Fault behaviour and fault detection in islanded inverter-only microgrids

The increase in popularity of the microgrid concept requires the analysis and solution of the numerous technical issues arising from the operation and integration of the microgrid into the original distribution network. The work presented in this thesis is centred on the study of the fault behaviour of inverter-only microgrids and on the development of a suitable fault detection technique. This task is approached by first understanding the behaviour of a microgrid during a fault and the factors affecting it. A complete description and analysis of the key elements in the study of microgrid fault behaviour is presented. Then, three microgrid models with different inverter control methods (i.e. Synchronous Reference Frame control, Natural Reference Frame control and droop control) and with various current limiting strategies are built in PSCAD and their fault behaviour is simulated, analyzed and compared. It is found that the control of the inverter is able to shape the response of the microgrid in the event of a fault. The constraints to this capability are the inverter’s ratings (current and voltage limits) and the characteristic changes in the network introduced by faults. Moreover, it is found that the control in the Natural Reference Frame gives better fault response, in terms of voltage control and simplicity in implementation, compared with the popular control in the Synchronous Reference Frame. The behaviour of the system is then further analyzed by developing quasi steadystate inverter models suitable for numerical fault analysis. The models are developed starting from the inverter control and analyzing how it changes in the event of a fault. By combining control gains and circuit parameters, they result in being capable of capturing the key features of inverters’ fault behaviour. Depending on the control strategy, some of these models are balanced and therefore are directly applicable in numerical fault analysis based on sequence components. Others are unbalanced and therefore require a fault analysis based on a direct phase coordinates representation of the network. Examples on how to perform numerical fault analysis calculations with balanced and unbalanced models are given and the numerical results well compare with the ones obtained from time-domain simulations using PSCAD. From the knowledge of the microgrid fault behaviour developed analyzing the responses in time-domain simulations and by using the developed inverter models to numerically calculate voltages and currents in the microgrid during different faults at various locations, a fault detection strategy based on voltage sequence components is proposed. Indeed, it is the behaviour of the inverter control during faults which makes the monitoring of voltage sequence components the best discriminator between normal operation and fault operation. The three building blocks of the fault detection strategy which are capable of a fast extraction and comparison of voltage sequence components are described and then the performance of the fault detection strategy for different faults and microgrid operating conditions is tested in PSCAD and discussed. Finally, examples are given on how this voltage detection can be used in the design of a microgrid protection system

Fault behaviour and fault detetion in islanded inverter-only microgrids

Imperial Users onl

Decision tree aided planning and energy balancing of planned community microgrids

Planned Communities (PCs) present a unique opportunity for deployment of intelligent control of demand-side distributed energy resources (DER) and storage, which may be organized in Microgrids (MGs). MGs require balancing for maintaining safe and resilient operation. This paper discusses the implications of using MG concepts for planning and control of energy systems within PCs. A novel tool is presented, based on decision trees (DTs), with two potential applications: (i) planning of energy storage systems within such MGs and (ii) controlling energy resources for energy balancing within a PC MG. The energy storage planning and energy balancing methodology is validated through sensitivity case studies, demonstrating its effectiveness. A test implementation is presented, utilizing distributed controller hardware to execute the energy balancing algorithm in real-time

Improving the feasibility of household and community energy storage : a techno-enviro-economic study for the UK

Rooftop photovoltaics (PV) have become widely adopted by domestic customers in tandem with energy storage systems to generate clean energy and limit import from the grid, however most applications struggle to achieve profitability. The level at which energy storage is deployed, be it household energy storage (HES), or as a community energy storage (CES) system, can potentially increase the economic feasibility. Furthermore, the introduction of a Time-of-Use (TOU) tariff enables households to further reduce their energy costs through demand side management (DSM). Here we investigate and compare the performance of HES and CES with DSM. The results suggest that TOU tariffs can effectively shave peak demand by up to 30% and lower energy bills by at least 20%, but do not improve self-consumption or selfsufficiency rate. This study indicates that all cases considered are environmentally friendly and can pay back the total CO2 emissions associated with the manufacturing within 8 years. However, the levelised cost of storage (LCOS) is still beyond a household’s affordability, ranging from £0.4 to £2.03 kWh-1, though CES is proven more effective at improving self-consumption for consumers and shaving peak demand for network operators. The feasibility can be improved by 1) combining different services and tariffs to obtain more revenues for households; 2) more legislative and financial support to reduce system costs; and 3) more innovative business models and policies to optimise revenues with existing resourc

