Analysis of a voltage instability incident in the Greek power system

The paper reports on a 1996 incident in the Greek power system that had all the characteristics of voltage instability and the subsequent analyses and countermeasures. Voltage stability analysis is performed by both NTUA and ULg software tools that give identical simulation results. After the upgrades performed by PPC a considerable increase in the maximum power that can be fed to the Athens area is achieved. Finally, secure operation limits of the upgraded system are calculated

Improvement of biomethane production from sewage sludge in co-digestion with glycerol and waste frying oil, using a design of experiments

A central composite design circumscribed method was used to define the experimental conditions that improve the methane production rate (kCH4, liters of methane per kilogram of VS of waste added and per day) and the cumulative methane production (cMP, liters of methane per kilogram of VS of waste added) of the co-digestion of sewage sludge (SS) with crude glycerol (cGly) and waste frying oil (WFO). Three factors were selected, i.e., SS concentration, global co-substrate concentration, and mass fraction of cGly (xcGly) in a mixture of cGly and WFO (in chemical oxygen demand, COD). SS digestion without co-substrate reached a cMP of (294±6) L·kg1 and a kCH4 of (64±1) L·kg1·d1, at standard temperature and pressure conditions and expressed relatively to the initial volatile solids. After statistical analysis, SS and co-substrate concentrations of 4.6 g·L1 and 8.8 g·L1 (in COD), respectively, with xcGly of 0.8, were defined to simultaneously boost cMP (91 % more) and kCH4 (3-fold increase). Application of these conditions would yield 214 MWh more in electricity per 1000 m3 of SS digested.This study was supported by the Portuguese Foundation for Science and Technology (FCT) under the scope of Project RECI/BBB-EBI/0179/2012 (FCOMP-01-0124-FEDER027462), the strategic funding of UID/BIO/04469/2013 unit and COMPETE 2020 (POCI-01-0145-FEDER-006684), and BioTecNorte operation (NORTE-01-0145-FEDER-000004) funded by European Regional Development Fund under the scope of Norte2020—Programa OperacionalRegional doNorte. The authors also acknowledge the financial support of the FCT (ESF) through the grant given to J.V. Oliveira (SFRH/BD/111911/2015). Research of A.J. Cavaleiro was supported by the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 323009.info:eu-repo/semantics/publishedVersio