DC grids for integration of large scale wind power

It is widely recognized that the future wind power development in the Nordic region will be to a large extent be offshore. The most promising technology for that is the voltage source converter (VSC) HVDC. A review of the most important options and state-of-the-art related to transients in HVDC systems, converters, grid topologies, control strategies and wind power clustering are presented in this paper

Physiological parameters for Prognosis in Abdominal Sepsis (PIPAS) Study : a WSES observational study

BackgroundTiming and adequacy of peritoneal source control are the most important pillars in the management of patients with acute peritonitis. Therefore, early prognostic evaluation of acute peritonitis is paramount to assess the severity and establish a prompt and appropriate treatment. The objectives of this study were to identify clinical and laboratory predictors for in-hospital mortality in patients with acute peritonitis and to develop a warning score system, based on easily recognizable and assessable variables, globally accepted.MethodsThis worldwide multicentre observational study included 153 surgical departments across 56 countries over a 4-month study period between February 1, 2018, and May 31, 2018.ResultsA total of 3137 patients were included, with 1815 (57.9%) men and 1322 (42.1%) women, with a median age of 47years (interquartile range [IQR] 28-66). The overall in-hospital mortality rate was 8.9%, with a median length of stay of 6days (IQR 4-10). Using multivariable logistic regression, independent variables associated with in-hospital mortality were identified: age > 80years, malignancy, severe cardiovascular disease, severe chronic kidney disease, respiratory rate >= 22 breaths/min, systolic blood pressure 4mmol/l. These variables were used to create the PIPAS Severity Score, a bedside early warning score for patients with acute peritonitis. The overall mortality was 2.9% for patients who had scores of 0-1, 22.7% for those who had scores of 2-3, 46.8% for those who had scores of 4-5, and 86.7% for those who have scores of 7-8.ConclusionsThe simple PIPAS Severity Score can be used on a global level and can help clinicians to identify patients at high risk for treatment failure and mortality.Peer reviewe

Converter interactions in VSC-based HVDC systems

The main objective of this thesis is to perform stability and control studies in the area of VSC-HVDC. A major part of the investigation focuses on the explanation of poorly-damped conditions and instability that are linked to dc-side resonances. Initially, a frequency domain approach is considered, applied to a two-terminal VSC-HVDC connection that is modeled as a Single-Input Single-Output (SISO) feedback system, where the VSC-system and dc-grid transfer functions are defined and derived. The passivity analysis and the net-damping criterion are separately applied, demonstrating the superiority of the latter as an analysis tool. Furthermore, it was discovered that the net-damping of a system and the damping factor of its poorly-damped dominant poles are correlated in an almost linear way.The occurrence of poorly-damped conditions is further analyzed from an analytical perspective, where the eigenvalues of a two-terminal VSC-HVDC system are approximated by closed-form expressions. This offers the benefit of a deeper understanding in the way selected parameters of the system can affect the frequency and damping characteristics of its eigenvalues. The Similarity Matrix Transformation (SMT) method is introduced in this thesis and applied to the reduced 4th order state-space model of a two-terminal VSC-HVDC system. The results show that the SMT offers improved accuracy in approximating the actual eigenvalues of the system, compared to the already established LR method.Finally, studies are performed in VSC-MTDC grids, with the main objective of proposing advanced control strategies that can offer robust performance during steady-state and transient conditions, with improved power flow and direct-voltage handling capabilities. The advantageous properties of the proposed controllers are proven through simulations of four- and five-terminal MTDC grids, in which their benefits compared to their conventional counterparts are shown

Converter interactions in VSC-based HVDC systems

The main objective of this thesis is to perform stability and control studies in the area of VSC-HVDC. A major part of the investigation focuses on the explanation of poorly-damped conditions and instability that are linked to dc-side resonances. Initially, a frequency domain approach is considered, applied to a two-terminal VSC-HVDC connection that is modeled as a Single-Input Single-Output (SISO) feedback system, where the VSC-system and dc-grid transfer functions are defined and derived. The passivity analysis and the net-damping criterion are separately applied, demonstrating the superiority of the latter as an analysis tool. Furthermore, it was discovered that the net-damping of a system and the damping factor of its poorly-damped dominant poles are correlated in an almost linear way.The occurrence of poorly-damped conditions is further analyzed from an analytical perspective, where the eigenvalues of a two-terminal VSC-HVDC system are approximated by closed-form expressions. This offers the benefit of a deeper understanding in the way selected parameters of the system can affect the frequency and damping characteristics of its eigenvalues. The Similarity Matrix Transformation (SMT) method is introduced in this thesis and applied to the reduced 4th order state-space model of a two-terminal VSC-HVDC system. The results show that the SMT offers improved accuracy in approximating the actual eigenvalues of the system, compared to the already established LR method.Finally, studies are performed in VSC-MTDC grids, with the main objective of proposing advanced control strategies that can offer robust performance during steady-state and transient conditions, with improved power flow and direct-voltage handling capabilities. The advantageous properties of the proposed controllers are proven through simulations of four- and five-terminal MTDC grids, in which their benefits compared to their conventional counterparts are shown

