Protection of the switches in a three-level inverter by using modified switching schemes for the redistribution of losses

Elektronische Ventile (IGBT, IGCT, usw.) eines Drei-Punkt-Wechselrichters (3L-NPC-VSI) für Mittelspannungsanwendungen werden üblicherweise auf separaten Kühlkörpern montiert, als Folge entsteht eine schwache thermische Kopplung zwischen den Ventilen. Wie aus der Literatur bekannt ist, kann im normalen Betrieb des Wechselrichters die Wärmeverteilung zwischen den Schaltern mit Hilfe verschiedener Verfahren verbessert werden. Trotzdem ist die Entstehung von lokalen Hotspots unter bestimmten Fehlersituationen wie: Störung im Kühlsystem, schlechter Platzierung der Ventile, Fehler in der Elektronik usw. unvermeidbar. Die lokale thermische Überlastung erhöht das Ausfallrisiko der betroffenen Ventile und reduziert die Lebensdauer des gesamten Systems. In der vorliegender Arbeit wird ein neuartiger fehlertoleranter Regelungsansatz entwickelt, um das Problem anzugehen. Im Falle der lokalen thermischen Überlastung wird die Pulsweitenmodulation bzw. werden die Schaltmuster so adaptiert, dass die thermisch belasteten Ventile geschont werden und ein Teil der Wärme auf andere, nicht überlastete Schalter verlagert wird. Dabei werden redundante Zustände des 3L-NPC-VSI ausgenutzt, welche dieselbe Ausgangsspannung haben, aber unterschiedlichen Einfluss auf die Verlustverteilung des Wechselrichters nehmen. Dank der aktiven Verlustverteilung zwischen den Ventilen bewegt sich der Temperaturwert der überlasteten Ventile im erlaubten Bereich und das System kann im Betrieb bleiben. Darüber hinaus wird die Lebensdauer des Wechselrichters selbst im Fehlerzustand maximiert. Der vorgeschlagene Ansatz wird in zwei verschiedenen Varianten untersucht. Die erste betrachtet den 3L-NPC-VSI als Stellglied einer feldorientiert geregelten Asynchronmaschine und modifiziert die Pulsweitenmodulation entsprechend. Im zweiten Fall wird die Direkte Drehmomentregelung (DTC) betrachtet und die Schalttabelle modifiziert. Die vorgeschlagene fehlertolerante Prozedur minimiert auch ihre Auswirkungen auf die Mittelpunktspannung des Zwischenkreises, ohne dass zusätzliche Hardware notwendig ist. Das Verfahren wird mittels Simulation und zahlreicher Messungen an einem Laborprüfstand validiert.Switching devices (IGBT, IGCT, etc.) of Three-level Neutral-Point-Clamped Voltage Source Inverters (3L-NPC-VSI) for high-power applications are typically mounted on dedicated heatsinks, which results in a lack of heat sharing among the switches. Several control strategies have been proposed to enable even distribution of losses among the switches in normal operation, however a local thermal overload can occur due to adverse operating conditions such as: degradation of cooling system, inappropriate placing of semiconductor device, failures in electronics, etc. The thermal overload increases the risk of thermal breakdown of the affected device and reduces consequently the expected lifetime of inverter. A new fault-tolerant control approach is proposed in the work to deal with this matter. In case of thermal overload, the control method of inverter is immediately adapted, so that the stressed switch is relieved, the heat is distributed to the other devices which are not affected by the overload. Here, redundant states of multi-level inverter are taken advantage of, since redundancies of a voltage space vector deliver the same line-to-line output voltage, but having different impact on the loss distribution among the switches of the inverter. Thanks to the active redistribution of losses among the switches, the temperature of the affected device is kept under the critical limit. In this way, the lifetime of the inverter is maximized even in case of thermal overload. The proposed strategy will be examined with two control methods for the inverter feeding an induction machine: Fields Oriented Control (FOC) and Direct Torque Control (DTC), which are very widespread in the practice. The implementation of each control method requires different considerations and strategies. Finally, the impact of the optimized switching pattern on loss distribution among the switches will be verified by means of simulation and measurements on a laboratory set-up. During operation of the proposed control methods, the stability of the neutral-point potential of the inverter is ensured without any additional hardware

