MVDC Marine Electrical Distribution: Are we ready?

Marine high-power on-board electrical systems are predominantly utilizing three-phase medium voltage alternating current (MVAC) distribution. Depending on the size and purpose of the ship, on-board electrical systems provide supply to loads in excess of 60MW. Increasingly, medium voltage direct current (MVDC) distribution systems are being considered as an alternative. An increase in the fuel efficiency of the prime movers, the removal of bulky low frequency transformer and the easier integration of different storage technologies are especially attractive. This paper discusses the opportunities but also the technical difficulties associated with the transition from an MVAC to an MVDC system, using an existing LNG tanker MVAC on-board distribution system, as example

Systemic inflammation and acute-on-chronic liver failure: too much, not enough

ACLF is a specific, but complex and multifactorial form of acute decompensation of cirrhosis and is characterized by an extraordinary dynamic natural course, rapidly evolving organ failure, and high short-term mortality. Dysbalanced immune function is central to its pathogenesis and outcome with an initial excessive systemic inflammatory response that drives organ failure and mortality. Later in its course, immuno-exhaustion/immunoparalysis prevails predisposing the patient to secondary infectious events and reescalation in end-organ dysfunction and mortality. The management of patients with ACLF is still poorly defined. However, as its pathophysiology is gradually being unravelled, potential therapeutic targets emerge that warrant further study such as restoring or substituting albumin via plasma exchange or via albumin dialysis and evaluating usefulness of TLR4 antagonists, modulators of gut dysbiosis (pre- or probiotics), and FXR-agonists

Dynamic Assessment of Source–Load Interactions in Marine MVDC Distribution

Medium-voltage direct-current (MVDC) distribution is a possible replacement due to the advancements in power electronic technologies, for existing medium-voltage ac distribution on ships. The new systems based on MVDC are expected to increase fuel efficiency, remove bulky low frequency transformers used for voltage coordination, and integrate storage technologies. These expected benefits of MVDC come with challenges such as stability and reliability of the new distribution system. In this paper, the effect of three different source-side converters, based on commercially available technology, on the MVDC distribution grid and their interactions with the constant power loads (propulsion drives) are investigated. Additionally, the effect of variations in the filtering effort and the distribution lengths on the system stability is analyzed using the impedance-based stability assessment

Marine MVDC Multi-Phase Multi-Pulse Supply

This paper presents comparative evaluation and explores the different technologies suitable for the implementation of the Medium Voltage Direct Current (MVDC) electrical supplies for the marine on-board electrical distribution network. To realize the MVDC supply in the range from 5 kV to 25 kV, various technological choices are available during the design process and have a direct influence on the overall system performances. In this work we discuss different prime movers, choice of electrical generators and type of rectifiers. Different configurations are characterized with different metrics, in terms of commercial availability, quality of supply, efficiency, dynamic performance and volume. Multi-phase multi-pulse supply configurations are identified to provide high quality dc supply and recommended for marine MVDC distribution

MVDC Supply Technologies for Marine Electrical Distribution Systems

The increase in the popularity of the medium voltage dc (MVDC) electrical distribution, as a possible evolution of the medium voltage ac (MVAC) electrical distribution for the ship on-board power systems, arises a need for a comparative evaluation and demonstration of feasible technologies for the MVDC supplies. For designing the MVDC supplies in the range from 5 - 35 kV, different available technologies can be considered for the designs and have a direct influence on the overall system performance. In this paper, different technologies for prime movers, electrical generators and rectifiers are discussed in terms of feasibility for the MVDC supplies. Different supply configurations can be envisioned from these based on the commercial availability, quality of supply, efficiency, dynamic performance and volume. Multi-phase multi-pulse supply configurations are identified and proposed for the marine MVDC systems. Combination of multi-phase generators and multi-pulse rectifiers offer reliable, simple and fault tolerant solution with acceptable dynamics. To explore and highlight these benefits, a six-pulse rectifier sub-module is designed and analysed in two arrangements for multi-pulse configurations, namely parallel and series. It has been shown that with appropriate selection of semiconductor devices, coupled with properly selected fast fuses, excellent fault current (thermal) withstand capabilities can be achieved

