An Enhancing Fault Current Limitation Hybrid Droop/V-f Control for Grid-Tied Four-Wire Inverters in AC Microgrids

Microgrid integration and fault protection in complex network scenarios is a coming challenge to be faced with new strategies and solutions. In this context of increasing complexity, this paper describes two specific overload control strategies for four-wire inverters integrated in low voltage four-wire alternating current (AC) microgrids. The control of grid-tied microgrid inverters has been widely studied in the past and mainly focused on the use of droop control, which hugely constrains the time response during grid-disconnected operation. Taking into account the previous knowledge and experience about this subject, the main contribution of these two proposals regards providing fault current limitation in both operation modes, over-load capability skills in grid-connected operation and sinusoidal short-circuit proof in grid-disconnected operation. In the complex operation scenarios mentioned above, a hybrid combination of AC droop control based on dynamic phasors with varying virtual resistance, and voltage/frequency master voltage control for grid-(dis)connected operation modes are adopted as the mechanism to enhance time response. The two proposals described in the present document are validated by means of simulations using Matlab/Simulink and real experimental results obtained from CENER (The National Renewable Energy Centre) experimental ATENEA four-wire AC microgrid, obtaining time responses in the order of two-three grid cycles for all cases.

Progressive Data Synchronization Model for Mobile Devices

Mobile data synchronization is an important technique used to replicate or synchronize data between a mobile client and a remote server. It helps overcome unstable wireless networks and support the disconnected operation. The current state of mobile data synchronization is most successful at transparently synchronizing all data. Generally, once a synchronization feature enabled, data is automatically synchronized without offering a fine grained and customizable synchronization policy. Furthermore, the context information, such as localization, date, device overload and resource consumption, type and quality of the connection, etc., are not equally taken into account or not considered at all. One solution is to develop a synchronization model that rely on a fine grained and flexible synchronization policy and where context information is considered as first-class citizen. This article puts forward a new model of data synchronization in mobile devices based on progressive data access schema.

Remote Digital Signing for Mobile Commerce

Mobile agents (MAs) are a promising technology which directly address physical limitations of mobile devices such as limited battery life, intermittent and low-bandwidth connections, with their capability of providing disconnected operation. This chapter addresses the problem of digital contract signing with MAs, which is an important part of any mobile commerce activity and one special challenging case of computing with secrets remotely in public. The authors use a multi-agent model together with simple secret splitting schemes for signing with shares of a secret key carried by MAs, cooperating to accomplish a trading task. In addition to known key splitting techniques of RSA, authors introduce similar techniques for El Gamal and DSS public key cryptosystems. The objective is to achieve a simple and ubiquitous solution by using the well-known public-key cryptosystem implementations, which conform to the established standards.