Structure of krypton isotopes within the interacting boson model derived from the Gogny energy density functional

The evolution and coexistence of the nuclear shapes as well as the corresponding low-lying collective states and electromagnetic transition rates are investigated along the Krypton isotopic chain within the framework of the interacting boson model (IBM). The IBM Hamiltonian is determined through mean-field calculations based on the several parametrizations of the Gogny energy density functional and the relativistic mean-field Lagrangian. The mean-field energy surfaces, as functions of the axial $\beta$ and triaxial $\gamma$ quadrupole deformations, are mapped onto the expectation value of the interacting-boson Hamiltonian that explicitly includes the particle-hole excitations. The resulting boson Hamiltonian is then used to compute low-energy excitation spectra as well as E2 and E0 transition probabilities for $^{70-100}$Kr. Our results point to a number of examples of the prolate-oblate shape transitions and coexistence both on the neutron-deficient and neutron-rich sides. A reasonable agreement with the available experimental data is obtained for the considered nuclear properties.Comment: 13 pages, 9 figures, 2 table

A power management system for interconnected AC islanded microgrids using back-to-back converter

This is the author accepted manuscript. The final version is available from IET via the DOI in this recordIslanded AC microgrids are mini-power grids formed by assembling distributed generation sources, energy storage systems and loads. They are reliable and can operate at different voltages and frequencies to meet the requirements of the load. Interconnected Islanded Microgrids consist of multiple islanded microgrids interconnected to improve power system availability and stability, control flexibility, resiliency and reserve capacity. Islanded microgrids can be interconnected using back-to-back power converters to decouple connecting frequencies whose active and reactive power control can be done wirelessly and autonomously under a low communication link. This paper proposes a novel structure and control strategy using back-to-back converters and power transformers to interconnect multiple islanded AC microgrids to a global AC bus. Microgrids and global bus can utilise their resources to improve their operation and benefits. The control strategy uses a frequency signalling mechanism to limit the power demand of individual global converters and adjusts its droop coefficients accordingly and in proportion to deviation in frequency. The global droop controllers of the global connecting converters receive information about the status of the frequencies of individual microgrids using a low bandwidth communication link to enhance network power flow. MATLAB/Simulink results validate the performance of the proposed structure and control strategy

Impedance interaction between islanded parallel voltage source inverters and the distribution network

In an islanded microgrid consisting of parallel-connected inverters, the interaction between an inverter’s output impedance (dominated by the inverter’s filter and voltage controller) and the impedance of the distribution network (dominated by the other paralleled inverters’ output impedances and the interconnecting power cables) might lead to instability. This paper studies this phenomenon using root locus analysis. A controller based on the second derivative of the output capacitor voltage is proposed to enhance the stability of the system. Matlab simulation results are presented to confirm the validity of the theoretical analysis and the robustness of the proposed controlle

Hybrid Generators-based AC Microgrid Performance Assessment in Island Mode

Achieving an accurate steady-state averaged active power sharing between parallel inverters in islanded AC microgrids could be realized by a traditional droop control. For identical inverters having the same droop gains, it is assumed that the transient average power responses will be similar, and no circulating current will flow between the units. However, different line impedances could influence the instantaneous power significantly and thus circulating power flows among the inverters particularly during sudden disturbances such as load changes. This power, if absorbed by an inverter, will lead the DC link voltage to rise abruptly and trip the inverter, thus, degrading the performance of the whole microgrid. The problem becomes worse when hybrid generators are serving as unidirectional power source. This paper assesses the performance of hybrid generators within an islanded microgrid against the mismatch in line impedances. Two schemes to stabilize the microgrid are proposed. In addition, a participation factor analysis is developed to select the most effective controller scheme to bound the DC link voltage and minimize the circulating power. Simulation and experimental results are presented to verify the analysis and the capability of the proposed controller

Speed control of synchronous machine by changing duty cycle of DC/DC buck converter

Global Conference on Energy and Sustainable Development, 2015-02-24, 2015-04-26, Coventry, UKRenewable energies such as wind or solar energy are naturally intermittent and can create technical challenges to interconnected grid in particular with high integration amounts. In addition, if wind or solar is used to supply power to a stand-alone system, continuous power supply will be met only if sufficient energy storage system is available. The global penetration of renewable energy in power systems is increasing rapidly especially wind and solar photovoltaic (PV) systems. Hybrid wind and solar PV generation system becomes very attractive solution in particular for stand-alone applications. It can provide better reliability since the weakness of one system could be complemented by the strength of the other one. When wind energy is integrated into grid, maximum power point tracking control could be used to optimize the output of wind turbine. In variable speed wind turbine, the turbine speed is varied according to the wind speed. This paper presents a comparison between two methods of controlling the speed of a wind turbine in a microgrid namely; Proportional-Integral (PI) control of the tip speed ratio and stored power curve. The PI method provides more controllability, but it requires an anemometer to measure the wind speed. The stored power curve method, however, is easier to implement, but the amount of energy extracted can be less. The system has been modelled using Matlab/Simulink.The work is financially supported by the Government of Oman, which provides a PhD grant for Rashid Al Badwawi. Also, financial support from EPSRC-DST funded RESCUES project (EP/K03619X/1)