Analysis of VSC-based HVDC systems

The main objective of this thesis is to perform stability and control studies in the area of VSC-HVDC systems. A major part of the investigation focuses on the development of procedures, whose aim is to understand, explain and avoid poorly-damped conditions or instability that may appear due to dc-side resonances, which stem from the interaction of converters and passive elements in such systems.An analytical approach is initially considered, where the eigenvalues of VSC-HVDC systems are approximated by analytical closed-form expressions. The Similarity Matrix Transformation (SMT) method is introduced and applied to the reduced 4th order state-space model of a two-terminal VSC-HVDC system. The results show that the SMT offers improved accuracy in approximating the actual eigenvalues of the system, compared to the already established LR method. Nevertheless, the two analytical methods are not free of limitations. The increase in modeling accuracy of a system renders the analytical approach impractical or impossible to use. A frequency-domain approach proves ideal in performing a stability analysis in such cases, and is therefore considered and applied to a detailed two-terminal, two-level converter-based VSC-HVDC system. The latter is modeled as a Single-Input Single-Output (SISO) feedback system, where the VSC-system and dc-grid transfer functions are defined and derived. The passivity analysis and the net-damping criterion are separately utilized and assessed on their potential to be adequate analysis tool in VSC-HVDC stability studies.In contrast with the typical Two-Level Converter (2LC), the Modular Multilevel Converter (MMC) has a fundamentally different structure that introduces internal dynamics and requires additional control for the converter to operate properly. The dc-side input admittance of the MMC is analytically derived, allowing the dynamic impact of MMCs in two-terminal VSC-HVDC systems to be analyzed from a frequency-domain perspective. The contribution of the MMC\u27s circulating-current control to the closed-loop system stability is investigated and the differences of the MMC and the 2LC in terms of their passivity characteristics are highlighted.Finally, studies are performed in VSC-based Multiterminal grids, with the objective of proposing advanced control strategies that can offer robust performance during steady-state and transient conditions, with improved power flow and direct-voltage handling capabilities. The properties of the proposed controllers are assessed through simulations of four- and five-terminal grids, where their benefits compared to those of their conventional counterparts are shown

Decentralized converter controller for multiterminal HVDC grids

Multiterminal High Voltage Direct Current (MTDC) grids based on Voltage Source Converters (VSC) are considered ideal for the future integration of large scale and highly distributed renewable power generation, as well as interconnection of remote power systems. The control of such grids is challenging as it is necessary to provide accurate power flow control and multiple voltage controlling stations. Existing control techniques either fail to fulfill those requirements or achieve them at a cost of maintaining a communication channel between the local station controllers and a master central control level. This paper suggests a decentralized controller for MTDC grids that ensures accurate power flow control, provides voltage control capabilities to all stations and requires no need for communication between the stations. The validity of the controller was tested in a five-terminal MTDC model proving its functionality

Contribution of vestibular evoked myogenic potentials in the evaluation of patients with idiopathic sudden hearing loss

