hybrid renewable energy systems by proposing methodologies to optimise the sizing and operation of hydrogen-based systems. These include using intelligent techniques such as Genetic Algorithm (GA), Particle Swarm Optimisation (PSO) and Neural Networks (NNs). Three design aspects: component sizing, renewables forecasting, and operation coordination, have been investigated. The thesis includes a series of four journal articles. The first article introduced a multi-objective sizing methodology to optimise standalone, hydrogen-based systems using GA. The sizing method was developed to calculate the optimum capacities of system components that underpin appropriate compromise between investment, renewables penetration and environmental footprint. The system reliability was assessed using the Loss of Power Supply Probability (LPSP) for which a novel modification was introduced to account for load losses during transient start-up times for the back-ups. The second article investigated the factors that may influence the accuracy of NNs when applied to forecasting short-term renewable energy. That study involved two NNs: Feedforward, and Radial Basis Function in an investigation of the effect of the type, span and resolution of training data, and the length of training pattern, on shortterm wind speed prediction accuracy. The impact of forecasting error on estimating the available wind power was also evaluated for a commercially available wind turbine. The third article experimentally validated the concept of a NN-based (predictive) PMS. A lab-scale (stand-alone) hybrid energy system, which consisted of: an emulated renewable power source, battery bank, and hydrogen fuel cell coupled with metal hydride storage, satisfied the dynamic load demand. The overall power flow of the constructed system was controlled by a NN-based PMS which was implemented using MATLAB and LabVIEW software. The effects of several control parameters, which are either hardware dependent or affect the predictive algorithm, on system performance was investigated under the predictive PMS, this was benchmarked against a rulebased (non-intelligent) strategy. The fourth article investigated the potential impact of NN-based PMS on the economic and operational characteristics of such hybrid systems. That study benchmarked a rule-based PMS to its (predictive) counterpart. In addition, the effect of real-time fuel cell optimisation using PSO, when applied in the context of predictive PMS was also investigated. The comparative analysis was based on deriving the cost of energy, life cycle emissions, renewables penetration, and duty cycles of fuel cell and electrolyser units. The effects of other parameters such the LPSP level, prediction accuracy were also investigated. The developed techniques outperformed traditional approaches by drawing upon complex artificial intelligence models. The research could underpin cost-effective, reliable power supplies to remote communities as well as reducing the dependence on fossil fuels and the associated environmental footprint

An Assessment of PIER Electric Grid Research 2003-2014 White Paper

This white paper describes the circumstances in California around the turn of the 21st century that led the California Energy Commission (CEC) to direct additional Public Interest Energy Research funds to address critical electric grid issues, especially those arising from integrating high penetrations of variable renewable generation with the electric grid. It contains an assessment of the beneficial science and technology advances of the resultant portfolio of electric grid research projects administered under the direction of the CEC by a competitively selected contractor, the University of California’s California Institute for Energy and the Environment, from 2003-2014

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

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

Resilience-driven planning and operation of networked microgrids featuring decentralisation and flexibility

High-impact and low-probability extreme events including both man-made events and natural weather events can cause severe damage to power systems. These events are typically rare but featured in long duration and large scale. Many research efforts have been conducted on the resilience enhancement of modern power systems. In recent years, microgrids (MGs) with distributed energy resources (DERs) including both conventional generation resources and renewable energy sources provide a viable solution for the resilience enhancement of such multi-energy systems during extreme events. More specifically, several islanded MGs after extreme events can be connected with each other as a cluster, which has the advantage of significantly reducing load shedding through energy sharing among them. On the other hand, mobile power sources (MPSs) such as mobile energy storage systems (MESSs), electric vehicles (EVs), and mobile emergency generators (MEGs) have been gradually deployed in current energy systems for resilience enhancement due to their significant advantages on mobility and flexibility. Given such a context, a literature review on resilience-driven planning and operation problems featuring MGs is presented in detail, while research limitations are summarised briefly. Then, this thesis investigates how to develop appropriate planning and operation models for the resilience enhancement of networked MGs via different types of DERs (e.g., MGs, ESSs, EVs, MESSs, etc.). This research is conducted in the following application scenarios: 1. This thesis proposes novel operation strategies for hybrid AC/DC MGs and networked MGs towards resilience enhancement. Three modelling approaches including centralised control, hierarchical control, and distributed control have been applied to formulate the proposed operation problems. A detailed non-linear AC OPF algorithm is employed to model each MG capturing all the network and technical constraints relating to stability properties (e.g., voltage limits, active and reactive power flow limits, and power losses), while uncertainties associated with renewable energy sources and load profiles are incorporated into the proposed models via stochastic programming. Impacts of limited generation resources, load distinction intro critical and non-critical, and severe contingencies (e.g., multiple line outages) are appropriately captured to mimic a realistic scenario. 2. This thesis introduces MPSs (e.g., EVs and MESSs) into the suggested networked MGs against the severe contingencies caused by extreme events. Specifically, time-coupled routing and scheduling characteristics of MPSs inside each MG are modelled to reduce load shedding when large damage is caused to each MG during extreme events. Both transportation networks and power networks are considered in the proposed models, while transporting time of MPSs between different transportation nodes is also appropriately captured. 3. This thesis focuses on developing realistic planning models for the optimal sizing problem of networked MGs capturing a trade-off between resilience and cost, while both internal uncertainties and external contingencies are considered in the suggested three-level planning model. Additionally, a resilience-driven planning model is developed to solve the coupled optimal sizing and pre-positioning problem of MESSs in the context of decentralised networked MGs. Internal uncertainties are captured in the model via stochastic programming, while external contingencies are included through the three-level structure. 4. This thesis investigates the application of artificial intelligence techniques to power system operations. Specifically, a model-free multi-agent reinforcement learning (MARL) approach is proposed for the coordinated routing and scheduling problem of multiple MESSs towards resilience enhancement. The parameterized double deep Q-network method (P-DDQN) is employed to capture a hybrid policy including both discrete and continuous actions. A coupled power-transportation network featuring a linearised AC OPF algorithm is realised as the environment, while uncertainties associated with renewable energy sources, load profiles, line outages, and traffic volumes are incorporated into the proposed data-driven approach through the learning procedure.Open Acces