A BRANCH AND BOUND ALGORITHM FOR WORKFLOW SCHEDULING

Nowadays, people are connected to the Internet and use different Cloud solutions to store, process and deliver data. The Cloud consists of a collection of virtual servers that promise to provision on-demand computational and storage resources when needed. Workflow data is becoming an ubiquitous term in both science and technology and there is a strong need for new  tools and techniques to process and analyze large-scale complex datasets that are growing exponentially. scientific workflow is a sequence of connected tasks with large data transfer from parent task to children tasks. Workflow scheduling is the activity of assigning tasks to execution on servers and satisfying resource constraints and this is an NP-hard problem. In this paper, we propose a scheduling algorithm for workflow data that is derived from the Branch and Bound Algorithm

Incorporate ACO routing algorithm and mobile sink in wireless sensor networks

Today, science and technology is developing, particularly the internet of things (IoT), there is an increasing demand in the sensor field to serve the requirements of individuals within modern life. Wireless sensor networks (WSNs) was created to assist us to modernize our lives, saving labor, avoid dangers, and that bring high efficiency at work. There are many various routing protocols accustomed to increase the ability efficiency and network lifetime. However, network systems with one settled sink frequently endure from a hot spots issue since hubs close sinks take a lot of vitality to forward information amid the transmission method. In this paper, the authors proposed combining the colony optimization algorithm ant colony optimization (ACO) routing algorithm and mobile sink to deal with that drawback and extend the network life. The simulation results on MATLAB show that the proposed protocol has far better performance than studies within the same field

Neural-BO: A Black-box Optimization Algorithm using Deep Neural Networks

Bayesian Optimization (BO) is an effective approach for global optimization of black-box functions when function evaluations are expensive. Most prior works use Gaussian processes to model the black-box function, however, the use of kernels in Gaussian processes leads to two problems: first, the kernel-based methods scale poorly with the number of data points and second, kernel methods are usually not effective on complex structured high dimensional data due to curse of dimensionality. Therefore, we propose a novel black-box optimization algorithm where the black-box function is modeled using a neural network. Our algorithm does not need a Bayesian neural network to estimate predictive uncertainty and is therefore computationally favorable. We analyze the theoretical behavior of our algorithm in terms of regret bound using advances in NTK theory showing its efficient convergence. We perform experiments with both synthetic and real-world optimization tasks and show that our algorithm is more sample efficient compared to existing methods

Efficiency of Farmer Organisations in Supplying Supermarkets with Quality Food in Vietnam

The development of supermarkets in Vietnam, as in other emerging countries, goes along with an increasing concern on the part of purchasers for food quality. The paper investigates whether farmer organisations are able to help small-scale farmers get access to supermarkets, and the role that supermarkets and public support play in their emergence and development. It is based on case studies involving a number of stakeholders marketing vegetables, flavoured rice and litchi fruit in Vietnam. The interviews investigated patterns of horizontal and vertical coordination that link farmers to supermarkets, the distribution of costs and benefits between farmers and traders along the chains in relation to the strategy of quality differentiation. Eight farmer associations that work in the form of private commercial organisations are regular supermarket suppliers for the selected products. Their ability to supply supermarkets is related to the combination of functions they make available to their members, especially as regards training to improve quality (appearance, taste, safety), quality promotion and control, for which they receive public support, as well as their participation in flexible contracts with supermarkets, shops and schools. Supermarket supply through farmer associations increases farmer incomes when compared with traditional chains, yet the situation is reported to change with the increase in supermarket competition. The paper argues that changes in farmer organisation are not only due to supplying supermarkets, but also to public and international support to food quality improvement, which have been of benefit to supermarkets.Agribusiness, Industrial Organization,