Rituximab and improved nodular regenerative hyperplasia-associated non-cirrhotic liver disease in common variable immunodeficiency: a case report and literature study

Common variable immunodeficiency (CVID) associated liver disease is an underrecognized and poorly studied non-infectious complication that lacks an established treatment. We describe a CVID patient with severe multiorgan complications, including non-cirrhotic portal hypertension secondary to nodular regenerative hyperplasia leading to diuretic-refractory ascites. Remarkably, treatment with rituximab, administered for concomitant immune thrombocytopenia, resulted in the complete and sustained resolution of portal hypertension and ascites. Our case, complemented with a literature review, suggests a beneficial effect of rituximab that warrants further research

PWM converter for a highly non-linear plasma load

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005.This thesis discuss an investigation into the applicability of modern high frequency power conversion technology in the plasma mineral processing industry. The physics governing the plasma in a processing environment are analysed to provide a clear understanding of this plasma as electrical load. This was done to create an electrical model for the plasma as load and gain understanding into the electrical supply requirements. Modern high frequency power conversion technology is contrasted with thyristor controlled line frequency technologies to provide a suitable starting point for the study. A 3 kW soft switched converter is proposed for application with a plasma load. This converter is designed and verified. The small-signal signature of the proposed converter under peak current mode control is investigated and a new model is proposed to describe this control configuration

Natural balancing mechanisms in converters

Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2011.AFRIKAANSE OPSOMMING: Hierdie proefskrif handel oor die natuurlike balanserings meganismes van veelvlakkige en modulêre omsetters wat fase-skuif dragolf puls wydte modulasie gebruik. Die meganismes kan in twee hoof groepe verdeel word: ‘n swak balanserings meganisme wat afhanklik is van die oorvleuling van die skakelfunksies en ‘n sterk meganisme wat voorkom ongeag of die skakelfunksies oorvleul al dan nie. Die sterk meganisme verdeel verder in twee subgroepe, ‘n direkte oordrag van onbalans energie en ‘n meganisme wat afhang van die verliese in die stelsel. Elkeen van die meganismes word aan die hand van ‘n omsetter topologie waarin die spesifieke meganisme oorheers beskryf en ontleed. In die ondersoek word klem geplaas op die daarstelling van uitdrukkings om die tydskonstantes van herbalansering na ’n afwyking vir elk van die omsetter toplologieë te beskryf.ENGLISH ABSTRACT: This thesis investigates the natural balancing mechanisms in multilevel and modular converters using phase shifted carrier pulse width modulation. Two groups of mechanisms are identified; a weak balancing mechanism that is only present when the switching functions are interleaved and a strong mechanism that occurs irrespective of the interleaving of the switching functions. It is further shown that the strong balancing mechanism can be divided into a balancing mechanism that depends on the direct exchange of unbalance energy and a loss based balancing mechanism. Each of the mechanisms is discussed and analysed using a converter where the specific mechanism dominates as example. Emphasis is placed on the calculation of the rebalancing time constant following a perturbation. Closed form expressions for the rebalancing time constants for each of the analysed converters are presented

Calculating boundaries for the natural voltage balancing time-constant of the constant duty cycle single leg flying capacitor converter

Purpose - The flying capacitor converter (FFC) balances the clamping capacitor voltages naturally when phase shifted carrier modulation is used. Several models that describe this mechanism, and to estimate the time constants following a perturbation, are discussed in the literature. However, due to the model complexity, numerical methods must be used to evaluate these models. This paper aims to present a closed form expression, using a reference table, that describes a maximum bound for the voltage balancing time constant. Design/methodology/approach - The FCC is analysed in the frequency domain. A decomposition of the characteristic matrix that describes the voltage balancing mechanism is used. The resulting real symmetric matrix is factorised by using approximations of the load characteristics at the frequencies of interest. Findings - The minimum eigenvalue of the factorised matrix is used to determine a maximum bound for the time constant of the voltage balancing. Since the factorised matrix is independent of variations in switching frequency and load, the eigenvalue of interest can be calculated once and tabulated. Originality/value - The closed form expression can be used for quick calculations of the maximum time constant under different operating conditions. Furthermore, the expression provides considerable insight into the influences circuit design choices have on the balancing mechanism