Line-Interactive UPS for Microgrids

Line interactive Uninterruptable Power Supply (UPS) systems are good candidates for providing energy storage within a microgrid to help improve its reliability, economy and efficiency. In grid-connected mode, power can be imported from the grid by the UPS to charge its battery. Power can also be exported when required, e.g., when the tariffs are advantageous. In stand-alone mode, the UPS supplies local distributed loads in parallel with other sources. In this paper, a line interactive UPS and its control system are presented and discussed. Power flow is controlled using the frequency and voltage drooping technique to ensure seamless transfer between grid-connected and stand-alone parallel modes of operation. The drooping coefficients are chosen to limit the energy imported by the USP when re-connecting to the grid and to give good transient response. Experimental results of a microgrid consisting of two 60kW line interactive UPS systems are provided to validate the design

A novel Kalman filter based technique for calculating the time history of vertical displacement of a boat from measured acceleration

This is the final version of the article. Available from Science and Engineering Publishing Company via the link in this record.Accelerometers are used to measure velocity and displacement in many applications such as ship motion, monitoring of civil and mechanical structure, seismology and machine condition monitoring. However, using direct numerical integration to calculate velocity and displacement from the acceleration signal is known to suffer from low frequency noise amplification and wind-up. In this paper, a Kalman filter based method is proposed for calculating displacement from measured acceleration. Integration wind-up is eliminated by incorporating an additional state variable, namely the integral of the displacement whose "measured" value is assumed to be equal to the known average value of the displacement. In many applications, such as those in marine environment, this average value can be assumed to be constant, usually conveniently assigned to be zero if non-linear behaviour and permanent deformations are deemed negligible. The paper describes the technique and investigates its performance under different conditions of amplitude and frequency of vibrations and sampling rate and validates it by conducting two laboratory experiments. In the first experiment the displacement of a small shaker is calculated from a relatively high frequency (tens of Hz) acceleration signal sampled at 1 kHz with a resolution of 1 g. The calculated displacement of the shaker is found to agree well with that measured using a high resolution laser. In the second experiment, the proposed method is applied to the calculation of the vertical displacement of a boat from a low frequency (less than 1 Hz) acceleration signal sampled at 5 Hz and a resolution of 0.01g. An experimental set up designed to mimic typical motion of a boat is used to validate the results. Although the method explained in this paper is used to calculate the vertical displacement of a boat, it can be applied for calculating the displacement in a wide range of applications with reciprocating movement.The authors wish to thank Mr Mike Russell for his financial support and for collecting boat motion data. They also wish to thank Mr L. Auboin for his help with collecting boat motion data and conducting simulated boat motion lab experiments. Thanks are also due to Dr Jamil Renno for facilitating the high-frequency vibration experiments

Fission barriers in covariant density functional theory: extrapolation to superheavy nuclei

Systematic calculations of fission barriers allowing for triaxial deformation are performed for even-even superheavy nuclei with charge number $Z=112-120$ using three classes of covariant density functional models. The softness of nuclei in the triaxial plane leads to an emergence of several competing fission pathes in the region of the inner fission barrier in some of these nuclei. The outer fission barriers are considerably affected by triaxiality and octupole deformation. General trends of the evolution of the inner and the outer fission barrier heights are discussed as a function of the particle numbers.Comment: 24 pages, 8 tables, 12 figure

Towards real-time reinforcement learning control of a wave energy converter

The levellised cost of energy of wave energy converters (WECs) is not competitive with fossil fuel-powered stations yet. To improve the feasibility of wave energy, it is necessary to develop effective control strategies that maximise energy absorption in mild sea states, whilst limiting motions in high waves. Due to their model-based nature, state-of-the-art control schemes struggle to deal with model uncertainties, adapt to changes in the system dynamics with time, and provide real-time centralised control for large arrays of WECs. Here, an alternative solution is introduced to address these challenges, applying deep reinforcement learning (DRL) to the control of WECs for the first time. A DRL agent is initialised from data collected in multiple sea states under linear model predictive control in a linear simulation environment. The agent outperforms model predictive control for high wave heights and periods, but suffers close to the resonant period of the WEC. The computational cost at deployment time of DRL is also much lower by diverting the computational effort from deployment time to training. This provides confidence in the application of DRL to large arrays of WECs, enabling economies of scale. Additionally, model-free reinforcement learning can autonomously adapt to changes in the system dynamics, enabling fault-tolerant control