Fatality rate and predictors of mortality in an Italian cohort of hospitalized COVID-19 patients

Clinical features and natural history of coronavirus disease 2019 (COVID-19) differ widely among different countries and during different phases of the pandemia. Here, we aimed to evaluate the case fatality rate (CFR) and to identify predictors of mortality in a cohort of COVID-19 patients admitted to three hospitals of Northern Italy between March 1 and April 28, 2020. All these patients had a confirmed diagnosis of SARS-CoV-2 infection by molecular methods. During the study period 504/1697 patients died; thus, overall CFR was 29.7%. We looked for predictors of mortality in a subgroup of 486 patients (239 males, 59%; median age 71 years) for whom sufficient clinical data were available at data cut-off. Among the demographic and clinical variables considered, age, a diagnosis of cancer, obesity and current smoking independently predicted mortality. When laboratory data were added to the model in a further subgroup of patients, age, the diagnosis of cancer, and the baseline PaO2/FiO2 ratio were identified as independent predictors of mortality. In conclusion, the CFR of hospitalized patients in Northern Italy during the ascending phase of the COVID-19 pandemic approached 30%. The identification of mortality predictors might contribute to better stratification of individual patient risk

On the utilisation of the pseudo-capacitive capabilities of Li-ion cells for the provision of frequency response services

This work investigates the capacitive capabilities of Li-ion pouch and cylindrical cells in respect to the provision of Frequency Response services and a potential for reduction in battery ageing effects. This is achieved using Electrochemical Impedance Spectroscopy (EIS) and a novel method of identifying and defining the threshold frequency between pseudo-capacitive and diffusion processes of the cell. It is found that this threshold frequency is independent of current intensity up to 1 C, showing that even at high power, pseudo-capacitance has significant impact. However, a severe dependency upon relative cell surface area and State of Charge (SoC) is identified. Symmetrical charge-discharge pulses of up to 10 s utilise primarily cell capacitance. Literature indicates, that this level of utilisation reduces the electrochemical ageing impact significantly. This article displays a method to identify and isolate these processes for any given cell and to allow enhancement of conventional ageing modelling

Structural Characterization and Antimicrobial Evaluation ofAtractyloside, Atractyligenin, and 15-DidehydroatractyligeninMethyl Ester

e report thefirst complete structure elucida-tion of theent-kaurane diterpenoid glycoside atractyloside (1)by means of NMR and X-ray diffractometry techniques.Extensive one- and two-dimensional NMR experiments wereemployed to assign the proton and carbon signals of1, andcrystallography experiments established the configurations ofall stereogenic centers. Furthermore, we present a novelsemisynthetic route for the preparation of the highly cytotoxicaglycone derivative of1, 15-didehydroatractyligenin methylester (3). All compounds were tested for their antibioticactivity againstEnterococcus faecalis,Escherichia coli, and several strains ofStaphylococcus aureus, includingfluoroquinolone-resistant (SA1199B) and two epidemic MRSA (EMRSA-15 and -16) strains. Compound3exhibited moderate activity against allof theStaph. aureusstrains with an MIC value of 128 mg/

Balancing urban micro-grids in future planned communities

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Planned communities as microgrid applications

The microgrid (MG), as an operational paradigm for the exploitation of distributed energy resources (DER), has been thoroughly studied the past few years. However, applications are largely limited to test sites and demo installations, lacking the business argument in favor of the wider deployment of the MG concept. Planned communities (PC), a residential development concept aiming to channel the population growth away from the overcrowded and polluted city centers, can serve as a business model for the application of the MG control architectures and philosophies. This paper intends to link the core characteristics of the PC to existing MG sites and installations, providing evidence of their suitability for MG development