Research on microgrids at the University of Malta

This article presents some of the work done in recent years by the microgrids research team at the Department of Industrial Electrical Power Conversion (IEPC). Research activities are dedicated towards enabling secure, reliable, and carbon free electricity systems. To date, the main contributions by the department were made to the operation, control and management of microgrids in low voltage (LV) distribution networks, ship electrification (shipboard microgrids) and low voltage DC microgrids. The paper also presents a few of the significant results achieved by the department to date including a hierarchical control architecture for single phase microgrids, control algorithms of power electronic converters for AC and DC microgrids, energy and power management strategies, power quality improvement algorithms and technologies, demand response strategies and devices, and the effective integration of renewables and energy storage systems.peer-reviewe

Advances in friction stir welding of steel : Project HILDA

A microstructure and property evaluation of friction stir welded DH36 6mm plate has been undertaken. The study examined a wide range of process parameters and, from this, a process parameter envelope has been developed and an initial process parameter set established that gives good welding properties. Thermo-mechanical deformation studies were developed to generate flow stress regimes over a range of stain rates and temperatures and these data will support the on-going local numerical modelling development. A preliminary thermo-fluid model has been developed to predict temperature and material flow during the FSW of steel grade DH36. In this model, materials are considered as highly viscous incompressible fluid. The welded material is flowing around the rotating tool thanks to the modelling of the friction at tool/workpiece interface. In parallel, a global numerical model is being developed to predict the inherent residual stresses and distortion of FSW butt welded assemblies often in excess of 6m long plate

The meaning of the 1971 Paris Conventions on copyright

Many complex problems in the field of International Copyright arise because the matter is governed by two different conventions. While most of the European States continued to adhere to the International Conventions signed in Berne in· the 19th Century, the United Nations had signed soon after World War II another international instrument known as the Universal Copyright Convention. An attempt has now been made in Paris to bring into closer association these two international agreements and setting up of an international centre as a link between publishing houses and the developing countries.peer-reviewe

Single-phase microgrid with seamless transition capabilities between modes of operation

Microgrids are an effective way to increase the penetration of DG into the grid. They are capable of operating either in grid-connected or in islanded mode thereby increasing the supply reliability for the end user. This paper focuses on achieving seamless transitions from islanded to grid-connected and vice versa for a single phase microgrid made up from voltage controlled voltage source inverters (VC-VSIs) and current controlled voltage source inverters (CC-VSIs) working together in both modes of operation. The primary control structures for the VC-VSIs and CC-VSIs is considered together with the secondary control loops that are used to synchronize the microgrid as a single unit to the grid. Simulation results are given that show the seamless transitions between the two modes without any disconnection times for the CC-VSIs and VC-VSIs connected to the microgrid.peer-reviewe

Numerical modelling techniques applicable for the prediction of residual stresses and distortion due to mild steel DH36 frictions stir welding

Friction stir welding involves a multi-physics phenomena, including visco-plasticity, material flow, metallurgical transformation, heat generation, thermal straining and structural interaction. Numerical modelling provides an efficient and cost effective tool capable to analysis and predict the different phenomena. This study integrates different numerical modelling strategies to ultimately develop a robust yet computationally efficient modelling technique capable of predicting residual stresses and distortion due to FSW. A computational efficient local-global numerical model capable of predicting the material visco-plastic flow, thermal transients, stir/heat affected zone, residual stresses and distortion developed due to friction stir welding of DH36 plates is described. Different thermo-elasto-plastic modelling strategies ranging from analytical to transient numerical models are explored and the most robust and computational efficient strategy is identified through cross-reference with the realistic experimental test results

Modelling magnetohydrodynamics to investigate variation of shielding gases on arc characteristics in the GTAW process

Gas tungsten arc welding requires a gas shield to be present in order to protect the arc area from contamination by atmospheric gases. As a result of each gas having its own unique thermophysical properties, the shielding gas selected can have a major influence on the arc stability, welding speed, weld appearance and geometry, mechanical properties and fume generation. Alternating shielding gases is a relatively new method of discreetly supplying two different shielding gases to the welding region in order to take advantage of the beneficial properties of each gas, as well as the inherent pulsing effects generated. As part of an ongoing process to fully evaluate the effects of this novel supply method, a computational fluid dynamics model has been generated to include the gas dependent thermodynamic and transport properties in order to evaluate the effects that an alternating gas supply has on the arc plasma. Experimental trials have also been conducted to validate the model arc profile predictions

Reactive power sharing and voltage harmonic distortion compensation of droop controlled single phase islanded microgrids

When paralleling multiple inverters that are capable of operating as an island, the inverters typically employ the droop control scheme. Traditional droop control enables the decentralized regulation of the local voltage and frequency of the microgrid by the inverters. The droop method also enables the inverters to share the real and reactive power required by the loads. This paper focuses on some of the limitations of parallel islanded single phase inverters using droop control. Algorithms with the aim to address the following limitations in islanded operation were proposed: reactive power sharing and reduction of the voltage harmonic distortion at the point of common coupling (PCC). Experimental results were then presented to show the suitability of the proposed algorithms in achieving reactive power sharing and in improving the voltage harmonic distortion at the PCC.peer-reviewe