Objective: To evaluate Vestibular Evoked Myogenic Potentials (VEMP) testing results in patients with unilateral Idiopathic Sudden Hearing Loss (ISHL) and to correlate them with the findings of caloric testing, the clinical appearance of vertigo, the influence of age, the grade and type of initial hearing loss and finally, the early recovery of subjects.Materials and Methods: One hundred and four (104) patients with unilateral ISHL and 35 healthy controls were subjected to a standard protocol of neurotological evaluation. VEMP responses were measured and compared to caloric responses, to epidemiological and clinical parameters and to the initial and final audiograms of each subject. Results: Overall, abnormal VEMP responses were observed on the affected side in 28.8% of patients whereas abnormal Caloric findings occurred in 50% of the subjects studied. A significant relationship was found between the results of the two tests, meaning that the highest probability is either of the lesion to spare both otoliths and semicircular canals or, to involve both labyrinthic structures at the same time. A significant relationship was found between the type of vestibular lesion and subjective vertigo. Advancing age correlated statistically with more extended labyrinthic lesion, mostly including both semicircular canals and otolithic organs. Moreover, a statistically significant relationship was found between the type of inner ear lesion and the incidence of profound hearing loss. The type of vestibular lesion did not correlate with the occurrence of high frequency hearing loss, and the audiogram type had no significant effect on the likelihood of hearing recovery. Finally, a negative correlation was found between the extent of inner ear lesion and the likelihood of early recovery.Conclusions: The combination of VEMP and ENG testing has proven vestibular involvement in many ISHL patients. Both tests are necessary in order to obtain a more thorough and deep knowledge of the pathophysiology of ISHL. The extent of inner-ear lesion tends to correlate with the severity of cochlear damage in ISHL. Vestibular assessment can be extremely valuable in prediction of the final outcome.Εισαγωγή: Σκοπός της μελέτης είναι η διερεύνηση των αποτελεσμάτων των Αιθουσαίων Προκλητών Μυογενών Δυναμικών (ΑΠΜΔ) σε ένα μεγάλο πληθυσμό ασθενών με ιδιοπαθή μονόπλευρη αιφνίδια βαρηκοΐα και η συσχέτιση των αποτελεσμάτων αυτών με τα ευρήματα της ηλεκτρονυσταγμογραφίας με ψυχροθερμοδιακλυσμούς (ΗΝΓ), με την κλινική εμφάνιση διαταραχών ισορροπίας, την επίδραση του παράγοντα της ηλικίας, το βαθμό και τον τύπο της βαρηκοΐας και τέλος με την πιθανότητα βελτίωσης της ακοής των ασθενών αυτών. Υλικό και Μέθοδοι: Πλήρης νευρο-ωτολογικός έλεγχος διενεργήθηκε σε 104 ασθενείς με αιφνίδια βαρηκοΐα, καθώς και σε 35 φυσιολογικούς ενήλικες (70 αυτιά), αντίστοιχου φύλου και ηλικίας που αποτέλεσαν την ομάδα των φυσιολογικών μαρτύρων. Τα αποτελέσματα των ΑΠΜΔ μετρήθηκαν και στη συνέχεια συγκρίθηκαν με αυτά της ΗΝΓ, με επιδημιολογικούς και κλινικούς παράγοντες και τέλος με το αρχικό και το τελικό τονικό ακοόγραμμα των ασθενών. Αποτελέσματα: Συνολικά, παθολογικά αποτελέσματα στη δοκιμασία των ΑΠΜΔ στην πλευρά της βλάβης παρουσιάσθηκαν στο 28.8% των ασθενών, ενώ παθολογικά ευρήματα στη δοκιμασία της ΗΝΓ εμφανίσθηκαν στο 50% των πασχόντων. Τα αποτελέσματα των δύο αιθουσαίων δοκιμασιών παρουσίασαν στατιστικά σημαντική συσχέτιση μεταξύ τους, καταδεικνύοντας την υψηλή πιθανότητα η βλάβη να προσβάλει ή να αφήνει ταυτόχρονα άθικτα τα ωτολιθοφόρα όργανα και τους ημικυκλίους σωλήνες. Στατιστικά σημαντική συσχέτιση παρουσίασε επίσης ο τύπος της αιθουσαίας συμμετοχής και η κλινική εκδήλωση ιλίγγου. Η πάροδος της ηλικίας επέδειξε στατιστική συσχέτιση με πιο εκτεταμένη προσβολή του λαβυρίνθου, περιλαμβάνοντας συνήθως τόσο το σφαιρικό κυστίδιο όσο και τους ημικυκλίους σωλήνες. Επιπλέον, στατιστικά σημαντική συσχέτιση παρουσιάσθηκε μεταξύ του τύπου της δυσλειτουργίας του έσω ωτός και της εμφάνισης πρακτικής κώφωσης στο τονικό ακοόγραμμα. Αντίθετα, η έκταση της αιθουσαίας βλάβης δεν εμφάνισε συσχέτιση με την εμφάνιση αιφνίδιας βαρηκοΐας υψηλών συχνοτήτων, ενώ ο τύπος της αρχικής βαρηκοΐας δεν σχετίζεται με την πιθανότητα βελτίωσης της ακοής. Τέλος, η έκταση της παθολογίας του λαβυρίνθου και η πρόγνωση της νόσου παρουσίασαν αντίστροφη συσχέτιση μεταξύ τους.Συμπεράσματα: Ο συνδυασμός των διαγνωστικών δοκιμασιών των ΑΠΜΔ και της ΗΝΓ ανέδειξε τη συμμετοχή του αιθουσαίου συστήματος σε σημαντικό ποσοστό των ασθενών με αιφνίδια βαρηκοΐα. Με βάση τα αποτελέσματα της μελέτης, αμφότερες οι αιθουσαίες δοκιμασίες συμβάλλουν στην βαθύτερη μελέτη και κατανόηση της παθοφυσιολογίας της νόσου. Η έκταση της δυσλειτουργίας του έσω ωτός σχετίζεται με τη βαρύτητα της κοχλιακής βλάβης στις περιπτώσεις της αιφνίδιας βαρηκοΐας. Η εργαστηριακή διερεύνηση της αιθουσαίας λειτουργίας στους ασθενείς αυτούς αποδεικνύεται ιδιαίτερα χρήσιμη στην πρόγνωση της αποκατάστασης της απώλειας της ακοής